



Produced by Chris Curnow, Harry Lame and the Online
Distributed Proofreading Team at http://www.pgdp.net (This
file was produced from images generously made available
by The Internet Archive)







  Transcriber's Notes:

  Text printed in italics, bold face and small capitals in the original
  work have been transcribed between _underscores_, =equal signs= and
  ALL CAPITALS, respectively.

  [pi] represents the Greek letter pi, _{x} indicates a subscript x, and
  ^{x} a superscript x.

  More Transcriber's Notes may be found at the end of this text.




  Books by
  Horstmann & Tousley


  Motion Picture Operation, Stage Electrics and Illusions.    $2.00

  (Ready July 1, 1914.)

  Alternating Current Theory, Practice and Diagrams.          $2.00

  Modern Electric Illumination.                               $2.00

  Practical Armature and Magnet Winding.                      $1.50

  Modern Electrical Construction.                             $1.50

  Electrical Wiring and Construction Tables.                  $1.50

  Dynamo Tending for Engineers.                               $1.50

  Modern Wiring Diagrams and Descriptions.                    $1.50

  Electricians' Operating and Testing Manual.                 $1.50




  Motion Picture Operation
  STAGE ELECTRICS
  AND ILLUSIONS


  A Practical Hand-book and Guide for Theater
  Electricians, Motion Picture Operators and
  Managers of Theaters and
  Productions


  By
  Henry C. Horstmann
  and
  Victor H. Tousley


  Authors of
  "Alternating Current," "Modern Wiring Diagrams," "Modern
  Electrical Construction," "Electrical Wiring and Construction
  Tables," "Practical Armature and Magnet
  Winding," "Electricians' Operating and Testing
  Manual," "Modern Illumination."


  ILLUSTRATED

  [Illustration]


  Chicago
  FREDERICK J. DRAKE & CO.
  Publishers


  Copyright 1914, by
  HENRY C. HORSTMANN
  and
  VICTOR H. TOUSLEY




PREFACE


In this volume the authors have attempted to lay before the
Motion-Picture Operators and Theatrical Employes generally, a reference
and handbook making a specialty of electrical requirements about
theaters.

A working knowledge of electricity in general is assumed, and therefore
elementary ideas have been treated sparingly. A specialty, however, has
been made of all matters peculiar to theaters, and it is thought that
theater electricians will find in this volume everything that they need
whether they be operating motion-picture machines or switchboards in
first-class houses.

The two special chapters "Portable Stage Equipment" and "Theater Wiring"
have been arranged so that they are particularly valuable for reference.
They should be consulted before undertaking any electrical construction
work, either for the stage or for the auditorium. These chapters embody
all of the practical knowledge that has come to the notice of the
authors in many years of actual experience with theatrical construction.

The aim of this volume has been to present in a simple and practical way
the essential principles of Motion-Picture Work.

  THE AUTHORS.




Table of Contents


                                                                   PAGE

  CHAPTER I
  The Electrical Circuit and Electrical Hazards                       9

  CHAPTER II
  The Arc Lamp                                                       19

  CHAPTER III
  Projection                                                         31

  CHAPTER IV
  Motion Pictures                                                    55

  CHAPTER V
  The Motion-Picture Machine                                         62

  CHAPTER VI
  The Film                                                           89

  CHAPTER VII
  General Hints on Installation, Operation and Care of Machines      96

  CHAPTER VIII
  Light                                                             113

  CHAPTER IX
  Principles of Vision                                              122

  CHAPTER X
  Reflection                                                        126

  CHAPTER XI
  Refraction                                                        137

  CHAPTER XII
  Optical Instruments                                               147

  CHAPTER XIII
  Optical Illusions                                                 155

  CHAPTER XIV
  Theater Buildings                                                 163

  CHAPTER XV
  Operating Room Equipment                                          176

  CHAPTER XVI
  Current Control for Arc Lamps                                     190

  CHAPTER XVII
  Management of Generators and Motors                               213

  CHAPTER XVIII
  Theater Wiring                                                    218

  CHAPTER XIX
  Portable Stage Equipment                                          311

  CHAPTER XX
  Useful Facts and Formulas                                         353

  CHAPTER XXI
  Glossary of Electrical, Mechanical and Optical Words, Terms and
  Phrases                                                           358

  Index                                                             385




  MOTION PICTURE OPERATION
  STAGE ELECTRICS AND
  ILLUSIONS




CHAPTER I.

THE ELECTRICAL CIRCUIT AND ELECTRICAL HAZARDS.


_Two and Three-Wire Systems._--If the theater electrician will take the
trouble to trace the circuits in the building to their supply, he will
find them entering the building either as two-wire or three-wire
circuits.

[Illustration: FIGURE 1.]

A two-wire circuit is diagrammatically shown in Figure 1. The circuit,
coming from _1_, enters the building, passes through the fuses _2_, and
through switch 3 to the lights. A two-wire system will ordinarily be
found operating at 110 volts, the current varying according to the
number of lights turned on. In the drawing, for instance, only one light
is shown with the switch closed, the other three switches being open.
The current in the circuit is equal to that which passes through the
single lamp. If another switch be closed, another light will burn and
the current will be increased, so that the more lights be turned on, the
greater will be the current.

[Illustration: FIGURE 2.]

The three-wire system, Figure 2, is almost universally used where the
supply is from the outside and where any considerable number of lights
are connected. The chief advantage of the three-wire system lies in its
economy of copper. The middle or neutral wire ordinarily does not carry
current, but it is a necessity whenever the number of lights burning on
the two sides of the system are not equal.

With the neutral wire omitted, we have a straight two-wire system using
double the voltage of the ordinary two-wire system and always operating
two 110-volt lamps in series. Two lamps would always have to be turned
on at the same time and if one of them should burn out, the other would
be extinguished also.

A system using double voltage requires only half the current and
consequently but half the copper. In order to obviate the necessity of
always using two lamps together and at the same time economizing in
copper, the neutral wire is provided. As long as the same number of
lamps are burning on each side of the neutral wire, the same current
always passes through two lamps in series and there is no current in
the neutral. Should, however, the group on one side be turned out, the
other would still continue to burn; but the path of the current to the
dynamo, or bank of transformers, would be through the neutral wire.

The system is thus seen to possess all the advantages of the ordinary
two-wire system since each lamp can be operated by itself and at the low
voltage, while the actual supply voltage for the whole system is double
that which is actually used at any lamp. We have thus two voltages at
our command; 110 and 220 being the voltages in common use.

_Electrical Hazards._--Since this work is not intended for mere
beginners, we shall not enter into elementary considerations, but shall
take up the matter of fire and life hazard, both of which are important
items to which attention cannot be too often called.

The electrical current may cause fire by overheating the wires. This
overheating may be due to the willful overloading of circuits; and to
prevent this, no wire should ever be used to carry more current than is
allowed by the table of carrying capacities given on page 238.

The overheating may also be due to an unknown load which is caused by a
"ground" or a partial short circuit. "Ground" is the technical term used
to designate the connection of a wire to any substance over which
electricity may be carried to the wire of opposite polarity. A ground
may be caused by a bare wire coming in contact with the iron framework
of a building, wet wood, or moist substances of any kind. One such
ground on a circuit can do no harm; but, if one ground exists, in the
course of time another one may come on and, when the second one
appears, if it is on the side of the circuit opposite to the first,
there will be trouble at once.

If the two grounds are both "good"--that is, if they are of low
resistance--we shall have a short circuit and probably blow a fuse; but
if they are not "good," we may have but a small current which may
continue unnoticed for months. Such a current may eat away the copper of
the positive pole and in time cause the wire to break, creating an
electric arc and perhaps causing a fire. It may also cause wet wood
through which it flows to become charred and finally ignited.

The ground is the bane of the electrical worker. If a system can be kept
free of grounds, the chances of trouble are vastly reduced. The cause of
most grounds is moisture. Nearly all substances except metals are fairly
good insulators if dry; and nearly all of them are fairly good
conductors if sufficiently wet.

Another very prolific source of fires is the electrical spark, large and
small. The spark, due to the breaking of an incandescent lamp, often
causes fires when it comes in contact with inflammable material or
gases. The ordinary lamp cord also causes many fires because it is
easily damaged and liable to short circuits which often result in
arcing. Short circuiting two wires or breaking a wire carrying current
may easily ignite inflammable material in the vicinity.

The best way to reduce the fire hazard to a minimum, is to install all
electrical work carefully according to the rules laid down in the
"National Electrical Code".

The life hazard is one which concerns the operator personally and is
especially great to those traveling with shows. Traveling men are often
obliged to get along with all sorts of make shifts, especially in the
smaller towns where one-night stands are the rule. Here it is often
necessary to connect to trolley circuits or power lines of different
voltages, frequencies, etc.

A person may suffer injury directly from a current of electricity by
making himself part of the circuit. If the system on which he is working
is alive and grounded, he may easily cause injury to himself by touching
a live wire with his hands while standing upon anything that is
grounded. By so doing he completes a path for the current through his
body.

He may also become part of a circuit by holding a wire with both hands
while someone is cutting it between his two hands. As long as a wire so
held is intact, no shock can be received except to ground, but when the
wire is cut or breaks, a very high voltage may be produced for an
instant which will cause a current through the body of the man holding
the wire. The extra high voltage is produced only if the wire is
carrying current at the time it is being cut. Under these circumstances
there will often be a strong flash, due to the momentary increase in
voltage, produced by the breaking of the circuit, which may be
excessive, especially if there is considerable inductance in the
circuit.

The most frequent cause of injury is due to making contact with the two
opposite polarities of a system. As a rule circuits, with which
operators ordinarily have to work, are low voltage, i.e., not over 220
volts. But many deaths have been caused by this voltage, sometimes
directly and at other times indirectly as, for instance, by causing a
fall. People whose hearts are in any way defective should be careful
about exposing themselves to shocks even at 110 volts.

It is true that many wiremen are in the habit of testing 220-volt
circuits by allowing the current to pass through their bodies, but it
will be noticed that they are very careful not to make a good contact.
The current which passes through the body, when one touches two wires of
opposite polarity very lightly with the finger tips, is but a fraction
of what one would receive if he were unwittingly to grab two wires of
opposite polarities with the hands, especially if the hands were moist.

Numerous cases are on record of persons having been killed by 110 volts
under favorable circumstances; as, for instance, while in the bath
receiving a shock from a so-called vibrator. The body partly immersed in
water and perhaps a foot resting against a water pipe forms a conductor
of very low resistance, and a comparatively strong current may pass
through the body.

The most important precautions against injury while working on live
circuits are:

(1) Insulate yourself from the ground.

(2) Handle only one side of a line at a time.

(3) If possible, work with only one hand at a time in contact with the
wires.

(4) Use rubber gloves, or rubber boots where necessary, but bear in mind
that they are of little value unless kept dry. Moisture will allow some
current to pass over the surface of any substance no matter how good an
insulator it may otherwise be.

(5) Always place yourself so that a slight shock which might cause you
to lose your balance will not give you a bad fall.

(6) Remember that if once you make good contact with an
alternating-current circuit, you cannot let go.

(7) Fix firmly in your mind the directions for resuscitation from
electric shock, on pages 15-18.

When energy is obtained through transformers, there is another danger to
be added to the above, viz., the possibility of the breaking down of the
insulation between primary and secondary wires of the transformer. If
this happens, we have suddenly and without warning, instead of the 110
or 220 volts supposed to exist between the wires forming the circuit,
2,000 or 3,000 volts. Such accidents are especially likely during
thunder storms when lightning often breaks down transformers.

In order to reduce this danger to a minimum, the secondaries of
transformers are grounded. It will be well for the electrician to assure
himself that the secondaries of the transformers from which he is
getting his supply are grounded. This can be tested by an incandescent
lamp. Connect the lamp to ground with one wire and, with the other, try
the two sides of the circuit. If the transformer secondaries are
properly grounded, the lamp will burn at full candle power from one of
the wires; this will show that the other wire is grounded.

A person working on such a circuit is of course more likely to receive a
low voltage shock than if the secondaries were not grounded, but he is
fairly well protected against the primary voltage or lightning.


RESUSCITATION FROM ELECTRIC SHOCK.

Rules recommended by commission on resuscitation from electric shock,
representing The American Medical Association, The National Electric
Light Association, The American Institute of Electrical Engineers: Dr.
W. B. Cannon, chairman; professor of physiology, Harvard University. Dr.
Yandell Henderson, professor of physiology, Yale University; Dr. S. J.
Meltzer, head of department of physiology and pharmacology, Rockefeller
Institute for Medical Research; Dr. Edw. Anthony Spitzka, director and
professor of general anatomy, Daniel Baugh Institute of Anatomy,
Jefferson Medical College; Dr. George W. Crile, professor of surgery,
Western Reserve University; W. C. L. Eglin, past-president National
Electric Light Association; Dr. A. E. Kennelly, professor of electrical
engineering, Harvard University; Dr. Elihu Thomson, electrician, General
Electric Company; W. D. Weaver, secretary, editor Electrical World.
Issued and copyrighted by National Electric Light Association. Reprinted
by permission. Follow these instructions even if victim appears dead.


I. IMMEDIATELY BREAK THE CIRCUIT.

With a single quick motion, free the victim from the current. Use any
_dry non-conductor_ (clothing, rope, board) to move either the victim or
the wire. Beware of using metal or any moist material. While freeing the
victim from the live conductor have every effort also made to shut off
the current quickly.


II. INSTANTLY ATTEND TO THE VICTIM'S BREATHING.

1. As soon as the victim is clear of the conductor, rapidly feel with
your finger in his mouth and throat and remove any foreign body
(tobacco, false teeth, etc.) Then _begin artificial respiration at
once_. Do not stop to loosen the victim's clothing now; _every moment of
delay is serious_. Proceed as follows:

_a._ Lay the subject on his belly, with arms extended as straightforward
as possible and with face to one side, so that nose and mouth are free
for breathing, see Figure on page 17. Let an assistant draw forward the
subject's tongue.

[Illustration: INSPIRATION--PRESSURE OFF.]

_b._ Kneel straddling the subject's thighs and facing his head; rest the
palms of your hands on the loins (on the muscles of the small of the
back), with fingers spread over the lowest ribs, as in Figure on page
17.

_c._ With arms held straight, swing forward slowly so that the weight of
your body is gradually, but _not violently_, brought to bear upon the
subject, see Figure on page 18. This act should take from two to three
seconds.

Immediately swing backward so as to remove the pressure, thus returning
to the position shown in the Figure on page 17.

_d._ Repeat deliberately twelve to fifteen times a minute the swinging
forward and back--a complete respiration in four or five seconds.

_e._ As soon as this artificial respiration has been started, and while
it is being continued, an assistant should loosen any tight clothing
about the subject's neck, chest or waist.

[Illustration: EXPIRATION--PRESSURE ON.]

2. Continue the artificial respiration (if necessary, at least an hour),
_without interruption_, until natural breathing is restored, or until a
physician arrives. If natural breathing stops after being restored, use
artificial respiration again.

3. _Do not give any liquid by mouth until the subject is fully
conscious._

4. Give the subject fresh air, but keep him warm.


III. SEND FOR NEAREST DOCTOR AS SOON AS ACCIDENT IS DISCOVERED.




CHAPTER II.

THE ARC LAMP.


_General Discussion of the Electrical Arc._--The name of the electrical
arc lamp is derived from the arch-like appearance of the vapors which
give out the light when the carbons are placed horizontally. The
horizontal arc was the earliest form, hence the name which it carries to
this day.

The arc proper is due to the vapors of volatilized carbon or other
materials forming the electrodes, which may be consumed by the passage
of an electrical current from one electrode to another through the
intervening medium. In order that an arc may be formed, it is necessary
first to bring the electrodes together. This, if the circuit is properly
arranged, starts the current and when the circuit is partly interrupted,
as by slowly separating the points of the electrodes, the current passes
through the intervening space, with the result that a high degree of
heat (about 3,500 centigrade) is produced. This results in volatilizing
the carbon or any other material of which one or both electrodes may
consist.

As long as the distance between the electrode points is small, the
current will be quite strong and a hissing or frying sound will be given
out. In order to keep the current within bounds during the time that the
electrodes are together or while they are separated only a very
short distance, some resistance, or reactance in the case of
alternating-current arcs, is always connected in series in the circuit.
If this were not done, there would be a short circuit at the time of
starting or striking the arc.

The arc formed with very short separation of electrodes is generally
spoken of as a low tension arc and requires very hard carbons and about
25 volts. This type of arc is very little used for illuminating
purposes.

If the distance between the electrodes is increased gradually, the light
becomes very unsteady and flickers considerably until at a certain point
it begins to improve and give the long quiet arc. This condition will
occur when, with direct current, the electrodes are about one-eighth of
an inch apart. It will then be found that the voltage across the arc is
from 45 to 50 volts, which is the best voltage to use with open arcs. If
the separation be carried still further, the arc will grow longer and
become flaming until finally it breaks entirely.

The resistance of the arc is closely proportional to the cross section
of the electrodes and increases with the distance of the arc gap. It
acts, however, very much as though there were a small counter e. m. f.
set up within it.

The color of the light given off varies with the length of the arc
somewhat, but depends mainly upon the material of which the electrodes
consist. In the so-called flaming arcs, the peculiar color is obtained
by certain chemicals imbedded in the material composing the electrodes.
Whenever an arc is allowed to burn down until it reaches the electrode
holders, a greenish light is given off which is due to the
volatilization of the metal--usually brass--in these holders.

The light of a strong arc is extremely injurious to the eyes and should
only be viewed through colored glass. Many very painful experiences have
resulted from persons gazing upon arcs of 200 or 300 amperes, such as
are used sometimes in cutting away metals of old buildings, etc.

The most powerful arcs known at the present time are those used in some
steel mills for refining steel. These use upward of 10,000 amperes.

[Illustration: FIGURE 3.]

[Illustration: FIGURE 4.]

The length of the ordinary arc varies from one thirty-second of an inch
to one inch. The light is not of much use and is rather unsteady until
the electrodes have assumed a shape somewhat similar to that shown in
Figure 3 for direct current, and Figure 4 for alternating current. With
direct-current arcs, a crater is formed at the bottom of the positive
electrode and, from this crater, about 80 per cent of the light is
emitted. Where the light is wanted in a downward direction, the crater
is always formed at the top and for this purpose the top electrode must
be made positive; that is, the electricity must flow from the top
electrode into the lower one. In some cases, where special illumination
effects are desired, the bottom electrode is made the positive with the
result that most of the light is thrown upward. In such cases strong
shadows are thrown against the ceiling and the lamp is said to be
burning "upside down."

The positive electrode can always be distinguished from the negative (a)
by the shadows cast; (b) by the form of the electrodes; and (c) by the
fact that since it is heated to a greater degree, it will, when the lamp
is turned off, remain hot for some time after the negative electrode has
cooled off.

In case the arc is drawn out very long and operated in this way for a
considerable time, the crater will almost wholly disappear and the
electrodes will appear rounded off.

In an alternating-current circuit, the positive and negative poles
reverse generally about 120 times per second and both electrodes in the
alternating-current arc are positive and negative to the same degree.
They are therefore very nearly alike, except that the heat rising from
the lower one increases slightly the volatilization of the upper. The
positive electrode in the direct-current arc is consumed approximately
twice as fast as the negative electrode. The consumption of the two
electrodes in an alternating-current arc is about equal and a crater
much smaller than the kind formed in a direct-current arc is, therefore,
formed on each electrode, instead of only on the positive electrode as
in the case of the direct-current arc.

The general form of alternating-current arc carbons is given in Figure
4. The small elevations shown in the cuts are due to impurities and do
not appear with first-class carbons.

When arc lamps are operated on alternating-current circuits, the best
voltage for the arc is about 28; and consequently, for the same quantity
of light, the current must be increased so that the amperage of
alternating-current lamps is always much greater than that of
direct-current lamps.

The alternating-current arc is much noisier than the direct-current arc,
but with very high frequencies this noise ceases.

In general, arc lamps do not work very well on low frequencies. The time
at which the current is practically zero is long enough to allow the
vapor between the electrode points to cool off sufficiently to interfere
with successful operation.

Any arc light is affected by draughts of air and can even be blown out.
If this occurs often, there will be rapid feeding, a short arc, and
great waste of electrode.

A magnet held close to an arc can be made to blow it out or force it to
one side. This fact is made use of in some lightning arresters.

Generally speaking, arc lamps are of two kinds, _open_ and _enclosed_.
The enclosed arc operates at a much higher voltage and is but little
used about theaters. The open arc is almost universally used for stage
work and this is about the only place where it is still considered
useful. This kind of arc lamp is, however, very hazardous in localities
where inflammable material abounds and for this reason it is always
enclosed with wire mesh when possible.

Lens lamps can be tightly enclosed since none of the light is wanted
except that which passes through the lens in front.

[Illustration: FIGURE 5.]

[Illustration: FIGURE 6.]

The so-called flood lamps are usually provided with wire gauze in front
of the arc, which prevents the escape of pieces of the electrodes and
also prevents parts of scenery, etc., from coming in contact with the
arc.

The lamp houses should be of such dimensions that, with the highest
amperage the lamp is capable of using, the outer walls will not become
excessively hot.

Illustrations of standard lens and flood lamps, as made by the Chicago
Stage Lighting Company, are shown in Figures 5 and 6.

_Operation of Arc Lamps._--From the standpoint of operation, arc lamps
may be divided into two classes, viz.: _hand-feed_ and _automatic-feed_.
The hand-feed lamp is generally used in theaters and is practically the
only kind admitted on the stage, or for stage illuminating purposes.
Only a very few houses now use arc lamps for general illumination.

The operation of hand-feed lamps[1] is ordinarily quite simple and will
be fully treated under the head of "Projection", so that we may now
consider only the automatic lamps. At the present time these are used
mostly, if at all, for the illumination of the exterior of the theater.

  [1] Full diagrams and descriptions are in another work of the
  authors', entitled "Electricians' Operating and Testing Manual," so
  that no space need be given to these here, save in a very general way.

The operator should first familiarize himself with the construction and
principles upon which the mechanism of his lamp is based. For this
purpose he should remove the outer jacket, thus exposing the working
mechanism; turn on the current; and endeavor to learn the significance
of each part. It is of course necessary that the operator understand the
hazards due to manipulating live wires and that he should be very
careful not to make short circuits or grounds which might destroy parts
of the lamp.

Automatic-feed lamps are usually trimmed in the following manner: Bring
the lamp within reach; remove the globe; take out the lower electrode;
let down the upper electrode rod and thoroughly clean it with crocus
cloth. This upper electrode rod is the principal thing that concerns the
lamp trimmer; it must be perfectly straight and care must be exercised
not to bend it accidentally; it must be clean so that the clutches may
properly grip it; it must not be greasy. If it grows dirty or greasy, it
will soon become pitted from the current that passes from the contacts
to it.

The next operation is to remove the upper electrode and place it in the
lower electrode holder. (The length of electrode necessary should be
known. The lower one generally burns out first--it being shorter--and if
the arc reaches the lower electrode holder, will begin to consume it; if
the lower carbon is too long, the arc is liable to reach the upper
electrode holder and destroy it.) The upper electrode may then be placed
in position and aligned with the lower. To do this it is best to turn it
about and try it until it aligns in all positions. The two electrodes
should form a straight line, up and down, no matter which way the upper
is turned.

In some forms of enclosed lamps, the clutch grips the electrode direct.
In such a case all of the upper electrode must be carefully examined to
see that it is straight and free from burs, and that it can pass freely
into the opening at the top of the inner globe. The successful operation
of enclosed arcs depends upon the confinement of the gas in the inner
globe. This globe must, therefore, be kept as tight as possible without
interfering with the operation of the electrodes which pass through it.

With enclosed arcs, the care of the inner globe is of great importance,
because impurities are cast off which soon coat the inner globe and
absorb much of the light.

The care of the outer globes in general is also an important matter. A
dirty globe looks very unsightly and absorbs much light.

The following points should be carefully considered in handling and
trimming lamps:

(1) Be sure that you understand your system and know whether it is a
constant-current or a constant-potential system of distribution. With
constant-current systems, the current is constant and the voltage over
the arc is regulated; while with constant-potential systems, the voltage
is constant and the current through the arc is regulated.

(2) With constant-current or series lamps, the line must never be
opened, but must be shunted around the lamp if a lamp is to be cut out.

(3) With constant-potential lamps, the lamp must never be shunted but
the circuit must be opened.

(4) In all cases each lamp should be controlled by a double pole switch.

(5) Constant-potential lamps cannot be operated without resistance in
the circuit; this resistance may be in the lamp itself or outside.

(6) Never handle high tension lamps without insulating yourself from the
ground; and handle live wires only with one hand at a time.

(7) Provide spark arresters for all open-arc lamps in the vicinity of
inflammable material.

(8) Never leave a lamp without globes where the wind can strike it. It
will be blown out or feed often, thus consuming the electrodes very fast
and at the same time yielding a very poor light.

Green light emitted by the lamp will indicate that the electrode
holders are burning. Strong shadows cast upwards indicate a lamp burning
"upside down". The positive electrode retains heat longer than the
negative. The quality and size of electrodes has much to do with
successful operation. Always use the kind of electrodes recommended by
the maker of the lamp.

Direct-current arc lamps do not require much in the shape of reflectors
as they naturally throw most of the light downward, when the upper
electrode is positive. They should as a rule be suspended high.

Alternating-current arc lamps throw most of the light from the upper
electrode slightly below the horizontal and that from the lower
electrode somewhat above. If the light is wanted in a downward
direction, suitable reflectors must be provided.

_Testing of Arc Lamps._--The constant-potential arc lamp is usually
designed for a certain current and voltage. The enclosed arcs as a rule
operate singly on 110 volts, while open arcs are run two in series on
the same voltage. In order to test and see that the voltage and current
are right, an ammeter and a voltmeter are needed. The current and
voltage can both be adjusted by altering the resistance, which is always
in series with such lamps. To get the correct voltage over the arc, be
sure to connect the voltmeter to the two electrode holders so as to
eliminate any other potential drops that may affect the reading.

_Testing Carbons._--The color of the light and the steadiness of it can
of course only be determined by actual operation tests. The arc obtained
by using large electrodes with low current density is liable to rotate
around the electrodes, burn unsteadily, and flicker. This is due to the
fact that the arc tends to establish itself at the point of least
resistance. In order that the arc may burn uniformly, the current
density must be great enough to force all of the electrode points into
use.

As a rule the best electrode is the one that has the longest range from
the low voltage point of hissing to the high voltage point of flaming.
With such an electrode the greatest range in light can be obtained
without either the hissing or the flaming.

The same qualities that give an electrode long range, as above, also
indicate its purity and if we make a test for range, we shall therefore
at the same time make a test for purity.

The test for range can be carried out by any ordinary hand-feed lamp. To
make it, the electrodes are inserted and allowed to burn until their
points have assumed the proper shape. The arc can then be shortened
until the familiar hissing sound is heard. Note the voltage at which
this occurs, being careful to have the voltmeter connected so as to get
the voltage across the arc only. Now separate the electrodes slowly
until they begin to flame and note this voltage. Ordinarily the hissing
voltage will be about 42 and the flaming voltage about 62. The greater
the difference between the two, the better the carbons are assumed to
be. In making comparative tests on electrodes in this manner, care
should be taken that all of the conditions of current and size of
electrodes be the same.

The test for comparative life of electrodes is best made by arranging
the different electrodes so that the same current will pass through each
for the same length of time. If this is done, all that is necessary is
to weigh the electrodes before and after burning. The approximate useful
life of an electrode can be easily determined by burning it for a stated
length of time, noting the length consumed and comparing it with the
length available for burning.




CHAPTER III.

PROJECTION.


_Setting and Adjustment of Carbons._--To project a picture upon a screen
properly is an art and requires close study and some knowledge of all
the factors involved. The most important factor is that of the light.
Electric light is so universally used at the present time that it is
hardly necessary to mention the other sources of illumination.

[Illustration: FIGURE 7.]

The electric current with which the operator has to deal may be either
alternating or direct, and the kind is of great importance. The color of
the light obtained from a direct-current arc is not only superior to
that obtained from an alternating-current arc but is obtained at a much
lower cost since, as we shall presently see, it is so much more
efficient.

To project clear white light upon the screen is impossible, some color
will always be in it. But by careful attention and by training himself
to notice slight degrees of color, the operator can learn to render a
light which will be clear enough to satisfy the majority of the
spectators. In order to obtain this light, the source from whence it
comes should be located exactly in the optical axis of the lens system;
that is, a straight line drawn through the center of all of the lenses
should pass also directly through the center of the arc as indicated in
Figure 7. (For comprehensive treatise on lenses, see Chapter XII.)

[Illustration: FIGURE 8.]

[Illustration: FIGURE 9.]

[Illustration: FIGURE 10.]

[Illustration: FIGURE 11.]

[Illustration: FIGURE 12.]

[Illustration: FIGURE 13.]

Most of the light, we have already seen, is emitted from the crater of
an arc of which there is but one in a direct-current arc and two in an
alternating current arc. In order to obtain the most light with the
least expenditure of current and heat in the lamp house, the crater must
be formed in such a manner as to face the center of the condensers as
nearly as possible. Since, however, there are always two electrodes and
the current must pass from one to the other, the crater always tends to
face the lower electrode if the upper one is positive. It is, therefore,
impossible to get the full benefit of the light for the condenser; we
must be satisfied with getting a part of it, and to do this such
settings of electrodes as are shown in Figures 8 to 13 are used. About
the relative merits of these various settings there is considerable
dispute and the best advice that can be given to any new comer in the
operating line is to make his own experiments and find out for himself.
The fact that a certain point is much disputed, alone indicates that
there is no exact knowledge available; for we very seldom have any
differences of opinion about the things that we can prove.

In the operating line very much depends upon the judgment of the
operator. Electrode setting like that of Figure 8 may be good for an
operator who is extremely careful and has a reliable machine which
requires a minimum of attention. But it can readily be seen that if the
top electrode were fed a trifle too far forward, the crater would form
underneath and the lenses would receive but a small part of the light.
Each of the settings given has its peculiarities and it is best for any
operator who has not done so, to try them all out and find which one
best suits him and his conditions.

Figures 8 to 10 show the settings used with direct-current arcs; while
those illustrated by Figures 11 to 13 are used with alternating-current
arcs.

With alternating-current arcs the problem is even more difficult than
with direct, for we have here two craters to deal with; and if we wish
to use the light from both, we shall have to be very careful about it.
If the electrodes are not set exactly right, we may get a double spot
and poor illumination at the center of the screen. Perhaps most
operators will soon give up the idea of using the light of both craters
and will settle down to an electrode setting something like that shown
in _A_, Figure 7. In this setting both electrodes are angled and the
lower one is set a little ahead of the upper. This has a tendency to
draw the crater of the upper electrode forward, thus improving the light
on the condenser; but if this be carried too far, the lower electrode
will obstruct the lower part of the lens. The lower electrode must
always be set so that it allows all parts of the condenser to receive
direct rays of light from the crater of the upper. The electrode must
align perfectly in the vertical plane as shown in _B_, Figure 7, or the
arc will move while burning.

In order to enable the operator to arrange his electrodes at any angle
and to bring them into the center of the optical system, arc lamps are
made up in various ways as illustrated in Figures 14 to 19. The simpler
types are used only in stage lighting lamps where the centering is not
so important. The more elaborate lamps are provided for motion picture
arc lamps and allow of all necessary adjustments which are: feed
electrodes; move lamps forward or back; up or down; sideways and angle
electrodes.

Where direct current is used, the upper electrode must be fed
approximately twice as fast as the lower; but with alternating current,
they both feed at practically the same rate.

[Illustration: FIGURE 14.]

Figure 14 shows a form of McIntosh stereopticon lamp.

[Illustration: FIGURE 15.]

Figure 15 is a Kliegl lamp for open arc lamps.

[Illustration: FIGURE 16.]

Figure 16 is an Edison lamp used for motion picture work.

[Illustration: FIGURE 17.]

Figure 17 is a Kliegl lamp used for focusing purposes.

[Illustration: FIGURE 18.]

Figure 18 shows the Powers lamp.

[Illustration: FIGURE 19.]

Figure 19 shows one of the Motiograph Company lamps.

_Optical System._--In Figure 20 we have the complete optical system of
the moving picture or stereopticon outfit. The crater of the arc lamp
and the center of the objective lens are at the conjugate focal points
(see Optics) and must always be in this relation. The size of the
picture projected upon the screen is governed entirely by the focal
length of the objective lens and the distance of the screen from this
lens. The shorter the focal length, the greater will be the bulging out
or rounding of the lens, and the larger the picture projected. The
objective lens is always fitted with an adjusting device of some kind by
which it can be moved forward or back a little to focus the picture
properly.

[Illustration: FIGURE 20.]

In order to project a picture properly, it is necessary that the center
of the arc or other illuminant, the center of the condensers, and the
center of the objective, all fall in one straight line as indicated in
Figure 20. The condensers are provided for the purpose of gathering and
condensing as many of the scattering light rays of the arc lamp as
possible and bringing them to bear upon the slide and the objective.

The light used must come either from a reasonably small source or from a
larger source far enough away so that the rays can be considered as
parallel. The focal point for parallel rays would, however, differ
somewhat from that of a point source and such illumination is seldom
used; in fact, it is used only where special arrangements are made for
it.

One of the principal points to be borne in mind in trying to project a
good clear picture is to keep the arc down to as small a point as is
practicable. A long arc can be tolerated only when it is absolutely
impossible to obtain sufficient illumination from a short arc; as, for
instance, in operating the Kinemacolor machines, in which from 80 to 100
amperes are used with a very long arc. The above expedient is imperative
because the colored discs through which the light must pass absorb a
great amount of it and the definition or outline of the picture is apt
to be poor.

The position of the arc with reference to the condensers is also an
important point to consider. The focal length of the condensers
determines the point at which the arc must be maintained. The flatter
the condensers are, the farther away the arc may be, and the less will
be the heating; but this position is accompanied with considerable loss
of light.

For the purpose of projection we can use only the light which strikes
the condensers direct from the arc. Rays reflected by the lamp house do
not pass through the condensers in the same direction as those coming
directly from the crater and will not focus with them. Hence, the
farther the arc is from the condensers, the smaller will be the
percentage of light used; the shorter the focal length of the
condensers, the closer to them must the arc be maintained, and the
greater will be the percentage of light used. But if the light is
brought too close, there will be undue heating of the condensers and
these, especially the one nearest the light, will be likely to break. So
great is the heat produced that sometimes the two lenses are partially
melted and welded together. This is a frequent occurrence in cases where
very heavy currents are used. It must be recalled that the heat produced
is proportional to the square of the current and that other things being
equal, 80 amperes would produce four times the heat of 40 amperes.

Condenser breakage is quite an important subject and one upon which
there is much argument among operators. Many of the theories held are,
however, not plausible enough to merit mention. The principal cause is
no doubt overheating without allowing sufficient room for expansion in
the setting. No lens should ever be set so that it does not move freely
even while it is hot. Even if free while cold, the expansion, where the
heating is great, may be sufficient to tighten it in the casing, and
this is likely to cause breakage. The best methods of preventing heating
are: a large lamp house well ventilated and condensers of such focal
length as to allow the arc to be maintained at some distance from them.
Drafts of air are often given as the cause of breakage, but the truth of
this is rather problematical. There is no doubt that sudden contraction,
due to rapid cooling, would have a strong tendency to break them; but
the air in operating rooms is not often cold and is not likely to
strike the lens anyway. It must be noted that it is usually the inner
lens, which is ordinarily enclosed, that breaks.

[Illustration: FIGURE 21.]

In the projection of moving pictures there are two important points that
must always be considered. (1) the size of the spot on the gate at which
the film appears, and (2) the clearness of the field or light on the
screen. By properly adjusting the arc, we can make the spot any size we
desire; and the smaller we make it, so long as it covers the whole
aperture, the brighter the light will be. But if we make this spot too
small, we shall bring in the fringe of color which always appears at the
outer edge. Color of this kind is objectionable and must be avoided as
much as possible; but it is not necessary to go to extremes. A little
coloring will not be noticed by the audience and will therefore not be
objectionable. With a given system there will thus be a certain size of
spot which gives the best results obtainable. Considering that if the
spot is increased in size, the light becomes clearer but also less
intense; and that if the spot is decreased in size, the light on the
screen, though more brilliant, is liable to show coloring, a good
operator should practice distinguishing the coloring and make himself as
proficient in this art as possible. The customary proportions of spot
and aperture are shown in Figure 21.

Coloring appears, however, from another cause also, viz., improper
centering or adjustment of the arc lamp with reference to the
condensers. If the arc is not properly adjusted, bands of color such as
are indicated in Figure 22 may appear in any of the positions shown.
This is commonly spoken of as the "ghost", and it must be eliminated. It
is not possible to get rid of it entirely, but by a little skill,
patience, and experience, it can be reduced to a negligible amount. When
the spot is right and the screen clear, the picture may be focused by
adjusting the objective lens.

[Illustration: FIGURE 22.]

To focus sharply, it is advisable to move the lens in one direction
until the picture appears a trifle blurred; then move it in the opposite
direction until at this point there is also a blurred picture. The
exact focus will be at a point half way between the two. To focus the
lens in this manner is important where the slide or film has some play,
as when the aperture plate on a machine is worn and allows the film some
movement.

_Current Required._--The measurement of the candle power of arc lamps
has never been satisfactorily taken, and the difficulties encountered in
determining it for a projecting arc are especially great because only a
small part of the total light can be utilized and this is constantly
varying. The light may, however, be assumed as proportional to the
wattage of the arc, hence, we can best judge it by noting the volts and
amperes. Where a very strong light is desirable, the arc is usually
drawn out to some length; and as there is a rise in voltage, with a long
arc, in such a case, the light increases at a greater rate than the
amperage. In ordinary projection work, the arc is kept quite short
because of the better definition obtainable by the use of such an arc;
and we may assume that the light obtained is nearly directly
proportional to the amperage. This relation of light and the current
input to the lamp will be practically correct, especially if the size of
the electrodes chosen is proportional to the amperage.

_Current Required for Projecting._--The value of the current to be used
for projection is a matter of some dispute among operators and probably
much of this is caused by the absence of ammeters, most operators merely
guessing at what they are using, or being guided by markings of
rheostats or compensators. In most cases something like 40 amperes seems
to be the rule.

In order to give the reader a clear understanding of the theoretical
requirements, Table I has been prepared. This table is not intended to
act as an accurate guide, but merely to show the amperage theoretically
required with different sized pictures, to bring about the same
illumination in each case.

TABLE I.

CURRENT REQUIRED FOR DIFFERENT SIZE PICTURES.

  -----------+--------------+------------------------
   Greatest  |              |       Amperes
  Dimension  |     Area     +----------+-------------
  of Picture | Illuminated. |  Direct  | Alternating
  in feet.   |              | Current. |  Current.
  -----------+--------------+----------+-------------
        5    |      39      |     8    |     12
        6    |      56      |    11    |     16
        7    |      77      |    15    |     22
        8    |     100      |    20    |     30
        9    |     127      |    25    |     37
       10    |     157      |    31    |     45
       11    |     189      |    38    |     57
       12    |     224      |    45    |     67
       13    |     260      |    52    |     78
       14    |     307      |    60    |     90
  -----------+--------------+----------+-------------

Two errors are very common in the computation of the light intensity for
a given picture: (1) the length of throw governs the amperage; and (2)
the amperage depends upon the actual space to be illuminated. Apparently
only an oblong square of exactly the proportions of the aperture in the
machine is illuminated, but in reality the light must be spread out so
that its total illumination covers a circle enclosing the actual visible
picture. This is illustrated in Figure 23 where the enclosed oblong
square represents the space illuminated on the screen and the circle
represents the area over which the light must be spread. The portion
shown by shading is nearly equal to the clear portion and shows that
half of the light is wasted since it is blocked out by the cooling plate
in the machine or the framework of the slides. With increasing size of
picture, the light is, however, diminished in proportion to the area of
the circle and not in proportion to the area of the picture. If, for
instance, the picture were to retain its width and be reduced in height
by one half, or even more, there would still be about the same quantity
of illumination required. For this reason we have, in Table I, given
only the maximum dimension of the picture and have based the amperage
calculation upon the area of the circle which encloses the picture.

[Illustration: FIGURE 23.]

The values given are less than are generally used for small pictures and
more than are generally used for large pictures. As a rule much light is
wasted on small pictures because the apparatus is at hand to deliver it;
with large pictures, the illumination is often poor because transformers
and rheostats are seldom fitted to deliver more than 60 amperes. Much
light can easily be wasted if the picture is made too bright. In such a
case, much of the light is reflected back to the auditorium and this in
turn makes the picture appear less bright.

In determining the amperage necessary to show a picture properly, the
following conditions must be borne in mind, any one of which may
appreciably affect the result:

(1) _Nature of Screen._--A good screen will reflect more light than a
poor one.

(2) _Size of Picture._--The larger the picture, the more light will be
required.

(3) _Character of Film._--Some films are very dark and require extra
illumination.

(4) _House Illumination._--In some cities the law requires fairly bright
illumination of auditoriums and this makes the picture appear less
bright.

(5) _Atmosphere._--Where the air is full of dust, or where smoking is
allowed; much light will be absorbed.

(6) _Lenses._--Some lenses are badly discolored and absorb much light.

(7) _Electrodes and Electrode Setting._--This is a very important factor
and one which a good operator will never neglect.

_Selection of Lenses._--Upon the proper selection of lenses depends very
much the quality of the picture. The size of the picture, under given
circumstances, depends entirely upon the focal length of the objective.
With a given distance between lens and screen there is practically but
one size of picture obtainable. If we wish to obtain a picture of
another size by the use of the same lens, this can be done only by
sacrificing the definition and had better not be attempted.

Very large pictures are desirable only in large halls in which portions
of the audience are very far from the screen. Such a picture requires
very much light and, on account of its size, shows many imperfections to
those who sit in the front rows. It is better to limit the size of the
picture to one which can be easily illuminated, and thus avoid such
imperfections.

TABLE II.

MOTION PICTURE LENSES.

TABLE SHOWING SIZE OF SCREEN IMAGE WHEN MOVING-PICTURE FILMS ARE
PROJECTED.

              Size of Mat opening 11-16 x 15-16 inch.
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  E.E. | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 60 | 70 | 80 | 90 |100
  In.  | ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  2-1/8| 4.8| 6.4| 8.0| 9.6|11.3|12.9|14.5|16.1|    |    |    |    |
       | 6.5| 8.7|11.0|13.2|15.4|17.6|19.8|22.0|    |    |    |    |
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  2-1/2|    | 5.4| 6.8| 8.2| 9.6|10.9|12.3|13.7|16.4|    |    |    |
       |    | 7.4| 9.3|11.2|13.1|14.9|16.8|18.7|22.4|    |    |    |
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  3    |    | 4.5| 5.7| 6.8| 8.0| 9.1|10.3|11.4|13.7|16.0|    |    |
       |    | 6.2| 7.7| 9.3|10.9|12.4|14.0|15.6|18.7|21.8|    |    |
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  3-1/2|    |    | 4.9| 5.8| 6.8| 7.8| 8.8| 9.8|11.7|13.7|15.7|    |
       |    |    | 6.6| 8.0| 9.3|10.6|12.0|13.3|16.0|18.7|21.4|    |
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  4    |    |    | 4.2| 5.1| 6.0| 6.8| 7.7| 8.5|10.3|12.0|13.7|15.4|
       |    |    | 5.8| 7.0| 8.1| 9.3|10.5|11.6|14.0|16.3|18.7|21.0|
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  4-1/2|    |    |    | 4.5| 5.3| 6.2| 6.8| 7.7| 9.1|10.6|12.2|13.7|15.4
       |    |    |    | 6.2| 7.2| 8.4| 9.3|10.5|12.4|14.5|16.6|18.7|21.0
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  5    |    |    |    |    | 4.8| 5.4| 6.1| 6.8| 8.2| 9.6|10.9|12.3|13.7
       |    |    |    |    | 6.5| 7.4| 8.4| 9.3|11.2|13.0|14.9|16.8|18.7
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  5-1/2|    |    |    |    | 4.3| 4.9| 5.6| 6.2| 7.4| 8.7| 9.9|11.2|12.4
       |    |    |    |    | 5.9| 6.7| 7.6| 8.4|10.2|11.9|13.6|15.3|17.0
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  6    |    |    |    |    |    | 4.5| 5.1| 5.7| 6.8| 8.0| 9.1|10.3|11.4
       |    |    |    |    |    | 6.2| 7.0| 7.7| 9.3|10.9|12.4|14.0|15.6
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  6-1/2|    |    |    |    |    |    | 4.7| 5.2| 6.3| 7.3| 8.4| 9.6|10.6
       |    |    |    |    |    |    | 6.4| 7.1| 8.6|10.0|11.4|13.0|14.5
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  7    |    |    |    |    |    |    | 4.4| 4.9| 5.8| 6.8| 7.8| 8.8| 9.8
       |    |    |    |    |    |    | 6.0| 6.6| 8.0| 9.3|10.6|12.0|13.3
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  7-1/2|    |    |    |    |    |    |    | 4.5| 5.4| 6.4| 7.3| 8.2| 9.1
       |    |    |    |    |    |    |    | 6.2| 7.4| 8.7|10.0|11.2|12.3
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----
  8    |    |    |    |    |    |    |    |    | 5.1| 6.0| 6.8| 7.7| 8.5
       |    |    |    |    |    |    |    |    | 7.0| 8.1| 9.3|10.5|11.6
  -----+----+----+----+----+----+----+----+----+----+----+----+----+----

=Example=: With a lens of 5-1/2 inch focus at a distance of 35 ft. the
screen image will be 4.3x5.9; at 40 ft., 4.9x6.7; at 45 ft., 5.6x7.6;
etc.

=Note=: When ordering lenses, give size of picture wanted, and distance
from machine to screen.

TABLE III.

STEREOPTICON LENSES.

TABLE SHOWING SIZE OF SCREEN IMAGE WHEN LANTERN SLIDES ARE PROJECTED.

                          Size of Mat opening 2-3/4 x 3 inches.
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   E.F. |15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 60 | 70 | 80 | 90 |100
   In.  |ft.| ft.| ft.| ft | ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   5    |8.0|10.8|13.5|16.3|19.0|    |    |    |    |    |    |    |
        |8.8|11.8|14.8|17.8|20.8|    |    |    |    |    |    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   5-1/2|7.3| 9.8|12.3|14.8|17.3|19.8|    |    |    |    |    |    |
        |7.9|10.7|13.4|16.1|18.8|21.6|    |    |    |    |    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   6    |6.6| 8.9|11.2|13.5|15.8|18.1|20.4|    |    |    |    |    |
        |7.3| 9.8|12.3|14.8|17.3|19.8|22.3|    |    |    |    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   6-1/2|6.1| 8.2|10.4|12.5|14.6|16.7|18.8|    |    |    |    |    |
        |6.7| 9.0|11.3|13.6|15.9|18.2|20.5|    |    |    |    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   7    |5.7| 7.6| 9.6|11.6|13.5|15.5|17.5|19.4|    |    |    |    |
        |6.2| 8.3|10.5|12.6|14.8|16.9|19.0|21.2|    |    |    |    |
  ------+---+----+----+----+----+----+----+----+----+---+----+----+----
   7-1/2|5.3| 7.1| 8.9|10.8|12.6|14.4|16.3|18.1|    |    |    |    |
        |5.8| 7.8| 9.8|11.8|13.8|15.8|17.8|19.8|    |    |    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   8    |   | 6.6| 8.4|10.1|11.8|13.5|15.2|17.0|20.4|    |    |    |
        |   | 7.3| 9.1|11.0|12.9|14.8|16.6|18.5|22.3|    |    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   8-1/2|   | 6.2| 7.9| 9.5|11.1|12.7|14.3|16.0|19.2|    |    |    |
        |   | 6.8| 8.6|10.3|12.1|13.9|15.6|17.4|20.9|    |    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   9    |   | 5.9| 7.4| 8.9|10.5|12.0|13.5|15.1|18.1|21.1|    |    |
        |   | 6.4| 8.1| 9.8|11.4|13.1|14.8|16.4|19.8|23.1|    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
   9-1/2|   | 5.6| 7.0| 8.5| 9.9|11.4|12.8|14.2|17.1|20.0|    |    |
        |   | 6.1| 7.6| 9.2|10.8|12.4|14.0|15.5|18.7|21.9|    |    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  10    |   | 5.3| 6.6| 8.0| 9.4|10.8|12.2|13.5|16.3|19.0|21.8|    |
        |   | 5.8| 7.3| 8.8|10.3|11.8|13.3|14.8|17.8|20.8|23.8|    |
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  12    |   |    | 5.5| 6.6| 7.8| 8.9|10.1|11.2|13.5|15.8|18.1|20.4|
        |   |    | 6.0| 7.3| 8.5| 9.8|11.0|12.3|14.8|17.3|19.8|22.3|
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  14    |   |    |    | 5.6| 6.6| 7.6| 8.6| 9.6|11.6|13.5|15.5|17.5|19.4
        |   |    |    | 6.2| 7.3| 8.3| 9.4|10.5|12.6|14.8|16.9|19.0|21.2
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  16    |   |    |    |    | 5.8| 6.6| 7.5| 8.4|10.1|11.8|12.5|15.2|17.0
        |   |    |    |    | 6.3| 7.3| 8.2| 9.1|11.0|12.9|14.8|16.6|18.5
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  18    |   |    |    |    | 5.1| 5.9| 6.6| 7.4| 8.9|10.5|12.0|13.5|15.1
        |   |    |    |    | 5.6| 6.4| 7.3| 8.1| 9.8|11.4|13.1|14.8|16.4
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  20    |   |    |    |    |    | 5.3| 6.0| 6.6| 8.0| 9.4|10.8|12.2|13.5
        |   |    |    |    |    | 5.8| 6.5| 7.3| 8.8|10.3|11.8|13.3|14.8
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  22    |   |    |    |    |    |    | 5.4| 6.0| 7.3| 8.5| 9.8|11.0|12.3
        |   |    |    |    |    |    | 5.9| 6.6| 7.9| 9.3|10.7|12.0|13.4
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----
  24    |   |    |    |    |    |    |    | 5.5| 6.6| 7.8| 8.9|10.1|11.2
        |   |    |    |    |    |    |    | 6.0| 7.3| 8.5| 9.8|11.0|12.3
  ------+---+----+----+----+----+----+----+----+----+----+----+----+----

=Example=: With lens of 10-inch focus at a distance of 20 ft. the screen
image will be 5.3x5.8; at 25 ft., 6.6x7.3; at 30 ft., 8.0x8.8; at 50
ft., 13.5x14.8.; etc.

Table II shows the size of picture obtainable from films, and Table III,
the size obtainable from lantern slides. Since the slide pictures must
be shown upon the same screen as the film, it can be seen from the
tables that lenses of different focal length must be used for the two.
The aim should be to get the two pictures to match as nearly as
possible, but as they are not of the same proportions, it is impossible
to match them exactly in both directions. The nearest approximation that
can be brought about by standard lenses is illustrated in Figure 24. The
heavy lines show the dimension of the picture projected through the
film, and the light and dotted lines show the dimensions obtainable by
the use of slides. If the slide picture is matched to the height of the
film, it will be considerably narrower; if it is matched to the sides,
it will be considerably higher. It would of course be possible to trim
down slides so that the dimensions of the two pictures would be exactly
alike; but as most all stereopticon slides belong to traveling actors
this is not practicable.

[Illustration: FIGURE 24.]

If the focal length of a lens is not known, it can easily be measured by
focusing some distant object, an incandescent lamp for instance, against
the wall of a room or against some screen placed upon a table as shown
in Figure 25. In the case of a single plano-convex lens, the
measurements must be made from both sides--first one side turned toward
the light, and then the other. There will always be some difference
between the two measurements and we must take the mean of the two. To
get the measurement accurately, place a rule upon a table and set up
some suitable object upon which the picture can be projected. Turn the
flat side of the lens toward the screen and focus some distant object by
moving the lens to a point at which the object selected will appear
clearly upon the screen. Note the distance of the flat side of the lens
from the picture. Now turn the lens half way around and focus again in
the same manner, noting this distance also. Add the two measurements and
divide by two; this will give the focal length of the lens. In the case
of an objective lens, we must turn the side which bulges out most toward
the screen and focus in the same manner.

[Illustration: FIGURE 25.]

With the objective lens we have two possible focal lengths to consider.
If we measure from the center of the lens to the screen, we shall obtain
what is called the _equivalent focal length_ (usually abbreviated E.F.
or e.f.). If, instead, we take measurements from the face of the lens
nearest the screen, we shall obtain what is termed the _back focus_, or
b.f., of the lens. In all cases it is important, when ordering, to state
which of the two is meant.

Lenses may also be tested for chromatic and spherical aberration.
Chromatic aberration is the fault of showing colors unduly. It is
impossible to avoid a fringe of color when using only a single lens, but
where we have a complete optical system, consisting of two condensers
and an objective, it must be possible to adjust the combination so that
practically no color is visible. Spherical aberration is best tested for
by laying out very accurately, as in Figure 26, a set of small squares
upon some material that will not be damaged by the heat of the
lamp--mica for instance--and projecting this upon the screen. If the
lenses are all good, the lines will all appear square; if the lenses are
poor, the lines will appear curved a little, or perhaps considerably.

[Illustration: FIGURE 26.]

The diameter of the ordinary condenser lens is 4-1/2 inches. Smaller
lenses than this cannot well be used because they would not cover the
diagonal of lantern slides. A very common focal length of condenser is
6-1/2 inches. There is no very accurate relation necessary between the
focal length of condenser and objective. There is considerable
difference of opinion on this subject and much of it is induced by the
possibility of condenser breakage which is increased by using
condensers of short focal length, but in this case, as in many others,
the operator must find out by his own experiments.

A very good plan--since, on account of breakage, extra lenses must be
carried anyway--is to carry two 7-1/2-inch and two 6-1/2-inch condensers
and experiment with these. The two of the same diameters may be tried
together and also those of different focal lengths, using the one of
shorter focal length either in front of or behind the other.


HINTS ON MANAGEMENT OF PROJECTING ARCS.

Before starting to work about the lamp, be sure the switch is off.

See that the lamp house is clean and spark tight.

The gauze provided at the top must be kept free from dirt and carbon
ash, or the house itself may get too hot.

The house should be of such dimensions, relative to the length of
electrodes used, that the latter cannot touch either at the top or
bottom and thus ground the circuit on the lamp house and possibly burn a
hole in it.

See that your lamp mechanism is well aligned so that electrodes center
at all positions.

All of the screws and adjustments should be well lubricated frequently.
The heat in the lamp house soon evaporates all lubrication.

Where lamps are used much and carry heavy currents, the leading in wires
will probably need reconnecting about once a week. It is best to
reconnect them some time before they burn off rather than be obliged to
do this during a show.

See that your polarity is right. With direct current, the upper
electrode will retain its heat longer than the lower if connections are
made properly. With alternating current the polarity is immaterial.

Always point your electrodes, especially the lower. If the lower
electrode is not pointed, it will interfere with the light of the
crater.

The recommendations for sizes of upper and lower electrodes vary
somewhat but run mostly to 5/8 inch for upper and 1/2 inch for lower.
The size depends very much upon the current used. If the electrodes are
too large, the arc will travel around the outside and yield a poor and
uneven light.

Always use cored carbons for alternating current.

The best length for electrodes is about 6 inches, if they do not strike
the lamp house.

Notch the carbon electrode a little before attempting to break it off.

Many operators are in the habit of watching the arc, opening the
lamp-house door to look at it. Not only is this injurious to the eyes
but it also interferes with proper judgment of the illumination of the
picture. A better way is to punch a very small hole in the lamp house
exactly opposite the arc. Over this opening a lens may be placed, and a
picture of the arc may be thrown against a wall or screen set up for
that purpose. A picture of the arc is also obtainable in another way: If
the lamp is once set exactly right, a cross may be painted at the proper
place on the screen which will indicate the exact point where the arc
should be maintained. The arc will of course appear inverted. Another
method of keeping the arc always in view without inconvenience consists
in arranging a small mirror, at an angle to the peep glass in the door,
so that it will reflect the arc towards the operator.

An adjustable resistance should always be kept in reach so that the
current may be varied to suit different films or stereopticon lamps.

Keep your hands as free from carbon dust as possible. This dust is
responsible for much damage to films.




CHAPTER IV.

MOTION PICTURES.


Strictly speaking there are no pictures of motion. What we see as such
is simply an optical illusion. This illusion is produced by presenting a
series of pictures of an object in a systematic manner, each picture
showing some slight change from the preceding one. If these changes be
all in a certain direction and brought before our eyes in regular order,
we shall perceive the appearance of motion in that direction. Such
pictures may be made by means of photography.

A very simple form of motion picture is made up in the form of a small
book containing a number of leaves that may be run off under the finger
of the holder. If these leaves contain such a series of pictures as is
mentioned above, the holder, on manipulating them properly, will see
motion reproduced quite naturally.

The manner in which the illusion of motion is produced can perhaps best
be illustrated by Figure 27. Here we have an ordinary film, or it may be
any piece of white paper, upon which are drawn a series of black dots as
shown. If this film--the observer being able to see only that part in
the aperture _A_--be drawn downward the length of one section very
quickly; allowed to rest a moment; then, in the same manner, be drawn
down another section; and this process repeated at proper speed, until
the full length of the film has passed the aperture, we shall have
received the impression that the black dot moved from the lower
left-hand corner to the upper right-hand corner of the aperture. In
order that such an illusion might be perfect, we should have to move the
film so rapidly that the eye would not perceive the movement. This is
not possible except with very weak illumination and we should actually,
in the above experiment, receive a blurred impression, because we could
not help seeing the dots while they were moving, and our eyes would
behold a mixture of stationary and moving dots. In order to produce the
impression of perfect motion, it is necessary to shut off the light
during the time that the film is actually in motion. Thus, paradoxical
as it may seem, in order to simulate motion, we must have the object
which is to appear in motion always perfectly still before our eyes.

[Illustration: FIGURE 27.]

In order that we may not notice that the film is out of sight, it must
be moved very quickly. The actual time during which the picture on the
average film is hidden from view, and in which the picture is changed,
is about 1/80th of a second and the time during which the picture is
stationary is about 4/80th of a second.

The possibility of the illusion of motion pictures depends upon a
faculty of the eye known as _persistence of vision_. The eye retains an
impression for something like 1/25th of a second. When an object is in
motion, we see, therefore, not only one position of the object but all
positions of it during the time of persistence of vision. This time
varies somewhat with the intensity of the light or the impression made
upon the eye. If it is equal to 1/25th of a second in the case of a ball
thrown at the rate of one hundred feet per second, then we should see,
instead of one ball, a large number of balls merging imperceptibly into
one another, or, in other words, a streak of balls four feet long. Thus,
in actual life, we obtain from the moving ball but a blurred impression.

We see thus that in order to produce the impression of motion, we must
present the picture to the eye long enough to stimulate it properly; we
must very quickly remove that picture and substitute another differing
to a slight extent from the former; and we must repeat this process a
number of times. The ordinary moving picture film contains 16 pictures
per foot, and is run off at the rate of about 60 feet per minute, so
that in one minute, we see 960 different pictures.

In order to make motion visible, we must bring it within a certain speed
limit. Thus, to show the motion of a swiftly thrown ball in detail, we
must make it appear to move more slowly than it really does; and to show
the development of a growing plant, it must appear to grow much faster
than it actually does. Both of these requirements can easily be
fulfilled by the motion picture camera and the projecting machine.

A man, walking at the rate of three miles an hour, displaces himself
about three inches during the time of the exposure of one picture, or
1/16th of a second. At this rate we obtain the impression of even and
continuous motion unless he be too close to the camera. In order to
obtain pictures of other objects moving at faster or slower rates, we
must take them at intervals in order that the displacement between
pictures will be about the same or at least not any more. This means
that pictures of rapidly moving objects must be taken at short intervals
and those of slowly moving objects, at long intervals. A kernel of corn
develops into a stalk six feet high in about ninety days. If a
photograph of this is taken every day during its growth and these
pictures arranged in proper order, they will be run off at normal speed
in less than six seconds, thus showing us in six seconds the growth
which actually takes place in ninety days.

The motion picture camera enables us not only to produce the illusion of
motion, but to see in detail what actually takes place in connection
with the moving object at any instant. If we take pictures of a running
horse, for instance, at short enough intervals, we shall be able to see,
on the films, just how he holds or places his feet or any other part of
his body at any time.

In order to obtain a perfect picture simulating motion, we must present
the first picture long enough to stimulate the eye; then we must shut
off the light, remove the first picture, and substitute the second;
remove the second and substitute the third, etc., as long as desired.
During the time that the light is shut off, the first picture must
persist in our vision until the new one has appeared. The two pictures
thus mix until the first one has faded, and thus we obtain the illusion
of motion.

If the bright picture remains too long, the pupil contracts--as
explained in the chapter on Optics--and when next the light is shut off,
the darkness is noticeable and gives rise to the disagreeable phenomenon
of flicker. In order to prevent this over-stimulation of the eye, the
long period of exposure is interrupted by a shutter at least once and,
in some cases, two times; and some machines are equipped with a
three-blade shutter. This three-blade shutter has a wide blade which
shuts off the light while the film is in motion and two narrower blades
which pass across the light during the time that the film is stationary,
to prevent the over-stimulation of the eye.

_Colored Pictures._--There are two general methods of producing colored
motion pictures: One is that of hand coloring or tinting, and the other
is what is known as the _Kinemacolor process_. In the latter process, no
color whatever is used on the film; the coloring is supplied by a
shutter with a green and a red blade which are alternately thrust into
the light by which the picture is projected upon the screen.

In order that this process may be used, the film pictures must first be
taken through screens of corresponding color. The film in the
Kinemacolor camera, or projecting machine, must run at more than double
the speed of that which is used in the ordinary process; and each
alternate picture must be photographed through a red screen; the others,
through a green screen.

The red screen will allow only red light to pass; hence, any part of an
object that contains no red will not affect the photographic emulsion.
Similarly, the green screen will allow only green light to pass; and
such parts of the objects as contain no green will not affect the
emulsion. The alternate sections of film will thus be entirely
different from each other.

In order to reproduce the original color of the object upon the screen,
it is but necessary to arrange that the pictures shall in turn be
projected through the same or similar color screens. In order to
accomplish this, the Kinemacolor machine has, in addition to the regular
shutter which cuts off the light during the time the film is in motion,
an additional two-wing shutter which inserts the properly colored
screens before each picture, as it comes to a standstill in the film
window. Thus we see in alternation, a red picture and a green.
Persistence of vision, which is explained in Chapter IX, helps us to mix
the two colors and we see the object approximately in its own colors.

The color effect is not very good, owing partly to the fact that only
two colors are used. If the three primary colors--red, blue, and
yellow--could be used, the effect would doubtless be better; but the
complications would be multiplied.

In order to get the best results with the Kinemacolor process, colors of
a certain shade are used and the size and depth of coloring in the two
shutters is variable. One of the colors is adjustable and must be so
arranged that when the machine is run without film, it will throw
approximately white light upon the screen. The high speed with which the
film must be run makes it impossible to turn the machine by hand and it
is always motor-driven.

The colored shutters are constantly in the light and absorb a large
portion of it and this must be compensated for by an extraordinary high
amperage. This process requires more than two times as much current as
the ordinary projection of black and white pictures. In order to obtain
the greatest possible amount of light, operators usually run a very long
arc and this often results in imperfect definition.

In arranging for an exhibition, it is important that the film and the
colored screens be correctly placed with reference to each other; to
facilitate this, an identifying mark is placed on the side of one of the
colors.




CHAPTER V.

THE MOTION-PICTURE MACHINE.


A diagrammatic sketch of the essentials of a good motion picture machine
is given in Figure 28. This does not represent any machine in particular
and no machine exactly like it will be found; it does, however, show the
theoretical elements necessary to the projection of motion pictures and
the usual safety devices with which standard machines are provided. As a
variation from Figure 28, the exact method of threading the film through
the Edison machine is given in Figure 29. In Figure 28, the parts are
designated as follows:

  _A_--feed reel or upper reel
  _B_--feed reel magazine or upper magazine
  _C_--magazine fire traps, or film valves, or fire valves
  _D_--upper steady-feed sprocket
  _E_--presser rollers, or friction rollers, or idlers
  _F_--upper feed loop or upper loop
  _G_--film-steady drum or film steadier
  _H_--film gate
  _I_--tension spring
  _J_--automatic fire shutter
  _K_--revolving shutter
  _L_--intermittent sprocket
  _M_--lower feed loop
  _N_--lower steady-feed sprocket
  _O_--take-up reel and lower magazine
  _P_--framing device or adjusting lever (not shown)
  _Q_--film shields

[Illustration: FIGURE 28.]

[Illustration: FIGURE 29.]

In addition to the above, there are the gearing and the belting which
transmit motion to the various sprockets and drums shown. The whole
function of the machine, however, is the proper moving of the film along
the lines indicated in the figure, the heavy black line representing the
film. The film is unwound from the upper reel by the upper steady feed
sprocket _D_. After forming the upper loop _F,_ it passes over _G,_
through the film gate _H,_ to the intermittent sprocket _L_. This
sprocket moves the film by an intermittent motion allowing the film to
remain stationary in the light for about four-eightieths of a second and
shifting it during about one-eightieth of a second. After leaving the
intermittent sprocket, the film forms the lower loop and then passes to
the lower steady feed sprocket, which prevents the take-up from pulling
the loop away from the intermittent sprocket. The object of the upper
and lower loops is to lighten the work of the intermittent sprocket as
much as possible by making it unnecessary to move anything but the film
between the two loops.

[Illustration: FIGURE 30.]

All of the sprockets and the shutter _K_ are connected together by a
train of gears (see Figure 30, which is the gearing of the Motiograph
machine) and when properly adjusted they all work in proper relation to
each other. The upper steady-feed sprocket feeds just as much film into
the upper loop as the intermittent sprocket takes away; while the lower
steady-feed sprocket takes away just as much as the intermittent
sprocket feeds to it. The film is unwound from the upper reel and
rewound upon the lower; but before it is used again, it must be rewound
from the lower reel upon another one. If it were to be exhibited from
the lower reel without this rewinding, the pictures would be exhibited
backwards.


DISCUSSION OF PARTS.

_A--Upper, or Feed Reel._--The feed reel is usually either 10 or 12
inches in diameter. A 10-inch reel accommodates 1,000 feet of film and a
12-inch reel, about 2,000 feet. The reel fits loosely upon bearings in
the upper magazine and the film is unwound from the reel, which
revolves, by the upper steady-feed sprocket. To prevent its unwinding
more film than is wanted, a small spring is arranged to cause a slight
friction. It is best to keep good reels in the operating room for use as
feed reels. A reel to be used for this purpose should be perfectly true
and in good order; reels sent out from the exchanges are often bent or
have loose parts which cause trouble. A good operator will keep a supply
of good reels always on hand.

_B--Upper Magazine._--The upper magazine is a steel box made up without
solder and fitted with a steel door on the crank side of the machine. It
exists for the purpose of protecting the film against fire or injury
from other causes. It is important that the door be kept closed for,
without this precaution, there is no fire protection. In some cities, it
is required that the door be provided with spring hinges to keep it
closed, but it is doubtful whether this is a wise provision; for since
the door must be opened to change films and since there is nearly always
great haste in making the change, it is likely that the operator will
block the door open if the spring hinges interfere with him. All doors
now swing to the side, but it seems as though it would be a great
improvement if the doors were arranged to drop down. The door would
then, even if open, prevent fire from below reaching the film. The
magazine might as well be entirely missing as to be left with the
side-swinging door open. Many operators have the habit of opening the
door to watch the progress of the reel instead of noting it on the
screen, or of preparing for a change of reel by opening the door long
before necessary and starting the new run before closing the doors.
Needless to say, this is highly reprehensible.

In order to put out a fire in case it should communicate to the reel,
the magazines have been connected with water piping and a valve so
arranged that the water could be turned on instantly thus filling the
magazines. If the door of the magazine is kept closed, the progress of a
fire will be comparatively slow.

_C--Fire Traps._--The fire traps are an important adjunct. Their object
is to prevent fire, which often occurs at the aperture, from reaching
the film on the reel. All of the various makes of fire traps have been
tested and will ordinarily prevent fire from passing through them. The
larger the roller and the smaller the space around the film, the better
the traps seem to be. The metal of the rollers has a cooling effect
upon the flame and this is undoubtedly one reason why they do their work
so well. None of them, however, is absolutely sure.

If the opening through which the film passes is made too narrow, the
film is likely to brush one side or the other and wear grooves in it, or
cut entirely through it, with the result that a splice may be caught in
the opening and the film torn. Fire traps, cut in this manner, have been
the cause of many fires. This trouble is due mainly to improper
alignment of the magazine with the fire traps. Examine them often and,
if the least wear shows, improve the alignment.

It is advisable that every operator test his traps with pieces of film
and assure himself that they will not, under ordinary circumstances,
carry fire; but he must never rely too much upon them as safeguards, for
although they will check a small blaze, such as would result from the
burning of a few inches of film, they would probably not extinguish a
fire occurring when there was much film crowded around them, as might be
the case when a take-up reel or the intermittent sprocket failed to
work. If the film catches fire and there is any possible chance to do
so, the operator should tear it off at the upper and lower magazines,
and thus break the communication. But if there happens to be a lot of
film lying loosely about, it is advisable for the operator to get away
as quick as possible.

_D--Upper Steady-Feed Sprocket._--The office of the upper steady-feed
sprocket is to unwind the film from the reel and feed it toward the
intermittent sprocket. In order that it may do its work well and relieve
the intermittent sprocket of all unnecessary strain, there must be a
loop _F_. The upper steady-feed sprocket is in continual steady motion
and feeds just as much film to the loop as the intermittent sprocket
takes from it by periodical jerks. The main trouble with all sprockets
is in the wear of the teeth, as in time they wear away near the body of
the sprocket and form hooks. The best sprockets have a number of teeth
to engage the film. In some of the older machines, only two teeth catch
the film, in which case, if two holes in the film are torn out, the film
may stand still.

_E--Presser or Friction Rollers._--The presser or friction rollers exist
merely to hold the film in its proper place. They are sometimes spoken
of as friction rollers although they have nothing to do with friction.

_F--Upper Loop._--The upper loop is provided for the purpose of storing
slack film which the intermittent sprocket may pull away with a very
rapid jerk. It avoids placing unnecessary strain upon the film. The
upper loop often causes trouble by enlarging; this occurs when the
intermittent sprocket fails to work while the steady sprocket continues
to feed. The most frequent cause of this is due to faulty films. In
order to prevent the excess film in the upper loop from falling over in
front of the light of the arc lamp (which would quickly set it on fire),
the film shields _Q_ are provided. In many machines they are too short
or too narrow to be of much use.

_G--Film-Steady Drum, or Film Steadier._--The film steadier is not found
in all machines. If the upper portion of the machine head is properly
arranged, the film may feed directly into the gate.

_H--Film Gate._--The film gate is to hold the film in position. In
order that the picture may be properly projected, the film must lie
perfectly flat and at a fixed distance from the lenses. If this distance
varies, there will be improper focusing of the picture. The film must
also be held in its proper position laterally and vertically. The film
gate has nothing to do with the height of the film. This is taken care
of by the framing device. The film gate wears quite rapidly and when
badly worn allows the film too much play. In order to avoid the wobbling
of the picture, a new gate must be provided. There should be
considerable metal surrounding the aperture and there should also be an
air space and some little ventilation. The metal surrounding the gate is
subject to the heat from the arc lamp rays and, unless properly
constructed, may overheat and damage the tension springs, if it does not
set fire to the film.

_I--Tension Springs._--The tension springs are provided to hold the film
flat against the film gate and also to check its motion as soon as the
intermittent sprocket has stopped pulling it. A certain intermittent
movement is advertised to make the change from one picture to another in
1/96 of a second at normal operating speed. The film moved at this rate
of speed acquires considerable momentum; and the office of the tension
spring is to bring it to rest as quickly as possible. If the springs are
set too tight, they will cause the machine to run hard and may also be
the cause of tearing out splices. They should be set just tight enough
to keep the picture steady; anything beyond this will merely cause
unnecessary wear of the parts, besides calling for unnecessary exertions
on the part of the operator.

_J--Automatic Fire Shutter._--The automatic fire shutter is provided
for the purpose of shutting off the light when the machine is not in
motion. It is required by most city ordinances. The ideal fire shutter
would be one so controlled by the film that it would remain up only
while the film is moving at operating speed. The shutters in use at the
present time vary widely in details of construction. Some of them are
raised and admit the light to the film the instant the handle is pressed
sufficiently to start the machine. Other types are raised only after the
machine has nearly attained its proper speed. Some operate as indicated
in the figure and others operate from the side. All of them are liable
to become deranged at times, and it is no unusual thing to find them
tied up in some way because they failed to work properly. The shutter is
a great convenience when in proper order and has probably prevented many
fires; but it is good practice to consider it as entirely absent and not
get into the habit of relying upon it. The operator should always push
his lamp to the side whenever anything is out of order. This act is
necessary in most houses at the end of every run of film showing
stereopticon slides. It would inconvenience the operator but little to
do this at all times, and then the habit would be formed and become
second nature. On the machines now in use, there is no automatic shutter
governed by the action of the film, which would give protection in case
the film should come to rest, as old films often do when their sprocket
holes are torn out, or, as often happens, when the film splits along the
row of holes. In some machines the fire shutter is made up of such thin
material and rests so close to the film that the light may heat it
sufficiently to fire the film below it. Pieces of film often become
detached at the aperture and remain in the light long enough to be
ignited. Against such occurrences as these there is no protection
whatever except the watchfulness of the operator. If the film fits
snugly into the gate and is well enclosed, such fires do not usually
spread.

_K--The Revolving Shutter._--The revolving shutter is sometimes arranged
in front of the machine and often between the film and the lenses. The
shutter _K_ is of the "barrel" type and is further illustrated in Figure
31 at the left. The object of the shutter is to shut off the light
during the time that the film is in motion so that the impressions made
upon the eye by the succession of stationary pictures may not be blurred
by the motion of the film when it is changed. The ideal shutter is one
that shuts the light off instantly and, at the expiration of the
necessary time, allows it as quickly to come to view again. The
barrel-type shutter allows the light to pass through while it is in the
position shown by solid lines, Figure 31, and has it entirely shut off
when in the position indicated by broken lines. When it is in motion,
the upper wing begins to shut off the light from the top and the lower
from the bottom, thus causing the total eclipse of light in one-half of
the time that a single wing shutter could do it.

[Illustration: FIGURE 31.]

At the right of Figure 31, we have a cone shutter such as is used in the
Motiograph. The stem and gearing of this shutter are set at an angle of
forty-five degrees to the rays of light, for the purpose of arranging
them inside of the mechanism without taking up too much room. There are
two cones of the kind illustrated in Figures 32 and 33 and they move in
opposite directions, thus shutting off the light in about the same time
that a barrel shutter could be made to do it. These two figures show the
positions of wings just before shutting off the last vestige of light
and just as they are beginning to admit it again.

[Illustration: FIGURE 32.]

[Illustration: FIGURE 33.]

The disk type of shutter is much used. It may have either one, two, or
three blades and may also be double, i.e., two disks revolving in
opposite directions so as to shut off the light from two sides of the
opening at once. Three types of disk shutters are shown in Figures 34 to
36. In order to obtain a flickerless picture, it is necessary to cut off
the light not only during the time that the film is in motion, but also
during part of the time that it is standing still. If the light were not
interrupted during the time of exposure, the light interval would be so
long that the difference between the dark period, when it is shut off
entirely, and the light period would be great enough to be noticed in
the form of a flicker.

[Illustration: FIGURE 34.]

[Illustration: FIGURE 35.]

[Illustration: FIGURE 36.]

If a single-blade shutter is used, it must make two revolutions during
the time that a single picture is exposed; once to shut off the light
while the film is being moved and once to interrupt the period of
illumination. This shutter would thus have to run twice as fast as the
two- or three-blade shutter, the three-blade shutter interrupting the
light twice during the time the film is standing still.

The three-blade shutter has one wide blade which must be in front of the
light while the film is moving and two narrower ones which interrupt the
light during the time of exposure.

The two-blade shutter must have both blades of the same size and run one
and one-half times as fast as the three-blade shutter to obtain the same
effect, i.e., cause two interruptions of light during the time of
exposure.

In order to shut the light off very rapidly with a blade, which
approaches the opening from one side only, the disk shutter must be of
sufficient size to produce great angular velocity. It can not well be
used, therefore, inside of the mechanism, but is usually placed in front
of the machine; and it is much larger than the aperture which it must
cover. The shutters must all be set so as to shut off the light during
the time that the film is in motion. When the dividing line between two
film pictures is in the center of the aperture, the shutter should have
it entirely covered. It is possible, however, to arrange the shutter so
that the film may move a trifle before the light is shut off and
continue in motion also for a very short time after the light is again
admitted. This is practicable with the "Geneva" intermittent movement
which starts and stops the film gradually.

A shutter should be adapted to the speed of the film movement. There is
no need of keeping the film covered any longer than it is in motion. A
quick film movement and a narrow shutter will add considerably to the
light obtained from a given lamp and a given current consumption. If the
shutter is not properly adjusted, there will be what is known as "travel
ghost", "light rain", or "halo". These are due to the improper timing of
the shutter, allowing a part of the picture to be seen while in motion.
The travel ghost may be seen either at the top or the bottom, according
to whether the shutter is set too fast or too slow.

[Illustration: FIGURE 37.]

[Illustration: FIGURE 38.]

_L--Intermittent Sprocket._--The intermittent sprocket is a very
important part of the machine. Owing to the swiftness with which it
strikes the film, the teeth on it are more subject to wear than those on
the other sprockets. The number of teeth that engage the film is also an
important item. In order to obtain the best picture with the least
expenditure of light, the film should be moved very rapidly so as to
allow the greatest possible length of time for the stationary picture.
The film should, furthermore, be started slowly; then increased in
speed; and the speed should decrease gradually until brought to rest,
and thus avoid unnecessary jerking. It is also necessary to prevent all
motion of the film during the time the mechanism which moves it is
preparing for the next succeeding movement. All of these conditions are
admirably fulfilled by the "Geneva" movement, Figure 37. The pin wheel
_W_ is in continuous motion and the pin is so placed upon it that it
enters one slot of the cross and carries it along with it, thus causing
a quarter revolution of the cross each time the pin wheel makes one
revolution. The cam band _P_ is cut away sufficiently to allow the cross
to make a quarter revolution, but the remaining portion of it is made to
fit the cross snugly, so that when the cross is not in motion it is held
rigid. Figure 37 shows the movement just starting and Figure 38 shows it
half completed. It can be seen that the motion begins very slowly; comes
to a maximum when it is in the middle; and ends slowly; thus subjecting
the film to the least possible strain. This movement is widely used and
may be arranged with one pin, as shown, or with two. If the pin wheel is
equipped with two pins, it will move only half as fast as with one pin
and thus the proportionate time that the film is in motion will be
lengthened. By making the pin wheel large as compared with the cross or
star, the time during which the motion takes place can be reduced as
much as desired; but the characteristic feature of starting and stopping
the film gradually will be lost directly in proportion as the relative
size of the pin wheel is increased.

[Illustration: FIGURE 39.]

The "Geneva" movement requires extremely accurate construction and
careful management. If dusty or insufficiently lubricated, it wears very
rapidly. It is often arranged so that it can be immersed in oil while
running. Figure 39 shows a practical application of the "Geneva" as used
in the Motiograph. The cover shown at the right entirely encloses it.

[Illustration: FIGURE 40.]

[Illustration: FIGURE 41.]

[Illustration: FIGURE 42.]

[Illustration: FIGURE 43.]

A form of intermittent movement which has lately come out and is used on
the Powers No. 6 Cameragraph is shown in Figures 40 to 42 inclusive. The
moving element of this intermittent movement is a diamond-shaped
projection on the surface of a revolving disk. The driven element is a
cross carrying four pins--termed a "pin cross"--and is indicated by
broken lines. The raised portion _A_ of the diamond strikes one of the
pins of the cross and starts it in motion as indicated in Figure 41.
The movement of the cross can be traced by the black dot indicating one
of the pins. At every revolution of the driving disk, the cross makes
one quarter of a revolution and when not in motion is held rigidly
locked by the locking ring _R_, which fits snugly between the pins. This
movement probably moves the film in the shortest time of any. It is
entirely encased, runs in oil, and is practically noiseless. The manner
in which it is installed in the machine is indicated in Figure 43.

_M--Lower Feed Loop._--The lower feed loop is provided to prevent the
lower sprocket from pulling on the film and thus interfering with the
work of the intermittent sprocket. The intermittent sprocket feeds the
loop, by jerks, three-fourths of an inch of the film at a time; and the
lower sprocket is geared to take away that much by steady motion. Thus,
everything working properly, the loop remains of the same size. In case
the lower sprocket is not working properly, the loop may become enlarged
and the film may run onto the floor. Sometimes the intermittent sprocket
does not work properly and the lower sprocket pulls away all of the
loop.

_N--Lower Steady-Feed Sprocket._--The lower steady-feed sprocket
maintains the loop and restrains the film. If it were not for this
sprocket, the take-up mechanism would pull directly on the intermittent
sprocket and keep the film jiggling.

[Illustration: FIGURE 44.]

_O--Take-up Reel._--The take-up reel is identical with the upper reel
and, in fact, the two are generally used interchangeably. In both cases,
only perfect reels should be used; and reels delivered by exchanges are
very often untrustworthy.

The principle of the take-up mechanism is shown in Figure 44. When a run
of film is started, the diameter of the roll of film being small, the
reel must travel comparatively fast to take up the film as fast as it is
unwound from the upper reel. As more and more film is gathered by the
lower reel, the travel must become slower until finally, with a
twelve-inch reel, the number of revolutions per minute are but about
one-fourth as many as at the beginning.

In Figure 44, _A_ is the reel which is fastened to the spindle _B_; _C_
is a collar also fastened to the same spindle. _D_ is a loose pulley
capable of revolving around the spindle without imparting motion to it
and arranged so that it can be driven by a belt. _E_ is a spiral spring
which is controlled by the small collar _F_, and it presses the loose
pulley against the collar _C_. If the loose pulley is pressed against
_C_ with sufficient force, it will cause _C_ to revolve with it; and the
movement of _C_ will impart motion to the reel and wind up the film
which is fastened to it. _D_ must revolve fast enough so that, even when
the reel is empty, it will take up all of the film fed to it. As the
film is wound up, the size of the roll of film increases and the speed
must become less; thus _D_ begins to slip a little on _C_ and this
slippage increases until the end of the run.

Friction in one form or another is the only method by which take-up
reels are now operated; but in some of them this friction may be in a
belt, as well as in the two disks as shown. Sometimes a fiber washer is
interposed between the two friction disks. In the Motiograph take-up,
Figure 45, the necessary friction and slip are all in the belting.

[Illustration: FIGURE 45.]

If the take-up is not handling the film properly, it is due to a loose
belt or to insufficient tension on the spring. The take-up probably
causes more trouble than any other part of the mechanism and it should
be carefully watched. On account of the trouble incurred from it, there
are still many operators who prefer to run films into a tank; although
the take-up saves them much time. A careful operator who understands the
care of machinery would have little trouble with a take-up; while the
one who has not the faculty of keeping things in order would, perhaps,
be better off (fire hazard neglected) with a tank. With most machines,
the take-up magazine is located where it is very difficult for the
operator to watch its operation. Those machines in which it is located
in easy sight of the operator have considerable advantage in this
respect. A careless operator is often compelled to open the lower
magazine door and turn the reel by hand when the take-up tension is out
of order. Remember that the take-up can cause trouble only when it is
out of order and that a good operator will never allow it to get out of
order and he will inspect it carefully before every show.

_P--Framing Device._--The aperture in the film gate is just large enough
to fit the picture on the film. If the film is accurately placed, the
whole picture will show upon the screen. In order to bring this about,
the machine must either be provided with a means of adjustment, or a
certain part of the film must be placed upon a certain sprocket tooth.
This latter method causes considerable annoyance and waste of time;
furthermore, every splice in a film would have to be made exactly right
or the whole succeeding part of the show would be "out of frame". In
order to avoid these troubles, every machine is provided with a
so-called framing device. Somewhere about the machine, convenient to the
operator, will be found a lever by which, during the operation of the
machine and without interruption, the position of the film with
reference to the film window and the lens may be adjusted either up or
down. The precise manner in which this is done varies with different
machines. The framing device must be capable of adjusting the film to
somewhat more than the width of one picture.

The splice should be so made that framing becomes unnecessary and this
is accomplished by cutting out full pictures so as to leave each picture
with the full four holes. If a picture is left, for instance, with only
two holes, it will appear upon the screen with the black line, which
should be hidden in the center. If this happens, by moving the framing
lever either up or down, the picture can again be centered upon the
screen.

[Illustration: FIGURE 46.]

_Q--Film Shields._--The film shield is an important part of the machine;
although it is often very much neglected and in many machines is so
small as to be practically useless. It should prevent films from
crowding out into the light when the loops enlarge. Many operators do
not take kindly to machines that enclose the film too tightly; for in
case the take-up fails to work properly, it is often a convenience to be
able to handle the film. This is also the case where very badly torn or
old film is used. Many old films are so badly worn that they will not
run properly on the sprockets and must be very closely watched.

[Illustration: FIGURE 47.]

[Illustration: FIGURE 48.]

It is not thought necessary or advisable to go into the details of
construction of any particular machine as changes are made nearly every
year so that such description would not be of great value and might, in
fact, be misleading. Any person possessing a little mechanical ingenuity
and taking an interest in such matters can readily learn the
peculiarities of any of the machines. They are, after all, when one has
mastered the principles, quite simple affairs. All progressive
manufacturers, furthermore, publish instruction books which are issued
whenever a change is made in any machine; and these are always
obtainable.

[Illustration: FIGURE 49.]

In Figure 46 we have a general view of the Edengraph machine. It will be
noticed that the total length of film between the two magazines is
exposed to view making threading of the film very easy; also that the
revolving shutter works between the objective lens and the film gate.

The well-known Edison machine is shown in Figure 47. This is one of the
best known machines; and it is very compact and durable.

[Illustration: FIGURE 50.]

The Motiograph is shown in Figure 48. The take-up, shown on the lower
magazine, is different in this machine from what it is in the others.
This machine also has a special rewind feature. By an adjustment of the
operating handle, the film, after having been run off onto the take-up
reel, can be rewound upon the upper reel without taking it out of the
magazines.

Figure 49 shows a general view of the Powers No. 6 Cameragraph
mechanism.

The Simplex mechanism is shown in Figure 50. In this machine the film is
entirely enclosed between the two magazines.




CHAPTER VI.

THE FILM.


_Film Making and Characteristics._--All film is of celluloid. The
commercial film used for the projection of motion pictures comes in long
strips, is about one and three-eighths inches wide, and about one
two-hundredths of an inch thick. Upon one side of this film is a
photographic emulsion of gelatine impregnated with a solution of bromide
of silver. This silver is sensitive to the light and has the property of
turning black, during the process of developing, in proportion to the
quantity of light that has struck the different parts of the emulsion.
Wherever the light has been very intense or has been applied for a
sufficiently long time, the film, after treatment with a proper
developing fluid, becomes very black; where no light has reached it, it
remains white; and in the intermediate portions, the shading is in
proportion to the light that has reached those parts.

Film that has not been exposed and developed is of a creamy yellowish
color and, if immersed in a solution of hyposulphite of soda, will turn
perfectly clear and offer no obstruction to the light. If, on the other
hand, it is completely exposed and developed, it will become solid
black; such black pieces are sometimes used as leaders and tails in
connection with motion pictures.

The emulsion on the film melts at a temperature of about 90 degrees
Fahrenheit and will then run off the film. It is also soluble in warm
water. The celluloid of the film is very inflammable and bursts into
flame at a temperature of 284 degrees Fahrenheit. If it is ignited while
tightly wound upon a reel, it burns somewhat slowly; if it is ignited in
a tank, the beginning of the fire is also slow; but as soon as the air
in the tank attains the proper temperature, the whole of the film bursts
into flame at once. It acts in this respect very much like gunpowder. In
burning, the film gives off a dense brownish mass of poisonous smoke.
Film will burn even when it has no access to the air and a film fire
cannot be put out by smothering.

Before sending out their films, the manufacturers treat them with
glycerine. This is for the purpose of keeping them pliable as long as
possible. In time, however, a film dries out and there is then very
little that can be done with it. At any rate, whatever is to be done
should be done by the exchange, as no operator has facilities for
handling it.

Some films are waterproofed and a little water will not damage them
much; but other films must be kept dry. If by accident a roll of film
should become wet, it can be saved by very quickly unrolling it and
spreading it out to dry. If it can be spread out quickly enough, it will
not be harmed; but if it is not spread out at once, the emulsion will
stick to the celluloid and will come off when the film is finally
unwound.

There is also a process of fireproofing film. This must not, however, be
too thoroughly relied upon, especially if the film is old. Fireproof
film is a valuable aid, but it is best to treat it exactly like other
film in respect to the fire hazard.

On account of its inflammable character, film should always be kept in
fireproof enclosures.

The film contains sixteen pictures to each foot and for every picture
there are four sprocket holes on each side of the film. The size of the
aperture plate and also the size of the actual picture on the film is
11/16 inch high by 15/16 inch wide. If the picture on the screen is
enlarged to 18 feet 9 inches in width and 13 feet 9 inches high, every
part of the picture on the film will appear 57,600 times as large on the
screen as it is in the film. In viewing a 1000-foot film, the audience
will see 16,000 separate pictures following each other so rapidly that
the change from one to the other will be unnoticeable. If a three-blade
shutter is used, the beholder will see the light shut off and turned on
48,000 times in the same run of film, but will not be able to notice it
if the speed with which it is done is correct.

New film coming directly from the exchange is likely to be too moist and
cause trouble from that condition; while old film is likely to be too
dry and cause trouble by warping or bending out of shape.

_Patching Film._--All reels of films contain patches and but little
trouble will arise from them, provided they are properly made and
reasonable care is exercised in watching them. In order to obtain a
satisfactory splice, it should not be made more than 3/16 inch long; a
greater length than this will reach two sprocket teeth and this is
objectionable because the splice stiffens the film and will prevent it
from fitting the teeth properly. The splice should be so made that it
does not throw the pictures out of frame. If made properly, it will
never be noticed in the running picture at all, unless a large quantity
of film is cut out. If however, the splice be made over part of a
picture--as would be the case if an improper number of holes were cut
out--the picture, when projected, would appear out of frame, i.e., the
dividing line between two pictures would then appear on the screen and
would continue to show until the framing lever had been adjusted to
cover the dividing lines again. Care must also be taken that the film is
straight after being spliced. If the ends are not fitted squarely to
each other, there will be a bend in the film at the point the splice is
made; and this may cause trouble.

Many operators have a plate of ground glass fitted into a work bench at
the place where splices are to be made and a small lamp under this
glass. This is very convenient for, as a rule, the illumination in an
operating room is very poor although a good light is necessary to match
film properly. If ground glass is used, the rough side must be at the
bottom; otherwise it will be difficult to clean off the cement which
adheres to the glass.

By making every splice in exactly the same order, the operator will
acquire the habit of doing it nicely and quickly. Most splices are made
in a hurry (before or after a show) and in this, as well as in all
things that must be done hastily and at a moment's notice, habit and
practice are very important. One does not become expert until he has
practiced it often enough to be able to do it mechanically.

[Illustration: FIGURE 51.]

Proceed as follows and as outlined in Figure 51: Cut exactly along the
dividing line of one picture and save this to become the top piece. (The
emulsion side of the two sections of film must always be either up or
down, and for convenience it is well to form the habit of having them
always one way or the other.) Now, with a sharp knife, scrape the
celluloid side of this piece until all of the gloss is removed for a
width of a little more than 3/16 inch. Do not scrape any more than
necessary, but be sure that all dirt and oil are removed. Now take up
the other part of the film and cut off the bad portion, leaving a strip
a trifle less than 3/16 inch beyond the dividing line. This strip need
not be scraped on the back, but all of the emulsion must be scraped from
the front side of it. To scrape the front side nicely, lay a short ruler
across it to act as a guide for the knife and scrape up to this ruler.
Scrape only to the dividing line, but give particular attention to the
sprocket holes and the edges, for it is at the edges and sprocket holes
that film first begins to work loose and where it is most difficult to
secure proper adhesion. Both parts of the film should be so scraped
that, when finished, each will furnish half of the dividing line. Now
apply a liberal coating of cement to the latter section and carefully
lay the first upon it, taking great care that the sprocket holes in the
two sections overlap and match perfectly. It is for this matching
purpose that the glass in the table (with a small lamp underneath it)
is valuable. The sprocket holes must match accurately and the films must
line up straight. After the upper film is placed, it must be pressed
down for a few seconds until the cement has set. Wipe off all
superfluous cement before it has time to set. Be sure that there are
four sprocket holes for each picture. Keep a knife on hand and use it
for no other purpose. Keep also a suitable stone on which to sharpen
this knife, for, unless the knife is very sharp, it will do only poor
work and the edge on it will not last long at any rate. Ordinary cement
will not hold non-inflammable film, and water-proof film must be more
thoroughly scraped than ordinary film. In the bench, provide a recess
into which the cement vessel may be set and where it will always remain.
Provide also a pair of scissors for cutting the film.

[Illustration: FIGURE 52.]

The Edison film mender is shown in Figure 52. This is made with three
gates or hinges. Those on the right and left clamp down to hold the
film, while the narrower one in the center clamps down on the cemented
edge. To mend a film, place one of the pieces to be joined in the
mender. Place that part of the gage marked "cut" in position, cutting
between first and second perforations above the floor line separating
the picture. The remaining piece of film should be placed in the other
side of the mender and cut exactly on the line. Set the opposite end of
the gage marked "scrape" on the first section; remove the emulsion; cut
to the floor line with a sharp pen knife, moistening it so that the
emulsion will come off easily. After applying the cement with a brush to
the scraped piece, lay the other piece over it in the mender, clamping
it down; and leave it for an instant to adhere.




CHAPTER VII.

GENERAL HINTS ON INSTALLATION, OPERATION, AND CARE OF MACHINES.


_Tools._--First of all provide a suitable and sufficient set of tools.
There should be an assortment of screw drivers, large and small, to fit
the different sizes of screws used not only with the machine but about
the operating room and with other devices as well. Keep these screw
drivers filed or ground to proper shapes. A screw driver rounded off, or
too small, or bent, will ruin the heads of the screws. There should also
be a collection of pliers (especially such as are commonly termed "gas
pliers"), files, raps, chisels (for wood and metal), punchers, drills,
and monkey wrenches; a hack saw, a blow torch, and a vise will also come
in handy. Tools, however, depend upon the man who uses them. There are
men who would be no better off with such a collection at their disposal;
on the other hand, a clever man would be able to make use of all these
tools and show his skill thereby.

_Aligning the Machine._--The first thing to do in setting up a new
machine, after it has been unpacked and put together, is to line it up
properly. This can be done by stretching a string from the lamp house,
through the center of the condenser opening, film gate, and objective
lenses. These must all be in such a position relative to one another
that a taut string will pass through the center of each. The center can
be determined by measurement either with a suitable rule or a pair of
calipers.

_Anchoring the Machine._--The next step is to anchor the machine so as
to reduce vibration to a minimum. Vibration is often due to a flimsy
floor which will not stand the strain of the unavoidable vibration and
thus helps to increase it. Often, however, the vibration is due to some
part of the running gear of the machine, such as the revolving shutter,
for instance, being out of balance. If there is undue vibration, the
machinery should be looked after first, for it is much better to prevent
the vibration by removing the cause than to seek to restrain it by bolts
and braces. If the machine runs very badly, it cannot be bolted down
with any satisfactory results.

_Inspection._--After the machine is lined up and running smoothly, the
next step should be to inspect every part of it thoroughly for loose
screws, pins, gears, or other parts. It is best to do this by beginning
at one end and continuing to the other so that no part may be
overlooked.

_Upper Magazine._--See that the spring in the upper magazine, or on the
door of it, which prevents the momentum of the upper reel from running
off film after the sprocket has ceased pulling, is in good order and the
reel true. A crooked reel will generally cause trouble and force the
operator to keep the magazine door open so as to be able to aid the
movement of the film.

_Fire Trap._--The fire trap must next be looked after. The upper
magazine, the fire trap, and the sprockets must all be in perfect
alignment and the film must run through the trap without touching either
side. Film wears the metal very fast and a day's run with the film
brushing one side of a valve will show decided wear. If it is allowed to
run this way, it will soon cut through the side of the valve, forming
an opening in which the film will run and in which it will often catch
and tear splices. Special care must be exercised to see that the traps
are kept clean and, when not in use, they should always be covered. The
film is drawn through the traps very rapidly and any kind of dirt
accumulating on them is apt to scratch it. After a fire, burned film is
likely to be found in the trap, and this, when cold, becomes hard and is
difficult to remove.

_Cleaning Machine._--After the machine has been used for a considerable
time, it is advisable to wash out the gears thoroughly. Not only the
gearing, but the bearings also, should be reached by this process. If
one has plenty of gasoline and a suitable receptacle, the quickest way
is to take out the lenses and immerse the whole machine for a while in a
vessel filled with gasoline. It can then be taken out, worked a little,
and immersed again; this process being repeated until all of the old and
hardened grease is worked out. If facilities for immersing the machine
are not at hand, the bearings may easily be cleaned by using kerosene or
gasoline to oil them and then running the machine a little; this will
work all of the old grease out of the bearings. The gearing can be
cleaned in the same way. If gasoline is used, it will quickly evaporate
and leave the machine in good order.

_Oiling Machine._--After the machine is thoroughly cleaned, it may be
oiled again. The best lubricants to be used range all the way from
sewing-machine oil to beeswax. Beeswax is used only for the gearing and
only where that is somewhat worn. It is solid enough to compensate for
some of the wear. That part of it which works out on the sides should
be wiped off at once. There are many lubricants to be recommended; in
general, however, the particular kind to be chosen depends upon the
parts to be lubricated. A part moving rapidly with no great friction can
be well served with a light lubricant, but heavier parts, or parts that
fit so closely as to cause much friction are best served by some
lubricant that has more body. If a light oil is used, it will of course
have to be applied oftener than a heavy oil. None of the extremely light
oils should be used because they run off too fast.

_Care of Sprockets._--Having thus cleaned and oiled the machine, the
next step is to look after the sprockets. In an old machine it is likely
that the sprocket teeth will be so worn as somewhat to resemble hooks. A
little wear of this kind will do no great harm, but beyond a certain
point of wear, the sprocket teeth tend to carry the film farther than is
necessary and in so doing tear out the holes. Some sprockets are made
reversible and the trouble can be remedied by turning the sprocket wheel
around. In most cases it will, however, be advisable, when the old teeth
are badly worn, to get a new set of sprockets.

The sprockets must also be examined for accumulation of dirt, for in
case there is much dirt on a sprocket, it will not guide the film
properly. The dirt will cause it to jump, and thus bring about an
unsteady picture; or, if the accumulation of dirt is sufficient, it may
even cause the film to jump off the sprocket. Hundreds of feet of film
have been ruined in this way. The sprockets should be cleaned every day
with a suitable brush. An operator too careless to clean his machine
properly, is also as a rule too careless to watch the picture properly;
and he suddenly finds himself with a length of film ruined by sprocket
teeth which have cut a row of holes through the center of it.

The sprocket should be made to align perfectly with the upper and lower
magazines, the fire valves, the film gate, and the path of the film.
They may be aligned with a straight edge made of a flexible piece of
steel or, if nothing else is available, a piece of film may be placed
upon the sprockets. Be sure to examine the film carefully before using
it for this purpose; old film is apt to be warped. The film must fit
perfectly in the aperture and on the different sprockets. The sprockets
must have little or no end play, especially the intermittent, or it may
cause the picture to wiggle sideways. The idlers, or presser rollers,
should not fit tight enough to bind, but must fit closely enough to hold
the film securely. In the adjustment of these and all other parts of a
moving picture machine, considerable practice and judgment are necessary
to estimate how much freedom of movement is permissible.

_Adjusting Intermittent Sprocket._--The "Geneva" wears rapidly when it
is not well oiled or protected; and it becomes necessary to adjust it.
There are other ways of doing this but generally it is done by means of
an eccentric bushing which must be set to compensate for the wear. In
many operating rooms, there is a wide range of temperature and, as heat
expands the metals, they will bind unduly when they become warmed up, if
the star and cam wheel are set too close while cold. In many machines
the heat from the lamp warms them up irrespective of the temperature in
the operating room.

_Automatic Fire Shutter._--Some means of adjusting and regulating the
automatic fire shutter is usually provided; this should be adjusted so
that it will not rise before the machine is in motion and will fall
before it has come to rest. Under no conditions should the stationary
film be allowed in the path of the light. There are fire shutters that
do not depend upon the motion of the machine, but are raised as soon as
the handle is pressed to give motion to the machine. This type of
shutter is raised as soon as the handle is pressed and may expose the
stationary film to the light. Such a shutter should not be tolerated at
all. The fire shutter should be carefully tested to see that it operates
properly under all conditions. If one is not acquainted with the
particular type of fire shutter in use, it is advisable for him to test
it with the lamp turned on it for some time. Some shutters are so thin
and are placed so close to the film they are supposed to protect, that
it is possible to fire the film through them.

_The Revolving Shutter._--In some machines the light is shut off before
the film begins to move, and in others a very slight movement of the
film is permitted before the shutter closes out the light entirely. The
general object of the shutter is to shut off the light while the film is
moving and to interrupt the long period of light so that it may not
affect the eye too strongly and give rise to the sensation of flicker.
The exact manner of setting the shutter varies with different machines,
but in general the above consideration must be kept in view. Where a
shutter has several blades, it is always the larger one that shuts off
the light while the film is moving and this is the only one to which any
attention must be paid while setting.

The best test for the setting of the shutter is the appearance of the
picture. The best kind of picture with which to judge a shutter is one
that has strong contrasts of light and darkness. If a picture contains a
very bright object, the travel ghost will appear if there is any motion
while the light is on the film. If the shutter is set too late, the
travel ghost will be in the lower part of the screen; and if too soon,
in the upper. If the shutter is made too small, the travel ghost cannot
be entirely eliminated but must be divided half on the lower and half on
the upper side of the picture. The travel ghost and other imperfections
of a picture are much more noticeable to persons close to the screen
than to those far away. As the operator himself is the farthest away of
all spectators, he is likely to miss many imperfections that are plainly
visible to the audience. For this reason many operators have opera
glasses through which they view their pictures.

_Framing._--When the machine is finally threaded, the film should be so
placed on the upper sprocket that, with a proper loop, the picture will
be in frame when the framing lever is about in the middle position. This
position will allow the operator to frame either up or down without
seriously affecting the loop. The framing is best done by looking at the
picture thrown upon the screen.

_Film Gate._--The film gate needs special attention when new film is
being used. Very often new film is so damp that some of the emulsion
sticks to the gate. Wet film running through a machine makes it work
hard and is likely to cause much trouble. With such film it is often
necessary to stop during a run and clean off the gate and it is always
necessary to do so after each run. Never use a sharp tool or anything
that may scratch the gate or roughen the bearing surfaces where the film
runs. The gate must be watched closely for wear; if worn too much, the
picture may be out of focus or jump in all directions.

_Tension Springs._--The tension springs will repay careful study by the
operator. The object of the springs is twofold: First, to hold the film
flat, and second, to introduce sufficient friction to overcome quickly
the slight momentum which the film has acquired while in motion. The
springs should be just tight enough to do this. The pressure of the
springs necessary to overcome the momentum of the film is proportional
to the speed at which the film moves. If the film is to be run at a high
rate of speed--as is often the case when a crowd is waiting outside the
doors--the tension must be increased, unless it was originally great
enough to hold the film at this rate of speed.

As the film varies in thickness, the adjustment cannot be made too
close; but the springs must be made tight enough so that they will hold
the film at the highest rate of speed when the thinnest part of the film
is passing through. If the springs are not sufficiently tight, the
picture will seem to move a trifle. They must be just tight enough to
prevent this at the highest rate of speed at which the machine is to
operate.

_The Take-Up._--The lower sprocket exists for the purpose of
restraining the take-up from pulling on the intermittent sprocket. There
is thus actually always a strain that the one puts on the other. For
this reason the tension on the take-up should be no greater than
necessary. Excessive tension will increase the wear on the machine; may
cause the tearing of many splices; and will cause the machine to run
hard, thus imposing extra labor on the operator. While the lower reel is
nearly empty, the strain is small; but as it fills up, the strain
becomes greater. The driving wheel or belt must move at a fixed rate of
speed while the slip of the driven wheel becomes greater with every
turn. Thus the energy expended in friction becomes greater as the
picture progresses. Defective belting, crooked reels, or poor adjustment
of tension are the most frequent cause of annoyance in the take-up and
the take-up as a whole gives probably more trouble than any other part
of the equipment.

_Cover._--Every machine should be covered when not in use and the cover
should be dust tight.

_Getting Ready for the Show._--The first thing for the operator to do
before entering the operating room is to put away all smoking
paraphernalia he may have about him. No smoking tobacco or matches
should ever be found inside of any operating room. After taking this
precaution the next thing will be to rewind and inspect the films to be
used for the next performance. If the reel happens to be properly wound
when received, it is not absolutely necessary to rewind it and, in an
emergency, one may take a chance and run it through without previous
inspection. When possible, however, the operator should always inspect
his film before using and also familiarize himself with its contents.

A film is best inspected by rewinding it slowly from one reel onto
another, gripping it with a gentle pressure, and allowing it to run
between the fingers. This method will catch all splices and also all
split films. Films are frequently split along the line of the sprocket
holes. The film should be run through the fingers in this manner twice:
once to catch such irregularities as make themselves manifest along the
outer edge and again to catch the defects that exist only in the middle
of the film. Every splice should be carefully inspected to note how the
sprocket holes match and whether the splice will throw the picture out
of frame. All splices that are not properly made should be re-made as
directed in Chapter VI.

If there is sufficient time, it would be well to run the film through
the machine in the regular way. Thus the operator can familiarize
himself with the picture and note every place where improvements can be
made by adjusting the speed, for very often a picture may be improved by
slowing down or increasing the speed in certain scenes. The flicker
varies with the illumination; the brighter the light the greater will be
the tendency to flicker, if the machine is not run at proper speed. The
operator should know before hand just when his brightest scenes will
come on so as to adjust the speed or, in case he must run slowly, so as
to reduce the flicker by reducing the illumination. Running the film
through will also show every place where the picture goes out of frame
and all such places can be corrected. All bad places should be cut out
of the film, but not more than necessary. Every additional section cut
out of a film will increase the jerkiness of the motion. A few are not
noticed, but one often sees in old reels, scenes that are simply
ridiculous because too much film has been cut out. Such effects are
especially noticeable if a picture is run slowly.

The operator must always be prepared to run pictures at abnormal speeds.
It is bad practice to do so but often unavoidable; for instance, if the
next number on the program is delayed, it is desirable to extend the
picture until it is ready in order that the audience may not be
dissatisfied; or if all the seats are filled and people are waiting
outside, the manager may demand extra speed so as to make room for the
outsiders.

The operator who is wise and has had experience will provide himself
with a receptacle in which he can store the pieces he cuts out of films.
Saving such pieces and taking them to the exchange will often make an
impression upon the exchange management and induce greater care in the
inspection of films at the exchange. Such pieces will sometimes be good
evidence in the hands of an operator, too, in case he is charged with
having damaged a film. While the film is being run through the machine,
it can also be measured. Every turn of the crank means one foot of film.

Before the film is wound upon the reel from which it is to be exhibited,
an opaque piece of film about two feet in length should be attached to
the tail end of the film. This piece of film will in some cases serve a
double purpose. With some machines the film can be rewound from the
lower magazine into the upper without taking it out of either, if this
tail piece is long enough to remain fast to the upper reel after the
whole film is run off the upper reel. Thus the operator when through
running the picture may immediately change his crank and, without delay,
begin to rewind. The main purpose of this tail piece, however, is to
shut off the light immediately after the last section of the picture has
been exhibited. If the light is left on the screen while the operator
slows down the machine, it will cause a disagreeable flicker to appear
before the eyes of the audience immediately after the picture is over,
and thus leave an unpleasant impression.

The beginning of the film should be provided with a leader from three to
five feet long. The purpose of this leader is to allow the operator time
to frame his picture and get his lamp adjusted before the title appears,
or is finished, so that when the picture actually begins, everything
will be running smoothly.

The film must be rewound upon the reel in such a manner that when
threaded, the emulsion side will face the lamp. If this is not done, the
titles will read from right to left instead of from left to right as
they should. It will be well for the operator to form the habit of
regularly testing his film by the actual projection to see that his
titles are right. There are often many other things to think of and
cases of absent-mindedness are not uncommon. If the title shows wrong,
the reel may be taken out and turned around; this will reverse it, but
ordinarily it is best to wind it in one certain way.

Every operator should have a full supply of good reels for use in the
machine and should never let these get out of his house. The reels
furnished by the exchanges are often very bad and if perchance a real
good one is sent out, some wide-awake operator will surely keep it.

Although it is best to rewind a reel as slowly as your time will permit,
it is necessary sometimes to do it very rapidly. The reel from which the
film is being unwound should be under a steady tension and some form of
brake is often provided. The practice of "pulling down," which is
indulged in by so many operators, is highly reprehensible. It causes the
emulsion side of the film to slide over the other and any dirt or dust
that may be between the two is thus ground into them. The scratches thus
caused soon fill up with dirt and cause streaks in the picture which are
commonly termed "rain." The dirt--especially the carbon dust, which is
so plentiful in an operating room and which is very sharp and cuts badly
into the emulsion or the celluloid--disfigures the picture very much.

A good film box should be located in every operating room and it should
be so conveniently located that the operator will use it in preference
to anything else. Such a box should be provided with automatic
self-closing covers and should be near the floor, as the film is less
liable to dry out there than if placed near the ceiling where the heat
is greater. Nothing whatever should be placed upon the floor itself and
it should be kept absolutely clean. Although it is very bad practice to
run film on the floor and it should not be done, there are occasions
when it will be done and, if the floor is clear of all obstruction, the
hazard of doing so is much reduced. An operator who, having trouble with
his take-up and having run the film on the floor, has had it entangled,
will appreciate this fact. The spasmodic bursts of applause which an
impatient audience is apt to offer while the show is being delayed are
not especially comforting to any one in such a position.

Under no circumstances should film ever be left exposed. Occasional
small fires at the aperture plate are unavoidable and in such cases the
operator will seek to tear off the burning film and throw it on the
floor, and it may then ignite the film lying about. Form the habit of
keeping your film under cover; and make up your mind that it is
absolutely necessary. Keeping films under cover will also tend to save
them from drying out and prevent accumulation of dust.

A slide with the words "Just a moment please," or some equivalent
expression, will reassure the audience; one should always be kept in
place and when any trouble causes an interruption of the show, the lamp
should be pushed over thus exhibiting this sign. The habit of pushing
the lamp over in case of interruption is also a very good one. It is the
simplest and most natural way of taking the light off the film and
satisfying the audience at the same time. This practice should become a
habit. It will be a great protection against fire in addition to the
other safety devices. All the work of making ready should if possible be
done before a considerable part of the audience is in the house. It is
best to let none of them see the process of getting the show in
readiness.

As has been stated before, an occasional fire at the aperture is
unavoidable. Sometimes pieces of film are torn loose, stick where they
are subject to the light, and in time catch fire. A fire thus started
may easily consume all of the film between the two magazines and will
progress slowly toward the lower one, but towards the upper one very
rapidly. If the doors of the magazines are tightly closed and fit tight
and if the fire traps are in good order, there will be but a small
chance of fire getting at the rolls of film that may be in either
magazine. Every effort should, however, be made to extinguish it before
it reaches the fire traps. Just what can be done depends upon the design
of the machine. In order to be prepared for such an emergency, the
operator should study his machine and decide what would be best to do.
In many cases the film can be torn off before the fire extends to the
magazine and thus break the connection. In some machines the film runs
in such a narrow groove that the fire is generally extinguished there
without any outside help. Some operators have been known to open their
magazines and attempt to remove the film after a fire had started,
resulting in burning all their film. It is far better to leave the film
in the magazine and trust to the fire traps than to attempt to remove
it.

No visitors should be allowed in any operating room during the time that
the audience is in the house, more especially when the time for opening
the show approaches. There should be nothing to distract the operator's
attention from his duties. The arc lamp should be started a few minutes
before the show is to begin so that the electrodes may burn to their
proper shape and the light be adjusted. Before the show begins, the
house should be made dark for a few minutes. This will make the picture
appear brighter.

In case the film tears during a run, some operators have the habit of
pinning the pieces together with the intention of making the splice
after the picture is run. This practice is well enough for an operator
who feels sure that he will not forget about the pin, but with one who
is forgetful or absent-minded, or who has a lot of other duties which
might take his mind off the film, it is a bad practice. The film may be
rewound with the pin remaining in position and cause very serious
trouble. It is safer to put the film on the take-up without pinning.

In a general way the points alluded to in the following list should be
carefully looked after. This is a list which is used by the Electrical
Inspection Bureau of the City of Chicago in checking up operators and
their equipment.

  Theatre address.......................................................

                  Perm.
  Operator's name      .................................................
                  Temp.

  Address.................................. Lic. No. ...................


                       MOVING PICTURE MACHINE INSPECTION.

  An x indicates defect; a check, satisfactory condition.

  -------------------------------+--+--------------------------------+--
   1 Smoking?                    |  |25 Fan Motor?                   |
  -------------------------------+--+--------------------------------+--
   2 Matches?                    |  |26 Guards on inc. lights?       |
  -------------------------------+--+--------------------------------+--
   3 Condition of Films?         |  |27 Unauthorized persons?        |
  -------------------------------+--+--------------------------------+--
   4 Magazine closed?            |  |28 Rewind approved?             |
  -------------------------------+--+--------------------------------+--
   5 Film exposed?               |  |29 Rewind location?             |
  -------------------------------+--+--------------------------------+--
   6 Arc enclosed?               |  |30 Rheostat, 5 ft. above floor? |
  -------------------------------+--+--------------------------------+--
   7 Guard fastened?             |  |31 Rheostat, contacts enclosed? |
  -------------------------------+--+--------------------------------+--
   8 Upper fire guard?           |  |32 Rheostat, safe location?     |
  -------------------------------+--+--------------------------------+--
   9 Lower fire guard?           |  |33 Box for films, approved?     |
  -------------------------------+--+--------------------------------+--
  10 Upper magazine approved?    |  |34 Box for films, cover?        |
  -------------------------------+--+--------------------------------+--
  11 Lower magazine approved?    |  |35 Shutters, guides?            |
  -------------------------------+--+--------------------------------+--
  12 Upper magazine, lock well?  |  |36 Shutt's self-closing,        |
                                 |  |   supports?                    |
  -------------------------------+--+--------------------------------+--
  13 Lower magazine, lock well?  |  |37 Booth lining?                |
  -------------------------------+--+--------------------------------+--
  14 Light shutter?              |  |38 Floor?                       |
  -------------------------------+--+--------------------------------+--
  15 Fuses enclosed?             |  |39 Shelving?                    |
  -------------------------------+--+--------------------------------+--
  16 Fuses, proper size?         |  |40 General appearance?          |
  -------------------------------+--+--------------------------------+--
  17 Inc. circuit?               |  |41 Door, self-closing?          |
  -------------------------------+--+--------------------------------+--
  18 Arc switches, enclosed?     |  |42 Box office?                  |
  -------------------------------+--+--------------------------------+--
  19 Arc switches, control       |  |43 Unapproved display?          |
     rheostat?                   |  |                                |
  -------------------------------+--+--------------------------------+--
  20 Arc switches, condition?    |  |44 Main cutout cabinet?         |
  -------------------------------+--+--------------------------------+--
  21 How much open arc wire?     |  |45 Emergency cutout cabinet?    |
  -------------------------------+--+--------------------------------+--
  22 How much open inc. wiring?  |  |46 Number emergency lights?     |
  -------------------------------+--+--------------------------------+--
  23 Open contacts?              |  |47 Fan motors auditorium?       |
  -------------------------------+--+--------------------------------+--
  24 Flexible or BX cord?        |  |48 Rheostat in base.   cond.?   |
  -------------------------------+--+--------------------------------+--
      49 Exits.....?  Gas.....?   Sperm Oil.....?   Electric.....?
  ----------------------------------------------------------------------

                                           .............................
                                                            Inspector.

  These defects must be corrected within..............days.

  The Electrical Inspection Bureau must be notified as soon as repairs
  are made.




CHAPTER VIII.

LIGHT.


Light is emitted from a luminous body. A luminous body is one in which
all of the particles are conceived to be in violent motion, which motion
is transmitted to a supposed ether. The existence of such ether cannot
be demonstrated, but it is supposed to exist because it is impossible to
think of anything being transmitted except through some medium. This
ether is a rather imponderable substance; it is supposed to pervade all
space; and it exists in all matter and in every vacuum. It is supposed
to be elastic without weight and capable of transmitting motion without
loss of energy or friction. It is, however, assumed to be modified to
some extent by the matter in connection with which it exists. Thus the
rate of transmission of light waves is different for air, glass, water,
and other substances through which they may pass and such bodies as are
entirely opaque are supposed to suppress the ether vibrations, resulting
in light entirely.

Light is then a mode of motion of this universal ether which pervades
all space even to the farthest star. The motion of this ether is
conceived to be about as illustrated in Figure 53. If we take a heavy
string or a small rope and, stretching it reasonably taut, jerk it
forward and back quickly a few times, it will be seen to move and assume
the appearance shown in the figure; and that portion shown between the
two vertical lines represents one complete wave.

[Illustration: FIGURE 53.]

The assumption of this ether and the vibratory motions of it form the
only explanation that is capable of accounting for all of the phenomena
of light. All other theories advanced have failed to stand the test;
sooner or later some phenomena have appeared which could not be
explained by them.

Light waves are known to travel through space in straight lines until
they meet with some medium which is capable either of reflecting,
refracting, or absorbing them. The rectilinear propagation of light,
consisting of vibratory motion, is one of the most difficult parts of
the theory to explain. It involves rather deep study and more
mathematics than the scope of this work will warrant. Suffice it to say
that the rectilinear propagation is brought about through the
interference of light waves. An analogy of this can be found in a stream
of water. It is well known that a stream of water issuing from the
nozzle of a garden hose moves in a straight line until gradually forced
into a spray by the resistance of the air. Yet the water passing through
the hose is interfered with on all sides and on all sides there is a
tendency to deflect it. If the water were to move along slowly so that
we could observe the action on one side independently of the balance of
the stream, we should see a series of waves being formed by every
particle of the hose which offers resistance to the flow. The waves
formed in the interior of the hose on all sides are interfered with by
waves from all other directions and the result is motion in a straight
line. In a similar way it may be conceived that the millions of light
waves emitted from a luminous body interfere with each other and thus
cause light rays to move in a straight line.

Light is a form of energy and can be converted into other forms of
energy. It can be converted into heat, for instance, or can be used to
produce chemical effects. Light differs from heat only in the rate of
vibration and the length of the ether waves. Heat can be reflected in
the same manner as light, as the following experiment will show: Arrange
two ordinary reflectors as shown in Figure 54. In the focal point of
one, place a heated iron ball or something of the kind; if this be hot
enough, it will ignite paper suspended in the focal point of the other
reflector, although a thermometer placed anywhere between the two
reflectors will give only a small indication of a rise in temperature.

[Illustration: FIGURE 54.]

All light rays as well as heat rays are in themselves invisible; we can
see only the object which emits or reflects them. If a beam of light be
allowed to enter a darkened room, as shown in Figure 55, we shall
probably be able to see the whole path of the rays illuminated, as well
as the spot on the floor. But this will be because of particles of dust
in the air which reflect the rays to our eyes. If we introduce some
smoke into the room, we shall see the light much more plainly because
there are now more particles of matter to reflect it. On the other hand,
if special precautions are taken to have the air absolutely clear of
dust, we shall be able to see nothing but the spot on the floor.

[Illustration: FIGURE 55.]

Light travels through space at the rate of about 186,000 miles per
second. White light is a combination of light of many colors, but the
speed of transmission is the same for all colors. The length of waves
and the rate of vibrations, however, vary. The red rays have the longest
waves and the slowest rate of vibration; they vibrate about three
hundred ninety-five billion times per second and the wave length is
about 0.0008 millimeters. The violet rays possess a wave length of about
seven hundred sixty-three billion per second. There are light waves
which are longer than the red rays and these are known as _infra red_.
They are not visible to the eye but their existence can be proved in
many ways. Light waves shorter than the violet are also invisible and
are known as _ultra violet_. These have much importance in photography
and to this class belong the X ray.

[Illustration: FIGURE 56.]

We have mentioned above that white light is a combination of light rays
of many colors. This can be proved by the following experiments: If we
arrange to have a beam of sunlight pass through a small hole into a
darkened room, it will pass to the wall on the opposite side in a
straight line and give us white illumination upon a small spot. If we
now arrange a prism in the path of this ray or beam of light, we shall
find that the light no longer passes straight to the wall but that
instead, it is bent in a certain direction and furthermore shows us a
brilliant array of colors. This is illustrated in Figure 56. The rays
are thus shown to be separated into their constituent colors; red is
shown at the top and the following colors merge imperceptibly into one
another--orange, yellow, green, blue, indigo, and finally violet at the
bottom.

The reason for this change is that the rays of light on entering the
glass are slowed down--those of the higher rate of vibration more than
the others. The violet rays are thus said to be more refrangible than
the red, for instance. The colors thus produced are simple colors. This
is proved by the fact that if the light is passed on through another
prism, it will be again reflected but will not be resolved into other
colors; although whichever color is carried to the next prism will
spread out and show finer gradations in its color.

[Illustration: FIGURE 57.]

The colors given above are those obtainable from the decomposition of
sunlight and make up what is known as the _solar spectrum_. If instead
of sunlight some other illuminant be used, the arrangement of colors
will be different; and it has been found possible to tell from the
colors of the spectrum what substances are burning, or heated to a
luminous degree, in the source from which the light comes. This method
is known as _spectrum analysis_.

There are several ways in which light, which has thus been separated
into its fundamental colors, can be re-composed so as to give us white
light again. One of these methods consists in arranging an inverted
prism to receive the light, as shown in Figure 57. The rays leave the
second prism parallel and produce the effect of white light. Another
method consists in gathering the rays from the prism by a lens, as
shown in Figure 58. Furthermore, if we take a disc and paint the colors
of the solar spectrum upon it in the proper proportions, as indicated in
Figure 59, and cause this disc to be rapidly revolved, we shall see it
as almost white.

[Illustration: FIGURE 58.]

[Illustration: FIGURE 59.]

Another fact which goes to prove the undulatory, or motion, theory of
light is that two sources of light arranged to oppose each other can
actually be made to produce darkness. To do this, the waves of one
source of light must be made so that they exactly oppose those of the
other; thus they destroy each other and destroy what light there is in
either. There are other methods, but this can be partially accomplished
in the following manner: Two small mirrors of black glass or of metal
are placed, as shown in Figure 60, very close together and so that they
form an angle of nearly 180 degrees. A beam of light arranged to fall
upon both of the mirrors will be reflected in such a manner that the two
halves interfere with each other and cause bands of light and darkness
to appear. The dark lines are due to the opposition and nullification of
certain of the light waves.

[Illustration: FIGURE 60.]

[Illustration: FIGURE 61.]

The intensity of light diminishes directly as the square of the distance
through which it is transmitted. This is illustrated in Figure 61. The
light, starting from a point, is limited by the size of the first square
at the left; it spreads out more and more, and illuminates larger and
larger spaces. Exact measurement will show that the spaces illuminated
by a ray of light are always exactly proportional to the square of the
distance from the point of light. This law, however, applies strictly
only if the distances considered are long compared to the source of
light, so that the light may be considered as being a mathematical
point, that is, having no physical dimensions. If the source of light,
for instance, were of the same size as the first opening and of uniform
intensity there would result the same intensity of illumination of a
similar space at all distances. There would, however, be an outer fringe
of light which would be proportional to the law of inverse squares. Many
reflectors are arranged to throw very nearly parallel rays; and with
these the intensity remains the same except for absorption, which is
ordinarily not very great.

We see things only through the rays of light they reflect. All colored
bodies have this peculiarity, viz., that they are capable of reflecting
only such rays as make up the color the body is said to possess. A red
body, for instance, absorbs all colors except red and reflects red only.
A black body absorbs all rays and a perfectly dull black body is visible
only by contrast; that is, we do not see it but we are aware that there
is something invisible before our eyes. When we are in a perfectly dark
room, we see nothing but we have blackness before our eyes. A perfectly
white body is one which reflects all of the rays of light and absorbs
none.

When we view things through colored glasses, we see them only in the
colors which the glass will transmit. If we view a red body through a
green glass or under a green light, it will appear black because it is
capable of reflecting only red rays and in the green light there are no
red rays; hence there is nothing to be reflected and the red appears
black.




CHAPTER IX.

PRINCIPLES OF VISION.


Through the medium of our eyes we see objects by means of the light
which is reflected from them. This light enters the eye and forms an
inverted image of the object upon the retina, just as an inverted image
is formed upon the ground glass of a camera. This impression made upon
the eye is corrected automatically, so that, although we see everything
upside down, we are not at all aware of so doing. The proof of this
peculiarity of the eye is found in cases where persons born blind have
later through operations acquired sight. In the eyes of slaughtered
cattle also the image can be seen inverted. A further proof that we are
able thus to adjust ourselves is found in the experience of persons
using cameras with ground glass screens. The image on these screens is
always inverted both horizontally and vertically. The user soon learns
to see his object, although inverted, in the natural way, that is,
vertically, because this is so plain that he must take it into account
at every focusing. He does not, however, accommodate himself to the
reversal from right to left because this is of no consequence ordinarily
and is not noticed. Many photographers, who have been accustomed to the
vertical inversion, still find themselves confused when trying to locate
the right and left of a view seen through the lens.

Unless a special arrangement of lenses is provided, all images cast
upon screens through small openings appear inverted. The reason for this
can be seen from Figure 62. It is obvious that no other light can reach
the bottom of the screen through the pin hole _O_, at the left of the
figure, except that coming from the flame of the candle; also that no
other light can reach the top of the screen except that reflected from
the bottom of the candle at the right. Hence the image of the candle
appears inverted.

[Illustration: FIGURE 62.]

[Illustration: FIGURE 63.]

A general understanding of the structure of the eye can be had from
Figure 63. _W_ indicates a watery substance in the front of the eye; _I_
is the iris which has power to contract or expand and thus regulate the
quantity of light admitted to the eye; _P_ is the pupil; _L_ is the
lens; and _R_ is the retina which connects with the optic nerve and the
brain. The lens is made up of several parts having different indexes of
refraction. The whole resembles an ordinary convex lens but has
considerable power of adjustment. When looking at objects close by, for
instance, the pupil can often be seen to bulge out which is its method
of accommodating itself to objects close at hand.

To the iris falls the duty of regulating the quantity of light which is
to reach the retina. If confronted by a bright light, it closes
partially; in a dim light, it opens out wide. When subject to a
flickering light, there is a tendency to follow the flickerings by rapid
opening and closing, which causes pain. If subject to flickerings long
enough, however, the pain becomes somewhat less, probably because the
iris has come to rest on an intermediate point.

An image formed upon the retina remains for some time, the time varying
with the intensity of the light. Very intense impressions are supposed
to last about one twenty-fifth of a second; milder ones as long as
one-tenth of a second. This tendency to retain images is known as the
_persistence of vision_ and can be noticed in many ways. A twenty-five
cycle alternating current falls to zero fifty times in one second; and
fifty times in each second there is a slight cooling off of the
incandescent filament. Yet the variation in the intensity of the light
is noticeable. Many of the sleight-of-hand tricks depend upon this
persistence of vision and the projection of moving pictures would be
impossible without it.

We are able to judge distance principally through the fact that we have
two eyes. If our eyes were immovable, we should see two images for
every object. But as they are movable and as both normally point
directly at the object we are looking at, their axes form angles with
each other and in this way we are enabled to judge the distance, as well
as other qualities of objects.

When both eyes are centered upon an object, the impressions received by
the brain from both sources are mixed, and the picture we become
conscious of is a composite of the two images in the eyes.

This is verified by the fact that many persons with defective vision can
see much more clearly with one eye than with both. They are not able to
focus both eyes upon the same point and thus the perfectly clear picture
which may exist in one eye is mixed with an uncertain picture in the
other.

In youth normal eyes are able to adjust themselves to different
intensities of light and different distances very rapidly. This power is
largely curtailed as age advances. Where a young person can almost
instantly, after gazing at some distant object, turn to a newspaper and
read, the eyes of a person of advanced age generally require
considerable time before they can adjust themselves in the same way.
Quite frequently, however, very old people regain their powers of vision
and become able to do without the glasses formerly used.

All of the above facts should be thoroughly understood by those having
to do with illumination used by a mixed audience. Light that may seem
perfectly satisfactory to one may be entirely unsuitable for another.




CHAPTER X.

REFLECTION.


Light may be reflected from opaque or transparent bodies such as glass.
In the case of transparent bodies, the reflected rays are not noticeable
unless the ground behind the reflecting body is dark. If there is much
light behind a pane of glass, for instance, the pupil of the eye will be
partially closed and not be able to see the faint light which is
reflected. As we gradually darken the space behind the glass, the image
begins to appear more and more distinct, partly from contrast with the
dark background and partly on account of the increased opening of the
pupil. This can be readily noticed if some evening out of a
dimly-lighted room we look at some object just discernible. If we then
turn on the light suddenly, the object will at once disappear but
reflections will appear in the glass where there were none before.

The reflections from clear glass are much stronger at an angle than when
the rays are thrown straight back. This can be seen by placing any
object directly in front of a pane of glass with a dark background. If
we place the eye so as to receive only those rays which are reflected
directly back, we shall obtain but a weak reflection. If, however, we
place the object a little to one side and stand close to the glass, we
shall see the object almost as plainly as in a regular mirror.

A ray of light is always reflected at exactly the same angle at which
it strikes, the reflecting body; that is, the angle of incidence is
equal and opposite to the angle of reflection. This can be illustrated
by Figure 64. If a mirror be attached to the pointer in the position
shown at an angle of exactly ninety degrees and a beam of light be
allowed to enter through the slit at the top, it will be reflected back
exactly to the spot at which it entered. If we then turn the pointer
slightly, we shall notice that the reflection of the beam of light moves
twice as fast as the pointer and, when the pointer occupies the position
indicated by broken lines, the light will be reflected at right angles
to the line along which it enters. If the mirror is turned still more,
the same law will hold; so that, if the mirror were turned through an
angle of nearly ninety degrees, the reflected beam of light would in the
same space of time make an angle of nearly one hundred eighty degrees.

[Illustration: FIGURE 64.]

Reflected light results in the formation of images in mirrors and other
reflecting bodies and, by bearing in mind the law of reflection given
above, we can readily explain how these images are formed and the manner
in which they appear to us.

[Illustration: FIGURE 65.]

Let _N_, Figure 65 be an object in front of the mirror. The only rays
that are reflected back to the eye are those that strike the mirror at
the proper angle. All others are wasted with reference to the
particular position of the eye. If the eye and the object reflected are
equally distant from the mirror, we need but draw a line at right angles
to the mirror and half way between the eye and the object and, from
these two, draw lines to the point at which the perpendicular line
strikes the mirror. The two lines thus drawn will give us the path of
the incident and the reflected rays. The image will appear to lie in the
direction from which the reflected ray comes and as far behind the
mirror as the object is in front of it. If the eye and the object to be
reflected are not equally distant from the mirror, it is more difficult
to find the paths of the rays and it simplifies matters very much to use
the following construction: Draw a line from the object before the
mirror at right angles to the mirror and extend it behind the mirror as
far as the object is in front of it. From this point behind the mirror,
draw another line to the eye. By drawing a third line from the object to
the point in the mirror where this line, from back of the mirror to the
eye, crosses it, we shall obtain the paths of the rays and the position
of the image in the mirror. The image will exist in the mirror at the
point where the reflected and incident rays meet but will have the
appearance of lying some distance behind the mirror. This is illustrated
in Figure 65, _N_ being the object reflected and _M_ the apparent
position of the image to the eyes as located in the cut.

[Illustration: FIGURE 66.]

[Illustration: FIGURE 67.]

[Illustration: FIGURE 68.]

In Figures 66 and 67 the same construction is used to show the
appearance of arrows as they are reflected from a mirror to the eye.
Objects standing erect over horizontal mirrors or arranged at right
angles to mirrors and looked at, as in Figure 68, always appear
inverted. This can be noticed in quiet ponds of clear water which give
reflections of trees and other objects. Figure 68 shows two arrows, one
horizontal, the other vertical; by the construction in the figure one
appears inverted, the other not. If the arrow were placed in the
position indicated by broken lines, the eye would see only the butt. If
the arrow were placed a little nearer the horizontal, it would appear in
its natural position; if a little more vertical, it would appear
inverted in the mirror.

[Illustration: FIGURE 69.]

All objects seen in mirrors are reversed with reference to right and
left. A pocket on the left side of a person facing a mirror will appear
to be on the right side. Printed matter held before a mirror will appear
just as it would if seen through the paper from the back side and will
have to be read from right to left.

[Illustration: FIGURE 70.]

[Illustration: FIGURE 71.]

[Illustration: FIGURE 72.]

If an object be placed between two parallel mirrors as _B_, Figure 69,
there will be a vast number of reflections visible at the point _D_.
Several reflections of _B_ will come to the eye in the manner indicated
but there will be a large number of additional reflections. If the
mirrors are exactly parallel and absolutely smooth, the number of
reflections would theoretically be infinite. At each reflection,
however, some light is absorbed and some diffused so that many of the
reflections are not discernible. Two mirrors set opposite each other
will also give many reflections of each other as indicated in Figure 70.
The images seen in parallel mirrors all appear arranged in straight
lines on both sides, as indicated in Figure 69. If now one of the
mirrors be inclined so as to form an angle with the other, the long line
of images will seem to become curved and finally lie in a circle. If the
mirrors be placed at right angles to each other, as in Figure 71, there
will be three reflections of the object _C_ visible and these will
reach the eye by the paths shown. If the mirrors be placed at an angle
of sixty degrees to each other, five images will appear as shown in
Figure 72, in which _A_ is the object being reflected.

The following tabulation shows the number of images obtainable at
different angles between the mirrors.

  +---------------+-----------+
  | Angle between | Number of |
  |    mirrors    |  images   |
  +---------------+-----------+
  |  90 degrees   |     3     |
  |  72 degrees   |     4     |
  |  60 degrees   |     5     |
  |  45 degrees   |     7     |
  |  30 degrees   |    11     |
  +---------------+-----------+

[Illustration: FIGURE 73.]

Instead of being plain, mirrors may be either _concave_ or _convex_. A
concave mirror is hollowed out in conformity with a small section of the
surface of a sphere. If a piece of glass be cut out of a hollow sphere,
the inner side of it will show the surface of a concave, and the outer
side, the surface of a convex mirror. A section of a concave mirror is
shown in Figure 73. _C_ is the center of curvature and any line drawn
from the surface of the mirror to this center is at right angles, or
normal, to the curvature of the mirror. A ray of light emanating from
this center will be reflected straight back to it. If the source of
light be moved a little nearer to the mirror, the light reflected will
be spread out more and come to a focus farther back from the glass; if
it be moved farther back from the glass, the rays will be focused nearer
the mirror. Thus if a light be placed at _A_, its rays will be focused
at _D_ and a light placed at _D_ will focus at _A_. This can be seen by
the lines which represent the rays of light. The two points at which a
source of light will thus focus are known as the _conjugate foci_ of the
mirror.

If such a mirror receives parallel rays of light, they will be reflected
and come to a focus at a point midway between the mirror and the center
of curvature. This point is known as the _principal focus_ of the
mirror, and the distance between it and the mirror is called the _focal
length_ of the mirror. A source of light placed at this point will throw
out parallel rays from the mirror. If the light be moved closer to the
mirror, the reflected rays will spread out; while if moved farther away,
the light will come to a focus at some distant point, as shown above.

Figure 74 can be used to illustrate the manner in which a concave mirror
reflects the light from an object placed before it. From the upper point
of the large arrow, rays of light emanate in all directions. All that
strike the face of the mirror are thrown to a certain point which can be
found by tracing out the lines, using the small arrows as guides. At
this point will appear the image of the top of the arrow. It will be
noted that it is inverted. The rays from the lower part of the arrow
are of course all reflected in the same manner.

[Illustration: FIGURE 74.]

With mirrors of this kind, the position of the object with reference to
the focal length and center of curvature is of great importance. If the
object be placed in the position shown as the image in Figure 74, the
image will appear as though it were in place of the object; it will be
much enlarged and also inverted. If the object is placed between the
principal focus and the mirror, it will appear to lie behind the mirror
as shown in Figure 75. In this case it will not be inverted.

[Illustration: FIGURE 75.]

When concave mirrors forming large sections of spheres are used, the
rays reflected from the outer edges will not all meet exactly at the
focal point. There will then be a somewhat fuzzy image formed. This is
illustrated in Figure 76. In order to obtain a perfectly clear and
distinct image, only the central part of concave mirrors should be used.

[Illustration: FIGURE 76.]

Convex mirrors are not much used. Sometimes glass spheres are set up to
show miniature reflections of scenery; convex mirrors are also found in
the lobbies of theaters and in places of amusement to amuse the patrons
with the caricatures of themselves reflected in them.

[Illustration: FIGURE 77.]

All of the rays that strike a convex mirror are reflected back in such a
manner that they seem to come from a common point behind the mirror.
This is shown in Figure 77. The center of curvature here is behind the
mirror but the paths of the various rays can be determined as before
explained. Thus we shall find that every ray, striking the mirror from a
certain point, is reflected back in a direction which gives it the
appearance of coming from a certain point behind the mirror. Two such
points are shown in Figure 77.

[Illustration: FIGURE 78.]

In Figure 78 we have drawn the arrow and the image it would produce in
the mirror. If the mirror forms a section of a sphere, the object
reflected will appear reduced in size in all directions. If the mirror
forms merely a section of a cylinder, a person standing in front of it
will appear very much shorter than natural but of full width thus
presenting a ridiculous appearance. Convex and concave mirrors are often
combined and if properly set, a person standing in front of one may see
himself either very much elongated or shortened.




CHAPTER XI.

REFRACTION.


If a straight stick or pencil be plunged into a vessel containing water,
it will appear to be bent. The reason for this appearance is given in
detail in Figure 79. The only light which can reach the eye from the
lower extremity of the stick must reach it by a path similar to that of
the bended ray at the left. According to this, the rays of light leaving
the bottom of the stick at _E_ bend at the water's edge and meet the eye
as shown. To the eye the rays, by which it sees the end of the stick,
appear to come from the direction _F_; hence the stick is seen crooked.
When a ray of light passes from air into a denser medium such as water
or glass, the ray appears to be bent somewhat, as illustrated at the
left of Figure 79. This bending of the rays of light is called
_refraction_.

[Illustration: FIGURE 79.]

The fact of refraction can be further verified by placing an object _G_
into an empty vessel in the position shown. In this position the object
is not visible to the eye. By slowly filling the vessel with water, the
object will gradually appear and will seem to lie in the direction of
the straight dotted line.

[Illustration: FIGURE 80.]

The bending or refraction of light rays is governed by a law which holds
in all cases. The bending differs with different kinds of glass, with
water, with oils, or even with water mixed with chemicals; but no matter
how much the bending may differ with different materials, for each
material there is a fixed ratio between the angle at which the light
enters and the angle at which it continues in the material. This ratio
is known as the _index of refraction_. For water it is about 1.33; for
glass about 1.5; for the diamond about 2.5; etc. This can perhaps be
most easily explained by reference to Figure 80. Here the top of the
shaded space represents the surface of the glass at which the light
enters. The line _E_ is drawn at right angles to this surface and the
various lines, meeting in the center of the circle, represent different
rays of light falling upon the glass at different angles. The heavy line
shows the ray of light striking the glass and passing through it.
Similarly each line is drawn differently and the nature of the lines
will act as a guide by which the angle of incidence upon the glass and
the angle at which the rays continue through the glass can be traced.

When it is said that glass has an index of refraction of 1.5, or 3/2, it
means that the sine of the angle of incidence is 1.5 times as great as
the sine of the angle of refraction. The sine of the angle of incidence
is proportional to the length of a line drawn from the vertical _E_ to
the periphery of a circle, outside of the glass or other substance, to
where it meets the ray; and the sine of the angle of refraction is
proportional to the length of a line drawn from the same vertical to the
periphery of the same circle, inside of the glass or other substance,
where it meets the ray.

In order to lay out the path of a ray through the material, its angle of
incidence being known, we may resort to the construction shown in Figure
80. Draw the incident ray at its proper angle and the vertical line at
right angles to the surface of the material. From the point _A_, where
the ray enters the circle, and at right angles to the vertical line,
draw the line _1_. Measure the distance from _A_ to the vertical line
and divide it by the index of refraction; and on a continuation of the
line _1_, lay off the distance as found. From the point so located, draw
a line parallel to the vertical until it intersects the circle at the
bottom. From the point at which the ray enters the medium, draw another
line to where the last line drawn strikes the circle. This line will
give the direction in which the ray is propagated in the medium. For
glass, the index of refraction is taken as 3/2; hence the line _1_ must
contain three parts, while the continuation contains two of the same
length. Upon leaving the medium, the ray is bent back again so that it
continues in a direction parallel to that at which it entered, as shown
in Fig. 81; and thus the case is reversed.

[Illustration: FIGURE 81.]

[Illustration: FIGURE 82.]

Figure 82 is an illustration of how both refracted and reflected rays
may be several times reflected. If one will hold a match close to a
mirror, looking at it somewhat from the side, there will be visible six
or eight distinct reflections of it. These will be of different
intensity.

If a source of light has its origin in a medium denser than air, it will
be bent as shown in Figure 83. In such a case some of the rays do not
leave the medium at all but are reflected back into it, as indicated at
the right. The heavy line indicates what is commonly known as the
_critical angle_. This angle varies with different substances and is the
angle at which the refracted ray of light skims along the surface and
does not pass out. All rays emitted at a lower angle are returned back
into the medium. If an electric light were immersed in water, only the
rays of light at the left of the black line would be visible to an eye
outside.

[Illustration: FIGURE 83.]

Figure 84 is an illustration of an equilateral prism. Here we have
refraction on both surfaces and a light placed at _A_ would be seen by
the eye as located at _B_. With prisms it is easily possible to locate a
light so that it may be seen in two positions at the same time.

[Illustration: FIGURE 84.]

Figure 85 shows a double convex lens. Both sides of it are segments of
circles or sections of a sphere. The center of curvature or the radius
of one side is at _A_ and that of the other at _B_. Each minute particle
on the surface of such a part of a sphere may be looked upon as the
surface of a prism, the inclination of it being indicated by a line
drawn at right angles to a line passing from the point through the
center of curvature. This is illustrated in the figure by the short
heavy lines at right angles to the broken lines centering at _A_ and
_B_.

[Illustration: FIGURE 85.]

By using the construction explained with Figure 80 and using for the
left-hand surface the circle of broken lines and for the surface at _E_
the circle of solid lines, we find the path of the ray coming from _C_
to cross the principal optical axis at _A_. A similar construction for a
ray at the lower side of the lens would bring it to the same point and
furthermore all parallel rays would focus at this point. This point is
thus known as the _principal focus of the lens_, and the distance
between this point and the lens is called the _focal length of the
lens_. Every lens has two principal foci, one on either side and at
equal distances from the lens.

[Illustration: FIGURE 86.]

[Illustration: FIGURE 87.]

If, instead of subjecting the lens to parallel rays, we use rays
emanating from a central point on the optical axis, they will come to a
focus at some different point on the other side of the lens, as
illustrated in Figure 86. If the light be placed at the principal focus
of the lens, the rays leaving the lens will be parallel. If the light be
brought nearer the lens, the rays leaving it will spread out as shown in
Figure 87; they will leave the lens in a direction such as to make upon
an observer from in front the impression that they are coming from the
point _C_ behind the lens. If the light be placed beyond the principal
focus of the lens, the rays will converge to a point some distance ahead
of it. This distance varies with the position of the light and the two
points (light on one side and focus on the other) are known as
_conjugate foci of the lens_.

[Illustration: FIGURE 88.]

If an object be projected through a lens as, for instance, the arrow at
the right of Figure 88, it will appear upon a screen placed on the
opposite side, but will be inverted. The reason for this is that the
rays of light striking the lens from the top of the arrow will be
refracted as shown, cross the focal point _F_, and meet those which come
from the same point and pass through the center of the lens in a
straight line; and thus the image of the arrow head will appear at the
bottom; in a similar manner the image of the tail of the arrow will
appear at the top. The flatter the lens, the farther away will be the
point at which the image is formed.

[Illustration: FIGURE 89.]

If such a lens be placed over an object, the light will come to the eye
as shown in Figure 89. The solid lines show the rays by which the eye
receives the light and the broken lines show the direction from which it
appears to come. Thus we see the object much enlarged. A lens used in
this manner is spoken of as a _reading glass_, and the greater the
curvature, the greater its magnifying power.

[Illustration: FIGURE 90.]

[Illustration: FIGURE 91.]

In Figure 90 is illustrated a double concave lens. Parallel rays
entering this lens are scattered by it, as shown. Therefore, if such a
lens be placed over an object, the light from the extremity of the
object will come to the eye as indicated in Figure 91, but will give the
appearance of coming along the lines _H_ and _I_. Thus the object will
be seen much reduced in size. Such lenses are sometimes used by artists
to bring landscapes to a reduced size so that they may be viewed as a
whole more easily.

[Illustration: FIGURE 92.]

The general forms of lenses are shown in Figure 92,

  (_a_) Is a plano-convex lens
  (_b_) Is a double convex lens
  (_c_) Is a convexo-concave lens, or convex meniscus
  (_d_) Is a plano-concave lens
  (_e_) Is a double concave lens
  (_f_) Is a concavo-convex lens, or concave meniscus.




CHAPTER XII.

OPTICAL INSTRUMENTS.


In most optical instruments, lenses are used for the purpose of
gathering a large number of rays of light and altering the apparent
direction of the rays so that an enlarged picture may be presented to
the eye. In order to accomplish this, it is necessary that the rays of
light be bent or refracted. This refraction, we have already seen, is
always accompanied by a dispersion which causes the light to be
dissolved into its original colors more or less. This has been
illustrated by means of prisms.

[Illustration: FIGURE 93.]

[Illustration: FIGURE 94.]

Wherever a single lens is used, the light around the edges of the
illuminated space is always more or less colored, varying with the
illuminant used. Such coloring is most noticeable along the edges of
projected pictures but it also exists, though to a less noticeable
degree, over the whole field, showing least in the center.

The light which is thus refracted and dispersed by one prism may be
gathered again by another, as shown in Figure 93, but the light rays
after passing through the second prism will be exactly parallel to the
ray striking the first. The light coming out of the second prism will
appear white but it will be impossible either to enlarge or diminish the
size of a picture in this way; hence lenses, corrected to give white
light in this manner, would be of no use.

Fortunately it has been found that, with different kinds of glass, the
ratio of refraction to dispersion is different; and by combining two
pieces of glass of different nature, it is possible to recombine the
colors without causing the emergent ray to become parallel to the
incident ray. Consider, for instance, Figure 94 in which we have drawn a
prism made up of two different kinds of glass. If the right half were of
a glass having the same index of refraction as the left, for the red
rays for instance, these would continue through both in a straight line.
If the dispersion were less in the right half, i.e., if the violet rays
were refracted less--sufficiently less to cause them to approach the
red--they would meet the latter at some point outside of the prism and
combine into white light again, thus eliminating the colors ordinarily
visible through single glass lenses.

[Illustration: FIGURE 95.]

Whenever it is necessary to project especially good pictures upon a
screen, lenses corrected in some such manner as outlined above are
always used and the lenses are often combined as shown in Figure 95. In
this figure, _R_ indicates the line, through the principal axis, at
which the red rays refracted by lens 1 alone would strike; and _V_, the
line where the violet rays would be projected. The addition of lens 2
brings the red and violet together again at _W_. A combination of two
such lenses, placed the proper distance apart and the surfaces properly
proportioned, may be made to combine any two of the colors of the
spectrum. Hence even with these corrected lenses there is always some
coloring on the screen although it is hardly noticeable.

[Illustration: FIGURE 96.]

[Illustration: FIGURE 97.]

[Illustration: FIGURE 98.]

Figures 96, 97, and 98 are drawings showing the manner in which
objective lenses are usually made up. The types at the right and left
are used for camera work, while the one shown in the center is used
mostly for moving picture and stereopticon projection. The end having
the separate lenses is turned towards the light. Those shown in contact
are glued together by the use of Canadian balsam.

[Illustration: FIGURE 99.]

The optical system of the ordinary telescope is shown in Figure 99.
Light from the distant object _A_ is gathered by the large lens _B_ and
an image is formed as indicated by the small arrow. This image acts as
the object to lens _C_ and is projected to lens _D_ where the rays of
light are strongly refracted, entering the eye by angles which cause an
enlarged view of the object at _E_, as indicated. There must of course
be some means by which the lenses may be adjusted to each other for
focusing.

[Illustration: FIGURE 100.]

The arrangement of the opera glass in Figure 100 is quite different from
the above because of the reduced size of the instrument and for the
reason that an erect picture is desired, whereas the telescope above
gives an inverted one. The principal difference between the two is in
the eyepiece. In the opera glass this is a concave lens while in the
other it is a convex lens. In this case it is necessary to have the eye
very close to the lens to catch the rays of light. The opera glass, as
well as the telescope, must be provided with means of varying the
distance between the lenses according to the distance of the object
viewed, for the purpose of focusing. In some telescopes and also opera
glasses, prisms are used for the purpose of obtaining erect images.
Figure 101 will show how the rays of light entering a prism are
reflected and the image reversed thereby.

[Illustration: FIGURE 101.]

Figure 102 is an explanation of the reflecting stereoscope. Let the
black circles represent the eyes of the observer and let _M_ represent
two mirrors placed as shown. If two pictures taken by a stereoscopic
camera are placed as indicated by the arrows at the right and the left,
they will appear superimposed in the position of the arrow in the rear.

[Illustration: FIGURE 102.]

[Illustration: FIGURE 103.]

The refracting stereoscope is the one mostly used and the plan of it is
shown in Figure 103. Pictures for use with stereoscopes are taken by
special cameras provided with two lenses placed about as far apart as
the human eyes and mounted together. Stereoscopic effects may, however,
be produced even without this precaution and it is possible to obtain
some queer results by combining certain pictures.

In the so-called "Camera Lucida", prisms of the type shown in Figures
104 and 105 are used. At the left is a combination lens and reflecting
prism which gives an erect image, and Figure 105 is a prism also
designed to give erect images. Such instruments are used for sketching.
They may be made to throw an image upon a small screen where its lines
may be traced out by the artist.

[Illustration: FIGURE 104.]

[Illustration: FIGURE 105.]

The most important optical instrument with which we have to deal is the
_projecting arc lamp_ and its optical system. The passage of light
through this system of lenses is altogether different from that passing
through a camera lens for instance. In a camera lens the picture is
formed upon the ground glass screen by the light reflected from the
object. From any single point of the object, rays of light strike all
parts of the lens, pass through it, and are recombined or focused at
some point behind it. Under these conditions, focus can be obtained only
at a certain distance behind the lens, this distance varying with the
distance of the object whose light is being received by the lens.

[Illustration: FIGURE 106.]

[Illustration: FIGURE 107.]

[Illustration: FIGURE 108.]

With our projecting lens we have no reflected light leaving the object
in all directions, but instead we have rays of light having definite
directions. This can be seen from Figure 106. The light used must come
from a point source, the smaller and the more intense it is the better.
This light is gathered by condensers, as shown at _C_, which are so
arranged as to focus the light in the center of the objective lens _D_.
In the moving-picture machine the light, before reaching the objective
lens, is passed through the film as indicated, the arrows representing
sections of film. The picture projected in this manner can be made to
appear upon a screen in front of the object lenses at any distance, but
the farther away it is, the larger it will be and the less bright the
illumination of it. A picture projected in this manner is always
inverted, and, in order to have it appear right side up, it must be
placed in position upside down. Figures 107 and 108 show arrangement of
lenses frequently used as condensers.




CHAPTER XIII.

OPTICAL ILLUSIONS.


[Illustration: FIGURE 109.]

[Illustration: FIGURE 110.]

[Illustration: FIGURE 111.]

[Illustration: FIGURE 112.]

The eye is easily deceived and is also very inaccurate in its judgment.
In Figures 109 to 111, all of the lines are of precisely the same
length, yet they appear to differ considerably. The reason for this
error in the estimation of objects is not known. It is especially
noticeable with such objects as a high hat; almost any one will estimate
the height of a silk hat as much greater than it really is. In general,
white objects also appear to be much larger than black objects. This can
be seen by the two inscribed squares one black and the other white in
Figure 112; both of these are of exactly the same size. Probably the
fact that more light reaches the eye from a light-colored object than
from a dark-colored one of equal proportions causes the impression of
greater size.

That the mind has the power of mixing contradictory or conflicting
impressions made upon it is proved by many facts. If a star, Figure 113,
be pinned to the center of a wheel and rapidly revolved, the center of
it will appear jet black; while the outer portions, made up of the
points and the white background, will appear grey which will gradually
fade to a lighter shade from the center black spot outward.

[Illustration: FIGURE 113.]

[Illustration: FIGURE 114.]

If a card be provided with a picture of a bird on one side and a ring on
the other, Figure 114, and this be rapidly spun on one corner, after the
manner of a top, the bird will appear to be in the center of the ring.
This effect is due to the persistence of vision; an image formed upon
the retina requires some time before it can be eliminated so that both
images appear together.

This persistence of vision and the power of suggestion are made use of
in a well-known act which consists of apparently throwing something, for
instance a guinea pig, into the air and causing it to vanish. To perform
this trick, the operator holds the pig in his hand and makes a few
motions suggestive of tossing it into the air. Then, with a final more
extreme motion on the downward swing of his hand, he drops the pig and
swiftly moves his empty hand upward. Due to the persistence of vision,
the audience actually still sees the pig and due to the suggestion of
tossing the eyes look upward, and thus the persistence of vision and the
power of suggestion create the illusion.

[Illustration: FIGURE 115.]

_Dissolving Views of Living Pictures._--This act requires a large glass
plate of good clear quality arranged upon the stage, as shown in Figure
115. _One_ of the poses is arranged as at the black circle behind the
glass and when illuminated is seen by the audience. The other is
arranged at one side. The lighting of both is connected to the same
dimmers in such a manner that when the light of one is increasing, that
of the other is diminishing. Thus the two figures are dissolved into one
another. The paths of the various rays of light to different parts of
the auditorium are shown in the drawing and it is possible to procure
perfect registry.

_Human Figure Floating or Performing in Air._--This act is arranged by
means of a large mirror placed upon the stage as shown in Figure 116.
In the _pit_ out of sight of the audience is a revolving table draped in
dull black and all of the pit is draped in the same manner. A figure
lying upon this table dressed in light clothes will appear erect in the
mirror as indicated by the arrow. If now the table is revolved, the
figure will be seen as turning over in the mirror. The figure upon the
table may perform a number of evolutions suggestive of floating, flying,
etc. The black cloth of the pit will reflect no light and only the
figure will be visible. If a glass plate is used instead of a mirror,
suitable scenery may be arranged back of it. The figure must of course
be brightly illuminated. By moving the table upon which the figure rests
across the pit, the figure will appear to move along.

[Illustration: FIGURE 116.]

_Head Suspended in Air._--This act is worked out in bright light. It
requires a mirror with a hole in the center of it large enough to admit
the head of a person, as shown in Figure 117. Above the mirror suitable
decorations are provided which cause one looking at the mirror to
imagine seeing the back wall of a room. The ceiling must of course be
kept out of view as much as possible. A person looking on sees the head
and the mirror gives him the impression of vacant space about it. The
head must be surrounded by a collar or drapery of some kind so that no
reflection of it will be visible.

[Illustration: FIGURE 117.]

[Illustration: FIGURE 118.]

_Magic Cabinet._--In this act a person enters the cabinet, Figure 118.
The outer doors are closed for an instant and then opened. The person
has disappeared! The disappearance is brought about by pulling the two
mirrors, arranged inside of the cabinet, into the position indicated by
the dotted lines. When the cabinet is open for inspection, these are
swung to the side and are invisible. The backs are of the same design as
the rest of the interior cabinet. When they are pulled together and hide
the person behind them, they reflect the side walls of the cabinet and
are not noticed. If the cabinet is well made and the act skilfully
performed, the outer doors may even be omitted.

_Head Resting on Table._--For this act a hole large enough to allow a
person's head to project through is cut in the top of a table, Figure
119. This table has only two legs and is fitted up with mirrors, as
indicated by shading. The reflection of the two legs in the mirror
causes the observer to see four and imagine that the space below the
table is vacant.

[Illustration: FIGURE 119.]

_Multiplication of Images._--If three large mirrors be arranged in the
form of an equilateral triangle, as indicated in Figure 120, a person
standing in the center will see his image reflected so many times that
he will receive the impression of being in the midst of a crowd. He will
see the reflections of reflections repeated until by absorption so much
light is lost that they finally become invisible.

[Illustration: FIGURE 120.]

[Illustration: FIGURE 121.]

_Trick Mirror._--Rather startling effects can be produced by a thinly
coated mirror _A_, Figure 121, behind which an electric light is
arranged so that it may be easily turned on. The space behind the
mirror being dark, no one suspects that it is anything but a common
looking glass. It is, however, quite transparent when the light behind
it is turned on. The person in front viewing himself may thus suddenly
be brought to see anything that is behind the mirror. If another set of
mirrors is arranged, as shown by shaded lines, the person in front of
the mirror may suddenly be made to see a head floating in the air in
front of him.

[Illustration: FIGURE 122.]

The manner in which one can see through a brick is illustrated in Figure
122. Four mirrors reflect the light around it. If the mirrors are
properly arranged, the person will imagine he is looking through the
brick.

[Illustration: FIGURE 123.]

The face of a person may be thrown upon a screen among clouds or other
pictures in the manner shown in Figure 123. The face occupies the space
marked by a half circle and two arc lamps with condensers are trained
upon it at very close range. The face itself acts as a reflector
projecting itself through the object lens in the center. This act is
very trying on the person whose face is reflected.

At a certain distance from a concave mirror, an object will be seen
inverted. If the mirror be brought closer, the image will gradually go
out of focus and, by bringing the mirror still closer, it will gradually
come into focus again but this time erect. Properly arranged such a
mirror can be made to give weird effects, for instance a skeleton,
rushing at the observer; and the image will appear to step out of the
mirror.




CHAPTER XIV.

THEATER BUILDINGS.


_General Requirements._--In most cities it is required that theaters be
built in locations which give free space for exits on two or more sides.
A common requirement is that two sides adjoin public streets or alleys
and one or both the other sides be provided with an open court allowing
space for fire escapes and connections to street or alley. The buildings
are also, as a rule, required to be of fireproof construction and
divided practically into two parts by strong fire walls; one of these
parts being the auditorium, that portion of the building used by the
public, and the other, the stage and its belongings.

The main fire hazard of course is on the stage and every possible
precaution should be taken, first to keep a fire from starting, and
second, to keep it, should it start, from communicating to the
auditorium. In order to protect the audience in case a fire starts doing
damage on the stage, a steel and asbestos fireproof curtain is generally
provided, large enough to cover the whole proscenium opening and
equipped with the necessary apparatus to lower it instantly in case of
necessity. In order to keep this curtain in first-class working order it
is lowered at the end of every act and in the case of continuous
vaudeville performances, which are not divided into acts, at least once
or twice during each performance.

This curtain must be strong enough to withstand the strain of air
pressure which would exist in case of a fire raging behind it. This
strain, when one considers the quantity of oil-painted scenery carried
by many of the large shows, is apt to be considerable, in case of fire.

In order further to protect the audience, a large vent flue is required
above the stage. The purpose of this vent is to carry off the smoke and
gases. The Chicago law requires the vent to be equal in area to
one-twentieth of the area of the stage and to extend fifteen feet above
the highest point of the roof.

The hazard to the audience is not so much that of the actual fire, as of
the rapid consumption of oxygen by the flames. This causes strangling.
In the case of the Iroquois Theater fire the death of many was due
almost entirely to this cause. The flames spreading rapidly consumed all
of the oxygen in the tightly closed theater; at the same time the smoke
and gases were spreading and hundreds of the audience inhaled this
heated and vitiated air with almost instantly fatal results.

As a further precaution against fire, it is now required in all large
cities, that all scenery be fireproofed to such an extent that it will
not carry fire. The usual test for this is, to hold a match to a part of
the cloth long enough to burn a hole through it. The fire must go out as
soon as the match is withdrawn.

The following are extracts from the Chicago ordinances governing
theaters of the larger class:

The building must be of fireproof construction. It must adjoin two
public highways one of which may be an alley.

There must be an open space on both sides of the audience room and in
front.

All balconies, galleries, main floor, and stage must connect with this
open space by means of doors or fire escapes.

There must be an opening into this free space also from both sides of
stage.

The floor level of the highest bank of seats on the main floor shall not
be more than three feet above the sidewalk level and the lowest bank of
seats not more than eight feet below this level.

All stairways must have a width of twenty inches for each one hundred
seats in the room, but no stairway shall be less than four feet in
width.

An iron stairway must lead from the stage to the fly floor, rigging
loft, and out onto the roof.

All openings leading from the stage to the outside must be vestibuled.

Above the stage there must be a ventilating flue which must extend
fifteen feet above the highest point of the roof and must equal in area
one-twentieth of the area of the stage.

No seat must be less than twenty inches wide and thirty-four inches from
back to back.

Every aisle must lead directly to an exit.

Between the stage and the auditorium there must be a wall of masonry and
all openings in this wall must be equipped with self-closing doors.

There must be a steel curtain provided to close the main stage opening
and the lowering of this curtain must be controlled at two different
places.

The use of wood is allowed only for the stage floor and this must be at
least two and three-fourths inches thick.

Automatic sprinklers must be provided in paint room, storeroom, property
room, scene storage room, carpenter shop, and dressing rooms.

A special fire alarm system must be provided on the stage.

All scenery must be treated with a suitable fireproofing compound.

All parts of the building used by the audience must be equipped with two
separate lighting systems, one of which is known as the "Emergency
Lighting" and must be kept lighted at all times while the audience is in
the building.

A suitable supply of axes, pike poles, and fire extinguishers must be
kept on hand.

There must be regular drills of employes on the use of these appliances
and in the operations of doors and vents.

All doors must swing to the outside.

[Illustration: FIGURE 124.]

_The Stage._--Figure 124 is a floor plan of a typical stage, showing the
orchestra pit _O_, foot lights _F_, steel curtain _C_, switchboard _S_,
stage pockets _Q_, proscenium side lights _P_, and general arrangement
of scenery. The foot lights are not always curved but it seems
advantageous to arrange them in this way. The rows of seats are of
necessity curved so as to face each patron squarely toward the stage. If
then the stage and foot lights are curved in the same way, the actors
will be able to come that much closer to the audience and can thus make
themselves more easily heard. The curving of the foot lights will have
the further advantage of illuminating the sides of an actor more than
would be the case if they were laid out in a straight line.

At each side of the steel curtain are the proscenium side lights. In
some cases these lights are arranged on the audience side of the
curtain, the object being to arrange them as far in front of the stage
as possible. The location of these lights is awkward and it is difficult
to get light from them in a useful direction.

In most theaters the switchboard is located on the right-hand side of
the stage, facing the audience. This is the side from which the stage
manager prefers to work and the operating electrician should be close
to him. In most well-arranged theaters, the switchboard is raised above
the stage level so that it may not obstruct the exits of the actors.
Where practicable, the board should be let into the proscenium wall so
as to allow the operator to stand as close to the proscenium opening as
possible. He should have a full view of the stage at all times, since
many of his cues are given by movements of the actors.

The stage pockets are laid out far enough from the center of the stage
to insure their being always behind the scenery. If they are brought in
too close, it is possible that, in a panorama setting, for instance,
they would be visible to the audience.

Doors leading from the stage to the outside are always vestibuled in
good houses. The vestibule prevents the wind from blowing the drapery
about unduly and also shields the actors from unpleasant drafts.

Dressing rooms are arranged wherever the conditions of the building
allow suitable space. Many of them are under the stage and others are
arranged on one or both sides of the stage, sometimes very high up.

A view of the stage looking from the rear is shown in Figure 125. This
figure shows the vent, the method of border light suspension, bridges
for support of arc lamps or other sources of illumination, the fly
floor, and the rigging loft, or "grid".

The rigging loft is generally constructed entirely of iron slats with
open spaces between them. This is necessary to insure ventilation in
case of fire.

[Illustration: FIGURE 125.]

In most theaters the height from stage to ceiling is about seventy feet
or more. The height must be great enough to allow the scenery to be
hoisted out of sight above the proscenium opening. There must also be
sufficient space above the rigging loft to allow men to walk about
straightening out ropes, etc. The total height should, therefore, be
nearly two and one-half times that of the proscenium opening.

[Illustration: FIGURE 126.]

As a rule no openings of any kind are arranged in any of the enclosing
walls of the stage, except those that are necessary as entrances. In
houses which are likely to originate shows, a paint bridge is generally
provided along the back wall.

[Illustration: FIGURE 127.]

_Moving-Picture Theater._--Figure 126 is a plan of a typical small
moving-picture theater. A spacious lobby is an important adjunct to any
such theater and this space is often as valuable as the seating space
inside. It affords shelter to those who cannot be seated at the moment.
It is especially convenient in localities where race suicide is not yet
very noticeable and where it forms a repository for many baby carriages.

A view showing the location of a picture-machine booth is given in
Figure 127. This is the arrangement most in use although there are
localities where it is required that the operating room be at the end of
the theater opposite the main entrance, the object being to avoid the
necessity of the audience passing under the fire to the outside in case
a fire should occur in the operating room. This plan has so many
disadvantages that it is never followed except where the law requires
it. If a properly arranged operating room is provided, there is no
necessity whatever for it.

[Illustration: FIGURE 128.]

The chief use of Figure 127 is to illustrate the importance of adjusting
the height at which the picture is thrown upon the curtain to the pitch
of the floor. The curved lines in the upper part of the picture
indicate the floor conditions which would give each seat the same
clearance over-the-one-in-front-of-it to a view at the bottom of the
picture. The upper line shows the bottom of the picture, level with
highest seat, the other with the same point somewhat below it.

[Illustration: FIGURE 129.]

Figures 128 and 129 show the plan and the elevation of a large operating
room. Every operating room should be of ample size to allow the operator
to walk on all sides of the machine. It should be strictly fireproof,
well-ventilated, and provided with a door giving easy exit to a floor
outside, level with the floor of the room. In altogether too many rooms
the only means of getting in or out is by a ladder which leads directly
into the room through a small trap door. Such an arrangement is
extremely dangerous in case of fire. The fumes of burning film are
poisonous if inhaled and spread very fast. If two men should happen to
be in such a room at the time the film is ignited, it is likely that
they would wedge each other into the narrow opening and both perish. The
door to the operating room should open outward and be self-closing. All
openings in the operating room must be fitted with fireproof shutters by
which they can be instantly closed in case of fire. There should also be
suitable ventilation to the outside to carry off all smoke from burning
film.

_The Screen._--The screen upon which the picture is projected should be
a clear white but of a dull finish. All glare should be avoided and
wherever there is a polished surface, this glare is always conspicuous.

There are a number of patent or special makes of screens on the market,
but it is not the purpose at this time to discuss any of them. It will
be sufficient to point out some of the simpler methods by which screens
may be made.

A simple plaster wall is often used and is very serviceable, but it must
not be polished and must be coated in some way to allow of washing off
occasionally.

Perhaps, all things considered, a muslin screen is the best of all. Such
a screen, if well stretched, gives a good flat surface entirely without
glare and has the further advantage that it can easily be taken down and
laundered as often as necessary. It is best to provide duplicate sets so
that there may be one for use while the other is in the wash.

The only real objection to a muslin screen is the loss of light which it
entails. It is not only a poor reflector of light, but it allows much of
the light to pass through it. With many such screens, the picture is as
well visible behind the screen as it is in front of it. This indicates a
great loss of light and it is to prevent this loss of light that the
other forms of screens have been devised. The most noticeable of these
is the _mirror screen_, which is a mirror of ground glass. It is very
economical in light but expensive in first cost and hard to keep clean.
It must be washed quite frequently and careless men are apt to use too
much water and get some of it on the back where it will soon begin to
loosen the silver coating and ruin the mirror.

The screen should be surrounded so that no other light except that
coming from the picture can reach the audience. To this end there should
be a black frame around it and this black frame must also be a dull
black and without glare. Velvet drapery is the very best thing that can
be provided and should be brought in so close from all sides that no
white rim around the picture will be visible, as such a white rim would
detract greatly from the illumination of the picture.

The decorations around the proscenium should preferably be of a dark
color and kept dull, especially in cities where it is necessary to
maintain emergency illumination. If the front is light in color, there
will be considerable reflection which will detract from the picture and
require a higher illumination of it.

Wherever there is an opportunity to do so, the screen should be set well
away from the nearest row of seats. This will not affect the view of
those in the rear, and it will very much improve the view of those in
front. It must of course be seen to that a clear view from both sides is
possible.




CHAPTER XV.

OPERATING ROOM EQUIPMENT.


_Construction and Ventilation._--The operating room should be built of
fireproof material, such as cement, brick, or tile. If it is of wood, it
may be made nearly fireproof by lining the inside with heavy sheet iron.
It will be well if asbestos is placed between the sheet iron and the
wood which it covers. All woodwork, whether window frames, doors, or
shelving should be thus lined. It may be taken as practically certain
that sooner or later one or more reels of film will be burned in the
room and every precaution should be taken to prevent the fire from
spreading. If this is done there will probably be no serious results,
for in the past there have been so many film fires which have been
successfully confined to the operating room that audiences have become
pretty well accustomed to them. If all operating rooms are strongly
built and of ample dimensions, with all openings as small as
practicable, it will help to inspire confidence in the audience and
there will be but little likelihood of a panic.

The chief danger is to the operator. It is true that he is generally to
blame, but it is not always possible even with the utmost care to
prevent a fire. Above all the operator should be given every possible
chance to get away in case of fire. An operator who feels that he can
get away at the last moment is far more likely to stay and fight a fire
than one who is penned up in a coop from which it is difficult to get
out rapidly. There are in existence, at the present time, operating
rooms where it is necessary to crawl on hands and knees some twelve or
fifteen feet to get from the machine to the exit, and where the exit is
by means of a trap door at the head of a ladder. Such operating rooms
are an abomination and no self respecting operator would work in one of
them.

The entrance and exit from the booth should be by means of a door at the
right hand or crank side of the machine. The door should be self closing
and should open into a space having a floor level with the floor of the
operating room. The door should preferably be kept closed but if the
provision illustrated in Figure 128 is made there can be no serious
objection to leaving the door open for ventilation. A thin bar of iron
is fastened to the door by a screw eye and is used to block the door
open. A person rushing out will naturally knock this away and the door
will close. The door could also be arranged to be held open by means of
the strings, to be described later, which hold the shutters in place.

The ventilation of the operating room should be by means of a fireproof
duct leading to the outer air. This flue should preferably extend above
the highest point of the roof and should be of good size. The smoke from
burning film is heavy and does not rise very fast but it is generated
with extreme rapidity. Film burns at a temperature of 284 degrees
Fahrenheit. If it is incased in a tank and fire communicates with it,
the whole body of the film will be raised to this temperature in a very
short time and will burn with the rapidity of an explosion. In the
magazines, which are in general use in the newer houses, the combustion
will be slower but nevertheless will take not more than about one
minute.

It has often been suggested that a motor be provided in the ventilating
flue, this motor to be kept running during the time the machine is in
operation. This would no doubt help clear away all smoke very quickly
but there is no assurance that it will be running at the time when it is
most needed. A simple way to help out the ventilation without the
expense and noise of a motor, would be to arrange the ventilating flue
directly above the arc lamps so that heat rising from these would cause
a draft. This can be further helped out by locating all resistances in
this flue. They will then be out of the way in a safe place and assist
in ventilating the room. The ventilating flue will need to be fitted
with a damper for use in cold weather. This damper should be arranged to
open by gravity and be held closed by a string connected to the shutter
strings so that the damper may open when the shutters close.

The floor of the operating room is a very important part of it. It
should, above all, be extremely rigid so that the machine may be
fastened solid and thus obviate any vibration which will cause movement
of the picture. All floors that are constructed of wood are somewhat
elastic and aid this vibration. Movement of the picture is especially
noticeable and annoying to those compelled to sit close to it. Floors
must furthermore be lined with fireproof material and the only material
used is sheet iron which, upon the floor, is very noisy and can hardly
be nailed sufficiently to eliminate the noise. A metal floor lining has
the further disadvantage of affording a good "ground," so that any one
standing upon it and touching a live part of the lamp is liable to
receive a severe electrical shock. It will also soon wear through, make
pockets to catch dirt, and allow a fire to communicate with the woodwork
beneath it. The best floor material is cement, but this, when wet, also
becomes a fairly good conductor and shocks may easily be obtained while
standing upon it. There is no reason, however, why it should ever be wet
in an operating room and it is therefore advisable to use cement.

The floor of the operating room should be kept clear as far as possible.
Careless operators often allow their take-up magazines to become
deranged and when they fail to work, allow the film to run on the floor.
A thousand feet of film scattered loosely about occupies considerable
space and is very likely to become badly tangled in case there is
anything on the floor for it to tangle with. The writer has witnessed
cases where it was necessary to take some of the film outside of the
booth in order to get it straightened out. Any operator who has been in
a similar predicament will appreciate the advantage of having every
possible obstacle to a speedy re-adjustment removed. Above all,
resistances and other sources of heat should be kept off the floor.

Every moving-picture machine requires at least two openings; one through
which the picture is projected, which need not be very large; another
through which the operator must view his picture, and this must be large
enough to allow him to sit in comfort while watching the picture. In
some operating rooms the opening through which the picture is projected
is closed by a metal funnel which covers the opening and extends back
enclosing the front of the projecting lens. This arrangement is very
simple in the case of machines in which the revolving shutter is behind
the lens. With many machines, however, this shutter works in front of
the lens and the funnel would have to be extended so as to enclose the
revolving shutter.

There is no reason why strong clear glass could not be used in the
operator's peep hole. This would of course interfere slightly with the
clearness of his vision for focusing, but every operator should be
provided with an opera glass for this purpose anyway, so that this
objection would be easily overcome.

The same considerations which apply to motion-picture machines also
applies to stereopticon lamps.

In the more pretentious houses a spot light is generally maintained.
This is for the purpose of illuminating vaudeville performers. The
opening, provided for this lamp, must be large enough to allow the light
to follow an actor about the stage, and glass cannot well be used in it.

The best color for the operating room is green but it must be a dull
dark green. Every vestige of light maintained in the room helps dull the
vision of the operator to the light upon the screen. There is,
furthermore, much light reflected from the film where the light strikes
it and it is unavoidable that the operator should be bothered by this at
times. All other light should be kept away from his eyes while the
picture is running.

All of the openings in the room must be equipped with fire shutters.
These are generally made of heavy iron and are arranged so that, if
necessary, they can be instantly released and made to close all the
openings, thus preventing the escape of fire and smoke into the
auditorium.

[Illustration: FIGURE 130.]

The best arrangement of such a shutter is illustrated in Figure 130 at
_A_. The shaded portion shows the opening; above is the iron slide,
preferably one-sixteenth inch metal held up by a string. At the top is a
step which will prevent the shutter from being pulled up out of the
guides, for many operators would do this if the step were not there.

It is best to mount the shutter on a large piece of heavy metal which
can then be bolted to the wall of the room. The hole in the wall can
then be cut to the right proportions. When the machine is in place the
shutter can be placed very accurately. In this way it will be possible
to arrange for the smallest possible opening through which the machine
can work.

Making up the shutter, as suggested above, is further of great advantage
in old operating rooms of flimsy wood construction. If the guides in
such places are fastened independently to the wall it is possible that
they will not always remain in line with each other. A slight giving of
the flimsy wall on one side may cause them to bind the shutter and
prevent its free descent. If the metal on which the shutter guides are
mounted is strong and heavy and the guides themselves substantially
made, there will be but little chance to get them askew.

In that part of the guides in which the shutter runs there must be no
screws, rivets, or anything else that could work loose and obstruct the
shutters. Good methods of making up the guides are shown in Figure 130,
at _B_ and _C_. They should be loose enough to work without lubrication.
At the bottom there should be a bumper to stop the fall at the proper
place. This bumper should be padded with some fireproof or slow-burning
material and should be narrow. If it is made wide the operator is apt to
use it as a shelf for tools or other things and this would prevent it
from closing properly in case of fire.

All of the shutters must be normally self-closing and must be held open
by some arrangement which can readily be worked by the operator; or, in
case he fails to close them, they should be arranged so that the flames
will cause them to close automatically. This is often arranged for by
simply attaching a light strong string to each shutter, running it
through a hook eye directly above and attaching it to a master string
which is led over every place at which there is thought to be a
likelihood of fire. The idea is, that the first spread of flames shall
strike this string and by burning it, release all of the shutters. In
order to enable the operator to drop the shutter perhaps before the
flames have spread at all, the string is carried to the door and
fastened so that he, on leaving the room, may quickly release it. It is
even better to carry it still farther so that one can release it from
the outside.

Some people have recommended that these strings be soaked in oil to make
them more inflammable and tarred rope has even been made mandatory for
similar purposes in ordinances. This might make the string more readily
inflammable, but it would at the same time make it last longer. A string
or rope so treated would act like a lamp wick and the better soaked or
tarred it might be, the longer would it last and thus fail to its
purpose.

[Illustration: FIGURE 131.]

The principle of the arrangement of strings is shown in Figure 131. The
string is fastened at the door in a manner allowing of easy and quick
removal. Instead of the strings, light chains may be used provided that
at a sufficient number of places fusible links are inserted. Such links
are made up of an alloy which melts at a low temperature and allows the
shutters to drop just as they would in the case of a string burning in
two.

In many cases the strings are run over grooved pulleys. This is,
however, inadvisable. Experience has proved that very often the string
has come out of the groove and is wedged in between the shaft of the
pulley and the side so that it will not allow the shutter to descend at
all. A substantial hook eye is much better than any pulley. The strain
being taken off the string when it separates, there is no friction and
it slides through the eyes very readily. The door and the vent flue may
all be brought under control of the same string so that all will operate
together, the vent of course opening while the others close.

Where there is but one door there is little need of connecting it with
the shutters, since, if it is provided with spring hinges it will
naturally close after the operator has passed through it. In some
operating rooms, however, there are two doors and both are often found
open. In case of fire in such a booth it is extremely likely that one of
the doors would be left open unless it is provided with some automatic
manner of closing.

At _D_, Figure 130 is shown a shutter, provided with strong spring
hinges, which closes over the opening from the outside. Such shutters
should not be used where there is room for the drop shutters. Where it
is necessary to have them, they should always be arranged on the outside
of the booth; if on the inside there is too much chance of obstacles
being in the way to prevent their closing.

It is very desirable that all of the shutters be closed after each show.
If this is done there will be strong assurance that they will be in
working order in case of need and this is about the only way in which
they can be kept in order. The string arrangement above described is,
however, not well suited to this purpose unless each shutter string be
detachable from the main string. There are also various other ways of
accomplishing this which will readily suggest themselves to the
operator.

In every operating room there should be ample provision for keeping
tools but it is advisable to have all tools, that are not kept in the
lockers, on hooks instead of shelves. It is best to limit the amount of
shelving to what is actually necessary. Shelving offers great temptation
for the accumulation of rubbish. There must be a shelf upon which to
locate the rewinding apparatus and upon which to place the machine while
looking it over or making repairs, also a shelf to hold a small vise,
etc. But this is all; any additional shelving will be inexpedient. A
locker for tools can easily be arranged under the rewind shelf. A good
safe place is an encouragement to an operator to keep a supply of good
tools and nothing is more essential to the proper management of a
machine than good tools.

If the operating room is large enough, it may also contain a locker for
clothes but this should be fireproof. Preferably, however, the clothes
locker should be outside of the room, since a film fire gives one but
little time to pick up clothes or anything else.

There should, further, be a metal pail or receptacle of some kind for
hot carbons taken from the lamps from time to time. A wise operator will
also provide himself with a small receptacle in which he can keep the
pieces of film which he finds it is necessary to cut out of films
received from the exchanges. The inspection of films in the exchange is
often very perfunctory and many lengths are left in that should be cut
out. Cutting them out and saving them for a time will often help an
operator to win out in disputes with his film exchange. There are
operating rooms which have film boxes built into the wall in fireproof
manner, some of them even surrounded by water. At the present time there
are a number of good portable boxes on the market so that this is
unnecessary.

_Wiring of Operating Room._--Every operating room must have at least two
circuits run into it; one for the arc lamp and another for one or more
incandescent lights. In the larger cities an emergency lighting system
is required for all parts of the house used by the audience. This system
is always entirely independent of the main service and is supposed to
furnish sufficient light for the audience in case the other lighting
system should fail. It will be well if one light connected to the
emergency system is arranged in the operating room. If this is done,
there must be special provisions that no cords, fan motors, or portables
be connected to it. A short circuit, or trouble of any kind on this
light might put the whole emergency system out of operation. If such a
light is installed, it should be at the ceiling and protected by a
strong wire guard fastened to the outlet box. All wires carried into the
operating room should be in conduit, with the exception of the asbestos
covered wires used with arc lamps and resistances.

In addition to the incandescent light above mentioned, there should be
several other lights so arranged that good light can be obtained in any
part of the room. The color of walls being dark there is but little
reflection and a single light will only illuminate a small space. One
light should be arranged above the rewind reel and there is often
another arranged under a piece of glass in a shelf above which films are
patched. Only the best reinforced cord should be used and all work that
is stationary should be in conduit. Several outlets should be arranged
for portable cords so that one may have a light to bring down to the
take-up reel or close to the floor if necessary. Every incandescent lamp
in the operating room should be protected by a substantial wire guard
fastened to the socket.

Fan motors should be provided with strong brackets located near the
ceiling and close to the outlet from which the motor is to be run. Fan
motors should never be allowed upon the floor.

Where rewinding is to be done by motor, it will be well to arrange a
separate circuit for this motor independent of all lights. The
commutator part of the motor should be enclosed so that there may be no
possibility of a spark igniting the film.

[Illustration: FIGURE 132.]

Figure 132 is a sketch illustrating the arrangement of wiring by which
two arc lamps may be connected to mains of sufficient carrying capacity
for one only. _F_ is the fuse box in which the arc lamp fuses are
enclosed and may be large enough to accommodate a set of fuses for the
incandescent-lamp circuit also. In many cases these are, however, taken
from the house circuit since the meter rate for arc lamps is usually
different from that for incandescent lamps. _S_ is the main arc-lamp
switch by which the whole installation in the room can be cut off. This
is necessary especially if so-called compensarcs, economy coils, or
transformers are used, for these always take some current if left alive
even if the arc lamp be not burning. At _C_ is the resistance or economy
coil which serves for both lamps and at _S_ is the throw-over switch.
This switch is provided to make it impossible to burn the two arcs at
the same time as this would overload the small capacity mains. The above
is a cheap arrangement and is installed only where a set of mains exists
which is too small to supply more than one arc at a time. It is not to
be advised for a new installation because it is of great advantage to be
able to burn both arcs at the same time. When changing rapidly from one
machine to the other it is of advantage to have the second one on for a
short time before the first is off, not only to save a little time, but
also for bringing electrode points to the proper shape. Wherever it is
possible to do so always provide mains of sufficient size to feed all of
the arcs and give each lamp its own fuse, switch, and resistance or
transformer.

[Illustration: FIGURE 133.]

Figure 133 shows a method by which it is possible to arrange for either
one of two arcs to be run from a rectifier _R_, indicated by the square
at the left, or transformer _T_, at the right. In case rectifiers, heavy
transformers that cannot well be raised to the ceiling, or motor
generators are to be installed, they should be located in a separate
enclosure which must be fireproof.

In general, all of the wiring in the operating rooms must be installed
in accordance with the rules given in the chapter on Theater and Stage
Wiring. These are the rules of The National Board of Fire Underwriters
and are familiarly known as the "National Electrical Code," or "N.E.C."




CHAPTER XVI.

CURRENT CONTROL FOR ARC LAMPS.


_Voltages Required._--The commercial distribution of electrical energy
is at voltages of 110 or 220, in most cases, and occasionally runs as
high as 550. The direct-current arc requires for its best operation a
voltage of from 45 to 50, while the alternating-current arc uses from 30
to 40. In order to secure satisfactory operation of arc lamps, it
becomes necessary to provide some means of reducing the voltage at the
arc to the proper amount.

_Resistance Control._--The simplest method and the one universally
applicable is that of inserting resistance in series with the arc. The
drop in voltage is equal to the current multiplied by the resistance;
hence, if we wish to decrease our voltage, say 65 volts, as would be the
case in connection with a 110-volt circuit and a 45-volt arc, using 25
amperes, we should need a resistance of 2.6 ohms. Twenty-five times 2.6
equals 65 volts lost, which leaves 45 volts to operate the arc with. In
connection with arc lamps, however, it is not only necessary to lower
the voltage but some provision must be made so that the current, when
the electrodes are brought together, will not become excessive. At the
time the arc is struck, i.e., at the time the electrodes are brought in
contact with each other, the current is limited only by the extra
resistance in the circuit, for the electrodes then form a short circuit.
In the above case, 110 volts and a rheostat with 2.6 ohms resistance we
should obtain, during the time the electrodes are together, a current
equal to 110 divided by 2.6 which equals about 43 amperes. If it were
not for this resistance the current would rise to several times this
value and blow out any fuse we might provide. It is not necessary that
this resistance be in any particular place; if we have a very long run
of small wire from the service to the arc, there may be sufficient
resistance in this so that very little extra resistance is required.
Somewhere, however, there must be some provision inserted in the circuit
to prevent the current from becoming too great at the time the
electrodes are brought together. In passing it may be noted at this
point that an arc can be started without any resistance in the circuit
by bridging the space between the electrodes with a small fuse wire
which will melt the instant the current is turned on and establish the
arc.

The resistance method is very wasteful of energy, as the following
tabulation will show; but with direct currents it is the only method
available, unless we are willing to provide a motor generator to give us
the proper voltage. For alternating currents, resistances are not much
used, except with traveling shows where the portability of the control
as well as its fitness for all possible conditions is an important
consideration.

TABLE IV.

SHOWING WASTE OF ENERGY WITH USE OF RESISTANCES FOR VARIOUS VOLTAGES.

  +-------+---------+------------+--------------+
  | Volts | Current | Watts Lost | Useful Watts |
  +-------+---------+------------+--------------+
  |  110  |   30    |    1950    |     1350     |
  |  220  |   30    |    5250    |     1350     |
  |  550  |   30    |   15150    |     1350     |
  +-------+---------+------------+--------------+

The tabulation in Table IV shows that the higher our voltage the greater
the loss of energy caused by the use of resistances. The figures apply,
as given, only to cases where only a single arc lamp is used. Where
several can be used in series the loss due to high voltage need not be
greater than with lower voltage.

[Illustration: FIGURE 134.]

In Figure 134 we have shown diagrammatically the usual representation of
a resistance. The more wire there is in circuit, the higher will be the
resistance and the greater the drop caused by a given current. If we
lengthen the arc, the current will be somewhat decreased and the drop in
voltage over the resistance will be less, thus allowing a rise in the
voltage at the terminals of the lamp. The energy lost in resistances
takes the form of heat and all resistances, used for the control of arc
lamps, give off much heat and must be located in safe places. The
heating also makes them objectionable in small operating rooms in
summer, but somewhat welcome in winter. The heat generated in a wire is
proportional to the square of the current; hence if we double the
current through a certain resistance we shall have four times the heat.

If several resistances are connected in series the total resistance will
be equal to the sum of the individual resistances, and the current will
be correspondingly decreased. If we wish to get more current than can be
obtained with the use of one resistance we may connect up two or more
in parallel. Two equal resistances connected in parallel will give
approximately double the current that can be obtained through one of
them.

[Illustration: FIGURE 135.]

_Reactance Control._--A reactance, shown diagrammatically in Figure 135,
may take the place of the resistance in alternating-current circuits and
is preferable because it wastes comparatively little energy. It lowers
the voltage over the arc but its operation depends upon a counter e.m.f.
which opposes the impressed e.m.f. of the circuit and must be subtracted
from the latter. The nature of reactances as well as of transformation,
etc., has been fully treated in another work of the authors', entitled,
"Alternating Current Theory, Practice and Diagrams" and would carry us
too far were it to be discussed in this work. Every reactive coil is
made up of copper wire wound upon an iron core and contains both
resistance and reactance. So far as the resistance in it is concerned,
this causes a waste of energy, but it is always very small. There is
also a waste of energy due to the hysteresis and eddy-current losses in
the iron, but this is also small.

The reactance is proportional to the square of the number of turns of
wire, if the iron core is fixed, and may be controlled either by
adjusting the position of an iron core in a helix, or by adjusting the
number of turns of wire around a fixed iron core. The light obtainable
through a reactance is not of the best quality and reactances are not
much used.

_Transformation Control._--Another method of lowering the voltage is by
means of the transformer. A diagram of an ordinary transformer winding
is given in Figure 136. The fine-wire winding is the primary winding or
coil, if the transformer is used to lower the voltage; and the other is
known as the secondary winding or coil. The energy in both coils of the
transformer, neglecting the iron and copper losses, is always exactly
equal. The ratio of voltage between the primary and secondary terminals
is in direct proportion to the number of turns of wire in each. If there
are half as many turns in the secondary winding as in the primary, the
voltage will be just one-half, but the current will be double. The
transformer is self-regulating, within the limits of its capacity, and
whatever energy is taken from the secondary, the primary will
automatically supply.

[Illustration: FIGURE 136.]

A transformer must be specially built for the voltage and frequency, at
which it is to be used; but many of them are provided with taps, such as
shown in Figure 136, by which small adjustments of voltage or current
can be made. A transformer must always be connected so that the switch,
when open, will disconnect the primary wires. If these remain closed
there will be a small current through the primary winding which will
mean a considerable waste of energy.

_Auto-transformer Control._--The auto-transformer is a special type of
transformer used to obtain reduced voltage and increased current. Its
principle may be gathered from Figure 137. There is an iron core and two
coils of wire as in ordinary transformers but the two coils are
connected in series, as shown in the figure. It will also be noted that
the arc is connected directly across one of the coils. The lower portion
of the winding or coil is traversed by the alternating current from the
mains at all times and this current also passes through the arc lamp
when the circuit through it is closed. The current passing through the
lower coil and the arc induces a current in the upper portion of the
winding and these two currents then pass in parallel through the lamp.

[Illustration: FIGURE 137.]

[Illustration: FIGURE 138.]

When the arc circuit is open, both coils are in series and act as choke
coils so that but very little current is used. The auto-transformer may
be designed to reduce the voltage to any desirable amount and the
current will be correspondingly increased, neglecting all losses.

[Illustration: FIGURE 139.]

If the reduced voltage were to be obtained from an ordinary transformer,
the secondary coil would be called upon to carry the full current used
by the lamp, while with this connection it carries much less. If the two
coils are equal, the voltage will be reduced one-half, the current will
be doubled, and only half of the current will pass through the secondary
coil. The nearer equal the primary and secondary voltages are, the
greater the saving in copper in the secondary coil. If it were intended
to transform from 110 to 100 volts, the capacity of the secondary
winding would need to be only one-eleventh of the total capacity. The
auto-transformer is a very useful device but on account of the fact that
the high voltage exists in all of its parts, it is not safe to use with
the high commercial voltages outside.

[Illustration: FIGURE 140.]

If the auto-transformer is connected as shown in Figure 138, it can be
used to raise the voltage; but in this case the current will be
decreased. These auto-transformers, as well as the ordinary transformer,
must always be connected to the source of energy by means of a switch so
that they may be disconnected when not in use; otherwise there will be a
small current in the primary circuit all the time which will show up
quite strong on the watt hour meter. Transformers and auto-transformers
are arranged to be portable. A general view is given in Figures 139 and
140; the former being the Edison and the latter the Fort Wayne.

_Motor-Generator Control._--The proper voltage for the operation of arc
lamps can be obtained by the use of motor-generators. A motor-generator
is a generator driven by a motor, the two armatures being placed upon
one shaft or belted together. The motor may be driven by a current of
any voltage desired. The diagram of such an outfit for direct current is
shown in Figure 141. This type of machine is used, as a rule, only where
the supply voltage is much higher than that used at the arc. Resistance
must be used at each arc lamp.

[Illustration: FIGURE 141.]

Figure 142 shows the connections of an alternating-current to a
direct-current motor-generator of the Fort Wayne Electric Company. The
switch _A_ is used to start it and is shown connected to a three-phase
line. Aside from the field winding there are three wires leading to the
generator. The wire _B_ carries a compound winding inside of the
generator which opposes the magnetization of the shunt winding. The wire
_C_ carries another compound winding which is arranged to strengthen the
shunt field. _D_ is a box containing two resistances, one for each arc
lamp shown.

If only one lamp is to burn, the switch _E_ is closed and the arc
started in the usual way. When ready to change to the other arc lamp,
switch _E_ must be opened, the switch on the second arc lamp closed, and
the arc struck. Then extinguish the first arc and close the switch _E_
again. If both lamps are to be used continually, switch _E_ must be left
open.

[Illustration: FIGURE 142.]

As long as current is used through wire _B_, there is no loss of energy
in any resistance and should the current in the arc rise, as when the
electrodes are brought together, the increased current in the series
winding, cut into this wire, would weaken the field and thus keep the
current down. When current is used through the wire _C_, the series
field winding strengthens the field and builds up the voltage
sufficiently so that the lamps may be operated through the resistances.
The field strength may be further regulated by the rheostat _R_.

Another connection of the Fort Wayne motor-generator is shown in Figure
143. In this case the lamps may be operated either from the compensarc
_C_ or the generator. By throwing either one of the switches connected
to the arc lamps up, the corresponding arc lamp is connected to the
compensarc. By throwing the switch down it is fed from the generator.
The lamp, by which the picture is being projected, should be fed from
the generator and when nearly ready to change, the other may be started
on the compensarc. This lamp will burn with a short arc and when it is
connected in parallel with the one on the generator, it will immediately
extinguish the latter. No attempt must be made to burn both arcs from
either the compensarc or the generator. This generator is also wound to
protect itself against an overload.

[Illustration: FIGURE 143.]

Where these connections are to be installed, it will be best to consult
the local inspection departments concerning the necessary fusing for the
compensarc and the generator. In some localities the possibility of
throwing both arcs on either compensarc or generator, where these have
capacity for but one arc at a time, will be considered very
objectionable.

[Illustration: FIGURE 144.]

Another combination of motor and generator sometimes used is shown in
Figure 144. By tracing out the circuits it will be seen that the
armatures of both are in series and that the electrodes, when they come
together, form a shunt about _B_. With the electrodes separated, if
current is turned on, it must pass through both armatures in series.
Thus the counter e.m.f. of both armatures opposes that of the line and
they operate at a certain speed. Each motor has a natural tendency to
send current in opposition to that impressed upon it by the line. If
then the electrodes are brought together, they at once form a short
circuit around the armature of _B_. The current in _B_ reverses and it
then begins to act as a generator and sends current through the arc
lamp. The current which passes through the armature of _A_ also passes
through the arc lamp. _A_ is then a motor and operates _B_ as a
generator.

The voltage at the arc is less than the line voltage by as much as the
counter e.m.f. of motor _A_ amounts to, neglecting the drop in voltage
due to resistance. No resistance is needed if the winding is properly
arranged and there is not the less in heat which goes with the use of
resistances. This arrangement can be used with direct-current circuits
only. It is not suitable where the supply voltage is very much higher
than the voltage used at the arc. A field rheostat is provided to adjust
the field strength of _B_. _A_ is equipped with the ordinary
motor-starting rheostat only.

_Rotary Converter Control._--This is a machine used only where the
supply is alternating current. The voltage delivered to the converter
must be the same as that desired at the direct-current terminals. This
machine has an armature essentially similar to that of a direct-current
dynamo. Alternating current is supplied to it at one set of terminals
and direct current is taken from the others. This armature acts as motor
and generator at the same time. Whatever voltage regulating is necessary
with this machine must be done on the alternating-current side. Changing
the field strength does not materially affect the voltage so that no
means for regulating the field strength is provided.

The polarity of the direct-current terminals depends upon the position
the armature happens to be in when the alternating current is applied to
it and is very apt to come in wrong when the machine is started. It is
therefore necessary to have a polarity indicating voltmeter in the
circuit and to watch it when starting the machine. If the polarity is
wrong, the switch must be opened and in a moment thrown in again; and
if still wrong, this process must be repeated until the polarity comes
right. Each arc lamp fed from a converter must be equipped with
resistance.

[Illustration: FIGURE 145.]

The Martin rotary converter is especially designed for motion-picture
work and may be provided with the proper connections for either
single-phase, two-phase, or three-phase work. There is a stator ring
which entirely surrounds the armature. This ring is made up of laminated
disks with squirrel-cage bars and slots alternating. The squirrel-cage
bars are joined at the end to a copper bar and it is by the aid of this
squirrel-cage that the motor may be started and brought into step. The
squirrel-cage also prevents "hunting" which is one of the common
troubles experienced with synchronous motors or converters. Into the
slots are wound special compensating coils to balance the armature
reaction and keep the neutral point in constant position from no load to
full load. This prevents sparking at the brushes. On the outside of this
damper ring or squirrel-cage winding is the regular shunt-field winding
used with direct-current motors or generators.

Figure 145 is a diagram showing the connections of the Martin Rotary
Converter as installed by the Northwestern Electric Company of Chicago.
This switchboard is equipped to operate two moving-picture arcs, two
dissolving stereopticon lamps, and one spot light. Each lamp is provided
with a throw-over switch so that current may be used, either from the
alternating-current mains direct or from the direct-current side of the
converter.

Figure 146 is another panel board for moving-picture work made up by the
same company. In this case resistances are provided for use when the arc
lamps are operated from the converter. In case it is desired to run from
the alternating-current mains, transformers or compensarcs are used. The
emergency feature of these panel boards is highly to be recommended. It
must be borne in mind that one may suddenly be forced to deal with an
operator who has never seen a converter and knows nothing of its
operation; and there is also always the possibility of some trouble with
the machine.

[Illustration: FIGURE 146.]

A Martin rotary converter to be operated from a single-phase line is
shown in Figure 147. This machine is started through the commutator
side. In order to start this machine it is necessary first to close the
main switch. Next throw the switch 2 to the right and leave it there for
about five seconds. It may then be thrown over to the running position
at the left and allowed to remain in this position. If the polarity is
not correct, the switch must be opened again for an instant and closed
again; and this process must be repeated until the polarity comes in
right. To stop the converter, first open the main switch and then the
throw-over switch. The manner in which the above machines are preferably
set up is shown in Figure 148.

[Illustration: FIGURE 147.]

[Illustration: FIGURE 148.]

_The Mercury-Arc Rectifier Control._--The mercury-arc rectifier has
three essential parts: the rectifier tube, the main reactance, and the
panel. The rectifier tube, Figure 149, is a glass vessel from which the
air has been exhausted and in which there are two graphite electrodes,
_A_ and _A'_, and one mercury electrode _B_. From the two upper
electrodes current can pass in the direction of the mercury only. They
are always positive and the term _anode_ is usually applied to them. _B_
is always negative and the term _cathode_ applies to it. Each anode is
connected to a separate side of an alternating-current circuit and is
thus subject alternately to positive or negative potential.

When current has once been started, the tube is filled with ionized
mercury vapor through which the electricity can flow, from whichever of
the two anodes is positive, toward the cathode _B_. Under no conditions,
however, can electricity flow from the mercury in the tube toward the
anodes. The action of the tube is started by tilting it sufficiently, so
that the mercury in the bottom of it connects the starting anode _C_ to
_A_. This starts the current and when the tube is returned to its
upright position, the mercury bridge from _C_ to _A_ is interrupted; but
the current then continues from one or the other of the anodes.

Should the current be interrupted, even for an instant, the tube would
cease to work until it had been tilted again. In order to provide that
the current, which is alternating and comes to zero twice in every
cycle, may never cease in the tube, it is necessary to provide some
reactance. Such a reactance causes the current to lag behind the e.m.f.
and in consequence lap over the time when it would otherwise fall to
zero. While the current from the rectifier is always in the same
direction, positive from _B_ to the lamp, it is also a pulsating current
changing in value to some extent.

In Figure 149 a complete diagram of the connections of the General
Electric Company Mercury Arc Rectifier for moving-picture arcs is given.
This type of rectifier is entirely automatic and is much used. The
front and back connections are shown in Figure 150. The following
instructions are taken from a publication of the General Electric
Company:

The leads marked _A C_ should be connected to the lower side of a
double-pole switch located near the moving-picture machine. The upper
studs of the switch should be connected to the _A C_ source of supply.

[Illustration: FIGURE 149.]

The leads marked + and - should be connected, respectively, to the
positive (upper) and negative (lower) electrodes of the moving-picture
lamp.

If the _A C_ supply voltage is 110 volts; then connect the flexible lead
marked _Z_ to stud marked _12_; and flexible lead marked _Y_ to stud
marked _6_.

If the _A C_ supply voltage is 220 volts; then connect lead _Z_ to stud
_7_, and lead _Y_ to stud _1_.

NOTE:--Do not disturb the other connections that are made on studs _1_,
_6_, _7_, and _12_, but only place leads _Y_ and _Z_ as directed.

[Illustration: FIGURE 150.]

The tube holder should be reversed so that the clip and support will be
turned away from the panel instead of towards the panel, as it is when
shipped.

Remove the tube from its box, being very careful not to handle it
roughly and not to strain the seals in any way whatever. Care must also
be taken to prevent the mercury from suddenly flowing into any of the
arms; otherwise the resultant pounding might damage them.

Examine the tube for vacuum by noting the sound the mercury makes when
allowed to roll gently about in the large chamber. If it makes a clear,
metallic click, the vacuum is good; but, if the sound is dull and the
mercury sluggish in moving, the vacuum is either partially or wholly
destroyed. If the vacuum is poor, the life of the tube may be short or
it may not start at all. To insure careful handling and safe delivery,
Mercury-Arc Rectifier tubes are always shipped by express in the special
box as they come from the factory.

Place the tube in the holder by inserting the small part of the tube
just above the anode arms in the upper clip; then gently lower it until
it rests firmly on the lower support. Connect the tube and beaded leads
according to the above diagram.

Adjustment of current (number of amperes) at the arc is obtained by
connecting leads marked _X_ to studs marked _11_, _9_, _7_, _5_, _3_, or
_1_ of the regulating reactance. Stud _1_ gives the maximum and stud
_11_ the minimum number of amperes. In starting up the first time it is
best to start with lead _X_ on stud _11_ and move toward the maximum
position by steps until the desired current is obtained, as indicated by
the ammeter. For this adjustment it is advisable to connect an ammeter
in series with the arc in the moving-picture machine.

With the above instructions carried out, all that is necessary to start
is to close the switch in the _A C_ line; then bring the electrodes of
the arc together. The automatic shaking device should then rock the
tube until the arc in the tube starts; as soon as the arc in the tube
starts separate the electrodes.

The best and whitest light can be obtained when a 5/8-inch cored-carbon
electrode is used above and a 1/2-inch solid-carbon electrode below,
care being taken not to get solid carbons too hard. The average current
in the arc should not exceed 30 amperes and it will be found that
excellent pictures can be obtained with 25 amperes or even less and the
cost of energy, carbons, and condensers will be less.




CHAPTER XVII.

MANAGEMENT OF GENERATORS AND MOTORS.


_Operation of Generators._--The generator should be located in a clean
dry place. If it is belt driven, the belt should be run horizontal if
possible and so that the slack side will be on top. This increases the
arc of contact with the pulleys and allows the belt to run with less
tightening. The frame should be provided with a slide for the purpose of
adjusting and tightening. The proportion between the largest and the
smallest pulleys used close together should not be greater than about 6
to 1.

To start a dynamo it is best first to disconnect it from the
switchboard. Start it running and adjust the voltage by the field
rheostat. When the voltage has arisen to its proper value and everything
is running smoothly, the main switch may be closed. If there is much of
a load, it will probably be found that the voltage has fallen off a
little and it will be necessary to re-adjust it. Next, look carefully
after the brushes and set them at the points where there will be the
least sparking. A good modern generator should not spark at all. All of
the bearings must be carefully looked after and watched for heating. If
they are not properly oiled or in good condition, they may heat
considerably. The armature should run with considerable end play as this
helps to distribute the oil over the bearings and even up the wear on
the commutator surface.

Small generators sometimes lose their residual magnetism and it is then
impossible to start them generating. In such a case the fields may be
connected to a live-lighting circuit; or a small exciting current may be
obtained from a battery. One should know which is the positive pole of
the field and apply the battery or line current accordingly. A test for
polarity may be made by placing the ends of wires of opposite polarity
in a vessel with water and bringing them within an inch or so of each
other. Under these conditions bubbles will be given off at the negative
pole. The polarity of the generator will vary with the polarity of the
field, the direction of rotation, and the connection of the brushes. By
reversing any one of these we may reverse the polarity of the current
delivered. Shunt dynamos cannot well be operated in parallel; where it
is desired to operate several dynamos together, compound-wound machines
are used.

_Operation of Motors._--The speed of a direct-current motor is always
such that the counter e.m.f. of the motor becomes nearly equal to the
impressed e.m.f. of the line. In order to speed up a motor it is
necessary to weaken the fields; and conversely, to slow it down we must
strengthen the fields. The above methods are necessary if the motor is
to run at a nearly fixed speed with a variable load. The speed can also
be controlled by a variable resistance placed in the armature circuit.
This method, however, does not result in a steady speed with a variable
load. It has very little effect if the load is light, and very much if
it is heavy.

In order to start a motor it is necessary to have some resistance in the
armature circuit. In very small motors the armatures are usually wound
with sufficient resistance so that no external resistance is required.
The larger motors are, however, equipped with starting boxes which limit
the current through the armature until it has attained sufficient speed
so that its counter e.m.f. will keep the current in check.

These starting boxes are usually wound with fine wire and cannot stand
the starting current very long. The handle must be moved over steadily
and slowly and not allowed to remain on an intermediate position unless
it is known that the box is meant to be used as a speed controller as
well as a starting box. The direction of rotation of a direct-current
motor can be reversed by reversing either the field or the armature
current. If both are reversed, it will continue to run in the same
direction.

_Alternating-Current Motors._--Synchronous alternating-current motors
are not used in theater work but the rotary converter is frequently used
and may be considered as such. This machine must run at a certain speed
which depends upon the frequency of the current supplied and the number
of poles on the machine.

Rotary converters are of different types and may be started either from
the direct-current side or from the alternating-current side. Some of
them are provided with connections so that the alternating current may
be applied on the direct-current side of the armature. For starting and
operating these converters, the instruction of the maker should be
consulted.

For motor-generators where alternating current is used, the induction
motor is generally employed. It may be either single-phase, two-phase,
or three-phase. All of these are essentially constant-speed motors.
Simple repulsion motors and single-phase induction motors that start as
repulsion motors are reversed by shifting the brushes.

Alternating-current series motors are reversed in the same manner as
direct-current motors by reversing either field or armature. If both are
reversed, the motor will continue to run in the same direction.

A three-phase induction motor will reverse its direction of rotation if
any two of the line wires connected to the primary winding be reversed.
If all three wires are changed in order, the direction of rotation will
remain unchanged. Synchronous motors when started as induction motors
are also governed in this way.

These motors of the larger size are generally started through an
auto-transformer. For the smaller ones, it is customary to provide a
throw-over switch, one side only of the switch being fused. The motor at
starting takes very strong currents which would blow the running fuse.
To reverse a two-phase induction motor the two wires of one phase must
be reversed.


GENERAL HINTS.

Be sure that all belts are sufficiently tight.

See that all bearings are well oiled.

Let all shafts have sufficient end play.

Use no oil cans of iron around dynamos or motors.

Keep files and other iron or steel away.

See that all connections are good and tight.

Allow no metal dust or gritty substances to accumulate at the insulation
of exposed parts.

See that the brushes fit properly and do not cut or scratch.

Use no emery paper on commutators.

Lubricate the commutators very sparingly and wipe off as much of the
lubricant as you can.

Keep everything about the machines clean and allow no oil drippings to
accumulate.

Place the starting box for a motor so you can see the motor start from
the box.

Always place a switch which will disconnect all of the wires close to
the motor.

If possible arrange motors so they may start without load.

Allow no motor or generator to be placed in the operating room of a
moving-picture theater.




CHAPTER XVIII.

THEATER WIRING.


The wireman should not fail to consult local rules or inspection
departments as to whether any rules conflict with those given below. He
must be warned to consult local authorities or rules, too, because
safety rules are liable to change.

The purpose of this chapter is to furnish a ready reference work
concerning questions of electrical construction in theaters which come
up daily in all progressive play houses. To this end the subjects have
been arranged in alphabetical order and the practical considerations, as
well as extracts from the National Electrical Code governing
construction, have been given together. The aim has been to enable the
workman to find all information concerning construction work grouped
together, so as to obviate the necessity of looking through various
parts of the book for the information sought. This order of things will
probably avoid the troubles now often caused by overlooking certain
points that should be considered.

_Aisle Light._--Figure 151 is an illustration of an aisle light. Such
lights are often placed along steps and aisles. The light illuminates
only the floor. Aisle lights should be arranged on a separate circuit
and controlled by a switch at the door.

_Alternating Current._--All wires of any circuit or mains or sub-mains
of any system must be run in the same conduit. Failure to do this will
cause an unnecessary drop in voltage and heating of the conduit.

_Arc Lamps._--For treatment and construction of portable arc lamps, see
the chapter on "Portable Stage Equipment".

Permanently located arc lamps are used about theaters mostly for
out-door lighting. Very often, two or more lamps are arranged in front
of the house. Such lamps are mostly of the flaming arc lamp type and are
hung up high.

[Illustration: FIGURE 151.]

In some of the cheaper theaters a pair of arc lamps is used on the
stage, but they do not give satisfaction. The light is not even and
steady enough and cannot be properly "dimmed". Where arc lamps are to be
arranged for stage illumination they must be suspended amid the scenery
and enclosed with wire guards. In some cities the use of arc lamps
suspended above the stage is prohibited.

In the auditorium, arc lamps are sometimes installed, but this practice
can not be recommended and, with the present high efficiency
incandescent lamps, there is but little excuse for it. The only
advantage in using arc lamps lies in the first cost of wiring, and this
is more than balanced by the difficulties of trimming lamps located in
such places. Wherever arc lamps are used it is essential that they be
hung high and those that do not naturally throw the light downward must
be equipped with suitable reflectors. The question of drop in voltage
need not be considered with arc lamps unless runs are very long.


_National Electrical Code Rules for Arc Lamps._


_Arc Lamps in General._

Must be provided with reliable stops to prevent carbons from falling out
in case the clamps become loose.

All exposed parts must be carefully insulated from the circuit.

Must, for constant-current systems, be provided with an approved hand
switch, and an automatic switch that will shunt the current around the
carbons, should they fail to feed properly.

The hand switch to be approved, if placed anywhere except on the lamps
itself, must comply with requirements for switches on hanger-boards.

Terminals must be designed to secure a thoroughly good and permanent
contact with the supply wires, which contact must not become loosened by
motion of the lamp during trimming.

Spark arresters must so close the upper orifice of the globe that it
will be impossible for any sparks, thrown off by the carbons, to escape.


_Series Arc Lamps._

Must be carefully isolated from inflammable material.

Must be provided at all times with a glass globe surrounding the arc,
and securely fastened upon a closed base. Broken or cracked globes must
not be used.

Must be provided with a wire netting (having a mesh not exceeding one
and one-fourth inches) around the globe and an approved spark arrester,
when readily inflammable material is in the vicinity of the lamps, to
prevent escape of sparks of carbon or melted copper.

Outside arc lamps must be suspended at least eight feet above sidewalks.
Inside arc lamps must be placed out of reach or suitably protected.

Arc lamps, when used in places where they are exposed to flyings of
easily inflammable material, must have the electrodes enclosed
completely in a tight globe in such manner as to avoid the necessity for
spark arresters.

"Enclosed arc" lamps, having tight inner globes may be used, and the
requirements of b and c above would, of course, not apply to them.

Where hanger-boards are not used, lamps must be hung from insulating
supports other than their conductors.

Lamps when arranged to be raised and lowered either for carboning or
other purposes, shall be connected up with stranded conductors from the
last point of support to the lamp, when such conductor is larger than
No. 14 B. & S. gauge.


_Arc Lamps on Constant-Potential Circuits._

Must have a cut-out for each lamp or each series of lamps.

The branch conductors must have a carrying capacity about fifty per cent
in excess of the normal current required by the lamp.

Must only be furnished with such resistances or regulators as are
enclosed in non-combustible material, such resistances being treated as
sources of heat. Incandescent lamps must not be used for this purpose.


_Arc Lamps Used as a Part of a Moving-Picture Machine._

Must be constructed, so far as practicable, similar to arc lamps of
theaters, and wiring to same must not be of less capacity than No. 6 B.
& S. gauge. See "Portable Stage Equipment".


_Stage and Gallery Pockets._

Must be of approved type, insulated from ground and controlled from
switchboard, each receptacle to be of not less than 35 ampere rating for
arc lamps nor 15 amperes for incandescent lamps, and each receptacle to
be wired to its full capacity. Arc pockets to be wired with wire not
smaller than No. 6 B. & S. gauge and incandescent pockets with not less
than No. 12 B. & S. gauge.

Plugs for arcs and incandescent pockets must not be interchangeable.

_Armored Cable._--All wires in the stage part of theaters must be
enclosed in conduit or armored cable. Armored cable is thus the only
flexible conductor allowed for permanent work. This cable is very
convenient where wires must be "fished", or run around beams or other
obstacles making many bends necessary. It should, however, be used only
where the rigid conduit cannot well be installed and it is advisable to
use the latter, even where the additional expense is considerable. Wires
run in rigid conduit can be taken out and replaced by new ones at any
time, while this is not the case with armored cable. Should a wire
incased in armored cable develop a serious fault, the old cable would
have to be abandoned and a new circuit run, which in many cases would
mean the tearing up of parts of the building.

[Illustration: FIGURE 152.]

Where armored cable is to be used great care should be exercised to see
that bends are not made too short, and that each length of cable is
tested for grounds, short circuits, and open circuits. Special attention
must be given to the wires at the place where the armor has been cut.
Careless workmen can do great damage at this point. The manner of
cutting the armor is shown in Figure 152. Each strand of the armor is
partly cut with a saw and may then be broken off, taking care that no
sharp edge is left in position to pierce the wire.

Installation rules are given below. Before installing any armored cable,
be sure that it is of approved make and guaranteed to pass inspection.


_National Electrical Code Rule for Armored Cables._

Must be continuous from outlet to outlet or to junction boxes or
cabinets, and the armor of the cable must properly enter and be secured
to all fittings, and the entire system must be mechanically secured in
position.

In case of service connections and main runs, this involves running such
armored cable continuously into a main cut-out cabinet or gutter
surrounding the panel board, as the case may be.

Must be equipped at every outlet with an approved outlet box or plate,
as required in conduit work.

Outlet plates must not be used where it is practicable to install outlet
boxes.

For concealed work in walls and ceilings composed of plaster on wooden
joist or stud construction, outlet boxes or plates and also cut-out
cabinets must be so installed that the front edge will not be more than
one-fourth inch back of the finished surface of the plaster, and if this
surface is broken or incomplete it shall be repaired so that it will not
show any gaps or open spaces around the edges of the outlet box or plate
or of the cut-out cabinet. On wooden walls or ceilings, outlet boxes or
plates and cut-out cabinets must be so installed that the front edge
will either be flush with the finished surface or project therefrom.
This will not apply to concealed work in walls or ceilings composed of
concrete, tile or other non-combustible material.

In buildings already constructed where the conditions are such that
neither outlet box nor plate can be installed, these appliances may be
omitted by special permission, provided the armored cable is firmly and
rigidly secured in place.

Must have the metal armor of cables permanently and effectually grounded
to water piping, gas piping, or other suitable grounds, provided that
when connections are made to gas piping, they must be on the street side
of the meter. If the armored cable system consists of several separate
sections, the sections must be bonded to each other, and the system
grounded, or each section may be separately grounded, as required
above.

The armor of cables and gas pipes must be securely fastened in outlet
boxes, junction boxes, and cabinets, so as to secure good electrical
connection.

If armor of cables and metal of couplings, outlet boxes, junction boxes,
cabinets, or fittings having protective coating of non-conducting
material, such as enamel, are used, such coating must be thoroughly
removed from threads of both couplings and the armor of cables, and from
surfaces of the boxes, cabinets, and fittings where the armor of cables
or ground clamp is secured in order to obtain the requisite good
connection. Grounded pipes must be cleaned of rust, scale, etc., at
place of attachment of ground clamp. Connections to grounded pipes and
to armor of cables must be exposed to view or readily accessible, and
must be made by means of approved ground clamps, to which the ground
wires must be soldered.

Ground wires must be of copper, at least No. 10 B. & S. gauge (where
largest wire contained in cable is not greater than No. 0 B. & S.
gauge), and need not be greater than No. 4 B. & S. gauge (where largest
wire contained in cable is greater than No. 0 B. & S. gauge). They must
be protected from mechanical injury. The ground for the armored cable
system is not to be considered as a ground for a secondary system.

When installed in so-called fireproof buildings in course of
construction, or afterwards if exposed to moisture, or where it is
exposed to the weather, or in damp places, such as breweries, stables,
etc., the cable must have a lead covering placed between the outer braid
of the conductors and the steel armor. The lead covering is not to be
required when the cable is run against brick walls, or laid in ordinary
plaster walls unless same are continuously damp.

Where entering junction boxes, and at all other outlets, etc., must be
provided with approved terminal fittings which will protect the
insulation of the conductors from abrasion, unless such junction or
outlet boxes are specially designed and approved for use with the cable.

Junction boxes must always be installed in such a manner as to be
accessible.

For alternating-current systems must have the two or more conductors of
the circuit enclosed in one metal armor.

All bends must be so made that the armor of the cable will not be
injured. The radius of the curve of the inner edge of any bend not to be
less than 1-1/2 inches.

_Asbestos._--As all wiring in theaters is required to be run in conduit,
and metal cabinets are compulsory in this connection, there is but
little opportunity to use asbestos. Wherever the use of asbestos is
advisable it must conform to the general requirements as given for
wooden cutout cabinets, viz.: "for lining wooden cabinets, one-eighth
inch rigid asbestos board may be used when firmly secured in place by
screws or tacks".

_Attachment Plugs._--Attachment plugs should be used to connect all
portable apparatus. All plugs should be of approved type and constructed
so as to pull out in case strain is put on them. On the stage, pin-plug
connectors should be used in place of attachment plugs, as none of the
latter are sufficiently rugged to withstand the hard usage.


_National Electrical Code Rules for Attachment Plugs._

Link fuse attachment plugs of the types now on the market are considered
unsafe, as under entirely possible conditions an arc may be produced
when the fuses blow, damaging the plug and perhaps causing fire.
Attachment plugs are not approved for more than six hundred and sixty
watts or two hundred and fifty volts.

_Auditorium._--Two separate systems of lighting are required: See
emergency or exit lighting. Metal moulding, as well as armored cable or
conduit, is permissible in wiring the auditorium part of the theater.

_Auto-Starters._--Auto-starters perform the same service with
alternating-current motors that resistances do with direct-current
motors. They are used with motors from two or three horse power upward,
and not generally with the smaller motors.

The following are extracts from the "National Electrical Code"
concerning their use:

In all wet, dusty, or linty places, auto-starters, unless equipped with
tight casings enclosing all current-carrying parts, must be enclosed in
dust-tight fireproof cabinets. Where there is any liability of short
circuits, caused by accidental contacts, across their exposed live parts
a railing must be erected around them.

The switch on the auto-starter must provide an off position, a running
position, and at least one starting position. It must be so arranged
that it will be held in off and running position, but not in starting
position or without the proper running overload-protection devices in
the circuit.

For currents above 30 amperes, lugs, into which the connecting wires may
be soldered, or approved solderless connectors must be used. Clamps or
lugs will not be required when leads are provided as a part of the
device.

The following rules are drawn for rheostats but may also apply to
auto-starters:

Where the circuit-breaking device on the motor-starting rheostat
disconnects all of the wires of the circuit, the switch called for in
this section (to disconnect all apparatus) may be omitted.

Overload-release devices on motor-starting rheostats will not be
considered to take the place of the cutout required to protect the motor
and the rheostat.

_Balconies._--The illumination of balconies is a difficult matter. The
ceilings under galleries above are always low, and to obtain even
illumination requires the use of many small candle-power lamps. These
should be set well back so as not to be visible too much to the
audience.

Stage-pocket capacity for one or more arc lamps should always be
provided. Where there are galleries above, the arc lamps used for stage
illumination are generally placed there, but it often happens that a
moving-picture machine must be installed and it is very disadvantageous
if this must be located in the galleries. Balconies require the same
exit and emergency light service as is required in the auditorium.

_Batteries._--See Portable Stage Equipment.

_Bells._--Systems of call bells are generally arranged between the box
office or the manager's office and the stage switch board; also from the
stage switch board to the fly floor by which signals for raising and
lowering the curtain may be given; also to the orchestra leader. In some
cities all of this wiring is required to be in conduit. These signaling
circuits should be carefully installed, for they are as important as
any part of the wiring. Only the very best bells and push buttons should
be used, and it is advisable to avoid the use of the ordinary
annunciator wire so often seen in connection with bell work. Numerous
diagrams and much information concerning bell wiring is given in "Modern
Wiring Diagrams and Descriptions", which should be consulted in case
some complicated annunciator system is to be installed. Figure 153 is a
diagram of a simple call-and-return-call system.

[Illustration: FIGURE 153.]

[Illustration: FIGURE 154.]

_Borders._--The number of borders used in theaters varies from one to
six, the latter number being sufficient for almost any stage. They are
generally made of a length about equal to the proscenium opening. The
borders placed in the rear are of less importance than those in front
and consequently are made shorter and are not provided with so many
lights. Each border should have at least three circuits, one for each
color; each circuit being taken through a separate dimmer so that any
color may be used alone and dimmed as desired.

Figures 154 and 155 show types of borders, and the method of wiring is
shown in Figure 156.

[Illustration: FIGURE 155.]

[Illustration: FIGURE 156.]

Large borders are very heavy and are usually suspended by wire rope and
provided with counterweights to make handling easy. The wire rope should
be kept well protected by oiling; the moisture given off by fireproofed
scenery is liable to rust them very fast, and as the ropes are made up
of very fine strands the rust soon cuts entirely through them. The
suspending ropes and also cables supplying lights must be long enough to
admit of bringing the border within five or six feet of the stage floor
for lamp renewals and cleaning. The cleaning is very important as the
dust which accumulates may absorb a large part of the light.


_The National Electrical Code Rules Governing Border and Proscenium
Sidelight Construction._

Must be constructed of steel of a thickness not less than No. 20 U. S.
sheet metal gauge; treated to prevent oxidation; be suitably stayed and
supported; and so designed that flanges of reflectors will protect
lamps.

Must be so wired that no set of lamps requiring more than 1,320 watts
nor more than 26 receptacles shall be dependent upon one cut-out.

Must be wired in approved conduit or armored cable, each lamp receptacle
to be enclosed within an approved outlet box, or the lamp receptacles
may be mounted in an iron or steel box, metal to be of a thickness not
less than No. 20 U. S. sheet metal gauge treated to prevent oxidation,
so constructed as to enclose all wires. Wires to be soldered to lugs of
receptacles.

Must be provided with suitable guards to prevent scenery or other
combustible material coming in contact with lamps.

Cable for borders must be of approved type and suitably supported;
conduit construction must be used from switchboard to point where cables
must be flexible to permit of the raising and lowering of border.

For the wiring of the border proper, wire with approved slow-burning
insulation must be used.

Borders must be suitably suspended, and if a wire rope is used same must
be insulated by at least one strain insulator inserted at the border.

_Box Office._--The box office is often a very stuffy place in summer and
very cold in winter. There should be an outlet for a fan motor, and also
one for a heater. Very many box offices depend upon electric heating
for comfort in winter. Plenty of light should be provided and the light
should be at the ceiling, out of the way, and directly over the ticket
window.

_Brackets._--Any brackets used in theaters should be at least seven feet
above the floor. Brackets used for emergency or exit lights must be
fitted with keyless sockets. See "Fixtures".

_Branch Circuits._--The term, "branch circuit", as here used refers to
the wires leaving the last cut-outs and connecting directly to the lamps
or other devices. Mains are often run from the switchboard to various
parts of the building to feed cut-out centers, and the branch circuits
are then run from these centers. This is often the cheapest method, but
it is by no means the best. In a well designed theater all branch
circuits lead out from the vicinity of the switchboard so that, in case
of any trouble with fuses, they may be replaced without interfering with
any part of the audience and in the shortest possible time. This method
of wiring involves some long runs of branch circuit wires and Table V,
which shows the drop in voltage, was prepared for the convenience of
wiremen. If possible the circuits should be so arranged that the lamps
fed are closely together so all may receive nearly the same voltage. The
drop should not much exceed two per cent.

Table showing drop in voltage due to 6 amperes, with sizes of wire and
distances given:

TABLE V.

DROP IN VOLTAGE DUE TO 6 AMPERES WITH SIZE OF WIRE AND DISTANCE GIVEN.

  --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  Distance|  50 |  75 | 100 | 125 | 150 | 175 | 200 | 225 | 250 | 300
  in feet |     |     |     |     |     |     |     |     |     |
  --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  Size of |     |     |     |     |     |     |     |     |     |
  Wire    | 1.58| 2.37| 3.16| 3.95| 4.74| 5.53| 6.32| 7.11| 7.90| 9.48
  No. 14  |     |     |     |     |     |     |     |     |     |
  --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  Size of |     |     |     |     |     |     |     |     |     |
  Wire    | 0.99| 1.48| 1.98| 2.47| 2.97| 3.45| 3.96| 4.45| 4.95| 5.94
  No. 12  |     |     |     |     |     |     |     |     |     |
  --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  Size of |     |     |     |     |     |     |     |     |     |
  Wire    | 0.63| 0.94| 1.25| 1.56| 1.87| 2.19| 2.50| 2.81| 3.12| 3.75
  No. 10  |     |     |     |     |     |     |     |     |     |
  --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----
  Size of |     |     |     |     |     |     |     |     |     |
  Wire    | 0.39| 0.59| 0.78| 0.97| 1.17| 1.36| 1.56| 1.75| 1.95| 2.34
  No. 8   |     |     |     |     |     |     |     |     |     |
  --------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----


_National Electrical Code Rules Governing Size of Wire for Branch
Circuits and Fuses for Same._

No wire smaller than No. 14 B. & S. gauge must be used.

Each branch circuit must be protected by fuses, which must be so placed
that no set of small motors, small heating devices, or incandescent
lamps, whether grouped on one fixture or on several fixtures or pendants
(nor more than 16 sockets or receptacles) requiring more than 660 watts,
will be dependent upon one cut-out.

By special permission, in cases where wiring equal in size and
insulation to No. 14 B. & S. gauge approved rubber-covered wire is
carried direct into keyless sockets or receptacles, and where the
location of sockets and receptacles is such as to render unlikely the
attachment of flexible cords thereto, the circuits may be so arranged
that not more than 1,320 watts (or thirty-two sockets or receptacles)
will be dependent upon the final cut-out.

Except for signs and outline lighting, sockets and receptacles will be
considered as requiring not less than 40 watts each.

All branches or taps, from any three-wire system, which are directly
connected to lamp sockets or other translating devices, must be run as
two-wire circuits if the fuses are omitted in the neutral, or if the
difference of potential between the two outside wires is over 250 volts,
and both wires of such branch or tap circuits must be protected by
proper fuses.

The above shall also apply to motors, except that small motors may be
grouped under the protection of a single set of fuses, provided the
rated capacity of the fuses does not exceed 10 amperes.

When 1,320 watts are dependent upon one fusible cut-out, as is allowed
in theater wiring, outline lighting and large chandeliers, the fuses may
be in accordance with the following:

  125 volts or less      20 amperes
  125 to 250 volts       10 amperes

_Bus-Bars._--Bus-bars may be made of bare metal. They must, however, be
protected against accidental contact. The metal should be ample so that
it will not heat more than 50 degrees Fahrenheit above the surrounding
air.

It is customary to calculate the metal needed, on the basis of 1,000
amperes per square-inch cross section. On this basis bars of the sizes
given below would have the carrying capacities in amperes given in the
body of the table.

TABLE VI.

CURRENT-CARRYING CAPACITY OF BUS-BARS.

  -----------+----+----+----+----+----+----+----+----+-----+-----+-----
  Thickness  |2/32|3/32|4/32|5/32|6/32|7/32|8/32|9/32|10/32|12/32|16/32
  -----+-----+----+----+----+----+----+----+----+----+-----+-----+-----
       | 1/2 |  30|  45|  60|  75|  90| 105| 120| 135|  150|  165|  180
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
       | 5/8 |  37|  57|  75|  94| 108| 132| 150| 168|  188|  206|  225
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
       | 6/8 |  45|  68|  90| 112| 135| 158| 180| 202|  225|  248|  270
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
  Width| 7/8 |  53|  79| 105| 130| 158| 184| 210| 236|  263|  289|  315
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
   in  | 1.0 |  60|  90| 120| 150| 180| 210| 240| 270|  300|  330|  360
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
   In- |1-1/4|  75| 112| 150| 188| 225| 263| 300| 338|  375|  412|  450
  ches +-----+----+----+----+----+----+----+----+----+-----+-----+-----
       |1-1/2|  90| 135| 180| 225| 270| 315| 360| 405|  450|  495|  540
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
       |1-3/4| 105| 157| 210| 263| 315| 367| 420| 473|  525|  577|  630
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
       | 2.00| 120| 180| 240| 300| 360| 420| 480| 540|  600|  660|  720
       +-----+----+----+----+----+----+----+----+----+-----+-----+-----
       | 3.00| 180| 270| 360| 450| 540| 630| 720| 810|  900|  990| 1080
  -----+-----+----+----+----+----+----+----+----+----+-----+-----+-----

_Cabinets._--Cabinets are required to enclose all fuses. It is advised
that they be arranged so that it will not be necessary to open the fuse
compartment to operate switches.

Care should be exercised in locating cut-out cabinets. They should be in
a dry place, easily accessible to authorized persons, and not accessible
to the general public or miscellaneous employes. If too accessible they
are likely to be made receptacles for all sorts of rubbish. Wooden or
composition cabinets must not be used in theaters. Use only approved
cabinets.

In some cities special rules, governing the construction of cabinets for
theaters, exist and these should be looked up.

_Cables._--For construction rules, see "Stage Cables" in the chapter on
"Portable Stage Equipment". Cables are required to connect border
lights. The cable is usually run from an outlet on the grid floor above
the center of the border. It must be long enough to allow the border to
be lowered for the cleaning and renewal of lamps. The slack cable must
be taken up when the border is raised and some provision must be made to
support the cable without injury to the insulation. Wire ropes or cables
are generally used to support the borders, but the lower end, which is
handled by the stage crew, is of ordinary manilla rope. Wire cables must
be insulated from the border by strain insulators.

_Canopies._--Very many theaters are provided with canopies which extend
over the street. Sometimes these are fitted out with glass signs behind
which incandescent lamps are installed. Often, also, a row of lights is
arranged around the bottom of the canopy ceiling. The number of lights
to be used depends upon the design of the canopy, but the effect is best
if a large number of small candle-power lamps are used. These lamps
should be of low intrinsic brilliancy. If possible, the lamps and
sockets should be arranged to be out of the weather. In addition to the
small outline lamps, other lights are often placed under canopy to
provide bright illumination. Each circuit may carry 1,320 watts.


_National Electrical Code Rule._

Where insulating joints are required, fixture canopies of metal must be
thoroughly and permanently insulated from metal walls or ceilings, or
from plaster walls or ceilings on metal lathing, and from outlet boxes.

Canopy insulators must be securely fastened in place, so as to separate
the canopies thoroughly and permanently from the surfaces and outlet
boxes from which they are designed to be insulated.

_Carriage Call._--See "Program Board".

_Carrying Capacity of Wires._--Table VII is designed to help the wiremen
select the proper size of wire to use in supplying a certain number of
lights. The first column at the left gives the B. & S. gauge number. The
second column gives the number of amperes allowed on each wire according
to the National Electrical Code. The third column gives the distance in
feet which will cause a loss of 2 volts with the current given; thus a
current of 70 amperes in a No. 4 B. & S. gauge wire will lose 2 volts
over a distance of 56 feet. The proper size of wire to be used can
easily be determined from the table. The loss will always be in
proportion to the product of distance and current. The balance of the
table is devoted to showing the number of watts and lamps of various
sizes the wires will be allowed to supply.

_Ceiling Fans._--Must be hung from insulated hooks, or else there must
be an insulator interposed between the motor and its support. Not more
than 660 watts may be on one circuit.

_Chandeliers._--Large and elaborate chandeliers are sometimes used in
the center of the auditorium ceiling for purposes of illumination. Such
chandeliers should be suspended in a manner which will admit of readily
raising or lowering them for lamping up or repairs. For rules governing
wiring, see "Fixtures".

TABLE VII.

TABLE SHOWING CARRYING CAPACITY OF WIRES; DISTANCE TO WHICH FULL LOAD
MAY BE CARRIED AT 2 VOLTS DROP AND NUMBER OF LIGHTS EQUIVALENT TO FULL
CURRENT GIVEN.

  =======+=======+========+============+=============================
  B. & S.|Rubber |Distance|   Total    |  Total Number of Lamps of
  Gage   | Insu- |in Feet |Capacity in |   Different Voltages and
         |lation |Causing |   Watts    |Wattages that may be supplied
         |Amperes|a Loss  |            +---------+---------+---------
         |       | of 2   |            | 25-Watt | 40-Watt | 60-Watt
         |       | Volts  +-----+------+----+----+----+----+----+----
         |       |        | 110 |  220 | 110| 220| 110| 220| 110| 220
         |       |        | V.  |  V.  | V. | V. | V. | V. | V. | V.
  -------+-------+--------+-----+------+----+----+----+----+----+----
       14|   15  |    26  | 1650|  3300|  66| 132|  41|  82|  27|  54
       12|   20  |    30  | 2200|  4400|  88| 176|  55| 110|  36|  73
       10|   25  |    38  | 2750|  5500| 110| 220|  68| 137|  46|  91
        8|   35  |    43  | 3850|  7700| 154| 308|  96| 192|  64| 128
        6|   50  |    50  | 5500| 11000| 220| 440| 137| 275|  91| 183
        5|   55  |    56  | 6050| 12100| 242| 484| 151| 302| 100| 201
        4|   70  |    56  | 7700| 15400| 308| 616| 192| 385| 128| 256
        3|   80  |    61  | 8800| 17600| 352| 704| 220| 440| 146| 292
        2|   90  |    68  | 9900| 19800| 396| 792| 247| 494| 165| 330
        1|  100  |    67  |11000| 22000| 440| 880| 275| 550| 183| 366
        0|  125  |    78  |13750| 27500| 550|1100| 343| 686| 229| 458
       00|  150  |    82  |16500| 33000| 660|1320| 412| 824| 275| 550
      000|  175  |    89  |19250| 38500| 770|1540| 481| 962| 320| 640
     0000|  225  |    87  |24750| 49500| 990|1980| 618|1236| 412| 824
   200000|  200  |    92  |22000| 44000| 880|1760| 550|1100| 367| 734
   300000|  275  |   101  |30250| 60500|1210|2420| 756|1512| 504|1008
   400000|  325  |   114  |35750| 71500|1430|2860| 893|1786| 596|1192
   500000|  400  |   117  |44000| 88000|1760|3520|1100|2200| 733|1455
   600000|  450  |   123  |49500| 99000|1980|3960|1237|2474| 825|1650
   700000|  500  |   130  |55000|110000|2200|4400|1375|2750| 916|1832
   800000|  550  |   135  |60500|121000|2420|4840|1512|3024|1008|2016
   900000|  600  |   139  |66000|132000|2640|5280|1650|3300|1100|2200
  1000000|  650  |   143  |71500|143000|2860|5720|1787|3574|1191|2382
  1100000|  690  |   147  |75900|151800|3036|6072|1897|3794|1264|2528
  1200000|  730  |   151  |80300|160600|3212|6424|2007|4014|1338|2676
  1300000|  770  |   155  |84700|169400|3388|6776|2117|4234|1412|2824
  1400000|  810  |   161  |89100|178200|3564|7128|2227|4454|1485|2970
  1500000|  850  |   164  |93500|187000|3740|7480|2337|4674|1558|3116
  -------+-------+--------+-----+------+----+----+----+----+----+----

  =======+=======+========+============+=============================
         |Rubber |Distance|   Total    |  Total Number of Lamps of
         | Insu- |in Feet |Capacity in |   Different Voltages and
  B. & S.|lation |Causing |   Watts    |Wattages that may be supplied
  Gage   |Amperes|a Loss  |            +--------+--------+-----------
         |       | of 2   |            |100-Watt|150-Watt|250-Watt
         |       | Volts  +-----+------+---+----+---+----+---+-------
         |       |        | 110 |  220 |110| 220|110| 220|110|220
         |       |        | V.  |  V.  |V. | V. |V. | V. |V. |V.
  -------+-------+--------+-----+------+---+----+---+----+---+-------
       14|   15  |    26  | 1650|  3300| 16|  33| 11|  22|  6| 13
       12|   20  |    30  | 2200|  4400| 22|  44| 14|  29|  8| 17
       10|   25  |    38  | 2750|  5500| 27|  55| 18|  36| 11| 22
        8|   35  |    43  | 3850|  7700| 38|  77| 25|  61| 15| 30
        6|   50  |    50  | 5500| 11000| 55| 110| 36|  73| 22| 44
        5|   55  |    56  | 6050| 12100| 60| 121| 40|  80| 24| 48
        4|   70  |    56  | 7700| 15400| 77| 154| 49|  99| 30| 61
        3|   80  |    61  | 8800| 17600| 88| 176| 58| 117| 35| 70
        2|   90  |    68  | 9900| 19800| 99| 198| 66| 132| 39| 78
        1|  100  |    67  |11000| 22000|110| 220| 73| 146| 44| 88
        0|  125  |    78  |13750| 27500|137| 274| 91| 182| 55|110
       00|  150  |    82  |16500| 33000|165| 330|110| 220| 66|132
      000|  175  |    89  |19250| 38500|192| 384|128| 256| 77|154
     0000|  225  |    87  |24750| 49500|247| 404|165| 330| 99|198
   200000|  200  |    92  |22000| 44000|220| 440|146| 292| 88|176
   300000|  275  |   101  |30250| 60500|302| 604|201| 402|121|242
   400000|  325  |   114  |35750| 71500|357| 714|238| 476|143|286
   500000|  400  |   117  |44000| 88000|440| 880|293| 586|176|352
   600000|  450  |   123  |49500| 99000|495| 990|330| 660|198|396
   700000|  500  |   130  |55000|110000|550|1100|366| 732|220|440
   800000|  550  |   135  |60500|121000|605|1210|403| 806|242|484
   900000|  600  |   139  |66000|132000|660|1320|440| 880|264|528
  1000000|  650  |   143  |71500|143000|715|1430|476| 952|286|572
  1100000|  690  |   147  |75900|151800|759|1518|506|1012|303|606
  1200000|  730  |   151  |80300|160600|803|1606|535|1070|321|642
  1300000|  770  |   155  |84700|169400|847|1694|564|1128|338|676
  1400000|  810  |   161  |89100|178200|891|1782|594|1188|356|712
  1500000|  850  |   164  |93500|187000|935|1870|623|1246|374|748
  -------+-------+--------+-----+------+---+----+---+----+---+-------


_National Electrical Code Rules for Fusing Chandelier Circuits._

When 1,320 watts are dependent upon one cut-out, as is allowed in
theater wiring, outline lighting, and large chandeliers, the fuse may be
in accordance with the following:

  125 volts or less      20 amperes
  125 to 250 volts       10 amperes

_Circuit Breakers._--Circuit breakers are not used much about theaters.
They are used mostly in connection with motors and on switchboards of
isolated plants. They are more sensitive and quicker to act than fuses
and, in case of a blow-out, can be more easily replaced. Where circuit
breakers are used it is good practice to set them for a higher amperage
than the fuses so that, in case of a heavy short circuit, the breaker
may act while the fuse will take care of an overload which comes on more
gradually.


_National Electrical Code Rides for Circuit Breakers._

With motors an automatic circuit breaker, disconnecting all wires of the
circuit, may serve as both switch and cut-out.

Where the circuit-breaking device on the motor-starting rheostat
disconnects all wires of the circuit, the switch may be omitted.

Circuit breakers must not be set more than thirty per cent above the
allowable carrying capacity of the wires, unless a fusible cut-out is
also installed on the circuit.

When installed without other automatic overload protective devices,
automatic overload circuit breakers must have the poles and trip coils
so arranged as to afford complete protection against overloads and short
circuits and, if also used in place of the switch, must be so arranged
that no one pole can be opened manually without disconnecting all the
wires.

_Concealed Work._--All concealed work must be in conduit or armored
cable. Wherever possible, rigid conduit should be used. Armored cable
should be used only where the whole cable may at any time be withdrawn,
or where it is impracticable to use rigid conduit.

_Conduit Work._--This is now considered the standard method of
construction. It is, however, somewhat over-rated and, especially in wet
places, has led to much disappointment. It is generally recognized that
wires, placed in conduit, will cause more trouble than those placed in
any other standard manner. The trouble is, however, confined to the
interior of the conduit and, if the conduit system be kept in good
order, and fuses at their proper limit, there is no chance of fire from
the wire confined within the conduit.

There are three points which, if carefully observed, will avoid most, if
not all, of the conduit troubles:

Avoid as far as possible locating conduit in wet or damp places.

Run all conduits so that they will drain.

Give particular attention to joints in outlet or junction boxes; tape
them heavily; and arrange them so they are not pressed against metal
more than is necessary; use large boxes.

Wires used in conduits must have approved rubber insulation and double
braid.


_National Electrical Code Rules for Wires and Interior Conduits._


_Wires for Conduit Work._

Must have an approved rubber insulated covering (Type Letter R. D.), and
must within the conduit tubing be without splices or taps.

Must not be drawn in until all mechanical work on the building has been,
as far as possible, completed.

Conductors in vertical conduit risers must be supported within the
conduit system in accordance with the following:

  No. 14 to 0 every 100 feet
  No. 00 to 0000 every 80 feet
  No. 0000 to 350,000 C. M. every 60 feet
  350,000 C. M. to 500,000 C. M. every 50 feet
  500,000 C. M. to 750,000 C. M. every 40 feet
  750,000 C. M. every 35 feet

The following methods of supporting cables are recommended:

(1) A turn of 90 degrees in the conduit system will constitute a
satisfactory support.

(2) Junction boxes may be inserted in the conduit system at the required
intervals, in which insulating supports of approved type must be
installed and secured in a satisfactory manner so as to withstand the
weight of the conductors attached thereto, the boxes to be provided with
proper covers.

(3) Cables may be supported in approved junction boxes on two or more
insulating supports so placed that the conductors will be deflected at
an angle of not less than 90 degrees, and carried a distance of not less
than twice the diameter of the cable from its vertical position. Cables
so suspended may be additionally secured to these insulators by tie
wires. Other methods may be used if specially approved.

Must, for alternating-current systems, have two or more wires of a
circuit drawn in the same conduit. It is suggested that this be done for
direct-current systems, also, so that they may be changed to alternating
systems at any time, induction troubles preventing such a change if the
wires are in separate conduits.

Except in the case of stage pocket and border circuits the same conduit
must not contain more than four two-wire, or three three-wire circuits
of the same system, except by special permission, and must never contain
circuits of different systems.


_Interior Conduits._

No conduit smaller than one-half inch electrical-trade size shall be
used.

Must be continuous from outlet to outlet or to junction boxes or
cabinets, and the conduit must properly enter, and be secured to all
fittings and the entire system must be mechanically secured in position.
In case of service connections and main runs, this involves running each
conduit continuously into a main cut-out cabinet or gutter surrounding
the panel board, as the case may be. Departure from this rule may be
authorized in case of underground services by special permission.

Must be first installed as a complete conduit system, without the
conductors.

Must be equipped at every outlet with an approved outlet box or plate.
At exposed ends of conduit (but not at fixture outlets), where wires
pass from the conduit system without splice, joint, or tap, an approved
fitting, having separately bushed holes for each conductor, must be
used. Departure from this rule may be authorized by special permission.
Outlet plates must not be used where it is practicable to install outlet
boxes.

For concealed work in walls and ceilings composed of plaster on wooden
joist or stud construction, outlet boxes or plates, and also cut-out
cabinets must be so installed that the front edge will not be more than
one-fourth inch back of the finished surface of the plaster; and if this
surface is broken or incomplete it shall be repaired so that it will not
show any gaps or open spaces around the edges of the outlet box or plate
or of the cut-out cabinet. On wooden walls or ceilings, outlet boxes or
plates and cut-out cabinets must be so installed that the front edge
will either be flush with the finished surface or project therefrom.
This will not apply to concealed work in walls or ceilings composed of
concrete, tile or other non-combustible material.

In buildings already constructed where the conditions are such that
neither outlet box or plate can be installed, these appliances may be
omitted, providing the conduit ends are bushed and secured.

It is suggested that outlet boxes and fittings, having conductive
coatings, be used in order to secure better electrical contact at all
points through the conduit system.

Metal conduits, where they enter junction boxes and at all other
outlets, etc., must be provided with approved bushings or fastening
plates fitted so as to protect wire from abrasion, except when such
protection is obtained by the use of approved nipples, properly fitted
in boxes or devices.

Must have the metal of the conduit permanently and effectually grounded
to water piping, gas piping, or other suitable grounds, provided that
when connections are made to gas piping, they must be on the street side
of the meter. If the conduit system consists of several separate
sections, the sections must be bonded to each other, and the system
grounded; or each section may be separately grounded, as required above.
Where short sections of conduit (or pipe of equivalent strength) are
used for the protection of exposed wiring on side walls, and such
conduit or pipe and wiring is installed as required by the Code Rules,
the conduit or pipe need not be grounded.

Conduits and gas pipes must be securely fastened in outlet boxes,
junction boxes, and cabinets, so as to secure good electrical
connections.

If conduits, couplings, outlet boxes, junction boxes, cabinets or
fittings, having protective coating of non-conducting material such as
enamel, are used, such coating must be thoroughly removed from threads
of both couplings and conduit, and such surfaces of boxes, cabinets, and
fittings, where the conduit or group clamp is secured, in order to
obtain the requisite good connection. Grounded pipes must be cleaned of
rust, scale, etc., at place of attachment of ground clamp.

Connections to grounded pipes and to conduits must be exposed to view or
readily accessible, and must be made by means of approved ground clamps
to which the ground wires must be soldered.

Ground wires must be of copper, at least No. 10 B. & S. gauge (where
largest wire contained in conduit is not greater than No. 0 B. & S.
gauge), and need not be greater than No. 4 B. & S. gauge (where largest
wire contained in conduit is greater than No. 0 B. & S. gauge). They
shall be protected from mechanical injury. The ground on the conduit
system is not to be considered as a ground for a secondary system.

Junction boxes must always be installed in such a manner as to be
accessible.

All elbows or hands must be so made that the conduit will not be
injured. The radius of the curve of the inner edge of any elbow must not
be less than three and one-half inches, and must have not more than the
equivalent of four quarter bends from outlet to outlet, the bends at the
outlets not being counted.

_Contacts._--Must be mounted on non-combustible non-absorption insulated
bases. Other materials than slate, marble, or porcelain must be
submitted for special examination before being used.

_Cord._--The use of flexible cord should be reduced to a minimum.
Wherever drop lights are necessary they should be wired with reinforced
cord or stage cable.

_Current Taps._--Must be of approved construction and may be used if
properly installed.


_National Electrical Code Rules for Current Taps._

Where, in addition to sockets or receptacles already installed,
connections are desired to lighting circuits for portable lamps, for
motors, or other special appliances requiring only small amounts of
current, multiple-current taps may be used, provided the entire circuit
does not require more than 660 watts, and provided their use will not,
under any conditions, involve a departure from the requirements of Rule
23-d of the National Electrical Code. Current taps must not be used in
key or pull sockets if the device, controlled through such sockets,
requires more than 250 watts.

_Cut-Outs._--All cut-outs should be on the stage switchboard. This
requires considerable extra wire, but it will be profitable in the end.
Fuses should be inspected occasionally to see that contacts are bright
and screws kept tight. There is everywhere a very strong tendency to
over-fuse, and the principal reason for it is the failure to keep screws
tightly fastened.

There is no fuse better than the plug fuse but it is not permitted with
voltages higher than 125 or with more than 30 amperes. Refillable fuses
must not be used. Cartridge fuses, especially, require to be kept clean.
The spring contacts on these often grow weak and cause heating which
helps to blow the fuse. The neutral fuse in three-wire installations
should be larger than the outside fuses. Link fuses should be avoided on
account of the delay that may be caused through difficulty of
installing.

It is advisable not to place switches in the same cabinet with fuses.


_National Electrical Code Rules for Automatic Cut-outs (Fuses and
Circuit-Breakers)._


_Constant-Potential Systems._

Must be placed on all service wires, either overhead or underground, in
the nearest accessible place to the point where they enter the building,
inside the walls, and arranged to cut off the entire current from the
building. Departure from this rule may be authorized only under special
permission in writing. Where the service switch is inside the building,
the cut-out required by this section must be placed so as to protect it.

For three-wire (not three-phase) systems the fuse in the neutral wire
may be omitted, provided the neutral wire is of equal carrying capacity
to the larger of the outside wires and is grounded.

Must be placed at every point where a change is made in the size of wire
(unless the cut-out in the larger wire will protect the smaller). For
three-wire direct current or single-phase systems the fuse in the
neutral wire, except that called for under Section d, may be omitted,
provided the neutral wire is grounded.

Must be in plain sight, or enclosed in an approved cabinet, and readily
accessible. They must not be placed in the canopies or shells of
fixtures. Link fuses may be used only when mounted on approved bases and
must be enclosed in dust-tight, fireproofed cabinets, except on
switchboards.

Must be so placed that no set of small motors, small heating devices, or
incandescent lamps, whether grouped on one fixture or on several
fixtures or pendants (nor more than 16 sockets or receptacles) requiring
more than 660 watts, will be dependent upon one cut-out.

By special permission, in cases where wiring equal in size and
insulation to No. 14 B. & S. gauge approved rubber-covered wire is
carried direct into keyless sockets or receptacles, and where the
location of sockets and receptacles is such as to render unlikely the
attachment of flexible cords thereto, the circuits may be so arranged
that not more than 1,320 watts (or thirty-two sockets or receptacles)
will be dependent upon the final cut-out. Except for signs and outline
lighting, sockets and receptacles will be considered as requiring not
less than 40 watts each.

All branches or taps from any three-wire system which are directly
connected to lamp sockets or other translating devices, must be run as
two-wire circuits if the fuses are omitted in the neutral or if the
difference of potential between the two outside wires is over 250 volts,
and both wires of such branch or tap circuits must be protected by
proper fuses.

The above shall also apply to motors, except that small motors may be
grouped under the protection of a single set of fuses, provided the
rated capacity of the fuses does not exceed 10 amperes. When 1,320 watts
are dependent upon one fusible cut-out, as is allowed in theater wiring,
outline lighting, and large chandeliers, the fuses may be in accordance
with the following:

  125 volts or less      20 amperes
  125 to 250 volts       10 amperes

The rated capacity of fuses must not exceed the allowable carrying
capacity of the wire. Circuit breakers must not be set more than 30 per
cent above allowable carrying capacity of the wire, unless a fusible
cut-out is also installed on the circuit. Fixture wire or flexible cord
of No. 18 B. & S. gauge, will be considered as properly protected by
10-ampere fuses.

Each wire of motor circuits, except on main switchboard or when
otherwise subject to competent supervision, must be protected by an
approved fuse whether automatic overload circuit breakers are installed
or not. Single-phase motors may have one side protected by an approved
automatic overload circuit breaker only if the other side is protected
by an approved fuse.

For circuits having a maximum capacity greater than that for which
enclosed fuses are approved, circuit breakers alone will be approved.
The ordinary porcelain link fuse cut-out will not be approved. Link
fuses may be used only when mounted on slate or marble bases conforming
to the rules, and must be enclosed in dust-tight, fire-proofed cabinets,
except on switchboards located well away from combustible material, as
in the ordinary engine and dynamo room, and where these conditions will
be maintained.

_Damp Places._--It is best to avoid wiring in damp places if possible.
If wires must be run in such places, they should be lead covered. If
armored cable is used, the wires in cable must be incased in lead.
Weatherproof sockets must be used and the use of cords should be
avoided. Where cords must be used they should be of the brewery or
packing-house type. If outside of the theater, proper open work may be
used. Conduit work is permissible but not advisable, except where wires
are subject to mechanical injury.

_Decorative Lighting Systems._--The commercial decorative lighting
systems are not suitable for use inside of theaters and should be used
only outside.


_National Electrical Code Rule for Decorative Lighting Systems._

Special permission may be given in writing for the temporary
installation of approved systems of decorative lighting, provided the
difference of potential between the wires of any circuit shall not be
over 150 volts and also provided that no group of lamps requiring more
than 1,320 watts shall be dependent on one cut-out.

[Illustration: FIGURE 157.]

_Dimmers._--"Dimmer" is the name given to the resistances which are used
to dim the electric lights on the stage. They are usually mounted at the
top or bottom of the switchboard with the operating handles in easy
reach of the operator. A dimmer should be provided for each color in
every border light and also for the foot lights. In some cases the
proscenium side lights are also arranged for several colors. The dimmers
should be mounted so that each one is directly above the switch which
controls the circuit to which it belongs. They should be protected by
wire guards against rubbish, which is likely to accumulate among them if
they are not enclosed. Good ventilation must be provided. A certain
dimmer can be used only while the amperage remains within the proper
limitations. If carbon lamps, for instance, are changed for tungsten,
the amperage will be much reduced and the dimmer may not work properly.

Modern dimmers are all of the interlocking type so that the whole bank
may be worked together or any one singly. Figure 157 is an illustration
of a small bank of dimmers.

_Door Switches._--Door switches are used mostly in connection with
dressing rooms. When the light in the room is burning, opening and
closing the door will turn it out.

_Dressing Rooms._--Many dressing rooms are wired with lights on pendant
cords, but this method is not to be recommended. It is better to arrange
the lights in sockets fitted into the outlet boxes. It will be well if
each dressing room is provided with a circuit for flatirons. If this is
considered too expensive, the fuses on circuits leading into dressing
rooms must be kept small enough to blow in case a flatiron is connected.
Many actors are in the habit of carrying flatirons and will use them on
incandescent-light circuits if no other circuit is provided. If metal
guards are fastened on securely, much of the trouble from flatirons will
be avoided, but they must be fastened so that an ordinary screwdriver
will not loosen the screws.

Dressing room illumination, if properly carried out, will save the
proprietor much annoyance. Actors have ways of obtaining the light they
want by fair means or foul, and it is much better to arrange the
lighting so that it will be satisfactory to those using it, than to have
it continually tampered with.

A bright illumination is essential for making up and both sides of the
face should be fully illuminated. To accomplish this it is necessary to
provide at least two lights for each mirror. Every lamp should be
provided with a substantial guard which may be securely fastened to the
outlet box. This is especially necessary if small candle power lamps are
furnished. For, if the light furnished is insufficient, there will be
strong efforts made to replace the small lamps by larger ones. Guards
are also necessary as a protection against fire. Actors are in the habit
of drying small articles of clothing on the lamps, and fires have been
known to start from this cause. The guards on the lamps throw shadows on
the faces of the actors, and for this reason are not liked. In order to
avoid the shadows, frosted lamps may be used, or two lamps on each side
of the mirror. One lamp will then illuminate the shadows caused by the
other. Where there are long dressing tables, as in rooms occupied by the
chorus, mirrors may be placed between each two lamps on each side of the
table and the lamps staggered vertically. In this way each party will
receive the benefit of at least four lamps and the shadows will be less
noticeable.

The color value of the lamps used in dressing rooms should match that of
the lamps used on the stage. Some stages, for instance, are illuminated
by flaming arcs which may give an altogether different effect to a "make
up" than the tungsten lamps, for instance.

Where there are a large number of sockets without lamps in them, it is
advisable to close the socket by a fuse plug from which the fuse has
been removed, or in some similar manner in order to avoid the chance of
short circuits from hat pins or similar objects used in dressing rooms.

_Drop Lights._--Drop lights should be avoided as much as possible.
Where they must be installed reinforced cord, armored cable, or flexible
steel armored cord should be used.

_Emergency Lighting._--Every theater must have an emergency lighting
system as outlined in the rules below. The object of this is to provide
a double system of illumination so as to reduce the likelihood of the
theater being left in total darkness. This is a frequent occurrence
where the theater is supplied by an isolated plant, and may also occur
where the supply is obtained from a large outside lighting system
although the latter is usually more reliable.

Every unnecessary light that is added to an emergency system increases
the likelihood of trouble. A sufficient number of lights added to it to
illuminate the space used by the audience in getting out is, therefore,
necessary but more than this is a detriment rather than a help. The
practice of connecting all of the hall and lobby lights to the emergency
system is to be condemned, since it defeats the purpose of the rule
which provides a double illumination system for every part of the
theater used by the audience. The two systems should be carried into the
auditorium, all stairways, exits, and lobby clear to the street. They
should, furthermore, be kept entirely separate, in separate conduits and
using separate meters.

The rule does not specify the number of exit and emergency lights
required. Good practice requires a light for every 400 square feet or
so. If each of these outlets be fitted with a 50 watt lamp the
illumination will interfere with many of the acts and so, smaller lamps
are generally used. One emergency light for each 60 seats will fill the
bill nicely.

[Illustration: FIGURE 158.]

Over each exit, an exit light must be provided, but as these exit lights
are usually incased in ruby glass such as shown in Figure 158 they
cannot be counted as giving any illumination. In many theaters gas is
used for emergency and exit lighting and in many respects this seems to
be a good practice. In some quarters, however, it is believed that a
fire in the scenery of a theater might create such an air pressure in
the building as to overcome the low gas pressure and thus put out the
gas at the very time when it is most needed. The very best suggestion
that can be made is that of a storage battery to supply the emergency
system. Where alternating current is used this, however, introduces some
complications and becomes quite expensive. Avoid the use of key sockets,
fan motors, or any unnecessary device on emergency or exit light
circuits.


_National Electrical Code Rules for Services._

Where supply may be obtained from two separate street mains, two
separate and distinct services must be installed, one service to be of
sufficient capacity to supply current for the entire equipment of
theater, while the other service must be at least of sufficient capacity
to supply current for all emergency lights. Where supply cannot be
obtained from two separate sources, the feed for emergency lights must
be taken from a point on the street side of main service fuses. By
"emergency lights" are meant exit lights and all lights in lobbies,
stairways, corridors, and other portions of theater to which the public
have access, which are normally kept lighted during the performance.
Where the source of supply is an isolated plant within the same
building, an auxiliary service of at least sufficient capacity to supply
all emergency lights must be installed from some outside source, or a
suitable storage battery within the premises may be considered the
equivalent of such service.

[Illustration: FIGURE 159.]

Exit lights must not have more than one set of fuses between same and
service fuses. Exit lights and all lights in halls, corridors, or any
other part of the building used by audience, except the general
auditorium lighting, must be fed independently of the stage lighting,
and must be controlled only from the lobby or other convenient place in
front of the house. All fuses must be enclosed in approved cabinets.

Figure 159 shows a diagram of an emergency lighting system together with
that of the house lighting system. The main house switch is sometimes
arranged with two sets of fuses, so that it need only be thrown over
when a fuse blows. See Figure 159 at the right.

_Exit Lights._--Exit lights are usually required over all doors leading
from any part of the auditorium to the lobby or outside of the theater.
They are usually incased in ruby glass on which the word EXIT has been
etched in large letters. See also Emergency Lighting.

_Fan Motors._--In large theaters, sometimes as many as ten or twelve fan
motors are arranged along the walls of the auditorium, to blow air
toward the audience. In small theaters there is a strong temptation to
connect these to the emergency circuit, but this should never be done.
The motors are too noisy to be allowed to run during an act and must be
wired so that they can all be thrown on or off together from the stage
switchboard. Not more than 660 watts should be connected to any one
circuit and the connection should be by means of attachment plugs and
reinforced cord, a bracket for the motor being placed as near as
possible to the outlet, so as to reduce the amount of cord required to a
minimum.

_Fire Alarm._--In some of the cities a special signal system between
stage and box office is required and it is necessary to consult
authorities about this. Ordinarily a bell circuit, such as shown under
"Bells," will answer.

_Fish Work._--The fishing of wires incased in loom, as is permitted in
other places, is not permissible in theaters. If wires are to be
"fished" they must be incased in metal armor.

_Fixtures._--Fixtures require careful inspection before installing. It
is seldom that the piping is properly reamed out as required by the
rule. Chain fixtures especially, must be closely watched. There is
little if any trouble to be expected from the wires laced among the
chain links, but there is much trouble to be looked for from the wires
at the points where they leave the chain support and where they enter
the socket. The openings for the wires at these points are seldom large
enough to admit of the proper wire being used, and they are not well
reamed either. The result of these conditions is that the wire is very
quickly abraded and short circuits are common.

Fixtures may be tested for short circuit by connecting a magneto at the
feed end of the wires and turning it briskly. If a ring is obtained it
is an indication that something in wrong. The test for open circuit may
be made with the same connection. Insert a screw driver or a plug into
each socket; if it is not possible to obtain a ring there must be an
open circuit. The test for "ground" may be made by connecting both feed
wires to one pole of the magneto, and the other pole to the metal of the
fixture taking care that there is no lacquer on that part of the
fixture. If a ring is obtained it is an indication that some bare spot
on the wire is in contact with the fixture. Fixtures used for emergency
lighting must carry no key sockets and must not feed fan motors.


_National Electrical Code Rules for Fixture Wire and Fixtures._


_Wires for Fixture Work._

Must not be smaller than No. 18 B. & S. gauge, and must have an approved
rubber insulating covering. In wiring certain designs of show-case
fixtures, ceiling bull-eyes, and similar appliances in which the wiring
is exposed to temperature in excess of 120 degrees Fahrenheit (49
degrees centigrade), from the heat of the lamps, approved slow-burning
wire must be used. All such forms of fixtures must be submitted for
examination, test and approval before being introduced for use.

Supply conductors, and especially the splices to fixture wires, must be
kept clear of the grounded part of gas pipes, and, where shells or
outlet boxes are used, they must be made sufficiently large to allow the
fulfillment of this requirement.

Must, when fixtures are wired outside, be so secured as not to be cut or
abraded by the pressure of the fastenings or motion of the fixture.

Wires of different systems must never be contained in or attached to the
same fixture, and under no circumstances must there be a difference of
potential of more than 300 volts between wires contained in or attached
to the same fixtures.

On chains of similar parts where conductors are not completely enclosed
in metal, wires must be stranded and must have rubber insulation not
less than one thirty-second of an inch in thickness; or approved pendant
or portable cord may be used.


_Fixtures._

When supported at outlets in metal conduit, armored cable, or
metal-molding systems, or from gas piping or any grounded metal work, or
when installed on metal walls or ceilings, or on plaster walls or
ceilings containing metal lath, or on walls or ceilings in fireproof
buildings, fixtures must be insulated from such supports by approved
insulating joints placed as close as possible to the ceilings or walls.
The insulating joint may be omitted in conduit, armored cable, or
metal-molding systems with straight electric fixtures in which the
insulation of conductors is the equivalent of insulation in other parts
of the system, and provided that approved sockets, receptacles, or
wireless clusters are used of a type having porcelain or equivalent
insulation between live metal parts and outer metal shells, if any.

Where insulating joints are required, fixture canopies of metal must be
thoroughly and permanently insulated from metal walls or ceilings, or
from plaster walls or ceilings on metal lathing, and from outlet boxes.

Canopy insulators must be securely fastened in place so as to separate
the canopies thoroughly and permanently from the surfaces and outlet
boxes from which they are designed to be insulated.

Fixtures having so-called flat canopies, tops, or backs, will not be
approved for installation, except where outlet boxes are used.

In non-metallic fixtures wireways must be metal-lined unless approved
armored conductors with suitable fittings are used.

_Flashers._--Flashers are used to economize electrical energy in sign
illumination. An electric sign is often just as effective if only one
side or one letter is in use at a time and in such a case there is
considerable saving by the use of a flasher.

[Illustration: FIGURE 160.]

Flashers are made up in a variety of forms, one of them being shown in
Figure 160. The manner of wiring to the sign will be readily understood
by reference to the figure. The neutral wire is carried direct to the
sign and fuses for that wire are often installed up in the sign, while
those for the other side of the circuits must be close to the flasher to
comply with the rule which requires fuse protection where a small wire
is tapped off from a larger one, unless the fuse in the larger wire is
below the carrying capacity of the smaller.


_National Electrical Code Rule for Flashers._

Time switches, sign flashers, and similar appliances must be of approved
design and enclosed in approved cabinets.

_Flat Irons._--See Heaters.

_Flexible Cord._--The ordinary flexible cord should not be used in any
part of a theater. It is recommended that only those cords described
below be used, each of course in its proper place. Approved reinforced
cord for pendants. Approved asbestos-covered cord for portable heaters.
Approved Stage and Border Cables on Stage.

See "Stage Cables."

_Flexible Tubing._--Flexible tubing is not permissible in theaters.

_Floor Pockets._--These are always a source of trouble and their use
should be avoided as much as possible. If it is necessary to use any
they must be of a type approved by the inspection department having
jurisdiction.

_Fly Floor._--This is the name given to that part of the gallery above
the stage upon which those employes, whose duty it is to raise and lower
curtains and scenery, work. No great illumination is required, but the
lights should be arranged so as to be convenient to the men for reading
during long waits. If this is not done there will always be considerable
objectionable cord wiring found in this place. Fly-floor and
rigging-loft lights should be provided with three way switches so that
lights may be turned on or off either at the stage switchboard or on the
fly floor.

_Footlights._--These form the most important and effective part of the
permanently located stage lights. To locate them properly requires
careful consideration. If they are brought too high they will obstruct
the view of the audience; if located too low they will not illuminate
the lower part of the stage.

Opinion is somewhat divided as to whether they ought to be arranged in
circular form or be laid out straight across the stage. In large
theaters there is a circular space between the front seats and the front
of the stage. A circular stage will utilize this space and this form of
stage with footlights arranged in a similar manner will also allow the
actors to approach more closely to the audience and will procure for
them some illumination from the side. The difficulty of making up a
circular strip of footlights is somewhat greater than that of making
them up in a straight line.

[Illustration: FIGURE 161.]

[Illustration: FIGURE 162.]

The footlights are generally made to extend nearly the full opening of
the stage and the lights should be crowded into them as closely as
possible. The lighting should be divided into three groups suitable for
three colors. It is customary to have about half of the lights white and
the remaining half divided into two equal parts for different colors. A
small part of the white lights should be arranged upon a separate
circuit so that they may be used for rehearsals or other purposes, when
not very much light is needed.

No very definite recommendations concerning the type of lamp to be used
can be given. In some theaters the stage is very solid and tungsten
lamps are used to advantage; in others the jarring prevents their use.
The white lights should be arranged so that their light need not pass
through the colored lamps. In vaudeville houses some protection in the
form of chains or guards is often provided to prevent objects from being
rolled or thrown against the lights. Figures 161 and 162 show types of
footlights and the manner of installing the sockets.


_The National Electrical Code Rules for Wiring Footlights._

Must be wired in approved conduit or armored cable, each lamp receptacle
being enclosed within an approved outlet box, or the lamp receptacles
may be mounted in an iron or steel box, metal to be of a thickness not
less than No. 20 U. S. sheet metal gauge treated to prevent oxidation
and so constructed as to enclose all the wires. Wires to be soldered to
lugs of receptacles. Must be so wired that no set of lamps requiring
more than 1,320 watts nor more than 26 receptacles shall be dependent
upon one cut-out.

_Fuses._--See "Cut-outs."

_Gallery Lighting._--The proper lighting of galleries is a difficult
matter where there is another gallery above. In such a case the ceiling
is always very low. An even distribution of light requires the use of
many small incandescent lamps evenly distributed. This will make the
wiring expensive. The light should be taken pretty well from the rear so
as to avoid placing lights in the range of visions of the audience.
Emergency lights should be provided here as well as in the main
auditorium.

_Gallery Pockets._--Pockets for arc lamps are often provided, as it
often happens that a moving-picture machine must be placed in the
gallery. The stage pockets should be wired with number 6 B. & S. gauge
wire, this being the smallest wire allowed to feed a moving-picture arc.
The stage pocket should be controlled by a switch on the switchboard. It
often happens that the operator at the lamp has a wrong cue and his
light can then be controlled by the stage manager at the stage
switchboard.

_Gas Lighting._--Electric gas lighting is very little used at the
present time, but in many theaters the emergency lighting is by gas and,
in order to save the time necessary to light these by hand before every
performance, an automatic gas lighting system has been installed in some
theaters. A diagram of such a system is shown in Figure 163. A battery
of considerable capacity is necessary and also a spark coil _S_. At each
gas jet a magnet _M_ is installed which, when energized, is capable of
turning the gas on and at the same time producing a spark. This magnet
is also capable of turning the gas off. Two buttons are provided, one to
turn the gas on and the other to turn it off.

[Illustration: FIGURE 163.]

As the wiring is always used in connection with gas piping, grounds are
of frequent occurrence, and in order to indicate a ground as soon as it
comes on, the bell and battery shown are provided. If a ground causes a
continuous current, the spark coil will attract the armature, thus
causing the bell to ring continuously.


_National Electrical Code Rule for Gas Lighting._

Electric gas lighting, unless it is the frictional system, must not be
used on the same fixtures with the electric light.

_Grid Floor._--This is the term given to the framework which supports
the pulleys over which the cables for handling curtains and scenery run.
It is usually made up of parallel iron slats or bars; hence the name. A
fairly good illumination should be provided here and all of the lights
should be arranged on three-way switches. One light for every 400 square
feet will be sufficient.


_National Electrical Code Rule for Ground Clamps._

Connections to grounded pipes and to conduit must be exposed to view or
be readily accessible, and must be made by means of approved ground
clamps to which the ground wire must be soldered.


_National Electrical Code Rule for Grounding._

The metal of conduit, armored cable, or metal molding must be
permanently and effectually grounded to water piping, gas piping, or
other suitable grounds, provided that, when connections are made to gas
piping, they must be on the street side of the meter. If the conduit
system consists of several separate sections, the sections must be
bonded to each other, and the system grounded; or each section may be
separately grounded, as required above.

Conduits and gas pipes must be securely fastened in outlet boxes,
junction boxes, and cabinets, so as to secure good electrical
connections.

If conduit, couplings, outlet boxes, junction boxes, cabinets, or
fittings, having protective coating of non-conducting material such as
enamel, are used, such coating must be thoroughly removed from threads
of both couplings and conduit and such surfaces of boxes, cabinets and
fittings where the conduit or ground clamp is secured in order to obtain
the requisite good connection. Grounded pipes must be cleaned of rust,
scale, etc., at place of attachment of ground clamp.

Connections to grounded pipes and to conduit must be exposed to view or
readily accessible, and must be made by means of approved ground clamps
to which the ground wires must be soldered.

Ground wires must be of copper, at least No. 10 B. & S. gauge (where
largest wire contained in conduit is not greater than No. 0 B. & S.
gauge), and need not be greater than No. 4 B. & S. gauge (where largest
wire contained in conduit is greater than No. 0 B. & S. gauge). They
shall be protected from mechanical injury.

The ground on the conduit system is not to be considered as a ground for
a secondary system.

_Guards._--A guard rail should be provided around the stage switchboard
to prevent actors from coming in contact with any live part of the
switchboard. All incandescent lamps about the stage, dressing rooms,
fly-floor, grid-floor, etc., should be provided with guards.

_Hanger-Boards._--Hanger-boards are not compulsory but where not used,
arc lamps must be hung from insulating supports other than their
conductors.


_National Electrical Code Rule for Hanger-Boards for Series Arc Lamps._

Hanger-boards must be so constructed that all wires and current-carrying
devices thereon will be exposed to view and thoroughly insulated by
being mounted on a non-combustible, non-absorptive, insulating
substance. All switches attached to the same must be so constructed that
they shall be automatic in their action, cutting off both poles to the
lamps, not stopping between points when started, and preventing an arc
between points under all circumstances.

_Heaters._--Heaters are used sometimes in cold dressing rooms, in box
offices and in other small spaces where other adequate heating
arrangements have not been made. The fire hazard incident to the use of
electric heaters is considerable and they should be carefully installed
according to the rules given below.

From one-half to three watts per cubic foot will be required to heat a
room. The quantity of heat necessary to be supplied depends largely upon
the ventilation. It will be small in closed dressing rooms and large,
for instance, in the box office. Always place a heater where the air
enters a room; never where it leaves.


_National Electrical Code Rules for Electric Heaters._

Each heater of more than 6 amperes or 660 watts capacity must be
protected by a cut-out, and controlled by a switch or plug connector
plainly indicating whether "on" or "off" and located within sight of
the heater. Heaters of 6 amperes or 660 watts capacity, or less, may be
grouped under the protection of a single set of fuses, provided the
rated capacity of the fuses does not exceed 10 amperes; or may be
connected individually to lighting circuits.

Flexible conductors for smoothing irons and sad irons, and for all
devices requiring over 250 watts, must have an approved insulation and
covering.

With portable heating devices, approved plug connectors must be used so
arranged that the plug may be pulled out to open the circuit without
leaving any live parts so exposed as to render likely accidental contact
therewith. The connector may be located at either end of the flexible
conductor or inserted in the conductor itself.

Smoothing irons, sad irons, and other heating devices that are intended
to be applied to combustible articles, must be provided with approved
stands.

Stationary heaters such as radiators, ranges, plate warmers, etc., must
be so located as to furnish ample protection between the device and
surrounding combustible material.

Must each be provided with a name-plate, giving the maker's name and the
normal capacity in volts and amperes.

_High Potential._--The National Electrical Code classifies all voltages
below 550 as low. Nevertheless voltages above 220 should not be
considered in the auditorium, stage, or dressing rooms of any theater.
And this voltage only in connection with a three-wire system where the
high voltage exists only between the outside wires, and 110 volts are
used for lamps and other devices. High potential systems should be used
only on the outside.

_Illumination._--Illumination is more an art than a science and the
rules that can be given have only a very general application. The best
practice, where really good illumination is desired, is to install a
large number of circuits in proportion to the number of lights, so that
lamps of large candle power may be used wherever desirable. The quantity
of light needed in theaters varies greatly with the color of decorations
and with their condition. Dust accumulates rapidly in theaters and may
absorb half of the light. If there is plenty of capacity the candle
power of lamps may be changed to suit conditions and obtain any result
desired.

The number of sockets installed in the auditorium varies widely. In some
of the cheaper theaters one light for each twenty seats is considered
sufficient; while in elaborately lighted houses, there are cases where
the number of sockets is equal to about half the number of seats. Good
illumination requires that the light come from the back, but very often
the purpose is to obtain a brilliant effect by placing lights in front
of the audience. In such cases, however, the lamps are always of low
candle power and should be frosted.

_Incandescent Lamps._--Practically all incandescent lamps are now rated
in watts. They can be obtained in voltages ranging from 2 to 250 and may
be operated in series or in multiple. All but the tantalum lamp operates
equally well on alternating or direct current. The tantalum cannot be
recommended for alternating-current circuits. No incandescent lamps will
operate well on frequencies lower than forty cycles. The natural
distribution of light is mostly in the horizontal plane and for good
illumination reflectors should be provided.

The color value is in the following order: Tungsten, tantalum,
graphitized filament, carbon filament. None are equal for color-matching
purposes to the intensified arc or the Moore tube.

The life of all incandescent lamps varies inversely with the voltage. An
increase in voltage will produce an increase in the efficiency of the
lamp but shorten its life. The efficiency decreases with continued use,
and it is generally considered that the useful life of a lamp is over
when its efficiency has fallen to 80 per cent of its original value.
Frosting or coloring shortens the life of lamps from 30 to 50 per cent
and reduces the candle power from 3 to 10 per cent, but the lamp yields
a more pleasing light. Frosting is ordinarily used only where lamps are
placed so as to come in the line of vision. Bowl frosting does not
materially reduce the life of a lamp.


_Efficiency of Lamps._

With incandescent lamps the term "efficiency" has a meaning quite
different from that usually given, it being the number of watts per
candle power. The lower the efficiency of an incandescent lamp, the
better it is and the more light it yields per watt consumed.


_Mazda Lamp._

The Mazda lamp has an efficiency of about 1.25 and is the most efficient
of all. It may be recommended in all places where lamps may be suspended
vertically; where there is not too much jarring; where there is ordinary
care in handling; and where the lamps are burning a large part of the
time. The operating expense of this lamp is low, but the initial cost is
high and the breakage is likely to be considerable. If lamps are much
handled and not burned much, the cost of broken lamps may exceed the
saving in energy. It is best not to clean Mazda lamps when cold. Shock
absorbers should be used where there is much jarring. The illumination
should be laid out for the use of lamps not smaller than 60 watts. The
lamps should not be used for temporary work or for coloring. Arrange
lamps as far as possible to be controlled by switches. Broken filaments
can often be united again by shaking the lamps gently until the broken
ends come together. The current will then weld them. When a bank of
Mazda lamps is turned on there is an excessive current for an instant.


_Tantalum Lamp._

The efficiency of this lamp is from 1.8 to 2 watts per candle power.
This lamp is much used for street-car and similar illumination
because it stands jarring very well. It should not be used on
alternating-current circuits. The filament can often be united in the
same manner as the Mazda.


_Graphitized Filament Lamp._

This lamp has an efficiency of 2.5 watts per candle power.


_Carbon Filament Lamp._

This lamp used on 110-volt circuits has an efficiency of from 3 to 3.2
watts per candle power. The efficiency of the smaller sizes is from 4 to
5 watts per candle power. The carbon filament lamp is the most
expensive of all to operate, but on account of the strength and
cheapness of the lamp it can be recommended in places where the breakage
is liable to be great or where the light is used for comparatively brief
periods.


_National Electrical Code Rules for Incandescent Lamps._

Must be provided with guards when liable to come in contact with
inflammable material or subject to rough usage. Must be protected by
vapor-proof globes where inflammable gases exist.


_National Electrical Code Rules for Insulating Joints._

When supported at outlets in metal conduit, armored cable, or metal
molding systems, or from gas piping or any grounded metal work; or when
installed on metal walls or ceilings, or on plaster walls or ceilings
containing metal lath, or on walls or ceilings in fireproof buildings,
fixtures must be insulated from such supports by approved insulating
joints placed as close as possible to the ceilings or walls. The
insulating joint may be omitted in conduit, armored cable, or metal
molding systems with straight electric fixtures in which the insulation
of conductors is the equivalent to insulation in other parts of the
system, and provided that approved sockets, receptacles, or wireless
clusters are used of a type having porcelain or equivalent insulation
between live metal parts and outer metal shells, if any.

Where insulating joints are required, fixture canopies of metal must be
thoroughly and permanently insulated from metal walls or ceilings, or
from plaster walls or ceilings on metal lathing, and from outlet boxes.

Canopy insulators must be securely fastened in place, so as to separate
the canopies thoroughly and permanently from the surfaces and outlet
boxes from which they are designed to be insulated.

_Inverted Lighting._--In this method of lighting the light is first
thrown upward against the ceiling and then reflected back. This method
can be used to advantage with light colored ceilings only. It is
especially suited for low ceilings and with high ceilings the advantage
disappears. The light obtained in this manner is very even and almost
shadowless. Much light is lost through absorption but, owing to the fact
that the light is of such an even quality, the eye readily accommodates
itself to a lower quantity and the net increase in energy required to
illuminate suitable spaces is not so very great. Those who wish to go
into the subject of illumination thoroughly will find it treated fully
in "Modern Illumination Theory and Practice".

_Joints._--Methods of making joints are illustrated in Figure 164. Be
careful not to overheat, especially at points where there is a strain on
the wire.


_National Electrical Code Rule for Joints._

Wires must be so spliced or joined as to be both mechanically and
electrically secure without solder. The joints must then be soldered
unless made with some form of approved splicing device, and covered with
an insulation equal to that on the conductors.

[Illustration: FIGURE 164.]

Stranded wires (except in flexible cords) must be soldered before being
fastened under clamps or binding screws; and, whether stranded or solid,
when they have a conductivity greater than that of No. 8 B. & S. gauge
they must be soldered into lugs for all terminal connections, except
where an approved solderless terminal connector is used.

_Junction Boxes._--Junction boxes are installed in conduit systems for
the purpose of facilitating the drawing in of the wire or of branching
off from a main run. See "Conduit Work."

_Lamps._--See "Incandescent Lamps."

_Lobby._--The lobby generally requires a number of lights and the aim
often is to create a lavish display. Very often a cut-out center is
arranged at some convenient place. The exit and emergency lights must be
controlled from the lobby. Sometimes outlets are provided for electric
bulletin boards or small signs.

_Lugs._--For solderless lugs, such as are used on rheostats or for arc
lamps, see "Portable Stage Equipment."


_National Electrical Code Rule for Lugs._

For fuses rated over 30 amperes lugs, firmly screwed or bolted to the
terminals and into which the conducting wires are soldered, must be
used. On rheostats, resistances, etc., lugs will not be required when
leads are provided as a part of the device.

Switches for current of over 30 amperes must be equipped with lugs,
firmly screwed or bolted to the switch, and into which the conducting
wires shall be soldered. For the smaller sized switches simple clamps
can be employed, provided they are heavy enough to stand considerable
hard usage.

Where lugs are not provided, a rugged double-V groove clamp is advised.
A set screw gives a contact at only one point, is more likely to become
loosened, and is almost sure to cut into the wire. For the smaller
sizes, a screw and washer connection with up-turned lugs on the switch
terminal gives a satisfactory contact.


_National Electrical Code Rules for Metal Moldings._

Must not be used on stage side of proscenium wall.

Must not be used for circuits carrying more than 1,320 watts.

Wire used must be standard rubber covered, but may be single braid.

Must never be concealed or run in damp places.

Must not be used where the difference of potential exceeds 300 volts.

Must be continuous from outlet to outlet, to junction boxes, or approved
fittings designed especially for use with metal moldings, and must at
all outlets be provided with approved terminal fittings which will
protect the insulation of conductors from abrasion, unless such
protection is afforded by the construction of the boxes or fittings.

Such molding where passing through a floor must be carried through an
iron pipe extending from the ceiling below to a point five feet above
the floor, which will serve as an additional mechanical protection and
exclude the presence of moisture often prevalent in such locations.

Where the mechanical strength of the molding itself is adequate, this
ruling may be modified to require the protecting piping from the ceiling
below to a point at least three inches above the flooring.

Where such moldings pass through a partition the iron pipe, required for
passing through floors, may be omitted and the molding passed directly
through, providing the partition is dry and the molding is in a
continuous length with no joint or coupling within the partition.

Backing must be secured in position by screws or bolts, the heads of
which must be flush with the metal.

Must have the metal of molding permanently and effectually grounded to
water piping, gas piping, or other suitable grounds, provided that when
connections are made to gas piping, they must be on the street side of
the meter. If the metal molding system consists of several separate
sections, the sections must be bonded to each other and the system
grounded, or each section may be separately grounded, as required above.

Metal moldings and gas pipes must be securely fastened to outlet boxes,
junction boxes, and cabinets, so as to secure a good electrical
connection. Molding must be so installed that adjacent lengths of
molding will be mechanically and electrically secured at all points.

If metal molding, couplings, outlet boxes, junction boxes, cabinets, or
fittings having protective coating of non-conducting material such as
enamel are used, such coating must be thoroughly removed from threads of
couplings and metal moldings, and from the surfaces of boxes, cabinets,
and fittings, where the metal molding or ground clamp is secured in
order to obtain the requisite good connection. Grounded pipes must be
cleaned of rust, scale, etc., at the place of attachment of the ground
clamp.

Connection to grounded pipes and to metal moldings must be exposed to
view, or readily accessible, and must be made by means of approved
ground clamps, to which the wires must be soldered.

Ground wires must be of copper, at least No. 10 B. & S. gauge. They
shall be protected from mechanical injury.

[Illustration: FIGURE 165.]

Must be installed so that for alternating-current systems the two or
more wires of a circuit will be in the same metal molding. It is
suggested that this be done for direct-current systems also so that they
may be changed to the alternating-current system at any time, induction
troubles preventing such change if the wires are in separate moldings.

_Meters._--A good job of meter setting requires that the meter fittings
which are now on the market be used. Two separate meters will be
required in each theater; one for the general lighting and one for the
emergency system.

_Meter Reading._--Meter readings are indicated by pointers, arranged to
move over dials as shown in Figure 165. The various pointers are
connected together by gearing in such a manner, that alternate pointers
move in opposite directions, as indicated by the figures on the dials.
The gearing moving the pointers in Figure 165 is of such proportions
that a total revolution of any pointer represents one-tenth of a
revolution of the pointer to the left of it. Thus ten revolutions of one
pointer causes one revolution of the one at the left.

[Illustration: FIGURE 166.]

[Illustration: FIGURE 167.]

At the top of each dial the value of the reading of that dial is shown.
Where the figures given are followed by the letter "s", as in Figures
167 and 168, it signifies that each division of the dial represents the
amount of energy indicated by the figures at the top. For instance, in
Figure 168 each division of the dial at the right represents one-tenth
of one kilowatt hour and a total revolution of the pointer ten-tenths,
or one kilowatt hour.

If the figures given at the top of the dial are not followed by the
letter "s", or as shown in Figure 166, each division of the dial
represents one-tenth of the amount shown at the top of the dial, the
dial at the right of Figure 166 indicating nine-tenths of ten kilowatts
or nine kilowatts.

[Illustration: FIGURE 168.]

The meter must always be read from right to left, the lowest indicating
dial being the one at the extreme right, and the lower reading ones
always being used to check the higher ones just at the left. The
following example will illustrate the manner of reading meters:

In Figure 165 the right-hand pointer registers nine-tenths of 1000, or
900 watt hours; the pointer next to it registers eight, since it cannot
be considered as fully up to any number unless the pointer at the right
of it has just arrived at or passed 0. By the same token the middle
pointer also registers 8 and as the middle pointer has not reached 0 the
one at the left of it must be read as one, the last dial also indicates
one and the total reading is 1,188,900. On some types of meters a
multiplier is used. This is generally given on the meter dial and the
readings given by the pointers must be multiplied by this number to
obtain the correct reading of the meter.

_Motors._--Motors are used in theaters for ventilation, for raising and
lowering the steel curtain in cities where such are required, and also
in some cases for operating drop curtains, but for this last purpose
they have not found much favor. In some localities motors are required
to keep up a certain water pressure to be used in case of fire. Voltages
higher than 550 are not considered in theater work.


_National Electrical Code Rules for 550-volt Motors and Less._

Motors operating at a potential of 550 volts or less must be thoroughly
insulated from the ground wherever feasible. Wooden base frames used for
this purpose, and wooden floors, which are depended upon for insulation
where, for any reason, it is necessary to omit the base frames, must be
kept filled to prevent absorption of moisture, and must be kept clean
and dry. Where frame insulation is impracticable special permission in
writing may be given for its omission, in which case the frame must be
permanently and effectively grounded.

The motor leads or branch circuits must be designed to carry a current
at least 25 per cent greater than that for which the motor is rated.
Where the wires under this rule would be over-fused in order to provide
for the starting current, as in the case of many of the
alternating-current motors, the wires must be of such size as to be
properly protected by these larger fuses.

Each motor and resistance box must be protected by a cut-out and
controlled by a switch; said switch plainly indicating whether "on" or
"off".

Small motors may be grouped under the protection of a single set of
fuses, provided the rated capacity of the fuses does not exceed 6
amperes. With motors of one-fourth horse power or less, on circuits
where the voltage does not exceed 300, single-pole switches may be used.
The switch and rheostat must be located within sight of the motor,
except in cases where special permission to locate them elsewhere is
given, in writing.

Where the circuit-breaking device on the motor-starting rheostat
disconnects all wires of the circuit, the switch called for in this
section may be omitted. Overload-release devices on motor-starting
rheostats will not be considered to take the place of the cut-out
required by this section. An automatic circuit-breaker disconnecting all
wires of the circuit may serve as both switch and cut-out.

Auto starters, unless equipped with tight casings enclosing all
current-carrying parts, in all wet, dusty, or linty places, must be
enclosed in dust-tight, fireproof cabinets. Where there is any liability
of short circuits across their exposed live parts being caused by
accidental contacts, a railing must be erected around them.

Must not be run in series-multiple or multiple-series, except on
constant-potential systems, and then only by special permission.

Must, when combined with ceiling fans, be hung from insulated hooks, or
else there must be an insulator interposed between the motor and its
support.

Must each be provided with a name-plate, giving the maker's name, the
capacity in volts and amperes, and the normal speed in revolutions per
minute.

All varying (or variable) speed alternating-current motors except those
used for railway service must be marked with the maximum current which
they can safely carry for 30 minutes, starting cold.

Terminal blocks when used on motors must be made of approved
non-combustible, non-absorptive, insulating material such as slate,
marble, or porcelain.

Adjustable-speed motors, unless of special and appropriate design, if
controlled by means of field regulation, must be so arranged and
connected that they cannot be started under weakened field.

The use of soft rubber bushings to protect the lead wires coming through
the frames of motors is permitted, except when installed where oils,
grease, oily vapors, or other substances known to have rapid deleterious
effect on rubber are present in such quantities and in such proximity to
motors as may cause such bushings to be liable to rapid destruction. In
such cases hardwood properly filled, or preferably porcelain or micanite
bushings must be used.

The following table shows the sizes of wire recommended to be used with
motors of the horsepower given. This table is an extract from the rules
of the Department of Gas and Electricity of the City of Chicago. The
column headed "Mains" may be used when there are a number of motors fed
by a single line. For all lines which supply a single motor only, the
column headed "Branches" must be used.

The difference between the two is due to the fact that it is not
believed that several motors fed by a single line will all be started at
the same time; hence it is not necessary to provide the overload
capacity for all of the motors as it is where but a single motor is
installed.

TABLE VIII.

SIZE OF WIRES FOR MOTORS OF DIFFERENT HORSE POWER.

                                DIRECT CURRENT
  ------------------------------------+------------------------------
                  110 Volts           |          220 Volts
  -----+---------+---------+----------+---------+---------+----------
  H. P.|  Full-  | Size of | Size of  |  Full-  | Size of | Size of
       |  load   |  Wire   |  Wire    |  load   |  Wire   |  Wire
       | Current |  Mains  | Branches | Current |  Mains  | Branches
  -----+---------+---------+----------+---------+---------+----------
   1   |    8    |    14   |    14    |    4    |    14   |    14
   2   |   15    |    14   |    12    |    8    |    14   |    14
   3   |   23    |    10   |     8    |   12    |    14   |    14
   4   |   30    |     8   |     6    |   15    |    14   |    12
   5   |   38    |     6   |     6    |   19    |    12   |    10
   7.5 |   56    |     5   |     4    |   28    |     8   |     8
  10   |   75    |     3   |     1    |   38    |     6   |     6
  -----+---------+---------+----------+---------+---------+----------
                                 SINGLE-PHASE
  -----+---------+---------+----------+---------+---------+----------
   1   |   12    |   ...   |    12    |    6    |   ...   |    14
   2   |   23    |   ...   |     8    |   11    |   ...   |    12
   3   |   33    |   ...   |     6    |   16    |   ...   |    10
   4   |   44    |   ...   |     4    |   22    |   ...   |     8
   5   |   53    |   ...   |     3    |   26    |   ...   |     6
  -----+---------+---------+----------+---------+---------+----------
                                THREE-PHASE
  -----+---------+---------+----------+---------+---------+----------
   1   |   ...   |   ...   |   ...    |    3    |    14   |    14
   2   |   ...   |   ...   |   ...    |    5    |    14   |    14
   3   |   ...   |   ...   |   ...    |    8    |    14   |    14
   4   |   ...   |   ...   |   ...    |   10    |    14   |    14
   5   |   ...   |   ...   |   ...    |   13    |    14   |    12
   7.5 |   ...   |   ...   |   ...    |   19    |    12   |     8
  10   |   ...   |   ...   |   ...    |   26    |     8   |     6
  -----+---------+---------+----------+---------+---------+----------

_Music Stands._--Music stands are used by the musicians and are
generally placed between the first row of seats and the stage. Each
musician should be provided with an individual stand, although if
necessary, two or three can get along with one stand.

No first-class theater should be fitted up with less than twenty outlets
for musicians' lights. In the Metropolitan Opera House of New York there
are one hundred. Houses used exclusively for vaudeville do not, however,
need that many. In houses devoted to grand opera, often as many as one
hundred or more musicians are employed at the same time. To take care of
such a number, the music stands should be wired with pin-plug connectors
so that connections may be made from one to the other. Long flexible
connections are necessary for most music stands.

Sometimes it is necessary to crowd the orchestra under the stage and at
other times, with musical comedies, for instance, they must be brought
out where they can see the movements of the actors.

The more the circuits are subdivided, and the different lights made
independent of each other, the better it will be and the less the
annoyance in case a fuse blows. The fuses should always be arranged at
the switchboard so that it will not be necessary for the electrician to
work in front of the audience in cases of trouble. For this reason extra
stands should also be kept on hand.

A main switch controlling the lights should be placed where one of the
musicians can handle it. In dark scenes these lights must often be
turned out. If these lights are left under the control of the stage
electrician they will be more likely to be forgotten at the critical
moment than if under the control of the men who need them.

An eight candle-power lamp for each stand will be sufficient and this is
usually placed inside a special reflector which allows the light to fall
upon the music sheet only. Stage cable of good quality should be used
for the connections; there is too much rough handling for reinforced
cord. The use of the ordinary attachment plug should be avoided; use
approved pin-plug connectors.

_Open Work._--Open work is not allowed in theaters.

_Operating Room._--See special chapter on "Operating Room."

_Panel Boards._--Panel boards are really small switchboards, the
switches and cut-outs being mounted usually upon slate. The slate must
be free of metal seams; these, if present, often manifest themselves by
heating. Panel boards, unless located in the immediate vicinity of the
main switchboard, and where they are enclosed in a compartment, must
always be placed in standard metal cabinets.


_National Electrical Code Rules for Panel Boards._

The following specifications are intended to apply to all panel and
distributing boards used for the control of light and power circuits,
but not to such switchboards in central stations, sub-stations, or
isolated plants as directly control energy derived from generators or
transforming devices.


_Design._

The specifications for construction of switches and cut-outs given in
the following pages must be followed as far as they apply.

In the relative arrangement of fuses and switches, the fuses may be
placed between the bus-bars and the switches, or between the switches
and the circuits, except in the case of service switches. When the
branch switches are between the fuses and the bus-bars, the connections
must be so arranged that the blades will be dead when the switches are
open.

When there are exposed live-metal parts on the back of a board, a space
of at least one-half inch must be provided between such live metal parts
and the cabinet in which the board is mounted.


_Spacings._

TABLE IX.

THE MINIMUM DISTANCE THAT MUST BE MAINTAINED BETWEEN BARE LIVE METAL
PARTS (BUS-BARS, ETC.)

  -------------------+---------------------------+--------------
                     | Between Parts of Opposite |
                     |     Polarity, Except      |
                     |      at Switches and      |    Between
                     |        Link Fuses         |    Parts of
                     +------------+--------------+ Same Polarity
                     |    When    |              |    at Link
                     | Mounted on |  When Held   |     Fuses
                     |  the Same  | Free in Air  |
                     |  Surface   |              |
                     +------------+--------------+--------------
  Not over 125 volts |   3/4 inch |   1/2 inch   |   1/2 inch
  Not over 250 volts | 1-1/4 inch |   3/4 inch   |   3/4 inch
  Not over 600 volts |     2 inch | 1-3/4 inch   |
  -------------------+------------+--------------+--------------

At switches or enclosed fuses, parts of the same polarity may be placed
as close together as convenience in handling will allow. It should be
noted that the above distances are the minimum allowable, and it is
urged that greater distances be adopted wherever the conditions will
permit.

The spacings given in the first column apply to the branch conductors
where enclosed fuses are used. Where link fuses or knife switches are
used, the spacings must be at least as great as those given in the
following section on fuse spacing. The spacings given in the second
column above apply to the distance between the raised main bars and
between these bars and the branch bars over which they pass. The
spacings given in the third column are intended to prevent the melting
of a link fuse by the blowing of an adjacent fuse of the same polarity.

Panel boards of special design, in which the insulation and separation
between the bus-bars, and between the other current-carrying parts is
secured by means of barriers or insulating materials instead of by the
spacings given above, must be submitted for special examination and
approved before being used.


_Fuse Spacings._

Spacings must be at least as great as those given in Table X, which
applies only to plain, open link fuses. The spaces given are correct for
fuse blocks to be used on direct-current systems, and can therefore be
safely followed in devices designed for alternating currents. If the
copper fuse tips overhang the edges of the fuse block terminals, the
spacings should be measured between the nearest edges of the tips.

A space must be maintained between fuse terminals of the same polarity
of at least one-half of an inch for voltages up to 125 and of at least
three-quarters of an inch for voltages from 125 to 250. This is the
minimum distance allowable, and greater separation should be provided
when practicable.

For 250 volts, boards, or blocks with the ordinary front-connected
terminals, except where these have a mass of compact form equivalent to
the back-connected terminals usually found in switchboard work, a
substantial barrier of insulating material, not less than one-eighth of
an inch in thickness, must be placed in the "break" gap--this barrier to
extend out from the base at least one-eighth of an inch farther than any
bare live part of the fuse-block terminal, including binding screws,
nuts and the like.

For three-wire systems cut-outs must have the break-distance required
for circuits of the potential of the outside wires.

TABLE X.

FUSE SPACINGS.

  ---------------------+----------------------+-------------
                       |Minimum Separation of |   Minimum
                       |Nearest Metal Parts of|    Break
                       |Opposite Polarity     |   Distance
  Not over 125 volts:  +----------------------+-------------
    10 amperes or less |       3/4 inch       |   3/4 inch
    11-100 amperes     |     1     inch       |   3/4 inch
    101-300 amperes    |     1     inch       | 1     inch
    301-1,000 amperes  |     1-1/4 inches     | 1-1/4 inches
  ---------------------+----------------------+-------------
                       |Minimum Separation of |   Minimum
                       |Nearest Metal Parts of|    Break
                       |Opposite Polarity     |   Distance
  Not over 250 volts:  +----------------------+-------------
    10 amperes or less |     1-1/2 inches     | 1-1/4 inches
    11-100 amperes     |     1-3/4 inches     | 1-1/4 inches
    101-300 amperes    |     2     inches     | 1-1/2 inches
    301-1,000 amperes  |     2-1/2 inches     | 2     inches
  ---------------------+----------------------+-------------


_Switch Spacings and Dimensions._

When designed with 250-volt spacings between adjacent blades,
triple-pole switches must be marked 250 volts and may be used on 3-wire
D. C. or single-phase systems having not more than 250 volts between
adjacent wires and not more than 500 volts between the two outside
wires.

Spacings and dimensions must be at least as great as those given below:

TABLE XI A.

SWITCH SPACINGS AND DIMENSIONS FOR SWITCH-BOARDS AND PANEL BOARDS.

             _Not Over 125 Volts D. C. and A. C._
  --------+--------------------------+---------------+-----------
          |                          |    Minimum    |
          |                          | Separation of |
   Current| Width and Thickness      | Nearest Metal | Minimum
  --------+--------------------------+ parts of      | Break
          |   Blades    | Clips and  |   Opposite    | Distance
          |             | Hinges     |   Polarity    |
  --------+-------------+------------+---------------+-----------
  30 amp. | 1/2 inch by | 1/2 inch by|  1 inch       | 3/4 inch
  60 amp. | 5/64 inch   | 3/64 inch  |  1-1/4 inch   | 1-3/4 inch
  --------+-------------+------------+---------------+-----------

TABLE XI B.

SWITCH SPACINGS AND DIMENSIONS FOR INDIVIDUAL SWITCHES.

           _Not Over 125 Volts D. C. and A. C._

                          Inch        Inch     Inch   Inch
   30          amperes 1/2 by 5/64 1/2 by 3/64 1-1/4  1
   60 and  100 amperes                         1-1/2  1-1/4
  200          amperes                         2-1/4  2
  400 and  600 amperes                         2-3/4  2-1/2
  800 and 1000 amperes                         3      2-3/4

A 300-ampere switch with the spacings of the 200-ampere switch above may
be used on switchboards.

TABLE XI C.

SWITCH SPACINGS AND DIMENSIONS FOR ALL SWITCHES.

            _250 Volts Only D. C. and A. C._

                 Inch        Inch     Inch   Inch
  30 amperes  1/2 by 5/64 1/2 by 3/64 1-3/4 1-1/2

TABLE XI D.

SWITCH SPACINGS AND DIMENSIONS FOR ALL SWITCHES.

         _Not Over 250 Volts D. C. nor Over 500 Volts A. C._

                             Inch          Inch       Inch      Inch
   30          amperes    5/8 by 1/8    5/8 by 1/16   2-1/4     2
   60 and  100 amperes                                2-1/4     2
  200          amperes                                2-1/2     2-1/4
  400 and  600 amperes                                2-3/4     2-1/2
  800 and 1000 amperes                                3         2-3/4

A 300-ampere switch with the spacings of the 200-ampere switch above may
be used on switchboards.

Cut-out terminals on switches for over 250 volts must be designed and
spaced for 600-volt fuses.

TABLE XI E.

SWITCH SPACINGS AND DIMENSIONS FOR ALL SWITCHES.

  _Not Over 600 Volts D. C. and A. C._

                  Inch         Inch       Inch      Inch
   30 amperes  5/8 by 1/8   5/8 by 1/16   4         3-1/2
   60 amperes                             4         3-1/2
  100 amperes                             4-1/2     4

_Paint Bridge._--The paint bridge is usually a scaffold which can be
raised and lowered, and which serves the purpose of carrying scene
painters at work on the curtains. A long strip is the best means of
illumination and it must be connected with long stage cable so as to be
portable.

_Paint Room._--Rooms in which paints are stored should not contain
switches or cut-outs. Lamps should be incased in vapor-tight globes.

_Program Board._--A simple form of program board is shown in Figure 169.
Except at the top and the bottom, where three lamps are shown together
in one compartment, each lamp is incased by itself. In front of the
lamps is usually colored glass, bearing numbers or letters. The lamp
behind any number being turned on, that number becomes visible. The top
and bottom are usually labeled, "Special", or "Extra".

[Illustration: FIGURE 169.]

Another form of program board is shown in Figure 170. This requires much
more wiring. Each lamp on the board is connected to a plug or switch at
the operator's station. Whichever switch is closed on the small board at
the right causes a corresponding light on the large board to appear.

[Illustration: FIGURE 170.]

Two program boards are usually installed, one at each side of the stage,
facing the audience at angles, so that one can be seen from the extreme
right, the other from the extreme left.

[Illustration: FIGURE 171.]

A system that is often used for carriage call is shown in Figure 171.
The lights are arranged on circuits as indicated by numbers. All lamps
bearing the same number being connected to one wire leading to the
controlling station. Thus there are eight controlling or switch wires
and there is one wire common to all. A special switch is provided which
has a contact point for each wire. Paper cards, perforated specially for
the numbers they represent, are used. If a card bearing a certain number
is inserted in the proper place and the switch closed, the number
printed on the card will appear on the sign. If, for instance, the
shaded circles in the card at the left of the figure allow contact to be
made, the lights indicated by black circles will light up and make the
figure "6".

Another form of program or call board is shown in Figure 172. This
requires more wires than the one just described, one wire being carried
to the controlling board from each lamp. The connections may be made in
various ways. Sometimes they are made with knife switches. If it is
desired to make the figure 1, for instance, only the three lights in a
vertical row in the center need be lighted, and to do this the three
wires from those lamps must be connected to one switch. To make the
figure 2, seven lamps would have to be lit by another switch. It is
capable of making any letter or number and when used as a program board
only may be somewhat simplified.

[Illustration: FIGURE 172.]

In connection with these signs a large number of wires may be required
and attention should be given to the following rule:


_National Electrical Code Rule for Wiring Program and Call Boards._

Except in the case of stage pockets the same conduit must never contain
more than four two-wire, or three three-wire circuits of the same system
except by special permission, and must never contain circuits of
different systems.

_Proscenium Side Lights._--These lights are located on each side of the
stage, as near as possible to the edge of the proscenium opening and
often in front of the steel curtain. They are of more use in musical
productions, where choruses spread over the whole stage, than in
dramatic productions where the interest is more in the center of the
stage. In some theaters the proscenium side lights extend the full
height of the opening and are arranged for three colors in the same
manner as footlights and borders. Very often, however, only ten or
twelve lights are arranged on each side of the stage.

The construction rules are the same as those governing foot-lights.

Lamps must be protected by strong wire mesh. This is essential as in
many cases actors are very likely to back into them while passing off
the stage. Portable strips are sometimes used as proscenium side lights.

_Receptacles._--See "Stage Pockets".

_Retiring Rooms._--In these places a quiet subdued illumination is
usually desired. There are also often quaint objects and effects to be
illuminated. A large number of outlets conveniently located for the use
of table lamps, fire places, and illumination of pictures, should be
provided.

_Rheostats._


_National Electrical Code Rules for Rheostats._


_Location._

Must be placed on a switchboard, or at a distance of at least one foot
from combustible material, or separated therefrom by a slab or panel of
non-combustible, non-absorptive, insulating material such as slate,
soapstone, or marble, somewhat larger than the rheostat, which must be
secured in position independently of the rheostat supports. Bolts for
supporting the rheostat shall be countersunk at least one-eighth inch
below the surface at the back of the slab and the bolt heads shall be
covered with insulating material. For proper mechanical strength, slab
should be of a thickness consistent with the size and weight of the
rheostat, and in no case to be less than one-half inch.

If resistance devices are installed in rooms where dust or combustible
flyings would be liable to accumulate on them, they must be equipped
with dust-proof face plates.


_Materials._

Must be made entirely of non-combustible materials, except such minor
parts as handles, magnet insulation, etc. All segments, lever arms,
etc., must be mounted on non-combustible, non-absorptive, insulating
material.


_Connections._

Clamps for connecting wires to the terminals must be so designed as to
insure a thoroughly good connection and must be sufficiently strong and
heavy to withstand hard usage. For currents above 30 amperes, lugs into
which the connecting wires may be soldered, or approved solderless
connectors must be used. Clamps or lugs will not be required when leads
are provided as a part of the device.


_No-Voltage Release._

Motor-starting rheostats must be so designed that the contact arm cannot
be left on intermediate segments, and for direct-current circuits must
be provided with an automatic device which will interrupt the supply
circuit before the speed of the motor falls to less than one-third of
its normal value. In motor-starting rheostats for alternating-current
circuits the automatic interrupting device may be omitted.


_Overload Release._

Overload-release devices which are inoperative during the process of
starting a motor will not be approved, unless other circuit-breakers or
fuses are installed in connection with them.

_Signals._--Signals are necessary between the manager's office and the
stage electrician's station; for this purpose telephones are generally
used. In many cities a fire-alarm signal system is also required and it
is specified to what part of the house it shall lead. In addition to the
above, a means of signalling the fly floor and the orchestra leader must
be provided. Buzzers are used for these signals. It is best to use
batteries for all these signals instead of attempting to run them from
lighting circuits. Means of signalling the arc-lamp station in the
gallery or the balcony should also be provided for use in connection
with lantern slides, etc. Telephone connection to this point will also
be very useful in order to give instructions to the lamp operator.
Return-call systems are advisable in all places so that the electrician
can be called from any point.

In some cities the local laws require these wires to be in conduit so as
to preclude any possibility of their communicating fire to anything. At
any rate the wires should be very carefully installed.

_Signs._--In nearly all electric signs used about theaters the letters
are outlined by incandescent lamps. Small candle-power lamps are always
used. They are not only economical but much more attractive because the
glare from too much light is very annoying. Many signs are equipped with
low candle-power tungsten lamps wired in series. In order that the
letters should show to full advantage they should be glossy white and be
kept clean. Such letters will show as bright as the lamps. It is best to
wire the sign so that either side can be used independent of the other.
In many cases, also, signs are wired so that each letter is independent
of the others for use with flashers.

Attraction signs are used to advertise the attractions which appear from
time to time. Independent portable letters are used and changed as often
as the attractions change. Each letter must be fitted with cable and
attachment plug of some kind and should have a separate circuit.
Cut-outs are generally located on the frame and must be protected from
the weather.

_Sockets._--Sockets must be of approved construction. Key sockets must
not be used for emergency or exit lighting. Sockets with fiber linings
must not be used unless protected by insulating joints. In damp places
weatherproof sockets must be used. Where inflammable gases exist, lamps
must be incased in vapor-proof globes. Porcelain sockets should not be
used in theaters. Weatherproof sockets are generally used for portable
purposes because they have no screws, etc., to work loose.

_Stage Cable._--Stage cables are allowed in theaters only where flexible
conductors are absolutely necessary. All wiring that can possibly be
installed in conduit or strip construction should be so arranged. By the
use of plugging boxes the quantity of stage cable necessary can be much
reduced. Plugs for arc and incandescent lights must not be
interchangeable.

_Stage Flues._--In most cities stage flues, as described in the chapter
on "Theater Buildings", are required. The requirement is also, often,
that they be released by electrical means. The rule below contemplates a
circuit containing an electro-magnet and at least two switches in
series. The magnet must be energized to hold the ventilator closed. If
the current fails or a switch is opened the dampers must fly open at
once. Where direct current is available the magnet is usually wound for
110 volts and connected to a circuit that is always alive. In case of an
isolated plant which is shut down at night some arrangement for tying
the dampers down should be made, but it is not always advisable, as once
tied they are likely to remain so and sure to fail when needed. Magnets
do not work well on alternating currents and in such cases the magnet
should be wound for a small current to be obtained from a gravity
battery which may be allowed to be always working. Locate the battery
where it will not freeze.


_National Electrical Code Rules for Control of Stage Flues._

In cases where dampers are released by an electric device, the electric
circuit operating same must be normally closed. Magnet operating dampers
must be wound to take full voltage of circuit by which it is supplied,
using no resistance device, and must not heat more than normal for
apparatus of similar construction. It must be located in loft above
scenery, and be installed in a suitable iron box with a tight,
self-closing door.

Such dampers must be controlled by at least two standard single-pole
switches mounted within approved iron boxes provided with self-closing
doors without lock or latch, and located, one at the electrician's
station and the others as designated.

_Stage and Gallery Pockets._--Stage pockets are usually located on each
side of the stage just far enough from the center to be safely back of
the scenery. The more pockets there are the better it will be. Each
pocket usually contains four circuits, and one of these at least should
be for incandescent lights. It will be convenient if the stage pockets
are divided into four groups, each group under control of a separate
switch and the whole again under control of a main switch. This will
enable the operator to throw off all of the pockets at once, if
necessary, or to make any combination desired.

[Illustration: FIGURE 173.]

Sometimes lights fed through stage pockets require to be dimmed; but it
is hardly practicable to arrange dimmers for apparatus carried by
traveling companies, since a dimmer must be selected with respect to the
amperage it is to control.

In addition to the rules given below, some cities require the bottom of
stage pockets to be open, so as to avoid an accumulation of dust. This
is a very essential precaution, as otherwise the pockets soon fill up or
require very frequent cleaning. The covers must be substantial as heavy
trucks are sometimes run over them. Two illustrations of stage pockets
are given in Figures 173 and 174. The plugs used with these boxes are
shown in Figure 175.

[Illustration: FIGURE 174.]

[Illustration: FIGURE 175.]

Gallery pockets are not usually laid in the floor but along the railing.
The gallery pockets should be under separate control from the stage
switchboard.


_National Electrical Code Rules for Stage and Gallery Pockets._

Must be of approved type, insulated from ground and controlled from
switchboard, each receptacle to be of not less than 35-ampere rating for
arc lamps nor 15-ampere for incandescent lamps, and each receptacle to
be wired to its full capacity. Arc pockets to be wired with wire not
smaller than No. 6 B. & S. gauge and incandescent pockets with not less
than No. 12 B. & S. gauge. Plugs for arcs and incandescent pockets must
not be interchangeable.

_Switches._--All knife switches should be enclosed in cabinets unless
they are located on a switchboard. In exposed places, flush switches
should be used. The usual height at which scattered switches are located
is about four and one-half feet above floor. Door switches are of use
principally in dressing rooms.

[Illustration: FIGURE 176.]

Diagrams of three-way switches are given in Figures 176 and 177. In
Figure 177, the wiring by which a light can be controlled from three
different points is shown. The middle switch must be a pole-changing
switch. In this case a throw-over knife switch is wired to perform this
function. Omitting the knife switch we have two three-way switches
controlling the light. When both switches (indicated by broken lines)
connect to the same wire the light burns.

[Illustration: FIGURE 177.]

In Figure 176, when both switches connect to the same polarity the light
is out; when one of them is connected to the other pole the light burns.
Under certain conditions this arrangement is more economical in wire but
has the disadvantage which the other method avoids, of bringing both
poles to each switch. This method cannot be used with direct-current
arc lamps. Three-way switches are useful on stairways, enabling one to
turn on a light at the bottom of the stairway and turn it off when at
the top or vice versa.


_National Electrical Code Rules for Switches and Connections._


_Switches._

Must be placed on all service wires, either overhead or underground, in
the nearest readily accessible place to the point where the wires enter
the building, and arranged to cut off the entire current. Departure from
this rule may be authorized only under special permission in writing.
Switches controlling emergency lighting must be located only in the
lobby.

Must when used for service switches, indicate, on inspection, whether
the current be "on" or "off". Switch cabinets must be deep enough to
allow the door to be closed when 30 ampere branch-circuit switches are
in any position, and when larger single-throw switches are thrown open
as far as their construction and installation will permit.

Must, for constant-current systems, close the main circuit and
disconnect the branch wires when turned "off"; must be so constructed
that they shall be automatic in action, not stopping between points when
started, and must prevent an arc between the points under all
circumstances. They must indicate whether the current be "on" or "off".
Service cut-out and switch must be arranged to cut off current from all
devices including meters.

Must always be placed in dry, accessible places, and be grouped as far
as possible. Single-throw knife switches must be so placed that gravity
will not tend to close them. Double-throw knife switches may be mounted
so that the throw will be either vertical or horizontal as preferred,
but if the throw be vertical a locking device must be provided, so
constructed as to insure the blades remaining in the open position when
so set.

When practicable switches must be so wired that blades will be "dead"
when the switch is open.

When switches are used in rooms where combustible flyings would be
likely to accumulate around them, they must be enclosed in dust-tight
cabinets.

Up to 250 volts and thirty amperes, approved indicating snap switches
are suggested in preference to knife switches on lighting circuits.

Single-pole switches must never be used as service switches nor for the
control of outdoor signs or circuits located in damp places, nor placed
in the neutral wire of a three-wire system, except in the two-wire
branch or tap circuit supplying not more than 660 watts.

Where flush switches or receptacles are used, whether with conduit
systems or not, they must be enclosed in an approved box constructed of
iron or steel, in addition to the porcelain enclosure of the switch or
receptacle.

Hinges of knife switches must not be used to carry current unless they
are equipped with spring washers, held by lock-nuts or pins, or their
equivalent, so arranged that a firm and secure connection will be
maintained at all positions of the switch blades.

Spring washers must be of sufficient strength to take up any wear in the
hinge and maintain a good contact at all times.


_Connections._

Switches for currents of over thirty amperes must be equipped with lugs,
firmly screwed or bolted to the switch, and into which the conducting
wire shall be soldered. For the smaller sized switches simple clamps can
be employed, provided they are heavy enough to stand considerable hard
usage.

Where lugs are not provided, a rugged double-V groove clamp is advised.
A set screw gives a contact at only one point, is more likely to become
loosened, and is almost sure to cut into the wire. For the smaller
sizes, a screw and washer connection with up-turned lugs on the switch
terminal gives a satisfactory contact.


_Spacings._

See "Panel Boards".

_Switchboards._--The stage switchboard is usually located on the
right-hand side of the stage. This position enables the operator to
watch cues easily and handle his switches with the right hand. This side
is also the one from which the stage manager prefers to work. It is
preferable to have the board raised high enough above stage level to
allow all space on the stage to be used by actors. If the switchboard is
located on stage level it takes up much room that is valuable and makes
stage exits for actors difficult.

The wiring of a good switchboard should be divided into three parts,
each part independent of the others, so that any one part can be
entirely cut off without interfering with the other. All of the house
lights should be fed through one main switch, and all of the stage light
connected to another set of bus-bars, but it is best not to have the
stage side of the board as a whole controlled by a main switch. If the
stage side of the board is controlled by a single switch there exists
the possibility that this switch may be used to cut off everything for a
dark scene and when light is again wanted a part of the lighting of the
previous setting may be left on and possibly interfere with the new
scene.

[Illustration: FIGURE 178.]

The stage lighting is usually divided into four groups, viz.: three
color groups and a group of stage pockets. The number of white lights
is usually equal to that of all of the colors.

Figure 178 shows a well laid out switchboard. All of the lights in the
auditorium are controlled by switches shown in the upper right-hand
corner and all of these are under the control of the main switch. The
house lights are usually thrown on or off as a whole and this main
switch must be convenient for the operator.

The stage pockets are controlled by the bank of switches shown at _E_.
Lights burning off the stage pockets generally are controlled by special
operators on the stage or by actors, and need not be so very convenient
to the switchboard operator. They must, however, be under his control
when necessary; but here also a main switch is not advisable.

In addition to the three groups of lights spoken of above, there are a
number of lights which should be controlled from the switchboard but
must be so connected that they will be left on, no matter what is done
with either house or stage light.

In this group belong a few lights mounted at the top of the switchboard
which enable the operator to see his switches when all else is dark.
These lights are generally shielded so as to throw no light in the
direction of the stage opening. The orchestra lights also belong in this
group, but a special switch may, with advantage, be arranged on this
circuit to bring it under the control of one of the musicians. In some
dark scenes these lights are turned out for a short time, but it is very
important that they be turned on again at the proper moment, and this is
more certainly assured, by giving the musicians control, than by
leaving it to an electrician on the stage who may be very busy with
other matters at the time.

The lights on the fly floor and in the rigging loft, as well as those in
dressing rooms and basement, must also be independent. Fan motor
circuits should also be independent of the house circuit. They are often
needed when the house lights are thrown off.

With the switchboard illustrated in Figure 178 we have shown only the
switches governing the house and stage proper. The switches governing
the other lights are usually worked into the vacant spaces between the
two groups. Each switch should be labeled in such a manner that a new
operator can learn the board easily.

The most important part of the board is that part governing the stage
lighting, and this should always be placed as conveniently as possible
to the operator. A stage switchboard is like a house in that no one is
wholly satisfied with the one he has, but thinks he could improve it a
little if he could build it over again. The arrangement shown in Figure
178 is, however, quite satisfactory for general purposes. The white
lights predominate in the ratio of two to one and are laid out in two
groups _A_ and _B_. Both groups are controlled by the switch _C_. The
switches _A_ and _B_ do not control the lights at all if the smaller
throw-over switches at the right are thrown downward. A diagram of these
switch connections is given in Figure 179 where the switches _B_ and _C_
are indicated. The object of the switches _A_ and _B_ is to help in
quickly increasing or decreasing the illumination on the stage. If in
the beginning of a certain scene, for instance, only a small quantity of
light is wanted, while a little later full illumination is desired, the
low illumination may be obtained by throwing the proper switches down;
the additional illumination which will be needed a few minutes later may
be prepared for by setting the other switches needed to the upward
position and at the proper moment closing switch _B_; this will
instantly produce the effect which otherwise would be possible only by
operating a number of switches in succession. In the same way, by a
reversal of this process, the illumination may be reduced in any amount.
This feature is very valuable in many stage settings.

[Illustration: FIGURE 179.]

To throw off all of the white lights the switch _C_ must be opened. The
switches _D_ and _F_ are main switches controlling colored lamps. All
lamps of one color should be connected to one or the other of these
groups unless some special color effect is desired.

From the three groups of switches, circuits extend into the foot lights,
into every border, and into the proscenium side-light strips so that the
color scheme may be carried out in any of these.

The handles of all switches in a row should be of the same height so
that the operator may more readily accustom himself to catch them
quickly. It is better to have switches of some excess capacity, as the
heavy metal stands the arcing much better.

A main switch by which the whole board may be cut off should be
installed but this need not be on the front of the board.




CHAPTER XIX.

PORTABLE STAGE EQUIPMENT.


The general rule is that all wires in theaters must be in conduit or
armored cable. Metal molding is not considered suitable for the stage
side of the theater. There are, however, stage requirements which make
the use of some sort of flexible and portable connection necessary and
for such purposes stage cables are allowed. All wiring that can be
permanently fastened to battens or fastened in other ways; or that is
run inside of posts or statuary, must be in conduit or armored cable.

The National Electrical Code Rules are quoted in this chapter wherever
they apply and should be followed, not only because these rules outline
the best practice but because, throughout the country, inspections will
generally be governed by these requirements. Even where the local rules
vary somewhat from these rules, it will be found that all apparatus
constructed according to them will be favored to a considerable extent.

_Anvil Sparks._--These are sometimes produced by arranging the anvil as
one pole of a circuit and the hammer as the other. When the hammer
leaves the anvil, the spark occurs. A resistance must always be inserted
in the circuit if a light or power circuit is employed. One of the most
successful methods of producing anvil sparks is shown in Figure 180.
When the hammer strikes the anvil the spring follows and strikes the
head of the hammer; and when it rebounds the spark occurs. If it is not
desired to use a power circuit the spark may be obtained from batteries
by the use of a spark coil. Sparks in dueling scenes may be produced in
much the same manner.

In some cities the inspection laws require all arcs or sparks, used on
the stage, to be enclosed. So far as the authors are aware no really
satisfactory method of enclosing anvil sparks has ever been made up. If
such an enclosure is to be constructed it must be borne in mind that it
must not spoil the illusion nor mar the tone in case the anvil is tuned
for use in singing acts. A special combination of metals is on sale at
stage-supply dealers by which sparks can be produced without the use of
current.

[Illustration: FIGURE 180.]

_Arc Lamps._--In Figure 181, there is shown an open or flood lamp, also
known as the "Olivet". This style of lamp is used on the stage only for
general illumination. As the light from this lamp spreads in all
directions it must be brought close to the part to be illuminated and
this usually brings it close to the scenery.

All portable stage lamps are hand feed and require close attention on
the part of operators. While traveling it sometimes occurs that only
high voltage such as 500 or 600 volts is available and in such a case a
suitable number of lamps must be connected in series. Hand-feed arc
lamps are difficult to start under such circumstances and the following
method is often resorted to: Place the carbons of all the lamps
connected in series the proper distance apart and connect a small fuse
across each gap. When the current is turned on it will melt the fuses
and establish the arc.

[Illustration: FIGURE 181.]

[Illustration: FIGURE 182.]

In Figure 182 we have an illustration of what is known as the _spot_ or
_lens lamp_. This style of lamp is used on the stage, on bridges, or "in
front" (in front being the term by which the balcony or galleries are
designated in this case). In the figure a "Sciopticon" attachment is
shown which is removable. Even very unpretentious shows carry at least
one spot lamp and the largest shows often carry 30 or 40 open and spot
lamps. Arc lamps are often imitated by an incandescent lamp placed
within a suitable globe with the hood made up in the shape of an arc
lamp.

[Illustration: FIGURE 183.]

Figure 183 is a cut of a stereopticon outfit. Dissolving views require
at least two lamps and sometimes three, the third being for the purpose
of providing a frame for the views or adding an extra effect, such as
falling snow. Two lamps at least are always in use at the same time and
it is necessary to provide cables of sufficient carrying capacity for
two or three lamps, as the case may be.


_The National Electrical Code Rules for Arc Lamps to be Used on the
Stage._

Must be constructed entirely of metal except where the use of approved
insulating material is necessary.

Must be substantially constructed, and so designed as to provide for
proper ventilation, and to prevent sparks being emitted from lamps when
same are in operation; and mica must be used for frame insulation.

Front opening must be provided with self-closing hinged door frame, in
which wire gauze or glass must be inserted, except in the case of lens
lamps, where the front may be stationary, and a solid door be provided
on back or side.

Must be so constructed that neither carbons nor live parts will be
brought into contact with metal of hood during operation, and arc-lamp
frames and standards must be so installed and protected as to prevent
the liability of their being grounded.

Switch on standard must be so constructed that accidental contact with
any live portion of same will be impossible.

All stranded connections in lamp and at switch and rheostat must be
provided with approved lugs.

Rheostats must be plainly marked with their rated capacity in volts and
amperes, and, if mounted on a standard, must be raised to a height of at
least three inches above the floor. Resistance must be enclosed in a
substantial and properly ventilated metal case which affords a clearance
of at least one inch between the case and the resistance element.

A competent operator must be in charge of each arc lamp, except that one
operator may have charge of two lamps, when they are not more than ten
feet apart and are so located that he can properly watch and care for
both lamps.

Each lamp should be provided with its own fuses and switch.

_Baby Spot Lamp._--This is the name given to a small lens lamp in which
an incandescent lamp is used instead of an arc lamp. Such a lamp cannot
be used for projection very well but is well suited to illuminate parts
of an act that do not require very bright illumination. It is sometimes
used to take the place of proscenium side lights. In one well-known
production twelve such lamps were used upon scaffolding directly above
the proscenium opening. The incandescent lamp is movable for the purpose
of focusing; but it not being a "point source", no such accurate
focusing is possible as with arc lamps.

_Batteries._--Both primary and secondary, or storage, batteries are
extensively used upon the stage. Small dry cells are often used to ring
bells or light miniature lamps in the hair or clothes of the chorus. The
electromotive force of a cell is independent of the size and depends
only upon the materials used; but the current which a battery can
deliver varies directly with its size. If we wish to increase the
voltage we must connect several cells in series, as indicated in Figure
184; if we have sufficient voltage but lack current we must connect, as
in Figure 185. As a general rule, in any circuit, we must connect the
battery so that its resistances shall most nearly equal the resistance
of the device which it feeds. This means that if the resistance of the
device is greater than that of the battery we must connect our battery
in series; if it is less we must connect the cells in multiple, as
indicated by Figure 185.

When connecting cells in multiple those of the same kind should always
be selected. Any cell whose electromotive force is not fully equal to
the others will absorb energy and be charged by the others. The
resistance of small cells is high and it is best, where size is not
objectionable, to use large cells. Storage, or secondary batteries, have
low resistance and are capable of delivering very strong currents, even
though the voltage be low. The heat is proportional to the square of the
current and a certain amperage, with two volts back of it, will produce
just as much heat in given resistance as the same amperage with several
hundred volts. The sparking in case of a broken circuit or a fuse
blowing would, however, be hardly noticeable with the low voltage.

[Illustration: FIGURE 184.]

[Illustration: FIGURE 185.]

No definite rule is given concerning the fusing of low-voltage storage
batteries, but it is good practice to insert a double-pole fuse in each
branch circuit; the fuse to be small enough to protect the smallest wire
fed through it. The capacity of storage batteries is usually given in
"ampere hours". A 60-ampere-hour battery, for instance, can deliver
three amperes for twenty hours or ten amperes for six hours. With every
battery there is, however, a certain maximum rate of discharge which
should not be exceeded. The battery should not be allowed to run down
too low and should be kept well charged. It is best to follow the
instructions which the makers send out with them.

_Bells._--Electric bells are used on the stage for many purposes.
Sometimes they are used as musical instruments, the gongs being properly
tuned; at other times they are provided to ring in connection with
certain acts; and they are also used for the purpose of signaling. When
signals are to be given, which the audience must not hear, it is better
to provide buzzers.

[Illustration: FIGURE 186.]

Bells can be arranged to operate from power circuits by placing an
incandescent lamp, or some other suitable resistance, in series with
them; but it is not advisable to do so. It is not safe from a
fire-hazard point of view, and also not very practical, since in many
places alternating current only can be had and the ordinary bell does
not operate well with it. Batteries should always be used for bells and
other low-voltage devices.

Where it is desired to operate a number of bells together, they may be
arranged in series, as shown in Figure 186, in which the vibrator of one
bell is cut out of circuit so that the vibrations of this bell are
governed by those of the other. Sets of bells properly tuned are often
scattered about and controlled from a keyboard. There are usually
eighteen bells in such a set.

_Blinding Lights._--In certain acts it is necessary for the actor to
work part of the time unseen by the audience. This may easily be
arranged for by the use of a number of bright lights placed between him
and the audience. These lights, when burning, blind the audience so that
they cannot see beyond them. The lamps must be placed in reflectors
which throw all light toward the audience. They are arranged either upon
the floor of the stage or around cabinets or other devices. The blinding
effect depends upon the contrast between the front and the rear of the
lamps. Lights should be very bright but the quantity of light must not
be sufficient to cause reflected light from the front to illuminate the
back of the stage. Where emergency lights are kept burning during the
performance it is quite difficult to carry out the blinding effect for
any considerable length of time.

_Brackets._--Brackets are made up in any of the three ways shown in
Figure 187. If the stem is large enough to admit the cable, which should
be used to the pin-plug connector, no splice of any kind will be
necessary. If the bracket stem is too small for this, fixture wire must
be used and a connection from it to the cable must be made. If this is
made, as at the extreme right, the back of the canopy must be enclosed
in substantial metal. On no account should fixture wire be allowed to
extend outside of the stem. The bracket must have some provision by
which it can be made to hold its proper position when in place.
Sometimes hooks are provided for this purpose and at other times it is
held in place by screws in the scenery, as suggested by the back of the
canopy shown at _A_.


_National Electrical Code Rule for Lights on Scenery._

Where brackets are used they must be wired entirely on the inside;
fixture stem must come through to the back of the scenery and end of
stem be properly bushed.

[Illustration: FIGURE 187.]

_Bunch Lights._--Bunch lights are made up according to the plans shown
in Figure 188. They are used only on the stage. No very brilliant
illumination can be obtained from them. In one well-known production a
bunch light containing one hundred 32 c.-p. lamps was used without
obtaining the result desired. The light cannot be concentrated as can
that from arc lamps. Bunch lights, however, have the advantage of not
requiring so much attention as arc lamps and can often be used on a
shallow stage where arc lamps would be out of place. Bunch lights are
sometimes made double, two on one stand; two colors can thus be used at
the same time and certain color changes facilitated. Color effects or
changes in colors are brought about by inserting colored gelatine slides
in front of the lights.


_National Electrical Code Rules for Bunch Lights._

Must be substantially constructed of metal and must not contain any
exposed wiring.

[Illustration: FIGURE 188.]

The cable, feeding same, must be bushed in an approved manner where
passing through the metal, and must be properly secured to prevent any
mechanical strain on the connection.

_Chandeliers._--The rule which requires all wiring about brackets to be
enclosed, naturally would apply here also. In connection with chain
fixtures, which are at present very popular, this requirement is
generally overlooked if good reinforced cord is used. Ordinary fixture
wire or flexible cord should not be used. The weakest part of a chain
fixture is indicated in Figure 189 by the arrow. The opening at this
place is, as a rule, too small and not well reamed out. Fixtures used on
the road are constantly giving way at this point. The opening should be
large enough to pass reinforced cord and should be well reamed out. If
the opening cannot be made large enough to give the wire a little free
movement, it is advisable to wrap tape around the wire and the metal in
such a way as to prevent all motion of the wire at this point. The same
observations also apply to the other end where the cord enters the
socket.

[Illustration: FIGURE 189.]

[Illustration: FIGURE 190.]

[Illustration: FIGURE 191.]

Another frequent source of trouble with chandeliers is found at the end
of the stem where connection is made to the cable which feeds the
fixture. See Figure 190. Very often fixture wire is brought out here,
but this will soon break. The stage cable, or reinforced cord, should be
run down to the bottom of the fixture and connections there made to the
wires leading to the various arms. There are many fixture hooks made up
after the manner shown in Figure 191 which have a small opening for the
wire at the point indicated by the arrow. This opening is always too
small and in such a case it is best to arrange a tee as shown. A
chandelier must always be hung in such a manner as to avoid strain
coming on the electrical conductors.

[Illustration: FIGURE 192.]

_Colors._--Where a certain color is wanted, during the whole show, from
a certain light source, the incandescent lamps themselves may be
colored. Prepared colors for this purpose are on the market and it is
best to use them. Where the colors in a scene change rapidly there must
be some colored material available which can be quickly interposed
between the light and the scene. The manner in which this is done for
bunch lights is indicated in Figure 188. The same method is also used in
connection with open-arc lamps, but for spot lights there is another
method, shown in Figure 192. This is a so-called color wheel and the
colors can be changed by simply revolving the wheel until the proper
color is in front of the lens. An automatic color wheel is on the market
which allows the operator to control colors at a distance by means of an
electric circuit and magnet. This device cannot be used with flood lamps
and has the further disadvantage of casting unavoidable shadows during
the change of colors. Where detachable color holders are used, it is
possible to change from one color to another by almost imperceptible
degrees by holding the frame in the hand and bringing it gradually from
a distance to the front of the lamp. In this manner the shadows cast by
the frame can be avoided and the color brought very gradually into the
scene.

[Illustration: FIGURE 193.]

_Dimmer Box._--The dimmer box, as the name implies is a box which
contains the resistances required for dimming lights used on the stage.
It comes into service only with the more pretentious shows which carry
foot-lights or border-lights. A typical wiring diagram is shown in
Figure 193. This diagram supposes that not more than three colors are
used. A small candle power incandescent lamp should always be installed
in the box so that the operator may see what is necessary in dark
scenes. This lamp should have its own fuse protection. This is a point
very often overlooked by operators. The box is usually made of wood and
lined with metal. It should be deep and nearly square, so as to occupy a
minimum of space. If a shallow box is used, it will be necessary to
place the switches in a separate compartment from the fuses, so that the
latter may be always enclosed, even while the dimmers and fuses are
being operated.

[Illustration: FIGURE 194.]

Provisions should be made for the cables to lead out through suitable
bushings at the side instead of over the top of the box as is so often
done. In some dimmer boxes the slip connectors are fitted into the side
of a box so that the male end may be inserted from outside. This makes
it unnecessary to bush the cables through the box.

_Electric Sign Imitations._--These are used mostly in representations of
city night scenes. A flasher sign can be imitated by cutting small round
holes in the curtain to outline the reading matter on the sign. Behind
these holes a moon box is placed, the lamps in it being on a flasher or
controlled by "skeedoodle" sockets, thus giving the effect of an
electric sign on a flasher. In case the effect of a steady burning sign
is wanted the flasher is omitted.

Electric signs imitating falling water have in turn been imitated
according to the plan outlined in Figure 194. On the side nearest the
audience is the curtain _A_, upon which the sign is painted and in which
there are a number of holes representing the electric lamps in the
original sign; behind this curtain is a grating _B_, also full of holes;
and behind this revolves a spiral _C_, so arranged as to obstruct
intermittently the light which is behind it. The object is to imitate,
as nearly as possible, the action of the lights in the original sign.

_Electric Stage Effects._--A large number of stage effects can be
produced by properly painted mica discs which are caused to move in
front of the lens of a spot light arranged for the purpose. A view of
such a contrivance is given in Figure 182 and is known as the
"Sciopticon". The effects are changed by clockwork which is adjustable
as to speed. The device is arranged to slip in front of the lenses and
can be attached in a moment.

A list of some of the effects obtainable is given below:

  Avalanche
  Climbing monkeys
  Cyclone with flying objects
  Falling leaves
  Fireworks
  Flame
  Flowing water
  Flying birds
  Lightning
  Moving clouds
  Niagara Falls
  Ocean waves
  Rainbow
  Rain
  Rising fire and smoke
  Rolling lava
  Sand storm
  Snow storm
  Swimming fish
  Twinkling stars
  Volcano
  Water falls

The above list is given to illustrate the possibilities of this method
of producing effects. They are, however, rather expensive and some of
them require several machines to be in operation at the same time.

_Explosions._--Nearly all explosions are set off by the melting of a
fuse through an electrical short circuit. The explosive powder is placed
above, encloses the fuse, and is ignited when the fuse blows.

[Illustration: FIGURE 195.]

Such explosions are often forbidden by inspectors but there is no danger
from them if the powder, etc., is enclosed in a suitable wire mesh open
on all sides. This will not interfere with the explosion and will
prevent the flame, which lasts but an instant, from coming in contact
with scenery or people. Where such explosions are used they are
generally the most important part of the act, and every possible
precaution must be taken to avoid the chance of failure. They are,
therefore, often arranged on two circuits with a throw-over switch so
that in case one fuse fails the circuit can be immediately closed on the
other. See Figure 195. It requires some time to throw-over a switch, and
in order to make doubly sure that the fuse will blow at the right time,
two fuses and two separate wires may be used and connected at the same
time by a simple double-pole switch. The latter arrangement has the
disadvantage that one of the wires might be broken, or the contacts bad,
without the defect ever being noticed until the other wire also fails
and the explosion misses entirely.

It is best to use the throw-over switch and to use the wires for the
first connection in alternation so that a defect may be noticed at once.
If the same wire were used each time the throw-over switch would be open
to the same objection as the double-pole switch. In selecting fuses for
this work it must be borne in mind that the powder which is placed over
the fuse has a certain cooling and conducting effect, and that often,
therefore, a fuse which would readily blow if in the open would not blow
if covered with metallic powder.

_Face-O-Graph._--This is an arrangement of two powerful arc lamps with
the necessary lenses and objective by which the face of an actor can be
projected upon a screen. This effect is very trying to the face and eyes
of the actor. The face must be brought very close to the arc lamps and
must be held upside down during the performance. From forty to fifty
amperes are used in each arc lamp. The optical principle is explained in
another chapter.

_Festoons._--Festoons should be made up as shown in Figure 196.
Weatherproof sockets are generally used because the common brass shell
sockets have too many parts that are liable to work loose on account of
the constant handling the festoons are subjected to with traveling
productions. The use of porcelain sockets, however, should be avoided;
mica sockets seem to be generally preferred. The soldering also should
be carefully done and only at the end, as indicated by the arrow in the
cut. The branch wires are often taped to the main cable as indicated,
but often also, circular loom is slipped over the cable in such a manner
that the branch wires are supported by it. Short lengths of loom must be
slipped on the cable before the tapes are made. A really well made
festoon will last a long time and give very little trouble; while a
poorly made one will cause much annoyance and much unfavorable comment
from inspectors. Most of the festoon troubles are due to poor support of
the branch wires. A single festoon does not often carry more than ten or
twelve lights and usually not more than six or eight.

[Illustration: FIGURE 196.]

A certain style of shade is very much in vogue in connection with
festoon lamps and this shade makes the use of lamp guards impossible. If
the shade is made of gelatine there will be but little need for the
guard, but celluloid shades should not be allowed. All combustible
material that is to come near the lamp or the socket should be
fireproofed.


_The National Electrical Code Rule for String or Festooned Lights._

Wiring of same must be of approved type; joints to be properly made,
soldered, and taped, and staggered where practicable. Where lamps are
used in lanterns or similar devices, approved guards must be employed.

_Fire Dance._--This effect is generally illuminated from below the
stage, the light being thrown upward through glass. Special arc lamps,
equipped with powerful reflectors, are often used and generally several
of them are required to produce the proper effect. If spot lamps are to
be used they should not be turned upward but instead reflectors placed
at an angle of forty-five degrees, should be provided to throw the light
upward. See Figure 197. If a spot lamp is turned upward, the heat of the
arc rising will be great enough to make condenser breakage a very
serious problem. It is also difficult to keep a horizontal arc working
properly.

[Illustration: FIGURE 197.]

_Fireflies._--The "fireflies" are miniature incandescent lamps strung
about among the scenery and on the clothing or in the hair of
performers. No attempt should be made to operate these from any other
source than a low-voltage, low-amperage battery. It is best to subdivide
them as much as possible to avoid short circuits. A short circuit on one
lamp would put out a group of one hundred as quickly as it would ten.
Such lamps are often arranged on a keyboard by which single lamps or
small groups may be lighted intermittently, thus giving the effect of
fireflies flitting about.

_Fireplaces._--The commonest and best fireplace is made up of a few
lights arranged in regular strip fashion, the strip being placed in a
practical fireplace filled with coal or large pieces of glass, such as
broken pieces of lenses. For a very simple effect, red colored globes
are used. The effect can be enhanced by using lamps of amber, red, and
white and connecting each lamp to a dimmer which can be worked off the
stage. This is capable of producing a flaming effect. A further
enhancement of the effect is sometimes produced by placing colored
asbestos about the lights. This or other suitable material can be made
to represent glowing ashes very nicely. Very often the illumination of
the fireplace is by means of colored globes strung about among the logs.
For this purpose they must be equipped with strong guards and wired with
stage cable. Actual gas flames are also often mixed in with the electric
illumination. In a mechanical way the effect of flame is often imitated
by colored cloth so arranged that a small fan motor placed beneath it
can blow it upward.

In a certain well-known production, an imitation of the crater of a
volcano is made by arranging a number of fan motors under a large piece
of silk which rests upon a frame and revolves over the motors. Parts of
this silk are provided with pockets and are loose so that they may be
blown upward to represent flame while the major portion only undulates,
representing a sea of molten lava. Strips with red lights are used to
intensify the effect.

A small motor operating a suitable ratchet has been used to imitate the
sounds of wood crackling in the fire. "Chemical smoke" is also often
used to enhance the effect. This can be purchased of supply houses.

_Flat Irons._--See "Heaters".

_Flowers._--On the vaudeville stage, electrically lighted flowers are
sometimes used. These are often wired by simply running fixture wire or
flexible cord along a wire stem and fastening a small socket to it.
Flowers lighted in this manner, however, will not be allowed in theaters
where there is inspection. A better method is shown in Figure 198.
Armored cord is used for the stem and a miniature socket is placed at
the end. All splices are within the junction box which supports the
stems. Instead of the armored cord ordinary fixture tubing may be used.
Baskets of flowers and small bouquets are generally lighted from
batteries.

[Illustration: FIGURE 198.]

_Fountains._--The illumination for electrically lighted fountains is
sometimes obtained from incandescent lamps, but no very showy effects
can be produced with them. Where they are used in a manner which
subjects them to moisture they must be wired in waterproof conduit and
fittings and the lamps must be covered by a watertight globe. For
brilliant effects arc-lamp illumination is necessary and the arc must of
course be below the water. As arc lamps when tilted upward break
condensers very rapidly, the light is usually reflected upward by means
of mirrors set as indicated in Figure 197.

_Fuses._--There is no part of the electrical equipment of a traveling
production so much abused as the fuse. It is provided primarily for the
protection of apparatus and should never be of greater capacity than is
needed to carry the load put upon it. There is nothing whatever gained
by the use of larger fuses. If the load is not in excess of six amperes,
a six ampere fuse will carry it as readily as a twenty ampere fuse. If a
short circuit occurs the twenty ampere fuse will blow as readily as the
six ampere fuse and, in addition, will cause destruction of a socket or
whatever may be the cause of the trouble as well as an increased fire
hazard. Notwithstanding the above, it is a rare exception when one finds
the circuits of a traveling outfit properly fused. For this there are
two reasons. One is that, owing to frequent handling and moving, fuses
work loose; and thus, to the natural heat produced by the load, there is
added a certain amount of heat produced in the poor contacts. Trouble of
such a nature can be lessened by the use of larger fuses, but only to a
very small extent. It is far better to go over one's fuses two or three
times a week and tighten the screws and contacts to avoid this trouble
than to use over-large fuses.

Another reason for the overfusing, of plugging boxes especially, is
found in the fact that arc and incandescent plugs are interchangeable.
If, therefore, each circuit is fused at thirty amperes one need not
worry about which pocket an arc lamp is connected to. This practice is,
however, very bad, a table lamp, for instance, with a thirty-ampere fuse
behind it would be likely to cause flash and smoke enough to alarm an
audience. Some provisions should be made so that one can easily
distinguish between arc and incandescent pockets or divisions in
plugging boxes. The main requisite which determines the style of fuse to
be installed for stage effects is that of quick renewal in case of
blow-out.

[Illustration: FIGURE 199.]

_Gas Globes._--Electric light is so convenient on the stage that even
gas light is imitated by it. In one production in which it was desirable
to show the front of a theater illuminated by gas, the gas globes were
made up of cloth cut as shown in Figure 199. Behind each imitation globe
a small incandescent lamp was arranged.

_Glass Shades._--An imitation of these for small desk or table lamps is
often made by placing a strip of colored gelatine between two layers of
wire mesh. The wire screen keeps the gelatine straight and the whole is
more serviceable than glass and less liable to break.

_Heaters._--Under this head come flatirons and water heaters. Flatirons
are carried by many actors who cause much trouble by connecting them to
lighting circuits. In many places the house electrician prevents their
use by fusing his lighting circuits so low that the addition of a
flatiron immediately blows the fuse. If used at all, the flatirons
should be of the smaller kind. Small heaters are also extensively used
upon the stage.


_National Electrical Code Rule for Electric Heaters._

It is often desirable to connect in multiple with the heaters and
between the heater and the switch controlling the same, an incandescent
lamp of low-candle power, as this shows at a glance whether or not the
switch is open, and tends to prevent its being left closed through
oversight.

Must be protected by a cut-out and controlled by indicating switches.
Switches must be double-pole except when the device controlled does not
require more than 660 watts of energy.

Must never be concealed; but must at all times be in plain sight.
Special permission may be given in writing, by the Inspection Department
having jurisdiction, for departure from this rule.

Flexible conductors for smoothing irons and sadirons and for all devices
requiring over 250 watts must have an approved insulation and covering.

For portable heating devices the flexible conductors must be connected
to an approved plug device, so arranged that the plug will pull out and
open the circuit in case any abnormal strain is put upon the flexible
conductor. This device may be stationary, or it may be placed in the
cord itself. The cable or cord must be attached to the heating
apparatus in such a manner that it will be protected from kinking,
chafing, or like injury at or near the point of connection.

Smoothing irons, sadirons, and other heating appliances that are
intended to be applied to inflammable articles, such as clothing, must
conform to the above rules so far as they apply. They must also be
provided with an approved stand, on which they should be placed when not
in use.

Stationary electric heating apparatus, such as radiators, ranges, plate
warmers, etc., must be placed in a safe location, isolated from
inflammable materials, and be treated as sources of heat. Devices of
this description will often require a suitable heat-resisting material
placed between the device and its surroundings. Such protection may best
be secured by installing two or more plates of tin or sheet steel with a
one-inch air space between or by alternate layers of sheet steel and
asbestos with a similar air space.

Must each be provided with name-plate, giving the maker's name and the
normal capacity in volts and amperes.

_Lighthouse Effect._--To increase the effect of a lighthouse painted
upon the scenery, a strip of suitable length with one light at each end
is provided. The strip circuit contains a flasher so that both lights
are turned on and off at regular intervals. One lamp must be behind the
head of the lighthouse, the other where the reflection of the upper
light would appear in the water.

_Lightning._--The standard method of producing lightning effects upon
the stage is by means of the device shown in Figure 200. A magnet and
the arc-striking carbons are arranged in series and take current through
a resistance. Normally the carbons are in contact and when the switch is
closed the current passes through the magnet and carbons. The current
energizes the magnet and causes the upper carbon to separate from the
lower, thus striking the arc. The arrangement must be such that the arc
is immediately broken, thus de-energizing the magnet; this allows the
movable carbon to close the circuit again, thus repeating the operation.
This device if uninterrupted will give a rapid succession of lightning
effects and it may be suspended in the wings as it is entirely
automatic. At the right of Figure 200 is shown another lightning striker
which is operated by hand.

[Illustration: FIGURE 200.]

Where it is not prohibited, a very good lightning effect can be obtained
by arranging a file and a piece of carbon as opposite poles of the
circuit and scraping them together. It is a bad practice, however, to
strike an open arc on any stage. There should always be wire mesh around
it to prevent the escape of sparks.

Automatic strikers can be purchased which will make an open arc lamp
produce good lightning effects. Another method of producing lightning is
that of flashing some of the border lights. This will do in an emergency
but does not compare with the other methods. Sometimes the effect of a
bolt of lightning striking a certain place is imitated by an arc sliding
swiftly down a wire. In order to strike the arc at the proper moment,
the apparatus being somewhere up in the flies, a small fuse is connected
from one carbon to the other. The current when turned on passes through
the fuse, melts it, and establishes the arc. The carbons are incased in
a very small wire-mesh enclosure and the whole device is made to slide
swiftly down a tight wire leading to the proper place. The more
pretentious shows often use several lightning machines to avoid the
monotonous effect of all lightning strokes coming from the same
direction. One resistance may be made to serve for any number of
lightning machines provided only one machine is used at a time.

[Illustration: FIGURE 201.]

_Lugs._--Lugs must be provided for all leads where more than 30 amperes
are used. It is even more necessary to follow this rule in connection
with portable theater equipment than with other wires because they are
more apt to work loose.

For the connections to the arc lamp and rheostats it is not practicable
to use soldered lugs because no solder will stand the heat. Two styles
of lugs that may be used are shown in Figure 201. The cuts will also
explain the manner of connecting the lugs. At the left, the bare lugs
are shown; in the center the copper of the conductors is shown inserted
into the lugs, and at the right the whole conductor is shown bent back,
the metal of the lug catching over the insulation to keep it from
fraying out. The wires feeding arc lamps are always asbestos covered and
this covering fits loosely to the wire and must be held in some such
manner. A stock of such lugs should always be carried, as the leading-in
wires to arc lamps, using heavy currents, frequently burn off.

[Illustration: FIGURE 202.]

_Moon Boxes._--These boxes are generally made up as shown at the left in
Figure 202. The opening is from twelve to fourteen inches in diameter.
Four lamps give very good illumination. The frame must be provided with
hooks so that it may be suspended at any desired part of the scenery. It
is always hung close behind a curtain and must be so arranged that the
lamps cannot touch the curtain. The wires should be entirely enclosed
in metal of No. 20 sheet metal gauge after the manner of strips. Very
cheap productions often carry moon boxes made up of an incandescent lamp
fastened in a tin dish, similar to the one shown at the right in Figure
202.

_Moonlight on Water._--This and all similar effects can be produced
through the "Sciopticon" but this requires the use of an arc lamp and is
expensive. A cheap way of producing the effect is illustrated in Figure
203, which represents a metal cylinder, perforated as shown, inside of
which there are a number of incandescent lamps colored to suit. The
cylinder is slowly revolved and the moving shadows and streams of light
thrown upon the scene produce the effect.

[Illustration: FIGURE 203.]

[Illustration: FIGURE 204.]

A somewhat similar method of producing this effect, as also that of a
waterfall, is illustrated in Figure 204. In this case a spiral capable
of being moved by clock work is revolved about an electric lamp and
gives the effect of falling water. Both of these devices work behind
curtains.

_Musical Acts._--There are numerous musical acts upon the road in which
the beat of the music is emphasized by lighting colored lamps in time
with the tunes. It is advisable to arrange all such acts to operate by
batteries. In many places inspectors will not allow high-voltage
contacts to be used open, and it is often difficult to enclose them in a
practical manner. By using dry batteries the apparatus is taken out of
the jurisdiction of inspectors and all danger of fire is also
eliminated.

_Musical Bells._--Sets of musical bells properly tuned and connected to
a keyboard from which they can be operated electrically are carried in
stock by dealers in theatrical supplies. Sets are usually made up of
eighteen bells and they may be played from a distance.

_Oil Lamps._--These have been imitated by arranging a small battery
inside of the base of the lamp. This battery would supply current only
during the short time that the lamp was being carried about. The base of
the lamp is also fitted with electrical contacts which rest upon similar
contacts fitted into the table, and through which the lamp is kept
burning by a storage battery while it is resting upon the table. A small
dry battery, such as is placed in the base of an oil lamp, would not
keep the light burning for a long enough time.

_Pin-Plug Connectors._--These are provided to connect cables to effects
used on the stage. The essential points of a good connector are: It
should grip the whole cable, thus making it unnecessary to strip the
outer braid from any exposed part of the cable; and it should offer no
live screws or contacts on the outside. These conditions are fulfilled
by the connector shown in Figure 205. In some of these connectors,
however, the part that grips the cable (indicated by the arrow) is made
of metal, and this, if forced down too tight, may cut into the cable
and cause a short circuit. Several instances of this kind have been
known to occur. Pin-plug connectors must always be connected so that the
projecting pins are dead when the plug is disconnected.

[Illustration: FIGURE 205.]

[Illustration: FIGURE 206.]

A special warning against the use of such connectors as are illustrated
at the right of Figure 205 is necessary. Their cheapness brings them
into use very often, especially with acts that do not carry competent
electricians. The wires are always working loose; projecting ends are
bare, and if the connector is twisted, a short circuit results. Where
they must be used in an emergency, they should be heavily taped, the
tape being placed between the conductors and all around the plug, as
shown in the figure.

Branch-off connectors such are shown in Figure 206 are often used to tap
off lights from the main cable. Where plug connectors are used for arc
lamps they must be made up as shown in Figure 207. The asbestos-covered
wires necessary to be used with arc lamps cannot be used in cable but
must be separately connected.

[Illustration: FIGURE 207.]


_The National Electrical Code Rules for Pin-Plug Connectors._

Must be of an approved type, so installed that the female part of the
plug will be on the live end of the cable, and must be so constructed
that tension on the cable will not cause serious mechanical strain on
the connections.

_Plugging Boxes._--Plugging boxes are used to obviate the necessity of
running a large number of wires over the stage to the different stage
pockets. They are also a necessity because many theaters are not
equipped with a sufficient number of stage pockets to accommodate all of
the devices large productions carry. The plugging box should be
fireproof; should have self-closing doors so that there may be no
liability of open fuses being left among the scenery where they are
used. Most of the plugging boxes in use at the present time are arranged
for link fuses. A link fuse, however, requires more time to install than
a cartridge fuse and the latter would therefore seem much more
desirable. A cut of a much-used plugging box is shown in Figure 208.

[Illustration: FIGURE 208.]


_The National Electrical Code Rules for Portable Plugging Boxes._

Must be constructed so that no current-carrying part will be exposed,
and each receptacle must be protected by approved fuses mounted on slate
or marble bases and enclosed in a fireproof cabinet equipped with
self-closing doors. Each receptacle must be constructed to carry thirty
amperes without undue heating, and the bus-bars must have a carrying
capacity equivalent to the current required for the total number of
receptacles, and approved lugs must be provided for the connection of
the master cable.

_Rheostats._--Portable stage equipment being intended for use in all
parts of the country must be adjustable to all kinds of lighting
systems. For this reason it is not practicable to use auto-transformers
as is commonly the case where arc lamps are fed from alternating-current
circuits. Simple rheostats or resistances are therefore always used.
With these it is immaterial whether the current be alternating or direct
except in so far as the quantity is concerned.

Alternating current being less efficient in the production of useful
light than direct current, the amperage must be greater and this
produces a greater loss in voltage, the loss being always proportional
to the product of current times resistance. The drop in voltage over a
certain resistance is therefore always greater, when an
alternating-current arc is used, than it is with direct; but as the
alternating-current arc operates at a lower voltage than the direct this
is not objectionable and simple resistances give satisfactory results
whether used with one kind of current or the other. The traveling show
must also be prepared for different voltages and a suitably arranged
group of resistances is very convenient in this respect.

Figure 209 shows a box made by the Chicago Stage Lighting Company
containing a number of resistances which may be connected in various
ways to obtain different results. In Figure 210 at _A_ is given a
diagram representing the wire in a single section. One such section
placed in series with an arc lamp will give a certain current. If we
connect two of them in parallel, as in _B_, we shall obtain not quite
double the current, since this arrangement does not affect the
resistance of the arc which also influences the current strength. If the
connections are made as in _C_, we shall have but half the current
obtainable by the use of one section, or the same current in case we are
dealing with double the voltage. If _A_ is suitable for 110 volts, then
_C_ is suitable for 220 volts and _D_ for 550. In _E_ we have the
connection which would give us, for 220 volts, the same current _B_
would give with 110. It can thus be seen that a group of such
resistances can be made to fit an arc lamp to almost any voltage that is
likely to be met with.

[Illustration: FIGURE 209.]

Nothing but metal and porcelain must be used in the construction of
these resistances and they must be on stands which raise them well above
the floor. The connections to the resistances should ordinarily be made
with asbestos cable. It is necessary to keep close watch of the wire
used to make up the resistance especially if it becomes very hot during
a run. A resistance in which the wire becomes red hot should not be
used. There are productions in which it is necessary to place the arc
lamps and their rheostats directly in the way of flying paper used to
make "snow". In such cases the resistances must be tightly enclosed.
This destroys the ventilation and they will readily become overheated.
For use with such productions resistances may be specially wound and
arranged, or they may be made separate from the arc and placed in a safe
location.

[Illustration: FIGURE 210.]

_Stage Cables._--Stage cables are subject to very rough usage, but there
are some very good makes on the market, and by purchasing the best, one
can expect long service. Cuts of various kinds of cable likely to be
used upon the stage are given in Figure 211. _A_ is the ordinary stage
cable; _B_ is used for heaters; _C_ is reinforced cord; and _D_ is the
parallel cord. The last two must not be used on the floor. The size of
the wire to be used depends upon the apparatus which the cable must
supply and can be determined from Table VII which gives the carrying
capacities. Only approved cables should be used.

[Illustration: FIGURE 211.]

_Strips._--Strips are used for the illumination of special parts of the
scenery. They usually contain from one to twelve lights. A strip is
subject to rough usage and should be strongly made. Cuts of several
kinds of strips are given in Figure 212. The chief trouble with strips
is found to be in the working loose of screws which hold the sockets in
place, and the connection at the point at which the cable leaves the
strip.


_The National Electrical Code Rules for Strips._

Must be constructed of steel of a thickness not less than No. 20 U. S.
sheet metal gauge, treated to prevent oxidation, and suitably stayed and
supported and so designed that flanges will protect the lamps.

[Illustration: FIGURE 212.]

Cables must be bushed in a suitable manner where passing through the
metal, and must be properly secured to prevent serious mechanical strain
on the connections.

Must be wired in approved conduit or armored cable, each lamp receptacle
being enclosed within an approved outlet box, or the lamp receptacles
may be mounted in an iron or steel box, metal to be of a thickness not
less than No. 20 U. S. sheet metal gauge, treated to prevent oxidation,
so constructed as to enclose all wires. Wires to be soldered to lugs of
receptacles.

[Illustration: FIG. 213.]

_Sunrise._--Sunrise effects may be produced by means of the Sciopticon,
but a cheaper method is often resorted to. This is illustrated in Figure
213 and consists of a roll of suitably colored silk which may be
unrolled in front of an arc lamp. To produce sunrise it is unrolled in
one direction, the darkest colors coming first and gradually giving way
to lighter. For sunset it is worked the other way. In special cases a
similar sheet of colored silk wide enough to cover open arcs has been
stretched from the floor to the top of the scenery and several arc lamps
arranged behind it in such a manner that they could be gradually raised,
thus producing the same effect with more impressiveness, but with a much
greater quantity of light.

[Illustration: FIGURE 214.]

[Illustration: FIGURE 215.]

_Switches._--Knife switches on the stage are generally required to be
enclosed. It is not sufficient to provide a box with a cover that can be
closed, but the switch should be in a box which need not be opened to
operate the switch. See Figure 214. Often it is necessary to install
switches in a length of stage cable so that the switch may be portable.
Such switches should be installed as shown in Figure 215, in which a
flush switch is shown in a regular metal enclosure.

[Illustration: FIGURE 216.]

_Table Lamps._--The table lamp is a small device, but it causes a great
deal of annoyance and trouble. The bottom of the lamp should be enclosed
with metal, especially if there are any splices. The stem where the wire
enters should be well reamed out (see the arrow in Figure 216) and there
should be a strong well-fastened bushing where the wire leaves the lamp.
The hard-rubber bushings which come with the common table lamps do not
remain in place very long and are frequently broken or lost. Stage cable
is generally required upon the stage, but it is customary, in connection
with these lamps, to allow a good grade of reinforced cord or even fine
silk-covered cord, provided it is not long enough to trail on the floor
where it might be stepped upon. The glass shades which are often called
for in connection with the lamps also cause much trouble; and to avoid
this trouble, colored gelatine has been used, backed up with fine wire
screen to keep it in place. This of course cannot always be done because
many lamps are not suited to it.

_Time Beaters._--This is a small keyboard located at the orchestra
leader's stand; it controls the light which may be back of the stage and
indicates the time to the chorus behind scenes.

_Water Falls._--The devices shown in Figure 204 for use as moonlight on
the water can also be used to produce the effect of a waterfall or of
water rippling downstream.




CHAPTER XX.

USEFUL FACTS AND FORMULAS.


One turn of the crank runs off 1 foot of film.

One foot of film contains 16 pictures.

The audience sees about 960 different pictures in every minute that a
film is being run.

The relative number of revolutions made by two pulleys connected
together by belting or friction is in proportion to their diameters.

The relative number of revolutions made by two gears connected together
is in proportion to their number of teeth.

A theater seat occupies from 4 to 5 square feet.

One 25-watt tungsten lamp or its equivalent provided for every 20 seats
will give fairly good illumination in a small theater.

3 to 5 watts per cubic foot will be required to heat small spaces by
electricity.


_Ohm's Law._

Direct current equals e.m.f. divided by resistance.

Alternating current equals e.m.f. divided by impedance.

Electromotive force, d.c., equals current time resistance.

Electromotive force, a.c., equals current time impedance.

Resistance equals e.m.f. divided by current.

Impedance equals e.m.f. divided by current.

The joint resistance of two conductors connected in parallel is equal
to the product of their resistances divided by their sum:

  _r_ = (_r__{1} x _r__{2}) / (_r__{1} + _r__{2})

The joint resistance of any number of resistances connected in parallel
is the reciprocal of the sum of the reciprocals. The reciprocal of a
number is 1 divided by that number:

                          1
  _r_ = -----------------------------------
           1         1         1
        ------- + ------- + ------- + .....
        _r__{1}   _r__{2}   _r__{3}

The total resistance of a number of resistances in series is equal to
the sum of all of them:

  _r_ = _r__{1} + _r__{2} + _r__{3} + .....

The heating of a rheostat is proportional to the square of the current
it carries.

Drop in voltage is proportional to the product of the current and
resistance for a direct-current circuit, and the product of current and
impedance for an alternating-current circuit.

If a reactance is used in place of a rheostat in an alternating-current
circuit, the loss of energy is greatly reduced.

If the same intensity of illumination is to be provided for pictures of
different sizes, the amperage must vary about as:

  (_W_ x 1.2)^{2}

where _W_ is the width of picture on screen.

For a given power the current in conductors is:

                               _W_
  for direct current     _I_ = ---
                               _E_


                                   _W_
  for single-phase a.c.  _I_ = -------------
                               _E_ x _p. f._

                                         _W_
  for two-phase a.c.     _I_ = .5 x -------------
                                    _E_ x _p. f._

                                          _W_
  for three-phase a.c.   _I_ = .58 x -------------
                                     _E_ x _p. f._

in which _I_ is the current in amperes, W is the watts, E is the
voltage, and p. f. is the power factor.


_Lens Formulas._

To find the size of a picture obtainable under given conditions and
lens: _Multiply distance from center of lens to screen by one dimension
of slide or film and divide by e.f. focal length of lens, taking all
measurements in inches._

Example: Slide 2-3/4 inches. Length of throw 360 inches. e. f. 10 inches

2-3/4 x 360 / 10 = 99 inches.

To find focal length needed for a given slide or film to produce a given
size of picture: _Multiply slide or film dimension by length of throw
and divide by dimension of picture, taking all measurements in inches._

Example: Same dimensions as above:

2-3/4 x 360 / 99 = 10 inches.

To find length of throw needed to obtain a certain size of picture:
_Multiply required picture dimension by focal length of lens and divide
by slide or film dimension._

Example: Same dimensions as above:

99 x 10 / 2-3/4 = 360 inches.

To find slide size necessary to produce a certain size of picture under
fixed conditions: _Multiply focal length of lens by size of picture and
divide by length of throw._

Example: Same dimensions as above:

10 x 99 / 360 = 2-3/4 inches.


_Measurement of Surfaces._

To find the area of a parallelogram: _Multiply the base by the
altitude._

To find the area of a triangle when base and altitude are given:
_Multiply the base by the altitude and take half the product._

To find the area of any angular surface: _Divide it into triangles and
find the area of the different triangles and add them together._

To find the circumference of a circle: _Multiply the diameter by [pi];
or 3.1416._

To find the diameter of a circle when the circumference is given:
_Divide the circumference by [pi], or 3.1416._

To find the area of a circle when the radius is given: _Multiply the
square of the radius by 3.1416._ When diameter is given: _Multiply the
square of the diameter by .7854._

To find the radius of a circle when the area is given: _Divide the area
by 3.1416 and extract the square root of the quotient._


_Measurement of Solids._

To find the lateral area of a right prism: _Multiply the perimeter of
the base by the altitude._

To find the lateral area of a right cylinder: _Multiply the
circumference of the base by the altitude._

To find the volume of a cylinder or prism: _Multiply the area of the
base by the altitude._

To find the lateral area of a right pyramid: _Multiply the perimeter of
the base by the slant height and take half the product._

To find the lateral area of a cone: _Multiply the circumference of the
base by the slant height and take half the product._

To find the volume of a pyramid or cone: _Multiply the area of the base
by the altitude and take one-third of the product._

To find the surface of a sphere: _Multiply the square of the diameter by
3.1416._

To find the volume of a sphere: _Multiply the cube of the diameter by
one-sixth of 3.1416 or .5236._

In a right-angled triangle, the sum of the squares of the base and the
perpendicular equals the square of the hypotenuse. To find the length of
the hypotenuse: _Extract the square root of the sum of the squares of
the base and the perpendicular._

To find the base or the perpendicular: _From the square of the
hypotenuse subtract the square of the other given side and extract the
square root of the remainder._




CHAPTER XXI.

GLOSSARY OF ELECTRICAL, MECHANICAL, AND OPTICAL WORDS, TERMS, AND
PHRASES.


=Aberration.=--The convergence to different foci, by a lens or mirror,
of rays of light emanating from one and the same point or the deviation
of such from a single focus.

=a. c.=--An abbreviation for alternating current.

=Accumulator.=--A term often used to designate a storage battery.

=Achromatic.=--Free from false coloration.

=Achromatic Lens.=--A lens which does not show color in the light which
it projects; usually a combination of lenses of different kinds of
glass.

=Actinic Rays.=--The rays of light that cause chemical changes.

=Airdome.=--A theater arranged in the open air.

=Alloy.=--Any compound of two or more metals.

=Alternating Current.=--An electric current undergoing periodical
changes in direction.

=Amalgam.=--A compound of mercury with other metals.

=Ammeter.=--An instrument used for measuring the value of an electric
current.

=Amperage.=--The number of amperes; the current strength in the circuit;
not the quantity but the rate of flow.

=Ampere.=--The unit of current strength. It is equal to a rate of flow
of electricity of one coulomb per second.

=Ampere Hour.=--One ampere flowing for one hour; or its equivalent.

=Anamorphosis.=--A grossly distorted image, such as can be obtained from
a curved mirror.

=Angular Velocity.=--The rate at which a body revolves around a fixed
axis, usually measured in radians per second. The radian is equal to 360
/ 2 x [pi] degrees.

=Annular Space.=--The space between an inner and an outer ring or
cylinder.

=Annunciator.=--An electric device provided with one or more magnets and
indicators, which serve to indicate the points at which one or more
electric contacts have been closed.

=Anode.=--The positive pole; a term used mostly with electrolytic
devices, also with the mercury arc rectifier and the electric arc.

=Aplanatic.=--Free from spherical aberration.

=Apparent Power.=--The product of volts and amperes in an
alternating-current circuit. The true power in an alternating-current
circuit is always less than the apparent power if pressure and current
are not in phase.

=Arbor.=--An axle or spindle on which a wheel turns.

=Arc, Electric.=--The heated portion of an electric circuit between two
electrodes where the current passes from one to the other; the source of
light of the electric arc lamp.

=Armature.=--That part of a generator, in which the electrical pressure
is generated, which causes a current if the armature winding forms part
of a closed circuit.

=Automatic.=--A term applied to machinery in which certain movements,
usually made by hand, are made by the machine.

=Auto-Transformer.=--A transformer provided with only one winding; a
part of the winding being traversed by both the primary and secondary
currents.

=Axis.=--The straight line, real or imaginary, which passes through a
body, and on which the body revolves, or may be supposed to revolve.

=Axis of Lens.=--The straight line passing through its center and
perpendicular to its surface.

=Axle.=--A transverse bar connecting opposite wheels.

=Babbitt Metal.=--A soft white antifriction metal of varying
composition; as 4 parts of copper, 8 of antimony, and 24 or 96 of tin.
The larger the amount of tin, the harder the resultant alloy.

=Baby Spot Lamp.=--An incandescent lamp placed within a hood similar to
that of a theater arc lamp; also usually provided with a lens.

=Back Focus.=--This term is used to designate the distance from the lens
nearest the slide to the position of the slide when the picture is in
focus on the screen. See also Equivalent Focus.

=Balance Wheel.=--A wheel added to machinery for the purpose of
preventing too sudden variations in speed; also called fly wheel.

=Ball Bearing.=--A journal fitted with balls upon which the shaft turns.

=Balsam, Canadian.=--A cement used to cement lenses, etc.

=Band.=--A small belt passing over two pulleys.

=Battery.=--A term used to describe a number of voltaic cells connected
so as to act as a single cell; should never be used in reference to a
single cell.

=Bayonet.=--A pin which plays in and out of holes made to receive it and
serves to engage parts of machinery.

=Bearing.=--That part of a machine in contact with which a shaft moves;
the journal boxes.

=Bed.=--The foundation or main fixed part of a machine.

=Bell Crank.=--A rectangular lever by which the direction of rotation is
changed through an angle of ninety degrees.

=Belt.=--A strap or band of flexible material passing around two or more
wheels and communicating motion to them.

=Bevel Gear.=--A gear in which the two wheels working together lie in
planes that are not parallel.

=Bifocal.=--Having two foci.

=Binocular.=--Pertaining to both eyes, as binocular vision in microscope
or telescope.

=Blinding Lights.=--Lights used on the stage in certain acts to blind
the audience; thus allowing persons to work behind them unseen by the
audience.

=Boss.=--The enlarged part of a shaft on which a wheel is keyed.

=Brace.=--A curved tool of iron or wood adapted to turn and hold bits.

=Brake.=--A piece of mechanism adapted to stop or hold parts of
machinery.

=Brushes.=--That part of a dynamo by which the current is taken to or
from the commutator.

=Buffer.=--A cushion, with springs or other material, arranged to deaden
the blow of a moving body.

=Buffing Wheel.=--A rapidly revolving wheel adapted to buff or polish
materials.

=Burner; Automatic, Electric.=--An electric device, for turning on and
lighting gas, which may be operated from a distance by pressing a
button.

=Bushing.=--The lining of a hole with a ring or a perforated substance
of metal or other material.

=Calipers.=--A compass with curved legs; used for measuring round bars
or openings, the thickness of objects, and the distance between two
points.

=Cam.=--A projecting part of a wheel or other moving piece, so shaped as
to give an alternating or variable motion to another piece pressing
against it.

=Cam Wheel.=--A wheel, or part of a wheel, of irregular outline employed
to produce a variable or alternating motion in machinery.

=Canadian Balsam.=--A cement used to cement lenses, etc.

=Candle Power.=--This term, as commonly used, denotes the light-giving
power of a lamp in terms of a standard candle. Its meaning is not very
definite unless qualified by some of the following terms: apparent
candle power; equivalent candle power; mean horizontal candle power;
mean lower hemispherical candle power; mean spherical candle power;
maximum candle power; mean zonular candle power. All of these terms are
fully explained in "Modern Electric Illumination, Theory and Practice."

=Carbons.=--The rods of artificial carbon used as electrodes in arc
lamps.

=Carbons, Cored.=--Carbons, as above, but provided with a core of softer
material.

=Case Hardening.=--The act or process of converting the surface of iron
into steel while leaving the center soft and malleable.

=Cathode.=--The negative pole. A term usually used in connection with
electrolytic apparatus, mercury arc rectifiers, and electric arcs.

=Cell, Voltaic.=--A cup or jar containing electrodes and an electrolyte
either for producing an electromotive force or electrolysis. In the
latter case it is called an electrolytic cell. A number of cells
connected together make a battery.

=Centi.=--As a prefix it means the hundredth part; as centimeter.

=Choke Coil.=--A coil of wire wound upon an iron core in such a manner
as to acquire a very high self-inductance; similar to a reactance.

=Chuck.=--A contrivance fixed to the mandril of a turning lathe for
holding the material to be worked upon.

=Clutch.=--A projecting tooth, or other piece of machinery, for
connecting shafts with each other or with wheels in such a manner as to
be readily disengaged.

=Cog Wheel.=--A wheel with cogs or teeth.

=Collar.=--A ring-like part of a machine usually fastened to some part
of it and used for keeping something in place.

=Commutator.=--That part of a dynamo-electric machine to which
all of the armature wires are connected and in which the
alternating-electromotive force, actually generated in the armature
winding, is made to appear as a direct-electromotive force in the
external circuit.

=Compasses.=--An instrument for describing circles, measuring, etc.,
consisting of two pointed branches fastened together at the top by a
rivet or screw.

=Compensarc.=--A trade term used to describe a transformer or an
auto-transformer; used in connection with projection-arc lamps.

=Compound Winding.=--A generator, in which the fields are provided with
a shunt winding and in addition with a series winding which carries all
of the current around the fields; is said to be _compound wound_, and
the winding is referred to as a _compound winding_.

=Concave.=--Curved and hollow; the inside of a hollow sphere or
cylinder.

=Concentric.=--Having a common center.

=Condenser.=--A lens or a set of lenses, used to gather the divergent
rays from a source of illumination and bring them to a focus.

=Conductor.=--Any substance used to carry electricity. Most of the
commercial conductors are of copper.

=Cone.=--A solid body having a plane for its base and a point for its
top, the surface being formed by line segments joining every point of
the outline of the base to the point top.

=Conical.=--Approximating a cone in shape.

=Conjugate Foci.=--The two points in the principal axis of a lens and at
opposite sides of it; one point being the light source and the other the
point at which rays of light from this source are brought to a focus.

=Controller.=--A resistance used for controlling the speed of a motor or
the field strength of a dynamo.

=Converter.=--Any apparatus used for changing from one form of current
to another; generally used to indicate a synchronous or rotary
converter.

=Convex.=--Rising or swelling; the outside of a sphere or cylinder.

=Convexo-Plano.=--Convex on one side and plane on the other;
plano-convex is more often used.

=Cooling Plate.=--The metal around the projection aperture of a
motion-picture machine which protects the film from the rays of light.

=Cotter.=--A wedge-shaped piece of wood, iron, or other material used
for fastening the parts of a structure; a key.

=Cotter Pin.=--A pin made up of doubled wire expanded at top and bottom
so as to hold its place when inserted in a hole.

=Coulomb.=--The unit of quantity of electricity. It is equal to the
current multiplied by the time that it flows; viz., ampere times second.

=Countershaft.=--A secondary shaft driven from another shaft; not the
main or driving shaft.

=Countersink.=--A drill or cutting tool used to enlarge the outer edges
of a hole in countersinking.

=Crown Wheel.=--A wheel, with cogs or teeth set at right angles to its
plane.

=Cycle.=--A complete recurrence of any periodic change. The two
immediately succeeding half waves of an alternating current or e.m.f.
constitute a cycle.

=Damping Coils, Dampers.=--Coils of wire or bars of copper placed on, or
imbedded in, the pole pieces of a dynamo-electric machine; used mostly
in connection with synchronous motors or rotary converters to prevent
hunting.

=Dead Center.=--Either of the two opposite points, in the orbit of a
crank, at which the crank and a driving rod lie in a straight line.

=Diffusing Globes.=--Globes of ground glass or other medium which causes
diffusion of light. Used to reduce the intrinsic brilliancy of
illuminants. The light is emitted from a larger surface and is less
trying to the eyes.

=Diffusion.=--The scattering of light by diffuse reflection, as from an
irregular surface such as ground glass.

=Diffusion Projection.=--A method of projection sometimes resorted to
when condensers are broken. Ground glass is used in place of the
condensers.

=Direct Current.=--A current of electricity maintained in one direction
as distinguished from an alternating current which is regularly
alternating in direction.

=Dog.=--A part of machinery acting as a catch or a clutch.

=Dovetail.=--To unite by a tenon, in the form of a pigeon's tail spread
out, let into a board or timber.

=Dowel Pin.=--A pin of wood or metal used for joining two pieces; part
of its length entering one piece, and the rest entering a corresponding
hole in the other.

=Dowser.=--A hand shutter arranged in front of the lens of an arc lamp
by which the light may be shut off.

=Drift.=--A conical tool of steel for enlarging or shaping a hole in
metal by being driven into or through it.

=Drill.=--A pointed instrument used for boring holes; especially in hard
substances.

=Drop in Potential.=--A term often used to designate a loss of
electrical pressure. In d.c. circuits it is equal to current times
resistance. In a.c. circuits it is equal to current times impedance.

=Drum.=--A short cylinder revolving on an axis.

=Dynamo.=--A dynamo electric machine for converting electrical energy
into mechanical, or vice versa. When the transformation is from
electrical to mechanical energy the machine is operating as a motor; and
when the transformation is from mechanical to electrical energy the
machine is operating as a generator.

=e. f.=--Abbreviation for equivalent focus.

=e. m. f.=--Abbreviation for electromotive force. The unit of
electromotive force. The unit of electromotive force is the volt.

=Eccentric.=--A wheel or disc having its axis of revolution out of its
center; used to convert circular motion into reciprocating.

=Economizer.=--A trade name applied to a transformer or
auto-transformer, used in connection with projection-arc lamps.

=Economy Coils.=--An auto-transformer. This term is used very much in
connection with auto-transformers, and also transformers used in
connection with arc lamps.

=Efficiency of Incandescent Lamps.=--The efficiency of incandescent
lamps is expressed in watts per candle power. The lower the numerical
expression for efficiency, the greater is the light obtained per unit of
energy.

=Electrodes.=--The terminals of an electric source; in an arc lamp, the
two carbons.

=Electrolysis.=--Chemical decomposition by an electric current. The
positive pole, or anode, is eaten away with direct current; the effect
with alternating current is very slight.

=Emergency Lights.=--Those lights arranged in theaters, the object of
which is to furnish the necessary illumination by which the audience may
leave the theater in case of a failure of the regular illumination.
Emergency and exit lights are generally grouped together but must not be
confused with each other.

=Equivalent Focus.=--The distance from the center of a set of lenses to
the slide, when the picture on the slide is in focus on the screen.

=Exit Lights.=--The lights placed directly above exits; usually colored
red.

=Expansion Bolt.=--A bolt adapted to be inserted into a socket which,
expanded by the insertion, thus holds the bolt in place.

=Face Plate.=--The disc attached to the revolving spindle of a lathe to
which the work is often fastened.

=Feeders.=--The main wires extending from the supply to the distributing
center.

=Field.=--A term commonly used to designate the space traversed by the
lines of the force of magnetism which is supposed to exist. The fields
of a dynamo consist of the pole pieces around which the exciting current
circulates.

=Fire Trap.=--The arrangement of rollers and their enclosure, usually
placed on magazine or film tanks, through which the film is led, to
prevent a flame from following the film in case of fire.

=Flange.=--A projecting edge or rim on a wheel or shaft by which it may
be kept in place or fastened.

=Flasher.=--The term by which a contrivance for regularly, and at
frequent intervals, turning lights on and off is described; used mostly
in connection with electric signs.

=Flicker.=--The unsteady illumination of a motion picture; caused mostly
by insufficient speed of film or too brilliant illumination.

=Fly Floor.=--A floor above the stage upon which the scene shifters,
whose duty it is to raise and lower the curtains, work.

=Footlights.=--A row of lights placed at the stage level and directly in
front of the actors.

=Forging.=--A piece of metal shaped by hammering; not a casting.

=Framing Device.=--A device attached to every motion-picture machine by
which the relative position of the film to the optical system may be
adjusted to make the picture appear in its proper place on the screen.

=Frequency.=--The number of cycles per second; also spoken of as the
periodicity. The most common frequencies are 25 and 60 cycles per
second.

=Gearing.=--The parts by which motion given to one part of a machine is
transmitted to other parts. In a narrower sense it refers only to cog
wheels.

A _spur gear_ is one in which the teeth are arranged either on the
outside or inside of the rim in the direction of radii from the center
of the wheel.

A _bevel gear_ is one in which the shafts of the two gears are not
parallel.

A _friction gear_ is one in which friction takes the place of the teeth.

A _worm gear_ is one in which one of the members is in the form of a
screw. The pitch of the screw may, or may not, be such as to prevent it
from being turned by the other.

A _herring-bone gear_ is one in which the teeth are cut across the face
of the wheel at an angle. The teeth may be made to converge toward the
center of the face or may be cut in a line across it. This gear is used
where noiseless operation is required.

=Generator.=--A term frequently used to describe a dynamo-electric
machine used in converting mechanical energy into electrical energy.

=Geneva.=--A form of intermittent movement first used in Geneva,
Switzerland; hence the name. It is the form of intermittent mechanism
most used.

=Governor.=--A contrivance used in connection with machinery to regulate
the speed.

=Grid, or Grid Floor.=--This term refers to the topmost floor above the
stage of a theater where the pulleys and the cables, by which scenery is
raised and lowered, are placed. It is usually made up of iron bars with
spaces between them; hence the name.

=Ground.=---This term is used to describe any connection of an
electrical conductor to any conducting medium, other than that to which
it belongs, which may be common to other conductors or which may convey
current to the opposite pole in a manner not desired.

=Gudgeon.=--The piece of iron in the end of a wooden shaft on which it
turns in a collar or on a gudgeon block.

=Heat Shield.=--A part of a motion-picture machine which shields the
film and adjacent parts from the heat of the light.

=Helix.=--See Solenoid.

=Horsepower.=--A unit or standard by which the capability of machinery
is measured. Power sufficient to raise one pound 33,000 feet in one
minute is called one horsepower. Electrically it is equal to 746 watts.

=Hunting.=--This term describes the regular and persistent changes in
the speed of synchronous apparatus running in parallel, which are due to
fluctuations in the load on the generators or in the speed of the prime
mover; and causes cross currents. In theaters they would be noticeable
only in connection with rotary converters which are sometimes used to
convert a.c. into d.c. for arc lamps.

=Idler.=--A wheel placed between two other wheels to convey motion from
one to the other without changing the direction of motion.

=Impedance.=--Impedance is to an alternating-current circuit what
resistance is to a direct-current circuit. The e.m.f. divided by the
impedance equals the current. The symbol for impedance is _Z_.

=Impressed e.m.f.=--The electromotive force brought to bear on any
circuit to produce a current therein; the applied e.m.f.

=Index of Refraction.=--The index of refraction, or the refraction
index, is the ratio the sine of the angle of incidence bears to the sine
of the angle of refraction. For glass the index of refraction is about
1.5.

=Indirect Lighting.=--A system of illumination in which all of the light
is first thrown to the ceiling and reflected therefrom.

=Inductance.=--That property of an electric circuit which causes a
current in it to create lines of force and thus generate a counter
e.m.f. when the current changes in value.

=Induction Motor.=--A much used alternating-current motor in which there
is no electrical connection between the primary and secondary or rotor
and stator windings. It may be either single or polyphase.

=Intermittent Movements.=--There are numerous types of intermittent
movements but the Geneva and the pin cross are the only ones used with
the latest types of motion-picture projection machines. The other
movements have mostly come into use in connection with cameras. The
beater movement is one in which a certain length of film is pushed to
one side at each movement. The claw movement is one in which claws reach
up and pull down a fixed amount of film at each movement. Other
intermittent movements are known as, the drunken screw; ratchet; pitman
and grip. They have mostly only a historical interest.

=Intrinsic Brilliancy.=--This term refers to the candle power per unit
area of a source of light. A light having a large candle power in
proportion to its size is said to have a high intrinsic brilliancy.
Lamps of high intrinsic brilliancy should never be placed in the line of
vision.

=Inverted Lighting.=--Synonymous with indirect lighting.

=Key.=--A wedge-shaped piece of wood or metal used, to hold something in
place; a cotter.

=Key Seat.=--A groove cut into a shaft to fit a similar groove in a
wheel or flange, and into which a key may be fitted.

=Key to Curtain.=--A fictitious article, in quest of which novices
employed in theaters are often sent; a hoax.

=Kilowatt.=--1000 watts.

=Lagging Current.=--A current, the maximum value of which occurs later
than that of the e.m.f. producing it.

=Laminated.=--Made up of sheets. All iron parts subject to the influence
of alternating fields must be laminated to reduce the losses due to
Foucault or eddy currents.

=Leading Current.=--A current, the maximum value of which occurs earlier
than that of the e.m.f. producing it. In connection with synchronous
converters the current may be made either lagging or leading by
adjusting the field excitation.

=Lens.=--A piece of glass, or other transparent substance, ground with
two opposite regular surfaces. In practice the curved surfaces are
generally spherical but sometimes they are made cylindrical.

Of spherical lenses there are six varieties: _plano-concave_;
_double-concave_; _plano-convex_; _double-convex_; _meniscus_; and
_concave-convex_. A _double-convex lens_, with one radius six times that
of the other, is called a _crossed lens_.

A _multiplying lens_ or glass is one consisting of a number of plane
faces on one side and a convex surface on the other. The plane faces are
inclined to one another and each presents a different picture; thus
giving the appearance of the multiplication of images.

A _polyzonal lens_ is one that is made up of a large number of pieces
arranged in zones or rings. This kind of lens is used only for sizes in
which it is difficult to use pieces large enough to have the lens in
one piece.

=Lime Light.=--A light produced by means of a cylinder of lime which is
subjected to a flame of gas or of oxygen and hydrogen.

=Lines of Force.=--Every magnetic field is supposed to be made up of
imaginary lines called lines of force. The number of these lines per
unit area perpendicular to the field is a measure of the strength of the
field.

=Lost Motion.=--Refers to the loss or irregularity of motion caused by
wear of machinery or poor adjustment.

=Magic Pane.=--This is a condenser formed of a sheet of glass, one side
of which is covered by pieces of tin foil with spaces left between. The
spaces between may be arranged to represent any design and on the
discharge of the condenser the design is illuminated by the sparks which
jump over the spaces.

=Male Thread or Coupling.=--In connection with piping, hose, etc., a
coupling arranged to enter into another, is spoken of as "male"; that
part which is entered into is known as "female."

=Mandrel.=--A bar of metal inserted in the work to hold it, as in a
lathe, during the process of manufacture. Also the spindle which carries
the center chuck of a lathe, an arbor.

=Matched Lenses.=--The lenses in two stereopticon lamps to be used
together must be matched to give the same size of picture.

=Micrometer.=--An instrument used for measuring very small diameters or
thicknesses, the adjustments being made by a screw arranged to indicate
very slight changes in the opening in which the object is inserted.

=Mil.=--The one-thousandth part of an inch. The circular-mil area of a
circular area, such as the cross section of a wire, can be found by
squaring the diameter.

=Mirage.=--An optical illusion due to an unequal refraction in the lower
atmosphere which causes remote objects to be seen double and suspended
in the air. A mirage can be arranged for by a prism or by a piece of
beveled glass.

=Miter Wheels.=--A pair of bevel wheels of equal diameter working
together, usually with their axes at right angles.

=Motion Head.=--By this term the whole of the motion-picture machine is
usually designated.

=Motor.=--See dynamo.

=Motor Generator.=--A generator driven by a motor.

=Multiple Series.=--A system of wiring in which several lamps are
connected in series and a number of such series combinations connected
in multiple.

=Neutral Wire.=--The wire of a three-wire system which ordinarily
carries no current. In a three-wire system it is usually run in the
center.

=Ohm.=--The ohm is the unit of electrical resistance.

=Ohmic Drop.=--This is the loss of potential due to the resistance, as
distinguished from that due to the reactance. It is always equal to the
current multiplied by the resistance, the current being measured in
amperes and the resistance in ohms.

=Ohm's Law.=--This is a statement defining the relations existing
between electrical pressure, current, and resistance.

_The current is equal to the electrical pressure divided by the
resistance._

_The resistance is equal to the electrical pressure divided by the
current._

=p. d.=--An abbreviation for difference of potential.

=Pawl.=--A short movable piece or bar connected at one end, by a joint,
with some part of a machine; while the other end falls into notches or
teeth on another part, in such a manner as to permit motion in one
direction only.

=Period.=--The time required for one complete cycle.

=Photometer.=--An instrument used for comparing or measuring the candle
power of illuminants.

=Pinion.=--A small wheel with leaves or teeth working into the teeth of
a larger wheel or rack; especially such a wheel having its leaves formed
of the substance of the arbor or spindle on which it turns; a toothed
arbor.

=Pitch.=--The distance from center to center of any two adjacent teeth
of gearing measured on the pitch line. The distance measured on a line
parallel to the axis between two adjacent threads or convolutions of a
screw.

=Pitch Line.=--A circle concentric with the circumference of a toothed
wheel and cutting its teeth at such a distance from their points as to
touch the corresponding circle of the gear working with it. The two
circles must have a common velocity.

=Plunger.=--A part of machinery which is subject to a plunging motion.

=Polarity.=--A difference of polarity exists between any two wires, or
points in one wire, where there is any tendency to produce a current. In
practice, however, the term is used to designate wires pertaining to
the two opposite poles of a system.

=Polyphase.=--Referring to more than one phase, multiphase.

=Power.=--This term designates the rate of doing work.

=Power Factor.=--This term expresses the ratio of the true power in an
alternating-current circuit to the apparent power. It is always less
than 1. The apparent power (product of volts and amperes measured by
independent instruments) must be multiplied by the power factor to
obtain the true power in alternating-current circuits.

=Primary Battery.=--A battery in which the p.d. is produced by primary
action of the ingredients used, instead of by secondary action as in
secondary batteries or accumulators.

=Primary Coil.=--That coil of a transformer, induction motor, or
induction coil which is connected to the source of electrical energy.

=Prism.=--A transparent body usually having three rectangular faces and
triangular ends. It is used in experiments on optics to show refraction
and may also be made to produce total reflection.

=Prismatic Colors.=--The seven colors into which sunlight is resolved
when passed through a prism.

An achromatic prism is one made up of two kinds of glass of different
dispersive powers by which a ray of light may be refracted without
showing color.

=Pulley.=--A broad-rimmed wheel used for transmitting power by means of
a belt.

A narrow-rimmed wheel with a groove used for the same purpose is termed
a _sheave_.

A _cone pulley_ is a pulley made in the shape of a cone and used in
conjunction with a similar pulley and a belt connecting them for the
purpose of varying the velocity.

A _fast pulley_ is one which is rigidly attached to the shaft.

A _loose pulley_ is one which is used in conjunction with another pulley
of the same size to carry the belt while the shaft attached to the other
is still.

=Pulsating Current.=--A current of electricity varying regularly in
strength but not in direction.

=Rack and Pinion.=--A straight bar with teeth on its edge to work with
the teeth of a pinion which is to drive or follow it.

=Ratchet.=--A bar or piece of mechanism turning upon a pivot while the
other end falls into the teeth of a wheel or rack allowing the latter to
move in one direction only. It may be used for preventing backward
motion in machinery or for converting reciprocating motion into forward
motion.

=Ratchet Drill.=--A drill attached to a ratchet.

=Ratchet Wheel.=--A wheel having teeth by which it may be moved forward
as by a lever and catch, or by a pawl arranged to engage the teeth. The
pawl may also be arranged to engage the teeth for the purpose of
preventing backward motion.

=Reactance Coil.=--See Choke Coil.

=Reactive Drop.=--The drop in potential caused through the presence of
reactance in the circuit, as distinguished from that caused by ohmic
resistance which is known as the _ohmic drop_.

=Rectifier.=--Any apparatus used to change an alternating current to a
direct current by periodically interrupting or changing the direction of
the alternating current so that the derived current is always in the
same direction.

=Relay.=--A device actuated by an electric current to open or close
another circuit.

=Rheostat.=--An adjustable resistance.

=Rock Shaft.=--A shaft that vibrates or oscillates instead of making
complete revolutions. Called also rocker or rocking shaft.

=Rotary Converter.=--A rotating machine having a field excited by a
direct current and an armature winding connected to a commutator and a
set of collector rings. It is used to convert alternating current into
direct, or _vice versa_; also called _synchronous converter_.

=Rotor.=--The moving part of an alternating-current generator or motor.

=Secondary Battery.=--A storage battery; an accumulator.

=Secondary Coil.=--The coil of a transformer or an induction coil into
which currents are induced; the coil which is not connected to the
source of energy.

=Series Connection.=--A connection in which a number of devices are
joined so that the same current passes through all of them.

=Series Multiple.=--A system of wiring in which a number of devices are
connected in multiple and several of such groups then connected in
series. The same current must pass through each group.

=Shaking Coil.=--A coil used in connection with a mercury-arc rectifier
for tilting or shaking the tube and starting the apparatus.

=Shunt.=--A by-pass. A wire which carries part or all of the current in
a circuit around some other device connected in the circuit.

=Single-Phase.=--An alternating-current system using only two wires and
only one e.m.f. It is sometimes termed uniphase or monophase.

=Slip of Induction Motor.=--The difference in the number of revolutions
made by the revolving field and the rotor of the motor. It is usually
expressed in per cent of the synchronous speed which is that of the
revolving field.

=Solenoid.=--A round coil of wire generally used to control an iron
core; an electromagnetic helix.

=Spider.=--A casting or part of a machine having projections or arms
radiating from it so as somewhat to resemble a spider.

=Spindle.=--The revolving arbor of a machine tool; an axis.

=Spiral Gear or Gearing.=--A gearing consisting of wheels working
together with their axes at an angle to each other double that of the
angle of the teeth with the axes. It is sometimes used in light
machinery in place of bevel gearing.

=Spiral Wheel.=--A wheel having its teeth cut at an angle with its axis
or so that they form small parts of screws or spirals.

=Spline.=--A rectangular piece fitting the key seats of a hub and shaft
so that, while the one may slide endwise on the other, both must revolve
together.

=Spur Wheel.=--A wheel in which the teeth are perpendicular to the axis
and in the direction of the radii.

=Static Electricity.=--The name usually given to electricity generated
by friction, as from belting in machine shops, etc.

=Stator.=--The stationary part of an alternating-current motor or
generator.

=Stay Bolt.=--A bolt connecting opposite plates to keep them from
bulging.

=Stepdown Transformer.=--A transformer used for reducing voltage.

=Stepup Transformer.=--A transformer used for increasing voltage.

=Striking the Arc.=--The act of bringing the carbons of an arc lamp
together and immediately separating them and establishing the arc is
called _striking the arc_.

=Stud.=--A projection of a bolt adapted to hold a lug or connection of
some kind.

=Stud Bolt.=--A bolt with threads on both ends, to be screwed into a
fixed part at one end and receive a nut upon the other.

=Sustaining Coil.=--The name sometimes given to a reactance used in
connection with a mercury-arc rectifier to cause an overlapping of the
two currents from the anodes.

=Swivel or Swivel Joint.=--A joint, the two pieces of which can be
turned around independently of each other.

=Synchronism.=--Being in time or occurring at the same time in regular
order with some other event.

=Synchronous Converter.=--See rotary converter.

=Tap Bolt.=--A bolt with a head on one end and a thread on the other to
be screwed into some part instead of passing through and being fastened
with a nut.

=Three-Phase.=--Refers to a system of electrical distribution making use
of three separate currents. These currents may be superimposed and
generally only three wires are used in the transmission.

=Thumb Screw.=--A screw arranged so that it can be set up with the
fingers. Similar to a wing nut.

=Thumb Spot.=--A distinguishing mark usually placed on stereopticon
slides so as to mark the spot at which the thumb of the right hand
should be placed when inserting the slide to insure its proper placing
in the holder.

=Travel Ghost.=--The peculiar hazy appearance often seen in a motion
picture; it is produced by the improper setting of the revolving
shutter.

=Trunnion.=--Projection lugs on the sides of some piece of machinery
upon which it is supported and upon which it can move.

=Two-Phase.=--This term refers to an a.c. system of electrical
distribution making use of two currents of different phase. It may be
arranged with three or four wires.

=Unbalanced.=--This term is used in connection with three-wire and
three-phase systems.

The three-wire system is said to be unbalanced when one of the outside
wires is carrying more current than the other.

The three-phase system is said to be unbalanced when the load is
unevenly distributed over the three-phase wires.

=Universal Joint.=--A contrivance used for joining two shafts or parts
of machinery endwise so that one may give rotary motion to the other
while at an angle.

=Volt.=--The unit of electromotive force.

=Voltmeter.=--An instrument used for measuring potential difference.

=Water Rheostat.=--A resistance in which water is used to carry the
current. Plates of metal proportional to the current to be used are
inserted in the water and by inserting them _to a greater extent_ or
moving them closer together the current can be increased.

=Watt.=--The unit of power.

=Wattless Current.=--That part of an alternating current which is either
ahead of or behind the e. m. f. of the circuit.

=Watt Meter.=--An instrument used for measuring electrical energy.

=Yoke.=--A clamp resembling an ox yoke somewhat and adapted to join two
pieces of a machine.




INDEX


  PAGE
  Aberration, chromatic,                                              51
  Aberration, spherical,                                              51
  Adjusting intermittent sprocket,                                   100
  Aisle lights,                                                      218
  Aligning machine,                                                   96
  Alternating current,                                               218
  Alternating current motor,                                         215
  Amperage for projection,                                        43, 44
  Amperage, determination of,                                     45, 46
  Anchoring machine,                                                  97
  Anvil sparks,                                                      311
  Arc, color of,                                              19, 20, 31
    Enclosed,                                                         23
    Form of carbons,                                                  22
    Length of,                                                        21
    Low tension,                                                      20
    Open,                                                             23
    Resistance of,                                                    20
    Upside down,                                                      22
  Arc lamps,                                                19, 218, 312
    Automatic,                                                        25
    Edison,                                                           36
    Hand feed,                                                        25
    Kliegl,                                                       35, 36
    McIntosh,                                                         34
    Motiograph,                                                       37
    Operation of,                                                 25, 27
    Powers,                                                           37
    Testing of,                                                       28
  Arc light, injurious,                                               21
  Armored cable,                                                     222
  Asbestos,                                                          226
  Attachment plugs,                                                  226
  Auto-starters,                                                     227
  Automatic fire shutter,                                62, 70, 71, 101
  Axis of lens,                                                      360

  Baby spot lamp,                                                    316
  Balconies,                                                         228
  Batteries,                                                         316
  Bells,                                                        228, 318
  Blinding lights,                                                   319
  Borders,                                                           229
  Brackets,                                                     232, 319
  Branch circuits,                                                   232
  Branch-off connectors,                                             343
  Bunch lights,                                                      320
  Bus-bars,                                                          234

  Cabinets,                                                          235
  Cables,                                                            235
  Call-board,                                                        293
  Canopies,                                                          236
  Camera lucida,                                                     152
  Carbons, adjusting of,                                              31
    Best to use,                                                      29
    Cored,                                                            53
    Setting of,                                               31, 32, 34
    Testing of,                                                       28
  Care of sprockets,                                                  99
  Carriage call,                                                     293
  Carrying capacity,                                                 237
  Ceiling fans,                                                      237
  Chandeliers,                                             237, 321, 322
  Chromatic aberration,                                               51
  Cleaning machine,                                                   98
  Circuit breakers,                                                  239
  Colors for arc lamps,                                              323
  Colored pictures,                                                   59
  Comparison of stereopticon and motion picture machine pictures,     49
  Concavo-convex lens,                                               146
  Concave meniscus lens,                                             146
  Concealed work,                                                    240
  Condensers,                                                    38, 153
    Breakage,                                                         40
    Diameter of,                                                      51
    Focal length of,                                                  50
  Conduit work,                                                      240
  Conjugate foci of lens,                                            143
  Conjugate foci of mirror,                                          133
  Contacts,                                                          245
  Convexo-concave lens,                                              146
  Convex meniscus lens,                                              146
  Cord,                                                              245
  Corrected lenses,                                                  148
  Cover for machine,                                                 104
  Critical angle of refraction,                                      140
  Current control for arc lamps,                                     190
    Auto-transformer,                                                195
    Mercury-arc rectifier,                                           207
    Motor generators,                                                198
    Reactances,                                                      193
    Resistances,                                                     190
    Rotary converters,                                               202
    Transformation,                                                  194
  Current required for projection,                                43, 44
  Current taps,                                                      245
  Cut-outs,                                                          246

  Damp places,                                                       249
  Decorative lighting systems,                                       249
  Dimmers,                                                           250
  Dimmer box,                                                        324
  Door switches,                                                     251
  Double concave lens,                                               146
  Double convex lens,                                                146
  Dressing rooms,                                               168, 251
  Drop lights,                                                       252

  Edengraph motion head,                                          83, 86
  Edison motion head,                                             64, 84
  Efficiency of incandescent lamps,                                  270
  Electric sign imitations,                                          325
  Electric stage effects,                                            326
  Emergency lighting system,                                         253
  Equivalent focal length,                                            50
  Exit lights,                                                       256
  Explosions,                                                        327
  Eye, the,                                                          124
    Mixing of impressions,                                           125
    Regaining of vision by aged,                                     125

  Face-o-graph,                                                 161, 328
  Fan motors,                                                        256
  Festoons,                                                          328
  Film,                                                               89
    Box for,                                                         108
    Burning of,                                             90, 173, 177
    Emulsion on,                                                      89
    Fire,                                                            110
    Gate,                                                62, 69, 70, 102
    Inspection of,                                                   105
    Leaders and tails,                                          106, 107
    Mender,                                                           94
    Patching,                                                     91, 93
    Pictures, per foot,                                               57
    Pinning of,                                                      110
    Receptacle for,                                                  106
    Rewinding,                                                       108
    Shields,                                                      62, 83
    Steadier drum,                                                62, 69
  Fire alarm,                                                        256
  Fire dance,                                                        330
  Fire hazard,                                                        11
  Fire flies,                                                        330
  Fire places,                                                       331
  Fire proof film,                                                    90
  Fire traps,                                                 62, 67, 97
    Testing of,                                                       68
  Fish work,                                                         256
  Fixtures,                                                          257
  Flashers,                                                          259
  Flexible cord,                                                     261
  Flexible tubing,                                                   261
  Flood lamps,                                                        24
  Floor pockets,                                                     261
  Flowers,                                                           332
  Fly door,                                                          261
  Focal length of concave mirror,                                    133
  Focusing,                                                           42
  Foot-lights,                                                  261, 262
  Formulas,                                                          353
  Fountains,                                                         332
  Framing,                                                           102
  Framing device,                                                 62, 82
  Friction rollers,                                               62, 69
  Fuses,                                                             333
  Fuse spacings,                                                     288

  Gallery lighting,                                                  263
  Gallery pockets,                                              263, 300
  Gas globes,                                                        334
  Gas lighting,                                                      264
  Generators, management of,                                         213
  "Geneva" movement,                                              76, 77
  Getting ready for the show,                                        104
  "Ghost",                                                            42
  Glass shades,                                                      334
  Glossary of mechanical, electrical and optical words, terms and
  phrases,                                                           358
  Grid floor,                                                        265
  Ground,                                                             11
  Grounding,                                                         265
  Guards,                                                            266

  Hanger boards,                                                     266
  Heat,                                                              115
  Heaters,                                                      267, 335
  High potential,                                                    268
  Hints on management of projecting arcs,                             52
  Hissing voltage of arc,                                             29

  Illumination,                                                      269
  Illusions, optical,                                                155
    Dissolving living pictures,                                      157
    Face in clouds,                                                  161
    Figure floating in air,                                          157
    Head resting on table,                                           160
    Head suspended in air,                                           158
    Magic cabinet,                                                   159
    Of motion,                                                        55
    Multiplication of images,                                        160
    Trick mirror,                                                    160
    Seeing through a brick,                                          161
  Image inversion, by lens,                                          144
    By pin hole,                                                     123
    By prism,                                                        151
  Incandescent lamps,                                                269
  Index of refraction,                                               138
  Inspecting films,                                                  105
  Inspecting machine,                                                 97
  Inspection memoranda,                                              112
  Insulating joints,                                                 272
  Interior conduit,                                                  242
  Intermittent sprocket,                                          62, 75
  Inversion of images,                                     123, 144, 151
  Inverted lighting,                                                 273
  Iris, the,                                                         124

  Joints,                                                            273
  Junction boxes,                                                    275

  Kinemacolor process,                                                59

  Lens lamp,                                                          24
  Lenses, conjugate foci of,                                         143
    Double concave,                                                  145
    Double convex,                                              141, 142
    Focal length of,                                                 142
    Heating of,                                                       40
    Inversion of image by,                                           144
    Measuring focal length,                                           49
    Objective,                                                       149
    Principal focus of,                                              142
    Selection of,                                                     46
    Tables of,                                                    47, 48
    Types of,                                                        146
  Life hazard,                                                        13
  Light,                                                             113
    Infra red rays of,                                               117
    Intensity of,                                                    120
    Interference of,                                                 120
    Law of inverse squares,                                          121
    Reflection of,                                              121, 127
    Re-composition of,                                          118, 148
    Source best for projection,                                       39
    Ultra violet rays of,                                            117
    Vibratory, theory of,                                            114
    Waste of, in projection,                                          45
    Wave lengths,                                                    116
  Lighthouse effect,                                                 336
  Lightning,                                                         336
  Lobby,                                                             275
  Lower feed loop,                                                62, 79
  Lower magazine,                                                     62
  Lower steady feed sprocket,                                     62, 79
  Lugs,                                                         275, 338

  Magnifying glass,                                                  145
  Metal molding,                                                     276
  Meters,                                                            278
  Meter reading,                                                     279
  Mirrors,                                                           128
    Concave,                                                         132
    Convex,                                                     132, 135
    Conjugate foci of,                                               133
    Principal focus of,                                              133
  Moon boxes,                                                        339
  Moonlight on water,                                                340
  Motiograph motion head,                                             85
  Motion pictures,                                                    55
  Motion picture machine,                                             62
    Diagram of,                                                       63
    Discussion of parts of,                                           66
    Gearing of,                                                       65
  Motors,                                                            281
    Operation of,                                                    214
    Starting boxes for,                                              215
  Musical acts,                                                      340
  Musical bells,                                                     341
  Music stands,                                                      285

  Objective lenses,                                                  149
  Ohm's law,                                                         353
  Oiling machine,                                                     98
  Oil lamps, imitation of,                                           341
  Open work,                                                         286
  Opera glass,                                                       150
  Operating room,                                                    172
    Color of,                                                        180
    Construction of,                                                 176
    Entrance and exit,                                               177
    Equipment of,                                                    176
    Fire shutters for,                                               181
    Floor of,                                                   178, 179
    Shutter, strings for,                                            183
    Tools for,                                                       185
    Ventilation of,                                                  176
    Wiring for,                                                 186, 189
  Operation of motors,                                               214
  Operation of motion picture machines,                               96
  Optical illusions,                                                 155
  Optical instruments,                                               147
  Optical system of projecting arcs,                                  38

  Panel boards,                                                      286
  Paint bridge,                                                      291
  Paint room,                                                        291
  Persistence of vision,                                         56, 124
  "Pin cross" movement,                                           77, 78
  Pin plug connectors,                                               341
  Plano-concave lens,                                                146
  Plano-convex lens,                                                 146
  Plugging boxes,                                                    343
  Portable stage equipment,                                          311
  Powers motion head,                                                 86
  Presser rollers,                                                    69
  Principles of vision,                                              122
  Prisms,                                        117, 118, 141, 147, 151
  Program board,                                                     291
  Projection,                                                         31
  Projecting arc lamps,                                           38, 52
  Proscenium side lights,                                       167, 294

  Reactances,                                                        193
  Reading glass,                                                144, 145
  Reflection,                                                        126
    Inversion of image by,                                      129, 130
    Multiplication of image by,                                 130, 131
  Refraction,                                                        137
    Critical angle of,                                               140
    Index of,                                                        138
    Paths of rays of light,                                          139
  Refraction and reflection combined,                                140
  Retiring rooms,                                                    295
  Resistance,                                                        190
  Resuscitation,                                                      15
  Revolving shutter,                                         62, 72, 101
    Setting of,                                                       75
  Rewinding,                                                         108
  Rheostats,                                                    295, 345
  Rigging loft,                                                      168

  Sciopticon,                                                        313
  Screens,                                                           173
  Signals,                                                           297
  Signs,                                                             297
  Size of picture,                                                    38
  Simplex motion head,                                                87
  Slides,                                                            109
  Sockets,                                                           298
  Solar spectrum,                                                    118
  Spectrum analysis,                                                 118
  Spherical aberration,                                               51
  Spot on gate, size of,                                              41
  Stage cable,                                                  298, 347
  Stage flues,                                                       299
  Stage, plan of,                                                    167
    Pockets,                                                    168, 300
    Switchboard,                                                167, 305
  Stereopticon lamps,                                                314
  Stereoscope, reflecting,                                           151
    Refracting,                                                      151
  Strips,                                                            348
  Sunrise,                                                           350
  Switch board,                                                      167
  Switches,                                                     301, 350
    Knife,                                                           301
    Spacings of,                                                     289
    Three way,                                                       302

  Table of amperage for projection,                                   44
    Capacity of bus-bars,                                            235
    Capacity of wires,                                               238
    Drop due to 6 amperes,                                           233
    Fuse spacings,                                                   289
    Motion picture lenses,                                            47
    Of multiple reflections,                                         132
    Panel board spacings,                                            287
    Size of wire for motors,                                         284
    Stereopticon lenses,                                              48
    Waste of energy by resistance,                                   191
  Table lamps,                                                       351
  Take up,                                              62, 79, 103, 104
  Telescope,                                                         150
  Tension springs,                                           62, 70, 103
  Testing arc lamps,                                                  28
    Carbons,                                                      28, 29
    Fire traps,                                                       68
  Theater buildings,                                                 163
    For motion pictures,                                             171
    Wiring of,                                                       218
  Three-wire system,                                                  10
  Time beaters,                                                      352
  Tools for operating motion picture machine,                         96
  Transformers,                                                      194
  Trick mirror,                                                      160
  Two-wire system,                                                     9

  Upper feed reel,                                                    66
  Upper loop,                                                     62, 69
  Upper magazine,                                             62, 66, 97
  Upper steady feed sprocket,                                     62, 68
  Useful light of projecting arc,                                     40

  Ventilating flues,                                                 178
  Vision, persistence of,                                             56
  Visitors,                                                          110

  Water falls,                                                       352
  Waterproof film,                                                    90
  Wire systems,                                                        9




  _Books That Really Teach_
  =you the things you want to know, and in a
  simple, practical way that you can understand=


[Illustration]

Our illustrated catalogue, which will be sent you free upon request,
tells all about the Practical Mechanical Books for Home Study that =we
publish=. There are popular priced books on the operation of trains and
station work, practical mechanical drawing and machine designing,
pattern making, electrical railroading, power stations, automobiles, gas
engines, electrical wiring, armature and magnet winding, dynamo tending,
elementary electricity, wireless telegraphy and telephony, carpentry and
architecture, concrete construction, plumbing and heating, sign and
house painting, amusements, etc., etc.

No matter what your ambition or desire for knowledge may be, we publish
books =written by authorities in their different lines= that will give
you just the training and information that you want and need.

  Write today for this up-to-date and complete illustrated catalogue and
  popular price list. It is free.

  FREDERICK J. DRAKE & CO.
  PUBLISHERS OF SELF-EDUCATIONAL BOOKS
  1325 Michigan Avenue CHICAGO


  FREDERICK J. DRAKE & CO.'S
  PRACTICAL MECHANICAL BOOKS
  FOR
  HOME STUDY


                                                               Price.
  Titles.                                                 Cloth.    Lea.

  Air Brake Practice, Modern--Dukesmith. Illustrated       1.50      ...

  Air Brake, Complete Examinations, Westinghouse and New
  York.                                                     ...     2.00

  Air Brake, Westinghouse System.                          2.00      ...

  Air Brake, New York System.                              2.00      ...

  American Homes, Low Cost--Hodgson. Illustrated           1.00      ...

  Architectural Drawing, Self-Taught--Hodgson. Illustrated 2.00      ...

  Architecture, Easy Steps to--Hodgson. Illustrated        1.50      ...

  Architecture, Five Orders--Hodgson. Illustrated          1.00      ...

  Armature and Magnet Winding--Horstmann & Tousley.         ...     1.50

  Artist, The Amateur--Delamotte.                          1.00      ...

  Automobile Hand Book--Brookes. Illustrated                ...     2.00

  Automobile, The Mechanician's Catechism--Swingle          ...     1.25

  Blacksmithing, Modern--Holmstrom. Illustrated            1.00      ...

  Boat Building, for Amateurs--Nelson. Illustrated         1.00      ...

  Bricklayers' and Masons' Assistant, The 20th Century
  --Hodgson. Illustrated                                   1.50      ...

  Bricklaying, Practical, Self-Taught--Hodgson.
  Illustrated                                              1.00      ...

  Bungalows and Low Priced Cottages--Hodgson               1.00      ...

  Calculation of Horse Power Made Easy--Brookes.
  Illustrated                                               .75      ...

  Carpentry, Modern. Vol. I--Hodgson. Illustrated          1.00      ...

  Carpentry, Modern. Vol. II--Hodgson. Illustrated         1.00      ...

  Chemistry, Elementary, Self-Taught--Roscoe. Illustrated  1.00      ...

  Concretes, Cements, Plasters, etc.--Hodgson. Illustrated 1.50      ...

  Correct Measurements, Builders' and Contractors' Guide
  to--Hodgson.                                             1.50      ...

  Catechism, Swingle's Steam, Gas and Electrical
  Engineering                                               ...     1.50

  Cabinet Maker, The Practical, and Furniture Designer
  --Hodgson. Illustrated                                   2.00      ...

  Dynamo Tending for Engineers--Horstmann & Tousley.
  Illustrated                                              1.50      ...

  Dynamo--Electric Machines--Swingle. Illustrated          1.50      ...

  Electric Railway Troubles and How To Find Them--Lowe     1.50      ...

  Electric Power Stations--Swingle                         2.50      ...

  Electrical Construction, Modern. Illustrated              ...     1.50

  Electrical Dictionary. Handy, Weber.                      .25      .50

  Electrical Wiring and Construction Tables--Horstmann &
  Tousley.                                                  ...     1.50

  Electricity, Easy Experiments in--Dickinson. Illustrated 1.00      ...

  Electricity Made Simple--Haskins. Illustrated            1.00      ...

  Electric Railroading--Aylmer-Small. Illustrated           ...     3.50

  Electro-Plating Hand Book--Weston. Illustrated           1.00     1.50

  Elementary Electricity, Up To Date--Aylmer-Small         1.25      ...

  Estimator, Modern, for Builders and Architects--Hodgson  1.50      ...

  Examination Questions and Answers for Locomotive
  Firemen--Wallace. Illustrated                             ...     1.50

  Examination Questions and Answers for Marine and
  Stationary Engineers--Swingle. Illustrated                ...     1.50

  Elevators, Hydraulic and Electric--Swingle. Illustrated  1.00      ...

  Electrician's Operating and Testing Manual--Horstmann &
  Tousley. Illustrated                                      ...     1.50

  Farm Engines and How to Run Them--Stephenson.
  Illustrated                                              1.00      ...

  Furniture Making, Home--Raeth. Illustrated                .60      ...

  First Steps in Electricity, or Electricity for Beginners
  --Harrison                                               1.00      ...

  Gas and Oil Engine Hand Book--Brookes. Illustrated       1.00     1.50

  Hand Book for Engineers and Electricians--Swingle.
  Illustrated. Pocket Book Style                            ...     3.00

  Hardwood Finishing, Up-to-date--Hodgson. Illustrated     1.00      ...

  Horse Shoeing, Correct--Holmstrom. Illustrated           1.00      ...

  Hot Water Heating, Steam and Gas Fitting--Donaldson.
  Illustrated                                              1.50      ...

  Heating and Lighting Railway Passenger Cars--Prior       1.25      ...

  Locomotive Breakdowns, with Questions and Answers--
  Wallace. Illustrated                                      ...     1.50

  Locomotive Fireman's Boiler Instructor--Swingle           ...     1.50

  Locomotive Engineering--Swingle. Illustrated. Pocket Book
  Style.                                                    ...     3.00

  Machine Shop Practice--Brookes. Illustrated              2.00      ...

  Mechanical Drawing and Machine Design--Westinghouse.
  Illustrated                                              2.00      ...

  Motorman, How to Become a Successful. Aylmer-Small.
  Illustrated                                               ...     1.50

  Motorman's Practical Air Brake Instructor--Denehie.       ...     1.50

  Modern Electric Illumination, Theory and Practice--
  Horstmann & Tousley. Illustrated                          ...     2.00

  Millwright's Practical Hand Book--Swingle. Illustrated   2.00      ...

  Modern American Telephony In All Its Branches--Smith.
  Illustrated                                               ...     2.00

  Operation of Trains and Station Work--Prior. Illustrated  ...     1.50

  Painting, Cyclopedia of--Maire. Illustrated              1.50      ...

  Pattern Making and Foundry Practice--Hand. Illustrated    ...     1.50

  Picture Making for Pleasure and Profit--Baldwin.
  Illustrated                                              1.25      ...

  Plumbing, Practical, Up-to-Date--Clow. Illustrated       1.50      ...

  Railway Roadbed and Track, Construction and Maintenance
  of--Prior. Illustrated                                    ...     2.00

  Railway Shop Up-to-Date--Haig. Illustrated               2.00      ...

  Sheet Metal Workers' Instructor--Rose. Illustrated       2.00      ...

  Signist's Book of Modern Alphabets--Delamotte            1.50      ...

  Sign Painting, The Art of--Atkinson                      3.00      ...

  Stair Building and Hand Railing--Hodgson. Illustrated    1.00      ...

  Steam Boilers--Swingle. Illustrated                       ...     1.50

  Steel Square, A Key to--Woods.                           1.50      ...

  Steel Square, Vol. I--Hodgson. Illustrated               1.00      ...

  Steel Square, Vol. II--Hodgson. Illustrated              1.00      ...

  Steel Square, A B C--Hodgson.                             .50      ...

  Steel Construction, Practical--Hodgson. Illustrated       .50      ...

  Storage Batteries--Niblett                                .50      ...

  Sho' Cards, A Show At--Atkinson and Atkinson             3.00      ...

  Stonemasonry, Practical, Self-Taught--Hodgson.
  Illustrated                                              1.00      ...

  Telegraphy Self-Taught--Edison. Illustrated              1.00      ...

  Telephone Hand-Book--Baldwin. Illustrated                1.00      ...

  Timber Framing, Light and Heavy--Hodgson                 2.00      ...

  Toolsmith and Steel Worker--Holford. Illustrated         1.50      ...

  Turbine, The Steam--Swingle. Illustrated                 1.00      ...

  Walschaert Valve Gear Breakdowns and How to Adjust Them
  --Swingle. Illustrated                                   1.00      ...

  Wiring Diagrams, Modern--Horstmann & Tousley.
  Illustrated                                               ...     1.50

  Wireless Telegraphy and Telephony--V. H. Laughter        1.00      ...

  Wood Carving, Practical--Hodgson. Illustrated            1.50      ...


THE RED BOOK SERIES OF TRADE SCHOOL MANUALS

By F. Maire

  =16 mo., Cloth, Illustrated.= =Price, each, $0.60=

  Exterior Painting, Wood, Iron and Brick.

  Interior Painting, Water and Oil Colors.

  Colors, What They Are and What to Expect from Them.

  Graining and Marbling.

  Carriage Painting.

  The Wood Finisher.




THE AUTOMOBILE HAND-BOOK

OVER 200,000 SOLD

By ELLIOTT BROOKES, Assisted by Other Well-Known Experts


  =Revised and Enlarged New Edition--The largest and most practical work
  published. Used by all up-to-date automobile schools as their
  every-day text-book. over 720 pages and over 329 illustrations. Full
  Leather Limp, Round Corners, Red Edges. Price, $2.00.=

[Illustration]

At the present time nearly all automobile troubles or breakdowns may, in
almost every case, be traced to the lack of knowledge or carelessness of
the owner or operator of the car, rather than to the car itself.

The automobile hand book is a work of practical information for the use
of owners, operators and automobile mechanics, giving full and concise
information on all questions relating to the construction, care and
operation of gasoline and electric automobiles, including road troubles,
motor troubles, carburetor troubles, ignition troubles, battery
troubles, clutch troubles, starting troubles. With numerous tables,
useful rules and formulae, wiring diagrams and over 329 illustrations.

Special efforts have been put forth to treat the subjects of ignition,
and ignition devices, in a manner befitting their importance. A large
section has been devoted to these subjects, including batteries, primary
and secondary, magnetos, carburators, spark plugs, and in fact all
devices used in connection with the production of the spark. Power
transmission is thoroughly discussed, and the various systems of
transmitting the power from the motor to the driving axle are analyzed
and compared.

The perusal of this work for a few minutes when troubles occur, will
often not only save time, money, and worry, but give greater confidence
in the car, with regard to its going qualities on the road, when
properly and intelligently cared for.


A WORD TO THE WISE

The time is at hand when any person caring for and operating any kind of
self-propelling vehicle in a public or private capacity, will have to
undergo a rigid examination before a state board of examiners and secure
a license before they can collect their salary or get employment.

Already New York State has enacted such a law and before long, with a
positive certainty every state in the Union will pass such an ordinance
for the protection of life and property.

Remember this is a brand new book from cover to cover, just from the
press--=New Edition=--and must not be confounded with any former
editions of this popular work.

  =Sent prepaid to any address upon receipt of price=

  =FREDERICK J. DRAKE & CO., Publishers=
  =1325 Michigan Avenue= =CHICAGO, U. S. A.=


  COMPLETE EXAMINATION
  QUESTIONS AND ANSWERS
  FOR MARINE AND
  STATIONARY ENGINEERS

By Calvin F. Swingle, M. E. Author of =Swingle's Twentieth Century Hand
Book for Steam Engineers and Electricians. Modern Locomotive Engineering
Handy Book, and Steam Boilers--Their construction, care and management=

[Illustration]

This book is a compendium of useful knowledge, and practical pointers,
for all engineers, whether in the marine, or stationary service. For
busy men and for those who are not inclined to spend any more time at
study than is absolutely necessary, the book will prove a rich mine from
which they may draw nuggets of just the kind of information that they
are looking for.

The method pursued by the author in the compilation of the work and in
the arrangement of the subject matter, is such that a man in search of
any particular item of information relative to the operation of his
steam or electric plant, will experience no trouble in finding that
particular item, and he will not be under the necessity of going over a
couple of hundred pages, either, before he finds it because the matter
is systematically arranged and classified.

The book will be a valuable addition to any engineer's library, not
alone as a convenient reference book, but also as a book for study. It
also contains a complete chapter on refrigeration for engineers. 367
pages fully illustrated, durably bound in full Persian Morocco limp,
round corners, red edges.

  =PRICE= =$1.50=

Sold by Booksellers generally or sent postpaid to any address upon
receipt _of_ price by the Publishers

  FREDERICK J. DRAKE & CO.
  CHICAGO. U. S. A.




  Transcriber's Notes:

  The structure of the printed book regarding headings, sections and
  (sub-)sub-sections was inconsistent and occasionally unclear. As a
  consequence, there are some sub-sub-sections that are not part of
  sub-sections (for example, sub-sub-section national code rules for
  grounding, where there is no sub-section grounding), and in-line
  section headers are not marked as such.

  Inconsistent spacing of abbreviations, capitalisation, spelling and
  hyphenation have been retained.

  The alphabetic order of sections in Chapters XVIII and XIX is not
  always correct, this has not been changed.

  On various pages there are references to other sections and rules of
  the National Electric Code. These sections and rules are not listed as
  such, and it is therefore unclear whether the section or rule referred
  to is given in this work.

  Page 319: ... which should be used to the pin-plug connector ...:
  there is probably a word missing ("connect" or similar).

  Advertisement Automobile Hand-Book, ... over 720 pages: there is
  probably a word missing.


  Changes made

  Illustrations have been moved out of paragraphs, footnotes have been
  moved to directly below the paragraph they refer to.

  Obvious minor typographical and punctuation errors have been corrected
  silently.


  Other changes:

  Page 10: ... lights turned on ... changed to ... lights be turned on
  ...

  Page 25, footnote: author's changed to authors'

  Page 26: misplaced line moved to proper place on page

  Page 193: author's changed to authors'

  Page 55: aperature changed to aperture (2x)

  Page 209: "_Y_" changed to _Y_ as other reference letters

  Page 238, last row, column 25-Watt, 110 V.: 7430 changed to 3740

  Page 373: misplaced line moved to proper place on page.





End of the Project Gutenberg EBook of Motion Picture Operation, Stage
Electrics and Illusions, by Henry C. Horstmann and Victor H. Tousley

*** 