Unit of permeability

[AHaumer]

• SIunits.Permeability is defined with unit "H/m".
• Constants.mu_0 is defined as real with unit "N/A2".
• Both units are correct, we could also use "V.s/(A.m)".

Two questions:

1. Do all tools figure out that N/A2 = H/m = V.s/(A.m) ?
2. Shouldn't Constants.mu_0 define SIunits.Permeability mu_0 instead of Real mu_0(unit="N/A2")?

[dietmarw]

1. My personal preference would be to use V.s/(A.m) as they are SI units and not derived.
2. Yes I agree.

[HansOlsson]

Two questions:

Do all tools figure out that N/A2 = H/m = V.s/(A.m) ?

Yes, if tools are able to do any reasonable unit-checking (which I believe most are) that is trivial.

https://en.wikipedia.org/wiki/Vacuum_permeability uses both H/m and N/A2, and give reasonable explanations for both, and also indicates a larger issue - the constant no longer has a fixed simple value, but now seems to be 4π × 1.000 000 000 82 (20) 1e-7.

I can make a PR. (Just changing the value, not adding the indirect definition, since that definition requires yet another measured constant that I don't see any libraries using.)

Shouldn't Constants.mu_0 define SIunits.Permeability mu_0 instead of Real mu_0(unit="N/A2")?

That would work as well, but it depends on how dependent we want to make the libraries.
However, since we use it for the other constants it would make sense.

[HansOlsson]

Two questions:
Do all tools figure out that N/A2 = H/m = V.s/(A.m) ?

Yes, if tools are able to do any reasonable unit-checking (which I believe most are) that is trivial.
https://en.wikipedia.org/wiki/Vacuum_permeability uses both H/m and N/A2, and give reasonable explanations for both, and also indicates a larger issue - the constant no longer has a fixed simple value, but now seems to be 4π × 1.000 000 000 82 (20) 1e-7.

Have made PR with the correct value, now it is just a matter of fixing the bad value on Wikipedia!

[beutlich]

I guess you mean:

[HansOlsson]

I guess you mean:

I meant changing it from 4π × 1.000 000 000 82 (20) 1e-7 to 4π × 1.000 000 000 55 (15) 1e-7; as given in https://physics.nist.gov/cuu/pdf/all.pdf

https://en.wikipedia.org/w/index.php?title=Vacuum_permeability&type=revision&diff=934922863&oldid=921057044

[HansOlsson]

Reading more I'm not sure if we should change the type of the constant, as there are some indications that mu_0 isn't seen as "permeability of vacuum" anymore, but as a "magnetic constant".

https://en.wikipedia.org/wiki/Vacuum_permeability

I don't know if there is a deep reason for that (and if so what it is), but assuming there is it seems odd to use the type "Permeability". On the other hand https://physics.nist.gov/cuu/Constants/codata.pdf uses "magnetic constant" and mu/mu_0 as a relative permeability - indicating that mu_0 is also a permeability.

[AHaumer]

In fact mu_0 is a permeability ... and I prefer unit = V.s/(A.m)