The principle of relativity in electrodynamics.
Using (4) and (5) this becomes
Since the polarization electrons have no velocity relative to the body, they contribute nothing to the magnetization in either system.
Consider next the magnetization electrons, that is, systems of electrons spinning round centres fixed in the body and having a magnetic moment; possibly also an electric moment in a moving system.
If these have an electric moment, consider the contribution to the polarization
where we consider in the first place only those electrons having a definite value of and afterwards effect a summation.
Similarly , i.e., , becomes, after reduction,
In the same manner for the same group of electrons
which, on reduction, becomes