Summing for all electrons, we obtain Minkowski's equation,
(7)
The Polarization p and Magnetization m.
So far no distinction has been made between various types of motion of the electrons; but now they must be differentiated.
Consider first the so-called polarization electrons which move with the body, but have an electric moment. Taking δa, δA to be the volumes occupied by such a polarized element, its contributions to the polarization in the two systems are related as follows:
where X is measured relative to the centre of the element.