Einstein, Laub and Unipolar Induction

In the first passages of 1905 paper “On the electrodynamics of moving bodies”, Einstein underscored the theoretical asymmetries in the Maxwell and Lorentz explanation of electromagnetic induction. In the second part of the paper, the elctrodynamical part, he stated that “questions as to the <<seat>> of electrodynamic electromotive forces (unipolar machines) become meaningless.” At that time, unipolar machines were well known devices: through two sliding electric contacts, connecting the pole and the equatorial circle of a cylindrical rotating magnet, flowed an electric current. Huge unipolar inductors had important technological applications in spite of some theoretical problem concerning the explanation of the emerging induced currents. Were lines of force in motion together with the magnet or were not? Were the electric wires or the magnet itself the <<seat>> of the electromotive force? In 1908, A. Einstein and J. Laub claimed that the different predictions, about unipolar induction in dielectric magnets, of Lorentz and Einstein electrodynamics, allowed scientists to set the comparison between them on experimental ground.