Stefano Bordoni (University of Bologna)
The paper focuses on the conceptual link among seemingly different issues: Einstein-Laub’s 1908 paper on the electrodynamics of moving media, the long lasting debate on unipolar machines and unipolar induction, the differences between Lorentz and Einstein’s electrodynamics of moving bodies, and the role of symmetries in electromagnetic theories. More specifically, the different expressions Lorentz and Einstein-Laub put forward for the electric field generated by the motion of dielectric magnets are explicitly discussed. Although the equations in the Einstein-Laub paper made reference to rectilinear motions, the experiments suggested and afterwards performed involved rotating magnets, and therefore dealt with unipolar machines. Starting from an overview of the history of unipolar machines and unipolar induction, I am analysing Hertz, Lorentz and Einstein’s theories, the experiments Harold A. Wilson performed in 1904 and 1913, and the interpretations of unipolar induction offered by authoritative advanced textbooks. The role of electromagnetic symmetries in Einstein’s theory is analysed in primary and secondary sources. In the end, the recent interest of physicists for the subject matter is briefly outlined.