Unipolar induction, discovered in 1832 by Michael Faraday, is the case of electromagnetic induction in which a conductor and magnet are in relative rotatory motion. Attempts by scientists and engineers in the nineteenth and twentieth centuries to understand unipolar induction by using magnetic lines of force displayed striking national differences that influenced where the first largescale unipolar dynamo was built. This episode is described, as well as the effect of unipolar induction on Albert Einstein's thinking toward the special theory of relativity, in sections 1–6. The analysis of electromagnetic induction in cases where the source of the magnetic field is in motion relative to the conductor is provided in sections 7–9.