ON THE ELECTRODYNAMICS OF MOVING BODIES

By A. Einstein June 30, 1905

It is known that Maxwell's electrodynamics--as usually
understood at the present time--when applied to moving bodies,
leads to asymmetries which do not appear to be inherent in the
phenomena. Take, for example, the reciprocal electrodynamic
action of a magnet and a conductor. The observable phenomenon
here depends only on the relative motion of the conductor and the
magnet, whereas the customary view draws a sharp distinction
between the two cases in which either the one or the other of these
bodies is in motion. For if the magnet is in motion and the
conductor at rest, there arises in the neighbourhood of the magnet
an electric field with a certain definite energy, producing a current
at the places where parts of the conductor are situated. But if the
magnet is stationary and the conductor in motion, no electric field
arises in the neighbourhood of the magnet. In the conductor,
however, we find an electromotive force, to which in itself there is
no corresponding energy, but which gives rise--assuming equality
of relative motion in the two cases discussed--to electric currents
of the same path and intensity as those produced by the electric
forces in the former case.