with respect to their mass is considerable. Any moving corpuscle thus represents an electric current. Now, it's known that electric currents show a special kind of inertia, which is denoted as *self-induction*. Once created, a current has the endeavor to conserve itself; from that it comes that one notices a jumping of a spark, when one cuts the conductor (which is traversed by a current) and thus current is interrupted. The current endeavors to remain its intensity in the same way as a moving body endeavors to remain its velocity. Also our cathode-corpuscle will have a certain resistance with respect to the influences which can change its velocity: first by its actual inertia, second by its self-induction – the latter is the case because every change of its velocity would be connected with a simultaneous change of the corresponding current. The *electrons* – the name of the corpuscles – thus would have two kinds of inertia: the mechanical inertia and the electromagnetic inertia.

The works of the theoretician *Abraham* and the experimentalist *Kaufmann* were aimed to specify these two kinds of inertia more closely. For this purpose they had to made a hypothesis; they assumed that all negative electrons are identical with each other, that they all have the same essentially constant charge, and that the differences which exist among them are only caused by their different velocities. When the velocity is changing then their real, *i.e.* their mechanical mass, remains constant; this is so to speak