As in the first section it will be convenient to lead up to the bearing of the Principle of Relativity on this subject by giving an account of the earlier work.

It had been noticed as early as 1881 (by J. J. Thomson) that the charge carried by a body has the effect of increasing the apparent inertia of the body, the electromagnetic field set up by the moving charge exercising a reaction on the charge itself.

When it was found in experimenting on the cathode rays that the carriers of the electric charge through the vacuum tube must be conceived as particles having an effective mass which was very small compared with the smallest material particle hitherto conceived, namely the atom of hydrogen, the possibility revealed itself that there might be cases in which the electromagnetic inertia of a charged particle was comparable with the whole inertia. This actually proved to be so when Kaufmann, experimenting on the β-rays emitted by radium, found them to be of similar character, save that their velocity was much greater— from about 2 to 6 of that of light—and that the apparent mass increased in a regular way with the velocity.