The apparent mass, it can be easily shown, increases
indefinitely when the velocity with which the electrified particle is
animated tends towards the velocity of light, and thus the work
necessary to communicate such a velocity to an electron would be
infinite. It is in consequence impossible that the speed of an
electron, in relation to the ether, can ever exceed, or even
permanently attain to, 300,000 kilometres per second.
All the facts thus predicted by the theory are confirmed by
experiment. There is no known process which permits the direct
measurement of the mass of an electron, but it is possible, as we have
seen, to measure simultaneously its velocity and the relation of the
electric charge to its mass. In the case of the cathode rays emitted
by radium, these measurements are particularly interesting, for the
reason that the rays which compose a pencil of cathode rays are
animated by very different speeds, as is shown by the size of the
stain produced on a photographic plate by a pencil of them at first
very constricted and subsequently dispersed by the action of an
electric or magnetic field.
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