tive
electrons (if any such there be) remain motionless. Yet in the
experiment known as Hall's, the current is deflected by the magnetic
field to one side of the strip in certain metals, and to the opposite
side in others. This seems to show that in certain cases the positive
electrons move instead of the negative, and Professor Lorentz
confesses that up to the present he can find no valid argument against
this. See _Archives Neerlandaises_ 1906, parts 1 and 2.--ED.]

Sec. 3. THE MASS OF ELECTRONS

Other conceptions, bolder still, are suggested by the results of
certain interesting experiments. The electron affords us the
possibility of considering inertia and mass to be no longer a
fundamental notion, but a consequence of the electromagnetic
phenomena.

Professor J.J. Thomson was the first to have the clear idea that a
part, at least, of the inertia of an electrified body is due to its
electric charge. This idea was taken up and precisely stated by
Professor Max Abraham, who, for the first time, was led to regard
seriously the seemingly paradoxical notion of mass as a function of
velocity. Consider a small particle bearing a given electric charge,
and let us suppose that this particle moves through the ether. It is,
as we know, equivalent to a current proportional to its velocity, and
it therefore creates a magnetic field the intensity of which is
likewise proportional to its velocity: to set it in motion, therefore,
there must be communicated to it over and above the expenditure
corresponding to the acquisition of its ordinary kinetic energy, a
quantity of energy proportional to the square of its velocity.
Everything, therefore, takes place as if, by the fact of
electrification, its capacity for kinetic energy and its material mass
had been increased by a certain constant quantity. To the ordinary
mass may be added, if you will, an electromagnetic mass.

This is the state of things so long as the speed of the translation of
the particle is not very great, but they are no longer quite the same
when this particle is animated with a movement whose rapidity becomes
comparable to that with which light is propagated.

The magnetic field created is then no longer a field in repose, but
its energy depends, in a complicated manner, on the velocity, and the
apparent increase in the mass of the particle itself becomes a
function of the velocity. More than this, this increase may not be the
same for the same velocity, bu