Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/443

 electrified corpuscle of charge e moving with velocity in a field defined by the electric force  and magnetic force.

In Lorentz' fundamental case, which has thus been examined, account has been taken only of the ultimate constituents of which the universe is supposed to be composed, namely, corpuscles and the aether. We must now see how to build up from these the more complex systems which are directly presented to our experience.

The electromagnetic field in ponderable bodies, which to our senses appears in general to vary continuously, would present a different aspect if we were able to discern molecular structure; we should then perceive the individual electrons by which the field is produced, and the rapid fluctuations of electric and magnetie* force between them. As it is, the values furnished by our instruments represent averages taken over volumes which, though they appear small to us, are large compared with molecular dimensions. We shall denote an average value of this kind by a bar placed over the corresponding symbol.

Lorentz supposed that the phenomena of electrostatic charge and of conduction-currents are due to the presence or motion of simple electrons such as have been considered above. The part of $$\bar\rho$$ arising from these is the measurable density of electrostatic charge; this we shall denote by. If denote the velocity of the ponderable matter, and if the velocity  of the electrons be written, then the quantity $$\bar{\rho\mathbf v}$$, so far as it arises from electrons of this type, may be written $$\rho_1\mathbf w + \bar{\rho\mathbf u}$$. The former of these terms represents the convection-current, and the latter the conduction-current.

Consider next the phenomena of dielectrics. Following Faraday, Thomson, and Mossotti, Lorentz supposed that each dielectric molecule contains corpuscles charged vitreously and also corpuscles charged resinously. These in the absence of an