Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/299

 The conception of the electrostatic state as a displacement of something from its equilibrium position was not altogether new, although it had not been previously presented in this form. Thomson, as we have seen, had compared electric force to the displacement in an elastic solid; and Faraday, who had likened the particles of a ponderable dielectric to small conductors embedded in an insulating medium, had supposed that when the dielectric is subjected to an electrostatic field, there is a displacement of electric charge on each of the small conductors. The motion of these charges, when the field is varied, is equivalent to an electric current; and it was from this precedent that Maxwell derived the principle, which became of cardinal importance in his theory, that variations of displacement are to be counted as currents. But in adopting the idea, he altogether transformed it; for Faraday's conception of displacement was applicable only to ponderable dielectrics, and was in fact introduced solely in order to explain why the specific inductive capacity of such dielectrics is different from that of free aether; whereas according to Maxwell there is displacement wherever there is electric force, whether material bodies are present or not.

The difference between the conceptions of Faraday and Maxwell in this respect may be illustrated by an analogy drawn from the theory of magnetism. When a piece of iron is placed in a magnetic field, there is induced in it a magnetic distribution, say of intensity I; this induced magnetization exists only within the iron, being zero in the free aether outside. The vector I may be compared to the polarization or displacement, which according to Faraday is induced in dielectrics by an electric field; and the electric current constituted by the variation of this polarization is then analogous to &part;I/&part;t. But the entity which was called by Maxwell the electric displacement in the dielectric is analogous not to I, but to the magnetic induction B: the Maxwellian displace-