Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/386

 Maxwell made no change in the other electromagnetic equations, which therefore retained the customary forms

Hertz, however, impressed by the duality of electric and magnetic phenomena, modified the last of these equations by assuming that a magnetic force is generated in a dielectric which moves with velocity in an electric field; such a force would be the magnetic analogue of the electromotive force of induction. A term involving is then introduced into the last equation.

The theory of Hertz resembles in many respects that of Heaviside, who likewise insisted much on the duplex nature of the electromagnetic field, and was in consequence disposed to accept the term involving in the equations of moving media. Heaviside recognized more clearly than his predecessors the distinction between the force, which determines the flux , and the force , whose curl represents the electric current; and, in conformity with his principle of duality, he made a similar distinction between the magnetic force , which determines the flux , and the force , whose curl represents the "magnetic current" This distinction, as Heaviside showed, is of importance when the system is acted on by "impressed forces," such as voltaic electromotive forces, or permanent magnetization; these latter must be included in and , since they help to give rise to the fluxes  and ; but they must not be included in  and  since their curls are not electric or magnetic currents; so that in general We have

where and  denote the impressed forces.

Developing the theory by the aid of these conceptions, Heaviside was led to make a further modification, An im-