Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/301

 the constant μ be supposed to have the value unity, the equations may be written

Eliminating E, we see that H satisfies the equations

But these are precisely the equations which the light-vector satisfies in a medium in which the velocity of propagation is c1: it follows that disturbances are propagated through the model by waves which are similar to waves of light, the magnetic (and similarly the electric) vector being in the wave-front. For a plane-polarized wave propagated parallel to the axis of z, the equations reduce to

whence we have

these equations show that the electric and magnetic vectors are at right angles to each other.

The question now arises as to the magnitude of the constant c1. This may be determined by comparing different expressions for the energy of an electrostatic field. The work done by an electromotive force E in producing a displacement D is

per unit volume, since E is proportional to D. But if it be assumed that the energy of an electrostatic field is resident in the dielectric, the amount of energy per unit volume may be