264. The Motion of Faraday Tubes. Dielectric Currents. In Chapter xi. we considered the relation between the currents in the primary and secondary circuits when an alternating current passes through the primary circuit, we did not however discuss the phenomena occurring in the dielectric between the circuits. As we regard the dielectric as the seat of the energy due to the distribution of the currents, the study of the effects in the dielectric is of primary importance. We owe to Maxwell a theory, now in its main features universally accepted, by which we are able to completely determine the electrical conditions, not merely in the conductors but also in every part of the field. We shall also see that Maxwell's views lead to a comprehensive theory of optical as well as of electrical phenomena, and enable us by means of electrical principles to explain the fundamental laws of Optics.

Before specifying in detail the principles of Maxwell's theory, we shall endeavour to show by the consideration of some simple cases that in considering the relation between the work done in taking unit magnetic pole round a closed circuit and the current flowing through that circuit (see Art. 203), we must include under the term current, effects other than the passage of electricity through conducting media, if we are to retain the conception that the dielectric is the seat of the energy in electric and magnetic phenomena.