The Followers of Maxwell. 367 



pressed force is best defined in terms of the energy which it 

 communicates to the system ; thus, if e be an impressed electric 

 force, the energy communicated to unit volume of the electro- 

 magnetic system in unit time is e x the electric current. 

 In order that this equation may be true, it is necessary to 

 regard the electric current in a moving medium as composed 

 of the conduction-current, displacement-current, convection- 

 current, and also of the term curl [D . w] , whose presence in 

 the equation we have already noticed. This may be called 

 the current of dielectric convection. Thus the total current is 



S = D + i + pw + curl [D . w] , 



where pw denotes the conduction-current ; and the equation 

 connecting current with magnetic force is 

 curl (H' - h ) = 4?rS, 



where h denotes the impressed magnetic forces other than that 

 induced by motion of the medium. 



We must now consider the advances which were effected 

 during the period following the publication of Maxwell's 

 Treatise in some of the special problems of electricity and 

 optics. 



We have seen* that Maxwell accounted for the rotation of 

 the plane of polarization of light in a medium subjected to a 

 magnetic field K by adding to the kinetic energy of the aether, 

 which is represented by Jpe*, a term J<r (e . curl 9e/90), where 

 cr is a magneto-optic constant characteristic of the substance 

 through which the light is transmitted, and d/dO stands for 

 Kxdl'dx + Kydldy + K-d/dz. This theory was developed further 

 in 1879 by Fitz Gerald,f who brought it into closer connexion 

 with the electromagnetic theory of light by identifying the curl 

 of the displacement e of the aethereal particles with the electric 

 displacement ; the derivate of e with respect to the time then 

 corresponds to the magnetic force. Being thus in possession of 

 a definitely electromagnetic theory of the magnetic rotation of 



* Cf . p. 308. 



t Phil. Trans., 1879, p. 691. Fitz Gerald's Scient. Writings, p. 45. 



