366 The Followers of Maxwell. 



Maxwell made no change in the other electromagnetic 

 equations, which therefore retained the customary forms 



D = f E'/47rc 2 , div D = 0, 47r(i 4 D) = curl H, 

 Hertz, however, impressed by the duality of electric and 

 magnetic phenomena, modified the last of these equations by 

 assuming that a magnetic force 4?r [D . w] is generated in a 

 dielectric which moves with velocity w in an electric field ; such 

 a force would be the magnetic analogue of the electromotive 

 force of induction. A term involving curl |D . w] 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 curl [D . w] in the equations of 

 moving media. Heaviside recognized more clearly than his 

 predecessors the distinction between the force E', which 

 determines the flux D, and the force E, whose curl represents 

 the electric current ; and, in conformity with his principle of 

 duality, he made a similar distinction between the magnetic 

 force H', which determines the flux B, and the force H, 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 E' and H', since they help to give rise to the fluxes 

 D and B ; but they must not be included in E and H, since their 

 curls are not electric or magnetic currents ; so that in general 



we have 



E' = E + e, H' = + h, 



where e and h denote the impressed forces. 



Developing the theory by the aid of these conceptions, 

 Heaviside was led to make a further modification. An im- 



* Heaviside's general theory was published in a series of papers in the 

 Electrician, from 1885 onwards. His earlier work was republished in his 

 Electrical Papers (2 vols., 1892), and his Electromagnetic Theory (2 vols., 1894). 

 Mention may be specially made of a memoir in Phil. Trans, clxxxiii (1892), 

 p. 423. 



