Maxwell. 279 



The conception of the electrostatic state as a displacement 

 of something from its equilibrium position was not altogether 

 new, although it had not been previously presented in this 

 form. Thomson, as we have seen, had compared electric force 

 to the displacement in an elastic solid ; and Faraday, who had 

 likened the particles of a ponderable dielectric to small con- 

 ductors embedded in an insulating medium,* had supposed that 

 when the dielectric is subjected to an electrostatic field, there 

 is a displacement of electric charge on each of the small 

 conductors. The motion of these charges, when the field is 

 varied, is equivalent to an electric current ; and it was from 

 this precedent that Maxwell derived the principle, which became 

 of cardinal importance in his theory, that variations of displace- 

 ment are to be counted as currents. But in adopting the 

 idea, he altogether transformed it ; for Faraday's conception of 

 displacement was applicable only to ponderable dielectrics, and 

 was in fact introduced solely in order to explain why the 

 specific inductive capacity of such dielectrics is different from 

 that of free aether; whereas according to Maxwell there is 

 displacement wherever there is electric force, whether material 

 bodies are present or not. 



The difference between the conceptions of Faraday and 

 Maxwell in this respect may be illustrated by an analogy 

 drawn from the theory of magnetism. When a piece of iron 

 is placed in a magnetic field, there is induced in it a magnetic 

 distribution, say of intensity I ; this induced magnetization 

 exists only within the iron, being zero in the free aether 

 outside. The vector I may be compared to the polarization 

 or displacement, which according to Faraday is induced in 

 dielectrics by an electric field; and the electric current con- 

 stituted by the variation of this polarization is then analogous 

 to dl/dt. But the entity which was called by Maxwell the 

 electric displacement in the dielectric is analogous not to I, 

 but to the magnetic induction B : the Maxwellian displace- 



* Cf. p. 210. 



