The Followers of Maxwell. 365 



content of Maxwell's theory for bodies at rest, proceeded* to 

 extend the equations to the case in which material bodies are 

 in motion in the field. 



In a really comprehensive and correct theory, as Hertz 

 remarked, a distinction should be drawn between the quantities 

 which specify the state of the aether at every point, and those 

 which specify the state of the ponderable matter entangled with 

 it. This anticipation has been fulfilled by later investigators ; 

 but Hertz considered that the time was not ripe for such a 

 complete theory, and preferred, like Maxwell, to assume that 

 the state of the compound system matter plus aether can be 

 specified in the same way when the matter moves as when it is 

 at rest ; or, as Hertz himself expressed it, that " the aether 

 contained within ponderable bodies moves with them." 



Maxwell's own hypothesis with regard to moving systemsf 

 amounted merely to a modification in the equation 



B = - curl E, 



which represents the law that the electromotive force in a 

 closed circuit is measured by the rate of decrease in the number 

 of lines of magnetic induction which pass through the circuit. 

 This law is true whether the circuit is at rest or in motion ; but 

 in the latter case, the E in the equation must be taken to be the 

 electromotive force in a stationary circuit whose position 

 momentarily coincides with that of the moving circuit; and 

 since an electromotive force [w . B] is generated in matter by 

 its motion with velocity w in a magnetic field B, we see that E 

 is connected with the electromotive force E' in the moving 

 ponderable body by the equation 



E' = E + [w . B], 



so that the equation of electromagnetic induction in the moving 

 body is 



B = - curl E' + curl [w . B]. 



* Ann. d. Phys. xli (1890), p. 369 ; Electric Waves (English ed.), p. 241. 



The propagation of light through a moving dielectric had been discussed 

 previously, on the basis of Maxwell's equations for moving bodies, by J. J. Thomson, 

 Phil. Mag. ix (1880), p. 284 ; Proc. Camb. Phil. Soc. v (1885), p. 250. 



tCf. p. 288. 



