356 The Followers of Maxwell. 



It is evident that the quantity a thus defined satisfies the 

 equation 



or v*a - - a = - 47ri. 



c 2 dt' 



This equation may be written 



while the equations H = curl a, E = - a give 



curl E = - H. 



These are, however, the fundamental equations of Maxwell's 

 theory in the form given in his memoir of 1868,* 



That Hertz's deduction is ingenious and interesting will 

 readily be admitted. That it is conclusive may scarcely be 

 claimed : for the argument of Helmholtz regarding the induc- 

 tion of currents is not altogether satisfactory; and Hertz, in 

 following his master, is on no surer ground. 



In the course of a discussion^ on the validity of Hertz's 

 assumptions, which followed the publication of his paper, 

 E. AulingerJ brought to light a contradiction between the 

 principles of the unity of electric and of magnetic force and 

 the electrodynamics of Weber. Consider an electrostatically 

 charged hollow sphere, in the interior of which is a wire 

 carrying a variable current. According to Weber's theory, 

 the sphere would exert a turning couple on the wire; but 

 according to Hertz's principles, no action would be exerted, 

 since charging the sphere makes no difference to either the 

 electric or the magnetic force in its interior. The experiment 

 thus suggested would be a crucial test of the correctness of 

 Weber's theory ; it has the advantage of requiring nothing 

 but closed currents and electrostatic charges at rest ; but 

 the quantities to be observed would be on the limits of 

 observational accuracy. 

 Cf. p. 287. 



f Lorberg, Ann. d. Phys. xxvii (1886), p. 666; xxxi (1887), p. 131. 

 Boltzmann, ibid, xxix (1886), p. 598. + Ann. d. Phys. xxvii (1886), p. 119. 



