The Followers of Maxwell. 353 



or, by virtue of the fundamental equations for dielectrics, 

 [- B . D] + [D . B] , or (a/ft) [D . B]. 



This result compels us to adopt one of three alternatives: 

 either to modify the theory so as to reduce to zero the resultant 

 force on an element of free aether ; this expedient has not met 

 with general favour ;* or to assume that the force in question 

 sets the aether in motion: this alternative was chosen by 

 Helmholtz,f but is inconsistent with the theory of the aether 

 which was generally received in the closing years of the century; 

 or lastly, with Thomson^ to accept the principle that the aether 

 is itself the vehicle of mechanical momentum, of amount [D . B] 

 per unit volume. 



Maxwell's theory was now being developed in ways which 

 could scarcely have been anticipated by its author. But although 

 every year added something to the superstructure, the founda- 

 tions remained much as Maxwell had laid them ; the doubtful 

 argument by which he had sought to justify the introduction 

 of displacement- currents was still all that was offered in their 

 defence. In 1884, however, the theory was established on a 

 different basis by a pupil of Helmholtz', Heinrich Hertz 

 (b. 1857, d. 1894). 



The train of Hertz' ideas resembles that by which Ampere, 

 on hearing of Oersted's discovery of the magnetic field produced 

 by electric currents, inferred that electric currents should exert 

 ponderomotive forces on each other. Ampere argued that a 

 current, being competent to originate a magnetic field, must be 

 equivalent to a magnet in other respects ; and therefore that 

 currents, like magnets, should exhibit forces of mutual attraction 

 and repulsion. 



* It was, however, adopted by G. T. "Walker, Aberration and the Electromagnetic 

 Field, Camb., 1900. 



t Berlin Sitzungsberichte, 1893, p. 649; Ann. d. Phys. liii (1894), p. 135. 

 Helmholtz supposed the aether to behave as a frictionless incompressible fluid. 



+ Loc. cit. 



Ann. d. Phys. xxiii (1884), p. 84: English version in Hertz's Miscellaneous 

 Papers, translated by D. E. Jones and G. A. Schott, p. 273. 



2 A 



