338 The Followers of Maxwell. 



his theory was furnished by experiment. That an electric field 

 is closely concerned with the propagation of light was demon- 

 strated in 1875, when John Kerr* showed that dielectrics 

 subjected to powerful electrostatic force acquire the property 

 of double refraction, their optical behaviour being similar to 

 that of uniaxal crystals whose axes are directed along the lines 

 of force. 



Other researches undertaken at this time had a more direct 

 bearing on the questions at issue between the hypothesis of 

 Maxwell and the older potential theories. In 1875-6 Helmholtzf 

 and his pupil Schiller^ attempted to discriminate between the 

 various doctrines and formulae relative to unclosed circuits by 

 performing a crucial experiment. 



It was agreed in all theories that a ring-shaped magnet, 

 which returns into itself so as to have no poles, can exert no 

 ponderomotive force on other magnets or 011 closed electric 

 currents. Helmholtz had, however, shown in 1873 that accord- 

 ing to the potential-theories such a magnet would exert a 

 ponderomotive force on an unclosed current. The matter was 

 tested by suspending a magnetized steel ring by a long fibre 

 in a closed metallic case, near which was placed a terminal of 

 a Holtz machine. No ponderomotive force could be observed 

 when the machine was put in action so as to produce a brush 

 discharge from the terminal : from which it was inferred that 

 the potential-theories do not correctly represent the phenomena, 

 at least when displacement-currents and convection -currents 

 (such as that of the electricity carried by the electrically repelled 

 air from the terminal) are not taken into account. 



The researches of Helmholtz and Schiller brought into 

 prominence the question as to the effects produced by the 



* Phil. Mag. (4) 1 (1875), pp. 337, 446 ; (5) viii (1879), pp. 85, 229 ; xiii (1882), 

 pp. 153, 248. 



t Monatsberichte d. Acad. d. Berlin/1875, p. 400. Ann. d. Phys., clviii (1876), 

 p. 87. t Ann. d. Phys. clix (1876), pp. 456, 537 ; clx (1877), p. 333. 



\ The valuable memoirs by Helmholtz in Journal fiir Math. Ixxii (1870), 

 p. 57 ; Ixxv (1873), p. 35 ; Ixxviii (1874), p. 273, to which reference has already 

 been made, contain a full discussion of the various possibilities of the potential- 

 theories. 



