Models of the Aether. 323 



performed in the present system, the poles of the electro- 

 magnet are exchanged, while in the dielectric no change takes 

 place. 



We must now consider the bearing of Hall's effect on 

 the question as to whether magnetism is a rotatory or a 

 linear phenomenon.* If magnetism be linear, electric currents 

 must be rotatory; and if Hall's phenomenon be supposed to take 

 place in a horizontal strip of metal, the magnetic force being 

 directed vertically upwards, and the primary current flowing 

 horizontally from north to south, the only geometrical entities 

 involved are the vertical direction and a rotation in the east- 

 and-west vertical plane ; and these are indifferent with respect 

 to a rotation in the nor th-and- south vertical plane, so that there 

 is nothing in the physical circumstances of the system to 

 determine in which direction the secondary current shall flow. 

 The hypothesis that magnetism is linear appears therefore 

 to be inconsistent with the existence of Hall's effect, f There 

 are, however, some considerations which may be urged on the 

 other side. Hall's effect, like the magnetic rotation of light, 

 takes place only in ponderable bodies, not in free aether ; and 

 its direction is sometimes in one sense, sometimes in the other, 

 according to the nature of the substance. It may therefore be 

 doubted whether these phenomena are not of a secondary 

 character, and the argument based on them invalid. Moreover, 

 as Fitz Gerald remarked,^ the magnetic lines of force associated 

 with a system of currents are circuital and have no open ends, 

 making it difficult to imagine how alteration of rotation inside 

 them could be produced. 



Of the various attempts to represent electric and magnetic 

 phenomena by the motions and strains of a continuous medium, 

 none of those hitherto considered has been found free from 



* Of. F. Kol&cek, Ann. d. Phys. Iv (1895), p. 503. 



t Further evidence in favour of the hypothesis that it is the electric phenomena 

 which are linear is furnished by the fact that pyro-electric effects (the production of 

 electric polarization by warming) occur in acentric crystals, and only in such. Cf. 

 M. Abraham, Encyklopiidie der rnrith. Wiss. iv (2), p. 43. 



I Cf. Larmor, Phil. Trans, clxxxv, p. 780. 



Y 2 



