Models of the Aether. 321 



repeating an experiment which had been previously suggested 

 by H. A. Kowland, obtained a new action of a magnetic field 

 on electric currents. A strip of gold leaf mounted on glass, 

 forming part of an electric circuit through which a current 

 was passing, was placed between the poles of an electro- 

 magnet, the plane of the strip being perpendicular to the 

 lines of magnetic force. The two poles of a sensitive galvano- 

 meter were then placed in connexion with different parts of the 

 strip, until two points at the same potential were found. When 

 the magnetic field was created or destroyed, a deflection of the 

 galvanometer needle was observed, indicating a change in the 

 relative potential of the two poles. It was thus shown that 

 the magnetic field produces in the strip of gold leaf a new 

 electromotive force, at right angles to the primary electromotive 

 force and to the magnetic force, and proportional to the product 

 of these forces. 



From the physical point of view we may therefore regard 

 Hall's effect as an additional electromotive force generated by 

 the action of the magnetic field on the current ; or alternatively 

 we may regard it as a modification of the ohmic resistance of 

 the metal, such as would be produced if the molecules of the 

 metal assumed a helicoidal structure about the lines of magnetic 

 force. From the latter point of view, all that is needed is 

 to modify Ohm's law 



S = E 



(where S denotes electric current, k specific conductivity, and E 

 electric force) so that it takes the form 



S = KE + h [E . H] 



where H denotes the imposed magnetic force, and h denotes a 

 constant on which the magnitude of Hall's phenomenon 

 depends. It is a curious circumstance that the occurrence, in 

 the case of magnetized bodies, of an additional term in Ohm's 

 law, formed from a vector-product of E, had been expressly 

 suggested in Maxwell's Treatise*: although Maxwell had not- 

 indicated the possibility of realizing it by Hall's experiment. 



* Elect, and Mag., 303. Cf. Hopkinson, Phil. Mag. x (1880), p. 430. 



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