( 310 ) 



CHAPTEK IX. 



MODELS OF THE AETHER. 



THE early attempts of Thomson and Maxwell to represent the 

 electric medium by mechanical models opened up a new field of 

 research, to which investigators were attracted as much by its 

 intrinsic fascination as by the importance of the services which 

 it promised to render to electric theory. 



Of the models to which reference has already been made, 

 some such as those described in Thomson's memoir* of 1847 

 and Maxwell's memoirf of 1861-2 attribute a linear character 

 to electric force and electric current, and a rotatory character 

 to magnetism; others such as that devised by Maxwell in 

 1855J and afterwards amplified by Helmholtz regard mag- 

 netic force as a linear and electric current as a rotatory 

 phenomenon. This distinction furnishes a natural classification 

 of models into two principal groups. 



Even within the limits of the former group diversity has 

 already become apparent ; for in Maxwell's analogy of 1861-2, 

 a continuous vortical motion is supposed to be in progress about 

 the lines of magnetic induction ; whereas in Thomson's analogy 

 the vector-potential was likened to the displacement in an 

 elastic solid, so that the magnetic induction at any point would 

 be represented by the twist of an element of volume of the 

 solid from its equilibrium position ; or, in symbols, 



a = e, E = - e, B = curl e, 



where a denotes the vector-potential, E the electric force, B the 

 magnetic induction, and e the elastic displacement. 



Thomson's original memoir concluded with a notice of his 

 intention to resume the discussion in another communication 

 His purpose was fulfilled only in 1890, when|| he showed tha 



* Cf. p. 270. t Of. p. 276. % Cf. p. 271. Cf. p. 274. 



|| Kelvin's Math, and Phys. Papers, iii, p. 436. 



