Models of the Aether. 311 



in his model a linear current could be represented by a piece 

 of endless, cord, of the same quality as the solid and embedded 

 in it, if a tangential force were applied to the cord uniformly 

 all round the circuit. The forces so applied tangentially pro- 

 duce a tangential drag on the surrounding solid ; and the 

 rotatory displacement thus caused is everywhere proportional 

 to the magnetic vector. 



In order to represent the effect of varying permeability, 

 Thomson abandoned the ordinary type of elastic solid, and 

 replaced it by an aether of Mac Cullagh^s type; that is to say, 

 an ideal incompressible substance, having no rigidity of the 

 ordinary kind (i.e. elastic resistance to change of shape), but 

 capable of resisting absolute rotation a property to which the 

 name gyrostatic rigidity was given. The rotation of the solid 

 representing the magnetic induction, and the coefficient of 

 gyrostatic rigidity being inversely proportional to the permea- 

 bility, the normal component of magnetic induction will be 

 continuous across an interface, as it should be.* 



We have seen above that in models of this kind the electric 

 force is represented by the translatory velocity of the medium. 

 It might therefore be expected that a strong electric field would 

 perceptibly affect the velocity of propagation of light ; and that 

 this does not appear to be the case,f is an argument against the 

 validity of the scheme. 



We now turn to the alternative conception, in which electric 

 phenomena are regarded as rotatory, and magnetic force is 

 represented by the linear velocity of the medium; in symbols, 



4-TrD = curl e, 

 H = e, 



where D denotes the electric displacement, H the magnetic 

 force, and e the displacement of the medium. In Maxwell's 

 memoir of 1855, and in most of the succeeding writings for 



* Thomson inclined to believe (Papers, iii, p. I&5) that light might he correctly 

 represented by the vibratory motion of such a solid. 



t Wilberforce, Trans. Camb. Phil. Soc. xiv (1887), p. 170 ; Lodge, Phil. Trans, 

 clxxxix (1897), p. 149. 



