Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/331

 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 produce 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 MacCullagh'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 permeability, 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, 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,

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