278 Maxwell. 



action of the particles and vortices. This action is determined, 

 as Maxwell showed, hy the relation 



fj$L = - curl E 



which connects E, the force exerted on a unit quantity of 

 particles at any place in consequence of the tangential action 

 of the vortices, with H, the rate of change of velocity of the 

 neighbouring vortices. It will be observed that this equation 

 is not kinematical but dynamical. On comparing it with the 

 electromagnetic equations 



curl a = /*H, 



Induced electromotive force = - a, 

 it is seen that E must be interpreted electromagnetically as the 

 induced electromotive force. Thus the motion of the particles 

 constitutes an electric current, the tangential force with which 

 they are pressed by the matter of the vortex-cells constitutes 

 electromotive force, and the pressure of the particles on each 

 other may be taken to correspond to the tension or potential of 

 the electricity. 



The mechanism must next be extended so as to take account 

 of the phenomena of electrostatics. For this purpose Maxwell 

 assumed that the particles, when they are displaced from their 

 equilibrium position in any direction, exert a tangential action 

 on the elastic substance of the cells ; and that this gives rise 

 to a distortion of the cells, which in turn calls into play a 

 force arising from their elasticity, equal and opposite to the 

 force which urges the particles away from the equilibrium 

 position. When the exciting force is removed, the cells recover 

 their form, and the electricity returns to its former position. 

 The state of the medium, in which the electric particles are 

 displaced in a definite direction, is assumed to represent an 

 electrostatic field. Such a displacement does not itself con- 

 stitute a current, because when it has attained a certain value 

 it remains constant ; but the variations of displacement are to 

 be regarded as currents, in the positive or negative direction 

 according as the displacement is increasing or diminishing. 



