280 Maxwell. 



merit-current corresponds to d'B/dt, and may therefore have a 

 value different from zero even in free aether. 



It may be remarked in passing that the term displacement, 

 which was thus introduced, and which has been retained in 

 the later development of the theory, is perhaps not well chosen ; 

 what in the early models of the aether was represented as an 

 actual displacement, has in later investigations been conceived 

 of as a change of structure rather than of position in the 

 elements of the aether. 



Maxwell supposed the electromotive' force acting on the 

 electric particles to be connected with the displacement D 

 which accompanies it, by an equation of the form 



where c, denotes a constant which depends on the elastic 

 properties of the cells. The displacement-current D must now 

 be inserted in the relation which connects the current with 

 the magnetic force ; and thus we obtain the equation 



curl H = 47rS, 



where the vector S, which is called the total current, is the 

 sum of the convection-current i and the displacement-current 

 D. By performing the operation div on both sides of this 

 equation, it is seen that the total current is a circuital vector. 

 In the model, the total current is represented by the total 

 motion of the rolling particles ; and this is conditioned by the 

 rotations of the vortices in such a way as to impose the 

 kinematic relation 



div S = 0. 



Having obtained the equations of motion of his system 

 of vortices and particles, Maxwell proceeded to determine the 

 rate of propagation of disturbances through it. He considered 

 in particular the case in which the substance represented is a 

 dielectric, so that the conduction-current is zero. If, moreover, 



