540 JAMES CLERK MAXWELL. 



the direction of their length, somewhat in the same way as 

 elastic strings would be affected by the wire before they 

 broke and allowed it to pass through. Behind the wire the 

 lateral pressure will be relieved, the vortices will contract 

 in the direction of their axes and expand equatorially. But 

 we have seen that the effect of stretching a rotating elastic 

 body in the direction of its axis of rotation, and compressing 

 it at right angles to this direction, increases the velocity of 

 rotation so that the actual velocity of every point on the 

 surface is increased ; while the contraction of the body along 

 the axis of rotation diminishes the velocity. Hence as long 

 as the wire is moving across the lines of force the velocity 

 of the vortices in front of the wire will be greater than that 

 of the vortices behind, and the electric particles in the wire, 

 coming, as they do, between two sets of vortices, which are 

 rotating with different velocities, will flow in a stream along 

 the wire. The direction of the current in the wire will be 

 that which would cause the vortices in front to rotate more 

 rapidly than those behind, and therefore to exert a greater 

 pressure on the wire ; in other words, there will be a current 

 induced in such direction as to oppose the motion of the 

 wire. We arrive at a similar result if we suppose the lines 

 of force to be cut obliquely instead of orthogonally. Thus 

 Lenz's law is a consequence of the hypothesis of molecular 

 vortices. If we suppose the magnetic force to act from 

 south to north horizontally, the wire to be vertical and to 

 move from west to east, we have magnetic force acting from 

 south to north, mechanical force acting from east to west, 

 and opposing the motion of the wire, and electro-motive force 

 acting in the wire vertically upwards. 



Suppose that all over a certain area the electricity is 

 pushed forwards through a very small distance along the 

 normal, so that it does not pass from molecule to molecule 

 of the substance, but in each molecule undergoes a displace- 

 ment from back to front. The electric particles pressing 

 tangentially on the walls of the elastic cells are unable to 

 set them rotating, because each cell is acted upon equally 

 all round in the direction in which the electricity tends to 



