478 ON PHYSICAL LINES OF FORCE. 



be discovered, whether the current is kept constant or made to vary in strength ; 

 but if a conducting wire C be made to embrace the ring any number of times, 

 an electromotive force will act on that wire whenever the current in the coil is 

 made to vary ; and if the circuit be closed, there will be an actual current in 

 the wire C. 



This experiment shews that, in order to produce the electromotive force, it 

 is not necessary that the conducting wire should be placed in a field of magnetic 

 force, or that lines of magnetic force should pass through the substance of the 

 wire or near it. All that is required is that lines of force should pass through 

 the circuit of the conductor, and that these lines of force should vary in quantity 

 during the experiment. 



In this case the vortices, of which we suppose the lines of magnetic force 

 to consist, are all within the hollow of the ring, and outside the ring all is at 

 rest. If there is no conducting circuit embracing the ring, then, when the 

 primary current is made or broken, there is no action outside the ring, except 

 an instantaneous pressure between the particles and the vortices which they 

 separate. If there is a continuous conducting circuit embracing the ring, then, 

 when the primary current is made, there will be a current in the opposite 

 direction through C; and when it is broken, there will be a current through C 

 in the same direction as the primary current. 



We may now perceive that induced currents are produced when the elec- 

 tricity yields to the electromotive force, this force, however, still existing 

 when the formation of a sensible current is prevented by the resistance of the 

 circuit. 



The electromotive force, of which the components are P, Q, R, arises from 

 the action between the vortices and the interposed particles, when the velocity 

 of rotation is altered in any part of the field. It corresponds to the pressure 

 on the axle of a wheel in a machine when the velocity of the driving wheel 

 is increased or diminished. 



The electrotonic state, whose components are F, G, II, is what the electromotive 

 force would be if the currents, &c. to which the lines of force are due, instead 

 of arriving at their actual state by degrees, had started instantaneously from 

 rest with their actual values. It corresponds to the impulse which would act 

 on the axle of a wheel in a machine if the actual velocity were suddenly given 

 to the driving wheel, the machine being previously at rest. 



