ON PHYSICAL LINES OF FORCE. 487 



The only process in which electro-magnetic energy is lost and transformed 

 into heat, is in the passage of electricity from one molecule to another. In all 

 other cases the energy of the vortices can only be diminished when an equivalent 

 quantity of mechanical work is done by magnetic action. 



(6) The effect of an electric current upon the surrounding medium is to 

 make the vortices in contact with the current revolve so that the parts next 

 to the current move in the same direction as the current. The parts furthest 

 from the current will move in the opposite direction ; and if the medium is a 

 conductor of electricity, so that the particles are free to move in any direction, 

 the particles touching the outside of these vortices will be moved in a direction 

 contrary to that of the current, so that there will be an induced current in 

 the opposite direction to the primary one. 



If there were no resistance to the motion of the particles, the induced 

 current would be equal and opposite to the primary one, and would continue 

 as long as the primary current lasted, so that it would prevent all action of 

 the primary current at a distance. If there is a resistance to the induced 

 current, its particles act upon the vortices beyond them, and transmit the motion 

 of rotation to them, till at last all the vortices in the medium are set in 

 motion with such velocities of rotation that the particles between them have no 

 motion except that of rotation, and do not produce currents. 



In the transmission of the motion from one vortex to another, there arises a 

 force between the particles and the vortices, by which the particles are pressed 

 in one direction and the vortices in the opposite direction. We call the force 

 acting on the particles the electromotive force. The reaction on the vortices is 

 equal and opposite, so that the electromotive force cannot move any part of 

 the medium as a whole, it can only produce currents. When the primary 

 current is stopped, the electromotive forces all act in the opposite direction. 



(7) When an electric current or a magnet is moved in presence of a 

 conductor, the velocity of rotation of the vortices in any part of the field is 

 altered by that motion. The force by which the proper amount of rotation is 

 transmitted to each vortex, constitutes in this case also an electromotive force, 

 and, if permitted, will produce currents. 



(8) When a conductor is moved in a field of magnetic force, the vortices 

 in it and in its neighbourhood are moved out of their places, and are changed 

 in form. The force arising from these changes constitutes the electromotive 



