ON PHYSICAL LINES OF FORCE. 477 



We shall in the first place examine the process by which the lines of force 

 are produced by an electric current. 



Let AB, Plate VIII. , p. 488, fig. 2, represent a current of electricity in the 

 direction from A to B. Let the large spaces above and below AB represent the 

 vortices, and let the small circles separating the vortices represent the layers of 

 particles placed between them, which in our hypothesis represent electricity. 



Now let an electric current from left to right commence in AB. The 

 row of vortices gh above AB will be set in motion in the opposite direction 

 to that of a watch. (We shall call this direction +, and that of a watch .) 

 We shall suppose the row of vortices kl still at rest, then the layer of particles 

 between these rows will be acted on by the row gh on their lower sides, and 

 will be at rest above. If they are free to move, tKey will rotate in the 

 negative direction, and will at the same time move from right to left, or in 

 the opposite direction from the current, and so form an induced electric current. 



If this current is checked by the electrical resistance of the medium, the 

 rotating particles will act upon the row of vortices Id, and make them revolve 

 in the positive direction till they arrive at such a velocity that the motion of 

 the particles is reduced to that of rotation, and the induced current disappears. 

 If, now, the primary current AB be stopped, the vortices in the row gh will 

 be checked, while those of the row kl still continue in rapid motion. The 

 momentum of the vortices beyond the layer of particles pq will tend to move 

 them from left to right, that is, in the direction of the primary current ; but 

 if this motion is resisted by the medium, the motion of the vortices beyond pq 

 will be gradually destroyed. 



It appears therefore that the phenomena of induced currents are part of the 

 process of communicating the rotatory velocity of the vortices from one part of 

 the field to another. 



As an example of the action of the vortices in producing induced currents, 

 let us take the following case : Let B, Plate VIII., p. 488, fig. 3, be a circular 

 ring, of uniform section, lapped uniformly with covered wire. It may be shewn 

 that if an electric current is passed through this wire, a magnet placed within 

 the coil of wire will be strongly affected, but no magnetic effect will be produced 

 on any external point. The effect will be that of a magnet bent round till 

 its two poles are in contact. 



If the coil is properly made, no effect on a magnet placed outside it can 



