186 ON FARADAY'S LINES OF FORCE. 



through it does not change during the motion, the electro-motive forces in the 

 circuit will be in equilibrium, and there will be no current. Hence the electro- 

 motive forces depend on the number of lines which are cut by the conductor 

 during the motion. If the motion be such that a greater number of lines pass 

 through the circuit formed by the conductor after than before the motion, 

 then the electro-motive force will be measured by the increase of the number 

 of lines, and will generate a current the reverse of that which would have 

 produced the additional lines. When the number of lines of inductive magnetic 

 action through the circuit is increased, the induced current will tend to diminish 

 the number of lines, and when the number is diminished the induced current 

 will tend to increase them. 



That this is the true expression for the law of induced currents is shewn 

 from the fact that, in whatever way the number of lines of magnetic induction 

 passing through the circuit be increased, the electro-motive effect is the same, 

 whether the increase take place by the motion of the conductor itself, or of other 

 conductors, or of magnets, or by the change of intensity of other currents, or 

 by the magnetization or demagnetization of neighbouring magnetic bodies, or 

 lastly by the change of intensity of the current itself. 



In all these cases the electro-motive force depends on the cliange in the 

 number of lines of inductive magnetic action which pass through the circuit*. 



* The electro-magnetic forces, which tend to produce motion of the material conductor, must be 

 carefully distinguished from the electro-motive forces, which tend to produce electric currents. 



Let an electric current be passed through a maas of metal of any form. The distribution of 

 the currents within the metal will be determined by the laws of conduction. Now let a constant 

 electric current be passed through another conductor near the first If the two currents are in the 

 same direction the two conductors will be attracted towards each other, and would come nearer if 

 not held in their positions. But though the material conductors are attracted, the currents (which 

 are free to choose any course within the metal) will not alter their original distribution, or incline 

 towards each other. For, since no change takes place in the system, there will be no electro-motive 

 forces to modify the original distribution of currents. 



In this case we hare electro-magnetic forces acting on the material conductor, without any 

 electro-motive forces tending to modify the current which it carries. 



Let us take as another example the case of ft linear conductor, not forming a closed circuit, 

 and let it be made to traverse the lines of magnetic force, either by its own motion, or by changes 

 in the magnetic field. An electro-motive force will act in the direction of the conductor, and, as it 

 cannot produce a current, because there is no circuit, it will produce electric tension at the extremi- 

 ties. There will be no electro-magnetic attraction on the material conductor, for this attraction 

 depends on the existence of the current within it, and this is prevented by the circuit not being closed. 



Here then we have the opposite case of an electro-motive force acting on the electricity in the 

 conductor, but no attraction on its material particles. 



