j^ 



Mh maxwell, on FARADAY'S LINES OF FORCE. 



second current alone were in action. Hence the whole number of cells will be increased 

 by any motion which causes more lines of force to pass through either circuit, and therefore 

 the resultant force will tend to produce such a motion, and the work done by this force 

 during the motion will be measured by the number of new cells produced. All the actions 

 of closed conductors on each other may be deduced from this principle. 



On Electric Currents produced hy Induction. 



Faraday has shewn * that when a conductor moves transversely to the lines of magnetic 

 force, an electro-motive force arises in the conductor, tending to produce a current in it. If the 

 conductor is closed, there is a continuous current, if open, tension is the result. If a closed 

 conductor move transversely to the lines of magnetic induction, then, if the number of lines 

 which pass 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 the 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 change in the number of lines of 

 inductive magnetic action which pass through the circuit ■[•. 



• Exp. Res. (3077), &c. 



+ 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 mass 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 have 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 a linear con- 

 ductor, 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 pro- 

 duce a current, because there is no circuit, it will produce 

 electric tension at the extremities. 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. 



