CONTRIBUTIONS TO SCIENCE. 527 



If, then, the number of magnetic lines of force passing 

 through a circuit is diminished, there will be an electro- 

 motive force round the circuit in the direction in which a 

 right-handed screw would rotate if advancing along the lines 

 of force ; a line of force being always supposed to be drawn 

 in the direction in which a north magnetic pole tends to 

 move along it. If the number of lines of force passing 

 through the circuit is increased, the electro-motive force will 

 be in the opposite direction. This law can be deduced from 

 that which expresses the mechanical action upon a circuit 

 conveying a current when placed in a magnetic field together 

 with the principle of the conservation of energy. That it 

 may be numerically true all the quantities involved must be 

 expressed in terms of the electromagnetic system of units. 



The telephone is a beautiful example of the application of 

 this law. Every movement of the iron disc in front of the 

 pole of the magnet alters the number of magnetic lines of 

 force passing through the coils of wire surrounding the pole, 

 and hence induces a current in one direction or the other in the 

 coil, which current, increasing or diminishing the strength 

 of the magnetism in the receiving telephone, causes a cor- 

 responding motion in the iron disc of the receiver, which 

 therefore emits sounds similar to those incident upon the 

 receiving instrument. 



From what has been stated it will appear that the 

 motion of a conductor will produce a current therein only 

 when the conductor is moving in a magnetic field, that is, a 

 portion of space through which magnetic lines of force pass. 

 Faraday supposed that a conductor under these circumstances 

 was thrown into a peculiar condition, which he termed " the 

 electrotonic state," and that a current was induced whenever 

 this state varied. Maxwell showed that this electrotonic 

 state, on the variations of which the induced current in a 

 circuit depends, corresponds to the number of magnetic lines 

 of force which pass through the circuit. Because every 

 change in this quantity involved the action of electromotive 

 force, its relations to electromotive force being the same as 

 those of momentum to force in dynamics, he called the 



