506-511] Physical Principles 443 



If there is no battery in the circuit, the total electromotive force will be 

 -7- , and the current originated by this electromotive force is spoken of as 



CLu 



an " induced " current. 



508. In order that the phenomena of induced currents may be consistent 

 with the conservation of energy, it must obviously be a matter of indifference 

 whether we cause the magnetic lines of induction to move across the 

 circuit, or cause the circuit to move across the lines of induction. Thus 

 Neumann's law must apply equally to a circuit at rest and a circuit in 

 motion. So also if the circuit is flexible, and is twisted about so as to change 

 the number of lines of induction which pass through it, there will be an in- 

 duced current of which the amount will be given by Neumann's Law. 



509. For instance if a metal ring is spun about a diameter, the number 

 of lines of induction from the earth's field which pass through it will change 

 continuously, so that currents will flow in it. Furthermore, energy will be 

 consumed by these currents so that work must be expended to keep the ring 

 in rotation. Again the wheels and axles of two cars in motion on the same 

 line of rails, together with the rails themselves, may be regarded as forming 

 a closed circuit of continually changing dimensions in the earth's magnetic 

 field. Thus there will be currents flowing in the circuit, and there will be 

 electromagnetic forces tending to retard or accelerate the motions of the cars. 



510. If, as we have been led to believe, electromagnetic phenomena are 

 the effect of the action of the medium itself, and not of action at a distance, 

 it is clear that the induced current must depend on the motion of the lines of 

 force, and cannot depend on the manner in which these lines of force are pro- 

 duced. Thus induction must occur just the same whether the magnetic field 

 originates in actual magnets or in electric currents in other parts of the field. 

 This consequence of the hypothesis that the action is propagated through the 

 medium is confirmed by experiment indeed in Faraday's original investiga- 

 tions on induction, the field was produced by a second current. 



511. Let us suppose that we have two circuits 1, 2, of which 1 contains 

 a battery and a key by which the circuit 



can be closed and broken, while circuit 2 

 remains permanently closed, and contains a 

 galvanometer but no battery. On closing 

 the circuit 1, a current flows through circuit 

 1, setting up a magnetic field. Some of the 

 tubes of induction of this field pass through 

 circuit 2, so that the number of these tubes 

 changes as the current establishes itself in 

 circuit 1, and the galvanometer in 2 will 

 accordingly shew a current. When the current in 1 has reached its steady 



