49 2 INDUCTION. 



circuit, in reference to the magnetic field of a system of magnets or 

 of currents, is ordinarily sufficient to give rise to an induced current 

 in this circuit, which comes under one of the preceding heads. 



4th. Finally, the mere fact of altering in any way the strength of 

 the current in a circuit, even when it is withdrawn from any external 

 action, produces an induction current in this circuit which adds itself 

 to the principal current, and always tends to counteract the change 

 of strength which it experiences ; it is a current of self-induction 

 or an extra-current. 



509. Experiment has established the following general facts in 

 reference to induction currents : 



i st. Whatever be the kind of variation which gives rise to an 

 induction current, two equal variations in opposite directions always 

 give rise to equal and opposite currents. 



2nd. The duration of the induced current is equal to that of the 

 variation of the inducing system. 



3rd. The quantity of electricity set in motion in the induced 

 current by any operation is independent of the duration of the 

 variation, and therefore of that of the induced current itself. 



4th. Lastly, the nature of the conductor in which the induction 

 currents are transmitted is only of importance in so far as it affects 

 the resistance which it brings into the circuit. 



510. Examining the various circumstances in which induction 

 currents are produced, it is easily seen that their common charac- 

 teristic is that of corresponding to a variation in the flow of magnetic 

 force which traverses the induced circuit. This is evident for all 

 the phenomena of the relative displacement of currents or of 

 magnets; experiment shows, moreover, that any displacement, or 

 any deformation of the induced circuit which does not modify the 

 value of the flow which traverses it, never produces induced currents. 



This is also the case with the extra current. For a current gives 

 rise to a magnetic field, and therefore to a flow of force in the 

 surface of the circuit which it traverses. It is easily seen that 

 any change of intensity, or of shape, which modifies this flow, may 

 produce an effect analogous to that which would be produced by the 

 displacement of an external magnet, giving rise to the same variation. 



We are thus led to define the phenomena of induction in the 

 following manner : 



When the flow of magnetic force which traverses a closed circuit is 

 in any way modified, this circuit becomes the seat of a temporary 

 current, the duration of which is equal to that of the variation of 

 the flow. 



