INDUCTION OF CURRENTS. 749 



duces an INVERSE momentary current in a neighbour- 

 ing conducting circuit. 



II. A current which ceases, a current which recedes, or 

 a current which decreases in strength from any cause, 

 induces a DIRECT momentary current in a neighbouring 

 circuit. 



The ' induced ' current has not the same duration 

 as the ' inducing ' current ; it lasts only as long as the 

 change lasts which the inducing current undergoes. 

 If the circuit is closed or broken, the duration of the 

 induced current is exceedingly short ; it is somewhat 

 longer when the induced current is developed by gradual 

 changes in the relative position of the two conductors. 

 As long as the distance between two circuits is the 

 same, and one is traversed by a continuous current 

 of constant strength, no current is induced in the 

 adjacent conductor. 



In order to produce by induction currents of suffi- 

 cient intensity to be rendered manifest without specially 

 delicate instruments, two spiral coils of wire are used 

 as conductors, one of which is within the other. In 

 such an arrangement, each turn of one coil acts upon all 

 neighbouring turns in the other coil, and a much greater 

 effect is thereby obtained than would be produced 

 if two wires were stretched side by side, so that any 

 portion of one wire could act inductively only upon 

 an equal length of the other. 



The coil through which the inducing current of the 

 battery passes is called the primary coil, the current 

 itself the primary current. The second coil, in which 

 the induced current is developed, is termed the secondary 

 coil, the current itself the secondary current. The 



