36 PHYSIOLOGICAL PHYSICS. [Chap. iv. 



there is no current in the secondary coil. If, 

 however, the secondary coil be removed to a little 

 distance, a current immediately appears in it, which 

 continues while the removal is taking place, and 

 ceases when it stops. 4. Or if the secondary coil be 

 approximated to the primary circuit, again a current 

 appears, to cease as soon as the movement is suspended. 

 The same effects will follow if it is the primary coil 

 that is moved in the neighbourhood of the secondary. 

 5. If, while the current is flowing round the battery 

 circuit, its strength be increased, there is again a 

 current induced in the secondary coil, to disappear as 

 soon as the battery current is constant. 6. If, on the 

 other hand, the primary current be diminished, a 

 secondary current is induced, which lasts only while 

 the change is being effected. 



The direction of the induced currents 

 varies. The current that appears on the establish- 

 ment of the primary circuit is in the opposite direc- 

 tion to that of the primary current, and is, therefore, 

 called inverse ; that which appears on interruption is in 

 the same direction as the primary, and is called direct. 

 The current induced by separating the two circuits is 

 direct, that by approximating them is inverse. The 

 current induced by increase of battery strength is in 

 the opposite direction (inverse), that by diminution is 

 in the same direction (direct), to that of the battery 

 current. The direction of the induced currents is 

 easily remembered by the law of Lenz, according 

 to which the direction of the induced current is 

 always in a direction tending -to oppose the change 

 in the primary current. Thus, on the establishment 

 of the primary circuit, there is an induced current in 

 the opposite direction in the secondary circuit, to 

 oppose the establishment ; and on interruption, a cur- 

 rent in the same direction as the primary, i e. to 

 oppose the interruption, to continue the primary. 



