526 JAMES CLERK MAXWELL. 



direction in the neighbouring circuit, while on stopping the 

 " primary " current a transient current in the same direction 

 as the primary occurs in the other or " secondary " circuit. 

 This experiment was the origin of the now well-known 

 induction coil. Again, when the current was flowing 

 steadily in the primary circuit, if the secondary circuit were 

 brought nearer to it, a current was induced in the secondary 

 in the direction opposite to that in the primary, and con- 

 tinued during the approach of the circuits. On removing 

 the secondary circuit a transient current was set up in the 

 same direction as that in the primary. 



We cannot here spare space to trace the development 

 of the laws of induced currents. The character of the action 

 may in all cases be inferred from the very concise state- 

 ment of Lenz, generally quoted as Lenz's law, and which 

 may be thus expressed : 



If a conductor move in a magnetic field, an electro- 

 motive force will be induced in the conductor which will tend 

 to produce a current in such direction that the mechanical 

 force upon the conductor tends to oppose its motion. 



This law, taken in conjunction with the statements made 

 above respecting the mechanical action in a magnetic field 

 upon a conductor conveying a current, serves to determine 

 the character of the induced current whenever a conductor 

 moves in the neighbourhood of magnets or electric currents. 

 Moreover, the starting of a current in a neighbouring circuit 

 must have the same effect upon the wire as if the conductor 

 were suddenly brought from an infinite distance into the 

 position which.it actually occupies. Hence Lenz's law will 

 apply to every case of induced currents. 



Maxwell's statement expresses the laws of induced 

 currents quantitatively as well as qualitatively. It is as 

 follows : 



Whenever the number of lines of magnetic force passing 

 through a closed circuit is changed there is an electro-motive force 

 round the circuit represented by the rate of diminution of the 

 number of lines of force which pass through the circuit in the 

 positive direction. 



