362 ELEMENTARY LESSONS ON [CHAP. x. 



lines-of-force^ then if he and the conductor be moved towards his 

 left hand he will be swimming with the current induced by this 

 motion ; if he be moved toward his right hand, the current will 

 be against him. *0^J 



396. Lenz's Law. In Art. 320 it was laid down that a 

 circuit traversed by a current experiences a force tending to 

 move it so as to include the greatest possible number of lines- 

 of-force in the embrace of the circuit. But if the number of 

 lines-of-force be increased, during the increase there will be an 

 opposing (or negative) electromotive -force set up, which will 

 tend to stop the original current, and therefore tend to stop the 

 motion. If there be no current to begin with, the motion will 

 generate one, which being in a negative direction will tend to 

 diminish the number of lines-of-force passing through the 

 circuit, and so stop the motion. Lenz, in 1834, summed up 

 the matter by saying that in all cases of electromagnetic induction 

 the induced currents have such a direction that their reaction 

 tends to stop the motion which produces them. This is known 

 as Lenz's Law. 



397. Mutual Induction of Two Circuits. In 

 Art. 3 20 it was shown that when two circuits, in which 

 currents of unit strength are flowing, are placed near 

 together, they have a mutual potential whose value we 

 called M. This symbol M, upon investigation, was 

 found to represent the number of lines-of-force which 

 each circuit induced through the other circuit, or was 

 " the number of each other's lines - of- force mutually 

 intercepted by both circuits when each carries unit 

 current." This number depended upon the form and 

 position of the circuits, and was greatest when they 

 were brought as near together as possible. Hence 

 we may regard this quantity M as the " coefficient 

 of mutual induction " of the two circuits ; and any 

 movement of either circuit which alters the number 

 of lines-of-force passing mutually through them, will 

 be accompanied by the production of induced cur- 

 rents in each. It can be shown mathematically that, 

 in the case of two simple circular circuits of equal size, 

 enclosing area S, the greatest number of lines-of-force 



