8o 



ADVANCED ELECTRICITY AND MAGNETISM. 



dt 



ei, or Z X i, to keep the current from increasing. That is: 



dW _ d$ 

 dt ~~ Zl 'Tt 



(3) 



But Zi is a constant, because i is being kept from changing. 

 Therefore by integrating (3) from W = o and $ = o we get: 



W = Zi$ (4) 



in which W is the work in ergs done to keep a current of i 

 abamperes constant in a coil (of zero resistance) while the mag- 



8< 



oo< 

 oooz 



000 



Fig. 58. 



netic flux through the opening of the coil increases by the amount 

 of <f> maxwells, and Z is the number of turns of wire in the coil. 

 We shall now proceed to the proof of equation (i). Let ZZ, 

 Fig. 58, represent a coil of Z turns of wire. Imagine NS to 

 be a flexible magnet. Let the north pole of this flexible magnet 



Fig. 59. 



be carried through the coil and around to its initial position along 

 the dotted path. The flexible magnet will then link with the 



