140 THE TRANSFORMER. 



In an alternating circuit the values of C and B are, however, 

 constantly changing. Hence if B is the maximum value of 



the induction, C will be the maximum value of the current. 



(7 

 Thus a virtual value of c will correspond to a maximum 



V* 



induction of B lines as given by the formula above. 



Hence if we use the virtual value of the current which 

 would be read on an ammeter, we must write the formula 

 thus : 



c t = *i- x -313 



y-2 n 



1 ~R" 



i.e., c t = x -221 



p- 



or, ct = x 1-435 



M 



or, c t = ^ B x -565 



M 



Thus in order to produce a given alternating flux in the 

 magnetic circuit of a transformer there must be supplied a 

 definite magnetising current, the actual value of which will 

 depend on the number of magnetic lines and upon the per- 

 meability of the core. Actually the value of the permeability 

 will not be constant, since it will vary during each period 

 as the instantaneous values of the current and field fluctuate. 

 For low values of B, such as are used in practice for trans- 

 formers, no great error is introduced by taking the per- 

 meability as constant, and equal to its value at the maximum 

 induction due to the current employed. 



As in the case of a continuous-current electromagnet, no 

 power is spent in the production of the magnetic field directly, 

 although in both cases a loss necessarily occurs in the 

 resistance of the windings. An additional source of loss 

 exists with an alternating field, owing to the imperfect nature 

 of the iron, which absorbs power in overcoming its molecular 

 friction or hysteresis, and also has local eddy currents pro- 

 duced in it, which entail a loss of power. The power, 

 which is thus always required in order to produce an alternat- 

 ing field in the core of a transformer, is really spent in over- 

 coming incidental losses, and not in producing the alternating 

 flux, which, as already stated, does not require an expenditure 

 of energy. The magnetising current, which is given by the 

 formula above, is an idle current, not requiring any power 



