128 THE TRANSFORMER. 



In other words, this current will be | period out of phase 

 with the current in the primary (this is shown to be the case 

 in the diagram given later), and hence the field due to the 

 secondary current varies similarly to that due to the primary 

 winding, but is always opposite to it in direction. The 

 effect of the secondary current is consequently to weaken 

 the main field produced by the primary current in the magnetic 

 circuit of the transformer. The weakening of the alternating 

 field by the secondary current wiU at once diminish the back 

 electromotive force. in the primary winding, since this is due 

 to the alternating flux. In consequence of this the applied 

 voltage, being now opposed by a less back electromotive force, 

 will send an increased current through the primary winding ; 

 this will in turn again increase the strength of the field. Matters 

 will finally adjust themselves to the condition in which the 

 alternating flux is equal to its former value* (i.e., practically of 

 the strength sufficient to produce a back electromotive force 

 equal to the applied electromotive force), the primary current 

 having increased exactly in the proportion necessary to 

 counteract the opposing magnetising effects of the current 

 in the secondary winding. 



The magnetising effect of the current in either winding 

 is proportional to the number of turns in the winding 

 multiplied by the current flowing in the winding. Hence 

 a certain current in the secondary will not produce an equal 

 increase of current in the primary, but an increase which 

 bears to the secondary current the inverse ratio of the 

 number of turns in the two windings. Thus if the 

 primary have 10 times the number of turns of the secondary, 

 the increase in current in the primary will be one-tenth of 

 the current of the secondary. This may be put more briefly 

 by saying that the total magnetic flux is constant for a con- 

 stant applied voltage, and consequently the resultant ampere- 

 turns acting on the circuit must be a constant, and hence, 

 if we neglect the no-load current, N 1 C t = N., C. 2 In general, 

 the following relation is very nearly true f 



Primary current Secondary turns >. _/V 2 

 Secondary current ~ Primary turns JVj 



The experimental verification of the statements made 

 regarding the voltage and current ratios will be found in all 



*The question of drop in the conductors is considered later, 

 t The no-load current which makes it not exactly true is discussed later. 



