156 THE TRANSFORMER. 



If the reluctance of the iron path is ^th of the 

 reluctance of the leakage path A B through the air, then 

 xotfth of the lines will pass through the air from A to B, 

 the remainder following the iron path. If the secondary 

 coil carries current, its effect is to oppose the flux produced 

 by the primary, and the current in the primary has to increase 

 in order to force the same number of lines round the circuit. 

 The primary ampere-turns acting on the circuit are thus 

 increased, while the difficulty of sending lines from A to 

 B on account of the secondary reverse ampere turns, 

 through the iron path increases in the same proportion. 

 The reluctance of the air path from A to B remains, however,, 

 as before, so that the increased magnetising force applied 

 will send a correspondingly increased leakage field through 

 this path. Similar reasoning may be applied to trace the 

 effect of load on the secondary leakage field. In either case 

 the amount of leakage increases in practically direct propor- 

 tion with the load. 



Since both primary and secondary leakage are practically 

 equivalent to the addition of an inductance in these circuits, 

 it is evident that they both produce the effect of lessening 

 the secondary terminal voltage, and thus increase the ratio 

 of transformation of a step-down transformer, or decrease 

 the ratio of transformation if the transformer is used for 

 transforming up. 



Effect of Lag in Secondary Circuit. It has already been 

 shown that any current in the secondary circuit must corre- 

 spond to a current in the primary, whose value is equal to 

 the secondary current x by the ratio of transformation. This 

 is due to the fact that the secondary ampere-turns must always 

 be neutralised by an equal number of primary ampere-turns. 



If the secondary circuit is inductive, the secondary current 

 will lag behind the secondary voltage, and, consequently, the 

 secondary ampere-turns will tend to produce a magnetic 

 flux later in phase than the main flux, due to the primary 

 magnetising current. This lagging field must be counteracted 

 by a corresponding extra current in the primary circuit. 

 This primary current will be in phase with the field 

 which it overcomes, and will lag behind the primary voltage. 

 The effect on the primary circuit will be the same as if 

 the secondary current had been in phase with its voltage, 

 producing a non-lagging current in the primary, and an 



