THE TRANSFORMER. 127 



due almost entirely to self-induction. Consequently, the 

 applied electromotive force is nearly all spent in over- 

 coming the self-induction, and only an exceedingly small 

 part of it is required to produce an energy voltage over- 

 coming the resistance of the winding. 



Thus in the case of a transformer with unloaded secondary 

 winding, the back voltage very nearly equals the applied 

 voltage. In such a case if v is the alternator voltage e l = v 

 very nearly, consequently the current in the winding is 

 exceedingly small. 



In a transformer that winding which has a voltage applied 

 to it from an external source and which carries the magnetis- 

 ing current is called the Primary winding. 



The Secondary winding is the winding which also embraces 

 the magnetic circuit, and through which consequently the 

 magnetic lines also pass, and which has a voltage induced 

 in it due to the magnetic changes set up by the change of 

 current in the primary. 



Thus with no load on the secondary the ratio of voltages 

 will be very nearly 



Primary voltage 4:'^nZN l _ N 1 



Secondary voltage 4-44 rc 



No. of primary windings. 

 No. of secondary windings. 



Effect of Current in Secondary Circuit. The preceding 

 statements were based on the assumption that the magnetis- 

 ing current in the primary winding was the only current 

 acting upon the magnetic circuit of the transformer. If the 

 secondary circuit is closed through a resistance, the voltage 

 induced in the winding by the alternating field will produce 

 a current in it. 



The direction of this current will be opposite to that of 

 the current in the primary, because the voltage induced in 

 the secondary is due to the same causes, and is in the 

 same direction relative to the magnetic circuit as the back 

 voltage of the primary. 



The current in the primary is seat by the alternator in 

 opposition to the back voltage of the winding, but in the 

 secondary it is the induced voltage which produces the 

 current, which, therefore, circulates in the opposite direction 

 round the magn tic circuit to the current in the primary 

 winding. 



