ISO 



THE TRANSFORMER. 



If this is done, the windings of the transformer may be re- 

 duced until the total voltage of the two windings equals 

 the voltage of the original primary. 



Thus, suppose a transformer with a ratio of trans- 

 formation of 5 to 1 to be wound with separate primary and 

 secondary coils of 500 and 100 turns, i.e., 600 turns total. 

 If the two windings are joined in series as shown in Fig. 84, 



*500V* 



I A 



VwwW 



5A 



-100V- 



Fio. 84. PRINCIPLE OF THE AUTO-TBANSFOBMER. 



the number of turns required is only a total of 500, if the 

 voltage is applied to the ends of the whole winding, since 

 the ratio of primary turns to secondary turns still remains 

 5 to 1, and with the same magnetic induction in the 

 core the primary back voltage and the secondary voltage 

 will both remain exactly as before. The ratio between 

 primary and secondary currents remains also the same as 

 before, but is not produced quite in the same manner, since 

 the primary current will flow into the secondary winding, 

 and take the place of some of the current which would have 

 to be induced hi the secondary winding if the two coils had 

 remained separated. 



This will be rendered clear by a consideration of the 

 currents indicated by arrows in Fig. 84, together with the 

 following explanation. 



The primary and secondary coils still act in the same 

 way as before they were joined, as regards their magnetising 

 effect on the core. The windings are so connected that when 

 joined to the source of supply on no-load, the magnetising 

 current flows through them all, and in doing so produces a 

 magnetic flux in the core, which in turn produces the back 

 electromotive force in the windings, opposing the applied 

 voltage. This back electromotive force is induced equally 

 in all the windings, and is the voltage in virtue of which 

 the secondary coil is able to supply current. 



