i68 



ALTERNATING CURRENTS 



coil of the wattmeter. It is evident that Wi measures the sum of the 

 iron or core losses in the transformers. Let next the balance of the 

 secondary e.m.f.'s be disturbed by means of an auxiliary transformer, 

 as shown in Fig. 122, the resistance E in series with the primary P 

 of the auxiliary transformer being so adjusted as to produce the 

 desired current in the primary PI of the first transformer, the current 



FIG. 122. Sumpner's Transformer Efficiency Test. 



being measured by the ammeter AM. The wattmeter W 2 measures 

 the power supplied by the auxiliary transformer. This power 

 corresponds to the copper losses occurring in the transformers as 

 will be evident from Fig. 123, where the two transformers under test 

 are shown entirely disconnected from the mains. Since the secondary 

 e.m.f.'s are now added together instead of opposing each other, it is 



FIG. 123. Measurement of Copper Losses of Two Similar Transformers. 



evident that each transformer behaves as if its secondary were inde- 

 pendently short-circuited, and the wattmeter W 2 measures the sum of 

 the copper losses. Now, the mere connection of the primaries of the 

 transformers across the mains as in Fig. 122 simply introduces 

 two equal and opposite p.d.'s into the local circuit formed by PI, P 2 , 

 and S, and in no way affects the local current, i.e. the current 

 passing through AM and the thick-wire wattmeter coil. Thus the 



