THE TEANSFORMEE. 



153 



It must be remembered that in this experiment the 

 losses which are being measured are due to a high current 

 flowing through a low resistance. Consequently the resistance 

 of leads or of the ammeter in the secondary circuit may be 

 quite appreciable compared with the resistance of the trans- 

 former windings, and if not allowed for may introduce serious 

 errors. The watts supplied to the transformer should conse- 

 quently be measured at the terminals of the transformer itself, 

 so as to eliminate as far as possible losses in the leads. 

 The resistance of the leads forming the secondary circuit, 

 including the ammeter, should be measured, and the watts 

 spent in them subtracted from the watts supplied in order to 

 obtain the watts actually spent in the windings alone. 



In order to illustrate what has just been said, the following 

 figures, taken from an actual test, are given, together with the 

 curve showing the results of the experiment in Fig. 70. The 



140 



120 



100 



80 



60 



40 



20 



6 



8 1O 12 14 

 -Amperes. 

 Fm. 70. COPPER LOSSES IN A TRANSFORMER. 



16 18 2O 



