176 



THE TRANSFORMER. 



The action of the auxiliary transformer is to add to the 

 primary voltage of transformer T l the voltage necessary 

 to overcome the copper losses in both transformers. The 

 power read on the wattmeter W., is consequently equal to 

 the losses due to the resistance of the windings of both trans- 

 formers. Included in the reading of W, will also be the 

 losses in the resistance of the conductors joining the trans- 

 formers and in the instruments (with the exception of W } ). 

 To determine the losses in the leads and instruments alone, 

 another measurement should be made. 



(3) Open all the switches and short-circuit the primary 

 windings of the test transformers. Close switches S l and S 3 . 

 The power supplied by the auxiliary transformer and measured 

 by W^ will now be only spent in overcoming the circuit and 

 instrument resistance, and is therefore to be subtracted 

 from the readings of Wz in (2) in order that these readings 

 may give the copper losses of the transformers. 



This test of the instrument and lead losses should be 

 carried out with the voltage of the auxiliary transformer 

 adjusted to give a fairly large current through the instru- 

 ments, in order to make the losses sufficiently large to be 

 easily read. If the instruments or connections are changed 

 during the test the losses should be read after each alteration. 



In calculating the final results, the losses in the instruments 

 for each value of the load should be calculated from the 

 readings. It must be remembered that these losses will be 

 proportional to the square of the load current. 

 DETERMINATION OF EFFICIENCY BY SUMPNER'S METHOD. 



Transformer No Type 



Output .... kw cycles per second. 



Transformation . . volts to . . . volts. 



Supply voltage was maintained constant at 50. 



