170 



THE TRANSFORMER. 



them and measuring the drop of volts in the windings, or by 

 a potentiometer if the resistance is very low. 



Tabulate the results, calculating the efficiency in each 

 case from the formula given above. 



The following table shows a convenient form for entering 

 up the results of observation and calculation. Two sets of 

 readings are given as specimens. The figures employed in 

 calculating the results from the readings were as follows : 



Resistance of windings r l = -685 ohm r 2 = -183 ohm. 



Constant of dynamometer = K = -975 K* = -95. 



Ratio of windings = 1-935 N t = 248 turns N t = 128. 



DETERMINATION OF TRANSFORMER EFFICIENCY BY SPLIT 

 DYNAMOMETER. 



Transformer No Type 



Output kw cycles per second. 



Transformation. . . .volts to. . . .volts. 



The curves to be plotted from this experiment are the 

 same as those shown in Fig. 73, for the direct reading 

 measurement of efficiency. In performing the experiment 

 for the first time it is advisable to insert a wattmeter in the 

 primary circuit and to check the accuracy of the calculated 

 results by the wattmeter readings. When carefully carried 

 out, the results may be made to agree very closely. 



If sufficient instruments are available, it is convenient 

 to employ dynamometers for measuring the primary and 

 secondary currents, since the value of the mean squares of 

 these currents is required, and is given directly by the 

 dynamometers . 



Sumpncr's Method of Testing Two Transformers This is 

 a method of testing transformers which is analogous to 

 the Hopkinson method of testing dynamos, and founded 



