166 



THE TRANSFORMER. 



voltage as E l and the drop in volts as the length cut off 

 between the two circles. 



Fig. 79 shows the construction just described, the 

 method of drawing it is as follows : From measurements 

 made on the transformer with short-circuited secondary 

 construct the triangle / e, corresponding to full load, as 



FIG. 79 CONSTRUCTION FOR FINDING TERMINAL VOLTS AT ANY POWER FACTOR. 



explained for Fig. 75. From and e describe circles of 

 radius equal to the no-load secondary voltage. For any 

 value of the power-factor, draw from a line E d, making 

 the angle d c equal to the angle of lag or lead. E then 

 gives the terminal voltage and E d the drop corresponding 

 to that power-factor. 



Efficiency of Two Transformers by Double Conversion. Some- 

 times it is found desirable to measure both input and output 

 of transformers on the low-pressure side, so that both quan- 

 tities can be measured with equal accuracy and possibly on 

 the same instruments. In this case two similar transformers 

 may be connected with their high-tension windings together. 

 The low-tension winding of one is then supplied with power 

 and the low-tension side of the other is loaded. No 

 measurement is made on the high-tension windings, and the 

 ratio of low-tension input to low-tension output gives the 

 joint efficiency of the two transformers. The efficiency of 

 either is taken as the square root of this. 



Split Dynamometer Method of Measuring Efficiency. A 

 dynamometer consists of a fixed and movable coil through 



