134 THE TRANSFORMER. 



leakage drop, since the copper drop and the applied 

 voltage are not exactly in opposition of phase, although 

 they are nearly so at full load. 



The method just given for determining the drop in a 

 transformer is difficult to carry out with great accuracy, 

 especially if the voltages to be dealt with are fairly high. 



For instance, an error of 1 per cent, in the readings of the 

 voltmeter may produce an error of 10 to 20 per cent, or more 

 in the value of the drop. 



Thus if the secondary voltage is 200, it will probably be 

 difficult to read reliably the difference between 199 and 200 

 volts on the voltmeter. Since the total drop even at high 

 loads would possibly be only six to eight volts, it is evident 

 that the percentage accuracy in readings of the drop will 

 not be high! Also, if the primary voltage is high, say 1,000 

 volts, it is impossible to regulate this so as to maintain it at 

 exactly the same value. 



The following method has the advantage that the drop of 

 voltage is measured directly, instead of being only a small 

 fraction of the total volts registered. In this case two 

 similar transformers, or two transformers giving the same 

 ratio of transformation, are required, and are both connected 

 together to the supply mains primary and primary or 

 secondary and secondary, whichever is the most convenient. 



When two similar transformers have their primary wind- 

 ings connected to the same supply, their secondaries will then 

 give the same voltage, and if joined together in opposition 

 would exactly neutralise each other. Consequently a volt- 

 meter connected in series with the two windings under these 

 conditions would read zero. 



If. now, one transformer be loaded, it will give a slightly 

 lower secondary voltage on account of the drop in its windings. 

 The voltmeter would then give a reading which is the difference 

 between the secondary voltage of the transformer without 

 drop and that of the loaded transformer. That is to say, 

 the voltmeter will read the drop in the loaded transformer 

 directly. 



It is obvious that the second transformer is prevented 

 from contributing current to the load circuit by the volt- 

 meter in series with it. (See Fig. 62.) 



