174 THE TRANSFORMER. 



both will tend to send current round the circuit in the same 

 direction, and a short circuit will result. 



In order to test whether the connections have been 

 correctly made, before commencing the test close only the 

 switches S v S^. Under these conditions transformer T% 

 will receive current, and its secondary will excite the secondary 

 of T r inducing a voltage in the primary of T l equal to that of 

 the mains. If the connections are correct, the phase of the 

 voltage will make the voltage always opposite to that of the 

 mains. 



In order to test if this is so connect a voltmeter or incan- 

 descent lamp to the terminals of the open switch S 3 . If no 

 voltage is found to exist between them the connections are 

 correct, and the measurements may be proceeded with. 

 If the connections have been reversed, there will be a voltage 

 at the switch equal to twice the voltage of the mains. If 

 this is found, the connections to the secondary of one of the 

 test transformers must be reversed. 



Method of Measurement. After making the connections 

 as above, connect to an alternating supply giving a constant 

 voltage of the required value and frequency. 



(1) Close all switches except $ r Under these conditions 

 T n will receive a magnetising current, and will in addition 

 supply magnetising current to the secondary of T r The 

 total current taken by T 2 will thus be the no-load current 

 of the two transformers. 



The transformer T will not take its full current on account 

 of the self-induction of the secondary winding of the auxiliary 

 transformer which is in series with it. If this winding is 

 short-circuited both transformers T l and T 2 will take equal 

 currents, and there will be no current in their secondary 

 windings (if their ratios of transformation are identical), 

 and it will be immaterial whether the switch S 4 is open or 

 closed. 



In any case, the current taken from the mains will be 

 equal to twice the no-load current of one transformer, and 

 the reading of wattmeter W l will be twice the no-load losses. 

 The no-load current of both transformers is shown on 

 ammeter A if the secondary of the auxiliary transformer is 

 not short-circuited ; if it is short-circuited the current of T 2 

 only will be indicated. 



