THE TRANSFORMER. 171 



on Hopkinson's suggestions for testing transformers. It has 

 the advantage that each of the two transformers being tested 

 supplies power to the other, so that each transformer, although 

 fully loaded, derives its power from the other and at the 

 same time supplies power to it. Consequently the energy 

 required from the supply circuit is only small, being that 

 necessary to make up the losses in the two transformers. 

 Also the losses are measured directly and are not obtained 

 by subtracting the output from the input. In this way 

 the result of a small percentage error in measurement 

 affects the total efficiency to a much smaller extent. 



The method is an indirect one, i.e., the input and output 

 are not measured directly, but the losses in the transformers 

 are measured and the output of one of them is measured ; 

 the input is then calculated thus 



power supplied = output -(- losses in transformers. 



The joint efficiency of the transformers is then 

 output output 



input output -f losses 



Since the two transformers are taken to be similar, they 

 may be assumed to have the same losses, and 



the efficiency of each = or 



output + total losses 



efficiency of each = ^ j o i nt efficiency. 



By the use of two wattmeters suitably connected it is 

 further possible to measure the power spent in overcoming 

 the iron and copper losses separately. 



The method of carrying out the test is in outline as follows. 

 Two similar transformers are both connected to the same 

 alternating circuit usually the low-pressure winding is the 

 one supplied with current, but this is only a matter of conr 

 venience. For the purposes of explanation the winding 

 supplied with current will be called the primary. The 

 other windings of the transformers are connected so as to be 

 in opposition, i.e., the pressure generated in each will tend 

 to send a reverse current through the other. If the pressure 

 applied to both transformers is the same, no current will 

 flow in the circuit formed by the windings, which are con- 

 nected together. In order that a current may flow through 

 these windings, an auxiliary transformer is put in series 

 with the mains and the primary of one transformer, so as 

 to make its voltage either higher or lower than that given 



