;>ss 



ELECTRICAL MEASUREMENTS 



with c. When this has been done, that is, at balance, if L 2 is the 

 effective inductance of Z 2 , 



7 2 2 = /2(# 8 2 + w Ll 2) 



and 



Fi 

 F, 



l + /? 2 ) 2 + " 2 L 2 2 Ri + #2 

 *,,'+-=*:?- -^approximately. 



FIG. 365. Vector diagram for Fig. 363. 

 To obtain the phase angle, 



At balance, tan a = -~ and tan 6 = 



Kz 



V z leads Fi by 





T 



tan 



- = tan- 1 



1 



Comparison Tests of Instrument Transformers. 11 Current 

 and potential transformers may be compared with standard 

 transformers of the same rating whose constants are known, by a 

 method developed by Agnew. 



When dealing with current transformers the two primaries are 

 connected in series, while the two secondaries are connected to the 

 current coils of two induction watt-hour meters whose potential 

 coils are excited from a common source. The phase difference 

 between the P.D. applied to the meters and the current in the 

 primaries of the transformers should be adjustable. Fig. 366 

 shows the connections for testing both current and potential trans- 

 formers by this method. If the applied voltage and current were 

 in phase and the meters were perfectly accurate, the relative value 



