INSTRUMENT TRANSFORMERS 581 



When two alternating currents are to be compared, the re- 

 sistances Ri and # 2 should be non-inductive; /i and 7 2 must have 

 the same frequency and wave form and either be in time phase 

 or have a fixed phase difference. 



In applying this general method to the testing of current trans- 

 formers, 1 1 and 7 2 are the currents in the primary and secondary 

 circuits; they will ordinarily be in phase to within 2 or less. As 

 this time-phase difference exists, it is impossible by any adjust- 

 ment of the resistances RI and R 2 to balance the two IR drops. 



In order to bring the detector to zero, unless it be separately 

 excited, it is convenient to inject into the detector circuit an 

 e.m.f. in quadrature with 7 2 . This may be done by the method 



Current Trans. 



7i e fli a 



000000 



s 



a _ 



'ft \ Detector 



m 



Variable Mutual 

 Inductance 



FKJ. 358. Connection for determining Characteristics of current trans- 

 formers. 



used by Hughes and by Heaviside in their inductance bridges, by 

 employing a variable mutual inductance, or air-core trans- 

 former, m, see Fig. 358. 



The arrangement of apparatus shown diagrammatically in 

 Fig. 358 has been used by a number of experimenters, the chief 

 differences being in the form and manner of using the detector. 



Sharp and Crawford use a D'Arsonval galvanometer, the 

 current being rectified by a synchronous reversing key driven by a 

 synchronous motor. The brushes or their equivalent are so 

 mounted that the time phase of the commutation may be altered, 

 for it must be matched with the time phase of the potential 

 difference between a and b. 



Agnew and Silsbee use a vibration galvanometer of special 



design. 



Lot the circuits be as in Fig. 359. 



Then by a double adjustment of R* and m the vibration 

 galvanometer may be brought to zero. 



