AGNEW: TESTING INSTRUMENT TRANSFORMERS 509 



like those of Biirdwood and of Davis issued by the British Admir- 

 alty to the Royal Na\y, this simplified method of finding the 

 Simmer line is more generally applicable in the solution of obser- 

 vations upon celestial bodies situated within the limits of decli- 

 nation for which the tables are computed. 



PHYSICS. — A watthour meter method of testing instrument 

 transformers.^ P. G. Agnew", Bm'eau of Standards. 



Patio and phase-angle measurements of instrument trans- 

 formers can easily be made by the use of two precisely similar 

 watthour meters, which may be either of the portable type, or 

 of the house tj^De provided the disk has been graduated in 100 

 di\dsions. The meters should be adjusted to very nearly the 

 same rate. 



The method depends upon the use of a standard transformer 

 whose ratio and phase angle have been determined in some 

 laboratory equipped for the pm*pose. The standard transformer 

 must have the same nominal range as the one under test. 



For voltage transformers an auxiliary current is passed in 

 series through the cmTent coils of the two meters. The voltage 

 coil of each meter is connected to one of the transformers, and 

 readings are taken showing the difference in speeds of the meters. 

 The meters are then interchanged and readings again taken. 

 If the meters have been operating at, or very near unity power 

 factor, the ratio of the transformer under test is easily computed 

 in terms of the ratio of the standard. The formula is 



Ri — R^ ^ [ ax - Qi bi - 6 2 \ 



Ri \ a-i &2 / 



w^here Ri, R2 = ratios of the transformers, 



Qj, 02 = Number tm'ns of meter A when connected to 



transformers 1 and 2, respectively. 

 61, 62 = same for meter B. 

 Similarly the difference in phase angles can be determined by 

 running the meters at low power factor, which may be conveni- 

 ently done by taking the auxiliary current from one of the other 



1 Detailed paper co appear in the Bureau of Standards Bulletin. 



