506 Mr. A. Campbell on the 



I have tried the method with a ring of stampings from 

 ordinary transformer sheet, and obtained a result in very- 

 fair agreement with the ordinary wattmeter method; but 

 further experiments are desirable. I have found the method 

 convenient for testing small samples of iron for telephonic 

 work where the tests have to be made for very low values of 

 %C (say about 0*01), and in such cases the method is so 

 sensitive that it is easy to test very small rings weighing 

 only a gram or two. When the eddy currents are very 

 small, equation (24) becomes m = M ; and since 



m = 47r x 10~ 9 N 1 N' 2 )Lt5/(circum£. of ring), 



where s — section of ring, we can at once find the permea- 

 bility fi for various values of Ix (and hence of %). In such 

 cases I have found the values obtained at moderate fre- 

 quencies in agreement with the results of ballistic tests. 



When the eddy currents are large enough to affect the 

 phase of <£, but do not alter its magnitude appreciably, as 

 Mr. T. L. Eckersley has pointed out to me, we can still find jjl ; 

 in the galvanometer circuit in fig. 3 the vectors QI X , coMI ly 

 and ft>N 2 <I> form a right-angled triangle (<I> being the effective 

 value), and hence 



co 2 ^ 2 2 ^ 2 = (Q 2 + co 2 W)I 1 2 . .... (25) 



Thus we can find <E> and //, for any value of I, by observing 

 Q, M, and the frequency. 



• § 5. Tests of Current Transformers. 



In current transformers used with ammeters, wattmeters, 

 &c, it is necessary to be able to test accurately the ratio 

 of current transformation (I1/I2) and the angle of lag (<£) 

 between the secondary and the reversed • primary current. 

 The above method may be used for this as shown in fig. 4, 

 where the transformer has any desired load ; but part of the 

 load is the known resistance S. By considering the galvano- 

 meter circuit, when a balance is obtained, the vectors ©Mix, 

 QI 1} and SI 2 form a right-angled triangle, and hence 



tan<^ = ^ 1 and I 1 2 /I 2 2 = S 2 /(Q 2 + MW). . . (26) 



S 2 / M 2 ce 2 \ 

 When <£ is small, this ==— M — \ 



