Balance methods in alternating-current measurement

C. L. Fortescue: This question of Null measurement is one that has been discussed in the Institute in the past quite a good deal. I think that Null methods of calibrating current transformers are in general use now. We ourselves have used a method of calibrating current transformers which was described by me in the Institute JOURNAL some time ago, in which variable mutual inductances were used. At that time we used d'Arsonval portable type galvanometer, and a synchronous contactor. This method was quite satisfactory so long as the contactor was properly maintained; but as we were using this apparatus right on our test floor, with green operators from time to time, who had to be trained to use the device, it was difficult to get them familiar enough with it to be able to tell when it was working properly, and the results were sometimes erratic.