Discussion on “phase angle of current transformers” (Dawes), and “calibration of current transformers by means of mutual inductances” (Fortescue), Deer Park, Md., July 2, 1915. (see proceedings for May and June, 1915)

George A. Campbell: This paper is of great value to us, and we shall adopt the method of measurement which Mr. Fortescue has developed with results which are, from both the theoretical and the commercial standpoint, so eminently satisfactory. The method belongs in the category of null conjugate branch methods, together with the Wheatstone bridge and the induction balance, which are well known to be the most accurate of all methods, as neither deflections nor test current magnitudes are involved. Without constructing any special apparatus, we have recently been trying the method for measurements of mutual impedance, the circuit being similar to Fig. 12 of the paper, but with the non-inductive resistance R included as in Fig. 1 of the paper, and with a telephone substituted for the synchronous contactor. At telephonic frequencies, accurate measurements are easily and quickly made. In the development of permanent apparatus for the method, it will presumably be advantageous to adopt the circuit idea of Fig. 13, as well as the toroidal type of coil which Mr. Fortescue has shown to be so well adapted for accurate commercial work.