The Calibration of Current Transformers by Means of Mutual Inductances

Reasons are given for the selection of mutual inductances as a means for calibrating current transformers, the most important of which is the wide use to which they may be put besides the calibration of current transformers. The design and manufacture of the apparatus are described, reasons being given for the choice of the form of the secondary coil of the mutual inductance, which is a uniformly wound ring, and of the material used for the core, namely, marble. The formula used in calculating the values of mutual inductance is given and its limitations stated. Variations in physical dimensions are given and errors in calculated value discussed. The method of calibrating the mutual inductances is described, which illustrates also one method of using the apparatus for measuring mutual inductance. The method of measuring ratios and phase displacements by means of the apparatus, and the merits of various instruments for obtaining a balance, are described and discussed. An artificial method of loading the transformers under test is proposed. Its advantages are pointed out. The use of the apparatus for measuring mutual inductance, self-inductance and alternating-current resistance is described. In conclusion it is stated that the apparatus has been satisfactory, and has given little difficulty in manufacture.