Techniques for the calibration of standard current transformers up to 20 kc/s

When standard current transformers are tested at high audio frequencies the errors and uncertainties in the measurements are liable to be great owing to the effects of stray magnetic fields on the large unshielded resistors which of necessity have to be used. The sources of stray fields are examined and means are suggested for substantially reducing them. It is shown that the 4-terminal resistance standards used in current-transformer testing bridges are very frequency-dependent and present techniques limit the accuracy of measurement at 20 kc/s to 2 parts in 104 in ratio and 0.8¿ in phase. New techniques are described by which the errors and uncertainties can be reduced to give an accuracy of 5 parts in 105 and 0.1¿ at 20 kc/s.