A Direct-Reading Current-Comparator Bridge for Scaling Four-Terminal Impedances at Audio Frequencies

A direct-reading current-comparator bridge circuit, for scaling four-terminal impedances which do not deviate from nominal by more than a few thousand parts per million, is described. The scaling is performed at constant voltage and the impedances are treated as true four-terminal devices in that, at balance, no current is drawn from the potential terminals. The principal feature of the circuit is the use of a compensation winding to excite the magnetic shield of the current comparator and thus suppress the effect of lead unbalances. Possible applications of the bridge include the scaling of four-terminal resistance standards between 1 milliohm and 100 ohms at frequencies ranging from 50 Hz to 1600 Hz. Construction details and calibration of a current comparator for use in a 10-to-1 ratio bridge are given.