A Direct-Current-Comparator Bridge for Measuring Shunts up to 20000 Amperes

A direct-current-comparator bridge for calibrating four-terminal resistors or shunts at currents up to 20 000 amperes is described. Measurements can be made at up to full rated current of the shunts so that the effects of the load coefficient are included. The resistor under test is compared with a reference resistor of higher value by measuring the ratio of the currents through the two resistors required to produce equal voltages across them. A comparator bridge with a range of 100 amperes and errors of less than 1 ppm has been described previously. Improvements to this bridge have been made, the main one being a reversing feature, which permits the currents through the resistors to be reversed in a few milliseconds. This makes an accurate measurement easier, particularly if there is a change of resistance due to heating. By connecting a second comparator in cascade, the range has been extended to 20 000 amperes at an overall ratio up to 2 × 106: 1, with only a slight loss of accuracy; the errors may be a few parts per million. Other applications of the measuring system are the accurate measurement of large currents or the calibration of transductors.