A Low-Temperature Direct-Current Comparator Bridge

The application of superconducting direct-current comparators to the measurement of resistance ratios is described. One comparator consists of a binary set of ratios between 1:1 and 160:1 providing for self-calibration by a buildup procedure. A second comparator exhibiting discrete ratios of 1:1, 10:1, and 100:1 is also described. Ratio uncertainty of less than 1 part in 109 is achieved by enclosing the ratio windings in overlapping toroidal superconducting shields. Superconducting quantum interference devices (SQUID´s) serve as flux sensors for the comparators. One of these current comparators is used to calibrate a 100-¿:1-¿ resistive divider, which at a current of 10 mA exhibits a self-heating error of 0.0023 ppm.