Cryogenic direct current comparators and their applications

Cryogenic direct current comparators, utilizing superconducting shields and Superconducting Quantum Interference Devices (SQUIDS), provide current ratios of up to 100/1 or higher with accuracies of less than or approximately equal to 1 x 10-9 and curent resolutions of less than or approximately equal to 6 x 10-11 A. Two types of comparators differing in the shielding arrangement of the ratio windings are described. One type consists of unit windings inside a seamless Pb tube; the other consists of multi-turn ratio windings within an overlapping toroidal superconducting shield. For both shielding configurations, SQUIDS serve as flux sensors for the comparators. The application of these comparators to the measurement of resistance ratios is described