A Double Constant Current Source for Cryogenic Current Comparators and Its Applications

A simple double current source, designed for supplying two windings of a cryogenic current comparator (CCC) is described. The ratio of the two currents is preadjusted by means of an auxiliary resistor ratio and precisely regulated by a CCC associated with a SQUID detector. This enables accurate ratio calibration of four-terminal standard resistors at room temperature, taking advantage of high accuracy (better than 1 × 10-9) and resolution (5 × 10-9 A in a one-turn winding) of the CCC. Operational experience in the calibration of standard resistors (1:10 ratio) is reported. In addition, the use of this double source is suggested for the realization of a cryogenic voltage standard.