LEAKAGE CURRENT DETECTION IN CRYOGENIC CURRENT COMPARATOR BRIDGES

Several tests have been developed to locate leakage currents in cryoge nic current comparator (CCC) resistance ratio bridges used at NIST to measure ratios of 1000 OMEGA/100 OMEGA, 6453.2 OMEGA/100 OMEGA, and 10 kOMEGA/100 OMEGA. The major advantage of the tests is that they can b e performed in situ using the sensitivity of the CCC bridge. These tes t procedures have been used to reduce the leakage error uncertainty of CCC ratio measurements linking working standards to the quantized Hal l resistance (QHR) and to the NIST calculable capacitor experiment. CC C bridges require that the current which passes through a standard res istor must equal the current through the appropriate CCC winding to ve ry high precision. This can be difficult to verify at or below 1 pA be cause a large number of possible leakage paths exist. Errors due to si x important leakage current paths are given, and the calculated change s in the resistance ratio are compared with measurements made with a c ontrolled leakage resistance in a 100 OMEGA/1 OMEGA CCC bridge.