Automated direct comparison of two cryocooled 10 volt programmable Josephson voltage standards

We have performed direct dc comparisons between two cryocooled 10 V programmable Josephson voltage standards. Utilizing an automated synchronization scheme for the voltage reversals enables use of a high-sensitivity analog null detector on its 10 mu V range. No switches or manual operations are necessary to protect the null detector from overload signals. The agreement measured between the two voltage standards at 10 V is 9 parts in 10(12) with a relative combined uncertainty of 29 pV V-1 (k = 2 coverage factor). Since both systems can be operated floating from ground, various grounding configurations of the measurement circuit were investigated to evaluate possible leakage current paths to ground. Comparing the two systems under various grounding conditions enables minimization of leakage-current errors, optimization of system performance by revealing leakage-limiting components, and verification of key elements in the uncertainty budget for the measurement method(4).