Comparison of two Josephson array voltage standard systems using a set of Zener references

It is possible to compare Josephson-array voltage systems to parts in 10(9) via multi-Zener reference interchanges and ultra-lo cv thermal-emf sw itch ing. These techniques were used to achieve a Type A uncertainty of 6 nV/V ( k = 2) at the 1.018 V level, and to establish that the agreement between th e two systems fell within the estimated 4 nV/V Type B relative uncertainty (k = 2). The method is useful in checking system operation under normal mea surement configurations and programmed control. Data also indicated that th ermal-emfs greater than 25 nV have fluctuations that are not perfectly canc eled by either of the two differing measurement algorithms.