Direct comparison of two independent Josephson voltage standards at 1 V and precision measurements up to 10 V

Two Josephson voltage standards have been compared using a room-temperature electronic nanovoltmeter with a peak-to-peak noise of about 2 nV at the 1-V level corresponding to a RMS uncertainty of 4*10/sup -10/. The excellent stability in maintaining the desired voltage steps makes it possible to obtain recorder traces comparing Nb/Al/sub 2/O/sub 3//Nb Josephson standards with Weston cells and Zener reference standards at 1 V and 10 V. At 10 V the best result shows a peak-to-peak noise of 250 nV corresponding to a RMS uncertainty of 5*10/sup -9/ for a Zener reference and 50 nV corresponding to 1*10/sup -9/ for a series connection of nine Weston cells. As an example for the application of the Josephson standard as a potentiometer the deviation in the linearity of a digital voltmeter is confirmed to be on the order of 0.1 p.p.m. in the range from -10 V to +10 V.<
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