A transportable voltage standard based on a mercury battery

The NPL (National Physical Laboratory, United Kingdom) moving-coil measurements need a portable voltage source stable to 1 in 10/sup 9/ for a day to relate the results to a Josephson array voltage standard. A carefully-thermostated 1.35-V mercury cell has been shown to be superior to both Zener diodes and Weston cells if its decreasing voltage of about 0.3 mu V/h is compensated by a series voltage increasing at an equal and opposite rate. The nonlinearity with time of the compensation voltage is less than three in 10/sup 5/ of its maximum output, and the resulting nonlinearity of the total voltage from this cause is less than one in 10/sup 9/. The total RMS voltage noise in a 1-Hz bandwidth is less than 1 nV, and the isolation of all internal circuitry from the case is about 10/sup 11/ Omega . It is pointed out that this device could be more generally applicable as a transportable voltage standard.<>