The primary voltage standard system based on the Josephson junction array (JJA) produces the ultimate in accuracy for dc voltage calibration. However, it is only used in the highest level standards laboratories due to the expense and inconvenience of liquid helium cooling. In cooperation with the US Army Research Laboratory, HYPRES has developed a mobile JJA primary voltage standard system using a closed cycle refrigerator (CCR). The system performance is comparable to a conventional liquid helium system. The elimination of liquid helium makes the mobile system not only more economically attractive, but also more convenient to use. The mobile JJA voltage standard system is very suitable for users who lack screw to liquid helium, and for those who require transportability.
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Tang et al. (1997) studied this question.
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