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History of the present value of 2e/h commonly used for defining national units of voltage and possible changes in national units of voltage and resistance

The national standards laboratories of most major industrialized countries employ the Josephson effect to define and maintain their national or laboratory unit of voltage VLAB. The value of the Josephson frequency-voltage ratio commonly used for this purpose, 2e/h = 483 594 GHz/VLAB, is now known to be about 8 ppm less than the absolute or International System of Units (SI) value. Consequently, the different national units of voltage are smaller than the SI unit by the same amount. One of the purposes of this paper is to review how this value of 2e/h was selected and, hence, the origin of the present inconsistency between national voltage units and the SI unit. The motivation for such an historical study is the hope that it can benefit the selection of a new, more accurate value of 2e/h planned for the near future. Also discussed is the status of national units of resistance and the effect of defining and maintaining such units using a value of the quantized Hall resistance consistent with the SI, as may be suggested in the near future as well.