Summary of International Comparisons of As-Maintained Units of Voltage and Values of 2e/h

Using temperature-regulated transportable standard-cell enclosures, the National Bureau of Standards (NBS), under the auspices of the Bureau International des Poids et Mesures (BIPM), during the period June 1971 through June 1972 has carried out a series of direct comparisons of the units of voltage as-maintained by NBS and BIPM, Sèvres, France; the National Physical Laboratory, United Kingdom; the National Research Council, Canada; the National Standards Laboratory, Australia; and the Physikalisch-Technische Bundesanstalt, Germany. The main purpose of these comparisons was to provide a sound basis for intercomparing values of 2e/h obtained at the various national laboratories via the ac Josephson effect in superconductors. It was found that when converted to a common unit of voltage, most measured values of 2e/h agreed with each other to within the 1 to 2 parts in 107 estimated uncertainty (1 standard deviation) of the volt comparisons. This satisfying result would seem to indicate that serious consideration should be given to adopting a single international value of 2e/h for use in maintaining units of voltage.