The Maintenance and Comparison of Standards of Electromotive Force at the BIPM

This article describes improved standard cell comparison equipment, temperature-controlled enclosures, and measurement techniques used to relate reference groups of high-quality standard cells to the BIPM (Bureau International des Poids et Mesures) Josephson-effect standard with an uncertainty of less than 10 nV. This performance is demonstrated by results obtained over the last four years. The same techniques are now being applied in the routine comparison of international standards of electromotive force by the transfer of standard cells. An analysis of the behavior of the cells that have participated in comparisons over the last 12 years indicates that the uncertainty due to the instability of cells as transfer standards is about 0.5 μV, i.e., one hundred times greater than the best reproducibility in the laboratory. A comprehensive tabulation of uncertainties shows this to be by far the greatest single contribution.