For over ten years, routine resistance calibrations in the range 1 OMEGA to 10 k(OMEGA) have been carried out at NPL using an automated resistance bridge based around a cryogenic current comparator (CCC) [1]. A further CCC bridge has provided tracability for all measurements to the quantised Hall resistance (QHR) since the adoption of the internationally agreed value of the von Klitzing constant, R_(K-90), in 1990. Oxford Instruments have produced a commercial CCC bridge based on the NPL design which can be combined with a quantum Hall primary standard to provide a complete resistance measurement system. This paper describes the results of a side by side comparison of the Oxford Instruments and NPL CCC bridges and independent QHR systems carried out at NPL in July 1999.
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