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The introduction on 1 January 1990 of conventional values for the Josephson constant KJ-90 and the von Klitzing constant RK-90, following their adoption by the CIPM in 1988, has resulted in a significant improvement in the world-wide coherence of electrical measurements. For the BIPM, the formal introduction of these two macroscopic quantum effects into electrical metrology has changed the way in which international comparisons at the highest level are carried out.
The reproducibility of a voltage standard using an array of Josephson junctions is a few parts in 1010 and that of a resistance standard based on the quantum Hall effect is a few parts in 109. No secondary travelling standard, such as a Zener standard or a wire-wound resistor, has a stability comparable with this.
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Summary
