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.
Our most accurate international comparisons in electricity are made by taking the BIPM transportable Josephson and quantum-Hall systems to those national laboratories which have comparable systems. In this way world-wide uniformity has been demonstrated at the level of a few parts in 1010 in voltage standards and a few parts in 109 in resistance standards.
No secondary travelling standard, such as a Zener standard or a wire-wound resistor, has a stability comparable with this.