By international agreement, new practical reference standards for the volt and the ohm, based respectively on the Josephson effect and the quantum Hall effect, will be adopted worldwide on 1 January 1990. Until now, national standards laboratories have been using wirewound resistors as practical reference standards for the ohm. Practical reference standards for the volt, on the other hand, have been based on the Josephson effect since the early 1970s. But four different such standards are in use in different countries, and their values all differ significantly from the volt as defined in the Système International d'Unités (SI), the internationally accepted system of measurement units.
On New Year's Day, new practical reference standards for the volt and the ohm will take effect worldwide. Both will be based on highly reproducible quantum effects rather than artifacts. The practical temperature scale will also change.