New volt, new ohm, new temperature scale

Abstract

The article presents the revised practical definitions of the volt and the ohm that came into force on 1 January 1990, together with the introduction of the International Temperature Scale ITS-90. Although the fundamental definitions of the SI system remain unchanged, the new standards lead to small but significant corrections in high-precision measurements of voltage, resistance, and temperature. The electrical units are now realized using quantum phenomena: the Josephson effect for voltage and the quantum Hall effect for resistance, providing exceptional stability and reproducibility. The physical principles underlying these effects and their connection to fundamental constants are explained. Finally, the structure of ITS-90 is outlined, including fixed points, interpolation schemes, and the experimental techniques applied across different temperature ranges.