Quantum standards for electrical units

Abstract This article gives brief physical descriptions of the Josephson effects in superconductors and the quantum Hall effect in semiconductors and their subsequent development during the last twenty years to furnish quantum standards for voltage and resistance. The units of these electrical quantities, the volt and the ohm, which are derived from the standards are now (or shortly will be) based on combinations of two fundamental constants, h, the Planck constant and e, the electronic charge. Once prescribed but not particularly well defined physical conditions are realized, the values of the standards, unlike those that they replace, are not dependent on the materials or construction techniques of the fundamental devices. With the available measurement systems they can be used to define the values of conventional standards with relative inaccuracies which can be as small as 1 × 10−9.