Present state of the electrical units

The situation with regard to the basic electrical SI units has changed remarkably over the past five years. Emphasis has shifted from the definition of the ampere and its realisation using a current balance to realisation of the electrical watt in terms of the mechanical watt, or the volt in terms of mechanical potential energy, because these latter routes offer sub-part-per-million accuracy. Measurements of fundamental constants which involve a mixture of electrical and mechanical units have provided another accurate way of relating electrical units to the metre, kilogram and second. Moreover, once they have been realised in these new ways, the electrical units can be maintained, using the Josephson and quantum Hall effects, with 1 in 108 precision. The paper describes the hundredfold improvement in the accuracy with which the electrical SI units can now be realised, conserved and disseminated and which culminated in the change in the values of the maintained reference standards which took place on 1st January 1990.