An electromechanical apparatus for establishing the watt as an SI electrical unit. Prior to January 1, 1990, there were voltage units in use in the United States, France, the United Kingdom, Russia, and so on which differed from each other (and, with hindsight, from the SI volt) by up to 9 parts per million (ppm). These units were based on results from various current balances. Then results obtained from a different kind of apparatus at the National Physical Laboratory in the United Kingdom and the National Institute for Science and Technology in the United States, which were based on the simple principle described below, enabled the electrical units to be put on a sound SI basis. The accuracy in deriving the SI unit of voltage in this way was considered to be better than 0.2 ppm. Since that date these apparatuses have continued to be refined, and others are being developed in metrological laboratories, with the more ambitious objective of defining the kilogram in terms of fundamental physical constants instead of having to rely on a carefully preserved artifact, the cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures. To do this, the accuracy achieved by the apparatus will have to be demonstrated to be of the order of 0.01 ppm.