Ayrton-Jones ampere balance. This apparatus is used to measure a standard value for the ampere, the international (SI) unit of electric current. It balances the force between two current-carrying coils (left and right) and a known weight. Invented in 1898 by British physicists William Edward Ayrton and John Viriamu Jones, it was used from 1907 by the UK's National Physical Laboratory (NPL) to determine the ampere. Limitations in this method mean that a device called a watt balance is now used instead. New definitions of the ampere have been proposed for consideration in 2014. Photographed at the NPL, Teddington, UK.