Watt balance, close-up. This electromechanical apparatus is used to precisely measure the definition of an amp. One amp is presently one coulomb of electrical charge per second, which was defined using findings from ampere balances. The watt balance is expected to produce a more accurate definition than the ampere balances, as it has an extra calibration step to ensure greater precision. This research is part of attempts to redefine the standard international (SI) definition of units such as the kilogram, ampere and related units. Photographed in 2014, at the National Physical Laboratory, Teddington, UK.