Watt balance. Physicist adjusting a watt balance, electromechanical equipment 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. Photographed at the National Physical Laboratory in Teddington, UK.