In physics, the ampere (symbol: A, often informally abbreviated to amp) is the SI base unit used to measure electrical currentss. By definition, one ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length.
Due to the difficulty in measuring the force between two conductors, the so-called "international ampere" or "statampere" was proposed, defined in terms of deposition rate of silver. It is equal to 0.99985 ampere. This alternative unit is now considered obsolete.