, the ampere
, 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.
The ampere is named after André-Marie Ampère, one of the main discoverers of electromagnetism.
The unit of electric charge, the coulomb, is defined in terms of the ampere: one coulomb is the amount of electric charge carried in a current of one ampere flowing for one second.
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.