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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.

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.