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In physics, the ampere (symbol: A, often informally abbreviated to amp) is the SI base unit used to measure electrical currents. The present definition, adopted by the 9th CGPM in 1948 is: "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 definition for the ampere is equivalent to fixing a value of the permeability of vacuum to μ0 = 4π × 10−7 H/m. Prior to 1948, the so-called "international ampere" was used, defined in terms of the electrolytic deposition rate of silver. It is equal to 0.999 85 A.

The ampere is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

The ampere is most accurately realised using a ampere balance, but is in practice maintained via Ohm's Law from the units of voltage and resistance, the volt and the ohm.

The unit of electric charge, the coulomb, is defined in terms of the ampere: one coulomb is the amount of electric charge (formerly quantity of electricity) carried in a current of one ampere flowing for one second.

Since the current is the rate at which charge flows through a surface, and an ampere measures current, one ampere is equal to a flow of one coulomb per second:

\mathrm{A=C/s} \,

Since a coulomb is approximately equal to 6.24 × 1018 elementary charges, one ampere is equivalent to 6.24 × 1018 elementary charges moving through a surface in one second.

Using the SI definitions for the conventional values of the Josephson and von Klitzing constants, the ampere can be defined as exactly 6.241 509 629 152 65 × 1018 elementary charges per second.

For the ampere in electric circuits see Current (electricity)

SI electricity units

See also

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