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 × 10−7newton 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 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:
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.
André-Marie Ampere, the son of a Lyon city official, was born in Polemieux-au-Mont-d'Or, near Lyon.
In general Ampere's law is similar to Gauss's Law of electric fields, except for the fact that it deals with magnetic fields, and uses a line integral instead of the surface integral used in Gauss's Law.
Formerly, the definition involved the force that was produced between parallel wires carrying a current; still earlier, the ampere was defined as a flow of one coulomb per second, where the coulomb (a quantity of electrical charge) was taken as the basic unit.
One ampere is the current which, if it's flowing in these conductors, creates between them a force of 0.2 micronewtons per meter of length.
One ampere of current results from a potential distribution of one volt per ohm of resistance, or from a power production rate of one watt per volt of potential.
The ampere per meter is also the SI unit of "magnetization" in the sense of magnetic dipole moment per unit volume; in this context 1 A/m = 0.001 emu per cubic centimeter.
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