The ampere (symbol: A) is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics. In practice, its name is often shortened to amp.
In practical terms, the ampere is a measure of the amount of electric charge passing a point per unit time. Around 6.241 × 1018 electrons passing a given point each second constitutes one ampere.
Ampère’s force law states that there is an attractive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere which states that it is “the constant current which will produce an attractive force of 2 × 10–7 newtons per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum”.
The ampere was originally defined as one tenth of the CGS system electromagnetic unit of current (now known as the abampere), the amount of current which generates a force of two dynes per centimetre of length between two wires one centimetre apart. The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized.
The “international ampere” was an early realisation of the ampere, defined as the current that would deposit 0.001118000 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A.
The ampere is most accurately realised using a watt balance, but is in practice maintained via Ohm’s Law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively.
At present, techniques to establish the realisation of an ampere have a relative uncertainty of approximately a few parts in 107, and involve realisations of the watt, the ohm and the volt.
Proposed future definition
Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges. Since a coulomb is approximately equal to 6.24150948×1018 elementary charges, one ampere is approximately equivalent to 6.24150948×1018 elementary charges, such as electrons, moving past a boundary in one second. The proposed change would define 1 A as being the current in the direction of flow of a particular number of elementary charges per second. In 2005, the International Committee for Weights and Measures (CIPM) agreed to study the proposed change, and, depending on the outcome of experiments over the next few years, to formally propose the change at the 24th General Conference on Weights and Measures (CGPM) in 2011.