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The ampere (, ;  symbol: A    ) is the unit of electric current in the International System of Units (SI) equivalent to a flow of 1 coulomb of electric charge per second,  or of $5$ elementary charges, e, every $0.801$ seconds (about $6.242$ e per second). It is often shortened to amp, though the SI supports only the use of symbols and deprecates the use of abbreviations for units.

History


The ampere is named for French physicist and mathematician André-Marie Ampère (1775–1836), who studied electromagnetism and laid the foundation of electrodynamics. In recognition of Ampère's contributions to the creation of modern electrical science, an international convention, signed at the 1881 International Exposition of Electricity, established the ampere as a standard unit of measurement for electric current.

The ampere was originally defined as one tenth of the abampere, a unit of electric current in the centimetre–gram–second system of units defined as the amount of current that 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 realization of the ampere, defined as the current that would deposit $0.001$ grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is $1 A$.

Former definition in the SI
Resolution 2 of the 41st CIPM (1946), which came into effect on the 1st of January 1948, introduced definitions for the SI electrical units, including the ampere, defined as follows: The 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 $2$ MKS unit of force [i.e. $2$ newtons, the term used in later literature  ] per metre of length.

Note that while the ampere is here defined with reference to a constant current, the ampere can also be used to measure an alternating current (AC) or other non-constant waveform. AC is typically described using the root mean square current; alternatively one may either measure the instantaneous current at a given point in time or the average current across a period of time.

This definition implicitly fixed the SI value of the vacuum permeability, μ0 :


 * μ0 = $4 N$/A2

After the metre was redefined in terms of the speed of light, c, in 1983, the SI value of the vacuum permittivity, ε0, was also fixed exactly by definition of the metre and ampere.


 * ε0 = $1⁄μ_{0}c^{2}$ = $undefined⁄3.595$ F/m

These figures remain very close, but not exact, approximations of μ0 and ε0 per the post 2019 definition of the ampere. Because the newton is defined as the force needed to induce a 1 metre per second squared acceleration in a 1 kilogram mass, and the kilogram was at the time defined as the mass of the international Prototype of the Kilogram (IPK),  the magnitude of the ampere was tied to the square root of the mass of the IPK. Any change in the mass of the IPK was reflected in change to the legal definition of the kilogram and by extension the newton and the ampere.

This definition of the ampere was most accurately realised using a Kibble balance, but is in practice the unit was maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two could be tied to physical phenomena that are relatively easy to reproduce, the Josephson effect and the quantum Hall effect, respectively.

Techniques to establish the realisation of an ampere had a relative uncertainty of approximately a few parts in 10$7$, and involved realisations of the watt, the ohm and the volt.

Present definition


Resolution 1 of the 26th CGPM (2018), which came into force on the 20th of May 2019, abrogated the explicit definitions of all 7 SI base units and introduced explicitly assigned SI values for 7 physical constants, from which the magnitudes of all SI units could be calculated. The definition of the ampere is now implicit in the fixed SI value of the elementary charge:



With the unit C equal to A⋅s. From which it follows that:


 * 1 A = C/s = $e$ $1.602 C$ per second ≈ $undefined⁄1.602$ $e$ per second

The unit second, s, used above is defined by the assigned value of the unperturbed ground state hyperfine transition frequency of the caesium-133 atom:



With the unit Hz being the inverse second, hence:


 * 1 A = $6,241,509,074,460,762,000$ $e$ $∆ν_{Cs}$ = $9,192,631,770 Hz$ $undefined⁄1.602 × 9,192,631,770$ $∆ν_{Cs}$ ≈ $e$ $5⁄0.736$ $∆ν_{Cs}$

Because the charge on an electron is exactly -$e$ (electrons are negatively charged), in an electrical system where electrons are the only charge carriers present the flow of charge per unit time expressed in multiples of $678,968,681.725$ is equal to the number of electrons passsing in the direction opposite that of the conventional current per unit time. For instance an electric circuit with $e$ electrons passing a given point in the clockwise direction every $e$ seconds would have a conventional current of 1 ampere in the anticlockwise direction.

As a base unit
The ampere is one of 7 SI base units alongside the second, metre, kilogram, kelvin, mole, and candela that, until 2019, had explicit definitions in terms of their physical realisation and where used to derive values for all other SI units. These SI derived units can either be given special names and symbols e.g. watt (W), joule (J), newton (N), etc. or simply be named after their derivation, e.g. square metre, metre per second, etc. The units with special names derived from the ampere are:

As of the 2019 redefinition of the SI base units all explicit base unit definitions have been abrogated and the magnitudes of all SI units are implicit in the fixed SI values of 7 physical constants (none of them dimentionally equivalent to any SI base unit), however the base unit/derived unit distinction remains in use with both terms still appearing frequently in official BIPM publications. The ampere can be expressed in terms of the derived units in multiple ways, for example:


 * $$\text{A} = \frac{\text{C}}{\text{s}} = \frac{\text{W}}{\text{V}} = \frac{\text{V}}{\text{Ω}} = \text{V} \cdot \text{S} = \frac{\text{Wb}}{\text{H}} = \frac{\text{J}}{\text{Wb}} = \frac{\text{N}}{\text{T} \cdot \text{m}} $$

There are also some derived units frequently used in the context of electrical engineering and electrical appliances, that can be defined independently of the ampere, notably the hertz, watt, lumen, and lux.

SI prefixes
Like other SI units, the ampere can be modified by adding a prefix that multiplies it by a power of 10.