Demonstration model of a moving iron ammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
As of the 2019 redefinition of the SI base units, the ampere is defined by fixing the elementary chargee to be exactly 1.602176634×10−19 C,[6][9] which means an ampere is an electric current equivalent to 1019 elementary charges moving every 1.602176634 seconds or 6.241509074×1018 elementary charges moving in a second. Prior to the redefinition the ampere was defined as the current passing through two parallel wires 1 metre apart that produces a magnetic force of 2×10−7newtons per metre.
The earlier CGS system has two units of current, one structured similarly to the SI's and the other using Coulomb's law as a fundamental relationship, with the CGS unit of charge defined by measuring the force between two charged metal plates. The CGS unit of current is then defined as one unit of charge per second.[10]
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 electrical measurement for electric current.
The ampere was originally defined as one tenth of the unit of electric current in the centimetre–gram–second system of units. That unit, now known as the abampere, was defined as the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart.[11] 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 0.001118 grams of silver per second from a silver nitrate solution. Later, more accurate measurements revealed that this current is 0.99985 A.[12]
Since power is defined as the product of current and voltage, the ampere can alternatively be expressed in terms of the other units using the relationship I = P/V, and thus 1 A = 1 W/V. Current can be measured by a multimeter, a device that can measure electrical voltage, current, and resistance.
Former definition in the SI
Until 2019, the SI defined the ampere 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 2×10−7newtons per metre of length.[13]: 113 [14]
Ampère's force law[15][16] states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere.
The SI unit of charge, the coulomb, was then defined as "the quantity of electricity carried in 1 second by a current of 1 ampere".[13]: 144 Conversely, a current of one ampere is one coulomb of charge going past a given point per second:
In general, charge Q was determined by steady current I flowing for a time t as Q = It.
Techniques to establish the realisation of an ampere had a relative uncertainty of approximately a few parts in 107, and involved realisations of the watt, the ohm and the volt.[17]
Present definition
The 2019 redefinition of the SI base units defined the ampere by taking the fixed numerical value of the elementary chargee to be 1.602176634×10−19 when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of ∆νCs, the unperturbed ground state hyperfine transition frequency of the caesium-133 atom.[18]
The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere".[19] Conversely, a current of one ampere is one coulomb of charge going past a given point per second:
In general, charge Q is determined by steady current I flowing for a time t as Q = It.
Constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2 A") and the charge accumulated (or passed through a circuit) over a period of time is expressed in coulombs (as in "the battery charge is 30000 C"). The relation of the ampere (C/s) to the coulomb is the same as that of the watt (J/s) to the joule.
There are also some SI units that are frequently used in the context of electrical engineering and electrical appliances, but are defined independently of the ampere, notably the hertz, joule, watt, candela, lumen, and lux.
^"2. SI base units", SI brochure (8th ed.), BIPM, archived from the original on 7 October 2014, retrieved 19 November 2011
^SI supports only the use of symbols and deprecates the use of abbreviations for units."Bureau International des Poids et Mesures"(PDF). 2006. p. 130. Archived from the original(PDF) on 14 August 2017. Retrieved 21 November 2011.