Demonstration model of a moving ironammeter. As the current through the coil increases, the plunger is drawn further into the coil and the pointer deflects to the right.
As of the2019 revision of the SI, the ampere is defined by fixing theelementary chargee to be exactly1.602176634×10−19 C,[6][9] which means an ampere is an electric current equivalent to1019 elementary charges moving every1.602176634 seconds, or approximately6.241509074×1018 elementary charges moving in a second. Prior to the redefinition, the ampere was defined as the current passing through two parallel wires 1metre apart that produces a magnetic force of2×10−7newtons per metre.
The earlierCGS system has two units of current, one structured similarly to the SI's and the other usingCoulomb'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 mathematicianAndré-Marie Ampère (1775–1836), who studiedelectromagnetism and laid the foundation ofelectrodynamics. In recognition of Ampère's contributions to the creation of modern electrical science, an international convention, signed at the 1881International 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 ofelectric current in thecentimetre–gram–second system of units. That unit, now known as theabampere, was defined as the amount of current that generates a force of twodynes 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 theMKSA system would be conveniently sized.
The "international ampere" was an early realisation of the ampere, defined as the current that would deposit0.001118 grams of silver per second from asilver nitrate solution. Later, more accurate measurements revealed that this current is0.99985 A.[12]
Sincepower is defined as the product of current and voltage, the ampere can alternatively be expressed in terms of the other units using the relationshipI =P/V, and thus 1 A = 1 W/V. Current can be measured by amultimeter, a device that can measure electrical voltage, current, and resistance.
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed onemetre apart in vacuum, would produce between these conductors a force equal to2×10−7newtons 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 was used in the formal definition of the ampere, giving thevacuum magnetic permeability (magnetic constant,μ0) a value of exactly 4π × 10−7henries per metre (H/m, equivalent to N/A2). The SI unit of charge, thecoulomb, was then defined as "the quantity of electricity carried in 1 second by a current of 1 ampere".[13]: 144 In general, chargeQ was determined by steady currentI flowing for a timet asQ =It.
Techniques to establish the realisation of an ampere had arelative uncertainty of approximately a few parts in 107, and involved realisations of the watt, the ohm and the volt.[17]
The2019 revision of the SI defined the ampere by taking the fixed numerical value of theelementary chargee to be1.602176634×10−19 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 thecaesium-133 atom.[18]
The SI unit of charge, thecoulomb, "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 (approximately6.241509×1018 elementary charges) going past a given point per second, or equivalently 1019 elementary charges every1.602176634 seconds:
With the second defined in terms of∆νCs, the caesium-133 hyperfine transition frequency, the ampere can be expressed in terms ofe and∆νCs:[20]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 "thebattery charge is30000 C"). The relation of the ampere (A = C/s) to the coulomb (C) is the same as that of thewatt (W = J/s) to thejoule (J).
There are also some SI units that are frequently used in the context ofelectrical engineering and electrical appliances, but are defined independently of the ampere, notably thehertz,joule,watt, candela,lumen, and lux.
^"2. SI base units",SI brochure (8th ed.), BIPM,archived from the original on 7 October 2014, retrieved19 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 fromthe original(PDF) on 14 August 2017, retrieved21 November 2011
