# Ampere

The ampere (/ˈæmpɛər/, ;[1][2][3] symbol: A),[4] often shortened to amp,[5] is the unit of electric current in the International System of Units (SI). One ampere is equal to 6.241509074×1018 electrons worth of charge moving past a point in a second.[6][7][8] It is named after French mathematician and physicist André-Marie Ampère (1775–1836), considered the father of electromagnetism along with Danish physicist Hans Christian Ørsted.

ampere
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.
General information
Unit systemSI
Unit ofelectric current
SymbolA
Named afterAndré-Marie Ampère

As of the 2019 redefinition of the SI base units, the ampere is defined by fixing the elementary charge e to be exactly 1.602176634×10−19 C (coulomb),[6][9] which means an ampere is an electrical 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 that would need to be passed through 2 parallel wires 1 metre apart to produce a magnetic force of 2×10−7 newtons per metre.

The earlier CGS system had two definitions of current, one essentially the same as the SI's and the other using electric charge as the base unit, with the unit of charge defined by measuring the force between two charged metal plates. The ampere was then defined as one coulomb of charge per second.[10] In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second.

## 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 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.[12] Later, more accurate measurements revealed that this current is 0.99985 A.

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 2×10−7 newtons 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:

${\displaystyle {\rm {1\ A=1{\frac {C}{s}}.}}}$

In general, charge Q was determined by steady current I flowing for a time t as Q = It.

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.[17]

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 charge e to be 1.602 176 634 × 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:

${\displaystyle {\rm {1\ A=1\,{\text{C/s}}.}}}$

In general, charge Q is determined by steady current I flowing for a time t as Q = I t.

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.

## Units derived from the ampere

The international system of units (SI) is based on 7 SI base units the second, metre, kilogram, kelvin, ampere, mole, and candela representing 7 fundamental types of physical quantity, or "dimensions", (time, length, mass, temperature, electrical current, amount of substance, and luminous intensity respectively) with all other SI units being defined using these. These SI derived units can either be given special names e.g. watt, volt, lux, etc. or defined in terms of others, e.g. metre per second. The units with special names derived from the ampere are:

Quantity Unit Symbol Meaning In SI base units
Electric charge coulomb C ampere second A⋅s
Electric potential difference volt V joule per coulomb kg⋅m2⋅s−3⋅A−1
Electrical resistance ohm Ω volt per ampere kg⋅m2⋅s−3⋅A−2
Electrical conductance siemens S ampere per volt or inverse ohm s3⋅A2⋅kg−1⋅m−2
Electrical inductance henry H ohm second kg⋅m2⋅s−2⋅A−2
Electrical capacitance farad F coulomb per volt s4⋅A2⋅kg−1⋅m−2
Magnetic flux weber Wb volt second kg⋅m2⋅s−2⋅A−1
Magnetic flux density tesla T weber per square metre kg⋅s−2⋅A−1

There are also some SI units that are frequently used in the context of electrical engineering and electrical appliances, but can be defined independently of the ampere, notably the hertz, joule, watt, candela, 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.

Submultiples Multiples Value SI symbol Name Value 10−1 A dA deciampere 101 A daA decaampere 10−2 A cA centiampere 102 A hA hectoampere 10−3 A mA milliampere 103 A kA kiloampere 10−6 A µA microampere 106 A MA megaampere 10−9 A nA nanoampere 109 A GA gigaampere 10−12 A pA picoampere 1012 A TA teraampere 10−15 A fA femtoampere 1015 A PA petaampere 10−18 A aA attoampere 1018 A EA exaampere 10−21 A zA zeptoampere 1021 A ZA zettaampere 10−24 A yA yoctoampere 1024 A YA yottaampere

## References

1. ^ Jones, Daniel (2011). Roach, Peter; Setter, Jane; Esling, John (eds.). Cambridge English Pronouncing Dictionary (18th ed.). Cambridge University Press. ISBN 978-0-521-15255-6.
2. ^ Wells, John C. (2008). Longman Pronunciation Dictionary (3rd ed.). Longman. ISBN 978-1-4058-8118-0.
3. ^ "ampere". Merriam-Webster Dictionary. Retrieved 29 September 2020.
4. ^ "2. SI base units", SI brochure (8th ed.), BIPM, archived from the original on 7 October 2014, retrieved 19 November 2011
5. ^ 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.
6. ^ a b BIPM (20 May 2019). "Mise en pratique for the definition of the ampere in the SI". BIPM. Retrieved 18 February 2022.
7. ^ "2.1. Unit of electric current (ampere)", SI brochure (8th ed.), BIPM, archived from the original on 3 February 2012, retrieved 19 November 2011
8. ^ Base unit definitions: Ampere Archived 25 April 2017 at the Wayback Machine Physics.nist.gov. Retrieved on 28 September 2010.
9. ^ Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018) (PDF), archived from the original (PDF) on 29 April 2018, retrieved 28 October 2018
10. ^ Bodanis, David (2005), Electric Universe, New York: Three Rivers Press, ISBN 978-0-307-33598-2
11. ^ Kowalski, L (1986), "A short history of the SI units in electricity", The Physics Teacher, Montclair, 24 (2): 97–99, Bibcode:1986PhTea..24...97K, doi:10.1119/1.2341955, archived from the original on 14 February 2002
12. ^ History of the ampere, Sizes, 1 April 2014, archived from the original on 20 October 2016, retrieved 29 January 2017
13. ^ a b International Bureau of Weights and Measures (2006), The International System of Units (SI) (PDF) (8th ed.), ISBN 92-822-2213-6, archived (PDF) from the original on 4 June 2021, retrieved 16 December 2021
14. ^ Monk, Paul MS (2004), Physical Chemistry: Understanding our Chemical World, John Wiley & Sons, ISBN 0-471-49180-2, archived from the original on 2 January 2014
15. ^ Serway, Raymond A; Jewett, JW (2006). Serway's principles of physics: a calculus based text (Fourth ed.). Belmont, CA: Thompson Brooks/Cole. p. 746. ISBN 0-53449143-X. Archived from the original on 21 June 2013.
16. ^ Beyond the Kilogram: Redefining the International System of Units, US: National Institute of Standards and Technology, 2006, archived from the original on 21 March 2008, retrieved 3 December 2008.
17. ^ a b "Appendix 2: Practical realisation of unit definitions: Electrical quantities", SI brochure, BIPM, archived from the original on 14 April 2013.
18. ^ "ampere (A)". www.npl.co.uk. Retrieved 21 May 2019.
19. ^ The International System of Units (SI) (PDF) (8th ed.), Bureau International des Poids et Mesures, 2006, p. 144, archived (PDF) from the original on 5 November 2013.