View source for Ampere

{{Short description|SI base unit of electric current}}
{{Use British English|date=October 2025}}
{{Other uses}}
{{Use dmy dates|date=January 2020}}
{{CS1 config|mode=cs2}}
{{Infobox Unit
| name = ampere
| image = File:Amperemeter hg.jpg
| caption = 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.
| standard = [[SI]]
| quantity = [[electric current]]
| symbol = A
| dimension = I
| namedafter = [[André-Marie Ampère]]
}}
The '''ampere''' ({{IPAc-en|ˈ|æ|m|p|ɛər|audio=LL-Q1860 (eng)-Naomi Persephone Amethyst (NaomiAmethyst)-ampere.wav}} {{respell|AM|pair}}, {{IPAc-en|us|ˈ|æ|m|p|ɪər|audio=LL-Q1860 (eng)-Naomi Persephone Amethyst (NaomiAmethyst)-ampere (alt).wav}} {{respell|AM|peer}};<ref>{{cite EPD|18}}</ref><ref>{{cite LPD|3}}</ref><ref>{{cite Merriam-Webster|ampere|access-date=29 September 2020}}</ref> symbol: '''A'''),<ref>{{Citation|chapter= 2. SI base units|chapter-url= http://www.bipm.org/en/publications/si-brochure/section2-1.html|title= SI brochure|edition= 8th|publisher= BIPM|access-date= 19 November 2011|url-status= live|archive-url= https://web.archive.org/web/20141007203046/http://www.bipm.org/en/publications/si-brochure/section2-1.html|archive-date= 7 October 2014|df= dmy-all}}</ref> often [[Clipping (morphology)|shortened]] to '''amp''',<ref name=BIPM2006>SI supports only the use of symbols and deprecates the use of abbreviations for units. {{cite web |url=https://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |title=Bureau International des Poids et Mesures |year=2006 |page=130 |access-date=21 November 2011 |df=dmy-all |archive-date=14 August 2017 |archive-url=https://web.archive.org/web/20170814094625/http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf |url-status=dead }}</ref> is the unit of [[electric current]] in the [[International System of Units]] (SI).  One ampere is equal to 1 [[coulomb]] (C) moving past a point per second.<ref name= "MEP for amp">{{Cite web|last=BIPM|date=20 May 2019|title=Mise en pratique for the definition of the ampere in the SI|url=https://www.bipm.org/documents/20126/41489676/SI-App2-ampere.pdf/0987a90e-051b-dd7f-827d-3f7b32751a61?version=1.5&t=1637237858322&download=false|access-date=18 February 2022|website=BIPM}}</ref><ref name="BIPMdefinition">{{Citation|chapter= 2.1. Unit of electric current (ampere)|chapter-url= http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ampere.html|title= SI brochure|edition= 8th|publisher= BIPM|access-date= 19 November 2011|url-status= live|archive-url= https://web.archive.org/web/20120203153904/http://www.bipm.org/en/si/si_brochure/chapter2/2-1/ampere.html|archive-date= 3 February 2012|df= dmy-all}}</ref><ref>{{citation|url=http://physics.nist.gov/cuu/Units/ampere.html|title=Base unit definitions: Ampere|archive-url=https://web.archive.org/web/20170425134625/http://www.physics.nist.gov/cuu/Units/ampere.html|archive-date=25 April 2017|website=Physics.nist.gov|access-date=28 September 2010}}</ref> It is named after [[France|French]] mathematician and physicist [[André-Marie Ampère]] (1775–1836), considered the father of [[electromagnetism]] along with [[Denmark|Danish]] physicist [[Hans Christian Ørsted]].

As of the [[2019 revision of the SI]], the ampere is defined by fixing the [[elementary charge]] {{var|e}} to be exactly {{physical constants|e|ref=no}},<ref name="MEP for amp" /><ref name=draft-resolution-A>{{citation
 |title=Draft Resolution A "On the revision of the International System of units (SI)" to be submitted to the CGPM at its 26th meeting (2018)
 |url=https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf
 |access-date=28 October 2018
 |archive-date=29 April 2018
 |archive-url=https://web.archive.org/web/20180429025229/https://www.bipm.org/utils/en/pdf/CGPM/Draft-Resolution-A-EN.pdf
 |url-status=dead
 }}</ref> which means an ampere is an electric current equivalent to {{val|e=19}} elementary charges moving every {{val|1.602176634}} seconds, or approximately {{val|6.241509074|e=18}} 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 {{val|2|e=-7}} [[newton (unit)|newtons]] per metre.

The earlier [[Centimetre–gram–second system of units|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.<ref>{{Citation | author-link = David Bodanis| last = Bodanis | first = David | year = 2005 | title = Electric Universe | place = New York | publisher = Three Rivers Press | isbn = 978-0-307-33598-2}}</ref>

== History ==
{{Main|International System of Electrical and Magnetic Units}}
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 [[dyne]]s per centimetre of length between two wires one centimetre apart.<ref>{{Citation|url=http://alpha.montclair.edu/~kowalskiL/SI/SI_PAGE.HTML |archive-url=https://web.archive.org/web/20020214114359/http://alpha.montclair.edu/~kowalskiL/SI/SI_page.html |url-status=dead |archive-date=14 February 2002 |title=A short history of the SI units in electricity |publisher=Montclair |last=Kowalski |first=L |journal=The Physics Teacher |year=1986 |volume=24 |issue=2 |pages=97–99 |doi=10.1119/1.2341955 |bibcode=1986PhTea..24...97K |url-access=subscription }}</ref> The size of the unit was chosen so that the units derived from it in the [[MKS system of units|MKSA]] system would be conveniently sized.

The "international ampere" was an early realisation of the ampere, defined as the current that would deposit {{val|0.001118|u=grams}} of silver per second from a [[silver nitrate]] solution. Later, more accurate measurements revealed that this current is {{val|0.99985|u=A}}.<ref>{{Citation |url=http://www.sizes.com/units/ampHist.htm |title=History of the ampere |publisher=Sizes |date=1 April 2014 |access-date=20 September 2023|url-status=live |archive-url=https://web.archive.org/web/20161020170913/http://sizes.com/units/ampHist.htm |archive-date=20 October 2016 |df=dmy-all }}</ref>

Since [[power (physics)|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 {{math|1=''I'' = ''P''/''V''}}, and thus 1&nbsp;A = 1&nbsp;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:
<blockquote>
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 {{val|2|e=-7}} [[newton (unit)|newtons]] per metre of length.<ref name="formerSI definition">{{SIbrochure8th}}</ref>{{rp|113}} <ref>{{Citation | first = Paul MS | last = Monk | title = Physical Chemistry: Understanding our Chemical World | publisher = John Wiley & Sons | year = 2004 | isbn = 0-471-49180-2 | url = https://books.google.com/books?id=LupAi35QjhoC&pg=PA16 }}</ref>
</blockquote>

[[Ampère's force law]]<ref name=Serway>{{cite book
 |first1      = Raymond A
 |last1       = Serway
 |last2       = Jewett
 |first2      = JW
 |title       = Serway's principles of physics: a calculus based text
 |url         = https://books.google.com/books?id=1DZz341Pp50C&q=wire+%22magnetic+force%22&pg=RA1-PA746
 |publisher   = Thompson Brooks/Cole
 |edition     = Fourth
 |location    = Belmont, CA
 |year        = 2006
 |page        = 746
 |isbn        = 0-53449143-X
 |archive-url  = https://web.archive.org/web/20130621210413/http://books.google.com/books?id=1DZz341Pp50C&pg=RA1-PA746&dq=wire+%22magnetic+force%22&lr=&as_brr=0&sig=4vMV_CH6Nm8ZkgjtDJFlupekYoA#PRA1-PA746,M1
 |archive-date = 21 June 2013
|url-status=live  
}}</ref><ref name = "Beyond">{{Citation | url = https://www.nist.gov/public_affairs/newsfromnist_beyond_the_kilogram.htm | title = Beyond the Kilogram: Redefining the International System of Units | year = 2006 | publisher = [[National Institute of Standards and Technology]] | place = US | access-date = 3 December 2008| archive-url= https://web.archive.org/web/20080321221139/https://www.nist.gov/public_affairs/newsfromnist_beyond_the_kilogram.htm| archive-date= 21 March 2008 <!--DASHBot-->|url-status=live  }}</ref> 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 the [[Vacuum permeability|vacuum magnetic permeability]] (magnetic constant, {{math|''μ''<sub>0</sub>}}) a value of exactly 4π&nbsp;×&nbsp;10<sup>−7</sup> [[Henry (unit)|henries]] per metre (H/m, equivalent to N/A<sup>2</sup>). 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".<ref name="formerSI definition" />{{rp|144}} In general, charge {{mvar|Q}} was determined by steady current {{mvar|I}} flowing for a time {{mvar|t}} as {{math|1=''Q'' = ''It''}}.

This definition of the ampere was most accurately realised using a [[Kibble balance]], but in practice the unit was maintained via [[Ohm's law]] from the units of [[electromotive force]] and [[Electrical resistance and conductance|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.<ref name="Electrical quantities">{{Citation|chapter-url=http://www.bipm.org/en/si/si_brochure/appendix2/electrical.html |title=SI brochure |chapter=Appendix 2: Practical realisation of unit definitions: Electrical quantities |publisher=BIPM|archive-url=https://archive.today/20130414093725/http://www.bipm.org/en/si/si_brochure/appendix2/electrical.html|archive-date=14 April 2013}}</ref>

Techniques to establish the realisation of an ampere had a [[Approximation error|relative uncertainty]] of approximately a few parts in 10{{sup|7}}, and involved realisations of the watt, the ohm and the volt.<ref name= "Electrical quantities"/>

=== Present definition ===
The [[2019 revision of the SI]] defined the ampere by taking the fixed numerical value of the [[elementary charge]] {{mvar|e}} to be {{physical constants|e|ref=no|unit=no}} when expressed in the unit C, which is equal to A⋅s, where the second is defined in terms of {{math|∆''ν''<sub>Cs</sub>}}, the unperturbed ground state hyperfine transition frequency of the [[caesium]]-133 atom.<ref>{{cite web |title=ampere (A) |url=https://www.npl.co.uk/si-units/ampere |website=www.npl.co.uk |access-date=21 May 2019 |archive-url=https://web.archive.org/web/20250325050434/https://www.npl.co.uk/si-units/ampere |archive-date=2025-03-25}}</ref>

The SI unit of charge, the [[coulomb]], "is the quantity of electricity carried in 1 second by a current of 1 ampere".<ref>{{Citation | publisher = [[Bureau International des Poids et Mesures]] | year = 2006 | url = http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf | title = The International System of Units (SI) | edition = 8th | page = 144 | url-status = live | archive-url = https://web.archive.org/web/20131105051930/http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf | archive-date = 5 November 2013 | df = dmy-all }}.</ref> Conversely, a current of one ampere is one coulomb of charge (approximately {{Val|6.241509E18}} elementary charges) going past a given point per second, or equivalently 10<sup>19</sup> elementary charges every {{val|1.602176634}} seconds:
:<math>1\text{ A} = 1\text{ C/s} = 6.241\,509 \times10^{18}\,e\text{/s} = \frac{10^{19}\,e}{1.602\,176\,634\text{ s}}.</math>
With the second defined in terms of {{math|∆''ν''<sub>Cs</sub>}}, the caesium-133 hyperfine transition frequency, the ampere can be expressed in terms of {{mvar|e}} and {{math|∆''ν''<sub>Cs</sub>}}:<ref>{{Cite web |date=2019 |title=SI base unit: ampere (A) |url=https://www.bipm.org/en/si-base-units/ampere |access-date=2025-06-03 |website=International Bureau of Weights and Measures (BIPM) |language=en-US}}</ref><math display="block">1\text{ A} = 1\text{ C/s} = \Big(\frac{10^{19}\,e}{1.602\,176\,634}\Big)\Big(\frac{\Delta\nu_\text{Cs}}{9\,192\,631\,770}\Big) \approx 6.789\,6868\times10^{8}\,e\,\Delta\nu_\text{Cs}.</math>Constant, instantaneous and average current are expressed in amperes (as in "the charging current is 1.2&nbsp;A") and the charge accumulated (or passed through a circuit) over a period of time is expressed in coulombs (as in "the [[battery (electricity)|battery]] charge is {{val|30000|u=C}}"). The relation of the ampere (A&nbsp;=&nbsp;C/s) to the coulomb (C) is the same as that of the [[watt]] (W&nbsp;=&nbsp;J/s) to the [[joule]] (J).

== Units derived from the ampere ==
The [[international system of units]] (SI) is based on seven [[SI base units]] the [[second]], metre, [[kilogram]], [[kelvin]], ampere, [[mole (unit)|mole]], and [[candela]] representing seven fundamental types of physical quantity, or [[Dimensional analysis|"dimensions"]], ([[time]], [[length]], [[mass]], [[temperature]], electric 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:

{| class=wikitable
! 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⋅m<sup>2</sup>⋅s<sup>−3</sup>⋅A<sup>−1</sup>
|-
| [[Electrical resistance]]
| [[ohm]]
| Ω
| volt per ampere
| kg⋅m<sup>2</sup>⋅s<sup>−3</sup>⋅A<sup>−2</sup>
|-
| [[Electrical conductance]]
| [[siemens (unit)|siemens]]
| S
| ampere per volt or inverse ohm
| s<sup>3</sup>⋅A<sup>2</sup>⋅kg<sup>−1</sup>⋅m<sup>−2</sup>
|-
| [[Electrical inductance]]
| [[henry (unit)|henry]]
| H
| ohm second
| kg⋅m<sup>2</sup>⋅s<sup>−2</sup>⋅A<sup>−2</sup>
|-
| [[Electrical capacitance]]
| [[farad]]
| F
| coulomb per volt
| s<sup>4</sup>⋅A<sup>2</sup>⋅kg<sup>−1</sup>⋅m<sup>−2</sup>
|-
| [[Magnetic flux]]
| [[weber (unit)|weber]]
| Wb
| volt second
| kg⋅m<sup>2</sup>⋅s<sup>−2</sup>⋅A<sup>−1</sup>
|-
| [[Magnetic flux density]]
| [[tesla (unit)|tesla]]
| T
| weber per [[square metre]]
| kg⋅s<sup>−2</sup>⋅A<sup>−1</sup>
|}

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 (unit)|lumen]], and lux.

== SI prefixes ==
{{Main|Orders of magnitude (current)}}
Like other SI units, the ampere can be modified by adding a [[Metric prefix|prefix]] that multiplies it by a [[power of 10]].
{{SI multiples
| unit=ampere
| symbol=A
}}

== See also ==
* {{annotated link | Ammeter}}
* {{annotated link | Ampacity}}
* {{annotated link | Electric current}}
* {{annotated link | Electric shock}}
* {{annotated link | Hydraulic analogy}}
* {{annotated link | Vacuum permeability}}
* {{annotated link | Orders of magnitude (current)}}

== References ==
{{Reflist}}

== External links ==
* [http://physics.nist.gov/cuu/ The NIST Reference on Constants, Units, and Uncertainty]
* [http://physics.nist.gov/cuu/Units/ampere.html NIST ''Definition of ampere and μ<sub>0</sub>'']

{{SI units}}
{{Authority control}}

[[Category:SI base units]]
[[Category:Units of electric current]]