Editing Ampere (section)

== 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 }}</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 realization 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 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".<ref name="formerSI definition"/>{{rp|144}} Conversely, a current of one ampere is one coulomb of charge going past a given point per second:
:<math>\rm 1\ A=1\frac C s.</math>
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}}</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 going past a given point per second:
:<math>\rm 1\ A=1\,\text{C/s}.</math>
In general, charge {{mvar|Q}} is determined by steady current {{mvar|I}} flowing for a time {{mvar|t}} as {{math|1=''Q'' = ''I'' ''t''}}.

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 (C/s) to the coulomb is the same as that of the [[watt]] (J/s) to the [[joule]].