Editing Ampere (section)

=== 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"/>