# Volt

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{{Short description|SI derived unit of voltage}}
{{Other uses}}
{{Update|date=November 2024}}
{{Use dmy dates|date=December 2024}}
{{Infobox Unit
| name = volt
| image = 240px
| caption = [Josephson voltage standard](/source/Josephson_voltage_standard) chip developed by the [National Bureau of Standards](/source/NIST) as a standard volt
| standard = [SI](/source/SI)
| quantity = [electric potential](/source/electric_potential), [electromotive force](/source/electromotive_force)
| symbol = V
| dimension = M⋅L<sup>2</sup>⋅T<sup>−3</sup>⋅I
| namedafter = [Alessandro Volta](/source/Alessandro_Volta)
| extralabel = [SI base unit](/source/SI_base_unit)s
| extradata = [kg](/source/kilogram)⋅[m](/source/metre)<sup>2</sup>⋅[s](/source/second)<sup>−3</sup>⋅[A](/source/ampere)<sup>−1</sup>
}}

The '''volt''' (symbol: '''V'''), named after [Alessandro Volta](/source/Alessandro_Volta), is the [unit of measurement](/source/unit_of_measurement) of [electric potential](/source/electric_potential), [electric potential difference](/source/Voltage) ([voltage](/source/voltage)), and [electromotive force](/source/electromotive_force) in the [International System of Units (SI)](/source/International_System_of_Units).<ref>{{cite web |url=http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html |title=SI Brochure, Table 3 (Section 2.2.2) |access-date=29 July 2007 |year=2006 |publisher=BIPM |archive-url=https://web.archive.org/web/20070618123613/http://www.bipm.org/en/si/si_brochure/chapter2/2-2/table3.html |archive-date=18 June 2007}}</ref>

== Definition ==
One volt is defined as the electric potential between two points of a [conducting wire](/source/electrical_conductor) when an [electric current](/source/electric_current) of one [ampere](/source/ampere) dissipates one [watt](/source/watt) of [power](/source/power_(physics)) between those points.<ref>[https://www.bipm.org/documents/20126/41483022/si-brochure-9-App1-EN.pdf BIPM SI Brochure: Appendix 1] {{webarchive |url=https://web.archive.org/web/20220227145519/https://www.bipm.org/documents/20126/41483022/si-brochure-9-App1-EN.pdf |date=27 February 2022}}, p. 144.</ref> It can be expressed in terms of SI base units ([m](/source/metre), [kg](/source/kilogram), [s](/source/second), and [A](/source/ampere)) as
: <math alt="volt equals kilogram times meter squared per ampere per second cubed">
\text{V} = \frac{\text{power}}{\text{electric current}} = \frac{\text{W}}{\text{A}} = \frac{\text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}}{\text{A}} = \text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}{\cdot}{\text{A}^{-1}}.</math>

Equivalently, it is the potential difference between two points that will impart one [joule](/source/joule) of [energy](/source/energy) per [coulomb](/source/coulomb) of charge that passes through it. It can be expressed in terms of SI base units ([m](/source/metre), [kg](/source/kilogram), [s](/source/second), and [A](/source/ampere)) as
: <math alt="volt equals kilogram times meter squared per ampere per second cubed">
\text{V} = \frac{\text{potential energy}}{\text{charge}} = \frac{\text{J}}{\text{C}} = \frac{\text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-2}}{\text{A}{\cdot}\text{s}} = \text{kg}{\cdot}\text{m}^2{\cdot}\text{s}^{-3}{\cdot}{\text{A}^{-1}}.</math>

It can also be expressed as amperes times [ohm](/source/ohm)s (current times resistance, [Ohm's law](/source/Ohm's_law)), [webers](/source/Weber_(unit)) per second (magnetic flux per time), watts per ampere (power per current), or joules per coulomb (energy per charge), which is also equivalent to [electronvolt](/source/electronvolt)s per [elementary charge](/source/elementary_charge):
: <math alt="volt equals ampere times ohm, watt per ampere, and joules per coulomb">
\text{V} = \text{A}{\cdot}\Omega = \frac{\text{Wb}}{\text{s}} = \frac{\text{W}}{\text{A}} = \frac{\text{J}}{\text{C}} = \frac{\text{eV}}{e}.</math>

{{SI unit lowercase|Alessandro Volta|volt|V}}

=== Josephson junction definition ===
{{Main|Josephson voltage standard}}
Historically the "[conventional](/source/Conventional_electrical_unit)" volt, ''V''<sub>90</sub>, defined in 1987 by the 18th [General Conference on Weights and Measures](/source/General_Conference_on_Weights_and_Measures)<ref name="cgpm-18">{{cite web |url=https://www.bipm.org/documents/20126/33145736/CGPM18.pdf/f461df63-75c1-c14d-e6b7-69867b79382f |title=Resolutions of the CGPM: 18th meeting (12–15 October 1987) |access-date=27 February 2022 |archive-date=27 February 2022 |archive-url=https://web.archive.org/web/20220227150143/https://www.bipm.org/documents/20126/33145736/CGPM18.pdf/f461df63-75c1-c14d-e6b7-69867b79382f |url-status=live}}</ref> and in use from 1990 to 2019, was implemented using the [Josephson effect](/source/Josephson_effect) for exact frequency-to-voltage conversion, combined with the [caesium frequency standard](/source/Caesium_standard). Though the Josephson effect is still used to realize a volt, the constant used has changed slightly.

For the [Josephson constant](/source/Magnetic_flux_quantum), ''K''<sub>J</sub> = 2''e''/''h'' (where ''e'' is the [elementary charge](/source/elementary_charge) and ''h'' is the [Planck constant](/source/Planck_constant)), a "conventional" value ''K''<sub>J-90</sub> = {{val|0.4835979|u=GHz/μV}} was used for the purpose of defining the volt. As a consequence of the [2019 revision of the SI](/source/2019_revision_of_the_SI), as of 2019 the Josephson constant has an exact value of {{math|''K''<sub>J</sub>}} = {{val|483597.84841698|end=...|u=GHz/V}}, which replaced the conventional value ''K''<sub>J-90</sub>.

This standard is typically realized using a series-connected array of several thousand or tens of thousands of [junctions](/source/Electrical_junction), excited by microwave signals between 10 and 80&nbsp;GHz (depending on the array design).<ref name="ieee-josephson">{{Citation |title=1 Volt DC Programmable Josephson Voltage Standard |first1=Charles J. |last1=Burroughs |first2=Samuel P. |last2=Bent |first3=Todd E. |last3=Harvey |first4=Clark A. |last4=Hamilton |journal=IEEE Transactions on Applied Superconductivity |date=1 June 1999 |volume=9 |number=3 |pages=4145–4149 |issn=1051-8223 |publisher=[Institute of Electrical and Electronics Engineers](/source/Institute_of_Electrical_and_Electronics_Engineers) (IEEE) |doi=10.1109/77.783938 |bibcode=1999ITAS....9.4145B |s2cid=12970127 |url=https://zenodo.org/record/1232191}}</ref> Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc., and no correction terms are required in a practical implementation.<ref>{{Citation |title=Current status of the quantum metrology triangle |first=Mark W. |last=Keller |url=http://qdev.boulder.nist.gov/817.03/pubs/downloads/set/Metrologia%2045,%20102.pdf |journal=Metrologia |volume=45 |number=1 |pages=102–109 |date=18 January 2008 |issn=0026-1394 |doi=10.1088/0026-1394/45/1/014 |quote=Theoretically, there are no current predictions for any correction terms. Empirically, several experiments have shown that ''K''<sub>J</sub> and ''R''<sub>K</sub> are independent of device design, material, measurement setup, etc. This demonstration of universality is consistent with the exactness of the relations, but does not prove it outright. |bibcode=2008Metro..45..102K |s2cid=122008182 |access-date=11 April 2010 |archive-url=https://web.archive.org/web/20100527094953/http://qdev.boulder.nist.gov/817.03/pubs/downloads/set/Metrologia%2045,%20102.pdf |archive-date=27 May 2010 |url-status=dead}}</ref>

== Water-flow analogy ==
In the ''[water-flow analogy](/source/hydraulic_analogy)'', sometimes used to explain electric circuits by comparing them with water-filled pipes, [voltage](/source/voltage) (difference in electric potential) is likened to difference in water pressure, while [current](/source/electric_current) is proportional to the amount of water flowing. A [resistor](/source/resistor) would be a reduced diameter somewhere in the piping or something akin to a radiator offering resistance to flow.

The relationship between voltage and current is defined (in ohmic devices like resistors) by [Ohm's law](/source/Ohm's_law). Ohm's law is analogous to the [Hagen–Poiseuille equation](/source/Hagen%E2%80%93Poiseuille_equation), as both are linear models relating [flux](/source/flux) and [potential](/source/potential) in their respective systems.

== Common voltages <span class="anchor" id="Common values"></span> ==
[[File:Electronic multi meter.jpg|thumb| A [multimeter](/source/multimeter) can be used to measure the voltage between two positions.]]
upright|thumb|1.5&nbsp;V C-cell batteries

The voltage produced by each [electrochemical cell](/source/electrochemical_cell) in a [battery](/source/Electric_battery) is determined by the chemistry of that cell (see {{Section link|Galvanic cell|Cell voltage}}). Cells can be combined in series for multiples of that voltage, or additional circuitry added to adjust the voltage to a different level. Mechanical generators can usually be constructed to any voltage in a range of feasibility.

Nominal voltages of familiar sources:
* [Nerve cell](/source/Neuron) [resting potential](/source/resting_potential): ~&nbsp;75&nbsp;mV<ref>Bullock, Orkand, and Grinnell, pp. 150–151; Junge, pp. 89–90; Schmidt-Nielsen, p. 484.</ref>
* Single-cell, rechargeable [NiMH](/source/Nickel%E2%80%93metal_hydride_battery)<ref>{{cite book |last1=Horowitz |first1=Paul |last2=Winfield |first2=Hill |title=The Art of Electronics |date=2015 |publisher=Cambridge Univ. Press |location=Cambridge [u.a.] |isbn=978-0-521-809269 |page=689 |edition=3.}}</ref> or [NiCd](/source/nickel%E2%80%93cadmium_battery) battery: 1.2&nbsp;V
* Single-cell, non-rechargeable (e.g., [AAA, AA, C and D cells](/source/Electric_battery)): [alkaline battery](/source/alkaline_battery): 1.5&nbsp;V;<ref>{{cite web |url=http://www.ti.com/lit/an/slva194/slva194.pdf |title=Single-cell Battery Discharge Characteristics Using the TPS61070 Boost Converter |author1=SK Loo |author2=Keith Keller |publisher=Texas Instruments |date=Aug 2004 |url-status=live |archive-url=https://web.archive.org/web/20231015141242/https://www.ti.com/lit/an/slva194/slva194.pdf |archive-date= 15 October 2023}}</ref> [zinc–carbon battery](/source/zinc%E2%80%93carbon_battery): 1.56&nbsp;V if fresh and unused
* [Logic voltage levels](/source/Logic_level): 1.2&nbsp;V, 1.5&nbsp;V, 1.8&nbsp;V, 2.5&nbsp;V, 3.3&nbsp;V, 5.0&nbsp;V
* [LiFePO<sub>4</sub>](/source/Lithium_iron_phosphate_battery) rechargeable battery: 3.3&nbsp;V
* [Cobalt](/source/Cobalt)-based [lithium polymer](/source/Lithium_polymer_battery) rechargeable battery: 3.75&nbsp;V (see [Comparison of commercial battery types](/source/Comparison_of_commercial_battery_types))
* [Transistor–transistor logic](/source/Transistor%E2%80%93transistor_logic)/[CMOS](/source/CMOS) (TTL) power supply: 5&nbsp;V
* [USB](/source/USB): 5&nbsp;V DC
* [PP3 battery](/source/Nine-volt_battery): 9&nbsp;V
* [Automotive battery](/source/Automotive_battery) systems use cells with 2.1&nbsp;volts per cell; a "12&nbsp;V" battery has six cells connected in series, which produces 12.6&nbsp;V; a "24&nbsp;V" battery has 12 cells connected in series, producing 25.2&nbsp;V. Some antique vehicles use "6&nbsp;V" 3-cell batteries, or 6.3&nbsp;volts.
* Household [mains electricity](/source/mains_electricity) AC (see ''[Mains electricity by country](/source/Mains_electricity_by_country)'' for a list of countries with mains power plugs, voltages and frequencies)
** 100&nbsp;V in Japan
** 120&nbsp;V in North America
** 230&nbsp;V in Europe, Asia, Africa and Australia
* [Rapid transit](/source/Rapid_transit) [third rail](/source/third_rail): 600–750&nbsp;V (see [List of railway electrification systems](/source/List_of_railway_electrification_systems))
* High-speed train overhead power lines: [25&nbsp;kV at 50&nbsp;Hz](/source/25_kV_AC_railway_electrification), but see the [List of railway electrification systems](/source/List_of_railway_electrification_systems) and [25&nbsp;kV at 60&nbsp;Hz](/source/25_kV_AC_railway_electrification) for exceptions.
* High-voltage [electric power transmission](/source/electric_power_transmission) lines: 110&nbsp;kV and up (1.15&nbsp;MV is the record; the highest active voltage is 1.10&nbsp;MV<ref>{{cite web |url=https://www.bloomberg.com/news/articles/2019-01-02/world-s-biggest-ultra-high-voltage-line-powers-up-across-china |title=World's Biggest Ultra-High Voltage Line Powers Up Across China |website=Bloomberg |url-access=subscription |access-date=7 January 2020 |date=1 January 2019}}</ref>)
* [Lightning](/source/Lightning): a maximum of around 150&nbsp;MV.<ref>{{cite web |url=https://www.riskva.com/fff/lightning_062613.html |author=Paul H. Risk |title=Lightning – High-Voltage Nature |website=RiskVA |date=26 June 2013 |access-date=23 April 2021 |archive-date=23 April 2021 |archive-url=https://web.archive.org/web/20210423220123/https://www.riskva.com/fff/lightning_062613.html |url-status=live}}</ref>

== History ==
upright|left|thumb|Alessandro Volta
[[File:PSM V85 D521 Group photograph of herman helmholtz and academic friends.png|thumb|Group photograph of [Hermann Helmholtz](/source/Hermann_von_Helmholtz), his wife (seated) and academic friends [Hugo Kronecker](/source/Hugo_Kronecker) (left), [Thomas Corwin Mendenhall](/source/Thomas_Corwin_Mendenhall) (right), [Henry Villard](/source/Henry_Villard) (center) during the International Electrical Congress]]

In 1800, as the result of a professional disagreement over the galvanic response advocated by [Luigi Galvani](/source/Luigi_Galvani), [Alessandro Volta](/source/Alessandro_Volta) developed the so-called [voltaic pile](/source/voltaic_pile), a forerunner of the [battery](/source/Electric_battery), which produced a steady electric [current](/source/Electric_current). Volta had determined that the most effective pair of dissimilar metals to produce electricity was [zinc](/source/zinc) and [silver](/source/silver). In 1861, [Latimer Clark](/source/Josiah_Latimer_Clark) and Sir [Charles Bright](/source/Charles_Tilston_Bright) coined the name "volt" for the unit of resistance.<ref>As names for units of various electrical quantities, Bright and Clark suggested "ohma" for voltage, "farad" for charge, "galvat" for current, and "volt" for resistance. See:
* Latimer Clark and Sir Charles Bright (1861) [https://www.biodiversitylibrary.org/item/93052#page/483/mode/1up "On the formation of standards of electrical quantity and resistance"] {{Webarchive|url=https://web.archive.org/web/20121108105352/http://www.biodiversitylibrary.org/item/93052#page/483/mode/1up |date=8 November 2012}}, ''Report of the Thirty-first Meeting of the British Association for the Advancement of Science'' (Manchester, England: September 1861), section: Mathematics and Physics, pp. 37–38.
* Latimer Clark and Sir Charles Bright (9 November 1861) [https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837166;view=1up;seq=15 "Measurement of electrical quantities and resistance"], ''The Electrician'', '''1''' (1): 3–4.</ref> By 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad.<ref>Sir W. Thomson, et al. (1873) [https://www.biodiversitylibrary.org/page/29853513#page/324/mode/1up "First report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units"] {{webarchive |url=https://web.archive.org/web/20170423152619/http://www.biodiversitylibrary.org/page/29853513#page/324/mode/1up |date=23 April 2017}}, ''Report of the 43rd Meeting of the British Association for the Advancement of Science'' (Bradford, September 1873), pp. 222–225. From p. 223: "The 'ohm', as represented by the original standard coil, is approximately 10<sup>9</sup> C.G.S. units of resistance; the 'volt' is approximately 10<sup>8</sup> C.G.S. units of electromotive force; and the 'farad' is approximately 1/10<sup>9</sup> of the C.G.S. unit of capacity."</ref> In 1881, the International Electrical Congress, now the [International Electrotechnical Commission](/source/International_Electrotechnical_Commission) (IEC), approved the volt as the unit for electromotive force.<ref>(Anon.) (24 September 1881) [https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837489;view=1up;seq=309 "The Electrical Congress"] {{webarchive |url=https://web.archive.org/web/20190306002556/https://babel.hathitrust.org/cgi/pt?id=nyp.33433090837489;view=1up;seq=309 |date=6 March 2019}}, ''The Electrician'', '''7''': 297.</ref> They made the volt equal to 10<sup>8</sup> [cgs units](/source/Centimetre%E2%80%93gram%E2%80%93second_system_of_units) of voltage, the cgs system at the time being the customary system of units in science. They chose such a ratio because the cgs unit of voltage is inconveniently small and one volt in this definition is approximately the emf of a [Daniell cell](/source/Daniell_cell), the standard source of voltage in the telegraph systems of the day.<ref name="Hamer">{{cite book |title=Standard Cells: Their Construction, Maintenance, and Characteristics |publisher=US National Bureau of Standards |last=Hamer |first=Walter J. |date=15 January 1965 |series=National Bureau of Standards Monograph #84 |url=https://www.nist.gov/calibrations/upload/mn84.pdf |access-date=13 July 2017 |archive-date=3 March 2016 |archive-url=https://web.archive.org/web/20160303203423/http://www.nist.gov/calibrations/upload/mn84.pdf |url-status=live}}</ref> At that time, the volt was defined as the potential difference [i.e., what is nowadays called the "voltage (difference)"] across a conductor when a current of one [ampere](/source/ampere) dissipates one [watt](/source/watt) of power.

The "international volt" was defined in 1893 as {{fraction|1.434}} of the [emf](/source/Electromotive_force) of a [Clark cell](/source/Clark_cell). This definition was abandoned in 1908 in favor of a definition based on the international [ohm](/source/ohm) and international ampere until the entire set of "reproducible units" was abandoned in 1948.<ref name="BLR47.12">{{cite journal |date=December 1947 |title=Revised Values for Electrical Units |journal= Bell Laboratories Record |volume=XXV |issue=12 |pages=441 |url=https://www.worldradiohistory.com/Archive-Bell-Laboratories-Record/40s/Bell-Laboratories-Record-1947-12.pdf}}</ref>

A [2019 revision of the SI](/source/2019_revision_of_the_SI), including defining the value of the [elementary charge](/source/elementary_charge), took effect on 20 May 2019.<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=2 November 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>

== See also ==
{{Portal|Energy}}

{{div col|colwidth=24em}}
* [Orders of magnitude (voltage)](/source/Orders_of_magnitude_(voltage))
* [Rail traction voltage](/source/List_of_railway_electrification_systems)
* [SI electromagnetism units](/source/SI_electromagnetism_units)
* [SI prefix](/source/Metric_prefix) for unit prefixes
* [Standardised railway voltages](/source/Railway_electrification)
* [Voltmeter](/source/Voltmeter)
{{div col end}}

== References ==
{{reflist|2}}

== External links ==
{{Wiktionary}}
* [http://histoires-de-sciences.over-blog.fr/2013/11/electrical-units-history.html History of the electrical units.]

{{SI units}}

Category:SI derived units
Category:Units of electrical potential
Category:Alessandro Volta

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Adapted from the Wikipedia article [Volt](https://en.wikipedia.org/wiki/Volt) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Volt?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.