# Ampere

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 system | SI base unit |

Unit of | Electric current |

Symbol | A |

Named after | André-Marie Ampère |

The **ampere** (/ˈæmpɪər, æmˈpɪər/ or /ˈæmpɛər/ (UK),^{[1]}^{[2]} symbol: **A**),^{[3]} often shortened to "amp",^{[4]} is the base unit of electric current in the International System of Units (SI).^{[5]}^{[6]} It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

The International System of Units defines the ampere in terms of other base units by measuring the electromagnetic force between electrical conductors carrying electric current. The earlier CGS measurement system had two different 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.^{[7]} In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second.

New definitions, in terms of invariant constants of nature, specifically the elementary charge, took effect on 20 May 2019.^{[8]}

## Definition

The ampere is defined 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 ∆*ν*_{Cs}, the unperturbed ground state hyperfine transition frequency of the caesium-133 atom.^{[9]}

The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere".^{[10]} Conversely, a current of one ampere is one coulomb of charge going past a given point per second:

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.

## 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:

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

## Realization

The standard ampere is most accurately realized using a Kibble balance, but is in practice maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are relatively easy to reproduce, the Josephson junction and the quantum Hall effect, respectively.^{[17]}

At present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 10^{7}, and involve realizations of the watt, the ohm and the volt.^{[17]}

## See also

- Ammeter
- Ampacity (current-carrying capacity)
- Electric current
- Electric shock
- Hydraulic analogy
- Magnetic constant
- Orders of magnitude (current)

## References

**^**"Ampère".*Random House Webster's Unabridged Dictionary.***^**OED entry has /ˈampɛː/**^**"2. SI base units",*SI brochure*(8th ed.), BIPM, archived from the original on 7 October 2014, retrieved 19 November 2011**^**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. Retrieved 21 November 2011.**^**"2.1. Unit of electric current (ampere)",*SI brochure*(8th ed.), BIPM, archived from the original on 3 February 2012, retrieved 19 November 2011**^**Base unit definitions: Ampere Archived 25 April 2017 at the Wayback Machine Physics.nist.gov. Retrieved on 28 September 2010.**^**Bodanis, David (2005),*Electric Universe*, New York: Three Rivers Press, ISBN 978-0-307-33598-2**^***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)**^**"ampere (A)".*www.npl.co.uk*. Retrieved 21 May 2019.**^***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.**^**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**^***History of the ampere*, Sizes, 1 April 2014, archived from the original on 20 October 2016, retrieved 29 January 2017- ^
^{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 14 August 2017 **^**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**^**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.**^***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.- ^
^{a}^{b}"Appendix 2: Practical realization of unit definitions: Electrical quantities",*SI brochure*, BIPM, archived from the original on 14 April 2013.