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The ampere (symbol A),[1] often shortened to "amp",[2] is the base unit of electric current in the International System of Units (SI).[3][4] It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

Amperemeter hg.jpg
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.
Unit information
Unit system SI base unit
Unit of Electric current
Symbol A 
Named after André-Marie Ampère

SI 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.[5] In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second.

In the future, the SI definition may shift back to charge as the base unit, with the coulomb defined in terms of the elementary charge on electrons and protons (one coulomb equals the charge of roughly 6.242×1018 protons).



Illustration of the definition of the ampere unit

SI defines 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.[6][3][7]

Ampère's force law[8][9] 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, "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 = It.

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.


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.


The standard ampere is most accurately realized using a watt 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.[13]

At present, techniques to establish the realization of an ampere have a relative uncertainty of approximately a few parts in 107, and involve realizations of the watt, the ohm and the volt.[13]

Proposed future definitionEdit

Rather than a definition in terms of the force between two current-carrying wires, it has been proposed that the ampere should be defined in terms of the rate of flow of elementary charges.[9] Since a coulomb is approximately equal to 6.2415093×1018 elementary charges (such as those carried by protons, or the negative of those carried by electrons), one ampere is approximately equivalent to 6.2415093×1018 elementary charges moving past a boundary in one second. (6.2415093×1018 is the reciprocal of the value of the elementary charge in coulombs.[14]) The proposed change would define 1 A as being the current in the direction of flow of a particular number of elementary charges per second. In 2005, the International Committee for Weights and Measures (CIPM) agreed to study the proposed change. The new definition was discussed at the 25th General Conference on Weights and Measures (CGPM) in 2014 but for the time being was not adopted.

Everyday examplesEdit

The current drawn by typical constant-voltage energy distribution systems is usually dictated by the power (watt) consumed by the system and the operating voltage. For this reason the examples given below are grouped by voltage level.

Portable devicesEdit

  • Hearing aid (typically 1 mW at 1.4 V): 700 µA
  • USB charging cable (typically 10.5 W at 5.0 V): 2.1 A

Internal combustion engine vehicles – 12 V DCEdit

A typical motor vehicle has a 12 V battery. The various accessories that are powered by the battery might include:

  • Instrument panel light (typically 2 W): 166 mA
  • Headlight (each, typically 60 W): 5 A
  • Starter motor (typically 1–2 kW): 80–160 A

North American domestic supply – 120 V ACEdit

Most Canada, Mexico and United States domestic power suppliers run at 120 V.

Household circuit breakers typically provide a maximum of 15 A or 20 A of current to a given set of outlets.

  • 22-inch/56-centimeter portable television (35 W): 290 mA
  • Tungsten light bulb (60–100 W): 500–830 mA
  • Toaster, kettle (1.5 kW): 12.5 A
  • Hair dryer (1.8 kW): 15 A
  • USB cell phone charger (typically 10 W): 83 mA

European & Commonwealth domestic supply – 230–240 V ACEdit

Most European domestic power supplies run at 230 V, and most Commonwealth domestic power supplies run at 240 V. For the same amount of power (in watts), the current drawn by a particular European or Commonwealth appliance (in Europe or a Commonwealth country) will be less than for an equivalent North American appliance.[Note 1] Typical circuit breakers will provide 16 A.

The current drawn by a number of typical appliances are:

  • 22-inch/56-centimeter portable television (35 W): 145–150 mA
  • Tungsten light bulb (60–100 W): 240–450 mA
  • Compact fluorescent lamp (11–30 W): 56–112 mA
  • Toaster, kettle (2 kW): 9 A
  • Immersion heater (4.6 kW): 19–20 A

See alsoEdit


  1. ^ The formula for power is given by
    so it follows that if the voltage is doubled and the power remains the same, the current will be halved.


  1. ^ "2. SI base units", SI brochure (8th ed.), BIPM, retrieved 19 November 2011 
  2. ^ 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. 
  3. ^ a b "2.1. Unit of electric current (ampere)", SI brochure (8th ed.), BIPM, retrieved 19 November 2011 
  4. ^ Base unit definitions: Ampere. Retrieved on 2010-09-28.
  5. ^ Bodanis, David (2005), Electric Universe, New York: Three Rivers Press, ISBN 978-0-307-33598-2 
  6. ^ "BIPM - ampere". Retrieved 2017-01-31. 
  7. ^ Monk, Paul MS (2004), Physical Chemistry: Understanding our Chemical World, John Wiley & Sons, ISBN 0-471-49180-2 
  8. ^ 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. 
  9. ^ a b Beyond the Kilogram: Redefining the International System of Units, USA: National Institute of Standards and Technology, 2006, archived from the original on 21 March 2008, retrieved 3 December 2008 .
  10. ^ The International System of Units (SI) (PDF) (8th ed.), Bureau International des Poids et Mesures, 2006, p. 144 .
  11. ^ Kowalski, L, A short history of the SI units in electricity, Montclair, archived from the original on 2002-02-14 
  12. ^ History of the ampere, Sizes, 1 April 2014, retrieved 2017-01-29 
  13. ^ a b "Appendix 2: Practical realization of unit definitions: Electrical quantities", SI brochure, BIPM .
  14. ^ "Value", Physics, USA: NIST .

External linksEdit