Current can be measured by a galvanometer, via the deflection of a magnetic needle in the magnetic field created by the current.

The ampere (symbol: A) is the SI unit of electric current.^[1] The ampere, in practice often shortened to amp, is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

In practical terms, the ampere is a measure of the amount of electric charge passing a point per unit time. Around 6.242 × 10¹⁸ electrons passing a given point each second constitutes one ampere.^[2] (Since electrons have negative charge, they flow in the opposite direction to the conventional current.)

[edit] Definition

Qualitatively, the ampere "is now defined in terms of a current that, if maintained in two straight parallel conductors of specific sizes and positions, would produce a certain amount of [magnetic] force between the conductors."^[3] Quantitatively, the ampere is defined to be the constant current which will produce an attractive force of 2 × 10^–7 newtons per metre of length between two straight, parallel conductors of infinite length and negligible circular cross section placed one metre apart in a vacuum.^[1][4][5] The definition is based on Ampère's force law.^[6] The ampere is a base unit, along with the metre, kelvin, second, mole, candela and the kilogram: it is defined without reference to the quantity of electric charge.

In terms of Ampère's force law,

so

The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere."^[7] Conversely, a current of one ampere is one coulomb of charge going past a given point per second:

That is, in general, charge Q is determined by steady current I flowing for a time t as Q = It.

[edit] History

The term honors André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.

The ampere was originally defined as one tenth of the CGS system electromagnetic unit of current (now known as the abampere), the amount of current which generates a force of two dynes per centimetre of length between two wires one centimetre apart.^[8] 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 0.001118000 grams of silver per second from a silver nitrate solution.^[9] Later, more accurate measurements revealed that this current is 0.99985 A.

[edit] Realisation

The 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.^[10]

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.^[11]

[edit] Proposed future definition

Rather than a definition in terms of the force between two current-carrying wires, it has been proposed to define the ampere in terms of the rate of flow of elementary charges.^[12] Since a coulomb is approximately equal to 6.24150948×10¹⁸ elementary charges, one ampere is approximately equivalent to 6.24150948×10¹⁸ elementary charges, such as electrons, moving past a boundary in one second. 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, and, depending on the outcome of experiments over the next few years, to formally propose the change at the 24th General Conference on Weights and Measures (CGPM) in 2011.^[13]

[edit] See also

• Ammeter
• Electric shock
• Hydraulic analogy
• Magnetic constant

[edit] References

1. ^ ^{a b} BIPM official definition
2. ^ Bodanis, David. (2005). Electric Universe. New York: Three Rivers Press
3. ^ Beyond the Kilogram: Redefining the International System of Units(2006). National Institute of Standards and Technology. Square brackets appear in original. Retrieved March 2008.
4. ^ The BIPM does not distinguish between quantum vacuum and free space.
5. ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical World, John Wiley and Sons, 2004 online.
6. ^ Raymond A Serway & Jewett JW (2006). Serway's principles of physics: a calculus based text (Fourth Edition ed.). Belmont, CA: Thompson Brooks/Cole. p. 746. ISBN 053449143X. 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. 
7. ^ Bureau International des Poids et Mesures. (2006).The International System of Units (SI), 8th ed. p. 144.
8. ^ A short history of the SI units in electricity
9. ^ History of the ampere
10. ^ Practical realization of unit definitions: Electrical quantities
11. ^ BIPM SI brochure; Appendix 2
12. ^ Beyond the Kilogram: Redefining the International System of Units
13. ^ International Committee for Weights and Measures (CIPM) Recommendation 1 (CI-2005): Preparative steps towards new definitions of the kilogram, the ampere, the kelvin and the mole in terms of fundamental constants

[edit] External links

• The NIST Reference on Constants, Units, and Uncertainty
• A short history of the SI units in electricity
• NIST Definition of ampere and μ₀