Difference between revisions of "Ampere"
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==Definition== | ==Definition== | ||
| − | The current definition of the ampere was approved by the 9th [[General Conference on | + | The current definition of the ampere was approved by the 9th [[General Conference on Weights and Measures]] in 1948:<ref>9th CGPM (1948), [http://www.bipm.org/en/CIPM/db/1946/2/ Resolution 2].</ref><ref name="SI">{{SIbrochure8th|page=113}}.</ref> |
| − | {{quotation|The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2{{e|−7}} newton per metre of length. | + | {{quotation|The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2{{e|−7}} newton per metre of length.{{#tag:ref|The original definition used the term "MKS unit of force" instead of "[[newton]]": the change of name for the unit of force was also approved by the 9th CGPM in 1948.<ref name="SI"/><ref>9th CGPM (1948), [http://www.bipm.org/en/CGPM/db/9/7/ Resolution 7].</ref>|group=Note}}}} |
| − | This definition, which is based on the [[Biot–Savart law]], has the effect of fixing the value of the [[magnetic constant]] ''μ''<sub>0</sub> in SI units, so that {{nowrap|''μ''<sub>0</sub> = 4π{{e|−7}} H m<sup>−1</sup>}}. | + | This definition, which is based on the [[Biot–Savart law]], has the effect of fixing the value of the [[magnetic constant]] ''μ''<sub>0</sub> in SI units, so that {{nowrap|1=''μ''<sub>0</sub> = 4π{{e|−7}} H m<sup>−1</sup>}}. |
| + | |||
| + | ===Conventional ampere=== | ||
| + | The conventional ampere is a [[derived unit]], defined by [[Ohm's law]]: | ||
| + | :''A''<sub>90</sub> = ''V''<sub>90</sub>/''Ω''<sub>90</sub> | ||
| + | |||
| + | ==Realization== | ||
| + | |||
| + | ==History== | ||
| + | A unit of electric current in the [[cgs system]] was first proposed by [[Wilhelm Weber]] in 1851.<ref>{{citation | first = Wilhelm | last = Weber | authorlink = Wilhelm Weber | title = Messungen galvanischer Leitungswiederstände nach einem absoluten Maasse | journal = Ann. Phys. Chem. | year = 1851 | volume = 82 | pages = 337–69 | url = http://gallica.bnf.fr/ark:/12148/bpt6k15167x/f353.image.langEN}}; {{citation | title = On the Measurement of Electric Resistance according to an absolute Standard | journal = Philos. Mag., Ser. 4 | year = 1861 | volume = 22 | pages = 226–240, 261–69}}.</ref> | ||
==Notes and references== | ==Notes and references== | ||
Latest revision as of 16:28, 26 March 2011
The ampere (symbol: A) is the SI unit of electric current, and one of the seven base units in the International System of Units (SI). It is named after the French physicist André-Marie Ampère (1775–1836).
A similar unit, used for pratical measurements of current, is the conventional ampere (A90).[Note 1] The 2006 CODATA recommended value for A90 is:[1]
- A90 = V90/Ω90 = (KJ–90RK–90/KJRK) V = [1 − 0.3(2.5) × 10−8] A
The international ampere (symbol: Aint) is now obsolete.
Contents
Definition
The current definition of the ampere was approved by the 9th General Conference on Weights and Measures in 1948:[2][3]
The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 × 10−7 newton per metre of length.[Note 2]
This definition, which is based on the Biot–Savart law, has the effect of fixing the value of the magnetic constant μ0 in SI units, so that μ0 = 4π × 10−7 H m−1.
Conventional ampere
The conventional ampere is a derived unit, defined by Ohm's law:
- A90 = V90/Ω90
Realization
History
A unit of electric current in the cgs system was first proposed by Wilhelm Weber in 1851.[5]
Notes and references
Notes
References
- ↑ Mohr, Peter J.; Taylor, Barry N.; Newell, David B. CODATA Recommended Values of the Fundamental Physical Constants: 2006. Rev. Mod. Phys. 2008, 80 (2), 633–730. doi:10.1103/RevModPhys.80.633, <http://physics.nist.gov/cuu/Constants/codata.pdf>.
- ↑ 9th CGPM (1948), Resolution 2.
- ↑ 3.0 3.1 The International System of Units (SI), 8th ed.; International Bureau of Weights and Measures: Sèvres, France, 2006; p 113. ISBN 92-822-2213-6, <http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf>.
- ↑ 9th CGPM (1948), Resolution 7.
- ↑ Weber, Wilhelm Messungen galvanischer Leitungswiederstände nach einem absoluten Maasse. Ann. Phys. Chem. 1851, 82, 337–69, <http://gallica.bnf.fr/ark:/12148/bpt6k15167x/f353.image.langEN>; On the Measurement of Electric Resistance according to an absolute Standard. Philos. Mag., Ser. 4 1861, 22, 226–240, 261–69.
External links
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See also the corresponding article on Wikipedia. |
- ampere at Sizes.com
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