Ampere

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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.

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

  1. ↑ A90 is considered to be a physical constant, hence the use of italics for the symbol.
  2. ↑ 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.[3][4]

References

  1. ↑ 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>.
  2. ↑ 9th CGPM (1948), Resolution 2.
  3. ↑ 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>.
  4. ↑ 9th CGPM (1948), Resolution 7.
  5. ↑ 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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