Dictionary:

ampere

  (ăm'pîr')

1. A unit of electric current in the meter-kilogram-second system. It is the steady current that when flowing in straight parallel wires of infinite length and negligible cross section, separated by a distance of one meter in free space, produces a force between the wires of 2 × 10^-7 newtons per meter of length.
2. A unit in the International System specified as one International coulomb per second and equal to 0.999835 ampere.

[After André Marie AMPèRE.]

(Amp), a unit of measurement of the quantity of electric current, equal to a flow of 1 coulomb per second or 6.25 time 10¹⁸ electrons per second. The current produced by 1 volt acting through a resistance of 1 ohm.

[Etymology: A. M. Ampère; France 1775-1836] electric current strength. Symbol A. The amperes of steady current crossing any cross-section of a circuit equals the ratio of the charge in coulombs to the time in seconds, identically the amperes of steady current produced between two points of a conductor equals the ratio of the potential difference in volts across these points to the intervening resistance in ohms (the conductor not being the seat of any electromotive force). However, this unit is defined as a base unit in any m.k.s. A. system (including the SI), and was for the e.m.u. system.

SI, Metric-m.k.s. A. 1948 the base unit for all electromagnetic units, defined as the constant current which, if maintained in two straight parallel conductors of infinite length and of negligible 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. (This number effectively set the magnetic permeability of vacuum at 4π × 10^-7 H·m^-1; see permittivity.) The following are among the coherent derived units:

• A·m^-1 for magnetic field strength, magnetization;
• A·m^-2 for current density;
• A·s = coulomb for quantity of electricity;
• A·s·V^-1 = farad for electric capacitance;
• A·V^-1 = siemens for electric conductance;
• A·H = weber for magnetic flux;
• A·turn for magnetomotive force.

This ampere is equatable with 6.241 45~ × 10¹⁸ electronic charges per second.

Metric-c.g.s. See abampere and statampere. See also practical unit.

History

The name ‘ampère’ was agreed, along with related units and the use of the c.g.s. system, in 1881 at the first International Electrical Conference,
[Nature Vol. 24, 512 (1881)] as the ‘current produced by a volt in an ohm’, with the implication that there should be both an absolute form and a corresponding practical unit. The former, later discriminated as the abampere, falls within the e.m.u. system, and is fundamentally definable in terms of purely mechanical units. The practical ampere = 10^-1 abampere.

To make it a base unit instead of a derived unit, a specification for a laboratory realization of the ampere was established. This was expressed in terms of the rate of electrolytic deposition of silver, so has often been called the silver ampere or Ag ampere; the definition was ‘the unvarying current which deposits 1.118 mg of silver by electrolysis from a silver nitrate solution in one second’. The specification was subsequently shown to have made the ampere slightly smaller than intended,
[Nature Vol. 78, 678-81 (1908)] prompting the adoption by the IEC of 1908 of the distinct name international ampere, with no reference to it being either absolute or practical (though it was the latter). Because of experimental vagaries, the value for conversions is normally referred to as the mean international ampere = 0.999 85~ A. There is also the US international ampere = 0.999 835~ A.

At the implementation of the Metric-m.k.s. A. system in 1948, with the ampere as the base electrical unit but its definition made compatible with the original absolute units, the modern ampere became essentially the old practical ampere; this became identically the ampere of the SI (again the base electric unit).

The calibration of reference electrical instruments from the fundamental definition presents obvious practical problems with accuracy, as well as the impossibility of literally infinite length. Until the 1980s the method involved weighing on a balance the magnetic force between two coils of carefully measured copper wire; this gave an accuracy of barely 1 in 10⁵. Discovery of the Josephson effect, then of the quantum Hall effect, applying at very low temperatures with superconductors, together with subsequent development of the moving-coil balance and related work with the volt, improved accuracies about a thousandfold for the ampere, volt, ohm, etc.
[Hartland A. Contemp. Phys. Vol. 29, 477 (1988) http://www.npl.co.uk/npl/publications/electricity/] For maximum accuracy, the ampere has been realized via the watt, by comparison of electrical power and mechanical power.
[Taylor B. N. Metrologia Vol. 21, 37-9 (1985)]

Previous to adoption of the ampere, the names weber, oersted, and oerstedt were applied to units of electric current strength.

The International Standard unit for electrical current. A unit of the rate of flow of electric current; an electromotive force of 1 volt acting across a resistance of 1 ohm results in a current flow of 1 ampere.

A unit of electric current strength, the current yielded by one volt of electromotive force against one ohm of resistance.

To convert from ampere-hours to:

coulombs, multiply by 3600.
faradays, multiply by 0.03731.

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

The ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current, or amount of electric charge per second. The ampere is an SI base unit, and is named after André-Marie Ampère, one of the main discoverers of electromagnetism.

Definition

The ampere is a constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 metre apart in a vacuum, would produce between these conductors a force equal to 2×10^–7 newton per meter of length.^[1]

The ampere is a base unit, along with the metre, the second, and the kilogram: it is defined without reference to the quantity of electric charge. The unit of charge, the coulomb, is defined, as a derived unit, to be the amount of charge displaced by a one ampere current in the time of one second.

As a result, electric current is also the time rate of change or displacement of electric charge. One ampere represents the rate of 1 coulomb of charge per second.

Explanation

Because it is a base unit, the definition of the ampere is not tied to any other electrical unit. The definition for the ampere is equivalent to fixing a value of the permeability of vacuum to μ₀ = 4π×10⁻⁷ H/m. Prior to 1948, the so-called "international ampere" was used, defined in terms of the electrolytic deposition rate of silver.^[2] The older unit is equal to 0.999 85 A.

The ampere is most accurately realized using an watt balance, but is in practice maintained via Ohm's Law from the units of voltage 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.

The unit of electric charge, the coulomb, is defined in terms of the ampere: one coulomb is the amount of electric charge (formerly quantity of electricity) carried in a current of one ampere flowing for one second.^[3] Current, then, is the rate at which charge flows through a wire or surface. One ampere of current (I) is equal to a flow of one coulomb of charge (Q) per second of time (t):

Proposed future definition

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.

As with other SI base units, there have been proposals to redefine the kilogram in such a way as to define some presently measured physical constants to fixed values. One proposed definition of the kilogram is:

“

The kilogram is the mass which would be accelerated at precisely 2×10-7 m/s2 if subjected to the per metre force between two straight parallel conductors of infinite length, of negligible circular cross section, placed 1 metre apart in vacuum, through which flow a constant current of exactly 6 241 509 479 607 717 888 elementary charges per second.

”

This redefinition of the kilogram has the effect of fixing the elementary charge to be e = 1.60217653×10^-19 C and would result in a functionally equivalent definition for the coulomb as being the sum of exactly 6 241 509 479 607 717 888 elementary charges and the ampere as being the electrical current of exactly 6 241 509 479 607 717 888 elementary charges per second. This is consistent with the current 2002 CODATA value for the elementary charge which is 1.60217653×10^-19 ± 0.00000014×10^-19 C.

CIPM recommendation

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

The International Committee for Weights and Measures (CIPM),

• approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre Convention in time for possible adoption by the 24th CGPM in 2011;
• give consideration to the possibility of redefining, at the same time, the mole in terms of a fixed value of the Avogadro constant;
• prepare a Draft Resolution that may be put to the 23rd CGPM in 2007 to alert Member States to these activities;

This SI unit is named after André-Marie Ampère. As for all SI units whose names are derived from the proper name of a person, the first letter of its symbol is uppercase (A). But when an SI unit is spelled out, it should always be written in lowercase (ampere), unless it begins a sentence or is the name "degree Celsius". — Based on The International System of Units, section 5.2.

See also

• SI
• Ohm's Law
• Hydraulic analogy
• Electric shock
• Ampère's law
• Ammeter

References

1. ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical World, John Wiley and Sons, 2004 online.
2. ^ Robert B. Northrop, Introduction to Instrumentation and Measurements, CRC Press, 1997 online
3. ^ Kuzman Ražnjević, Physical Quantities and the Units of the International System (Si), Begell House Publishers, 1995 online

External links

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

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - ampere

Nederlands (Dutch)
ampère

Français (French)
n. - ampère

Deutsch (German)
n. - Ampere

Ελληνική (Greek)
n. - αμπέρ

Italiano (Italian)
ampere

Português (Portuguese)
n. - ampère (m) (Eletr.)

Русский (Russian)
ампер

Español (Spanish)
n. - amperio

Svenska (Swedish)
n. - ampere

中文（简体） (Chinese (Simplified))
安培

中文（繁體） (Chinese (Traditional))
n. - 安培

한국어 (Korean)
n. - 암페어(전류의 실용 단위)

日本語 (Japanese)
n. - アンペア

العربيه (Arabic)
‏(الاسم) الأمبير : وحدة لقياس, قوة التيار الكهربائي‏

עברית (Hebrew)
n. - ‮אמפר (יחידת-זרם)‬

If you are unable to view some languages clearly, click here.

To select your translation preferences click here.