Ampere

The ampere, symbol A, is the base unit of electric current in the International System of Units (SI). It measures the rate at which electric charge is transferred, rather than an amount of charge or energy. Named after the French physicist André-Marie Ampère, it is commonly shortened to “amp.” Since 20 May 2019, its definition has been based on an exact value of the elementary charge, replacing an earlier definition involving the force between current-carrying wires. (bipm.org)

Definition and physical meaning

The modern definition fixes the elementary charge, $e$, at exactly

where the coulomb is related to the ampere and the second by $\mathrm{C}=\mathrm{A\,s}$. The second is independently defined through the caesium-133 hyperfine transition frequency. Together, these fixed constants establish the magnitude of the ampere without requiring a particular measuring instrument. (bipm.org)

One ampere corresponds to the transfer of one coulomb of electric charge per second:

For steady current, $I=Q/t$, where $Q$ is the transferred charge and $t$ the elapsed time. Thus a current of 2 A maintained for 3 s transfers 6 C. In elementary-charge units, one ampere corresponds to approximately $6.241509074\times10^{18}$ charges per second. The decimal approximation is rounded; the fixed value of $e$ is exact. (nist.gov)

This charge-transfer rate can be pictured as a stream of electrons crossing a boundary. It is a statement about the net quantity of charge transferred per unit time, not a specification of how rapidly individual electrons travel. (nist.gov)

Symbol, prefixes, and related units

The unit name is written ampere in lowercase except at the beginning of a sentence or in a title. Its symbol is uppercase because the unit commemorates a person. SI typography separates the numerical value from the symbol, as in “5 A.” Symbols are not pluralized, so “5 As” is not the standard notation. (nist.gov)

Decimal prefixes express currents across different scales: a milliampere (mA) is $10^{-3}$ A, a microampere (μA) is $10^{-6}$ A, a nanoampere (nA) is $10^{-9}$ A, and a kiloampere (kA) is $10^{3}$ A. These are scaled forms of the same unit, not independently defined quantities. (nist.gov)

The ampere participates in several electrical relationships. The volt measures electric potential difference, while the ohm measures electrical resistance. Their unit relations are

For an ohmic resistor, Ohm’s law gives $I=V/R$. For example, 12 V across a resistance of 6 Ω produces a current of 2 A. Current, voltage, and resistance therefore describe distinct, interrelated quantities. (nist.gov)

An ampere-hour is instead a unit of charge: $1\ \mathrm{A\,h}=3600\ \mathrm{C}$. It must not be confused with the ampere, which measures current, or with the joule, which measures energy. Converting a charge quantity into electrical energy also requires information about voltage. (nist.gov)

Historical development

The unit’s name reflects Ampère’s work on electromagnetism following Ørsted’s 1820 demonstration that an electric current deflects a compass needle. Ampère investigated the attraction and repulsion of parallel current-carrying wires, relating these effects to their magnetic fields. These interactions later supplied the basis of the mechanical definition of the ampere. (nist.gov)

Early international standardization relied on electrochemical measurements, including the mass of silver deposited by a current. The International Electrical Congress of 1893 selected the ampere and ohm as foundations for electrical units; an international conference in London formally accepted this arrangement in 1908. The resulting “international ampere” depended on specified experimental conditions and was distinct from the later absolute electrical unit. (nist.gov)

In 1948, the General Conference on Weights and Measures adopted a definition based on parallel conductors. One ampere was the constant current which, maintained in two infinitely long, straight, parallel conductors of negligible circular cross-section, one metre apart in vacuum, produced a force of $2\times10^{-7}$ newtons per metre between them. This idealized arrangement was difficult to realize directly. (bipm.org)

The 2018 decision to revise the SI replaced that definition, effective 20 May 2019. The change fixed $e$, rather than the vacuum magnetic permeability, whose value consequently became experimentally determined. (bipm.org)

Practical realization and measurement

In metrology, defining a unit and realizing it experimentally are different tasks. The exact definition does not eliminate measurement uncertainty in an actual current source or instrument. Laboratories can realize the ampere through accurately established voltage and resistance, using $I=V/R$. (nist.gov)

Quantum electrical standards connect these measurements to the elementary charge and the Planck constant. The Josephson effect supplies voltage standards, while the quantum Hall effect supplies resistance standards. Fixing both constants brought these practical standards into exact agreement with the revised SI framework. (bipm.org)

A complementary approach uses single-electron transport devices to transfer controlled numbers of electrons at a known repetition frequency. Ideally, transferring $n$ electrons per cycle at frequency $f$ produces $I=nef$. Comparisons among independently realized current, voltage, and resistance form the quantum metrology triangle, used to test the consistency of quantum electrical standards. (bipm.org)