Ampere (unit)
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| Ampere (unit) | |
|---|---|
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| Name | Ampere |
| Quantity | Electric current |
| Units1 | SI base unit |
| Units2 | International System of Units |
Ampere (unit) is the SI base unit of electric current, denoted by the symbol A. It quantifies the rate of charge flow in conductors and appears throughout physics, engineering, and technology. The ampere is defined via fundamental constants and underpins electrical standards used by laboratories, industries, and international organizations.
Definition and SI status
The ampere is one of seven SI base units whose formal definitions are maintained by the International Bureau of Weights and Measures and endorsed by the General Conference on Weights and Measures. Since the 2019 redefinition of SI base units, the ampere is defined by fixing the numerical value of the elementary charge as measured in coulombs, as established by the International Committee for Weights and Measures and adopted at the 26th CGPM. This modern definition relates the ampere to the second (SI), the kilogram via the fixed values of the Planck constant and the elementary charge, and the metre through electromagnetic relations. The SI system and the ampere’s status are discussed and implemented across national metrology institutes such as the National Institute of Standards and Technology, the Physikalisch-Technische Bundesanstalt, and the National Physical Laboratory (United Kingdom).
Historical development and naming
The unit is named after André-Marie Ampère, the French physicist and mathematician who made foundational contributions to electrodynamics during the early 19th century and who participated in scientific circles including the Académie des Sciences (France). The conceptual roots trace to experimental work by Hans Christian Ørsted, whose discovery of electromagnetism influenced contemporaries like Michael Faraday and Georg Ohm, and to formalizations by Wilhelm Eduard Weber and Lord Kelvin (William Thomson). The ampere was adopted as an SI unit through international agreements at assemblies of the International Electrotechnical Commission and the General Conference on Weights and Measures. Historical proposals and standards involved figures and entities such as James Clerk Maxwell, the Royal Society, and the British Association for the Advancement of Science. Proposals for realization included the ampere balance and the weber-related experiments influenced by the work of Gustav Kirchhoff and Carl Friedrich Gauss.
Realization and measurement methods
Realization of the ampere has transitioned from classical mechanical experiments—such as the current balance electromechanical methods used by national laboratories like Bureau International des Poids et Mesures predecessors—to quantum electrical standards. Quantum standards employ effects investigated by scientists including Brian Josephson and Hans Bethe: the Josephson effect underpins voltage standards via the Josephson junction, while the quantum Hall effect, discovered by Klaus von Klitzing, provides resistance standards. Combining these with the fixed value of the Planck constant and the elementary charge enables the realization of the ampere through standards such as the quantum metrology triangle and single-electron tunneling devices researched by groups at institutions like CERN, National Research Council (Canada), and Mikroelektronik. Practical measurement methods include current comparators, superconducting quantum interference devices developed using principles from Brian David Josephson and John Bardeen-related superconductivity research, and precision shunts calibrated against quantum Hall resistance standards at metrology centers.
Practical applications and standards
The ampere is central to specifications and standards in electrical engineering, telecommunications, power systems, and instrumentation overseen by organizations such as the Institute of Electrical and Electronics Engineers, the International Electrotechnical Commission, and the European Committee for Electrotechnical Standardization. Electrical wiring codes, circuit breaker ratings, and power transmission standards use ampere-based limits referenced in standards from entities like Underwriters Laboratories and national utilities including Électricité de France and TenneT. In electronics, components such as semiconductors designed by firms like Intel and Samsung Electronics are rated in amperes for current handling; battery capacities in consumer devices produced by Panasonic and LG Chem relate to current draw and discharge rates. Scientific instruments at facilities like CERN, Fermilab, and synchrotron centers report beam currents in amperes, and biomedical devices regulated by agencies like the Food and Drug Administration specify therapeutic currents accordingly.
Relationship to other SI units and constants
The ampere relates directly to the coulomb, the unit of electric charge, since one ampere equals one coulomb per second (SI). Via electromagnetic relations and Maxwell’s equations developed by James Clerk Maxwell and extended by Oliver Heaviside, the ampere connects to the volt, the ohm, the tesla, and the weber through SI derived units. The redefinition of the ampere in 2019 fixed the numerical value of the elementary charge and, together with the fixed Planck constant, links the ampere to quantum standards like the Josephson effect and the quantum Hall effect. International traceability of current measurements flows through comparisons among national metrology institutes including the National Institute of Standards and Technology, the Physikalisch-Technische Bundesanstalt, and the International Bureau of Weights and Measures, ensuring coherence with the SI system and constants such as the speed of light and the Boltzmann constant.