Ampere

The ampere (A) is the SI base unit of electric current. Since 2019, it has been defined by fixing the numerical value of the elementary charge.

Elementary charge: The positive charge magnitude of a proton, equal in magnitude to the negative charge of an electron. Fixing its value in coulombs defines the ampere.
André-Marie Ampère: A French physicist and mathematician whose work helped establish electrodynamics. The ampere is named in his honor.
Electrical metrology: The science of measuring electrical quantities and maintaining their standards. It realizes and disseminates accurate measurements of current in amperes.
Volt: The SI derived unit of electric potential difference and electromotive force. Voltage measures energy per charge, unlike the ampere's charge-per-time measure.
Electric current: The rate at which electric charge passes through a surface. The ampere measures this rate in coulombs per second.
International System of Units: The internationally agreed system of measurement units and their definitions. Its revisions set the ampere's present definition.
Quantum Hall effect: A quantized Hall resistance observed in two-dimensional electron systems under strong magnetic fields. Its resistance standard supports precise electrical measurements linked to the ampere.
Ohm: The SI derived unit of electrical resistance. Resistance is distinct from current, though volts divided by ohms yield amperes.
Coulomb: The SI unit of electric charge, equal to the charge of about 6.24 × 10¹⁸ elementary charges. The fixed elementary charge links this unit to one ampere-second.
2019 redefinition of SI base units: A revision that redefined four SI base units using fixed values of physical constants. It replaced the ampere's force-based definition with one based on elementary charge.

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