Ampere — SI unit of electric current

Definition and modern redefinition

Since the 2019 revision of the SI, the ampere is defined indirectly through the fixed numerical value of the elementary charge e, which is exactly 1.602176634×10⁻¹⁹ coulombs. This links the ampere to fundamental constants of nature rather than to a mechanical experiment. In everyday terms the definition ensures that the amount of charge conveyed by electrical currents can be traced back to the discrete charge of electrons.

Practical realization and measurement

Electric current is measured with instruments such as ammeters, clamp meters and multimeters. For primary standards and quantum metrology, realizations of the ampere make use of phenomena in condensed matter physics: single-electron transport devices (charge pumps), the quantum Hall effect and Josephson voltage standards are combined in precision experiments to relate current, voltage and resistance to fundamental constants. Historically, the ampere had been defined by the magnetic force between two parallel conductors, but that mechanical definition was superseded by the constant-based SI.

Common multiples, examples and typical magnitudes

• Milliampere (mA) = 10⁻³ A — typical for small electronics and signal currents.
• Microampere (µA) = 10⁻⁶ A — common in sensors and low-power circuits.
• kiloampere (kA) = 10³ A — used for very large currents such as lightning or industrial arcs.

Examples across scales: tiny control currents in microelectronics are measured in microamperes, household circuit breakers are typically rated in amperes (often tens of amperes), while natural events like lightning involve currents many orders of magnitude larger.

History and name

The unit is named after the French physicist André-Marie Ampère, a pioneer of the field of electromagnetism. His experiments in the early 19th century established the relationship between electric currents and magnetic forces, and his work laid a foundation for later formalization of electrical units. The ampere was adopted as an SI base unit well before the modern constant-based definition was introduced.

Distinctions and practical notes

It is important to distinguish current (measured in amperes) from charge (measured in coulombs) and from voltage (measured in volts). Current represents the rate at which charge moves, not the amount of charge itself. Safety considerations are associated with current: the effects of electricity on living tissue and on equipment depend largely on current magnitude and pathway. For authoritative standards and further technical details, consult metrology resources or national standards organizations.

Further reading: the unit symbol and conventions are summarized in international standards; related topics include circuits and electrical units, practical metrology of current, and the historical definitions of SI units. See also resources on unit conventions and quantum electrical standards.

Unit overview • Symbol guide • Charge relations • Biographical note • Field background