Ampere

The ampere (symbol: A), often shortened informally to "amp," is the base unit of electric current in the International System of Units (SI). It is named after the French mathematician and physicist André-Marie Ampère (1775–1836), one of the principal founders of the science of classical electromagnetism. Since the revision of the SI that took effect on 20 May 2019, the ampere has been defined by fixing the numerical value of the elementary electric charge, making it one of the seven SI base units upon which all other electrical units depend. Current is one of the most fundamental quantities in electrical science and engineering, and the ampere is accordingly among the most frequently encountered units in physics, electrical engineering, and everyday technical life.

Etymology and Naming

The unit takes its name from André-Marie Ampère, who in the early 1820s established the mathematical theory of electrodynamics, demonstrating that parallel current-carrying wires exert forces on one another and formulating the laws governing these interactions. His name was chosen for the unit of current in recognition of this foundational work. In English, the unit is conventionally written with a lowercase "a" when spelled out (ampere) and with a capital "A" as the symbol, following SI conventions. The name was formally adopted at the first International Electrical Congress in Paris in 1881, which established the ampere, together with the volt and the ohm, as practical electrical units.

Historical Development

Early Definitions and the International Ampere

The need for standardized electrical units became pressing in the late nineteenth century with the growth of the electrical industry and telegraphy. The 1881 Paris congress defined the ampere in principle as the current produced by one volt acting through one ohm, but practical realizations depended on laboratory standards of resistance and electromotive force. In 1893, the International Electrical Congress in Chicago ratified the "international ampere," defined operationally as the steady current that deposits silver from a standard solution of silver nitrate at a specified rate (approximately 0.001118 grams per second). This electrochemical definition was convenient because it could be reproduced in any suitably equipped laboratory without reference to mechanical measurements.

The Absolute Ampere (1948)

Measurements eventually revealed small inconsistencies between the electrochemical "international" units and their theoretical values. In 1948, the General Conference on Weights and Measures (CGPM) replaced the international ampere with the "absolute ampere," defined mechanically: one ampere is the constant current that, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 × 10⁻⁷ newtons per metre of length. This definition tied the ampere directly to the mechanical units of force and length, but it relied on the permeability of vacuum μ₀ having an exact value of 4π × 10⁻⁷ N/A², an assumption invalidated by later adjustments to other constants.

The 2019 Redefinition

On 16 November 2018, the 26th CGPM resolved to redefine all SI base units in terms of fixed values of fundamental physical constants, effective 20 May 2019. Under the new definition, one ampere is defined by taking the fixed numerical value of the elementary charge e to be exactly 1.602176634 × 10⁻¹⁹ when expressed in the unit coulomb, which is equal to the ampere-second. Equivalently, one ampere corresponds to the flow of approximately 6.241509074 × 10¹⁸ elementary charges (such as electrons) per second through a given surface. This definition links the ampere directly to the quantum of charge and removes any dependence on an arbitrary mechanical constant.

Definition and Realization

Although the ampere is now defined through the elementary charge, realizing it in practice historically involved demanding force measurements using the "ampere balance" or current balance, in which the force between current-carrying coils is weighed. After the 2019 revision, laboratories realize the ampere indirectly by combining two quantum effects: the Josephson effect, which provides an exact voltage standard via the Josephson constant, and the quantum Hall effect, which provides an exact resistance standard via the von Klitzing constant. By Ohm's law, the ratio of these quantities yields the current. A more direct route, so-called quantum metrology, uses single-electron pumps—devices that transfer electrons one at a time at a controlled clocked rate—to generate currents of a few picoamperes with high accuracy; devices based on graphene and other materials continue to improve this method. As of the 2019 redefinition, the values of e, the Planck constant h, and related quantities are exact, and the vacuum permeability μ₀ has become a measured quantity subject to experimental uncertainty.

Relation to Other Units

The ampere is related to the other SI units through precise identities:

• One ampere equals one coulomb per second (1 A = 1 C/s), expressing current as the rate of charge flow.
• By Ohm's law, one ampere equals one volt per ohm (1 A = 1 V/Ω), so a current of one ampere flows through a resistance of one ohm under a potential difference of one volt.
• One ampere equals one watt per volt (1 A = 1 W/V), connecting current to electrical power.
• The ampere is the only SI base unit that remains an electrical unit; the coulomb, volt, ohm, farad, henry, tesla, weber, and siemens are all derived units whose definitions trace back to the ampere.

Standard SI prefixes apply: common multiples and submultiples include the microampere (µA, 10⁻⁶ A), milliampere (mA, 10⁻³ A), kiloampere (kA, 10³ A), and megaampere (MA, 10⁶ A).

Measurement

Electric current is measured with an ammeter, which in practical instruments is connected in series with the circuit under test. Modern digital multimeters typically measure the small voltage developed across a precision shunt resistor. For alternating currents and high currents without circuit interruption, clamp meters based on transformer action or Hall-effect sensors are widely used. For metrological work, current comparators and cryogenic devices achieve the highest accuracies. The current-carrying capacity of a conductor or device—its ability to sustain a given current without excessive heating—is known as its ampacity, a term central to electrical wiring codes.

Typical Magnitudes

Everyday currents span many orders of magnitude. The current required to power a light-emitting diode is typically around 10 to 30 milliamperes; a smartphone battery may supply charging currents of one to several amperes; household lighting and appliance circuits in most countries are rated for currents of roughly 10 to 16 amperes; an electric kettle or heater draws on the order of 10 amperes; industrial motors and electric-arc furnaces require hundreds to tens of thousands of amperes; and a natural lightning stroke carries peak currents typically between about 20 and 40 kiloamperes, occasionally exceeding 200 kiloamperes. At the smallest scales, currents in integrated-circuit transistors and biological ion channels lie in the picoampere to nanoampere range.

Significance and Applications

As the SI base unit for electricity, the ampere underpins virtually all quantitative work in electrical science and technology. Power engineering specifies generators, transformers, cables, and protective devices in terms of their current ratings; electronics characterizes devices through their current–voltage behavior; electrochemistry quantifies processes by the electric current passed, as in Faraday's laws of electrolysis; and electromagnetism itself is formulated through quantities such as current density, measured in amperes per square metre, and Ampère's circuital law, which relates currents to magnetic fields. The ampere's redefinition in 2019 marked a milestone in metrology, tying the everyday unit of current to an invariant constant of nature and ensuring that its value remains stable and universally reproducible for the foreseeable future.