Ampere

The ampere is the fundamental SI base unit used to quantify electric current, forming a cornerstone of modern electrical science and engineering. It expresses the rate at which electric charge flows and underpins a wide range of physical laws, measurement systems, and technological applications. In the International System of Units, the ampere is represented by the symbol A, and one ampere corresponds to one coulomb of electric charge passing a given point in one second.

Definition and Concept of Electric Current

Electric current describes the movement of electric charge, typically electrons or ions, through a conductor or medium. In practical terms, an electric current of one ampere signifies that one coulomb of charge flows past a point each second. This relationship allows current, charge, and time to be connected through the simple expression:

• 1 A = 1 C s⁻¹

The contemporary SI definition, adopted in 2019, establishes the ampere by fixing the value of the elementary charge. The elementary charge is defined as an exact constant, and this fundamentally ties the ampere to atomic-scale physical properties. As a result, an ampere can be conceptualised as a precise number of elementary charges transported each second.
Before the 2019 revision, the ampere had been defined using the magnetic force between two long, parallel conductors placed one metre apart in a vacuum. This earlier definition relied on Ampère’s force law and connected electric current to the mechanical force produced by magnetic interactions. Although elegant, the realisation of this definition introduced practical challenges, motivating the transition to a definition based on immutable physical constants.

Historical Development

The ampere is named in honour of André-Marie Ampère, the French mathematician and physicist whose pioneering studies on electromagnetism laid the foundations of electrodynamics. Ampère’s work in the early nineteenth century demonstrated the relationship between electric currents and magnetic forces, a discovery central to the development of electrical science.
An international agreement in 1881 at the International Exposition of Electricity formalised the ampere as a standard unit for electric current. Early definitions were linked to the centimetre–gram–second (CGS) system. The abampere, for instance, represented a current that generated a force of two dynes per centimetre between parallel wires one centimetre apart. This early unit was deliberately sized so that values within the metre–kilogram–second (MKS) system would be practical.
Another historical form, the international ampere, was defined by the rate of silver deposition from a silver nitrate solution, providing an electrochemical method of realisation. Such definitions, while innovative for the period, were gradually superseded by electromagnetic and quantum standards enabling more consistent and reproducible measurements.

Former SI Definition and Realisation Techniques

Until the 2019 system overhaul, the SI defined the ampere through the force exerted between two parallel conductors carrying identical currents. Two infinitely long, straight wires with negligible diameter placed one metre apart in a vacuum were used as the conceptual experiment. A force of a specified magnitude per metre of wire served as the basis for defining the ampere.
This formulation linked the ampere to the coulomb, which was defined as the charge transported by a one-ampere current in one second. Charge was therefore determined by maintaining a steady current over a measurable period.
In practical metrology, the most accurate realisations of the ampere relied on the Kibble balance (formerly known as the watt balance). However, laboratories commonly realised the ampere indirectly using Ohm’s law, drawing upon the volt and the ohm. These two units benefited from reproducible quantum standards: the Josephson effect for the volt and the quantum Hall effect for the ohm. Even so, uncertainties of a few parts in 10⁷ remained, driving the need for the redefinition.

Present SI Definition

In the modern SI, the ampere is defined by fixing the elementary charge to an exact value expressed in coulombs. Because the coulomb is defined in terms of the ampere and the second, this approach connects the unit of current to a fundamental property of matter.
The second, in turn, is defined through the frequency of the ground-state hyperfine transition of caesium-133, meaning the ampere ultimately rests on an atomic time standard. This structure provides long-term stability and universality.
Current may be expressed in several forms:

• Instantaneous current, representing the current at a specific moment.
• Average current, the mean value over a defined interval.
• Constant current, maintained continuously during a measurement.

Charge accumulated through a circuit is expressed in coulombs and is obtained by multiplying the current by the duration of flow.

Derived Units Associated with the Ampere

The ampere is one of seven SI base units, which together represent the fundamental quantities used in dimensional analysis. Many important SI derived units are constructed from the ampere and other base units. Common units derived directly from current include:

• Volt (V) for electric potential difference, derived from the watt divided by the ampere.
• Ohm (Ω) for electrical resistance, given by the volt per ampere.
• Siemens (S) for electrical conductance, the reciprocal of the ohm.
• Farad (F) for capacitance, relating charge and voltage.
• Henry (H) for inductance, relating magnetic flux and current.
• Weber (Wb) for magnetic flux, connecting electromotive force and time.

Together, these units enable consistent descriptions of electrical and electromagnetic phenomena across scientific, industrial, and technological contexts.
Some frequently used units, such as the hertz, joule, watt, candela, lumen, and lux, do not derive directly from the ampere but appear widely in electrical engineering.
Metric prefixes may be applied to the ampere in the same way as to other SI units, allowing current to be expressed conveniently at different magnitudes—for example, milliampere (mA), microampere (µA), and kiloampere (kA).

Applications and Measurement

Electric current is measured using instruments such as multimeters, which can also measure voltage and resistance. Clamp meters, galvanometers, and ammeters provide additional means for assessing current in diverse circuits. Electrical devices from domestic appliances to high-power industrial systems rely on current ratings for safe and efficient operation.