Ampere (A)
Definition, realization and electric current measurement in the International System of Units
Electric current is a fundamental quantity in physics, engineering and modern infrastructure. Energy distribution, electronics, communication and measurement technology all rely on precise control of electric current. Its present definition, and how that definition is realized, is less familiar.
The modern ampere is no longer defined through mechanical forces between conductors, but through a fundamental constant of nature. To understand this it is, as with the other SI base units, necessary to distinguish between definition, realization and practical use.
The definition of the ampere in the SI system
Since 2019, the ampere has been defined within the International System of Units (SI) according to the following principle:
The ampere is the SI unit of electric current. It is defined by giving the elementary charge e the exact numerical value
1.602 176 634 × 10⁻¹⁹ coulomb.
Since one coulomb is defined as one ampere multiplied by one second (C = A·s), this definition means that the ampere is in practice defined through:
- the fixed elementary charge e,
- the second, defined via the hyperfine transition of the cesium atom.
Electric current can thus be interpreted as the number of elementary charges passing per unit of time.
Historical background and the 2019 SI reform
Before 2019, the ampere was defined through an idealized force between two infinitely long, parallel conductors in vacuum. This definition was theoretically clear but difficult to realize with high precision in practice.
The 2019 SI reform brought a shift in principle, in which the ampere, like the kilogram, was tied to a constant of nature. By fixing the value of the elementary charge, it became possible to realize current directly by counting charges, rather than through mechanical force measurements.
Definition, realization and use
For electric current too, it is important to distinguish between three levels:
The definition states what the ampere is in principle and is tied to the elementary charge.
The realization refers to the experimental methods by which this definition is turned into actual current standards.
The use refers to practical current measurement in electronics, industry, power systems and everyday applications.
Realizing the ampere
The definition of the ampere prescribes no specific experimental method. In practice, the ampere is realized through methods traceable to the elementary charge and the second. Three central approaches are used in modern electrical metrology.
Single-electron transport
A direct way to realize the ampere is through single-electron transport, in which individual electrons are moved in a controlled manner through a nanostructure.
If exactly n electrons are transported per cycle at frequency f, the current is given by:I=nef
Since both e and f are exactly known quantities, the current can be realized directly from the definition. This method represents the most direct interpretation of the ampere as charge per unit of time.
Voltage and resistance: Josephson and quantum Hall standards
In practical electrical metrology, the ampere is often realized indirectly via Ohm’s law:I=RU
The voltage U is realized through the Josephson effect, in which voltage is linked to frequency and the Planck constant. The resistance R is realized through the quantum Hall effect, in which resistance is expressed in terms of the Planck constant and the elementary charge.
The combination of these quantum effects provides a very stable and exact realization of electric current.
Primary and secondary current standards
Methods based on single-electron transport are regarded as primary realizations of the ampere, since they link current directly to the elementary charge.
Methods based on voltage and resistance are used as secondary standards and are today the most widely used at national metrology institutes and in calibration work, owing to their high stability and practical usability.
The role of the ampere in derived units
The ampere is a fundamental component of many derived SI units, including:
- the volt (electric potential),
- the ohm (electrical resistance),
- the watt (electric power),
- the coulomb (electric charge).
A stable definition of the ampere therefore matters for the whole electrical part of the SI system.
Practical current measurement
In everyday and industrial applications, current is usually measured by indirect methods, for example via the voltage drop across a known resistance. Although these measurements do not directly realize the SI definition, they are traceable to it through calibration against primary and secondary standards.
Summary
The ampere is today defined through an exact value of the elementary charge and is thus a unit directly linked to atomic physics. Through modern quantum metrological methods, this definition can be realized with very high precision.
The transition from a mechanical to a constant-based definition has made the ampere more reproducible, more exact and better suited to modern electronics and measurement technology.