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SI units > Ampere (A)

Ampere (A)

AmpereThe ampere is the SI base unit for electric current. It is currently defined by the force created between two infinitely long wires in a vacuum that are carrying the same current.

Amperes, or 'amps' for short, measure electric current, which is a flow of electrons along a wire or ions in an electrolyte, as in batteries. Electric current allows us to power electrical devices, like smartphones or laptops.

The ampere has only been in use for as long as we have had access to electricity – a small proportion of the history of measurement.

It has previously been defined as the current that would cause the deposition of a certain mass of silver per second from a silver nitrate.


In 2019, the definition of the ampere is expected to be replaced by a definition that is more intuitive and easier to realise. Electric current will instead be a measure of the number of electrons passing a certain point on a wire, per second.

Did you know?

  • A typical lightning strike measures at around 30,000 amps

The science behind the unit

The formal definition of the ampere is that it is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in vacuum, would produce between those conductors a force equal to 2 x 10-7 newtons per metre of length.

However, the ampere is difficult to realise in practise with sufficient accuracy, so it is realised via the watt (the SI unit for power). The electrical power generated in a controlled experiment is compared to mechanical power, and using an accurate measurement of resistance the ampere can be calculated via: Power = (Current)2 x Resistance.

At the National Physical Laboratory (NPL), the volt is realised from the AC Josephson effect. Due to this effect, the potential difference between two superconductors separated by a narrow gap and exposed to electromagnetic radiation, takes discrete values dependent on the Josephson constant (483597.9 gigahertz per volt) and the frequency of radiation. This gives the volt to an accuracy of 1 hundred millionth of a volt (0.000 000 01 volts).

From May 2019, it is expected that current will be based on counting the single electrons flowing in a sophisticated semiconductor circuit. In a device under investigation at NPL, electrons are pushed through a narrow channel using tiny oscillating barriers. The current is easily compared with theory as it is simply the charge on an electron times the number of electrons transported per cycle of the barrier (frequency).

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