SI units

ampere (A)

The 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.

The ampere, or 'amp' for short, measures 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 and can even produce enough power to run a bus or car.

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 solution.


In 2019, the definition of the ampere will exploit the fact that electric current is generally made up of a flow of billions of identical charged particles called electrons. We can create a standard ampere by using special nano-scale electric circuits that control the flow of electrons.

More about the redefinition in May 2019

Did you know?

  • A typical lightning strike has a current of 20 000 A. There are 300 000 a year in the UK, but only 15% reach the ground.
  • The ampere is named after André-Marie Ampère, who established the equation connecting the size of a magnetic field to the electric current that produces it.

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 current can be calculated via: Power = (Current)2 x Resistance.

At the 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 volt).

From May 2019, it is expected that the realisation of the ampere 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).