SI units

ampere (A)

The ampere is the SI unit of electric 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. The ampere definition exploits the fact that electric current is 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.


The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602 176 634 × 10−19 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν.

This was a new definition in May 2019.

More about the redefinition


Electricity (moving electrical charge) is a key way to transport information and energy. Our electricity bills charge us on current × voltage × time. 

Electrical measurements are found in the readout of almost every type of ‘sensor’ that exists. Accurate measurements support everything from telecommunications and security to automotive and aerospace technologies.

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
  • A wristwatch typically needs only one millionth of an ampere of current from its battery (1 μA)

The science behind the unit

The ampere 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 and the force created between two infinitely long wires in a vacuum that are carrying the same current. 

The ampere is now realised by controlling the flow of single electrons 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 simply the charge on an electron times the number of electrons transported per cycle of the barrier (frequency).