Ampere hero

The Unit – ampere, A (wae-iahiko)

The SI unit of electric current, the ampere, is named after André-Marie Ampère, a French physicist and mathematician who was one of the founders of classical electromagnetism.

Ampère’s name was first associated with a unit of current in 1893, but it was in 1948 that the 9th CGPM formally adopted the ampere for the unit of electric current, following a definition proposed by the CIPM (1946, Resolution 2):

“The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10–7 newton per metre of length.”

 It follows that the magnetic constant, µ0, also known as the permeability of free space, is exactly 4π x 10-7 henry per metre.

This definition is quite difficult to implement, but fortunately some Nobel prize winning discoveries in 1962 and 1980 offer a more reliable approach. In common with most other national metrology institutes, MSL realises the units of voltage and resistance separately using different macroscopic quantum phenomena for each unit. For voltage, the Josephson effect links frequency to voltage so that voltages can be generated that are measurable at the part-per-billion level. For resistance, semi-conducting devices designed to exhibit the quantum Hall effect are operated as resistors with values measurable at the part-per-billion level. With these quantum phenomena, the values of voltage and resistance are linked directly to internationally agreed values of Planck’s constant, h, and electronic charge, e.

In 1990 the CIPM recommended conventional values for the Josephson constant, KJ = h/2e, and the von Klitzing constant, RK = h/e2. Adoption of these conventional values enabled a high level of international consistency in voltage and resistance values that was not previously possible. This recommendation anticipated the changes to the SI that will be agreed later in 2018 and implemented in 2019. There will be very small shifts in the values of the Josephson and von Klitzing constants, but no change in how the volt, ohm and ampere are realised. A minor change is that the magnetic constant, µ0, will no longer be an exact number, but will have a small uncertainty because its value will be derived from experiment.

The ampere and all other quantities in electricity are derived from these realisations of voltage and resistance.

Technical Capability

We are leading experts in electrical measurements across a very wide range of activities. We can advise on the best measurements to support your decision making as well as identify and control sources of error in measurement systems. A wide range of calibration services is offered, primarily for laboratory standards of ac and dc current and voltage, resistance, capacitance, inductance, power and energy. We also offer support for businesses making energy measurements in compliance with the Electricity Industry Participation Code.


Continued improvement of measurement capabilities is supported by a range of research activities. Work is being carried out on electron transport in mesoscopic systems for metrology and other applications. This anticipates the adoption of new electrical measurement technologies. Studies are also being carried out into:

  • Independent calibration methods for dc current comparators.
  • Establishing dc voltage scales up to 1000 V with electronic instruments.
  • Characterisation of errors in current transformers based on improved circuit models.
  • Josephson systems for sampling ac voltage.
  • Digital sampling systems for power measurement.
  • Improved methods for determining resistance ratios.

Go to the Electrical Calibration services page.