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Unit of the month – the ampere

In preparation for the SI changes that are coming in on 20 May 2019, we are looking at a unit a month

20 November 2018 the metre (m)
20 December 2018 the candela (cd)
20 January 2019 the ampere (A)
20 February 2019 the kelvin (K)
20 March 2019 the second (s)
20 April 2019 the mole (mol)
20 May 2019 the kilogram (kg)








Electric current, for most of us, seems to appear mysteriously from electrical sockets and gives life to our appliances. Just like the blood in our veins, an electric current is flowing through the 'conductive arteries' in our homes, powering the everyday equipment we need.

Today, electricity is so common that we rarely give it a second thought. Electrical measurements are everywhere, found in the readout of almost every type of 'sensor' (e.g. light, heat, force) that exists. Accurate measurements support everything from telecommunications and security to automotive and aerospace technologies.

The earliest historical work on electricity comes from the ancient Greeks, who described static electricity. They observed that when amber was rubbed with a piece of fur it began to attract small objects such as hair or dust. In fact, the word 'electricity' comes from the Greek 'electron' – ηλεκτρον – meaning amber. After the phenomenon was first noted, 'amber electric current' remained only a strange curiosity. Further development on electricity only began in earnest in the 19th century.

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 rapid developments in electricity seen during that time also brought key advancements in science and measurement. The fact that electric current can produce a mechanical force, and vice versa, became the basis of electric generators and motors that transformed the world, and is the basis of the modern definition of the ampere.

The ampere is that 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 these conductors a force equal to 2 x 10-7 newton per metre of length.

Having an accurate and precise way to define the ampere meant measurement scientists were able to examine different physical phenomena that could be used in construction of more and more accurate measuring instruments. Quantum physics, for example, has been especially fruitful in providing new and innovative solutions for measurement challenges.

The current practical realisation of the ampere is based on the relation of electric current with voltage and resistance. A device called a 'Josephson junction' is used to generate the voltage, and the quantum Hall effect generates resistance. Both methods use physical phenomena that are well understood, and are dependent on physical constants: the Josephson constant and von Klitzing constant respectively (both Josephson and von Klitzing are Nobel Prize laureates), that can both be expressed in terms of fundamental constants of nature: e – elementary charge and h – Planck constant. Natural constants remain unchanged no matter what, so that using them to define a unit makes that unit measurable throughout whole universe (e.g. a measurement of the Planck constant yields the same value in London, Paris or on Mars). Units that are dependent on constants of nature also assure the long-term stability of measurement standards.

The proposed change in definition means that electrical units will not be linked to mechanical units any more; instead, the new 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'. See the BIPM's Mise en pratique for the definition of the ampere for more details.

The official wording of new ampere definition which comes in to effect on 20 May 2019 will be:

The ampere, symbol A, is the SI unit of electric current. It 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 ∆νCs.

This research has also been the basis of the Kibble balance developed at NPL and will form the basis of the newly-defined kilogram, as it compares the gravitational force on an object against an electromagnetic force calculating it in terms of h, the Planck constant.

NPL's continuing research on quantum Hall resistance standards, particularly involving graphene, ensures that quantum standards become more robust and are easily accessible to all.

Find out more about Redefining the SI units and the ampere

Find out more about NPL's work in Quantum Current Standards

21 Jan 2019