National Physical Laboratory

Special journal edition on the new SI

A special edition of the journal Measurement and Control, guest-edited by Graham Machin from the National Physical Laboratory (NPL), has been published describing the proposed changes to the SI.

An acoustic resonator used to measure the Boltzmann constant, which could be fixed to redefine the kelvin
An acoustic resonator used to measure
the Boltzmann constant, which could be
fixed to redefine the kelvin

The SI is a globally agreed system of units at the heart of the world's measurement system. It consists of seven base units: the metre (the unit of measurement of length), the kilogram (mass), the second (time), the ampere (electric current), the kelvin (temperature), the mole (amount of substance) and the candela (luminous intensity). The vast majority of measurements performed in industry, science and everyday life are derived from these seven units.

However, some of the unit definitions are based on artefacts, material properties or physical laws, which limit the accuracy with which measurements can be made. The kilogram, for example, is based on a platinum-iridium cylinder made in the 19th century and kept in a safe near Paris. But the mass of this artefact is not absolutely stable and has been changing over the 125 years since it was made.

Under the proposed changes to the SI, the classical definitions of the kilogram, ampere, kelvin and mole will give way to those based on defined values of fundamental constants. This follows the precedent set in 1983 when the metre was defined by fixing the value of the speed of light. Professor Machin explains in his introduction:

"In the new SI, all the units will be defined in terms of associated fundamental constants whose values, once fixed, need never change again. This will provide a new SI which is robust and coherent and fit for purpose for the foreseeable future."

Professor Ian Mills, of the University of Reading and former president of the Consultative Committee for Units (CCU), then sets the scene, giving the rationale and an overall outline of the redefinition.

Three articles from NPL researchers follow, focusing on the changes to the individual SI units: the ampere (JT Janssen, Stephen Giblin, Patrick See, Jonathan Fletcher and Masaya Kataoka), the kilogram (Stuart Davidson and Ian Robinson) and the kelvin (Michael de Podesta).

Measurement and Control is circulated to all members of the Institute of Measurement and Control This special edition demonstrates NPL's leading contributions to the SI redefinition, as well as keeping this section of the measurement community informed of the forthcoming changes.

Find out more about NPL's work on the SI Units

For further information, contact Graham Machin

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Last Updated: 17 Aug 2015
Created: 3 Feb 2015

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