Measurement Fundamentals

It is my pleasure to introduce the following set of four articles describing what will be a fundamental change in the International System of Units, the SI.

The SI is at the heart of the world’s measurement system. It comprised seven base units: the metre, the kilogram, the second, the ampere, the kelvin, the mole and the candela, from which the vast majority of measurements performed in industry, science and everyday life are derived. A summary of the SI can be found on the website of the International Bureau of Weights and Measures (BIPM) at http://www.bipm.org/utils/common/pdf/si_summary_en.pdf

However, some of the unit definitions are based on artefacts or material properties or physical laws, which limit the accuracy with which measurements can be made. Thus, the unit of mass, the kilogram, is still based on a cylinder of platinum–iridium alloy made in the 19th century and kept in a safe near Paris, but whose mass cannot be completely constant. The unit of temperature, the kelvin, is based on a particular property of water, which depends on the purity and isotopic composition of the sample, and the unit of electric current, the ampere, is based on a physical law which cannot be realised to better than a few parts in 10 million, and in practice was superseded by the application of quantum electronic phenomena about 30 years ago.

These classical definitions will give way to ones 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 in vacuum. In the new SI, all the units will be defined in terms of associated fundamental constant 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.

The articles which follow introduce the proposed changes to the SI. The article by Mills sets the scene, giving the rationale and an overall outline of the redefinition. This is followed by three articles each focusing on particular units: Davidson and Robinson on the kilogram (and also the mole), Janssen et al. on the ampere, while de Podesta describes work leading to the kelvin redefinition. For the interested reader, more information can be found on the BIPM website http://www.bipm.org/en/si

The National Physical Laboratory (NPL) continues to play a leading role in the unit redefinitions and will, in the future, be working on improved realisations of the units to ensure that measurement requirements in the United Kingdom are met in an appropriate and reliable way. Our vision for this is laid out in our Metrology for the 2020s document http://www.npl.co.uk/upload/pdf/metrology-2020vision.pdf.