National Physical Laboratory

The kilogram's constant struggle

Stuart Davidson and Ian Robinson of the National Physical Laboratory (NPL) explain how vacuum and surface science is key to realising a new definition of the kilogram based on fundamental constants, in a feature for Physics World. The NPL-authored article forms part of a special focus issue devoted to vacuum science and technology.

The Kilogram

The unit of mass, the kilogram, is the last of the seven base units of the International System of Units (SI) to be based on a physical object rather than a more robust definition in terms of an invariant of nature. Progress is being made towards a redefinition of the kilogram in terms of the Planck constant, realised via the watt balance and Avogadro experiments. Linking the kilogram to unchanging physical parameters ensures long-term stability, but it also presents significant opportunities to improve the way in which the mass scale is disseminated.

NPL and other European national measurement institutes, in collaboration with the Bureau International des Poids et Mesures (BIPM), academia and industry, are undertaking the research necessary to ensure that the transition to the new kilogram - scheduled to take place in 2018 - is seamless. The NewKILO project, coordinated by Stuart Davidson, aims to provide a harmonised approach to the way in which the primary realisation experiments are compared and how the mass scale is disseminated following the redefinition.

Both the watt balance and Avogadro experiments operate in a vacuum in order to remove uncertainties due to the density and refractive index of air. It is therefore vital that we understand the behaviour of the surfaces of primary mass standards used in vacuum and transferred between vacuum and air, where the calibration of secondary standards takes place.

An important goal of NewKILO, due to finish later this year, has been to investigate materials for next-generation mass standards that are compatible with use in vacuum. The project has also optimised vacuum-air transfer protocols, developed mass storage and transfer apparatus, and investigated new non-contact cleaning techniques for primary mass standards, which are being implemented at a number of national measurement institutes.

NPL, together with the national measurement institute of Switzerland METAS, has recently published a paper in the journal Vacuum, which provides further detail on preparations for the implementation and dissemination of the redefined kilogram. A paper published last year by Stuart Davidson and Ian Robinson - Preparations for the forthcoming redefinition of the kilogram - was awarded the Worshipful Company of Scientific Instrument Makers' Prize for the best paper published by the Institute of Measurement and Control in 2014 on 'the development or application of scientific instrumentation'.

Read the full feature in Physics World

Find out more about the redefinition of the kilogram

For more information, contact Stuart Davidson or Ian Robinson

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Last Updated: 17 Aug 2015
Created: 14 Aug 2015

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