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NPL at the Royal Institution of Great Britain

Supporting the definition and realisation of the kilogram

The Kibble balance, formerly the watt balance and renamed after Bryan Kibble who developed the apparatus at NPL in the 1970s, is an instrument which compares electrical and mechanical power. It will allow the SI unit of mass to be realised in terms of quantum electrical units, the Josephson voltage and quantum Hall resistance, with relation to a fixed value of the Planck constant.

Current iterations of the Kibble balance, including the world-leading NPL MkII Kibble balance which is now at NRC in Canada, were designed primarily to measure the Planck constant with relation to the current mass scale. NPL's next-generation balance has been specifically designed to realise the unit of mass following the revision of the SI. It employs novel operational principles and mechanical design features which result in a balance which is more cost-effective, easier to manufacture and more convenient to use than current methods. It also equals or improves on the level of accuracy currently achievable.

Features of NPL's MkII Kibble balance include a novel mechanical design using flexure strips to minimise errors due to the misalignment of the coil in the magnet field. It has been specifically designed for multiple unit production with the intention of distributing the means to realising the unit of mass widely. This will make the global mass scale much more robust.

Looking forward, NPL intends to build on this work by designing Kibble balance instruments to give SI traceable mass measurements at the point of need, such as in a production process. We also want to miniaturise the apparatus for use in areas such as pharmaceutical research and production, biotechnology, personalised medicine, microfabrication and robotics.

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