National Physical Laboratory

Weighing the options

Dr Ian Robinson, NPL Fellow in Electrical Metrology and the work to redefine the kilogram features in the April edition of New Electronics magazine. The kilogram is the last SI unit to be represented by a physical artefact. But that may not be the case for much longer. Dr Robinson outlines the current situation and the international attempts to bring things into agreement and get a consensus value for Planck's constant that will, in future, determine mass.

The UK kilogram in its storage unit

The kilogram is the International System of Units (SI) unit for mass. It is the only remaining base unit to be defined by a physical object. All standards of mass must ultimately be traceable to this one object, a cylinder of platinum-iridium alloy kept at the International Bureau of Weights and Measures (BIPM) in France. The UK's National Standard Kilogram is stored in a basement at NPL and is a copy of the international prototype and the standard for all mass measurements in the UK.

As the current kilogram is defined by a solid object, its mass could change due to loss of material or contamination from the surrounding environment. It is also possible, however unlikely, that it could be lost or damaged. To overcome such risks and to improve our system of units, a definition is required that depends on nothing other than the value of a fundamental constant, e.g. the Planck constant h.

The search is on in a number of scientific laboratories to try to find a way of defining the kilogram in terms of a fundamental constant. Today, two key approaches are being pursued internationally: watt balances and the Avogadro project

Read the full article 'Weighing the options' in New Electronics

Find out more about the Redefining the kilogram

Contact: Ian Robinson

Last Updated: 2 May 2013
Created: 2 May 2013

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