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Boffins mount campaign against France's official kilogramme

'Eet cannot be changing. Eet weighs exactly 1 kg - sacré vache'

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International boffins are mounting a determined diplomatic push to end the practice of measuring mass by reference to a 130-year-old metal cylinder kept in France, saying that the French ingot is no longer up to the job.

The Consultative Committee for Units, whose chairman is Blighty's Professor I M Mills FRS, and which counts among its members the UK National Physical Laboratory and the US National Institute for Standards and Technology, has recommended that the Système International d'Unités (SI Units system) move to define the unit of mass - the kilogramme - more accurately.

At the moment, the official SI definition of a kilogramme is a mass equal to that of the international prototype kilogramme, a cylinder of platinum and iridium kept inside several nested glass cases in an environmentally monitored vault in France at the Bureau International des Poids et Mesures outside Paris. The kilogramme was made in 1879.

Elaborate precautions are taken to ensure that the kilogramme doesn't change mass - a complicated official cleaning procedure is carried out to remove atmospheric oxidants from the surface. Officially, of course, it can't change - it will always mass exactly one kilogramme, by definition. But it is compared from time to time with other exact copies held by other nations, and it is known that in reality slight changes on the order of microgrammes - a billionth of a kilo - do occur.

All this simply won't do, according to the boffins of the Consultative Committee for Units. They have now proposed to the Comité International des Poids et Mesures (CIPM - the body in charge of SI units) that the kilo should instead be defined in terms of the Planck constant - a well-known physical constant expressed in kilogramme square-metres per second.

All the other base units of the SI system have already been moved off physical artifacts - there's no official metre any more - so this would remove the last of the unsatisfactoriness from measurement.

It seems that the CIPM approved of substituting a quantum Planck for a platinum-iridium chamfered cylinder, and has now passed the matter on for action by international diplomats at the General Conference on Weights and Measures, which will meet in October 2011.

All the signs are that the Planck mass plan will become official at that point, and by the middle of the decade the French official kilogramme can be retired. ®

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