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takyon writes:

The future of the kilo: a weighty matter

First, the platinum-iridium cylinder is rubbed with a chamois that has been soaked in alcohol and ether. Then it is steam rinsed using boiling, double-distilled water. Finally, the 1kg cylinder is returned, carefully, to its resting place. Such reverence for a lump of metal is unusual, but has a purpose. The pavilion houses the International Bureau of Weights and Measures and that piece of platinum-iridium alloy is its holiest relic. It is the defining mass against which all other kilograms are measured. This is the international prototype of the kilogram. The IPK, in short.

Dozens of copies of this carefully calibrated piece of metal have been made. They are stored around the world and used to standardise individual nations' weights and measures systems. Britain's copy is kept at the National Physical Laboratory (NPL) at Teddington, near London. But the Parisian version is the king of the kilograms. "All mass measurements, anywhere on the planet, are traceable to that one unit in the Pavillon de Breteuil," says NPL scientist Tim Prior.

But the days of le grand K, as it is known, are numbered. Later this month, at the international General Conference on Weights and Measures, to be held in France, delegates are expected to vote to get rid of this single physical specimen and instead plump to use a fundamental measurement – to be defined in terms of an electric current – in order to define the mass of an object. The king of kilograms is about to be dethroned.

[...] In replacing the Paris kilogram with units measured in terms of Planck's constant, scientists have had to push technology to its limits, though in one sense they are merely catching up with all the other methods now used to define the other basic units with which we measure our existence, such as time and length. These are determined today in terms of fundamental processes.

Also at Inverse and Physics World.

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