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The international prototype of the kilogram at the Pavillon de Breteuil in Saint-Cloud . AFP/Getty Images

A lump of metal stored in a building near Paris has long served as the global standard for the kilogram. But no longer.

In a vault inside the elegant Louis XIV Pavillon de Breteuil , outside Paris, a small metal cylinder rests on a shelf beneath a double set of bell jars. It has lain there for more than a century, its repose only occasionally disturbed when the vault’s three key holders perform a co-ordinated opening ceremony to let technicians enter and clean the ingot.

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 1 kg 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-iridiu...

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