The future of the kilo: a weighty matter

A lump of metal in a building near Paris has long served as the global standard for the kilogram. That’s about to change…

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 1kg cylinder is returned, carefully, to its resting place.

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