Hidden in a vault in the Pavillon de Breteuil in Sèvres, outside Paris, vacuum-sealed under three bell jars, sits a palm-sized metal cylinder known as the International Prototype Kilogram. Forged in 1879 from an alloy of platinum and iridium, it was hailed as the 'perfect' kilogram - the standard by which other kilograms would be judged.
Although it's arguably the world's most famous weight, it doesn't get out much. Since hydrocarbons on fingertips or moisture in the air could contaminate its pristine surface, it goes untouched for decades, under triple lock and key. Every 40 years, however, it makes an appearance.
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