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Scientists agree to update the definition of a kilogram

A replica of Le Grand K (Photo: Japs 88)A replica of Le Grand K (Photo: Japs 88)Scientists will soon be updating the definition of a kilogram. Since 1889, the exact mass of one kilogram has been based on a lump of metal known as Le Grand K, a cylindrical object made of an alloy of platinum and iridium. Le Grand K is kept under three glass bell jars in a locked vault at the Pavillon de Breteuil in Saint-Cloud, near Paris, France. Over the years, there have been minute variations in its weight caused by pollutants in the air, despite regular cleanings. This has led to the decision to change the definition of a kilogram from a physical object to one derived from the fundamental laws of physics.


A kilogram weight (Coyau)A kilogram weight (Coyau)The kilogram was originally defined as the mass of a litre (a cubic decimetre) of water at 0°C (32°F). But because this quantity was difficult to replicate exactly, in 1799 an object made of platinum was used instead. This was later replaced by Le Grand K, or, to give it its official name, the International Prototype of the Kilogram (IPK). All modern measurements of mass are based on Le Grand K.

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