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Object

International Prototype of the Kilogram

A metallic cylinder, part of the International Prototype of the Kilogram, sits on a circular base with two identical flasks positioned symmetrically behind it against a dark background.

A cylinder of 90% platinum and 10% iridium, 39 millimetres in both height and diameter, cast in London in 1879 and adopted in 1889 as the world's mass standard. Stored in a triple-locked vault at the BIPM in Sèvres, France, under three nested bell jars. Affectionately known as Le Grand K. Demoted to a calibration artefact when the kilogram was redefined in May 2019, but still in its vault.

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