PLATINUM, Pt

NATIONAL PHYSICAL LABORATORY, UK

Platinum was discovered in Europe around 1750, although it had been known to South American metalsmiths long before this. Because of its resistance to corrosion, it was chosen as the material from which the early standard metre and kilogram were made. This piece of platinum, weighing exactly 1 kilogram, is a standard by which other weights are regularly measured. While the standard shown here is made from pure platinum, the international prototype kilogram, is made from a mix of platinum and iridium, increasing its hardness and stability. Maintained in Paris, the international prototype kilogram is the defining kilogram against which every other kilogram on the planet has been judged.

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STANDARD KILOGRAM NATIONAL PHYSICAL LABORATORY, UK