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Prototype Kilogram 20, replica
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Prototype Kilogram 20, replica

 ca. 1875-1889  On virtual loan from the NIST Automated Production Technology Division
After the Treaty of the Meter had been signed in 1875, the International Bureau of Weights and Measures (BIPM) in Sevres, France, procured 43 prototype kilograms of platinum-iridium in the form of right circular cylinders of equal diameter and height (approximately 39 mm) with slightly rounded edges. One of the group, having essentially the same mass as the kilogramme des Archives, was selected as the International Prototype Kilogram, and is now maintained at BIPM. National Prototype Kilograms No. 20 and No. 4 were received by the United States from BIPM in 1890. After The Mendenhall Order in 1893, Kilogram No. 20 became the primary national standard for all mass measurements. Its mass relative to the International Prototype Kilogram was reported as 0.999 999 961 kg. The reported displacement volume at a temperature of 0°C was 46.402 cm3.