File:National prototype kilogram K20 replica.jpg

English: Replica of the national prototype kilogram standard no. K20 kept by the US government National Institute of Standards and Technology (NIST), Bethesda, Maryland, shown as it is normally stored, inside two glass bell jars. This is the primary standard defining all units of mass and weight in the US. It is a polished cylinder made of 90% platinum - 10% iridium alloy, 39 mm (1.5 inches) in diameter and 39 mm high. It is an exact copy of the International Prototype Kilogram (IPK) kept at the International Bureau of Weights and Measures (Bureau International des Poids et Measures) in Sevres, France. It was one of 43 copies which were presented by France in 1889 to different nations as national standards, and became the primary standard of mass for the US in 1893 when the US switched to metric standards. Its exact mass, carefully compared to the IPK every 40 years, is 39 micrograms less than the IPK.