Image: National prototype kilogram K20 replica

Description: 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.
Title: National prototype kilogram K20 replica
Credit: Downloaded 2013-01-17 from Prototype Kilogram No. 20 Replica, Room 4, Weights and Measures, NIST Virtual Museum website. No copyright notice accompanied image. Disclaimer on NIST site: "These World Wide Web pages are provided as a public service by the National Institute of Standards and Technology (NIST). With the exception of material marked as copyrighted, information presented on these pages is considered public information and may be distributed or copied."
Author: Unknown
Permission: Product of US Government employee
Usage Terms: Public domain
License: Public domain
Attribution Required?: No