File:National prototype kilogram K20 replica.jpg

Summary

Description 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. It's exact mass, carefully compared to the IPK every 40 years, is 39 micrograms less than the IPK.

The handling of actual prototype kilograms like K20 is kept to an absolute minimum to avoid wear that results in changes in mass; even a single atomic layer of atoms removed from the bottom surface by a slight abrasion during lifting would result in a measureable decrease in mass of 10 μg. So the actual prototype is left in a locked basement vault safe from damage. This is an exact replica fabricated for public display. Date Unknown date Source

   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 (Reusing this file)

Product of US Government employee

Copyright status:

This image is in the public domain because it is a work of the United States Federal Government, specifically an employee of the National Institute of Standards and Technology, under the terms of Title 17, Chapter 1, Section 105 of the US Code.

Source:

http://commons.wikimedia.org/wiki/File:National_prototype_kilogram_K20_replica.jpg