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File:National prototype kilogram K20 replica.jpg

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National_prototype_kilogram_K20_replica.jpg (212 × 300 pixels, file size: 65 KB, MIME type: image/jpeg)

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. Its 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
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(Reusing this file)
Product of US Government employee

Licensing

Public domain
This image is in the public domain in the United States 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.

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Date/TimeThumbnailDimensionsUserComment
current11:15, 17 January 2013Thumbnail for version as of 11:15, 17 January 2013212 × 300 (65 KB)ChetvornoUser created page with UploadWizard

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