Image:Platinum-Iridium meter bar.jpg
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International Prototype Metre standard bar made of platinum-iridium. This was the standard until 1960, when the new SI system used a krypton-spectrum measurement as the base. In 1983 the current metre was defined by a relationship to the speed of light in a vaccuum. (NIST)
http://www.mel.nist.gov/div821/webdocs-14/lsi_2.htm
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This image is in the public domain because it contains materials that originally came from the National Institute of Standards and Technology. |
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