Hình:Platinum-Iridium meter bar.jpg

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

This image is in the public domain because it contains materials that originally came from the National Institute of Standards and Technology.