Historical length standards. Standards established for the yard and the metre, along with their cross-sections (right). The Henry VII Yard (1497) and
Historical length standards. Standards established for the yard and the metre, along with their cross-sections (right). The Henry VII Yard (1497) and the Elizabeth Yard (1588) were followed by the Imperial Standard Yard (officially recognised in 1855). The Metre Des Archives (platinum, standard from 1799 to 1889) was followed by the International Prototype Metre (iridium-platinum, standard from 1889 to 1960). Modern end standards are cylindrical bars of carbon steel accurately machined to precise lengths and used to measure larger lengths. Photographed at the National Physical Laboratory, Teddington, UK.
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Photo credit: © NATIONAL PHYSICAL LABORATORY © CROWN COPYRIGHT/SCIENCE PHOTO LIBRARY / Alamy / Afripics
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