National Physical Laboratory

Do you still use the 'metre bar' as the length standard? (FAQ - Length)

No. Since 1983 the metre has been re-defined in terms of the speed of light and is realised in practical applications using laser wavelengths.

The UK copy of the metre is stored in a safe at NPL but we no longer use it nor do we have any equipment suitable for measuring the metre bar. The accuracy of the laser wavelengths far surpasses the accuracy achievable using the previous krypton wavelength definition, and the definition involving the metal bar. See also The Metre (m) and Beginner's Guide to Length.

Last Updated: 25 Mar 2010
Created: 17 Jul 2007

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