International yard

The international yard and pound are two units of measurement that were the subject of an agreement between six nations signed on 1 July 1959. The six nations were the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa. The agreement defined the yard as exactly 0.9144 metres and the pound as exactly 0.45359237 kilogram.

In 1866 the United States legalized the use of the metric system, but at a time when metric units were poorly defined by antiquated physical standards. As a result, the US Congress supplied definitions of the meter and kilogram in terms of the yard and pound. By the 1890s the situation had reversed, with the metric system having superior physical standards. As a result, the Mendenhall Order of 1893 reversed the definitions of 1866, defining the yard and pound in terms of the meter and kilogram.

In the UK, a similar situation developed with the Weights and Measures Act of 1897 legalizing the metric system, and Order in Council 411 (1898) defining the meter and kilogram in terms of the yard and pound. As a practical matter the UK definitions were reversed, resulting in a de facto definition of the imperial yard as 36/39.370113 meter.

In the 1890s, Albert Michelson began conducting experiments in interferometry that led in 1903 to demonstrating the feasibility of using light waves as units of linear measurement. In 1908, two teams of researchers, one led by Michelson, defined the length of the international prototype meter in terms of light waves. In 1927 the International Bureau of Weights and Measures provisionally adopted the 1908 light-wave definition of the meter as a supplemental standard.

In 1930 the British Standards Institution adopted an inch of exactly 25.4 mm based on the 1927 light-wave definition of the meter. The American Standards Association followed suit in 1933. By 1935 industry in 16 countries had adopted the "industrial inch" as it came to be known.

In 1946 the Commonwealth Science Congress recommended a yard of exactly 0.9144 metres - i.e. exactly 36 industrial inches - for adoption throughout the British Commonwealth. This was adopted by Canada in 1951. In the late 1950s, the directors of the national standards laboratories of the United States, the United Kingdom, Canada, Australia, New Zealand and South Africa came to an agreement to adopt a uniform system of weights and measures in their respective countries.

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