International yard and pound

On July 1, 1959, six nations—Australia, Canada, New Zealand, South Africa, the United Kingdom, and the United States—signed an agreement that standardized the yard and the avoirdupois pound. Under this pact, the yard was set at exactly 0.9144 meters, and the pound at exactly 0.45359237 kilograms.

The push for a common standard began after a fire destroyed the British Houses of Parliament in October 1834, along with the official prototypes for imperial length and mass. New standards were crafted, including a yardstick in 1855, and two copies were sent to the United States, which adopted them as its national standard. In 1866, the U.S. Congress legalized metric use in commerce and included conversion tables linking customary and metric units. The Mendenhall Order of 1893 then shifted U.S. fundamental standards from English prototypes to metric ones, partly because the 1855 yardstick was shrinking, and partly because the U.S. had received meter prototypes as a signatory to the 1875 Treaty of the Metre.

In the United Kingdom, the Weights and Measures Act of 1897 legalized the metric system, and an 1898 Order in Council defined the meter and kilogram in terms of the yard and pound—effectively reversing the definitions and making the imperial yard equal to 36/39.370113 meters. Around the same time, Albert Michelson’s interferometry experiments (starting in the 1890s) showed that light waves could measure length. By 1908, two teams, including Michelson’s, had defined the international prototype meter using light waves, and in 1927 the International Bureau of Weights and Measures provisionally adopted that definition as a supplement.

In 1930, the British Standards Institution set the inch at exactly 25.4 millimeters, based on the 1927 light-wave meter. The American Standards Association followed in 1933, and by 1935, sixteen countries had adopted this “industrial inch.” In 1946, the British Commonwealth Scientific Conference recommended that Commonwealth members define the inch as 25.4 mm and the 36-inch yard as 0.9144 meters. Canada’s Standards Association accepted this in 1951.

In October 1958, the International Committee on Weights and Measures proposed defining the meter by atomic means (the orange line of krypton-86). To ensure identical values for the yard and pound in precise work, the six English-speaking nations agreed on the international yard and pound. The new yard was about two millionths of a meter longer than the old imperial yard, and the new pound was about six ten-millionths of a kilogram lighter than the imperial pound. The U.S. National Bureau of Standards adopted these values effective July 1, 1959. Australia made them law through Statutory Rule No. 142 of 1961, effective January 1, 1964. The UK’s Weights and Measures Act of 1963 also adopted them, effective the same date.