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National Prototype Meter No. 27
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National Prototype Meter No. 27

 ca. 1875-1889  NIST Museum Collection
After the Treaty of the Meter had been signed in 1875, the International Bureau of Weights and Measures (BIPM) in Sevres, France made 30 prototype line standards of platinum-iridium. The bars had a modified X cross section named for the French scientist, Henri Tresca, who proposed it. The Tresca section was designed to provide maximum rigidity. Small elliptical areas on the upper surface of the central rib at each end of the bars were highly polished, and three lines, nominally 0.5 mm apart, were ruled on these surfaces, the distance between the middle lines of each group defining the standard length. One of the bars was selected as the International Meter. The United States received National Prototype Meters No. 27 and No. 21 in 1890. When the Mendenhall Order in 1893 declared the meter to be the fundamental length standard, No. 27 became the primary national standard for all length measurements. It remained so until 1960. The relationship between No. 27 and the International Meter was certified to be 1 m-1.6 µm + 8.657 µm·T + 0.001 µm·T2 ± 0.2 µm with T in degrees centigrade. Intercomparison between the International Meter and No. 27 yielded a probable error of ±0.04 µm. The probable uncertainty of the length of No. 27 at temperatures between 20°C and 25°C was estimated by BIPM to lie between ±0.1 µm and ±0.2 µm.

Diagram of Tresca Cross Section of Meter Bar