metrology

284d

#OTD in 1889.

The General Conference on Weights and Measures (CGPM) defines the length of a metre.

The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus's publication of De revolutionibus orbium coelestium in 1543. The Mètre des Archives & its copies were replaced from 1889 at the initiative of the International Geodetic Association by 30 platinum-iridium bars kept across the globe.

https://en.wikipedia.org/wiki/History_of_the_metre#

#science #metrology

Closeup of National Prototype Meter Bar No. 27, made in 1889 by the International Bureau of Weights and Measures (BIPM) and given to the United States, which served as the standard for defining all units of length in the US from 1893 to 1960. In 1960 the SI changed the standard of length to define the meter by the wavelength of light of a spectral line of krypton 86. This bar is now in the collection of the NIST Museum, Gaithersburg, Maryland, USA. After the Treaty of the Meter had been signed in 1875, the BIPM in Sevres, France made 30 prototype standard bars of 90% platinum–10% iridium alloy. One of the bars was selected as the International Meter. 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 at the temperature of freezing water defining the meter. After selecting the bar to be used as the International Prototype Meter, the other bars were calibrated relative to it and given to nations to serve as their national standards. The United States received National Prototype Meters No. 27 and No. 21 in 1890.

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