National Standard of Mass, Kilogram

- Image ID: HRP330
Science History Images / Alamy Stock Photo
Image ID: HRP330
After the Treaty of the Meter was signed in 1875, the International Bureau of Weights and Measures (BIPM) in Sevres, France, procured 43 prototype kilograms of platinum-iridium in the form of right circular cylinders of equal diameter and height (approximately 39mm) with slightly rounded edges. One of the set, having essentially the same mass as the Kilogramme des Archives, was selected as the International Prototype Kilogram, and is now maintained at BIPM. National Prototype Kilograms No. 20 and No. 4 were received by the United States from BIPM in 1890. Benjamin A. Gould, US Delegate to the International Conference of Weights and Measures, supervised the packing of Prototype Kilogram K20 on October 27, 1889 for its transport to the US in care of George Davidson of the US Coast and Geodetic Survey. On January 2, 1890, US President Benjamin Harrison broke the seals on the case and gave the standard to Thomas Mendenhall, US Superintendent of Weights and Measures. Its mass relative to the International Prototype Kilogram was reported as 0.999 999 961 kilograms. Since it was the first of the two prototype kilograms to be received, K20 is sometimes called the national standard of mass. Of the seven base units in Le Systeme International, the kilogram is the only one currently defined by an artifact. After the Mendenhall Order in 1893, Kilogram No. 20 became the primary national standard for all mass measurements.