Kilogram No 20

Kilogram No 20, the national standard of mass, a cylinder measuring 39mm in diameter and 39mm high, with slightly rounded edges, made of an alloy of 90 percent platinum and 10 percent iridium, shown with its double bell jars removed, alongside Kilogram No 4, a duplicate of No 20, placed beneath its bell jars, at the US National Institute of Standards and Technology in Gaithersburg, Maryland, circa 1955. The chamois-faced lifting forceps with which the standards are handled are seen before the outer bell jar of No 20. (Photo by FPG/Archive Photos/Hulton Archive/Getty Images)