For 130 years, the kilogram was a single metal cylinder
From 1889 until 2019, the world's kilogram was a specific lump of platinum and iridium: whatever it weighed was, by definition, exactly one kilogram. Now the unit rests on a constant of nature, the Planck constant, yet it still matches the revolutionary French idea of a litre of water within 0.003 percent.
The story begins in revolutionary France. In 1793 a unit called the grave was set as the mass of a litre of water, measured at 18,841 grains. In 1795 a provisional gram was set equal to one cubic centimetre of water at 0 degrees Celsius, and French law replaced the grave and gravet with the kilogramme and gramme. In 1799 a platinum prototype, the Kilogramme des Archives, was made to match a litre of water at about 4 degrees, where water is densest.
International agreement followed. The Metre Convention of 1875 led to the manufacture of the International Prototype of the Kilogram in 1879 and its adoption in 1889. The platinum-iridium cylinder served for 130 years. On 16 November 2018 the General Conference on Weights and Measures approved a new definition, in force from 2019.
Today the kilogram depends on three fixed constants. A particular transition frequency of the caesium-133 atom defines the second; the speed of light, combined with the second, defines the metre; and the Planck constant, set at exactly 6.62607015 times ten to the minus 34 joule-seconds, then pins down mass. A suitably equipped laboratory can realise the unit with an instrument such as a Kibble balance, with no special artefact required.
The name has its own quirks. The kilogram is the only SI base unit with a prefix built into its name, from the Greek for a thousand plus a Late Latin word for a small weight. English borrowed the French spelling in 1795, while the United States adopted kilogram, now also the more common British form. The shortening kilo arrived in the 19th century. The US Congress allowed kilo as an alternative when it legalised the metric system in 1866, then revoked that status in 1990, and Canadian government style rules exclude it from technical writing.
Source: Kilogram
