The modern kilogram has its origins in the Age of Enlightenment and the French Revolution. In 1790 an influential proposal by Talleyrand called for a new system of units, including a unit of length derived from an invariable length in nature, and a unit of mass (then called weight) equal to the mass of a unit volume of water. In 1791, the Commission of Weights and Measures, appointed by the French Academy of Sciences, chose one ten-millionth of the quarter meridian as the unit of length, and named it metre. Initially a provisional value was used, based on the meridian measurement made in 1740 by Lacaille.
In 1793 the commission defined the unit of mass as a cubic decimetre of distilled water at 0 °C, and gave it the name grave. Two supplemental unit names, gravet (0.001 grave), and bar (1000 grave), were added to cover the same range as the old units, resulting in the following decimal series of units: milligravet, centigravet, decigravet, gravet, centigrave, decigrave, grave, centibar, decibar, bar. The mass of a unit volume of water at 0 °C was determined by Lavoisier and Haüy to be 18841 grains per cubic provisional decimetre. A prototype of the grave was made in brass.
After the grave
In 1795 a new law replaced the three names gravet, grave and bar by a single generic unit name: the gram. The new gram was equal to the old gravet. Four new prefixes (deca, hecto, kilo, and myria) were added to the metric system to cover almost the same range of units as in 1793 (milligram, centigram, decigram, gram, decagram, hectogram, kilogram, myriagram). The brass prototype of the grave was renamed to provisional kilogram.
In 1799 the provisional units were replaced by the final ones. Delambre and Méchain had completed their new measurement of the meridian, and the final metre was 0.03% smaller than the provisional one. Hence the final kilogram, being the mass of one cubic decimetre of water, was 0.09% lighter than the provisional one. In addition, the temperature specification of the water was changed from 0 °C to 4 °C, the point where the density of water is maximal. This change of temperature added 0.01% to the final kilogram. In 1799 a platinum cylinder was made that served as the prototype of the final kilogram. It was called the Kilogramme des Archives as it was stored in the Archives Nationales in Paris. This standard stood for the next ninety years, until being replaced in 1889 by the platinum–iridium International Prototype Kilogram (IPK).
Kilogramme des Archives
The Arago kilogram, an exact copy of the "Kilogramme des Archives" commissioned in 1821 by the US under supervision of French physicist François Arago that served as the US's first kilogram standard of mass until 1889, when the US converted to primary metric standards and received its current kilogram prototypes, K4 and K20.
On April 7, 1795, the gram was decreed in France to be "the absolute weight of a volume of pure water equal to the cube of the hundredth part of the metre, and at the temperature of melting ice".
Since trade and commerce typically involve items significantly more massive than one gram, and since a mass standard made of water would be inconvenient and unstable, the regulation of commerce necessitated the manufacture of a practical realisation of the water-based definition of mass. Accordingly, a provisional mass standard was made as a single-piece, metallic artefact one thousand times as massive as the gram—the kilogram.
At the same time, work was commissioned to precisely determine the mass of a cubic decimetre (one litre) of water.[Note 1] Although the decreed definition of the kilogram specified water at 0°C—its highly stable temperature point—the French chemist Louis Lefèvre-Gineau and the Italian naturalist Giovanni Fabbroni chose to redefine the standard in 1799 to water's most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4°C.[Note 2]
They concluded that one cubic decimetre of water at its maximum density was equal to 99.9265% of the target mass of the provisional kilogram standard made four years earlier.[Note 3] That same year, 1799, an all-platinum kilogram prototype was fabricated with the objective that it would equal, as close as was scientifically feasible for the day, the mass of one cubic decimetre of water at 4°C. The prototype was presented to the Archives of the Republic in June and on December 10, 1799, the prototype was formally ratified as the Kilogramme des Archives (Kilogram of the Archives) and the kilogram was defined as being equal to its mass. This standard stood for the next 90 years.
^Zupko, Ronald Edward (1990). Revolution in Measurement: Western European Weights and Measures Since the Age of Science. DIANE Publishing.
^Armoire de fer, (in English: Iron chest), vault built in 1790 and used to store kilogram and metre prototypes
The same decree also defined the litre as follows: "Liter: the measure of volume, both for liquid and solids, for which the displacement would be that of a cube [with sides measuring] one-tenth of a metre." Original text: "Litre, la mesure de capacité, tant pour les liquides que pour les matières sèches, dont la contenance sera celle du cube de la dixièrne partie du mètre."
^Modern measurements show the temperature at which water reaches maximum density is 3.984°C. However, the scientists at the close of the 18th century concluded that the temperature was 4°C.
^The provisional kilogram standard had been fabricated in accordance with a single, inaccurate measurement of the density of water made earlier by Antoine Lavoisier and René Just Haüy, which showed that one cubic decimetre of distilled water at 0°C had a mass of 18,841 grains in France's soon-to-be-obsoleted poids de marc system. The newer, highly accurate measurements by Lefèvre‑Gineau and Fabbroni concluded that the mass of a cubic decimetre of water at the new temperature of 4°C—a condition at which water is denser—was actually less massive, at 18,827.15 grains, than the earlier inaccurate value assumed for 0°C water. France's metric system had been championed by Charles Maurice de Talleyrand‑Périgord. On March 30, 1791, four days after Talleyrand forwarded a specific proposal on how to proceed with the project, the French government ordered a committee known as the Academy to commence work on accurately determining the magnitude of the base units of the new metric system. The Academy divided the task among five commissions. The commission charged with determining the mass of a cubic decimetre of water originally comprised Lavoisier and Haüy but their work was finished by Louis Lefèvre‑Gineau and Giovanni Fabbroni. Neither Lavoisier nor Haüy can be blamed for participating in an initial—and inaccurate—measurement and for leaving the final work to Lefèvre‑Gineau and Fabbroni to finish in 1799. As a member of the Ferme générale, Lavoisier was also one of France's 28 tax collectors. He was consequently convicted of treason during the waning days of the Reign of Terror period of the French Revolution and beheaded on May 8, 1794. Lavoisier's partner, Haüy, was also thrown into prison and was himself at risk of going to the guillotine but his life was spared after a renowned French naturalist interceded.