Grave (unit)

A replica of the prototype of the kilogram at the Cité des Sciences et de l'Industrie, Paris, France

The grave was the original name of the kilogram, in an early version of the metric system between 1793 and 1795.

History

The modern kilogram has its origins in the pre-French Revolution days of France. 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.[1] 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 meter.[2][3] Initially a provisional value was used, based on the old meridian measurement by Lacaille (1740).[4]

In 1793 the commission defined the unit of mass as a cubic decimeter of distilled water at 0°C, and gave it the name grave.[5] 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.[6][7] The mass of a unit volume of water at 0°C was accurately determined by Lavoisier and Haüy (18841 grains per cubic provisional decimeter). 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.[8] The new gram was equal to the old gravet. Four new prefixes were added to cover the same range of units as in 1793 (milligram, centigram, decigram, gram, decagram, hectogram, kilogram, myriagram).[9][10] 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 meter was 0.03% smaller than the provisional one. Hence the final kilogram, being the mass of one cubic decimeter 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.[11][12] In 1799 a platinum cylinder was made, and stored in the Archives of Paris, that served as the prototype of the final kilogram. It was called the Kilogramme des Archives, and this standard stood for the next ninety years.

See also

References

  1. ↑ May 1790 Proposal Talleyrand
  2. ↑ Rapport March 17, 1791 Ten millionth of meridian
  3. ↑ Rapport july 11, 1792 Introduction of the names metre, deci-centi-milli
  4. ↑ Decree of August 1, 1793
  5. ↑ Rapport january 19, 1793 Name grave and dm^3
  6. ↑ Instructions abrégée sur les mesures déduites de la grandeur de la terre et sur les calculs relatifs à leur division décimale, 1793: gravet, bar
  7. ↑ The obsolete unit bar (1000 kg, until 1795) is not to be confused with the newer unit of pressure that was introduced in 1909 (100 kPa)
  8. ↑ The 7 April 1795 Act: gramme
  9. ↑ Nouvelle instruction sur les poids et mesures, et sur le calcul décimal, adoptée par l'Agence temporaire des poids et mesures. Claude Antoine Prieur, 1795
  10. ↑ Decree relating to the weights and measurements
  11. ↑ L’Histoire Du Mètre, La Détermination De L’Unité De Poids
  12. ↑ History of the kilogram
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