page_title: History of the metre
section_title: Mètre des Archives
hierarchical_section_title: History of the metre / Mètre des Archives
caption_attribution_description: English: Copy of the first metre standard, sealed in the foundation of a building, 36 rue de Vaugirard, Paris
context_page_description: The history of the metre starts with the scientific revolution that began with Nicolaus Copernicus's work in 1543. Increasingly accurate measurements were required, and scientists looked for measures that were universal and could be based on natural phenomena rather than royal decree or physical prototypes. Rather than the various complex systems of subdivision in use, they also preferred a decimal system to ease their calculations.
  With the French Revolution came a desire to replace many features of the Ancien Régime, including the traditional units of measure. As a base unit of length, many scientists had favoured the seconds pendulum one century earlier, but this was rejected as it had been discovered that it varied from place to place with local gravity and that it could complement meridian arc measurements in determining the figure of the Earth. A new unit of length, the metre was introduced – defined as one ten-millionth of the shortest distance from the North Pole to the equator passing through Paris, assuming an Earth's flattening of 1/334.
  For practical purposes however, the standard metre was made available in the form of a platinum bar held in Paris.
context_section_description: While Méchain and Delambre were completing their survey, the commission had ordered a series of platinum bars to be made based on the provisional metre. When the final result was known, the bar whose length was closest to the meridional definition of the metre was selected and placed in the National Archives on 22 June 1799 (4 messidor An VII in the Republican calendar) as a permanent record of the result. This standard metre bar became known as the mètre des Archives.
  The metric system, that is the system of units based on the metre, was officially adopted in France on 10 December 1799 (19 frimaire An VIII) and became the sole legal system of weights and measures from 1801. After the restoration of the Empire, in 1812, the old names for units of length were revived but the units redefined in terms of the metre: this system was known as mesures usuelles, and lasted until 1840, when the decimal metric system was again made the sole legal measure. In the meantime, the Netherlands had adopted the metric system from 1816. The first of several countries to follow the French lead, the Helvetic Republic had adopted the metre shortly before its collapse in 1803.
  With the extension of the survey it became apparent that Méchain and Delambre's result (443.296 lignes) was slightly too short for the meridional definition of the metre. While the Ordnance Survey extended the British survey northward to the Shetland, Arago and Biot extended the survey southward in Spain to the island of Formentera in the western Mediterranean Sea (1806–1809), and found that one ten-millionth of the Earth's quadrant should be 443.31 lignes: later work increased the value to 443.39 lignes. Some thought that the base of the metric system could be attacked by pointing out some errors that crept into the measurement of the two French scientists. Méchain had even noticed an inaccuracy he did not dare to admit. Louis Puissant declared in 1836 in front of the French Academy of Sciences that Delambre and Méchain had made an error in the measurement of the French meridian arc. That is why from 1861 to 1866, Antoine Yvon Villarceau checked the geodesic opérations in eight points of the meridian arc. Some of the errors in the operations of Delambre and Méchain were corrected. In 1866, at the conference of the International Association of Geodesy in Neuchâtel Carlos Ibáñez e Ibáñez de Ibero announced Spain's contribution to the measurement of the French meridian arc. In 1870, François Perrier was in charge of resuming the triangulation between Dunkirk and Barcelona. This new survey of the Paris meridian arc, named West Europe-Africa Meridian-arc by Alexander Ross Clarke, was undertaken in France and in Algeria under the direction of François Perrier from 1870 to his death in 1888. Jean-Antonin-Léon Bassot completed the task in 1896. According to the calculations made at the central bureau of the international association on the great meridian arc extending from the Shetland Islands, through Great Britain, France and Spain to El Aghuat in Algeria, the Earth equatorial radius was 6377935 metres, the ellipticity being assumed as 1/299.15. The modern value, for the WGS 84 reference spheroid with an Earth's flattening of 1/298.257223563, is 1.00019657 × 10⁷ m for the distance from the North pole to the Equator.
  A more accurate determination of the Figure of the Earth resulted also from the measurement of the Struve Geodetic Arc (1816–1855) and would have given another value for the definition of this standard of length. This did not invalidate the metre but highlighted that progresses in science would allow better measurement of Earth's size and shape. The mètre des Archives remained the legal and practical standard for the metre in France, even once it was known that it did not exactly correspond to the meridional definition. When it was decided (in 1867) to create a new international standard metre, the length was taken to be that of the mètre des Archives "in the state in which it shall be found".
  One of the significant international use of the meridional definition o
caption: A copy of the "provisional" metre installed 1796–1797, located in the wall of a building, 36 rue de Vaugirard, Paris. These metres were based on the "provisional" metre, because the expedition to re-determine the metre wasn't completed until 1798.[39]
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