| In 1875 a number of countries signed the meter convention, soon to be followed by more. At that time the meter was defined as a forty millionth part of the length of a great circle through the meridian of Paris. A platinum specimen was kept in the archives of the "Bureau International des Poids et Mesures" in Sèvres. A new and defining platinum-iridium standard was made in 1889. In 1967 the meter was redefined as 1650763.73 times the wavelength of the orange-red light emitted by the 2p10-5d5 transmission of 86Kr (a natural Krypton isotope), as witnessed by the stamp below.
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In 1816 just after the French occupation Holland, at that time united with present day Belgium, accepted the "mètres des Archives" as legal standard.
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In 1736 a French expedition under Pierre de Maupertuis was sent to Lapland in the north of Finland to measure the earth's curvature along a meridian near the polar circle. Among the team were the Swedish mathematician and astronomer Anders Celsius and the young Alexis Clairaut [of the differential equations of type y = x*p + g(p), where p = y'] elected to the French Academy in 1731, below the legal age. |
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In 1790 a committee, including Lagrange, Laplace and Monge, was set up to standardise the weights and measures in France. It took them five years. [A first recommendation for the unit of length as that of a pendulum which at sealevel at a Northern latitude of 45o beats in seconds, was to be abandoned soon].
| In 1742 Celsius devised his temperature scale. A year later saw the appearance of Clairaut's classic book on the shape of rotating bodies under the influence of gravity and centrifugal force. Maupertuis became president of the Berlin Academy. [There he proposed the important "Principle of Least Action", later to be expanded by Hamilton].
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In 1983 the velocity of light in vacuum was defined by c0=299 792 458 m/s (exact) thereby providing a new, indirect, definition of the meter. The accuracy of its numerical value now depends only on the value of the unit of time, the second, as furnished by present day highly accurate atomic clocks. On the stamp we also see the symbols for the other basic quantities of the Système International d'Unités (SI):the units of
- mass,the kilogram (Kg)
- electric current, the ampère (A)
- thermodynamic temperature, the kelvin (K)
- luminous intensity, the candela (cd)
- amount of substance, the mole (mol)
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Another expedition, with Charles de La Condamine,[on the stamp below] travelled to Peru and Ecuador to measure an arc of a meridian near the equator. The work of both expeditions [contrary to Cassini's surveys] confirmed Newton's prediction, that the earth be flattened at the poles. It also showed the urgent need for a system of internationally accepted units of measure.
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Note that the Celsius temperature of the ice-point (0 oC) is equal to 273.15 K, while the triple point of water [where all three phases can exist in equilibrium] is 273.16 K at about 611.2 Pa (or .006 atm). Water boils at about 99.974 oC.
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The Ecuador team, which soon fell apart, further contained the mathematician Pierre Bouguer and the astronomer Louis Godin. [La Condamine finished his voyage by rafting down the Amazone river using astronomical observations to draw a map and bringing back many natural history specimens, such as caoutchouc (natural rubber) and curare, which he gave to Buffon for his Histoire Naturelle].
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