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The French Academy of Sciences and the systems of units: A long history!

2019; Elsevier BV; Volume: 20; Issue: 1-2 Linguagem: Inglês

10.1016/j.crhy.2019.05.017

1878-1535

Christophe Salomon, Christian Bordé, Pierre Fayet,

History and advancements in chemistry

May 2019: nothing will be as before in the realm of the units with which we measure everything!The 26th General Conference of Weights and Measures (CGPM), chaired by Sébastien Candel, president of the French Academy of Sciences, was held in Versailles in November 2018.The conference redefined four units of measurement, the kilogram, the ampere, the kelvin and the mole, from four fundamental constants of physics: the Planck constant h of quantum mechanics, the elementary charge e, the Boltzmann constant k B , and the Avogadro constant N A .This brings an end to the use of material artefacts such as the international prototype of the kilogram, carefully preserved since 1889 at the "pavillon de Breteuil" housing the International Bureau of Weights and Measures in Sèvres, near Paris.The 59 Member States of the Metre Convention unanimously approved this major change, the most important since its creation in 1875.The kilogram, unit of mass, is now defined from the second, unit of time, and the metre, unit of distance, thanks to quantum physics, by fixing the numerical value of Planck's constant h, the elementary action in quantum mechanics.But, in its 353-year history, which role has the "Académie" played in the creation and evolution of this system of units?Since the creation of the "Académie des sciences" in 1666 by King Louis XIV, the royal administration has been interested in measurement systems within the kingdom, for lengths, weights, volumes, and materials.It has solicited many times the scholars of the Academy to help in its desire to simplify their use and facilitate trade.There was a lot of disorder in the units, including the elbow, the foot, the ell, the yardstick, and many others.The historian Ken Adler has estimated that there were more than 250,000 units in use at that time, which often differed from one city to another across the kingdom, and of course from one country to another.The units of length and weight were the most important, simply to measure the lengths of pieces of fabric, or to map the kingdom of France or exchange material quantities.The French Revolution, with its ideals of equality and universality at the end of the 18th century, challenged academics to establish a system of units accessible "at all times and for all peoples".Thus, a commission of the Academy where the illustrious Borda, Lagrange, Laplace, Monge, and Condorcet were sitting recommended in 1791 to define the metre as 1/10 000 000 of the quarter of the terrestrial meridian.It was necessary for them to report all measures "to a unit of length taken in nature, [...] the only way to exclude any arbitrariness from the system of measurements".It had to be a universal quantity and accessible to all.The National Assembly voted in 1791 a large budget for a major expedition to measure the length of a meridian fraction between Dunkerque and Barcelona, in view of a practical realisation of this definition of the metre.Two scientists, Jean-Baptiste Delambre and Pierre Méchain, were sent to the field in 1792 to carry out these measurements by triangulation, which lasted for more than seven years.A prototype of the metre by Lenoir and a kilogram of platinum became in December 1799 the final standards of length and mass throughout the French Republic.The decimal metric system then prevailed.The next major step was the Metre Convention in 1875, with the creation of the International Bureau of Weights and Measures, one of the very first international bodies.The vision of its creators was to facilitate scientific and commercial developments between nations through a better international coordination on the unit system.The reader will find in the article by Suzanne Débarbat and Terry Quinn an exciting historical description of all these steps that led to the development of modern metrology and the recent redefinition of four basic units.This volume continues with two contributions that place the new unit system within a broader context.Christian Bordé shows how matter-wave interferometry allows for a unified interpretation of basic units in the context of a five-dimensional geometry involving proper time.This geometry is determined by fixing three constants, the speed of light in vacuum c, the elementary electric charge e, and the mass difference m Cs of a caesium atom between its fundamental level and the excited one serving in the definition of the second.The description of its physical content then requires adopting the Planck constant h and the Boltzmann constant k B as the elementary quantities of action and entropy.