Four measurement units, including one kilogram, will be redefined at the Weights and Measures Conference (GFCM) on November 16 in Versailles.
It is a small revolution in the field of metrology or science of measurement. One of the most significant changes in this discipline since 1795, and the institution of the metric system during the French Revolution. On November 16th, the 26th General Conference of Weights and Measures (GFCM), meeting in Versailles
, will redefine four of the seven basic units of the International System (SI), from which other quantities are calculated around the world. Among them: the kilogram, whose value will cease, on May 20, 2019, to be determined by comparison with a material artifact. In this case, the "big K", this piece of platinum and iridium enclosed in a pavilion Sèvres, whose mass served since 1889 to calibrate all the kilos of the planet!
Standardize measures
The disappearance of the kilo-étalon is a major step in a long process whose origins go back to the end of the Ancien Régime. "At the time, 700 units of length, mass or volume are used in the French territory, " said Jean-Philippe Uzan, director of CNRS research at the Institute of Astrophysics in Paris. And it is for the revolutionaries to put an end to this anomaly harmful to trade, by adopting for the calculation of lengths and masses of common references. These are the famous meter-stallion and kilo-stallion that the International Bureau of Weights and Measures (BIPM) has been keeping since 1889 in its premises in Sèvres.
Seven units of measurement
Since then, other basic units such as the second and the ampere or the kelvin and the mole have been adopted, bringing their number to seven in the international system (SI), which eventually succeeded the metric system. Their definitions have undergone many transformations as technical and scientific discoveries have progressed. Resulting, in 1960, in the disappearance of the meter-etalon and, in 1983, in a determination of the unit of length (the meter) from the speed of light whose value is then fixed, by convention, at 299 792,458 km / s.
A piece of metal that changes
But nothing had been done for the stallion. "This is only a piece of metal. And like any object, it is likely to undergo modifications " , explains Marc Himbert, director of the Joint Metrology Laboratory of LNE-CNAM. In fact, comparing in 1946, in 1989 and in 2014, the "Big K" to its copies, the physicists have each time noticed a difference. An unexplained drift of barely 38 micrograms in a century, but sufficient to alert the GFCM, aware of the disadvantages of this process, and requiring, to find the value of the kilogram, access to its prototype.
Imperceptible outside the labs
The 2018 reform aims to put an end to this archaism. In the same way that the meter was, in 1983, linked to the fixed speed of light, it aims to redefine the units of mass (kilogram), electrical current (amperes), temperature (kelvin). ) and quantity of matter (the mole), according to fundamental constants of nature, such as, for the kilogram, the "Planck constant", used in the field of quantum physics. But on one condition: these changes must be imperceptible outside specialized laboratories. Disappearance or not the kilo-standard, the amount of food available per kilo in the markets, it will not change!