The reference standard which is universal to weigh the mass will be replaced this week by a far more abstract, outcome of quantum physics. Other unit of measure such as temperature (kelvin), electrical current (ampere) or the amount of substance (mole) will also change.

This is the result of the decisions taken at the last general Conference of weights and measures, held on November 18. She had to find a replacement to the kilogram standard, a cylinder composed of 90% platinum and 10% iridium – the two materials chosen, originally, for their sustainability in time.

The long history of the kilogram standard dates back to the French Revolution. “By the end of this period, the men wear a ideal of universality, in particular, a system of units recognized by everyone,” says Thomas Grenon, director general of Laboratoire national de métrologie et d’essais (LNE). The first of these was the distance, with the establishment of the metric system. For mass, the gram, carried at the expense of an ounce or multiple definitions of the book. Until 1865, when the kilogram is needed in turn. The scientists then manufacture the famous cylinder-iridium, known as the “international prototype kilogram” (PIK), which is based in peace under a triple bell jar in a vault at the international Bureau of weights and measures (BIPM), the international body, established in 1875, located on the hill of Saint-Cloud (Hauts-de-Seine). “From this artefact reference, the scientists decide with caution of flow, by taking point of departure in the same sample, the other six standards identical entrusted to the six great powers of that time,” says Maguelonne Chambon, director of technical research of the ESA. France has, therefore, his jealously kept in a cabinet shielded.” In contrast to THE reference clodoaldienne, it can be shown.

The standard 1 kilogram of platinum, the Currency of Paris.


Charlotte and combination required

Claustrophobic refrain. At the headquarters of the ESA, in the Fifteenth arrondissement of Paris, you must go down to the first basement of the Art deco building, and then the second, then the third, passing through a maze of dark corridors. “I think you have put your wetsuit on backwards, but that should do the trick as long as you wear your overboots feet and your charlotte on the head”, smiled Tanguy Madec, an expert in metrology. To enter the interior of the laboratories in Mass, it is necessary to dress up as if we visited the clean room of a satellite manufacturer. Conditions drastic to avoid any dust outside are identical, as is the monitoring of temperature, humidity or air.

Here, the machines are scales – two, three or four trays and a curse -some have the air of stoves to fry, other, barbeque ; many look like nothing. “Our mass comparators are used to measure objects from 100 micrograms to 5 tonnes,” says Tanguy Madec. It is from these devices that the manufacturers and the authorities are calibrating their scales and weights with extreme accuracy.” The tour continues to the famous wardrobe armoured as the scientist opens religiously, thanks to a code, before presenting the French copy of the kilogram, called “K35”. “Ban to go further, and especially of touch,” warns Maguelonne Chambon, a good guardian of the temple. In the past, the K35 and his little brothers were three times compared to the master standard PIK : in the 1940s, in 1990 and in 2014. Terrible surprise : the metrologists have found a drift inexplicable to some 50 micrograms.

Study of the weight stability of mass standards.


A redefinition based on quantum physics

“An artifact physical wears out, loses atoms, resumes Maguelonne Chambon. Hence the idea to look for a standard more universal related to a fundamental constant of nature.” Such a redefinition was made, some years ago, with the meter that he, too, was based on a standard metal before being calculated with the speed of light. It will be the same for the kilogram, by referring to the Planck constant, a parameter of the quantum physics. To do this, the scientists have developed a “watt balance” is extra-sensitive. “It is to convert a mass into a electrical power”, explains Matthew Thomas, head of this great equipment on the site of Trappes (Yvelines) of the ESA. The device, which has requested a fifteen years of development, looks like a big bell, two meters high, contains a classical beam balance with two trays and works with an electric coil creating a magnetic field.

It took fifteen years at the national Laboratory of metrology and testing to develop its watt balance, here, in its premises in Trappes (Yvelines).


“There are four or five copies around the world, continues the researcher, particularly in the United States and Canada.” Each team has provided results that are convergent, last summer, with a measurement of the Planck constant to seven digits after the decimal point. “Sufficient accuracy to redefine the official kilogram, by setting a numerical value definitive, such as to accept the BIPM in November,” says Thomas Grenon. Greater precision, with the key, will especially be of immenses progress in the worlds of the infinitely light – new technology (create materials), medicine (manufacture of biomarkers) or the environment (measuring nanoparticles). But it should not change much on the scales of the market stalls.


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