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As is well known, most of the core standards of the International System of Measurement Units have been "dematerialized," with one exception: mass. The kilogram standard, which is a small platinum/iridium alloy cylinder (90% / 10%) 39 mm high and 39 mm in diameter, was manufactured in 1878 and has been housed since the start at the International Bureau of Weights and Measures (BIPM). To avoid any contamination (or dust deposit) that would alter its mass and undermine its status as a standard (which would be harmful for a stallion...), it almost never takes to the air. Six copies have been created to meet the need for referencing of the calibration chain. Since 1880, the stallion has been taken out of its shelter only three times to verify that the copies are in line with the original. These outputs revealed that the mass of copies drifted (compared to the benchmark) on average 0.5 g per year. In a century, this drift corresponds to the mass of a grain of sand 0.4 mm in diameter. This is little but too much because this variation can compromise the stability of the international system of IS units, especially since other IS units are dependent on the kilogram: the mole, ampere and candela are defiantly attached to the mass unit.
Regardless of these aspects, there is also the risk associated with the uniqueness of the mass standard. So far, he has withstood two world wars, but do you ever know...
All these reasons have led the BIPM to seek to achieve a new mass standard (by 2015), more practical and based on the major phenomena of physics, like the other stallions. Several approaches are underway, most not most in particular those based on a Watt scale. The principle is to balance the weight of a mass by the force of Laplace generated on a coil by a magnetic induction flow. The coil is moved vertically, in an application to accurately characterize the parameters of the magnetic circuit by an induced voltage measurement. The resistance and electrical voltage are measured very precisely through quantum standards involving the Planck constant. The mass is thus connected to the existing standards as well as to a constant of physics.
In Switzerland, the Metas (the equivalent of our metrology office) is at the heart of this project, with CERN supplying the magnetic circuit, Mettler-Toledo, which is building a mass comparator, and theEcole Polytechnique Fédérale de Lausanne, whose objective is to achieve the vertical translation mechanism of the coil.
The levels of uncertainty sought are very high, so the choice of materials and the mechanisms of operation of this machine must be controlled in every detail. Among the critical elements of the machine, there is a charging cell that must achieve a resolution of 1 g for a mass up to 2 kg. Mettler Toledo claims to have smashed this lens and reached a resolution of 0.3g...
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