on 03-Jun-2015 (Wed)

Cet étalon est un cylindre dont le diamètre est égal à sa hauteur (39 mm, ce qui minimise sa surface ex- térieure). Il est fait de platine allié à de l’iridium pour 10 % de la masse. Depuis plus d’un siècle, il est conservé dans l’air sous une triple cloche, dans un caveau du Bu- reau international des poids et mesures (BIPM) à Sèvres (France) ; par définition sa masse vaut un kilogramme exactement

Measurements made with watt balances are paving the way to a new definition of the kilogram based upon a fixed value of the Planck constant. The Bureau International des Poids et Measures (BIPM) are developing a simultaneous moving and weighing variant of the watt balance technique, which requires a superconducting coil to reach its ultimate uncertainty. The BIPM technique has the advantage that it eliminates the requirement for extreme stability in the magnetic field employed by conventional watt balances, but requires cryogenic operation of parts of the balance to achieve a low uncertainty.

The two most important forces acting on the upper-level air in the ‘free atmosphere’, which are above the frictional effects of the Earth’s boundary layer, are the pressure gradient force and the Coriolis force