Expérience française de balance du watt : conception d’un fléau de balance monobloc et étude de son comportement mécanique

¹ Conservatoire national des arts et métiers, Laboratoire commun de métrologie, 61 rue du Landy 93210 La Plaine Saint-Denis, France
² Laboratoire national de métrologie et d’essais, Laboratoire commun de métrologie, 29 avenue Roger Hennequin 78197 Trappes cedex, France

^a patrick.pinot@cnam.fr

Abstract

Since 2005, the French watt balance experiment, installed at the LNE, has been using a balance beam with removable flexure hinges designed at the CNAM. With this balance, a first determination of the Planck constant in air was performed in 2014. This balance beam presents some drawbacks. First, it is difficult to mount and adjust the hinges. Secondly, the use of clamped flexure strips used as pivots limits the beam sensitivity and measurement repeatability, moreover the central flexure strip twists under the effect of unwanted horizontal forces at the ends of the beam, which leads to a potential measurement error. For these reasons, a new, single-piece balance beam has been designed. It is not only easier to mount and adjust than its predecessor, but also free of unwanted stresses such as shear force on the hinges. This ensures that, once the beam is loaded, the rotation axes are by construction parallel and coplanar. In addition, the use of a double central hinge strongly reduces the torsion effect.