Fabrication of the Magnetic Circuit for the Bipm Watt Balance

INTRODUCTION Since 1889 the International Prototype of the Kilogram, kept at the BIPM, had served as the definition of the unit of mass in the International System of Units (SI). It is the last material artefact to define a base unit of the SI, and it influences several other base units. This situation is no longer adequate in a time of ever increasing measurement precision. It is therefore planned to redefine the unit of mass by fixing the numerical value of the Planck constant [1]. The kilogram redefinition requires a highly accurate determination of the Planck constant in the present SI system, with a relative uncertainty in the order of 1 part in 10 8 . A promising experiment for this purpose is the watt balance [2]. A watt balance is an electromechanical apparatus which compares mechanical and electrical power and makes use of two macroscopic quantum effects, thus creating a relationship between a macroscopic mass and the Planck constant. The BIPM is developing a watt balance to ensure that in the future the unit kilogram can be derived from the Planck constant. In the BIPM watt balance a current-carrying coil is placed in the circular air gap of a magnetic circuit (Fig. 1). The sources of magnetic flux are two disks of Sm2Co17 magnets, with opposite direction of magnetization. A closed yoke made of a high permeability Fe-Ni alloy (Supra 50) concentrates the flux and shapes the radial magnetic field within the air gap, where the coil will be placed. The mean diameter of the air gap is 250 mm, its width is 13 mm and the height is 80 mm. The flux density in the air gap is 0.5 T. An important requirement for the watt balance operation is that the flux density in the air gap is highly constant in the vertical direction, which requires that the width of the air gap is uniform to within several micrometers. In addition the outer and inner poles shall be centered to better than 50 micrometers. This requires machining tolerances at the level of 1 micrometer and the use of a dedicated assembly device, capable of accurately positioning the parts in the presence of very large magnetic forces, up to 10 kN.