Magnetic circuit design for the BNM Watt balance experiment

The Bureau National de Me/spl acute/trologie (BNM) has started since 2001 the development of a Watt balance experiment. We first remind the reader the basic principles of that experiment before describing in detail the design of the magnetic circuit which is a principal component of it. Several aspects of the design are described, in particular the circuit geometry, the materials, the influence of magnetostriction, the compensation of the magnet temperature coefficient, and the mounting.