The ultimate goal of the BIPM watt balance is to measure the Planck constant h with a relative uncertainty of 2 × 10^-8, following Recommendation G1 (2010) of the Consultative Committee for Mass and Related Quantities (CCM). Up to now, this level of uncertainty has never been reached by any watt balance.

All currently operational watt balances use the conventional two-phase scheme proposed by B.P. Kibble in 1975¹. In the two-phase scheme, force and velocity measurements are carried out non-simultaneously. Therefore, the experiment can be limited by magnetic field drifts and by changes in the coil orientation between the two phases.

The BIPM watt balance is unique in that it is designed to operate using either the conventional “two-phase” scheme or in an original “one-phase” mode. This “one-phase” mode consists in making simultaneous force and velocity measurements which allow the experiment to be practically independent from magnetic field drifts as well as largely insensitive to coil misalignments. Nevertheless, the drawback of the “one-phase” watt balance is the undesirable voltage drop in the coil as a result of the current flowing across it and of the non-zero resistance of the wire.

A possible approach to overcome this difficulty would be to use a cryogenic superconducting coil, with zero resistance, moving in the radial field of a permanent magnet (see image below).

¹Kibble B.P. (1975), Atomic Masses and Fundamental Constants 5, Sanders J.H., Wapstra, A.H., (Eds.), New York: Plenum, pp.545-551