We will pursue several new ideas with our experiment to overcome some of the difficulties found by the other watt balance groups working at the NPL (UK), the NIST (USA), METAS (Switzerland) and the LNE (France).

General concept

The main innovation is that we will try to carry out both measurement phases (weighing and moving experiment) simultaneously. This will remove the requirement for a perfectly stable magnetic field between the two phases and also the requirement for identical coil alignment in both phases. This concept, however, requires that the induced voltage is separated from the voltage drop related to the current flow in the coil. This is very difficult to achieve at the required uncertainty level of 1 part in 10⁸ in a room temperature environment because the resistance changes of the coil as a consequence of its temperature coefficient mask changes of the induced voltage. We therefore plan to ultimately develop an instrument with a superconducting coil, moving in a permanent magnet in a cryogenic environment. The following diagram shows the planned experiment in schematic form:

To demonstrate the feasibility of simultaneous force and velocity measurements, in mid-2005 we started to build a room-temperature experiment. If the results of this pilot experiment are positive we will start to develop the cryogenic experiment. Should there be principal problems related to this mode of operation, our watt balance can still be operated in the conventional way, by separating the weighing and moving modes.