The design of a superconducting coil watt balance presents a number of significant technological challenges as well as several physical fundamental questions. Therefore, before working on a full-scale cryogenic watt balance, the BIPM has decided to build a small-scale superconducting moving coil system. Its construction began in late 2010.

Extensive modelling and a literature review have been undertaken. Tests on superconducting wires in collaboration with NIST were performed to identify the appropriate wire diameter from the point of view of diamagnetic offset force effects. However, it is essential to test a real superconducting moving coil in our small-scale cryogenic watt balance environment to understand the many experimental aspects which cannot be modeled accurately a priori. In particular our model will provide essential information on cryogenic engineering issues, cooling techniques, coil temperature fluctuations, and vibration motion of the coil.

Below is a schematic view of the BIPM's existing cryogenic apparatus:

As a starting point, our apparatus will concentrate on the moving coil (dynamic) phase of the watt balance.

A superconducting and a non-superconducting coil of the same length L are wound together and placed in a cryostat filled with liquid helium. A radial magnetic field B is generated by two external field coils placed outside the cryostat. Relative velocity is generated by either moving the inside coils or the field coils. The relative velocity is measured by a Michelson interferometer. The induced voltage is measured separately in the superconducting and the non-superconducting coil. A first check will be to prove that the BL profile seen by a superconducting coil is the same as the one seen by a non-superconducting coil under the same conditions. We can also measure the voltage across the coil leads, under different cooling conditions and with wires of different diameter. First results from this experiment are expected in 2011.