An original procedure has been developed and tested to ensure that the electric force Fe=I∫dL×B exerted on the watt balance coil is parallel to the gravitational force Fg=mg exerted on the test mass.
For this procedure, the vectors L, B and g need to be aligned perpendicular to each other. As a first step, the vector L of the watt balance coil is aligned to be perpendicular to the local gravity field vector. In the second step, the radial magnetic field B of the watt balance magnet is aligned to be parallel to L.
This procedure is described in detail in: de Mirandés E., Fang H., Kiss A., Solve S., Stock M., Picard A., Alignment procedure used in the BIPM watt balance, Proc. 2010 Conference on Precision Electromagnetic Measurements (CPEM), 2011, 522-523.
The alignment procedure has been tested with a trial coil which has the same characteristics as the final watt balance coil (1200 turns, 250 mm diameter). The results were successful, proving that Fe can be made parallel to Fg to within 50 µrad. This accuracy is enough to allow the Planck constant to be measured at the 10-8 level. We plan to carry out the alignment with the real watt balance coil in the near future.
Long solenoid (long-term loan from NIST) used to generate a uniform
magnetic field for alignment purposes. Two length gauges scan
the solenoid axially to characterize its winding geometry.