An original procedure has been developed and tested to ensure that the electric force F_e=I∫dL×B exerted on the watt balance coil is parallel to the gravitational force F_g=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 F_e can be made parallel to F_g 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.