Alignment procedure used in the BIPM watt balance

We present an experimental method used for the BIPM watt balance to align with high accuracy the main vectors involved in the experiment. In particular, the electromagnetic force exerted on the watt balance coil needs to be aligned parallel to the gravitational force exerted on the test mass to within a maximum angle of 100 μrad. Our alignment method has been validated with a test coil which is a prototype for the ultimate watt balance coil. The result is accurate enough to enable a determination of the Planck constant with a relative uncertainty of 1·10^-8, which is the ultimate goal of the BIPM watt balance.