The Watt balance is comparing mechanical and electrical powers. In a first step, the mechanical force on a test mass in the earth’s gravitational field is balanced with an electro-magnetic force using a current-carrying coil. In a second step, the coil is moved through the magnetic field, which induces a voltage. The precise knowledge of the electrical, gravitational and dynamical properties allows to infer the mass by using natural constants only, which allows to redefine the kilogram - instead of using the prototype mass in Paris. One requirement during the two phases of the experiment is the accurate determination of the position and angular orientation of the coil. This measurement is performed using the PICOSCALE Interferometer.

For details please refer to https://www.metas.ch/metas/en/home/fue/forschungs-projekte/wattwaage.html