The distinguishing characteristic of the BIPM watt balance with respect to all other watt balances world-wide is that it carries out both measurement phases (weighing and moving experiments) simultaneously. The great advantage of this simultaneous measurement is to guarantee that the geometric properties of the coil, the coil orientation and the magnetic field, acting in both phases are strictly the same. The disadvantage is the appearance of an undesirable voltage drop in the coil, which is a result of the current flow and the non-zero resistance of the coil. This undesirable voltage drop needs to be separated from the induced voltage with an uncertainty of 1 part in 108. Such high accuracy is difficult to achieve at room temperature due to the resistance changes in the coil as a consequence of its temperature coefficient. One possibility is to ultimately develop a cryogenic watt balance with a superconducting coil (with zero resistance), moving in a permanent magnet in a cryogenic environment.
The BIPM watt balance