Precisely measuring the Planck constant by electromechanical balances

To eliminate the last artefact unit, kilogram, in the seven basic SI units, the possible new definition of kilogram by fixing the Planck constant is being considered. National Metrology Institutes, the BIPM and academic institutions have built several electromechanical balances such as watt balance to measure the Planck constant by an equivalence of mechanical and electrical energies. This paper reviewed the principle, apparatus and recent progress of different electromechanical balances. The tendency for measuring the Planck constant and the future of kilogram have also been predicted.