A simple watt balance for the absolute determination of mass

A watt balance is an electromechanical device that allows a mass to be determined in terms of measurable electrical and mechanical quantities, themselves traceable to the fundamental constants of physics. International plans are well advanced to redefine the unit of mass, the kilogram, in terms of a fixed numerical value for the Planck constant. A watt balance is one of the devices now being used to measure the Planck constant, but after redefinition will be used to make practical realizations of the kilogram. In this paper we describe a simple 'homemade' watt balance working at the level of about 10% which demonstrates the principle with a view to demystifying the proposed new definition of the kilogram to be based on the Planck constant.