Prototyping a new generation of watt balances

It has been shown that under specific circumstances the watt balance becomes immune to a range of errors that previously were thought to require careful alignment for their elimination. This opens the door to a new generation of watt balances which should be easier to build, more robust and easier to operate than previous designs. Possible new designs include an instrument based on a seismometer suspension and one based on a conventional knife edge balance with the coil attached rigidly to the balance beam and arranged to generate torque about the central knife. Work has commenced on the construction of a prototype balance based on the seismometer suspension. The paper describes the principles behind the new balances and outlines the balance designs.