Principles of a new generation of simplified and accurate watt balances

Abstract

It has been known for some time that the watt balance, which is based on the virtual work principle, is insensitive to some misalignments which would, at first sight, be expected to produce significant errors. In this paper we show that, in particular circumstances, this insensitivity applies to a whole range of misalignments. This effect can be exploited to design a watt balance, with a constrained coil motion, which offers advantages over conventional designs. We present three such new designs: one is based on a conventional balance and the other two are based on strip hinges, which are known to have excellent properties when measuring small force differences and with the production of precise linear movements having little motion in the other five degrees of freedom. A constrained design of this kind would have advantages for reproducing the forthcoming SI definition of mass in the range from milligrams to kilograms, whenever and wherever desired, and might well do so with improved accuracy.