The beam balance as an instrument for very precise weighing

An outline of the principles that should be followed in the design and operation of an equal-arm beam balance for very precise weighing is presented. A detailed discussion on the mechanical aspects of the design and on precautions to be taken to achieve mechanical stability is given. The effects of anelasticity in the pivots, ground vibrations and temperature changes are also described. Comparisons are drawn between the behaviour of knife edges and flexure strips as pivots and between their relative merits. Examples are given of some very precise weighings, reaching in one case a standard deviation of 4 parts in 10 12 . The factors that may ultimately limit the accuracy of weighing are indicated.