Roberval balance

Two horizontal plates, suitable for placing objects to be weighed, are fixed to the top of the two vertical beams.

An arrow on the lower horizontal beam (and perpendicular to it) and a mark on the vertical column may be added to aid in leveling the scale.

For maximum accuracy, Roberval balances require that their top fulcrum be placed on the line between the left and right pivot so that tipping will not result in the net transfer of weight to either the left or right side of the scale: a fulcrum placed below the ideal pivot point will tend to cause a net shift in the direction of any downward-moving vertical column (in a kind of positive feedback loop); likewise, a fulcrum placed above this point will tend to level out the arms of the balance rather than respond to small changes in weight (in a negative feedback loop).

For over three hundred years the Roberval balance has instead been popular for applications requiring convenience and only moderate accuracy, notably in retail trade.

Well known manufacturers of Roberval balances include W & T Avery Ltd. and George Salter & Co. Ltd. in the United Kingdom and Trayvou in France.

A sketch of a Roberval balance showing equal masses off-center on the plates. (This conceptual design, with rigid arms, equal distances, frictionless pivots, and the top 3 pivots colinear, is balanced in any position if the masses are exactly equal. There is no potential energy change if one side is pushed up.)
A Roberval balance made by W & T Avery Ltd. in England
Detail: the bottom horizontal beam is hidden under the protective cover
A Roberval balance shown responding to two masses of equal weight