To measure the mass of an object.
It is a balance of two plates which, unlike what happened in the Roman balances (steelyards balances, steelyards), are supported on the beam, instead of hanging from it.
This form, to make sure that the movement of the plates is carried out in a vertical direction, forces to place each of them on a vertical rod (AD), jointed to the beam (AB) and a backstop (beam) (DE). The set is subject to a central column (G), forming a parallelogram (parallelogram) with six pivots (A, B, C, D, E, F). In this way, the objects can be placed in any part of the pan without altering the measure, although the greater number of hinge points results in an increase in the friction of the assembly.
This instrument was invented by Gilles Personne, a French mathematician and physicist known by the name of Roberval because he was a native of Roberval, in Oise.
The theoretical principles that explain its functioning were the subject of a thesis presented, by Gilles himself, at the French Academy of Sciences, in 1669.