Roberval balance

Roberval balance, linked mechanism invented in 1669 by the French mathematician Gilles Personne de Roberval and used in commercial weighing machines. As shown in the figure, AB is an equal-armed beam pivoted to the vertical member G at C, while DE is an identical beam pivoted to G at F. The beams are connected by identical vertical links AD and BE, which are attached to the scale pans (platforms). The rectangle formed by the beams and links in the position shown becomes a parallelogram when one of the scale pans moves up and the other moves down. If the weight W and the load L are the same, the beams will remain horizontal regardless of the positions of W and L on the platforms.