Roberval Balance
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The Roberval Balance is a weighing scale invented by Gilles de Roberval in 1669. In this scale, two identical horizontal beams are attached, one directly above the other, to a vertical column, which is attached to a stable base. On each side, both horizontal beams are attached to a vertical beam. The six attachment points are pivots. 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.
The object to be weighed is placed on one plate, and calibrated masses are added to and subtracted from the other plate until level is reached. The mass of the object is equal to the mass of the calibrated masses - regardless of where on the plates items are placed. Since the vertical beams are always exactly vertical, the masses can only practically exert wholly downwards force (vertical vectors with no horizontal vectors).
The Roberval Balance is arguably less accurate and more difficult to manufacture than a beam balance with suspended plates. The suspended-mass balance, however, has the disadvantages of having strings in the way of the user and needing to be suspended. The Roberval Balance, therefore, has, for over three hundred years, been very popular for applications where convenience and moderate accuracy are required, notably in retail trade.
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