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peg-in-slot arrangement, so it effectively hangs beneath the top beam and stops the platforms from rotating. The torque on the column is taken by a pair of equal and opposite forces in the horizontal beams. If the offset weight sits toward the outside of the platform, further from the centre of the scales, the top beam will be in tension and the bottom beam will be in compression. These tensions and compressions are carried by horizontal reactions from the central supports; the other side of the scales is not affected at all, nor is the balance of the scales.
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extent, front–back as the arms move, but it experiences no up–down movement forces — in this arrangement, the entire pivot process takes place on the upper central pivot point, which acts as the single fulcrum for the entire balance; it is possible to reverse this, so that the lower pivot point acts as the fulcrum and the upper is only held in place so that it cannot sway left–right or front/back.
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that is in the actual center of the parallelogram and so that adding weights to these plates does not change that center of gravity. This produces the somewhat odd result that a correctly balanced
Roberval balance, unlike a beam balance, can be "balanced" in any arm position: so long as the masses of
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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
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with suspended plates. The beam balance, however, has the significant disadvantage of requiring suspensory strings, chains, or rods. For over three hundred years the
Roberval balance has instead been popular for applications requiring convenience and only moderate accuracy, notably in retail trade.
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An off-center weight on the plate exerts a downward force and a torque on the vertical column supporting the plate. The downward force is carried by the bearing at the top beam in most balance scales, the lower beam just being supported horizontally at midpoint by the body of the scales by a simple
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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
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The upper pivot point of the central supporting column is prevented from moving left–right and front–back by the fulcrum itself; it is prevented from moving up–down by gravitational pull. The lower pivot point of this column must be held in place so that it cannot sway left–right and, to a lesser
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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 the items are placed. Since the vertical beams are
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always vertical, and the weighing platforms always horizontal, the potential energy lost by a weight as its platform goes down a certain distance will always be the same, so it makes no difference where the weight is placed. For maximum accuracy, Roberval balances require that their top
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the objects on both sides are equal or the pans are empty, it will balance with the right arm up and the left arm down, as well as the left up and the right down, as well as any position in between, and all of these positions will be "correctly balanced".
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The correct method of using a precise but real
Roberval balance, then, is to place one of masses (either the known or the unknown) on one plate/pan and then add only enough of the other mass to the other pan until the balance just barely tips
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in the direction of the second added mass. If the arms finalize in a horizontal position, this only indicates friction in the pivot points somewhere. A well-made and precise
Roberval balance with a centralized center of gravity
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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.
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Certain presumptions are made in a theoretical
Roberval balance. In order for such a balance to appear level in its natural state and be able to balance theoretical masses, the following must be true:
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of difference. These effects must be distinguished from the feedback loops and the friction of the pivot points mentioned above, as those are undesirable effects caused by design weaknesses or flaws.
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All six pivot points must move without producing friction (since
Roberval balances often actually require twice this number, a total of 12 pivot points would need to be friction free)
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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
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In order to be balanced front-to-back the balance must either have two sets of two arms located around a central fulcrum or must have two fulcra supporting a single set of arms
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Roberval balances are frequently depicted with the "pan" as a plate or peg protruding from the center of each vertical column— this is so that the balance can have a
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If the weight of the pan above either vertical column is itself greater than zero and any weight placed on that pan is off-center then that pan's tendency to
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As a corollary, because no actual two masses can have exactly the same weight, a highly precise
Roberval balance measuring two such imprecise masses should
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The longer the arms generally, the more sensitive the balance, though longer arms usually entail greater arm weight, which tends to decrease sensitivity
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Sensitivity lost by increases in either arm weight or pan/column weight can be counteracted only through decreased static friction in the pivot points
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the ideal pivot point will tend to cause a net shift in the direction of any downward-moving vertical column (in a kind of
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this point will tend to level out the arms of the balance rather than respond to small changes in weight (in a
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at the pivot points below that pan. This tension will manifest as an increase in static friction.
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The
Roberval balance is arguably less accurate and more difficult to manufacture than a
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The vertical distance between each vertical set of pivot points must be exactly the same
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The lengths of the arms (left and right of the fulcrum) must be exactly equal unless
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The weight of the arms on each side of the fulcrum must be equal (unless see above)
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Physics
Demonstration showing surprising paradox of simple Roberval Balance design
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Detail: the bottom horizontal beam is hidden under the protective cover
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Heavier pans and vertical columns also tend to decrease sensitivity
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A Roberval balance shown responding to two masses of equal weight
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The weights of the vertical columns and/or pans is unequal
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Well known manufacturers of Roberval balances include
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170:will cause the balance to exist in a state of
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402:, Yale University Press, New Haven (1966)
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339:Learn how and when to remove this message
400:Scales and Weights. A Historical Outline
302:This article includes a list of general
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388:, Shire Publications, Aylesbury (1981)
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279:United States Department of Treasury
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267:George Salter & Co. Ltd.
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77:Gilles Personne de Roberval
29:A Roberval balance made by
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365:The Henry Troemner Company
269:in the United Kingdom and
70:French Academy of Sciences
369:Humboldt State University
361:The Tools of the Chemist
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115:Principles of operation
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440:Weighing instruments
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386:Scales and Balances
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