Editing Weighing scale (section)

===Mechanical balances===
[[File:Alte Dezimalwaage.JPG|thumb|Old decimal balance]]
The '''balance''' (also '''balance scale''', '''beam balance''' and '''laboratory balance''') was the first mass measuring instrument invented.<ref name="averyweigh-tronix1947">{{cite web|url=http://www.averyweigh-tronix.com/download.aspx?did=6249 |title=Download – A Short History to Weighing: AWTX Museum Book |publisher=Averyweigh-tronix.com |access-date=2015-03-05 |url-status=dead |archive-url=https://web.archive.org/web/20120302145357/http://www.averyweigh-tronix.com/download.aspx?did=6249 |archive-date=March 2, 2012 }}</ref> In its traditional form, it consists of a pivoted horizontal [[lever]] with arms of equal length{{spaced ndash}}the [[Beam (structure)|beam]] or [[Tron (Scotland)|tron]] {{spaced ndash}}and a weighing pan<ref>Or "scale", "scalepan" or the obsolete "basin" ([https://books.google.com/books?id=FuAwAAAAIAAJ&pg=PA1069&dq=bason+balance+scale&lr= ''A Practical Dictionary of the English and German Languages'' (1869), p.&nbsp;1069]).</ref> suspended from each arm (hence the plural name "''scales''{{-"}} for a weighing instrument). The unknown mass is placed in one pan and standard masses are added to the other pan until the beam is as close to [[Mechanical equilibrium|equilibrium]] as possible. In precision balances, a more accurate determination of the mass is given by the position of a sliding mass moved along a graduated scale. A [[decimal balance]] uses the lever in which the arm for weights is 10 times longer than the arm for weighted objects, so that much lighter weights may be used to weigh heavy object.<ref>''McGraw-Hill Dictionary of Scientific & Technical Terms''</ref> Similarly a [[centesimal balance]] uses arms in ratio 1:100.
[[File:Balance Scale Working Principle.svg|thumb|left|For a simple pan balance to be in equilibrium, the fulcrum must be offset from the lever arm. When this is the case, the higher arm gains a mechanical advantage over the lower because its horizontal separation from the fulcrum is greater.]]
[[File:Odważniki 2, ubt.jpeg|thumb|left|Two 10-[[Gram|decagram]] masses]]
[[File:Gewichtendoos B.jpg|thumb|left|Masses of 50, 20, 1, 2, 5 and 10 grams]]

Unlike spring-based scales, balances are used for the precision measurement of mass as their accuracy is not affected by variations in the local gravitational field. (On Earth, for example, these can amount to ±0.5% between locations.<ref>{{cite book |last=Hodgeman |first=Charles, Ed. |title=Handbook of Chemistry and Physics, 44th Ed. |publisher=Chemical Rubber Publishing Co. |year=1961 |location=Cleveland, USA }} pp.&nbsp;3480–3485.</ref>) A change in the strength of the gravitational field caused by moving the balance does not change the measured mass, because the [[Moment (physics)|moments of force]] on either side of the center balanced beam are affected equally. A center beam balance will render an accurate measurement of mass at any location experiencing a constant gravity or acceleration.

Very [[Accuracy and precision|precise]] measurements are achieved by ensuring that the balance's [[fulcrum (mechanics)|fulcrum]] is essentially [[friction]]-free (a [[knife]] edge is the traditional solution), by attaching a [[wikt:pointer|pointer]] to the beam which [[Amplifier|amplifies]] any [[Absolute deviation|deviation]] from a balance position; and finally by using the [[lever]] principle, which allows [[fraction (mathematics)|fractional]] masses to be applied by [[Motion (physics)|movement]] of a small mass along the measuring arm of the beam, as described above. For greatest accuracy, there needs to be an allowance for the [[buoyancy]] in air, whose effect depends on the densities of the masses involved.

[[File:Balance scales in China 02.jpg|thumb|Aluminum, mass-produced balance scale ([[steelyard balance]]) sold and used throughout China: the scale can be inverted and held by the larger ring beneath the user's right hand to produce greater leverage for heavier loads ([[Hainan]], [[China]], 2011)]]
[[File:Kubanino sur stratpesilo (Vieno).jpg|thumb|upright|Woman on a public weighing scale, [[Vienna]], [[Austria]], 2016]]

To reduce the need for large reference masses, an off-center beam can be used. A balance with an off-center beam can be almost as accurate as a scale with a center beam, but the off-center beam requires special reference masses and cannot be intrinsically checked for accuracy by simply swapping the contents of the pans as a center-beam balance can. To reduce the need for small graduated reference masses, a sliding weight called a poise can be installed so that it can be positioned along a calibrated scale. A poise adds further intricacies to the calibration procedure, since the exact mass of the poise must be adjusted to the exact lever ratio of the beam.

For greater convenience in placing large and awkward loads, a platform can be ''floated'' on a cantilever beam system which brings the proportional force to a ''noseiron'' bearing; this pulls on a ''stilyard rod'' to transmit the reduced force to a conveniently sized beam.
 
One still sees this design in portable beam balances of 500&nbsp;kg capacity which are commonly used in harsh environments without electricity, as well as in the lighter duty mechanical bathroom scale (which actually uses a spring scale, internally). The additional pivots and bearings all reduce the accuracy and complicate calibration; the float system must be corrected for corner errors before the span is corrected by adjusting the balance beam and poise. <!-- Such systems are typically accurate to at best 1/10,000 of their capacity, unless they are expensively engineered.{{citation needed|date=November 2012}} unnecessary as technical finepoint --->

====Roberval balance====
{{main|Roberval balance}}
[[File:Balance scale IMGP9755.jpg|thumb|left|A [[Roberval balance]]. The pivots of the parallelogram understructure makes it insensitive to load positioning away from center, so improves its accuracy, and ease of use. ]]
In 1669 the Frenchman [[Gilles Personne de Roberval]] presented a new kind of balance scale to the French Academy of Sciences. This scale consisted of a pair of vertical columns separated by a pair of equal-length arms and pivoting in the center of each arm from a central vertical column, creating a parallelogram. From the side of each vertical column a peg extended. To the amazement of observers, no matter where Roberval hung two equal weight along the peg, the scale still balanced.  In this sense, the scale was revolutionary: it evolved into the more-commonly encountered form consisting of two pans placed on vertical column located above the fulcrum and the parallelogram below them. The advantage of the Roberval design is that no matter where equal weights are placed in the pans, the scale will still balance.

Further developments have included a [https://www.besslerwheel.com/forum/download.php?id=14967&order=user_id&sid=e055c056e43d2d7da87d910766c9ff39 "gear balance"] in which the parallelogram is replaced by any odd number of interlocking gears greater than one, with alternating gears of the same size and with the central gear fixed to a stand and the outside gears fixed to pans, as well as the "sprocket gear balance" consisting of a bicycle-type chain looped around an odd number of [[sprocket]]s with the central one fixed and the outermost two free to pivot and attached to a pan.

Because it has more moving joints which add friction, the Roberval balance is consistently less accurate than the traditional beam balance, but for many purposes this is compensated for by its usability.

====Torsion balance====
[[File:DRX-3.jpg|thumb|Torsion balance scale made by Torbal]]
The torsion balance is one of the most mechanically accurate of analog balances. Pharmacy schools still teach how to use torsion balances in the U.S. It utilizes pans like a traditional balance that lie on top of a mechanical chamber which bases measurements on the amount of twisting of a wire or fiber inside the chamber. The scale must still use a calibration weight to compare against, and can weigh objects greater than 120&nbsp;mg and come within a margin of error +/- 7&nbsp;mg. Many microbalances and ultra-microbalances that weigh fractional gram values are torsion balances. A common fiber type is quartz crystal.<ref>{{cite web |url=https://www.grainger.com/know-how/equipment-information/kh-laboratory-balance-scale-types-care-terms |title = Types of Balances and Scales, Common Terms & Care - Grainger KnowHow}}</ref>