Weighing Scale
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This week we are launching Wikivoyage . Join us in creating a free travel guide that anyone can edit. Weighing scale From Wikipedia, the free encyclopedia Jump to: navigation, search Emperor Jahangir (reign 1605 - 1627) weighing his son Shah Jahan on a weighing scale by artist Manohar (AD 1615, Mughal dynasty, India). A weighing scale (usually just "scales" in UK and Australian English, "weighing machine" in south Asian English or "scale" in US English) is a measuring instrument for determining the weight or mass of an object. A spring scale measures weight by the distance a spring deflects under its load. A balance compares the torque on the arm due to the sample weight to the torque on the arm due to a standard reference weight using a horizontal lever. Balances are different from scales, in that a balance measures mass (or more specifically gravitational mass), where as a scale measures weight (or more specifically, either the tension or compression force of constraint provided by the scale). Weighing scales are used in many industrial and commercial applications, and products from feathers to loaded tractor-trailers are sold by weight. Specialized medical scales and bathroom scales are used to measure the body weight of human beings. Contents 1 History 2 Balance o 2.1 Analytical balance 3 Scales o 3.1 Spring scales o 3.2 Pendulum balance scales o 3.3 Electronic analytical "balance" scale o 3.4 Strain gauge scale o 3.5 Hydraulic or pneumatic scale 4 Testing and certification 5 Supermarket/retail scale 6 Sources of error 7 Symbolism 8 See also 9 Footnotes 10 External links History Balance scale in the Egyptian Book of the Dead The balance scale is such a simple device that its usage likely far predates the evidence. What has allowed archaeologists to link artifacts to weighing scales are the stones for determining absolute weight. The balance scale itself was probably used to determine relative weight long before absolute weight.[1] The oldest evidence for the existence of weighing scales dates to c. 2400-1800 B.C.E. in the Indus River valley (modern-day Pakistan). Before then no banking was performed due to lack of scales. Uniform, polished stone cubes discovered in early settlements were probably used as weight-setting stones in balance scales. Although the cubes bear no markings, their weights are multiples of a common denominator. The cubes are made of many different kinds of stones with varying densities. Clearly their weight, not their size or other characteristics, was a factor in sculpting these cubes.[2] In Egypt, scales can be traced to around 1878 B.C.E., but their usage probably extends much earlier. Carved stones bearing marks denoting weight and the Egyptian hieroglyphic symbol for gold have been discovered, which suggests that Egyptian merchants had been using an established system of weight measurement to catalog gold shipments and/or gold mine yields. Although no actual scales from this era have survived, many sets of weighing stones as well as murals depicting the use of balance scales suggest widespread usage.[3] Variations on the balance scale, including devices like the cheap and inaccurate bismar,[4] began to see common usage by c. 400 B.C.E. by many small merchants and their customers. A plethora of scale varieties each boasting advantages and improvements over one another appear throughout recorded history, with such great inventors as Leonardo da Vinci lending a personal hand in their development.[5] Even with all the advances in weighing scale design and development, all scales until the seventeenth century C.E. were variations on the balance scale. Although records dating to the 1600s refer to spring scales for measuring weight, the earliest design for such a device dates to 1770 and credits Richard Salter, an early scale-maker.[6] Spring scales came into common usage in 1840 when R. W. Winfield developed the candlestick scale for use in measuring letters and packages.[7] Postal workers could work more quickly with spring scales than balance scales because they could be read instantaneously and did not have to be carefully balanced with each measurement. By the 1940s various electronic devices were being attached to these designs to make readings more accurate. These were not true digital scales as the actual measuring of weight still relied on springs and balances.[8][9] Load cells, small nodes that convert pressure to a digital signal, have their beginnings as early as the late-nineteenth century, but it was not until the late-twentieth century that they became accurate enough for widespread usage.[10] Balance The balance (also balance scale, beam balance and laboratory balance) was the first mass measuring instrument invented.[11] In its traditional form, it consists of a pivoted horizontal lever of equal length arms, called the beam, with a weighing pan, also called scale, scalepan, or bason (obsolete[12]), suspended from each arm (which is the origin of the originally plural term "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 equilibrium as possible. In precision balances, a slider mass is moved along a graduated scale. The slider position gives a fine correction to the mass value. Although a balance technically compares weights, not masses, the weight of an object is proportional to its mass, and the standard weights used with balances are usually labeled in mass units. Two 10-decagram masses Masses of 50, 20, 1, 2, 5 and 10 gram Balances are used for precision mass measurement, because unlike spring scales their accuracy is not affected by differences in the local gravity, which can vary by almost 0.5%[13] at different locations on Earth. A change in the strength of the gravitational field caused by moving the balance will not change the measured mass, because the moments of force on either side of the balance beam are affected equally. In fact, a balance will measure the correct mass even on other planets or moons, or any location that experiences a constant gravity or acceleration. Very precise measurements are achieved by ensuring that the balance's fulcrum is essentially friction-free (a knife edge is the traditional solution), by attaching a pointer to the beam which amplifies any deviation from a balance position; and finally by using the lever principle, which allows fractional masses to be applied by 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. The original form of a balance consisted of a beam with a fulcrum at its center. For highest accuracy, the fulcrum would consist of a sharp V-shaped pivot seated in a shallower V-shaped bearing. To determine the mass of the object, a combination of reference masses was hung on one end of the beam while the object of unknown mass was hung on the other end (see balance and steelyard balance). For high precision work, the center beam balance is still one of the most accurate technologies available[citation needed], and is commonly used for calibrating test weights. 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. An aluminum, mass-produced balance scale sold and used throughout China. Note the larger ring beneath the user's right hand. The scale can be inverted, and held by this ring. This produces greater leverage, and is used for heavier loads. This scale is sold with an aluminum pan, but this is rarely used. The cost approximately 12 yuan ($2 USD) in 2011. Photo taken in Hainan Province, China. 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 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] Some mechanical balances also use dials (with counterbalancing masses instead of springs), a hybrid design with some of the accuracy advantages of the poise and beam but the convenience of a dial reading. Analytical balance An analytical balance is a class of balance designed to measure small mass in the sub- milligram range. The measuring pan of an analytical balance (0.1 mg or better) is inside a transparent enclosure with doors so that dust does not collect and so any air currents in the room do not affect the balance's operation.