History

The Ancient Egyptian Book of the Dead depicts a scene in which a scribe's heart is weighed against the feather of truth.

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 mass. The balance scale itself was probably used to determine relative mass long before absolute mass.^[1]

The oldest attested evidence for the existence of weighing scales dates to the Fourth Dynasty of Egypt, with Deben (unit) balance weights, from the reign of Sneferu (c. 2600 BC) excavated, though earlier usage has been proposed.^[2] Carved stones bearing marks denoting mass and the Egyptian hieroglyphic symbol for gold have been discovered, which suggests that Egyptian merchants had been using an established system of mass measurement to catalog gold shipments 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]

Examples, dating c. 2400–1800 BC, have also been found in the Indus River valley. Uniform, polished stone cubes discovered in early settlements were probably used as mass-setting stones in balance scales. Although the cubes bear no markings, their masses are multiples of a common denominator. The cubes are made of many different kinds of stones with varying densities. Clearly their mass, not their size or other characteristics, was a factor in sculpting these cubes.^[3]

In China, the earliest weighing balance excavated was from a tomb of the State of Chu of the Chinese Warring States Period dating back to the 3rd to 4th century BC in Mount Zuojiagong near Changsha, Hunan. The balance was made of wood and used bronze masses.^[4][5]

Variations on the balance scale, including devices like the cheap and inaccurate bismar (unequal-armed scales),^[6] began to see common usage by c. 400 B.C. 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.^[7]

Even with all the advances in weighing scale design and development, all scales until the seventeenth century AD were variations on the balance scale. The standardization of the weights used – and ensuring traders used the correct weights – was a considerable preoccupation of governments throughout this time.

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  Weighing dishes from the island of Thera, Minoan civilization, 2000–1500 BC
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  Assyrian lion weights (8th century BC) in the British Museum
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  Roman steelyard balance with two bronze weights, 50–200 A.D., Gallo-Roman Museum, Tongeren, Belgium
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  Emperor Jahangir (reigned 1605–1627) weighing his son Shah Jahan on a weighing scale by artist Manohar (AD 1615, Mughal dynasty, India).

Finely crafted pan balance or scales, with boxed set of standardized gram masses

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, such as empirical chemistry, the center beam balance is still one of the most accurate technologies available, and is commonly used for calibrating test masses.

However, bronze fragments discovered in central Germany and Italy had been used during the Bronze Age as an early form of currency.^[8] In the same time period, merchants had used standard weights of equivalent value between 8 and 10.5 grams from Great Britain to Mesopotamia.^[9]

Mechanical balances

The balance (also balance scale, beam balance and laboratory balance) was the first mass measuring instrument invented.^[1] In its traditional form, it consists of a pivoted horizontal lever with arms of equal length – the beam – and a weighing pan^[10] 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 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.

Two 10-decagram masses

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.^[11]) A change in the strength of the gravitational field caused by moving the balance does not change the measured mass, because the moments of force on either side of the beam are affected equally. A balance will render an accurate measurement of mass at any location experiencing 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.

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)

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 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.

Roberval balance

Main article: Roberval balance

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 "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 sprockets 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

Torsion balance scale made by Torbal

The torsion balance is one of the most mechanically accurate and 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 mg and come within a margin of error +/- 7 mg. Many microbalances and ultra-microbalances that weigh fractional gram values are torsion balances. A common fiber type is quartz crystal.^[12]

Electronic devices

Analytical balance

Main article: Analytical balance

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. This enclosure is often called a draft shield. The use of a mechanically vented balance safety enclosure, which has uniquely designed acrylic airfoils, allows a smooth turbulence-free airflow that prevents balance fluctuation and the measure of mass down to 1 μg without fluctuations or loss of product. Also, the sample must be at room temperature to prevent natural convection from forming air currents inside the enclosure from causing an error in reading. Single-pan mechanical substitution balances maintain consistent response throughout the useful capacity, which is achieved by maintaining a constant load on the balance beam and thus the fulcrum by subtracting mass on the same side of the beam to which the sample is added.

Electronic analytical scales measure the force needed to counter the mass being measured rather than using actual masses. As such they must have calibration adjustments made to compensate for gravitational differences.^[13] They use an electromagnet to generate a force to counter the sample being measured and output the result by measuring the force needed to achieve balance. Such a measurement device is called an electromagnetic force restoration sensor.^[14]

Symbolism

"Lady Justice" holding a 2-pan balance beam scale, and a sword: Statue of Justice, Central Criminal Court, London, UK

The scales (specifically, a two-pan, beam balance) are one of the traditional symbols of justice, as wielded by statues of Lady Justice. This corresponds to the use in a metaphor of matters being "held in the balance". It has its origins in ancient Egypt. ^[17]

Scales also are widely used as a symbol of finance, commerce, or trade, in which they have played a traditional, vital role since ancient times. For instance, balance scales are depicted in the seal of the U.S. Department of the Treasury and the Federal Trade Commission.

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  Seal of the U.S. Department of the Treasury
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  Seal of the U.S. Federal Trade Commission

Scales are also the symbol for the astrological sign Libra.

Scales (specifically, a two-pan, beam balance in a state of equal balance) are the traditional symbol of Pyrrhonism indicating the equal balance of arguments used in inducing epoche.^[18]

History

A simple balance from the 19th century

Although records dating to the 1700s refer to spring scales for measuring mass, the earliest design for such a device dates to 1770 and credits Richard Salter, an early scale-maker.^[3] Spring scales came into wide usage in the United Kingdom after 1840 when R. W. Winfield developed the candlestick scale for weighing letters and packages, required after the introduction of the Uniform Penny Post.^[19] 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.^[3][7] Load cells – transducers that convert force to an electrical signal – have their beginnings as early as the late nineteenth century, but it was not until the late twentieth century that their widespread usage became economically and technologically viable.^[20]

Mechanical scales

A mechanical scale or balance is used to describe a weighing device that is used to measure the mass, force exertion, tension, and resistance of an object without the need of a power supply. Types of mechanical scales include spring scales, hanging scales, triple beam balances, and force gauges.

Domestic Weighing Scale

A mechanical bathroom scale. Pressure on the internal springs rotates a disc displaying the user's weight in pounds.

Electronic digital scales display weight as a number, usually on a liquid crystal display (LCD). They are versatile because they may perform calculations on the measurement and transmit it to other digital devices. On a digital scale, the force of the weight causes a spring to deform, and the amount of deformation is measured by one or more transducers called strain gauges. A strain gauge is a conductor whose electrical resistance changes when its length changes. Strain gauges have limited capacity and larger digital scales may use a hydraulic transducer called a load cell instead. A voltage is applied to the device, and the weight causes the current through it to change. The current is converted to a digital number by an analog-to-digital converter, translated by digital logic to the correct units, and displayed on the display. Usually, the device is run by a microprocessor chip.

Testing and certification

Scales used for trade purposes in the United States, as this scale at the checkout in a cafeteria, are inspected for accuracy by the FDACS's Bureau of Weights and Measures.

Most countries regulate the design and servicing of scales used for commerce. For example, in the European Union weighing instruments are subject to 2014/31/EU and 2014/32/EU directives. A conformity assessment procedure is carried out before placing the instrument on the market, andv the instruments are verified after a given period of time in member states of the European Union. This has tended to cause scale technology to lag behind other technologies because expensive regulatory hurdles are involved in introducing new designs. Nevertheless, there has been a trend to "digital load cells" which are actually strain-gauge cells with dedicated analog converters and networking built into the cell itself. Such designs have reduced the service problems inherent with combining and transmitting a number of 20 millivolt signals in hostile environments.

Government regulation generally requires periodic inspections by licensed technicians, using masses whose calibration is traceable to an approved laboratory. Scales intended for non-trade use, such as those used in bathrooms, doctor's offices, kitchens (portion control), and price estimation (but not official price determination) may be produced, but must by law be labelled "Not Legal for Trade" to ensure that they are not re-purposed in a way that jeopardizes commercial interest. In the United States, the document describing how scales must be designed, installed, and used for commercial purposes is NIST Handbook 44. Legal For Trade (LFT) certification usually approve the readability by testing repeatability of measurements to ensure a maximum margin of error of 10%.

Because gravity varies by over 0.5% over the surface of the earth, the distinction between force due to gravity and mass is relevant for accurate calibration of scales for commercial purposes. Usually, the goal is to measure the mass of the sample rather than its force due to gravity at that particular location.

Traditional mechanical balance-beam scales intrinsically measured mass. But ordinary electronic scales intrinsically measure the gravitational force between the sample and the earth, i.e. the weight of the sample, which varies with location. So such a scale has to be re-calibrated after installation, for that specific location, in order to obtain an accurate indication of mass.

Elastic arm scale

In 2014 a concept of hybrid scale was introduced, the elastically deformable arm scale,^[26] which is a combination between a spring scale and a beam balance, exploiting simultaneously both principles of equilibrium and deformation. In this scale, the rigid arms of a classical beam balance (for example a steelyard) are replaced with a flexible elastic rod in an inclined frictionless sliding sleeve. The rod can reach a unique sliding equilibrium when two vertical dead loads (or masses) are applied at its edges. Equilibrium, which would be impossible with rigid arms, is guaranteed because configurational forces develop at the two edges of the sleeve as a consequence of both the free sliding condition and the nonlinear kinematics of the elastic rod. This mass measuring device can also work without a counterweight.