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A scale or balance is a device used to measure
weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar quan ...
or
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
. These are also known as mass scales, weight scales, mass balances, and weight balances. The traditional scale consists of two plates or bowls suspended at equal distances from a
fulcrum A fulcrum is the support about which a lever pivots. Fulcrum may also refer to: Companies and organizations * Fulcrum (Anglican think tank), a Church of England think tank * Fulcrum Press, a British publisher of poetry * Fulcrum Wheels, a bicy ...
. One plate holds an object of unknown
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
(or
weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar quan ...
), while known masses are added to the other plate until
static equilibrium In classical mechanics, a particle is in mechanical equilibrium if the net force on that particle is zero. By extension, a physical system made up of many parts is in mechanical equilibrium if the net force on each of its individual parts is zero ...
is achieved and the plates level off, which happens when the masses on the two plates are equal. The perfect scale rests at neutral. A
spring scale A spring scale, spring balance or newton meter is a type of mechanical force gauge or weighing scale. It consists of a spring fixed at one end with a hook to attach an object at the other. It works in accordance with Hooke's Law, which states ...
will make use of a spring of known
stiffness Stiffness is the extent to which an object resists deformation in response to an applied force. The complementary concept is flexibility or pliability: the more flexible an object is, the less stiff it is. Calculations The stiffness, k, of a bo ...
to determine mass (or weight). Suspending a certain mass will extend the spring by a certain amount depending on the spring's stiffness (or
spring constant In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of t ...
). The heavier the object, the more the spring stretches, as described in
Hooke's law In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of ...
. Other types of scales making use of different physical principles also exist. Some scales can be
calibrate In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known a ...
d to read in units of force (weight) such as
newtons The newton (symbol: N) is the unit of force in the International System of Units (SI). It is defined as 1 kg⋅m/s, the force which gives a mass of 1 kilogram an acceleration of 1 metre per second per second. It is named after Isaac Newton in ...
instead of units of mass such as
kilogram The kilogram (also kilogramme) is the unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquiall ...
s. Scales and balances are widely used in commerce, as many products are sold and packaged by mass.


Pan balance


History

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. The oldest attested evidence for the existence of weighing scales dates to the
Fourth Dynasty of Egypt The Fourth Dynasty of ancient Egypt (notated Dynasty IV) is characterized as a " golden age" of the Old Kingdom of Egypt. Dynasty IV lasted from to 2494 BC. It was a time of peace and prosperity as well as one during which trade with other ...
, with Deben (unit) balance weights, from the reign of
Sneferu Sneferu ( snfr-wj "He has perfected me", from ''Ḥr-nb-mꜣꜥt-snfr-wj'' "Horus, Lord of Maat, has perfected me", also read Snefru or Snofru), well known under his Hellenized name Soris ( grc-koi, Σῶρις by Manetho), was the founding pha ...
(c. 2600 BC) excavated, though earlier usage has been proposed. 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. Examples, dating , 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. In China, the earliest weighing balance excavated was from a tomb of the
State of Chu Chu, or Ch'u in Wade–Giles romanization, (, Hanyu Pinyin: Chǔ, Old Chinese: ''*s-r̥aʔ'') was a Zhou dynasty vassal state. Their first ruler was King Wu of Chu in the early 8th century BCE. Chu was located in the south of the Zhou he ...
of the Chinese
Warring States Period The Warring States period () was an era in ancient Chinese history characterized by warfare, as well as bureaucratic and military reforms and consolidation. It followed the Spring and Autumn period and concluded with the Qin wars of conquest ...
dating back to the 3rd to 4th century BC in Mount Zuojiagong near
Changsha Changsha (; ; ; Changshanese pronunciation: (), Standard Mandarin pronunciation: ) is the capital and the largest city of Hunan Province of China. Changsha is the 17th most populous city in China with a population of over 10 million, and th ...
, Hunan. The balance was made of wood and used bronze masses. Variations on the balance scale, including devices like the cheap and inaccurate ''bismar'' (unequal-armed scales), 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. 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. File:Archaeological site of Akrotiri - Museum of prehistoric Thera - Santorini - weighing dishes - 01.jpg, Weighing dishes from the island of
Thera Santorini ( el, Σαντορίνη, ), officially Thira (Greek language, Greek: Θήρα ) and classical Greek Thera (English language, English pronunciation ), is an island in the southern Aegean Sea, about 200 km (120 mi) southeast ...
,
Minoan civilization The Minoan civilization was a Bronze Age Aegean civilization on the island of Crete and other Aegean Islands, whose earliest beginnings were from 3500BC, with the complex urban civilization beginning around 2000BC, and then declining from 14 ...
, 2000–1500 BC File:Lionweights-BM.JPG,
Assyrian lion weights The Assyrian lion weights are a group of bronze statues of lions, discovered in archaeological excavations in or adjacent to ancient Assyria. The first published, and the most notable, are a group of sixteen bronze Mesopotamian weights found at ...
(8th century BC) in the
British Museum The British Museum is a public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is among the largest and most comprehensive in existence. It docu ...
File:Weegschaal (unster) met 2 gewichten in brons, 50 tot 200 NC, vindplaats- Onbekend, collectie Gallo-Romeins Museum Tongeren, GRM 48.jpg, Roman
steelyard balance A steelyard balance, steelyard, or stilyard is a straight-beam balance with arms of unequal length. It incorporates a counterweight which slides along the longer arm to counterbalance the load and indicate its weight. A steelyard is also known as ...
with two bronze weights, 50–200 A.D.,
Gallo-Roman Museum, Tongeren The Gallo-Roman Museum ( nl, Gallo-Romeins Museum, french: Musée Gallo-romain, german: Gallo-Römisches Museum) is an archeological museum in Tongeren. It is dedicated to the prehistorical times and Roman age of the region in South East Flanders. ...
, Belgium File:Meister der Jahângîr-Memoiren 001.jpg, Emperor
Jahangir Nur-ud-Din Muhammad Salim (30 August 1569 – 28 October 1627), known by his imperial name Jahangir (; ), was the fourth Mughal Emperor, who ruled from 1605 until he died in 1627. He was named after the Indian Sufi saint, Salim Chishti. Ear ...
(reigned 1605–1627) weighing his son
Shah Jahan Shihab-ud-Din Muhammad Khurram (5 January 1592 – 22 January 1666), better known by his regnal name Shah Jahan I (; ), was the fifth emperor of the Mughal Empire, reigning from January 1628 until July 1658. Under his emperorship, the Mugha ...
on a weighing scale by artist Manohar (AD 1615, Mughal dynasty, India).
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 Balance or balancing may refer to: Common meanings * Balance (ability) in biomechanics * Balance (accounting) * Balance or weighing scale * Balance as in equality or equilibrium Arts and entertainment Film * ''Balance'' (1983 film), a Bulgaria ...
and
steelyard balance A steelyard balance, steelyard, or stilyard is a straight-beam balance with arms of unequal length. It incorporates a counterweight which slides along the longer arm to counterbalance the load and indicate its weight. A steelyard is also known as ...
). 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 The Bronze Age is a historic period, lasting approximately from 3300 BC to 1200 BC, characterized by the use of bronze, the presence of writing in some areas, and other early features of urban civilization. The Bronze Age is the second pr ...
as an early form of currency. In the same time period, merchants had used standard weights of equivalent value between 8 and 10.5 grams from Great Britain to Mesopotamia.


Mechanical balances

The balance (also balance scale, beam balance and laboratory balance) was the first mass measuring instrument invented. In its traditional form, it consists of a pivoted horizontal
lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '' fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is ...
with arms of equal lengththe beamand a weighing pan 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. Technically, a balance compares
weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar quan ...
rather than mass, but, in a given
gravitational field In physics, a gravitational field is a model used to explain the influences that a massive body extends into the space around itself, producing a force on another massive body. Thus, a gravitational field is used to explain gravitational pheno ...
(such as
Earth's gravity The gravity of Earth, denoted by , is the net acceleration that is imparted to objects due to the combined effect of gravitation (from mass distribution within Earth) and the centrifugal force (from the Earth's rotation). It is a vector quantit ...
), the weight of an object is proportional to its mass, so the standard masses used with balances are usually labeled in
units of mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different elementa ...
(e.g. g or kg). 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.) 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 A fulcrum is the support about which a lever pivots. Fulcrum may also refer to: Companies and organizations * Fulcrum (Anglican think tank), a Church of England think tank * Fulcrum Press, a British publisher of poetry * Fulcrum Wheels, a bicy ...
is essentially
friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
-free (a
knife A knife ( : knives; from Old Norse 'knife, dirk') is a tool or weapon with a cutting edge or blade, usually attached to a handle or hilt. One of the earliest tools used by humanity, knives appeared at least 2.5 million years ago, as evidence ...
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 A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '' fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is ...
principle, which allows fractional masses to be applied by
movement Movement may refer to: Common uses * Movement (clockwork), the internal mechanism of a timepiece * Motion, commonly referred to as movement Arts, entertainment, and media Literature * "Movement" (short story), a short story by Nancy Fu ...
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 Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
in air, whose effect depends on the densities of the masses involved. 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

In 1669 the Frenchman
Gilles Personne de Roberval Gilles Personne de Roberval (August 10, 1602 – October 27, 1675), French mathematician, was born at Roberval near Beauvais, France. His name was originally Gilles Personne or Gilles Personier, with Roberval the place of his birth. Biography ...
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
"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 A sprocket, sprocket-wheel or chainwheel is a profiled wheel with teeth that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which radial projections engage a chain pa ...
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

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.


Electronic devices


Microbalance

A
microbalance A microbalance is an instrument capable of making precise measurements of weight of objects of relatively small mass: of the order of a million parts of a gram. In comparison, a standard analytical balance is 100 times less sensitive; i.e. it i ...
(also called an ultramicrobalance, or nanobalance) is an instrument capable of making precise measurements of the mass of objects of relatively small mass: on the order of a million parts of a gram and below.


Analytical balance

An analytical balance is a class of balance designed to measure small
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
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 Dust is made of fine particles of solid matter. On Earth, it generally consists of particles in the atmosphere that come from various sources such as soil lifted by wind (an aeolian process), volcanic eruptions, and pollution. Dust in ho ...
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 Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a person when they are wearing typical indoor clothing. Human comfort can extend beyond this range depending on ...
to prevent natural
convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conv ...
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. 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.


Pendulum balance scales

Pendulum type scales do not use springs. These designs use pendulums and operate as a balance that is unaffected by differences in gravity. An example of application of this design are scales made by the Toledo Scale Company.


Programmable scales

A programmable scale has a
programmable logic controller A programmable logic controller (PLC) or programmable controller is an industrial computer that has been ruggedized and adapted for the control of manufacturing processes, such as assembly lines, machines, robotic devices, or any activity tha ...
in it, allowing it to be programmed for various applications such as batching, labeling, filling, truck scales, and more.


Symbolism

The scales (specifically, a two-pan, beam balance) are one of the traditional symbols of
justice Justice, in its broadest sense, is the principle that people receive that which they deserve, with the interpretation of what then constitutes "deserving" being impacted upon by numerous fields, with many differing viewpoints and perspective ...
, as wielded by statues of
Lady Justice Lady Justice ( la, Iustitia) is an allegorical personification of the moral force in judicial systems. Her attributes are scales, a sword and sometimes a blindfold. She often appears as a pair with Prudentia. Lady Justice originates from the ...
. This corresponds to the use in a metaphor of matters being "held in the balance". It has its origins in ancient Egypt. 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 The Federal Trade Commission (FTC) is an independent agency of the United States government whose principal mission is the enforcement of civil (non-criminal) antitrust law and the promotion of consumer protection. The FTC shares jurisdiction ov ...
. Seal of the United States Department of the Treasury.svg, Seal of the U.S. Department of the Treasury Seal of the United States Federal Trade Commission.svg, Seal of the U.S. Federal Trade Commission Scales are also the symbol for the astrological sign
Libra Libra generally refers to: * Libra (constellation), a constellation * Libra (astrology), an astrological sign based on the star constellation Libra may also refer to: Arts and entertainment * ''Libra'' (novel), a 1988 novel by Don DeLillo Mus ...
. Scales (specifically, a two-pan, beam balance in a state of equal balance) are the traditional symbol of
Pyrrhonism Pyrrhonism is a school of philosophical skepticism founded by Pyrrho in the fourth century BCE. It is best known through the surviving works of Sextus Empiricus, writing in the late second century or early third century CE. History Pyrrho of ...
indicating the equal balance of arguments used in inducing epoche.


Force-measuring (weight) scales


History

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. 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 The Uniform Penny Post was a component of the comprehensive reform of the Royal Mail, the UK's official postal service, that took place in the 19th century. The reforms were a government initiative to eradicate the abuse and corruption of the ...
. 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.
Load cell A load cell converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. It is a force transducer. As the force applied to the load cell increases, the electrical signal change ...
s – 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.


Mechanical scales

A mechanical scale or balance is used to describe a weighing device that is used to measure the mass, force
exertion Exertion is the physical or perceived use of energy.Newton's Third Law, Elert, Glenn. “Forces.” ''Viscosity – The Physics Hypertextbook'', physics.info/newton-first/. Exertion traditionally connotes a strenuous or costly ''effort'', resulting ...
, 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.


Spring scales

A
spring scale A spring scale, spring balance or newton meter is a type of mechanical force gauge or weighing scale. It consists of a spring fixed at one end with a hook to attach an object at the other. It works in accordance with Hooke's Law, which states ...
measures mass by reporting the distance that a spring deflects under a load. This contrasts to a ''balance'', which compares the
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...
on the arm due to a sample weight to the
torque In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of t ...
on the arm due to a standard reference mass using a horizontal
lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '' fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is ...
. Spring scales measure
force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...
, which is the tension
force In physics, a force is an influence that can change the motion of an object. A force can cause an object with mass to change its velocity (e.g. moving from a state of rest), i.e., to accelerate. Force can also be described intuitively as a ...
of constraint acting on an object, opposing the local force of gravity. They are usually
calibrated In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known a ...
so that measured force translates to mass at earth's gravity. The object to be weighed can be simply hung from the spring or set on a pivot and bearing platform. In a spring scale, the spring either stretches (as in a hanging scale in the produce department of a
grocery store A grocery store ( AE), grocery shop ( BE) or simply grocery is a store that primarily retails a general range of food products, which may be fresh or packaged. In everyday U.S. usage, however, "grocery store" is a synonym for supermarket, an ...
) or compresses (as in a simple bathroom scale). By
Hooke's law In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of ...
, every spring has a proportionality constant that relates how hard it is pulled to how far it stretches. Weighing scales use a spring with a known spring constant (see
Hooke's law In physics, Hooke's law is an empirical law which states that the force () needed to extend or compress a spring by some distance () scales linearly with respect to that distance—that is, where is a constant factor characteristic of ...
) and measure the displacement of the spring by any variety of mechanisms to produce an estimate of the
gravitational In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
force applied by the object. Rack and
pinion A pinion is a round gear—usually the smaller of two meshed gears—used in several applications, including drivetrain and rack and pinion systems. Applications Drivetrain Drivetrains usually feature a gear known as the pinion, which may ...
mechanisms are often used to convert the linear spring motion to a dial reading. Spring scales have two sources of error that balances do not: the measured mass varies with the strength of the local gravitational force (by as much as 0.5% at different locations on Earth), and the elasticity of the measurement spring can vary slightly with temperature. With proper manufacturing and setup, however, spring scales can be rated as legal for commerce. To remove the temperature error, a commerce-legal spring scale must either have temperature-compensated springs or be used at a fairly constant temperature. To eliminate the effect of gravity variations, a commerce-legal spring scale must be calibrated where it is used.


Hydraulic or pneumatic scale

It is also common in high-capacity applications such as crane scales to use hydraulic force to sense mass. The test force is applied to a piston or diaphragm and transmitted through hydraulic lines to a dial indicator based on a
Bourdon tube Pressure measurement is the measurement of an applied force by a fluid ( liquid or gas) on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pre ...
or electronic sensor.


Domestic Weighing Scale

Electronic digital scales display weight as a number, usually on a
liquid crystal display A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but i ...
(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
transducer A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and cont ...
s called
strain gauge A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which support ...
s. A strain gauge is a conductor whose
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallel ...
changes when its length changes. Strain gauges have limited capacity and larger digital scales may use a
hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...
transducer called a
load cell A load cell converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. It is a force transducer. As the force applied to the load cell increases, the electrical signal change ...
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 In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provid ...
, translated by digital logic to the correct units, and displayed on the display. Usually, the device is run by a
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
chip.


Digital bathroom scale

A digital bathroom scale is a scale on the floor which a person stands on. The weight is shown on an LED or LCD display. The digital electronics may do more than just display weight, it may calculate body fat, BMI, lean mass, muscle mass, and water ratio. Some modern bathroom scales are wirelessly or cellularly connected and have features like smartphone integration, cloud storage, and fitness tracking. They are usually powered by a button cell, or battery of AA or AAA size.


Digital kitchen scale

Digital kitchen scales are used for weighing food in a kitchen during cooking. These are usually lightweight and compact.


Strain gauge scale

In electronic versions of spring scales, the deflection of a beam supporting the unknown mass is measured using a
strain gauge A strain gauge (also spelled strain gage) is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which support ...
, which is a length-sensitive
electrical resistance The electrical resistance of an object is a measure of its opposition to the flow of electric current. Its reciprocal quantity is , measuring the ease with which an electric current passes. Electrical resistance shares some conceptual parallel ...
. The capacity of such devices is only limited by the resistance of the beam to deflection. The results from several supporting locations may be added electronically, so this technique is suitable for determining the mass of very heavy objects, such as trucks and rail cars, and is used in a modern weighbridge.


Supermarket and other retail scale

These scales are used in the modern
bakery A bakery is an establishment that produces and sells flour-based food baked in an oven such as bread, cookies, cakes, donuts, pastries, and pies. Some retail bakeries are also categorized as cafés, serving coffee and tea to customers who w ...
,
grocery A grocery store ( AE), grocery shop ( BE) or simply grocery is a store that primarily retails a general range of food products, which may be fresh or packaged. In everyday U.S. usage, however, "grocery store" is a synonym for supermarket, ...
,
delicatessen Traditionally, a delicatessen or deli is a retail establishment that sells a selection of fine, exotic, or foreign prepared foods. Delicatessen originated in Germany (original: ) during the 18th century and spread to the United States in the m ...
,
seafood Seafood is any form of sea life regarded as food by humans, prominently including fish and shellfish. Shellfish include various species of molluscs (e.g. bivalve molluscs such as clams, oysters and mussels, and cephalopods such as octopus ...
,
meat Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as c ...
,
produce Produce is a generalized term for many farm-produced crops, including fruits and vegetables ( grains, oats, etc. are also sometimes considered ''produce''). More specifically, the term ''produce'' often implies that the products are fresh and ...
and other perishable goods departments. Supermarket scales can print labels and receipts, mark mass and count, unit price, total price and in some cases tare. Some modern supermarket scales print an
RFID Radio-frequency identification (RFID) uses electromagnetic fields to automatically identify and track tags attached to objects. An RFID system consists of a tiny radio transponder, a radio receiver and transmitter. When triggered by an electrom ...
tag that can be used to track the item for tampering or returns. In most cases, these types of scales have a sealed calibration so that the reading on the display is correct and cannot be tampered with. In the US, the scales are certified by the
National Type Evaluation Program The National Conference on Weights and Measures (NCWM) is a not-for-profit corporation dedicated to developing the United States technical standards for weights and measures in commerce. The organization's official mission is "Ensuring Equity and ...
(NTEP), in South Africa by the South African Bureau of Standards, in Australia, they are certified by the National Measurement Institute (NMI) and in the UK by the
International Organization of Legal Metrology The International Organization of Legal Metrology (french: Organisation Internationale de Métrologie Légale - OIML), is an intergovernmental organisation that was created in 1955 to promote the global harmonisation of the legal metrology procedur ...
.


Testing and certification

Most countries regulate the design and servicing of scales used for commerce (for example In the European Union weighing instruments are scope of 2014/31/EU and 2014/32/EU directives. Conformity assessment procedure is carried out before placing instrument on market. Instruments are verified in use after given period of time in member states of EU. ). 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 The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sc ...
''Handbook 44''. Legal For Trade (LFT) certification usually approve the readability as repeatability/10 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 Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
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 In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the strong ...
between the sample and the earth, i.e. the
weight In science and engineering, the weight of an object is the force acting on the object due to gravity. Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar quan ...
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.


Sources of error

Some of the sources of
error An error (from the Latin ''error'', meaning "wandering") is an action which is inaccurate or incorrect. In some usages, an error is synonymous with a mistake. The etymology derives from the Latin term 'errare', meaning 'to stray'. In statistics ...
in weighing are: *
Buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pr ...
– Objects in air develop a buoyancy force that is directly proportional to the volume of air displaced. The difference in density of air due to
barometric pressure Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, ...
and temperature creates errors. * Error in the mass of reference weight * Air gusts, even small ones, which push the scale up or down *
Friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of t ...
in the moving components that causes the scale to reach equilibrium at a different configuration than a frictionless equilibrium should occur. * Settling airborne dust contributing to the weight * Mis-calibration over time, due to drift in the circuit's accuracy, or temperature change * Mis-aligned mechanical components due to
thermal expansion Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature, usually not including phase transitions. Temperature is a monotonic function of the average molecular kinetic ...
or contraction of components *
Magnetic fields A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
acting on ferrous components * Forces from
electrostatic field An electric field (sometimes E-field) is the field (physics), physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the ...
s, for example, from feet shuffled on carpets on a dry day * Chemical reactivity between air and the substance being weighed (or the balance itself, in the form of
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engin ...
) *
Condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor to ...
of atmospheric water on cold items *
Evaporation Evaporation is a type of vaporization that occurs on the surface of a liquid as it changes into the gas phase. High concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when h ...
of water from wet items *
Convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conv ...
of air from hot or cold items * Gravitational differences for a scale which measures force, but not for a balance. * Vibration and seismic disturbances


Hybrid spring and balance scales


Elastic arm scale

In 2014 a concept of hybrid scale was introduced, the elastically deformable arm scale, 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 Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a fi ...
of the elastic rod. This mass measuring device can also work without a
counterweight A counterweight is a weight that, by applying an opposite force, provides balance and stability of a mechanical system. The purpose of a counterweight is to make lifting the load faster and more efficient, which saves energy and causes less wear ...
.


See also

* Ampere balance *
Apparent weight In physics, apparent weight is a property of objects that corresponds to how heavy an object is. The apparent weight of an object will differ from the weight of an object whenever the force of gravity acting on the object is not balanced by an equa ...
*
Auncel The auncel ( nrf, auncelle or ',  "little balance") was a balance scale formerly used in England. It consisted of a rod with the fulcrum near one end and a weight that could be moved along the graduated longer arm. The item to be weighed was ...
* Combination weigher * Digital
spoon scale A scale or balance is a device used to measure weight or mass. These are also known as mass scales, weight scales, mass balances, and weight balances. The traditional scale consists of two plates or bowls suspended at equal distances from a ...
* Digital Weight Indicator * Evans balance *
Faraday balance A Faraday balance is a device for measuring magnetic susceptibility. Magnetic susceptibility is related to the force experienced by a substance in a magnetic field. Various practical devices are available for the measurement of susceptibility, wh ...
*
Force gauge A force gauge (also called a force meter) is a measuring instrument used to measure forces. Applications exist in research and development, laboratory, quality, production and field environment. There are two kinds of force gauges today: mechanica ...
*
Gouy balance The Gouy balance, invented by the French physicist Louis Georges Gouy, is a device for measuring the magnetic susceptibility of a sample. Background Amongst a wide range of interest in optics, Brownian motion, and experimental physics, Gouy also ...
*
Kibble balance A Kibble balance is an electromechanical measuring instrument that measures the weight of a test object very precisely by the electric current and voltage needed to produce a compensating force. It is a metrological instrument that can realize ...
, also known as a Watt balance *
Mass versus weight In common usage, the mass of an object is often referred to as its weight, though these are in fact different concepts and quantities. Nevertheless, one object will always weigh more than a second object, if the first object has greater mass, and t ...
* Multihead weigher * Nutrition scale * Roberval balance *
Steelyard balance A steelyard balance, steelyard, or stilyard is a straight-beam balance with arms of unequal length. It incorporates a counterweight which slides along the longer arm to counterbalance the load and indicate its weight. A steelyard is also known as ...
*
Tare weight Tare weight , sometimes called unladen weight, is the weight of an empty vehicle or container. By subtracting tare weight from gross weight ( laden weight), one can determine the weight of the goods carried or contained (the net weight). Etymo ...
* Triple beam balance * Truck scale *
Weigh house A weighhouse or weighing house is a public building at or within which goods are weighed. Most of these buildings were built before 1800, prior to the establishment of international standards for weights, and were often a large and representative ...
- historic public building for the weighing of goods *
Weigh lock 200px, A weigh lock on the Lehigh Canal around 1873 A weigh lock is a specialized canal lock designed to determine the weight of barges in order to assess toll payments based upon the weight and value of the cargo carried. This requires that the ...
- for weighing canal barges *
Weigh station A weigh station is a checkpoint along a highway to inspect vehicular weights and safety compliance criteria. Usually, trucks and commercial vehicles are subject to the inspection. Weigh stations are equipped with truck scales, some of which are ...
, a checkpoint to inspect vehicular weights, usually equipped with a truck scale (weigh bridge)


References


External links

* This a comprehensive review of the history and contemporaneous state of weighing machines. * *
National Conference on Weights and Measures The National Conference on Weights and Measures (NCWM) is a not-for-profit corporation dedicated to developing the United States technical standards for weights and measures in commerce. The organization's official mission is "Ensuring Equity and ...
, NIST Handbook 44,
Specifications, Tolerances, And Other Technical Requirements for Weighing and Measuring Devices
', 2003


"The Precious Necklace Regarding Weigh Scales"
is an 18th-century manuscript by Abd al-Rahman al-Jabarti about the "design and operation" of scales {{DEFAULTSORT:Weighing Scale Ancient Egyptian technology Chinese inventions French inventions Italian inventions Professional symbols Weighing instruments