Pound (mass)

The pound or pound-mass (abbreviation: lb, lb_m, #) is a unit of mass used in the imperial, United States customary and other systems of measurement. A number of different definitions have been used, the most common today being the international avoirdupois pound of exactly 0.45359237 kilograms.

The unit is descended from the Roman libra (hence the abbreviation lb); the name pound is a Germanic adaptation of the Latin phrase libra pondo 'a pound weight'.^[1]

Usage of the unqualified term pound reflects the historical conflation of mass and weight resulting from the near uniformity of gravity on Earth. This accounts for the modern distinguishing terms pound-mass and pound-force.

[edit] Definitions

Historically, in different parts of the world, at different points in time, and for different applications, the pound (or its translation) has referred to broadly similar but not identical standards of mass or force. (The pound is often described as a unit of weight, and weight can be either mass or force depending on context.)

[edit] British pounds

A number of different definitions of the pound have been used in Britain. Amongst these are the avoirdupois pound and the obsolete tower, merchant's and London pounds.^[2] The weight of precious metals when given in pounds and/or ounces usually assumes Troy pounds and ounces; these units are not otherwise used today.

Historically the pound sterling was a Tower pound of silver (worth about £144 or about $225 US^[3] today). In 1528 the standard was changed to the Troy pound (worth about £60 or $100).

[edit] Avoirdupois pound

The avoirdupois pound was invented by London merchants in 1303. This invention was nearly four hundred years before Sir Isaac Newton defined the word mass to be a property of matter. Originally it was based on independent standards. During the reign of Henry VIII of England, the avoirdupois pound was redefined as 7,000 troy grains.^{[citation needed]} Since then, the grain has often been considered as a part of the avoirdupois system. By 1758, two standard weights for the avoirdupois pound existed, and when measured in troy grains they were found to be of 7,002 grains and 6,999 grains.^[4]

[edit] Imperial Standard Pound

In the United Kingdom, weights and measures have been defined by a long series of Acts of Parliament, the intention of which has been to regulate the sale of commodities. Materials traded in the marketplace must be quantified according to accepted units and standards in order to avoid fraud; the standards themselves must be legally defined so as to facilitate the resolution of disputes brought to the courts; only legally defined measures will be recognised by the courts. Quantifying devices used by traders (weights, weighing machines, containers of volumes, measures of length) are subject to official inspection, and penalties apply if they are fraudulent. The Weights and Measures Act of 1878 marked a major overhaul of the British system, and the definition of the Pound given there remained in force until modern times. The Pound was defined thus (Paragraph 4) ‘The … platinum weight … deposited in the Standards department of the Board of Trade … shall continue to be the imperial standard of ..weight… and the said platinum weight shall continue to be the imperial standard for determining the imperial standard pound for the United Kingdom’. Para 13 states that the weight ‘in vacuo’ of this standard shall be called the imperial standard pound, and that all other weights mentioned in the act and permissible for commerce shall be ascertained from it alone. The First Schedule of the Act gives more details of the standard pound:- It is a platinum cylinder nearly 1.35 inches high, and 1.15 inches diameter, and the edges are carefully rounded off. It has a groove about 0.34 inches from the top, to allow the cylinder to be lifted using an ivory fork. It was constructed following the destruction of the Houses of Parliament by fire in 1834, and is stamped P.S. 1844, 1lb (P.S. stands for 'Parliamentary Standard'). This definition of the imperial pound remains unchanged.

Relationship to the Kilogram. The 1878 Act says that contracts worded in terms of metric units will be deemed by the courts to be made according to the imperial units defined in the Act, and a table of metric equivalents is supplied whereby, in such cases, the imperial equivalents may be legally calculated. This effectively defines, for the UK courts and for commerce, metric units in terms of imperial ones. The equivalence for the pound is given as 1 lb = 453.592 65 g or 0.453 59 kg. which would make the kilogram weigh approximately 2.2046213 pounds. In 1883 it was determined jointly by the Standards Department of the Board of Trade and the Bureau International that 0.453 592 427 7kg was a better approximation, and this figure, rounded to 0.45359243 kg was given legal status by an Order in Council in May 1898.^[5] The Weights and Measures Acts (WMAs) of 1939 and 1958 defined the pound by reference to the WMA of 1878, so as late as 1963 the legal definition of the pound was the same as that given in 1878. However, in the WMA of 1963 the pound was redefined for the first time as a mass (not a weight) equal to 0.453 592 37 kg, and 'For the purposes of any measurement of weight, ... the weight of any thing may be expressed... in the same terms as its mass'. The definition of the Pound mass in terms of the imperial standard pound of 1844 was also ratified. This is its present status in the United Kingdom, the same dimension and value having been ratified in the Weights and Measures Act 1985.

In the United States, the (avoirdupois) pound as a unit of mass has been officially defined in terms of the kilogram since the Mendenhall Order of 1893. In 1893, the relationship was specified to be 2.20462 pounds per kilogram. In 1894, the relationship was specified to be 2.20462234 pounds per kilogram. This change followed a determination of the British pound.^[5]

According to a 1959 NIST publication, the international pound differed from the United States 1894 pound by approximately one part in 10 million.^[6] The difference is so insignificant that it can be ignored for almost all practical purposes.^[7]

[edit] International pound

The United States and countries of the Commonwealth of Nations agreed upon common definitions for the pound and the yard. Since 1 July 1959, the international avoirdupois pound has been defined as exactly 0.45359237 kilogram.^[8]

In the United Kingdom, the use of the international pound was implemented in the Weights and Measures Act 1963.^[9]

The yard or the metre shall be the unit of measurement of length and the pound or the kilogram shall be the unit of measurement of mass by reference to which any measurement involving a measurement of length or mass shall be made in the United Kingdom; and- (a) the yard shall be 0·9144 metre exactly; (b) the pound shall be 0·45359237 kilogram exactly.

—Weights and Measures Act, 1963, Section 1(1)

An avoirdupois pound is equal to 16 avoirdupois ounces and to exactly 7,000 grains. The conversion factor between the kilogram and the international pound was therefore chosen to be divisible by 7, and an (international) grain is thus equal to exactly 64.79891 milligrams.

[edit] Troy pound

The troy pound takes its name from the French market town of Troyes in France where English merchants traded at least as early as the time of Charlemagne (early ninth century). The system of Troy weights was used in England by apothecaries and jewellers.

A troy pound is equal to 12 troy ounces and to 5,760 grains. Today, the grain is common to the avoirdupois and troy systems of units of mass making an international troy pound equal to 373.2417216 grams.

The troy pound is no longer in general use. In Canada, Australia, the United Kingdom, and other places the troy pound is no longer a legal unit for trade (WMA 1878). In the United Kingdom, the use of the troy pound was abolished on 6 January 1879 in accordance with the WMA of 1878, though the troy ounce was retained. The troy ounce is still used for measurements of precious metals such as gold, silver, and platinum, and sometimes gems such as opals.

Most measurements of the mass of precious metals using pounds refer to troy pounds, even though it is not always explicitly stated that this is the case. Some notable exceptions are:

• Encyclopædia Britannica which uses either avoirdupois pounds or troy ounces, likely never both in the same article, and
• the mass of Tutankhamun's sarcophagus lid. This is 110 kilograms. It is often stated to have been 242 or 243 (avoirdupois) pounds but sometimes, much less commonly, it is stated as 296 (troy) pounds.

[edit] Tower pound

The tower system was based on the wheat grain (~50 mg), unlike all the other English systems of weight measurement, which were based on the barley grain (~65 mg). It was the system of measurement used by the Royal Mint until it was abolished in 1527. The tower pound is equivalent to about 350 grams.^[10]

[edit] Merchants' pound

The merchants' pound (mercantile pound, libra mercantoria or commercial pound) was equal to 9,600 wheat grains (15 tower ounces or 6,750 grains). It was used in England until the 14th century for most goods (other than money, spices and electuaries).^[11]

[edit] London pound

A London pound was equal to 7,200 troy grains (16 tower ounces or, equivalently, 15 troy ounces).

[edit] Wool pound

The wool pound was equal to 6,992 grains. It was a unit of mass used to measure the weight of wool.^[12]

[edit] Roman libra

Various historic pounds from a German textbook dated 1848

The libra (Latin for "scales / balance") is an ancient Roman unit of mass that was equivalent to approximately 327 grams. It was divided into 12 uncia, or ounces. The libra is the origin of the abbreviation for pound, lb. The commonly used abbreviation lbs to indicate the plural unit of measurement does not reflect Latin usage, in which lb is both the singular and plural abbreviation.

[edit] French livre

Since the Middle Ages, various pounds (livre) have been used in France. Since the nineteenth century, a livre has referred to the metric pound, 500g.

The livre esterlin was equivalent to about 367.1 grams (5,665 gr) and was used between the late ninth and the mid-fourteenth centuries.^[13]

The livre poids de marc or livre de Paris was equivalent to about 489.5 grams (7,555 gr) and was used between the 1350s and the late 18th century.^[13] It was introduced by the government of John II.

The livre métrique was set equal to the kilogram by the decree of 13 Brumaire an IX between 1800 and 1812. This was a form of official metric pound.^[13]

The livre usuelle was defined as 500 grams, by the decree of 28 March 1812. It was abolished as a unit of mass effective 1 January 1840 by a decree of 4 July 1837,^[13] but is still used informally.

[edit] German Pfund

Originally derived from the Roman libra, the definition varied throughout Germany in the Middle Ages and onward. While a Pfund might equal 510 grams in Nuremberg, it was only 467 grams in Berlin. In 1854 the German Customs Union decided that a Pfund must equal 500 grams in all of Germany. The Pfund is no longer an official measurement in Germany, but is still quite often used in everyday speech when buying food to describe half a kg.

[edit] Russian funt

The Russian pound (Фунт, funt) is an obsolete Russian unit of measurement of mass. It is equal to 409.51718 grams.^[14]

[edit] Skålpund

A Scandinavian measurement which varied in weight between regions. From the 17th century onward it was equal to 425.076 grams in Sweden. It was abandoned in 1889 when Sweden switched to the metric system. In Norway the same name was used for a weight of 498.1 grams, and in Denmark it equalled 471 grams. In the 19th century Denmark followed Germany's lead and redefined the pound as 500 grams.

20 skålpund = 1 lispund

[edit] Jersey pound

A Jersey pound is an obsolete unit of mass used on the island of Jersey from the 14th century to the 19th century. It was equivalent to about 7,561 grains (490 grams). It may have been derived from the French livre poids de marc.^[15]

[edit] Trone pound

The trone pound is one of a number of obsolete Scottish units of measurement. It was equivalent to between 21 and 28 avoirdupois ounces (about 600-800 grams).

[edit] Metric pounds

In many countries upon the introduction of a metric system, the pound (or its translation) became an informal term for 500 grams (half a kilogram, similar to the metric pint, being half a litre), often following an official redefinition of an existing unit during the 19th century.

The Dutch pond is an exception. It was officially redefined as 1 kilogram, with an ounce of 100 grams, but people seldom use it this way. In daily life pond is exclusively used for amounts of 500 grams, and to a lesser extent, ons for 100 grams.

In German the term is Pfund, in French livre, in Dutch pond, in Spanish and Portuguese libra, in Italian libbra, and in Danish and Swedish pund.

Though not from the same linguistic origin, the Chinese jin (also known a "catty") has a modern definition of exactly 500 grams, divided into ten cun. Traditionally about 605 grams, the jin has been in use for more than two thousand years, serving the same purpose as "pound" for the common-use measure of weight.

Hundreds of older pounds were replaced in this way. Examples of the older pounds are one of around 459 to 460 grams in Spain, Portugal, and Latin America; one of 498.1 grams in Norway; and several different ones in what is now Germany.

Although the use of the pound as an informal term persists in these countries to a varying degree, scales and measuring devices are denominated only in grams and kilograms. A pound of product must be determined by weighing the product in grams. The use of the term pound is usually forbidden for official use in trade.

[edit] Use in commerce

In the United States of America the United States Department of Commerce, the Technology Administration, and the National Institute of Standards and Technology (NIST) have defined the use of mass and weight in the exchange of goods under the Uniform Laws and Regulations in the areas of legal metrology and engine fuel quality in NIST Handbook 130.

NIST Handbook 130 states:

V. "Mass" and "Weight." [NOTE 1, See page 6]

The mass of an object is a measure of the object’s inertial property, or the amount of matter it contains. The weight of an object is a measure of the force exerted on the object by gravity, or the force needed to support it. The pull of gravity on the earth gives an object a downward acceleration of about 9.8 m/s². In trade and commerce and everyday use, the term "weight" is often used as a synonym for "mass." The "net mass" or "net weight" declared on a label indicates that the package contains a specific amount of commodity exclusive of wrapping materials. The use of the term "mass" is predominant throughout the world, and is becoming increasingly common in the United States. (Added 1993)

W. Use of the Terms "Mass" and "Weight." [NOTE 1, See page 6]

When used in this handbook, the term "weight" means "mass". The term "weight" appears when inch-pound units are cited, or when both inch-pound and SI units are included in a requirement. The terms "mass" or "masses" are used when only SI units are cited in a requirement. The following note appears where the term "weight" is first used in a law or regulation.

NOTE 1: When used in this law (or regulation), the term "weight" means "mass." (See paragraph V. and W. in Section I., Introduction, of NIST Handbook 130 for an explanation of these terms.) (Added 1993) 6"

U.S. federal law, which supersedes this handbook, also defines weight, particularly Net Weight, in terms of the avoirdupois pound or mass pound. From 21CFR101 Part 101.105 – Declaration of net quantity of contents when exempt:

(a) The principal display panel of a food in package form shall bear a declaration of the net quantity of contents. This shall be expressed in the terms of weight, measure, numerical count, or a combination of numerical count and weight or measure. The statement shall be in terms of fluid measure if the food is liquid, or in terms of weight if the food is solid, semisolid, or viscous, or a mixture of solid and liquid; except that such statement may be in terms of dry measure if the food is a fresh fruit, fresh vegetable, or other dry commodity that is customarily sold by dry measure. If there is a firmly established general consumer usage and trade custom of declaring the contents of a liquid by weight, or a solid, semisolid, or viscous product by fluid measure, it may be used. Whenever the Commissioner determines that an existing practice of declaring net quantity of contents by weight, measure, numerical count, or a combination in the case of a specific packaged food does not facilitate value comparisons by consumers and offers opportunity for consumer confusion, he will by regulation designate the appropriate term or terms to be used for such commodity.

(b)(1) Statements of weight shall be in terms of avoirdupois pound and ounce.

See also 21CFR201 Part 201.51 – "Declaration of net quantity of contents" for general labeling and prescription labeling requirements.

From paragraph "a" above, although the avoirdupois pound is a measure of mass, in commerce it is used with the term "Net Weight", because "there is a firmly established general consumer usage and trade custom of declaring the contents of a liquid by weight, or a solid..."

[edit] Weight, Mass, and Force

‘Weight’ in Commerce and Law. As mentioned above, UK Weights and Measures Acts have traditionally defined the pound as a ‘weight’. The meaning of this word, like that of many others in our language, has changed over the centuries, and in modern usage has become ambiguous. For commercial purposes, the word 'weight' refers to commonly used techniques for quantifying materials for trade, namely weighing, either with a yard-arm or other type of beam-balance, or with a spring-balance; some of these methods of quantifying traded commodities by weight have been in use for millennia. The various Acts provide a means whereby these techniques can be regulated. The use of a standard weight is an essential part of this regulation. Nowadays this item is the standard kilogram and its various substandards. Historically, the function of providing a standard for checking the accuracy of other weights or of weighing machines was provided by the Imperial Pound - a specific lump of metal preserved in a government office. It was regarded as a prototype 'weight', from which all others of its kind (pound weights, and fractions and multiples thereof) were legally derived. When necessary, other weights could be checked against the prototype and their accuracy determined. For all of these purposes, the term ‘weight’ has perfectly clear meaning, both as a specific tool used during weighing, and also as an attribute of an object – the quantity or measurement according to which a commodity could be traded.

‘Weight’, ‘Mass’ and ‘Force’ in Scientific Language. Since the time of Newton (and possibly before), the scientific community has found it necessary to clarify its thinking about ‘weight’, and it has become commonplace to give the term a specific meaning in a scientific or technical context. Newtonian mechanics makes a clear distinction between force and mass; the former is that which is required to make the motion of an object change, or to deviate from a straight line, and can be experienced as a push or a pull, while the latter is a way of quantifying the amount of matter present in a given object. ‘Mass’ therefore seems to be close to the meaning of ‘weight’ as discussed above (e.g. the 1963 WMA), and in the chemistry laboratory to 'weigh' an object is to determine its mass. However, language is not as simple as that. We live on the surface of the earth, and therefore in an environment where gravity has a profound effect on the behaviour of matter. Objects having mass experience a gravitational force which makes it difficult, in varying degrees (proportional to their mass), to lift them away from the surface of the earth. In common speech, this difficulty is called the weight of an object, and it is experienced in practice as a push or a pull. So ‘weight’ has been interpreted to mean ‘force’, and that is the accepted meaning of the word in technical language. Specifically, weight is the gravitational force which an object having mass exerts. Understood in this way, the weight of an object whose mass is fixed varies from place to place on the surface of the earth; this is because the force or acceleration of gravity (symbolised as g) is about 0.5% less at the equator than it is at the poles. This small difference can easily be detected by even quite simple weighing machines of the spring-balance type, and for much scientific work is far too large to be overlooked. The apparent weight (downward force) of a mass is also reduced by a buoyancy effect if the force is measured when the mass is surrounded by air. The downward force is reduced by an amount equal to the weight of the air displaced. This makes the apparent weight of a mass dependent on the density of the air, and thus on its temperature and pressure. Clearly the apparent weight of equal masses of different densities will also differ. The more dense mass will displace less air and will seem to have greater weight. This reduction in weight is only ‘apparent’ because weight has been defined as the gravitational force on the mass, and this does not depend on the surrounding air. But buoyancy provides an upward force which operates in opposition to weight, and makes it seem less in practice. The apparent weight of a fixed mass therefore depends on atmospheric pressure and temperature, on the density of the mass, and on the latitude at which the weight is measured; the weight proper varies with latitude alone. Away from the surface of the earth, weight depends on the local acceleration due to gravity (g), and on the acceleration of the local frame of reference.

Clearly, legally defined units of ‘weight’ can be used to specify either force or mass. Although modern laws describe such weights as masses, a degree of uncertainty remains in some minds, possibly as a consequence of the historical development of scientific language in different cultures or professional groups. To accommodate this diversity, while avoiding ambiguity, the technical community has recognised two usages of the Pound – either as a unit of mass (lb-mass) or of force (lb-wt or lb-f); various ways of symbolising these units have been in use. When the symbol lb was used, its meaning was deduced from the context. Thus lb/inch^2 was universally recognised as an abbreviation for lb-wt/inch^2 since pressure or stress is a force per unit area. Difficulties arise when units are mixed. Thus an equation for the thrust (force) created by a rocket can be expressed as: F = v dm/t + pA

Where v is the speed of the exhaust gas (feet/sec), dm/dt is the rate of burning (lb-m/sec), p is the average pressure of the exhaust gas (lb-wt/inch^2), and A is the projected area of the exhaust cone.

In this example, the pound unit has two different meanings, and values expressed in these units cannot be combined in the same equation.

To remedy this kind of problem, the pound has been incorporated into two coherent systems of units which are analogous to the cgs and International systems (SI) in the sense that mass and force units are given different names, and are related to each other in a rational way which avoids arbitrary constants.

In the foot-pound-second system (fps) the pound was considered to be a primary unit of mass, while the foot and second were primary units of length and time. In this system the derived unit of force was the poundal, defined as the force required to accelerate 1 pound mass at the rate of 1 foot/sec^2. This is 1/32.2 (i.e. 1/g) lb-wt. This system is analogous to both the cgs and SI systems in which the dyne and Newton are derived units of force.

In an alternative system, the pound was a primary unit of force. Then the derived unit of mass was that mass which will be accelerated at a rate of 1 foot/sec^2 by a force of 1 pound force. This mass is 32.2 (i.e. g) lb-m and was called a slug. The dependence of weight on environmental conditions makes the use of the pound (or any other weight) as a primary unit of force problematic, so this system is less attractive.

[edit] Use in weaponry

Smoothbore cannon and carronades are designated by the weight in Imperial pounds of round solid iron shot of diameter to fit the barrel. Standard sizes are 6, 12, 18, 24, 32 and 42 pounds, with some 68 pound weapons, and other nonstandard weapons using the same scheme. See Carronade#Ordnance.

[edit] Notes

[edit] External links

Look up pound in Wiktionary, the free dictionary.

[edit] Conversion between units

• Yahoo Conversion Calculator.
• U.S. National Institute of Standards and Technology Special Publication 811
• National Institute of Standards and Technology Handbook 130