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System of units of measurement

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Asystem of units of measurement,also known as asystem of unitsorsystem of measurement,is a collection ofunits of measurementand rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science andcommerce.Instances in use include theInternational System of UnitsorSI(the modern form of themetric system), theBritish imperial system,and theUnited States customary system.

History

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See also:Units of measurement § History
Further information:History of measurement

In antiquity,systems of measurementwere defined locally: the different units might be defined independently according to the length of a king's thumb or the size of his foot, the length of stride, the length of arm, or maybe the weight of water in a keg of specific size, perhaps itself defined inhandsandknuckles.The unifying characteristic is that there was some definition based on some standard. Eventuallycubitsandstridesgave way to "customary units" to meet the needs of merchants and scientists.

The preference for a more universal and consistent system only gradually spread with the growth of international trade and science. Changing a measurement system has costs in the near term, which often results in resistance to such a change. The substantial benefit of conversion to a more rational and internationally consistent system of measurement has been recognized and promoted by scientists, engineers, businesses and politicians, and has resulted in most of the world adopting a commonly agreed metric system.

TheFrench Revolutiongave rise to themetric system,and this has spread around the world, replacing most customary units of measure. In most systems,length(distance),mass,andtimearebase quantities.

Later, science developments showed that an electromagnetic quantity such aselectric chargeor electric current could be added to extend the set of base quantities.Gaussian unitshave only length, mass, and time as base quantities, with no separate electromagnetic dimension. Other quantities, such aspowerandspeed,are derived from the base quantities: for example, speed is distance per unit time. Historically, a wide range of units was used for the same type of quantity. In different contexts length was measured ininches,feet,yards,fathoms,rods,chains,furlongs,miles,nautical miles,stadia,leagues,with conversion factors that were not based on power of ten.

In the metric system and other recent systems, underlying relationships between quantities, as expressed by formulae of physics such asNewton's laws of motion,is used to select a small number of base quantities for which a unit is defined for each, from which all other units may be derived. Secondary units (multiples and submultiples) are derived from these base and derived units by multiplying by powers of ten. For example, where the unit of length is themetre;a distance of 1 metre is 1,000 millimetres, or 0.001 kilometres.

Current practice

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Main article:Metrication

Metrication is complete or nearly complete in most countries.

However,US customary unitsremain heavily used in theUnited Statesand to some degree inLiberia.TraditionalBurmese units of measurementare used inBurma,with partial transition to the metric system. U.S. units are used in limited contexts in Canada due to the large volume of trade with the U.S. There is also considerable use of imperial weights and measures, despitede jureCanadian conversion to metric.

A number of other jurisdictions have laws mandating or permitting other systems of measurement in some or all contexts, such as the United Kingdom whoseroad signage legislation,for instance, only allows distance signs displayingimperial units(miles or yards)[1]or Hong Kong.[2]

In the United States, metric units are virtually always used in science, frequently in the military, and partially in industry. U.S. customary units are primarily used in U.S. households. At retail stores, the litre (spelled 'liter' in the U.S.) is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather thangrains,are used for medications.

Some other non-SIunits are still in international use, such asnautical milesandknotsin aviation and shipping, andfeetfor aircraft altitude.

Metric system

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Main articles:Metric systemandInternational System of Units
A baby bottle that measures in three measurement systems—metric, imperial (UK), and US customary

Metric systemsof units have evolved since the adoption of the first well-defined system in France in 1795. During this evolution the use of these systems has spread throughout the world, first to non-English-speaking countries, and then to English speaking countries.

Multiples and submultiples of metric units are related by powers of ten and their names are formed withprefixes.This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units.

In the early metric system there were two base units, themetrefor length and thegramfor mass. The other units of length and mass, and all units of area, volume, and derived units such as density were derived from these two base units.

Mesures usuelles(Frenchforcustomary measures) were a system of measurement introduced as a compromise between the metric system and traditional measurements. It was used in France from 1812 to 1839.

A number of variations on the metric system have been in use. These includegravitational systems,thecentimetre–gram–second systems(cgs) useful in science, themetre–tonne–second system(mts) once used in the USSR and themetre–kilogram–second system(mks). In some engineering fields, likecomputer-aided design,millimetre–gram–second (mmgs) is also used.[3]

The current international standard for the metric system is theInternational System of Units(Système international d'unitésor SI). It is a system in which all units can be expressed in terms of seven units. The units that serve as theSI base unitsare themetre,kilogram,second,ampere,kelvin,mole,andcandela.

British imperial and US customary units

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Main articles:Imperial and US customary measurement systems,Imperial units,andUS customary units

BothBritish imperial unitsandUS customary unitsderive from earlierEnglish units.Imperial units were mostly used in the formerBritish Empireand theBritish Commonwealth,but in all these countries they have been largely supplanted by the metric system. They are still used for some applications in the United Kingdom but have been mostly replaced by the metric system incommercial,scientific,andindustrialapplications. US customary units, however, are still the main system of measurement in theUnited States.While some steps towardsmetricationhave been made (mainly in the late 1960s and early 1970s), the customary units have a strong hold due to the vast industrial infrastructure and commercial development.

While British imperial and US customary systems are closely related, there are a number ofdifferences between them.Units of length and area (theinch,foot,yard,mile,etc.) have been identical since the adoption of theInternational Yard and Pound Agreement;however, the US and, formerly, India retained older definitions for surveying purposes. This gave rise to the USsurvey foot,for instance. Theavoirdupoisunits of mass and weight differ for units larger than apound(lb). The British imperial system uses a stone of 14 lb, along hundredweightof 112 lb and along tonof 2,240 lb. Thestoneis not a measurement of weight used in the US. The US customary system uses theshort hundredweightof 100 lb andshort tonof 2,000 lb.

Where these systems most notably differ is in their units of volume. A USfluid ounce(fl oz), about 29.6millilitres(ml), is slightly larger than the imperial fluid ounce (about 28.4 ml). However, as there are 16 US fl oz to a USpintand 20 imp fl oz per imperial pint, the imperial pint is about 20% larger. The same is true ofquarts,gallons,etc.; six US gallons are a little less than five imperial gallons.

Theavoirdupoissystem served as the general system of mass and weight. In addition to this, there are thetroyand theapothecaries' systems.Troy weight was customarily used forprecious metals,black powder,andgemstones.The troy ounce is the only unit of the system in current use; it is used for precious metals. Although the troy ounce is larger than its avoirdupois equivalent, the pound is smaller. The obsolete troy pound was divided into 12 ounces, rather than the 16 ounces per pound of the avoirdupois system. The apothecaries' system was traditionally used inpharmacology,but has now been replaced by the metric system; it shared the same pound and ounce as the troy system but with different further subdivisions.

Natural units

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Natural unitsareunits of measurementdefined in terms of universalphysical constantsin such a manner that selected physical constants take on the numerical value of one when expressed in terms of those units. Natural units are so named because their definition relies on only properties ofnatureand not on any human construct. Varying systems of natural units are possible, depending on the choice of constants used.

Some examples are as follows:

Non-standard units

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Non-standard measurement unitsalso found in books, newspapers etc., include:

Area

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  • TheAmerican football field,which has a playing area 100yards(91.4 m) long by 160 feet (48.8 m) wide. This is often used by theAmericanpublic media for the sizes of large buildings or parks. It is used both as a unit of length (100 yd or 91.4 m, the length of the playing field excluding goal areas) and as a unit of area (57,600 sq ft or 5,350 m2), about 1.32acres(0.53ha).
  • British media also frequently uses thefootball pitchfor equivalent purposes, althoughsoccerpitches are not of a fixed size, but instead can vary within defined limits (100–130 yd or 91.4–118.9 m long, and 50–100 yd or 45.7–91.4 m wide, giving an area of 5,000 to 13,000 sq yd or 4,181 to 10,870 m2). However theUEFA Champions Leaguefield must be exactly 105 by 68 m (114.83 by 74.37 yd) giving an area of 7,140 m2(0.714 ha) or 8,539sq yd(1.764acres). For example, "HSS vesselsare aluminium catamaransabout the size of a football pitch."[4]
  • Larger areas are also expressed as a multiple of the areas of states and countries understood to be familiar to the reader.

Energy

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Units of currency

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A unit of measurement that applies tomoneyis called aunit of accountin economics and unit of measure in accounting.[5]This is normally acurrencyissued by acountryor a fraction thereof; for instance, theUS dollarand US cent (1⁄100of a dollar), or theeuroand euro cent.

ISO 4217is the international standard describing three letter codes (also known as the currency code) to define the names of currencies established by the International Organization for Standardization (ISO).

Historical systems of measurement

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Main article:History of measurement

Throughout history, many official systems of measurement have been used. While no longer in official use, some of thesecustomary systemsare occasionally used in day-to-day life, for instance incooking.

Africa

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Asia

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See also:History of measurement systems in India

Still in use:

Europe

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North America

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South America

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Ancient

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See also

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Notes and references

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  1. ^"Statutory Instrument 2002 No. 3113 The Traffic Signs Regulations and General Directions 2002".Her Majesty's Stationery Office (HMSO). 2002.Retrieved18 March2010.
  2. ^HK Weights and Measures Ordinance
  3. ^"Units and Dimension Standard - 2021 - SOLIDWORKS Help".help.solidworks.Retrieved2024-01-11.
  4. ^Henry, Lesley-Anne (23 June 2007)."Sad sight of a superferry laid up due to soaring jet fuel bills".TheBelfast Telegraph.Archived fromthe originalon 2008-10-06.
  5. ^Financial Accounting Standards Research Initiative: The Unit of Account Issue[permanent dead link]
  6. ^M. Ismail Marcinkowski,Measures and Weights in the Islamic World. An English Translation of Professor Walther Hinz's Handbook "Islamische Maße und Gewichte",with a foreword by Professor Bosworth, F.B.A. Kuala Lumpur, ISTAC, 2002,ISBN983-9379-27-5.This work is an annotated translation of a work in German by the late German orientalist Walther Hinz, published in theHandbuch der Orientalistik,erste Abteilung, Ergänzungsband I, Heft 1, Leiden, The Netherlands: E. J. Brill, 1970.

Bibliography

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Wikimedia Commons has media related toCustomary units of measurement.
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