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Asystem of units of measurement, also known as asystem of units orsystem of measurement, is a collection ofunits of measurement and 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 Units orSI (the modern form of themetric system), theBritish imperial system, and theUnited States customary system.
In antiquity,systems of measurement were 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 inhands andknuckles. The unifying characteristic is that there was some definition based on some standard. Eventuallycubits andstrides gave 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 Revolution gave rise to themetric system, and this has spread around the world, replacing most customary units of measure. In most systems,length (distance),mass, andtime arebase quantities.
Later, science developments showed that an electromagnetic quantity such aselectric charge or electric current could be added to extend the set of base quantities.Gaussian units have only length, mass, and time as base quantities, with no separate electromagnetic dimension. Other quantities, such aspower andspeed, 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.
Metrication is complete or nearly complete in most countries.
However,US customary units remain heavily used in theUnited States and to some degree inLiberia. TraditionalBurmese units of measurement are 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 jure Canadian 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-SI units are still in international use, such asnautical miles andknots in aviation and shipping, andfeet for aircraft altitude.
Metric systems of 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, themetre for length and thegram for 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 (French forcustomary 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'unités or SI). It is a system in which all units can be expressed in terms of seven units. The units that serve as theSI base units are themetre,kilogram,second,ampere,kelvin,mole, andcandela.
BothBritish imperial units andUS customary units derive from earlierEnglish units. Imperial units were mostly used in the formerBritish Empire and 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, andindustrial applications. US customary units, however, are still the main system of measurement in theUnited States. While some steps towardsmetrication have 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. Theavoirdupois units of mass and weight differ for units larger than apound (lb). The British imperial system uses a stone of 14 lb, along hundredweight of 112 lb and along ton of 2,240 lb. Thestone is not a measurement of weight used in the US. The US customary system uses theshort hundredweight of 100 lb andshort ton of 2,000 lb.
Where these systems most notably differ is in their units of volume. An imperial fluid ounce of 28.4130625 ml is 3.924% smaller than the USfluid ounce (fl oz) of 29.5735295625millilitres (ml). However, as there are 16 US fl oz to a USpint and 20 imp fl oz to an imperial pint, the imperial pint is 20.095% larger than a US pint, and the same is true forgills,quarts, andgallons: six US gallons (22.712470704 L) is only 0.08% less than five imperial gallons (22.73045 L).
Theavoirdupois system served as the general system of mass and weight. In addition to this, there are thetroy and 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 areunits of measurement defined in terms of universalphysical constants in 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 ofnature and 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 measurement units also found in books, newspapers etc., include:
A unit of measurement that applies tomoney is called aunit of account in economics and unit of measure in accounting.[5] This is normally acurrency issued by acountry or a fraction thereof; for instance, theUS dollar and US cent (1⁄100 of a dollar), or theeuro and euro cent.
ISO 4217 is 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).
Throughout history, many official systems of measurement have been used. While no longer in official use, some of thesecustomary systems are occasionally used in day-to-day life, for instance incooking.
Still in use:
