A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce. Systems of measurement in use include the International System of Units (SI), the modern form of the metric system, the imperial system, and United States customary units.
The French Revolution gave rise to the metric system, and this has spread around the world, replacing most customary units of measure. In most systems, length (distance), mass, and time are base quantities.
Later science developments showed that either electric charge or electric current could be added to extend the set of base quantities by which many other metrological units could be easily defined. (However, electrical units are not necessary for such a set. Gaussian units, for example, have only length, mass, and time as base quantities, and the ampere is defined in terms of other units.) Other quantities, such as power and speed, are derived from the base set: 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 in inches, feet, yards, fathoms, rods, chains, furlongs, miles, nautical miles, stadia, leagues, with conversion factors which were not powers of ten. Such arrangements were satisfactory in their own contexts.
The preference for a more universal and consistent system (based on more rational base units) only gradually spread with the growth of science. Changing a measurement system has substantial financial and cultural costs which must be offset against the advantages to be obtained from using a more rational system. However pressure built up, including from scientists and engineers for conversion to a more rational, and also internationally consistent, basis of measurement.
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 in hands and knuckles. The unifying characteristic is that there was some definition based on some standard. Eventually cubits and strides gave way to "customary units" to meet the needs of merchants and scientists.
In the metric system and other recent systems, a single basic unit is used for each base quantity. Often secondary units (multiples and submultiples) are derived from the basic units by multiplying by powers of ten, i.e. by simply moving the decimal point. Thus the basic metric unit of length is the metre; a distance of 1 m is 1,000 millimetres, or 0.001 kilometres.
Metrication is complete or nearly complete in almost all countries. US customary units are heavily used in the United States and to some degree in Liberia. Traditional Burmese units of measurement are used in Burma. U.S. units are used in limited contexts in Canada due to the large volume of trade; there is also considerable use of Imperial weights and measures, despite de 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 – whose road signage legislation, for instance, only allows distance signs displaying imperial units (miles or yards) – or Hong Kong.
In the United States, metric units are used almost universally in science, widely in the military, and partially in industry, but customary units predominate in household use. At retail stores, the liter is a commonly used unit for volume, especially on bottles of beverages, and milligrams, rather than grains, are used for medications. Some other standard non-SI units are still in international use, such as nautical miles and knots in aviation and shipping.
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 with prefixes. 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, the metre for length and the gram 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 for customary measurements) 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 include gravitational systems, the centimetre–gram–second systems (cgs) useful in science, the metre–tonne–second system (mts) once used in the USSR and the metre–kilogram–second system (mks).
The current international standard metric system is the International System of Units (Système international d'unités or SI) It is an mks system based on the metre, kilogram and second as well as the kelvin, ampere, candela, and mole.
The SI includes two classes of units which are defined and agreed internationally. The first of these classes includes the seven SI base units for length, mass, time, temperature, electric current, luminous intensity and amount of substance. The second class consists of the SI derived units. These derived units are defined in terms of the seven base units. All other quantities (e.g. work, force, power) are expressed in terms of SI derived units.
Both imperial units and US customary units derive from earlier English units. Imperial units were mostly used in the former British Empire and the British 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 in commercial, scientific, and industrial applications. US customary units, however, are still the main system of measurement in the United States. While some steps towards metrication 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 imperial and US customary systems are closely related, there are a number of differences between them. Units of length and area (the inch, foot, yard, mile etc.) are identical except for surveying purposes. The Avoirdupois units of mass and weight differ for units larger than a pound (lb). The imperial system uses a stone of 14 lb, a long hundredweight of 112 lb and a long ton of 2240 lb. The stone is not used in the US and the hundredweights and tons are short: 100 lb and 2000 lb respectively.
Where these systems most notably differ is in their units of volume. A US fluid ounce (fl oz), about 29.6 millilitres (ml), is slightly larger than the imperial fluid ounce (about 28.4 ml). However, as there are 16 US fl oz to a US pint and 20 imp fl oz per imperial pint, the imperial pint is about 20% larger. The same is true of quarts, gallons, etc. Six US gallons are a little less than five imperial gallons.
The Avoirdupois system served as the general system of mass and weight. In addition to this there are the Troy and the Apothecaries' systems. Troy weight was customarily used for precious metals, black powder and gemstones. 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 in pharmacology, 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 are physical units of measurement defined in terms of universal physical 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 of nature and not on any human construct. Various systems of natural units are possible.
Some other examples are as follows:
Non-standard measurement units, sometimes found in books, newspapers etc., include:
A unit of measurement that applies to money is called a unit of account in economics and unit of measure in accounting. This is normally a currency issued by a country or a fraction thereof; for instance, the US dollar and US cent (1⁄100 of a dollar), or the euro 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 these customary systems are occasionally used in day-to-day life, for instance in cooking.
An absolute scale is a system of measurement that begins at a minimum, or zero point, and progresses in only one direction. An absolute scale differs from an arbitrary, or "relative," scale, which begins at some point selected by a person and can progress in both directions. An absolute scale begins at a natural minimum, leaving only one direction in which to progress.
An absolute scale can only be applied to measurements in which a true minimum is known to exist. Time, for example, which does not have a clearly known beginning, is measured on a relative scale, with an arbitrary zero-point such as the conventional date of the birth of Jesus of Nazareth or the accession of an emperor. Temperature, on the other hand, has a known minimum, absolute zero (where all vibrational motion of atoms ceases), and therefore, can be measured either in absolute terms (kelvins or degrees Rankine), or relative to a reference temperature such as the freezing point of water at a specified pressure (Celsius and Reaumur) or the lowest temperature attainable in 1724 (Fahrenheit).
Pressure is a force that can be measured absolutely, because the natural minimum of pressure is total vacuum. Pressure is frequently measured with reference to atmospheric pressure rather than on any absolute scale, relative to complete and perfect vacuum; it is technologically simpler and cheaper. It may also be more convenient to use relative scales, because, with things like pneumatics and hydraulics, the amount of energy transferred is reduced by the relative "backpressure" of the atmosphere. (e.g.: 15 psi of air in a tank at sea level will become 30 psi in the vacuum of space.) Therefore, with measurements of things like blood pressure or tire pressure, a measurement relative to air pressure is a better indication of "burst pressure" (damage threshold) than an absolute scale. Absolute scales are typically used in science, deep vacuum measurements (where the fluctuating pressure of the atmosphere becomes a nuisance), aeronautics (where precise measurements of the atmosphere are needed to determine altitude), or lighting construction (where the relative pressure of the atmosphere is inconsequential), and are measured in units of "atmospheres" or torr. Barometers do measure absolute pressure by holding a vacuum at the top of the mercury column or one side of a diaphragm, but that vacuum is awkward to achieve and maintain. Thus, while the general public may be familiar with measurements of absolute pressure from weather forecasts, most pressures such as tire pressures and water pressures are measured relative to atmospheric pressure using cheaper and simpler pressure gauges. For this reason, the pressure relative to atmospheric pressure is called gauge pressure and measurements given in units like pounds per square inch (abbreviated lbf/in2 or psi) are often shown as psig (the "g" standing for gauge) or psia ("a" for absolute).
Absolute scales are used when precise values are needed in comparison to a natural, unchanging zero point. Measurements of length, area and volume are inherently absolute, although measurements of distance are often based on an arbitrary starting point. Measurements of weight can be absolute, such as atomic weight, but more often they are measurements of the relationship between two masses, while measurements of speed are relative to an arbitrary reference frame. (Unlike many other measurements without a known, absolute minimum, speed has a known maximum and can be measured from a purely relative scale.) Absolute scales can be used for measuring a variety of things, from the flatness of an optical flat to neuroscientific tests.Astronomical system of units
The astronomical system of units, formally called the IAU (1976) System of Astronomical Constants, is a system of measurement developed for use in astronomy. It was adopted by the International Astronomical Union (IAU) in 1976, and has been significantly updated in 1994 and 2009 (see astronomical constant).
The system was developed because of the difficulties in measuring and expressing astronomical data in International System of Units (SI units). In particular, there is a huge quantity of very precise data relating to the positions of objects within the Solar System which cannot conveniently be expressed or processed in SI units. Through a number of modifications, the astronomical system of units now explicitly recognizes the consequences of general relativity, which is a necessary addition to the International System of Units in order to accurately treat astronomical data.
The astronomical system of units is a tridimensional system, in that it defines units of length, mass and time. The associated astronomical constants also fix the different frames of reference that are needed to report observations. The system is a conventional system, in that neither the unit of length nor the unit of mass are true physical constants, and there are at least three different measures of time.English Engineering units
Some fields of engineering in the United States use a system of measurement of physical quantities known as the English Engineering units. The system is based on English units of measure.Imperial and US customary measurement systems
The imperial system of measurement and the US customary system of measurement are both derived from an earlier English system of measurement which in turn can be traced back to Ancient Roman units of measurement, and Carolingian and Saxon units of measure.
The US Customary system of units was developed and used in the United States after the American Revolution, based on a subset of the English units used in the Thirteen Colonies. The Imperial system of units was developed and used in the United Kingdom and its empire beginning in 1826. US Customary units are the predominant system of units in the United States and the metric system has, to varying degrees, replaced the imperial system in the countries that used it.
Most of the units of measure have been adapted in one way or another since the Norman Conquest (1066). The units of linear measure have changed the least – the yard (which replaced the ell) and the chain were measures derived in England. The foot used by craftsman supplanted the longer foot used in agriculture. The agricultural foot was reduced to 10⁄11 of its former size, causing the rod, pole or perch to become 16 1⁄2 (rather than the older 15) agricultural feet. The furlong and the acre, once it became a measure of the size of a piece of land rather than its value, remained relatively unchanged. In the last thousand years, three principal pounds were used in England. The troy pound (5760 grains) was used for precious metals, the apothecaries' pound, (also 5760 grains) was used by pharmacists and the avoirdupois pound (7000 grains) was used for general purposes. The apothecaries and troy pounds are divided into 12 ounces (of 480 grains) while the avoirdupois pound has 16 ounces (of 437.5 grains). The unit of volume, the gallon, has different values in the United States and in the United Kingdom – the US fluid gallon being about 0.83 imperial gallons and the US dry gallon being about 0.97 imperial gallons.
Both systems of measure were widely used in mechanical engineering, though not in electrical engineering. Some units of measure such as the horsepower or the British thermal unit (BTU) have special names but by and large unit names are generated from their constituent components – for example, pounds per square inch. In contrast, the metric system has a special name for pressure—the pascal.
After the United States Declaration of Independence the units of measurement in the United States developed into what is now known as customary units. The United Kingdom overhauled its system of measurement in 1826, when it introduced the imperial system of units. This resulted in the two countries having different gallons. Later in the century, efforts were made to align the definition of the pound and the yard in the two countries by using copies of the standards adopted by the British Parliament in 1855. However, these standards were of poor quality compared with those produced for the Convention of the Metre. In 1960 the two countries agreed to common definitions of the yard and the pound based on definitions of the metre and the kilogram. This change, which amounted to a few parts per million, had little effect in the United Kingdom, but resulted in the United States having two slightly different systems of linear measure – the international system, and the surveyors system.Imperial units
The system of imperial units or the imperial system (also known as British Imperial or Exchequer Standards of 1825) is the system of units first defined in the British Weights and Measures Act of 1824, which was later refined and reduced. The Imperial units replaced the Winchester Standards, which were in effect from 1588 to 1825. The system came into official use across the British Empire. By the late 20th century, most nations of the former empire had officially adopted the metric system as their main system of measurement, although some imperial units are still used in the United Kingdom, Canada and other countries formerly part of the British Empire. The imperial system developed from what were first known as English units, as did the related system of United States customary units.International Terrestrial Reference System and Frame
The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of that standard. The ITRS defines a geocentric system of coordinates using the SI system of measurement.
An International Terrestrial Reference Frame (ITRF) is a realization of the ITRS. New ITRF solutions are produced every few years, using the latest mathematical and surveying techniques to attempt to realize the ITRS as precisely as possible. Due to experimental error, any given ITRF will differ very slightly from any other realization of the ITRF. The difference between the latest WGS 84 and the latest ITRF is only a few centimeters.Practical navigation systems are in general referenced to a specific ITRF solution, or to their own coordinate systems which are then referenced to an ITRF solution.
The ITRS and ITRF solutions are maintained by the International Earth Rotation and Reference Systems Service (IERS).
The Galileo Terrestrial Reference Frame (GTRF) is used for the Galileo navigation system; currently defined as ITRF2005.
GTRF is defined by the European Space Agency (ESA).Luxembourgian units of measurement
Metric System has been compulsory as the system of measurement in Luxembourg since 1820.MKS system of units
The MKS system of units is a physical system of measurement that uses the metre, kilogram, and second (MKS) as base units.
Adopted in 1889, use of the MKS system of units succeeded the centimetre–gram–second system of units (CGS) in commerce and engineering. The metre and kilogram system served as the basis for the development of the International System of Units (abbreviated SI), which now serves as the international standard. Because of this, the standards of the CGS system were gradually replaced with metric standards incorporated from the MKS system.An advantage of MKS units is that since the derived units (like joules) are based on MKS units, using MKS units naturally gives answers in the appropriate SI unit. For example, the kinetic energy of an object is defined by 1/2 × mass × velocity2. If the calculation is done in MKS units of kilogram and meters per second then the result is in joules, the SI unit for energy. If the same calculation is done using CGS units the answer is in ergs (1 erg = 10−7 joules).
The exact list of units used in the MKS system changed over time. It incorporated base units other than the metre, kilogram, and second in addition to derived units. An incomplete list of the base and derived units appears below. Since the MKS system of units never had a governing body to rule on a standard definition, the list of units depended on different conventions at different times.
Cycle (This dimensionless quantity became synonymous with the term "cycle per second" as an abbreviation. This circumstance confused the exact definition of the term cycle. Therefore, the phrase "cycle per metre" became ill-defined. The cycle did not become an SI unit.)
Cycle per second
Cycle per metre (This measure of wavenumber became ill-defined due to the abbreviation of "cycle per second" as "cycle".)In 1901, Giovanni Giorgi proposed to the Associazione elettrotecnica italiana (AEI) that this system, extended with a fourth unit to be taken from the units of electromagnetism, be used as an international system.
This system was strongly promoted by electrical engineer George A. Campbell.Mesures usuelles
Mesures usuelles (French pronunciation: [məzyʁ yzɥɛl], customary measurements) were a system of measurement introduced by Napoleon I in 1812 to act as compromise between the metric system and traditional measurements. The system was restricted to use in the retail industry and continued in use until 1839.Metric
Metric or metrical may refer to:
Metric system, an internationally adopted decimal system of measurement
Metric (unit), a measure for quantitatively assessing, controlling or selecting a person, process, event, or institutionMetric system
The metric system is an internationally recognised decimalised system of measurement. It is in widespread use, and where it is adopted, it is the only or most common system of weights and measures (see metrication). It is now known as the International System of Units (SI). It is used to measure everyday things such as the mass of a sack of flour, the height of a person, the speed of a car, and the volume of fuel in its tank. It is also used in science, industry and trade.
In its modern form, it consists of a set of base units including metre for length, kilogram for mass, second for time and ampere for electrical current, and a few others, which together with their derived units, can measure any physical quantity. Metric system may also refer to other systems of related base and derived units defined before the middle of the 20th century, some of which are still in limited use today.
The metric system was designed to have properties that make it easy to use and widely applicable, including units based on the natural world, decimal ratios, prefixes for multiples and sub-multiples, and a structure of base and derived units. It is also a coherent system, which means that its units do not introduce conversion factors not already present in equations relating quantities. It has a property called rationalisation that eliminates certain constants of proportionality in equations of physics.
The units of the metric system, originally taken from observable features of nature, are now defined by phenomena such as the microwave frequency of a caesium atomic clock which accurately measures seconds. One unit, the kilogram, remains defined in terms of a man-made artefact, but scientists recently voted to change the definition to one based on Planck's constant via a Kibble balance. The new definition is expected to be formally propagated on 20 May 2019.
While there are numerous named derived units of the metric system, such as watt and lumen, other common quantities such as velocity and acceleration do not have their own unit, but are defined in terms of existing base and derived units such as metres per second for velocity.
Units of the British imperial system and the related US customary system are officially defined in terms of the metric system. Notably, as per the International Yard and Pound Agreement the base units of the Imperial and Customary system are defined in terms of the metre and kilogram.
The metric system is also extensible, and new base and derived units are defined as needed in fields such as radiology and chemistry. The most recent derived unit, the katal, for catalytic activity, was added in 1999. Recent changes are directed toward defining base units in terms of invariant constants of physics to provide more precise realisations of units for advances in science and industry.Myanmar units of measurement
The traditional Burmese units of measurement are still in everyday use in Myanmar (also known as Burma). According to the CIA Factbook, Myanmar is one of three countries that have not adopted the International System of Units (SI) metric system as their official system of weights and measures. However, in June 2011, the Burmese government's Ministry of Commerce began discussing proposals to reform the measurement system in Burma and adopt the metric system used by most of its trading partners, and in October 2013, Dr. Pwint San, Deputy Minister for Commerce, announced that the country was preparing to adopt the metric system.Most of the nation uses Burmese units only, although Burmese government web pages in English use imperial and metric units inconsistently. For instance, the Ministry of Construction uses miles to describe the length of roads and square feet for the size of houses, but square kilometres for the total land area of new town developments in Yangon City. The Ministry of Agriculture uses acres for land areas. The Ministry of Foreign Affairs uses kilometres (with mile equivalents in parentheses) to describe the dimensions of the country.Outline of the metric system
The following outline is provided as an overview of and topical guide to the metric system – various loosely related systems of measurement that trace their origin to the decimal system of measurement introduced in France during the French Revolution.Pitambar Pant
Pitambar Pant was an Indian independence activist, civil service officer and writer, best known for his contributions for the establishment of the Central Statistics Office and for changing Indian system of measurement to metric system. He served as the secretary to Jawaharlal Nehru, the then prime minister of India and headed the perspective planning division of the Planning Commission of India. He was also the author of a number of books on socialist economics. The Government of India awarded him the Padma Bhushan, the third highest civilian award, in 1973.Potrzebie
Potrzebie (; Polish pronunciation: [pɔtˈʂɛbʲe] dative/locative of potrzeba, "a need") is a Polish word popularized by its non sequitur use as a running gag in the early issues of Mad not long after the comic book began in 1952.Road signs in Colombia
Road signs in Colombia are similar to those of other South American countries. Many regulatory signs are based on European signs, while many warning signs are based on U.S. and Canadian signs.
Colombia uses the metric system of measurement and drives on the right.Thai units of measurement
Thailand adopted the metric system on 17 December 1923. However, old Thai units are still in common use, especially for measurements of land.Before metrication, the traditional system of measurement used in Thailand employed anthropic units. Some of these units are still in use, albeit standardised to SI/metric measurements. When the Royal Thai Survey Department began cadastral survey in 1896, Director R. W. Giblin, F.R.G.S., noted, "It so happens that 40 metres or 4,000 centimetres are equal to one sen," so all cadastral plans are plotted, drawn, and printed to a scale of 1:4,000. The square wa, ngan and rai are still used in measurements of land area.
The baht is still used as a unit of measurement in gold trading. However, one baht of 96.5% gold bullion is defined as 15.16 grams rather than the generic standard of 15 grams. The baht has also become the name of the currency of Thailand, which was originally fixed to the corresponding mass of silver.United States customary units
United States customary units are a system of measurements commonly used in the United States. The United States customary system (USCS or USC) developed from English units which were in use in the British Empire before the U.S. became an independent country. However, the United Kingdom's system of measures was overhauled in 1824 to create the imperial system, changing the definitions of some units. Therefore, while many U.S. units are essentially similar to their Imperial counterparts, there are significant differences between the systems.
The majority of U.S. customary units were redefined in terms of the meter and the kilogram with the Mendenhall Order of 1893 and, in practice, for many years before. These definitions were refined by the international yard and pound agreement of 1959.Americans primarily use customary units in commercial activities, as well as for personal and social use. In science, medicine, many sectors of industry, and some of government and military, metric units are used. The International System of Units (SI), the modern form of the metric system, is preferred for many uses by the U.S. National Institute of Standards and Technology (NIST). For newer units of measure where there is no traditional customary unit, international units are used, sometimes mixed with customary units, such as electrical resistance of wire expressed in ohms (SI) per thousand feet.
Systems of measurement
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with special names
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