Unit of measurement

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The former Weights and Measures office in Seven Sisters, London Weights and Measures office.jpg
The former Weights and Measures office in Seven Sisters, London
Units of measurement, Palazzo della Ragione, Padua Unita di misura - Palazzo della Ragione - Padova.jpg
Units of measurement, Palazzo della Ragione, Padua

A unit of measurement, or unit of measure, is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. [1] Any other quantity of that kind can be expressed as a multiple of the unit of measurement. [2]

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For example, a length is a physical quantity. The metre (symbol m) is a unit of length that represents a definite predetermined length. For instance, when referencing "10 metres" (or 10 m), what is actually meant is 10 times the definite predetermined length called "metre".

The definition, agreement, and practical use of units of measurement have played a crucial role in human endeavour from early ages up to the present. A multitude of systems of units used to be very common. Now there is a global standard, the International System of Units (SI), the modern form of the metric system.

In trade, weights and measures are often a subject of governmental regulation, to ensure fairness and transparency. The International Bureau of Weights and Measures (BIPM) is tasked with ensuring worldwide uniformity of measurements and their traceability to the International System of Units (SI).

Metrology is the science of developing nationally and internationally accepted units of measurement.

In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results is central to the scientific method. A standard system of units facilitates this. Scientific systems of units are a refinement of the concept of weights and measures historically developed for commercial purposes. [3]

Science, medicine, and engineering often use larger and smaller units of measurement than those used in everyday life. The judicious selection of the units of measurement can aid researchers in problem solving (see, for example, dimensional analysis).

In the social sciences, there are no standard units of measurement.

History

A unit of measurement is a standardized quantity of a physical property, used as a factor to express occurring quantities of that property. Units of measurement were among the earliest tools invented by humans. Primitive societies needed rudimentary measures for many tasks: constructing dwellings of an appropriate size and shape, fashioning clothing, or bartering food or raw materials.

The earliest known uniform systems of measurement seem to have all been created sometime in the 4th and 3rd millennia BC among the ancient peoples of Mesopotamia, Egypt and the Indus Valley, and perhaps also Elam in Persia as well.

Weights and measures are mentioned in the Bible (Leviticus 19:35–36). It is a commandment to be honest and have fair measures.

In the Magna Carta of 1215 (The Great Charter) with the seal of King John, put before him by the Barons of England, King John agreed in Clause 35 "There shall be one measure of wine throughout our whole realm, and one measure of ale and one measure of corn—namely, the London quart;—and one width of dyed and russet and hauberk cloths—namely, two ells below the selvage..."

As of the 21st century, the International System is predominantly used in the world. There exist other unit systems which are used in many places such as the United States Customary System and the Imperial System. The United States is the only industrialized country that has not yet at least mostly converted to the metric system. [4] The systematic effort to develop a universally acceptable system of units dates back to 1790 when the French National Assembly charged the French Academy of Sciences to come up such a unit system. This system was the precursor to the metric system which was quickly developed in France but did not take on universal acceptance until 1875 when The Metric Convention Treaty was signed by 17 nations. After this treaty was signed, a General Conference of Weights and Measures (CGPM) was established. The CGPM produced the current SI, which was adopted in 1954 at the 10th Conference of Weights and Measures. Currently, the United States is a dual-system society which uses both the SI and the US Customary system. [5] [6]

Systems of units

The use of a single unit of measurement for some quantity has obvious drawbacks. For example, it is impractical to use the same unit for the distance between two cities and the length of a needle. Thus, historically they would develop independently. One way to make large numbers or small fractions easier to read, is to use unit prefixes.

At some point in time though, the need to relate the two units might arise, and consequently the need to choose one unit as defining the other or vice versa. For example, an inch could be defined in terms of a barleycorn. A system of measurement is a collection of units of measurement and rules relating them to each other.

As science progressed, a need arose to relate the measurement systems of different quantities, like length and weight and volume. The effort of attempting to relate different traditional systems between each other exposed many inconsistencies, and brought about the development of new units and systems.

Systems of units vary from country to country. Some of the different systems include the centimetre–gram–second, foot–pound–second, metre–kilogram–second systems, and the International System of Units, SI. Among the different systems of units used in the world, the most widely used and internationally accepted one is SI. The base SI units are the second, metre, kilogram, ampere, kelvin, mole and candela; all other SI units are derived from these base units. [7] [8] :132

Systems of measurement in modern use include the metric system, the imperial system, and United States customary units.

Traditional systems

Historically many of the systems of measurement which had been in use were to some extent based on the dimensions of the human body. Such units, which may be called anthropic units, include the cubit, based on the length of the forearm; the pace, based on the length of a stride; and the foot and hand. [9] :25 As a result, units of measure could vary not only from location to location but from person to person. Units not based on the human body could be based on agriculture, as is the case with the furlong and the acre, both based on the amount of land able to be worked by a team of oxen.

Metric systems

Metric systems of units have evolved since the adoption of the original metric system in France in 1791. The current international standard metric system is the International System of Units (abbreviated to SI). An important feature of modern systems is standardization. Each unit has a universally recognized size.

An example of metrication in 1860 when Tuscany became part of modern Italy (ex. one "libbra" = 339.54 grams) Tabella conversione metrica 1860 MG 7771.jpg
An example of metrication in 1860 when Tuscany became part of modern Italy (ex. one "libbra" = 339.54 grams)

Both the imperial units and US customary units derive from earlier English units. Imperial units were mostly used in the British Commonwealth and the former British Empire. US customary units are still the main system of measurement used in the United States outside of science, medicine, many sectors of industry, and some of government and military, and despite Congress having legally authorised metric measure on 28 July 1866. [10] Some steps towards US metrication have been made, particularly the redefinition of basic US and imperial units to derive exactly from SI units. Since the international yard and pound agreement of 1959 the US and imperial inch is now defined as exactly 0.0254  m , and the US and imperial avoirdupois pound is now defined as exactly 0.45359237  kg . [11]

Natural systems

While the above systems of units are based on arbitrary unit values, formalised as standards, natural units in physics are based on physical principle or are selected to make physical equations easier to work with. For example, atomic units (au) were designed to simplify the wave equation in atomic physics. [12]

Some unusual and non-standard units may be encountered in sciences. These may include the solar mass (2×1030 kg), the megaton (the energy released by detonating one million tons of trinitrotoluene, TNT) and the electronvolt.

To reduce the incidence of retail fraud, many national statutes have standard definitions of weights and measures that may be used (hence "statute measure"), and these are verified by legal officers.[ citation needed ]

Informal comparison to familiar concepts

In informal settings, a quantity may be described as multiples of that of a familiar entity, which can be easier to contextualize than a value in a formal unit system. For instance, a publication may describe an area in a foreign country as a number of multiples of the area of a region local to the readership. The propensity for certain concepts to be used frequently can give rise to loosely defined "systems" of units. [13] [14]

Base and derived units

For most quantities a unit is necessary to communicate values of that physical quantity. For example, conveying to someone a particular length without using some sort of unit is impossible, because a length cannot be described without a reference used to make sense of the value given.

But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only a small set of units is required. These units are taken as the base units and the other units are derived units. Thus base units are the units of the quantities which are independent of other quantities and they are the units of length, mass, time, electric current, temperature, luminous intensity and the amount of substance. Derived units are the units of the quantities which are derived from the base quantities and some of the derived units are the units of speed, work, acceleration, energy, pressure etc. [7]

Different systems of units are based on different choices of a set of related units including fundamental and derived units.

Physical quantity components

Following ISO 80000-1, [15] any value or magnitude of a physical quantity is expressed as a comparison to a unit of that quantity. The value of a physical quantity Z is expressed as the product of a numerical value {Z} (a pure number) and a unit [Z]:

For example, let be "2 metres"; then, is the numerical value and is the unit. Conversely, the numerical value expressed in an arbitrary unit can be obtained as:

The multiplication sign is usually left out, just as it is left out between variables in the scientific notation of formulas. The convention used to express quantities is referred to as quantity calculus . In formulas, the unit [Z] can be treated as if it were a specific magnitude of a kind of physical dimension: see Dimensional analysis for more on this treatment.

Dimensional homogeneity

Units can only be added or subtracted if they are the same type; however units can always be multiplied or divided, as George Gamow used to explain. Let be "2 metres" and "3 seconds", then

.

There are certain rules that apply to units:

Converting units of measurement

Conversion of units is the conversion of the unit of measurement in which a quantity is expressed, typically through a multiplicative conversion factor that changes the unit without changing the quantity. This is also often loosely taken to include replacement of a quantity with a corresponding quantity that describes the same physical property.

Unit conversion is often easier within a metric system such as the SI than in others, due to the system's coherence and its metric prefixes that act as power-of-10 multipliers.

Real-world implications

One example of the importance of agreed units is the failure of the NASA Mars Climate Orbiter, which was accidentally destroyed on a mission to Mars in September 1999 (instead of entering orbit) due to miscommunications about the value of forces: different computer programs used different units of measurement (newton versus pound force). Considerable amounts of effort, time, and money were wasted. [16] [17]

On 15 April 1999, Korean Air cargo flight 6316 from Shanghai to Seoul was lost due to the crew confusing tower instructions (in metres) and altimeter readings (in feet). Three crew and five people on the ground were killed. Thirty-seven were injured. [18] [19]

In 1983, a Boeing 767 (which thanks to its pilot's gliding skills landed safely and became known as the Gimli Glider) ran out of fuel in mid-flight because of two mistakes in figuring the fuel supply of Air Canada's first aircraft to use metric measurements. [20] This accident was the result of both confusion due to the simultaneous use of metric and Imperial measures and confusion of mass and volume measures.

When planning his journey across the Atlantic Ocean in the 1480s, Columbus mistakenly assumed that the mile referred to in the Arabic estimate of 56+2/3 miles for the size of a degree was the same as the actually much shorter Italian mile of 1,480 metres. His estimate for the size of the degree and for the circumference of the Earth was therefore about 25% too small. [21] :1:17

See also

Related Research Articles

Conversion of units is the conversion of the unit of measurement in which a quantity is expressed, typically through a multiplicative conversion factor that changes the unit without changing the quantity. This is also often loosely taken to include replacement of a quantity with a corresponding quantity that describes the same physical property.

The centimetre–gram–second system of units is a variant of the metric system based on the centimetre as the unit of length, the gram as the unit of mass, and the second as the unit of time. All CGS mechanical units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover electromagnetism.

Length is a measure of distance. In the International System of Quantities, length is a quantity with dimension distance. In most systems of measurement a base unit for length is chosen, from which all other units are derived. In the International System of Units (SI) system the base unit for length is the metre.

<span class="mw-page-title-main">Measurement</span> Process of assigning numbers to objects or events

Measurement is the quantification of attributes of an object or event, which can be used to compare with other objects or events. In other words, measurement is a process of determining how large or small a physical quantity is as compared to a basic reference quantity of the same kind. The scope and application of measurement are dependent on the context and discipline. In natural sciences and engineering, measurements do not apply to nominal properties of objects or events, which is consistent with the guidelines of the International vocabulary of metrology published by the International Bureau of Weights and Measures. However, in other fields such as statistics as well as the social and behavioural sciences, measurements can have multiple levels, which would include nominal, ordinal, interval and ratio scales.

<span class="mw-page-title-main">Physical quantity</span> Measurable property of a material or system

A physical quantity is a property of a material or system that can be quantified by measurement. A physical quantity can be expressed as a value, which is the algebraic multiplication of a numerical value and a unit of measurement. For example, the physical quantity mass, symbol m, can be quantified as m=n kg, where n is the numerical value and kg is the unit symbol. Quantities that are vectors have, besides numerical value and unit, direction or orientation in space.

<span class="mw-page-title-main">International System of Units</span> Modern form of the metric system

The International System of Units, internationally known by the abbreviation SI, is the modern form of the metric system and the world's most widely used system of measurement. Coordinated by the International Bureau of Weights and Measures it is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce.

<span class="mw-page-title-main">SI base unit</span> One of the seven units of measurement that define the metric system

The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived. The units and their physical quantities are the second for time, the metre for length or distance, the kilogram for mass, the ampere for electric current, the kelvin for thermodynamic temperature, the mole for amount of substance, and the candela for luminous intensity. The SI base units are a fundamental part of modern metrology, and thus part of the foundation of modern science and technology.

<span class="mw-page-title-main">United States customary units</span> System of units of measurement commonly used in the United States

United States customary units form a system of measurement units commonly used in the United States and most U.S. territories, since being standardized and adopted in 1832. The United States customary system developed from English units that were in use in the British Empire before the U.S. became an independent country. The United Kingdom's system of measures was overhauled in 1824 to create the imperial system, which was officially adopted in 1826, changing the definitions of some of its units. Consequently, while many U.S. units are essentially similar to their imperial counterparts, there are noticeable differences between the systems.

<span class="mw-page-title-main">Volume</span> Quantity of three-dimensional space

Volume is a measure of regions in three-dimensional space. It is often quantified numerically using SI derived units or by various imperial or US customary units. The definition of length and height (cubed) is interrelated with volume. The volume of a container is generally understood to be the capacity of the container; i.e., the amount of fluid that the container could hold, rather than the amount of space the container itself displaces. By metonymy, the term "volume" sometimes is used to refer to the corresponding region.

<span class="mw-page-title-main">Metric system</span> Metre-based systems of measurement

The metric system is a decimal-based system of measurement. The current international standard for the metric system is the International System of Units, in which all units can be expressed in terms of seven base units: the metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (Cd). These can be made into larger or smaller units with the use of metric prefixes.

<span class="mw-page-title-main">Metrology</span> Science of measurement and its application

Metrology is the scientific study of measurement. It establishes a common understanding of units, crucial in linking human activities. Modern metrology has its roots in the French Revolution's political motivation to standardise units in France when a length standard taken from a natural source was proposed. This led to the creation of the decimal-based metric system in 1795, establishing a set of standards for other types of measurements. Several other countries adopted the metric system between 1795 and 1875; to ensure conformity between the countries, the Bureau International des Poids et Mesures (BIPM) was established by the Metre Convention. This has evolved into the International System of Units (SI) as a result of a resolution at the 11th General Conference on Weights and Measures (CGPM) in 1960.

<span class="mw-page-title-main">Chinese units of measurement</span> Traditional system of measurement used by Han Chinese

Chinese units of measurement, known in Chinese as the shìzhì, are the traditional units of measurement of the Han Chinese. Although Chinese numerals have been decimal (base-10) since the Shang, several Chinese measures use hexadecimal (base-16). Local applications have varied, but the Chinese dynasties usually proclaimed standard measurements and recorded their predecessor's systems in their histories.

<span class="mw-page-title-main">Unit of length</span> Reference value of length

A unit of length refers to any arbitrarily chosen and accepted reference standard for measurement of length. The most common units in modern use are the metric units, used in every country globally. In the United States the U.S. customary units are also in use. British Imperial units are still used for some purposes in the United Kingdom and some other countries. The metric system is sub-divided into SI and non-SI units.

A system of units of measurement, also known as a system of units or 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. Instances in use include the International System of Units or SI, the British imperial system, and the United States customary system.

The foot–pound–second system is a system of units built on three fundamental units: the foot for length, the (avoirdupois) pound for either mass or force, and the second for time.

Some fields of engineering in the United States use a system of measurement of physical quantities known as the English Engineering Units. Despite its name, the system is based on United States customary units of measure.

<span class="mw-page-title-main">History of the metric system</span>

The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature, along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios were added, and the system went on to be adopted across the world.

<span class="mw-page-title-main">Outline of the metric system</span> Overview of and topical guide to the metric system

The following outline is provided as an overview of and topical guide to the metric system:

<span class="mw-page-title-main">Coherence (units of measurement)</span> Type of system of units of measurement

A coherent system of units is a system of units of measurement used to express physical quantities that are defined in such a way that the equations relating the numerical values expressed in the units of the system have exactly the same form, including numerical factors, as the corresponding equations directly relating the quantities. It is a system in which every quantity has a unique unit, or one that does not use conversion factors.

<span class="mw-page-title-main">Imperial and US customary measurement systems</span> English (pre 1824), Imperial (post 1824) and US Customary (post 1776) units of measure

The imperial and US customary measurement systems 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.

References

  1. "measurement unit". International Vocabulary of Metrology – Basic and General Concepts and Associated Terms (VIM) (PDF) (in English and French) (3rd ed.). Joint Committee for Guides in Metrology. 2008. pp. 6–7. Archived from the original (PDF) on 7 June 2011..
  2. "Units of Measurement". www.ibiblio.org.
  3. "1.3 The Language of Physics: Physical Quantities and Units | Texas Gateway". www.texasgateway.org.
  4. Giunta, Carmen J. (2023). The Metric System and the United States. Cham: Springer International Publishing. pp. 69–78. doi:10.1007/978-3-031-28436-6_6. ISBN   978-3-031-28435-9.
  5. Yunus A. Çengel; Michael A. Boles (2002). Thermodynamics: An Engineering Approach (Eighth ed.). McGraw Hill. p. 996. ISBN   9780073398174.
  6. Dodd, Richard (2012). Using SI Units in Astronomy. Cambridge University Press. p. 246. doi:10.1017/CBO9781139019798. ISBN   9780521769174.
  7. 1 2 "Measurement in Physics & SI units of Measurement". HelpYouBetter. 15 November 2018. Retrieved 15 August 2019.
  8. "9th edition of the SI Brochure". BIPM. 2019. Retrieved 20 May 2019.
  9. Crease, Robert P. World in the balance: The historic quest for an absolute system of measurement. WW Norton & Company, 2011.
  10. "US Metric Act of 1866". Archived from the original on 10 October 2014. as amended by Public Law 110–69 dated 9 August 2007
  11. "NIST Handbook 44 Appendix B". National Institute of Standards and Technology. 2002. Archived from the original on 13 February 2007. Retrieved 18 February 2007.
  12. Hartree, D. R. (1928). "The Wave Mechanics of an Atom with a Non-Coulomb Central Field. Part I. Theory and Methods". Mathematical Proceedings of the Cambridge Philosophical Society. Vol. 24, no. 1. Cambridge University Press. pp. 89–110. Bibcode:1928PCPS...24...89H. doi:10.1017/S0305004100011919.
  13. Marsh, David (17 May 2010). "Mind your language: Wales, Belgium and other units of measurement". the Guardian. Retrieved 30 August 2018.
  14. "Size of Wales". The Economist. Retrieved 30 August 2018.
  15. "ISO 80000-1:2009(en) Quantities and units — Part 1: General". International Organization for Standardization. Retrieved 12 May 2023.
  16. "Unit Mixups". US Metric Association. Archived from the original on 23 September 2010.
  17. "Mars Climate Orbiter Mishap Investigation Board Phase I Report" (PDF). NASA. 10 November 1999. Archived from the original (PDF) on 16 March 2011.
  18. "Korean Air Flight 6316" (Press release). NTSB. Archived from the original on 6 October 2006.
  19. "Korean Air incident". Aviation Safety Net. Archived from the original on 31 July 2013.
  20. Witkin, Richard (30 July 1983). "Jet's Fuel Ran Out After Metric Conversion Errors". The New York Times . Retrieved 21 August 2007. Air Canada said yesterday that its Boeing 767 jet ran out of fuel in mid-flight last week because of two mistakes in figuring the fuel supply of the airline's first aircraft to use metric measurements. After both engines lost their power, the pilots made what is now thought to be the first successful emergency dead stick landing of a commercial jetliner.
  21. Nunn, George Emra. "The geographical conceptions of Columbus: a critical consideration of four problems". No. 14. New York: American Geographical Society, 1924.1–2 17-18

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