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The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1 299 792 458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
- Orders of Magnitude (Length)
To help compare different orders of magnitude, this section...
- Meter (Disambiguation)
Meter or metre is a unit of measurement of length in the...
- History of the metre
History of the metre. An early definition of the metre was...
- Orders of Magnitude (Length)
History of the metre. An early definition of the metre was one ten-millionth of the Earth quadrant, the distance from the North Pole to the Equator, measured along a meridian through Paris. The history of the metre starts with the Scientific Revolution that is considered to have begun with Nicolaus Copernicus 's publication of De revolutionibus ...
The metre is now defined as the distance light travels in a vacuum in 1/299,792,458 of a second. [1] In the imperial system of measurement, one yard is 0.9144 metres (after international agreement in 1959), so a metre is very near to 39.37 inches: about 3.281 feet, or 1.0936 yards . The bar (made of platinum and iridium) that defined the length ...
The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of 1 299 792 458 of a second, where the second is defined by a hyperfine transition frequency of caesium.
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- Definition
- Lexicographic Conventions
- Realisation of Units
- Organizational Status
- History
- Related Units
The International System of Units consists of a set of defining constants with corresponding base units, derived units, and a set of decimal-based multipliers that are used as prefixes.: 125
Unit names
According to the SI Brochure,: 148 unit names should be treated as common nouns of the context language. This means that they should be typeset in the same character set as other common nouns (e.g. Latin alphabet in English, Cyrillic script in Russian, etc.), following the usual grammatical and orthographical rules of the context language. For example, in English and French, even when the unit is named after a person and its symbol begins with a capital letter, the unit name in running text s...
Unit symbols and the values of quantities
Symbols of SI units are intended to be unique and universal, independent of the context language.: 130–35 The SI Brochure has specific rules for writing them.: 130–35 In addition, the SI Brochure provides style conventions for among other aspects of displaying quantities units: the quantity symbols, formatting of numbers and the decimal marker, expressing measurement uncertainty, multiplication and division of quantity symbols, and the use of pure numbers and various angles.: 147 In the Unite...
Metrologists carefully distinguish between the definition of a unit and its realisation. The SI units are defined by declaring that seven defining constants: 125–129 have certain exact numerical values when expressed in terms of their SI units. The realisation of the definition of a unit is the procedure by which the definition may be used to estab...
The International System of Units, or SI,: 123 is a decimal and metric system of units established in 1960 and periodically updated since then. The SI has an official status in most countries, including the United States, Canada, and the United Kingdom, although these three countries are among the handful of nations that, to various degrees, also c...
CGS and MKS systems
The concept of a system of units emerged a hundred years before the SI.In the 1860s, James Clerk Maxwell, William Thomson (later Lord Kelvin), and others working under the auspices of the British Association for the Advancement of Science, building on previous work of Carl Gauss, developed the centimetre–gram–second system of units or cgs system in 1874. The systems formalised the concept of a collection of related units called a coherent system of units. In a coherent system, base units comb...
Metre Convention
A French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention, also called Treaty of the Metre, by 17 nations.[e]: 353–354 The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presen...
Giovanni Giorgi and the problem of electrical units
At the close of the 19th century three different systems of units of measure existed for electrical measurements: a CGS-based system for electrostatic units, also known as the Gaussian or ESU system, a CGS-based system for electromechanical units (EMU), and an International system based on units defined by the Metre Convention for electrical distribution systems.Attempts to resolve the electrical units in terms of length, mass, and time using dimensional analysis was beset with difficulties –...
Non-SI units accepted for use with SI
Many non-SI units continue to be used in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The CIPM recognised and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI,including the hour, minute, degree of angle, litre, and decibel.
Metric units not recognised by SI
Although the term metric system is often used as an informal alternative name for the International System of Units, other metric systems exist, some of which were in widespread use in the past or are even still used in particular areas. There are also individual metric units such as the sverdrup and the darcythat exist outside of any system of units. Most of the units of the other metric systems are not recognised by the SI.
Unacceptable uses
Sometimes, SI unit name variations are introduced, mixing information about the corresponding physical quantity or the conditions of its measurement; however, this practice is unacceptable with the SI. "Unacceptability of mixing information with units: When one gives the value of a quantity, any information concerning the quantity or its conditions of measurement must be presented in such a way as not to be associated with the unit."Instances include: "watt-peak" and "watt RMS"; "geopotential...
Apr 12, 2024 · Metre is the fundamental unit of length in the metric system and the International Systems of Units. Learn how it was defined by the French Academy of Sciences, the International Bureau of Weights and Measures, and the speed of light.
The metric system is a decimal-based system of measurement. The system has units of measure for each quantity. The names of most units of measure in the metric system have two parts. One part is the unit name and the other part is the prefix. For example, in the name "centimetre", the word "centi" is the prefix and the word "metre" is the unit ...
