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 ≈ {{val|3.2808}} [[foot (unit)|ft]]
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The metre (British spelling and BIPM spelling ^{ [1] }) or meter (American spelling ^{ [2] }) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI). The SI unit symbol is m.^{ [3] } The metre is defined as the length of the path travelled by light in a vacuum in 1/299 792 458 second.^{ [1] }
The International Bureau of Weights and Measures is an intergovernmental organization that was established by the Metre Convention, through which member states act together on matters related to measurement science and measurement standards. The organisation is usually referred to by its French initialism, BIPM. The BIPM's headquarters is based at Sèvres, France. It has custody of the International Prototype of the Kilogram and houses the secretariat for this organization as well as hosting its formal meetings.
A base unit is a unit adopted for measurement of a base quantity. A base quantity is one of a conventionally chosen subset of physical quantities, where no quantity in the subset can be expressed in terms of the others.
Length is a measure of distance. In the International System of Quantities, length is any quantity with dimension distance. In most systems of measurement, the unit of length is a base unit, from which other units are derived.
The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole – as a result the Earth's circumference is approximately 40,000 km today. In 1799, it was redefined in terms of a prototype metre bar (the actual bar used was changed in 1889). In 1960, the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted.
An equator of a rotating spheroid is its zeroth circle of latitude (parallel). It is the imaginary line on the spheroid's surface, equidistant from its poles, dividing it into northern and southern hemispheres. In other words, it is the intersection of the spheroid's surface with the plane perpendicular to its axis of rotation and midway between its geographical poles.
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is defined as the point in the Northern Hemisphere where the Earth's axis of rotation meets its surface.
Earth's circumference is the distance around the Earth, either around the equator or around the poles.
The imperial inch is defined as 0.0254 metres (2.54 centimetres or 25.4 millimetres). One metre is about 3 ^{3}⁄_{8} inches longer than a yard, i.e. about 39 ^{3}⁄_{8} inches.
The inch is a unit of length in the (British) imperial and United States customary systems of measurement. It is equal to ^{1}⁄_{36} yard or ^{1}⁄_{12} of a foot. Derived from the Roman uncia ("twelfth"), the word inch is also sometimes used to translate similar units in other measurement systems, usually understood as deriving from the width of the human thumb. Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s it has been based on the metric system and defined as exactly 2.54 cm.
The yard is an English unit of length, in both the British imperial and US customary systems of measurement, that comprises 3 feet or 36 inches.
Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States^{ [4] }^{ [5] }^{ [6] } and the Philippines,^{ [7] } which use meter. Other Germanic languages, such as German, Dutch, and the Scandinavian languages^{ [8] } likewise spell the word meter.
Measuring devices (such as ammeter, speedometer) are spelled "-meter" in all variants of English.^{ [9] } The suffix "-meter" has the same Greek origin as the unit of length.^{ [10] }^{ [11] }
An ammeter is a measuring instrument used to measure the current in a circuit. Electric currents are measured in amperes (A), hence the name. Instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments which relied on the Earth's magnetic field for operation. By the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems. It is generally represented by letter 'A' in a circle. Ammeters have very low resistance and are always connected in series in any circuit.
A speedometer or a speed meter is a gauge that measures and displays the instantaneous speed of a vehicle. Now universally fitted to motor vehicles, they started to be available as options in the 1900s, and as standard equipment from about 1910 onwards. Speedometers for other vehicles have specific names and use other means of sensing speed. For a boat, this is a pit log. For an aircraft, this is an airspeed indicator.
The etymological roots of metre can be traced to the Greek verb μετρέω (metreo) (to measure, count or compare) and noun μέτρον (metron) (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (metior, mensura), French (mètre, mesure), English and other languages. The motto ΜΕΤΡΩ ΧΡΩ (metro chro) in the seal of the International Bureau of Weights and Measures (BIPM), which was a saying of the Greek statesman and philosopher Pittacus of Mytilene and may be translated as "Use measure!", thus calls for both measurement and moderation.
Pittacus was an ancient Mytilenaen military general and one of the Seven Sages of Greece.
In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, the universal measure or standard based on a pendulum with a two-second period.^{ [12] } The use of the seconds pendulum to define length had been suggested to the Royal Society in 1660 by Christopher Wren. Christiaan Huygens had observed that length to be 38 Rijnland inches or 39.26 English inches; that is, 997 mm.^{ [12] }^{ [13] }^{ [14] } No official action was taken regarding these suggestions.
In 1670 Gabriel Mouton, Bishop of Lyon, also suggested a universal length standard with decimal multiples and divisions, to be based on a one-minute angle of the Earth's meridian arc or (as the Earth's circumference was not easy to measure) on a pendulum with a two-second period. In 1675, the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the phrase metro cattolico ("universal measure"), derived from the Greek μέτρον καθολικόν (métron katholikón), to denote the standard unit of length derived from a pendulum.^{ [15] } As a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. On 7 October 1790 that commission advised the adoption of a decimal system, and on 19 March 1791 advised the adoption of the term mètre ("measure"), a basic unit of length, which they defined as equal to one ten-millionth of the distance between the North Pole and the Equator.^{ [16] }^{ [17] }^{ [18] }^{ [19] } In 1793, the French National Convention adopted the proposal; this use of metre in English began at least as early as 1797.^{ [20] }
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In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies slightly over the surface of the Earth, which affects the period of a pendulum.
To establish a universally accepted foundation for the definition of the metre, more accurate measurements of this meridian were needed. The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to accurately measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona to estimate the length of the meridian arc through Dunkerque. This portion of the meridian, assumed to be the same length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator. The problem with this approach is that the exact shape of the Earth is not a simple mathematical shape, such as a sphere or oblate spheroid, at the level of precision required for defining a standard of length. The irregular and particular shape of the Earth smoothed to sea level is represented by a mathematical model called a geoid, which literally means "Earth-shaped". Despite these issues, in 1793 France adopted this definition of the metre as its official unit of length based on provisional results from this expedition. However, it was later determined that the first prototype metre bar was short by about 200 micrometres because of miscalculation of the flattening of the Earth, making the prototype about 0.02% shorter than the original proposed definition of the metre. Regardless, this length became the French standard and was progressively adopted by other countries in Europe.
The expedition was fictionalised in Denis Guedj, Le mètre du Monde.^{ [21] } Ken Alder wrote factually about the expedition in The Measure of All Things: the seven year odyssey and hidden error that transformed the world.^{ [22] }
In 1867 at the second general conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth.^{ [23] }^{ [24] }^{ [25] } The conference recommended the adoption of the metre and the creation of an international metre commission, according to the proposal of Johann Jacob Baeyer, Adolphe Hirsch and Carlos Ibáñez e Ibáñez de Ibero.^{ [23] }
In the 1870s and in light of modern precision, a series of international conferences was held to devise new metric standards. The Metre Convention (Convention du Mètre) of 1875 mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organisation was to construct and preserve a prototype metre bar, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The organisation created such a bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of 90% platinum and 10% iridium, measured at the melting point of ice.^{ [26] }
The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.
In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of length. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1 650 763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.^{ [27] }
To further reduce uncertainty, the 17th CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second and the speed of light:^{ [1] }
This definition fixed the speed of light in vacuum at exactly 792458 metres per second (≈ 299000 km/s). An intended by-product of the 17th CGPM's definition was that it enabled scientists to compare lasers accurately using frequency, resulting in wavelengths with one-fifth the uncertainty involved in the direct comparison of wavelengths, because interferometer errors were eliminated. To further facilitate reproducibility from lab to lab, the 17th CGPM also made the iodine-stabilised 300helium–neon laser "a recommended radiation" for realising the metre.^{ [28] } For the purpose of delineating the metre, the BIPM currently considers the HeNe laser wavelength, λ_{HeNe}, to be 21258 nm with an estimated relative standard uncertainty (U) of 632.991×10^{−11}.^{ [28] }^{ [29] }^{ [30] } This uncertainty is currently one limiting factor in laboratory realisations of the metre, and it is several orders of magnitude poorer than that of the second, based upon the caesium fountain 2.1atomic clock (U = ×10^{−16} 5).^{ [31] } Consequently, a realisation of the metre is usually delineated (not defined) today in labs as 579800.762042(33) wavelengths of helium-neon laser light in a vacuum, the error stated being only that of frequency determination.^{ [28] } This bracket notation expressing the error is explained in the article on 1measurement uncertainty.
Practical realisation of the metre is subject to uncertainties in characterising the medium, to various uncertainties of interferometry, and to uncertainties in measuring the frequency of the source.^{ [32] } A commonly used medium is air, and the National Institute of Standards and Technology (NIST) has set up an online calculator to convert wavelengths in vacuum to wavelengths in air.^{ [33] } As described by NIST, in air, the uncertainties in characterising the medium are dominated by errors in measuring temperature and pressure. Errors in the theoretical formulas used are secondary.^{ [34] } By implementing a refractive index correction such as this, an approximate realisation of the metre can be implemented in air, for example, using the formulation of the metre as 579800.762042(33) wavelengths of helium–neon laser light in vacuum, and converting the wavelengths in a vacuum to wavelengths in air. Air is only one possible medium to use in a realisation of the metre, and any 1partial vacuum can be used, or some inert atmosphere like helium gas, provided the appropriate corrections for refractive index are implemented.^{ [35] }
The metre is defined as the path length travelled by light in a given time and practical laboratory length measurements in metres are determined by counting the number of wavelengths of laser light of one of the standard types that fit into the length,^{ [38] } and converting the selected unit of wavelength to metres. Three major factors limit the accuracy attainable with laser interferometers for a length measurement:^{ [32] }^{ [39] }
Of these, the last is peculiar to the interferometer itself. The conversion of a length in wavelengths to a length in metres is based upon the relation
which converts the unit of wavelength λ to metres using c, the speed of light in vacuum in m/s. Here n is the refractive index of the medium in which the measurement is made, and f is the measured frequency of the source. Although conversion from wavelengths to metres introduces an additional error in the overall length due to measurement error in determining the refractive index and the frequency, the measurement of frequency is one of the most accurate measurements available.^{ [39] }
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Basis of definition | Date | Absolute uncertainty | Relative uncertainty |
---|---|---|---|
1/10 000 000 part of the quadrant along the meridian, measurement by Delambre and Méchain (443.296 lines) | 1795 | 500–100 μm | 10^{−4} |
First prototype Mètre des Archives platinum bar standard | 1799 | 50–10 μm | 10^{−5} |
Platinum-iridium bar at melting point of ice (1st CGPM) | 1889 | 0.2–0.1 μm (200–100 nm) | 10^{−7} |
Platinum-iridium bar at melting point of ice, atmospheric pressure, supported by two rollers (7th CGPM) | 1927 | n.a. | n.a. |
Hyperfine atomic transition; 650763.73 wavelengths of light from a specified transition in 1krypton-86 (11th CGPM) | 1960 | 4 nm | ×10^{−9}^{ [44] } 4 |
Length of the path travelled by light in a vacuum in 1/299 792 458 second (17th CGPM) | 1983 | 0.1 nm | 10^{−10} |
SI prefixes are often employed to denote decimal multiples and submultiples of the metre, as shown in the table below. As indicated in the table, some are commonly used, while others are not. Long distances are usually expressed in km, astronomical units (149.6 Gm), light-years (10 Pm), or parsecs (31 Pm), rather than in Mm, Gm, Tm, Pm, Em, Zm or Ym; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.
The terms micron and (occasionally) millimicron are often used instead of micrometre (μm) and nanometre (nm), but this practice is officially discouraged.^{ [45] }
Submultiples | Multiples | |||||
---|---|---|---|---|---|---|
Value | SI symbol | Name | Value | SI symbol | Name | |
10^{−1} m | dm | decimetre | 10^{1} m | dam | decametre | |
10^{−2} m | cm | centimetre | 10^{2} m | hm | hectometre | |
10^{−3} m | mm | millimetre | 10^{3} m | km | kilometre | |
10^{−6} m | µm | micrometre | 10^{6} m | Mm | megametre | |
10^{−9} m | nm | nanometre | 10^{9} m | Gm | gigametre | |
10^{−12} m | pm | picometre | 10^{12} m | Tm | terametre | |
10^{−15} m | fm | femtometre | 10^{15} m | Pm | petametre | |
10^{−18} m | am | attometre | 10^{18} m | Em | exametre | |
10^{−21} m | zm | zeptometre | 10^{21} m | Zm | zettametre | |
10^{−24} m | ym | yoctometre | 10^{24} m | Ym | yottametre | |
Common prefixed units are in bold face. |
Metric unit expressed in non-SI units | Non-SI unit expressed in metric units | |||||||
---|---|---|---|---|---|---|---|---|
1 metre | ≈ | 1.0936 | yard | 1 yard | ≡ | 0.9144 | metre | |
1 metre | ≈ | 39.370 | inches | 1 inch | ≡ | 0.0254 | metre | |
1 centimetre | ≈ | 70 0.393 | inch | 1 inch | ≡ | 2.54 | centimetres | |
1 millimetre | ≈ | 370 0.039 | inch | 1 inch | ≡ | 25.4 | millimetres | |
1 metre | ≡ | 1 × 10^{10} | ångström | 1 ångström | ≡ | 1 × 10^{−10} | metre | |
1 nanometre | ≡ | 10 | ångström | 1 ångström | ≡ | 100 | picometres |
Within this table, "inch" and "yard" mean "international inch" and "international yard"^{ [46] } respectively, though approximate conversions in the left column hold for both international and survey units.
One metre is exactly equivalent to 10 000/254 inches and to 10 000/9 144 yards.
A simple mnemonic aid exists to assist with conversion, as three "3"s:
The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm). Scottish and English definitions of the ell (two cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively. The ancient Parisian toise (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly ^{1}⁄_{2} toise. The Russian verst was 1.0668 km. The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.
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Look up metre in Wiktionary, the free dictionary. |
The spelling of English words is in accordance with the United States Government Printing Office Style Manual, which follows Webster's Third New International Dictionary rather than the Oxford Dictionary. Thus the spellings "meter,"…rather than "metre,"...as in the original BIPM English text...
An identical metric system to that introduced in France was proposed in 1668 by Bishop John Wilkins, a founder of the Royal Society in England. ... he proposed an integrated system of measurement based on a decimal system and almost identical to the modern metric system. His unit of measurement was 997 millimeters - almost exactly a meter.
he [Wilkins] proposed essentially what became ... the French decimal metric system
The error [introduced by using air] can be reduced tenfold if the chamber is filled with an atmosphere of helium rather than air.
The ampere, often shortened to "amp", is the base unit of electric current in the International System of Units (SI). It is named after André-Marie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.
The candela is the base unit of luminous intensity in the International System of Units (SI); that is, luminous power per unit solid angle emitted by a point light source in a particular direction. Luminous intensity is analogous to radiant intensity, but instead of simply adding up the contributions of every wavelength of light in the source's spectrum, the contribution of each wavelength is weighted by the standard luminosity function. A common wax candle emits light with a luminous intensity of roughly one candela. If emission in some directions is blocked by an opaque barrier, the emission would still be approximately one candela in the directions that are not obscured.
Conversion of units is the conversion between different units of measurement for the same quantity, typically through multiplicative conversion factors.
The General Conference on Weights and Measures is the supreme authority of the International Bureau of Weights and Measures, the inter-governmental organization established in 1875 under the terms of the Metre Convention through which Member States act together on matters related to measurement science and measurement standards. The CGPM is made up of delegates of the governments of the Member States and observers from the Associates of the CGPM. Under its authority, the International Committee for Weights and Measures executes an exclusive direction and supervision of the BIPM.
The kilogram or kilogramme is the base unit of mass in the International System of Units (SI). Until 20 May 2019, it remains defined by a platinum alloy cylinder, the International Prototype Kilogram, manufactured in 1889, and carefully stored in Saint-Cloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants.
The litre or liter is an SI accepted metric system unit of volume equal to 1 cubic decimetre (dm^{3}), 1,000 cubic centimetres (cm^{3}) or 1/1,000 cubic metre. A cubic decimetre occupies a volume of 10 cm×10 cm×10 cm and is thus equal to one-thousandth of a cubic metre.
The Metre Convention, also known as the Treaty of the Metre, is an international treaty that was signed in Paris on 20 May 1875 by representatives of 17 nations. The treaty created the International Bureau of Weights and Measures (BIPM), an intergovernmental organization under the authority of the General Conference on Weights and Measures (CGPM) and the supervision of the International Committee for Weights and Measures (CIPM), that coordinates international metrology and the development of the metric system.
The International System of Units is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, kelvin, second, metre, kilogram, candela, mole, and a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units. The system also specifies names for 22 derived units, such as lumen and watt, for other common physical quantities.
The torr is a unit of pressure based on an absolute scale, now defined as exactly 1/760 of a standard atmosphere. Thus one torr is exactly 101325/760 pascals (≈ 133.32 Pa).
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. 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.
Metrology is the science 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 Conference Generale des Poids et Mesures (CGPM) in 1960.
The International Committee for Weights and Measures (ICWM) consists of eighteen persons, each of a different nationality, from Member States of the Metre Convention appointed by the General Conference on Weights and Measures (CGPM) whose principal task is to promote worldwide uniformity in units of measurement by taking direct action or by submitting proposals to the CGPM.
The standard acceleration due to gravity, sometimes abbreviated as standard gravity, usually denoted by ɡ_{0} or ɡ_{n}, is the nominal gravitational acceleration of an object in a vacuum near the surface of the Earth. It is defined by standard as 9.80665 m/s^{2}. This value was established by the 3rd CGPM and used to define the standard weight of an object as the product of its mass and this nominal acceleration. The acceleration of a body near the surface of the Earth is due to the combined effects of gravity and centrifugal acceleration from the rotation of the Earth ; the total is about 0.5% greater at the poles than at the Equator.
The Kelvin scale is an absolute thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The kelvin is the base unit of temperature in the International System of Units (SI).
In metrology, a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. Standards are the fundamental reference for a system of weights and measures, against which all other measuring devices are compared. Historical standards for length, volume, and mass were defined by many different authorities, which resulted in confusion and inaccuracy of measurements. Modern measurements are defined in relationship to internationally standardized reference objects, which are used under carefully controlled laboratory conditions to define the units of length, mass, electrical potential, and other physical quantities.
A redefinition of SI base units is scheduled to come into force on 20 May 2019. The kilogram, ampere, kelvin, and mole will then be defined by setting exact numerical values for the Planck constant, the elementary electric charge, the Boltzmann constant, and the Avogadro constant, respectively. The metre and candela are already defined by physical constants, subject to correction to their present definitions. The new definitions aim to improve the SI without changing the size of any units, thus ensuring continuity with existing measurements. In November 2018, the 26th General Conference on Weights and Measures (CGPM) unanimously approved these changes, which the International Committee for Weights and Measures (CIPM) had proposed earlier that year.
The history of the metric system began in the Age of Enlightenment with simple notions of length and weight taken from natural ones, and decimal multiples and fractions of them. The system was so useful it became the standard of France and Europe in half a century. Other dimensions with unity ratios were added, and it went on to be adopted by the world.
The metric system was developed during the French Revolution to replace the various measures previously used in France. The metre is the unit of length in the metric system and was originally based on the dimensions of the earth, as far as it could be measured at the time. The litre, is the unit of volume and was defined as one thousandth of a cubic metre. The metric unit of mass is the kilogram and it was defined as the mass of one litre of water. The metric system was, in the words of French philosopher Marquis de Condorcet, "for all people for all time".
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.
Data from Giacomo, P., Du platine à la lumière [From platinum to light], Bull. Bur. Nat. Metrologie, 102 (1995) 5–14.