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^{ [Note 1] } were added, and it went on to be adopted by the world.
The Age of Enlightenment was an intellectual and philosophical movement that dominated the world of ideas in Europe during the 18th century, the "Century of Philosophy".
The natural environment encompasses all living and nonliving things occurring naturally, meaning in this case not artificial. The term is most often applied to the Earth or some parts of Earth. This environment encompasses the interaction of all living species, climate, weather and natural resources that affect human survival and economic activity. The concept of the natural environment can be distinguished as components:
The decimal numeral system is the standard system for denoting integer and noninteger numbers. It is the extension to noninteger numbers of the Hindu–Arabic numeral system. The way of denoting numbers in the decimal system is often referred to as decimal notation.
The first practical realisation of the metric system came in 1799, during the French Revolution, when the existing system of measures, which had become impractical for trade, was replaced by a decimal system based on the kilogram and the metre. The basic units were taken from the natural world: the unit of length, the metre, was based on the dimensions of the Earth, and the unit of mass, the kilogram, was based on the mass of water having a volume of one litre or a cubic decimetre. Reference copies for both units were manufactured in platinum and remained the standards of measure for the next 90 years. After a period of reversion to the mesures usuelles due to unpopularity of the metric system, the metrication of France as well as much of Europe was complete by midcentury.
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.
The French Revolution was a period of farreaching social and political upheaval in France and its colonies beginning in 1789. The Revolution overthrew the monarchy, established a republic, catalyzed violent periods of political turmoil, and finally culminated in a dictatorship under Napoleon who brought many of its principles to areas he conquered in Western Europe and beyond. Inspired by liberal and radical ideas, the Revolution profoundly altered the course of modern history, triggering the global decline of absolute monarchies while replacing them with republics and liberal democracies. Through the Revolutionary Wars, it unleashed a wave of global conflicts that extended from the Caribbean to the Middle East. Historians widely regard the Revolution as one of the most important events in human history.
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 SaintCloud, a suburb of Paris. After 20 May, it will be defined in terms of fundamental physical constants.
In the middle of the 19th century, James Clerk Maxwell put forward the concept of a coherent system where a small number of units of measure were defined as base units, and all other units of measure, called derived units, were defined in terms of the base units. Maxwell proposed three base units: length, mass and time. Advances in electromagnetism in the 19th century necessitated new units to be defined, and multiple incompatible systems of such units came into usage; none could be reconciled with the existing system of mechanical units. This impasse was resolved by Giovanni Giorgi, who in 1901 proved that a coherent system that incorporated electromagnetic units had to have an electromagnetic unit as a fourth base unit.
James Clerk Maxwell was a Scottish scientist in the field of mathematical physics. His most notable achievement was to formulate the classical theory of electromagnetic radiation, bringing together for the first time electricity, magnetism, and light as different manifestations of the same phenomenon. Maxwell's equations for electromagnetism have been called the "second great unification in physics" after the first one realised by Isaac Newton.
Giovanni Giorgi was an Italian physicist and electrical engineer who proposed the Giorgi system of measurement, the precursor to the International System of Units (SI).
The seminal 1875 Treaty of the Metre resulted in the fashioning and distribution of metre and kilogram artefacts, the standards of the future coherent system that became the SI, and the creation of an international body Conférence générale des poids et mesures or CGPM to oversee systems of weights and measures based on them.
In 1960, the CGPM launched the International System of Units (in French the Système international d'unités or SI) which had six "base units": the metre, kilogram, second, ampere, degree Kelvin (subsequently renamed the "kelvin") and candela; as well as 16 further units derived from the base units. A seventh base unit, the mole, and six additional derived units were added in succeeding years through the close of the twentieth century. During this period, the metre was redefined in terms of the speed of light, and the second was redefined in terms of the microwave frequency of a cesium atomic clock. Since the end of the 20th century, an effort has been undertaken to redefine the ampere, kilogram, mole and kelvin in terms of invariant constants of physics.
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 second is the base unit of time in the International System of Units (SI), commonly understood and historically defined as ^{1}⁄_{86400} of a day – this factor derived from the division of the day first into 24 hours, then to 60 minutes and finally to 60 seconds each. Analog clocks and watches often have sixty tick marks on their faces, representing seconds, and a "second hand" to mark the passage of time in seconds. Digital clocks and watches often have a twodigit seconds counter. The second is also part of several other units of measurement like meters per second for velocity, meters per second per second for acceleration, and per second for frequency.
The ampere, often shortened to "amp", is the base unit of electric current in the International System of Units (SI). It is named after AndréMarie Ampère (1775–1836), French mathematician and physicist, considered the father of electrodynamics.
Foundational aspects of mathematics and culture, together with advances in the sciences during the Enlightenment, set the stage for the emergence in the late 18th century of a system of measurement with rationally related units and simple rules for combining them.
In the early ninth century, when much of what later became France was part of the Holy Roman Empire, units of measure had been standardised by the Emperor Charlemagne. He had introduced standard units of measure for length and for mass throughout his empire. As the empire disintegrated into separate nations, including France, these standards diverged. In England the Magna Carta (1215) had stipulated that "There shall be standard measures of wine, ale, and corn (the London quarter), throughout the kingdom. There shall also be a standard width of dyed cloth, russet, and haberject, namely two ells within the selvedges. Weights are to be standardised similarly."^{ [1] }
The Holy Roman Empire was a multiethnic complex of territories in Western and Central Europe that developed during the Early Middle Ages and continued until its dissolution in 1806 during the Napoleonic Wars. The largest territory of the empire after 962 was the Kingdom of Germany, though it also came to include the neighboring Kingdom of Bohemia, the Kingdom of Burgundy, the Kingdom of Italy, and numerous other territories.
Charlemagne or Charles the Great, numbered Charles I, was king of the Franks from 768, king of the Lombards from 774, and emperor of the Romans from 800. He united much of western and central Europe during the Early Middle Ages. He was the first recognised emperor to rule from western Europe since the fall of the Western Roman Empire three centuries earlier. The expanded Frankish state that Charlemagne founded is called the Carolingian Empire. He was later canonized by Antipope Paschal III.
Magna Carta Libertatum, commonly called Magna Carta, is a charter of rights agreed to by King John of England at Runnymede, near Windsor, on 15 June 1215. First drafted by the Archbishop of Canterbury to make peace between the unpopular King and a group of rebel barons, it promised the protection of church rights, protection for the barons from illegal imprisonment, access to swift justice, and limitations on feudal payments to the Crown, to be implemented through a council of 25 barons. Neither side stood behind their commitments, and the charter was annulled by Pope Innocent III, leading to the First Barons' War. After John's death, the regency government of his young son, Henry III, reissued the document in 1216, stripped of some of its more radical content, in an unsuccessful bid to build political support for their cause. At the end of the war in 1217, it formed part of the peace treaty agreed at Lambeth, where the document acquired the name Magna Carta, to distinguish it from the smaller Charter of the Forest which was issued at the same time. Short of funds, Henry reissued the charter again in 1225 in exchange for a grant of new taxes. His son, Edward I, repeated the exercise in 1297, this time confirming it as part of England's statute law.
During the early medieval era, Roman numerals were used in Europe to represent numbers,^{ [2] } but the Arabs represented numbers using the Hindu numeral system, a positional notation that used ten symbols. In about 1202, Fibonacci published his book Liber Abaci (Book of Calculation) which introduced the concept of positional notation into Europe. These symbols evolved into the numerals "0", "1", "2" etc.^{ [3] }^{ [4] } At that time there was dispute regarding the difference between rational numbers and irrational numbers and there was no consistency in the way in which decimal fractions were represented.
Simon Stevin is credited with introducing the decimal system into general use in Europe.^{ [5] } In 1586, Simon Stevin published a small pamphlet called De Thiende ("the tenth") which historians credit as being the basis of modern notation for decimal fractions.^{ [6] } Stevin felt that this innovation was so significant that he declared the universal introduction of decimal coinage, measures, and weights to be merely a question of time.^{ [5] }^{ [7] }^{:70}^{ [8] }^{:91}
Since the time of Charlemagne, the standard of length had been a measure of the body, that from fingertip to fingertip of the outstretched arms of a large man,^{ [Note 2] } from a family of body measures called fathoms, originally used among other things, to measure depth of water. An artefact to represent the standard was cast in the most durable substance available in the Middle Ages, an iron bar ^{[ citation needed ]}. The problems of a nonreproducible artefact became apparent over the ages: it rusted, was stolen, beaten into a mortised wall until it bent, and was at times lost. When a new royal standard had to be cast, it was a different standard than the old one, so replicas of old ones and new ones came into existence and use. The artefact existed through the 18th century, and was called a teise or later, a toise (from Latin tense: outstretched (arms)). This would lead to a search in the 18th century for a reproducible standard based on some invariant measure of the natural world.
The invention of the pendulum clock with its characteristic second pendulum gave rise to proposals to use its length as a standard unit. But it became apparent that the pendulum lengths of calibrated clocks in different locations varied, and the problem was irresolute. A more uniform standard was needed.
The invention of the pendulum clock in 1656 by Dutch scientist Christiaan Huygens realised for the first time the ancient sexagesimal second. It was soon evident that such clocks' periods varied with both latitude and altitude so that a clock's geometry, i.e. its pendulum length was local to the place where it was calibrated.^{ [Note 3] }
In 1670, Gabriel Mouton, a French abbot and astronomer, published the book Observationes diametrorum solis et lunae apparentium in which he proposed a decimal system of measurement of length for use by scientists in international communication, to be based on the dimensions of the Earth. The milliare would be defined as a minute of arc along a meridian and would be divided into 10 centuria, the centuria into 10 decuria and so on, successive units being the virga, virgula, decima, centesima, and the millesima. Mouton used Riccioli's estimate^{[ clarification needed ]} that one degree of arc was 321,185 Bolognese feet,^{[ clarification needed ]} and his own experiments showed that a pendulum of length one virgula would beat 3959.2 times^{ [Note 4] } in half an hour.^{ [9] }^{ [Note 5] } He believed that with this information scientists in a foreign country would be able to construct a copy of the virgula for their own use.^{ [10] } Mouton's ideas attracted interest at the time; Picard in his work Mesure de la Terre (1671) and Huygens in his work Horologium Oscillatorium sive de motu pendulorum (1673) both proposing that a standard unit of length be tied to the beat frequency of a pendulum.^{ [11] }^{ [10] }
Since at least the Middle Ages, the earth was perceived as eternal, unchanging and of symmetrical shape (close to a sphere), so it was natural that some fractional measure of its surface should be proposed as a standard of length. But first, scientific information about the shape and size of the earth had to be obtained.
In 1669, Jean Picard, a French astronomer, was the first person to measure the size of the earth accurately. In a survey spanning one degree of latitude, he erred by only 0.44%
In Philosophiæ Naturalis Principia Mathematica (1686), Isaac Newton gave a theoretical explanation for the "bulging equator"^{ [Note 6] } which also explained the differences found in the lengths of the "second pendulums",^{ [12] } theories that were confirmed by the Académie's^{[ clarification needed ]} expedition to Peru in 1735.^{ [13] }
This section is missing information about Jacques Cassini's survey of earth completed of 17131718; we refer to it below without defining what the survey did. (January 2018) 
By the mideighteenth century, it had become apparent that standardisation of weights and measures between nations who traded and exchanged scientific ideas with each other was necessary. Spain, for example, had aligned her units of measure with the royal units of France,^{ [15] } and Peter the Great aligned the Russian units of measure with those of England.^{ [16] } In 1783 the British inventor James Watt, who was having difficulties in communicating with German scientists, called for the creation of a global decimal measurement system, proposing a system which used the density of water to link length and mass,^{ [14] } and in 1788 the French chemist Antoine Lavoisier commissioned a set of nine brass cylinders—a [French] pound and decimal subdivisions thereof for his experimental work.^{ [17] }^{:71}
In 1790, a proposal floated by the French to Britain and the United States, to establish a uniform measure of length, a metre based on the period of a pendulum with a beat of one second, was defeated in the British Parliament and United States Congress. The underlying issue was failure to agree on the latitude for the definition, since gravitational acceleration and therefore the length of the pendulum, varies with latitude: each party wanted a definition according to a major latitude passing through their own country. The direct consequence of the failure was the French unilateral development and deployment of the metric system and its spread by trade to the continent, the British adoption of the Imperial System of Measures throughout the realm in 1824, and the United States retention of the British common system of measures in place at the time of the independence of the colonies. And that is the situation that pertained for nearly the next 200 years.^{ [Note 7] }
It has been estimated that on the eve of the Revolution in 1789, the eight hundred, or so, units of measure in use in France had up to a quarter of a million different definitions because the quantity associated with each unit could differ from town to town, and even from trade to trade.^{ [8] }^{:2–3} Although certain standards, such as the pied du roi (the King's foot) had a degree of preeminence and were used by scientists, many traders chose to use their own measuring devices, giving scope for fraud and hindering commerce and industry.^{ [18] } These variations were promoted by local vested interests, but hindered trade and taxation.^{ [19] }^{ [20] }
In 1790, a panel of five leading French scientists was appointed by the Académie des sciences to investigate weights and measures. Its members were JeanCharles de Borda, JosephLouis Lagrange, PierreSimon Laplace, Gaspard Monge and Nicolas de Condorcet.^{ [8] }^{:2–3}^{ [21] }^{:46} Over the following year, the panel, after studying various alternatives, made a series of recommendations regarding a new system of weights and measures, including that it should have a decimal radix, that the unit of length should be based on a fractional arc of a quadrature of earth's meridian, and that the unit of weight should be that of a cube of water whose dimension was a decimal fraction of the unit of length.^{ [22] }^{ [23] }^{ [7] }^{:50–51}^{ [24] }^{ [25] } The proposals were accepted by the French Assembly on 30 March 1791.^{ [26] }
Following acceptance, the Académie des sciences was instructed to implement the proposals. The Académie broke the tasks into five operations, allocating each part to a separate working group:^{ [7] }^{:82}
The panel decided that the new measure of length should be equal to one tenmillionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian passing through Paris.^{ [19] } Using Jean Picard's survey of 1670 and Jacques Cassini's survey of 1718, a provisional value of 443.44 lignes was assigned to the metre which, in turn, defined the other units of measure.^{ [8] }^{:106} While Méchain and Delambre were completing their survey, the commission had ordered a series of platinum bars to be made based on the provisional metre. When the final result was known, the bar whose length was closest to the meridianal definition of the metre would be selected.
After 1792 the name of the original defined unit of mass, "gramme", which was too small to serve as a practical realisation, was adopted, the new prefix "kilo" was added to it to form the name "kilogramme". Consequently, the kilogram is the only SI base unit that has an SI prefix as part of its unit name. A provisional kilogram standard was made and work was commissioned to determine the precise mass of a cubic decimetre (later to be defined as equal to one litre) of water. The regulation of trade and commerce required a "practical realisation": a singlepiece, metallic reference standard that was one thousand times more massive that would be known as the grave.^{ [Note 9] } This mass unit defined by Lavoisier and René Just Haüy had been in use since 1793.^{ [27] } This new, practical realisation would ultimately become the base unit of mass. On 7 April 1795, the gramme, upon which the kilogram is based, was decreed to be equal to "the absolute weight of a volume of pure water equal to a cube of one hundredth of a metre, and at the temperature of the melting ice".^{ [25] } Although the definition of the kilogramme specified water at 0 °C – a highly stable temperature point – it was replaced with the temperature at which water reaches maximum density, measured at the time as 4 °C.^{ [28] } They concluded that one cubic decimetre of water at its maximum density was equal to 99.92072% of the mass of the provisional kilogram made earlier that year.^{ [29] }
On 7 April 1795 the metric system was formally defined in French law.^{ [Note 10] } It defined six new decimal units:^{ [25] }
Decimal multiples of these units were defined by Greek prefixes: "myria" (10,000), "kilo" (1000), "hecto" (100) and "deka" (10) and submultiples were defined by the Latin prefixes "deci" (0.1), "centi" (0.01) and "milli" (0.001).^{ [30] }
The 1795 draft definitions enabled provisional copies of the kilograms and metres to be constructed.^{ [31] }^{ [32] }
The task of surveying the meridian arc, which was estimated to take two years, fell to Pierre Méchain and JeanBaptiste Delambre. The task eventually took more than six years (1792–1798) with delays caused not only by unforeseen technical difficulties but also by the convulsed period of the aftermath of the Revolution.^{ [8] } Apart from the obvious nationalistic considerations, the Paris meridian was also a sound choice for practical scientific reasons: a portion of the quadrant from Dunkirk to Barcelona (about 1000 km, or onetenth of the total) could be surveyed with start and endpoints at sea level, and that portion was roughly in the middle of the quadrant, where the effects of the Earth's oblateness were expected to be the largest.^{ [19] }
The project was split into two parts – the northern section of 742.7 km from the Belfry, Dunkirk to Rodez Cathedral which was surveyed by Delambre and the southern section of 333.0 km from Rodez to the Montjuïc Fortress, Barcelona which was surveyed by Méchain.^{ [8] }^{: 227–230}^{ [Note 11] }
Delambre used a baseline of about 10 km in length along a straight road, located close to Melun. In an operation taking six weeks, the baseline was accurately measured using four platinum rods, each of length two toise (about 3.9 m).^{ [8] }^{: 227–230} Thereafter he used, where possible, the triangulation points used by Cassini in his 1744 survey of France. Méchain's baseline, of a similar length, and also on a straight section of road was in the Perpignan area.^{ [8] }^{: 240–241} Although Méchain's sector was half the length of Delambre, it included the Pyrenees and hitherto unsurveyed parts of Spain. After the two surveyors met, each computed the other's baseline in order to crosscheck their results and they then recomputed the metre as 443.296 lignes,^{ [19] }^{ [Note 12] } notably shorter than the 1795 provisional value of 443.44 lignes
On 15 November 1798 Delambre and Méchain returned to Paris with their data, having completed the survey. The final value of the metre was defined in 1799 as the computed value from the survey.
In June 1799, platinum prototypes were fabricated according to the measured quantities, the metre des archives defined to be a length of 443.296 lignes, and the kilogramme des archives defined to be a weight of 18827.15 grains, and entered into the French National Archives. In December of that year, the metric system based on them became by law the sole system of weights and measures in France from 1801 until 1812.
Despite the law, the populace continued to use the old measures. In 1812, Napoleon revoked the law and issued one called the mesures usuelles , restoring the names and quantities of the customary measures but redefined as round multiples of the metric units, so it was a kind of hybrid system. In 1837, after the collapse of the Napoleonic Empire, the new Assembly reimposed the metric system defined by the laws of 1795 and 1799, to take effect in 1840. The metrication of France took until about 1858 to be completed. Some of the old unit names, especially the livre , originally a unit of mass derived from the Roman libra (as was the English pound), but now meaning 500 grams, are still in use today.
At the start of the nineteenth century, the French Academy of Sciences' artefacts for length and mass were the only nascent units of the metric system that were defined in terms of formal standards. Other units based on them, except the litre proved to be shortlived. Pendulum clocks that could keep time in seconds had been in use for about 150 years, but their geometries were local to both latitude and altitude, so there was no standard of timekeeping. Nor had a unit of time been recognised as an essential base unit for the derivation of things like force and acceleration. Some quantities of electricity like charge and potential had been identified, but names and interrelationships of units were not yet established.^{ [Note 14] } Both Fahrenheit (~1724) and Celsius (~1742) scales of temperature existed, and varied instruments for measuring units or degrees of them. The base/derived unit model had not yet been elaborated, nor was it known how many physical quantities might be interrelated.
A model of interrelated units was first proposed in 1861 by the British Association for the Advancement of Science (BAAS) based on what came to be called the "mechanical" units (length, mass and time). Over the following decades, this foundation enabled mechanical, electrical and thermal ^{[ when? ]} units to be correlated.
In 1832 German mathematician CarlFriedrich Gauss made the first absolute measurements of the Earth's magnetic field using a decimal system based on the use of the millimetre, milligram, and second as the base unit of time.^{ [33] }^{:109} Gauss' second was based on astronomical observations of the rotation of the earth, and was the sexagesimal second of the ancients: a partitioning of the solar day into two cycles of 12 periods, and each period divided into 60 intervals, and each interval so divided again, so that a second was 1/86,400th of the day.^{ [Note 15] } This effectively established a time dimension as a necessary constituent of any useful system of measures, and the astronomical second as the base unit.
In a paper published in 1843, James Prescott Joule first demonstrated a means of measuring the energy transferred between different systems when work is done thereby relating Nicolas Clément's calorie, defined in 1824 as "the amount of heat required to raise the temperature of 1 kg of water from 0 to 1 °C at 1 atmosphere of pressure" to mechanical work.^{ [34] }^{ [35] } Energy became the unifying concept of nineteenth century science,^{ [36] } initially by bringing thermodynamics and mechanics together and later adding electrical technology.
In 1861 a committee of the BAAS including William Thomson (later Lord Kelvin), James Clerk Maxwell and James Prescott Joule among its members was tasked with investigating the "Standards of Electrical Resistance".^{[ clarification needed ]} In their first report (1862)^{ [37] } they laid the ground rules for their work – the metric system was to be used, measures of electrical energy must have the same units as measures of mechanical energy and two sets of electromagnetic units would have to be derived – an electromagnetic system and an electrostatic system. In the second report (1863)^{ [38] } they introduced the concept of a coherent system of units whereby units of length, mass and time were identified as "fundamental units" (now known as base units ). All other units of measure could be derived (hence derived units ) from these base units. The metre, gram and second were chosen as base units.^{ [39] }^{ [40] }
In 1861, before a meeting of the BAAS, Charles Bright and Latimer Clark proposed the names of ohm, volt, and farad in honour of Georg Ohm, Alessandro Volta and Michael Faraday respectively for the practical units based on the CGS absolute system. This was supported by Thomson (Lord Kelvin)^{ [41] } The concept of naming units of measure after noteworthy scientists was subsequently used for other units.
In 1873, another committee of the BAAS that also counted Maxwell and Thomson among its members and tasked with "the Selection and Nomenclature of Dynamical and Electrical Units" recommended using the cgs system of units. The committee also recommended the names of "dyne" and "erg" for the cgs units of force and energy.^{ [42] }^{ [40] }^{ [43] } The cgs system became the basis for scientific work for the next seventy years.
The reports recognised two Centimetregramsecond based systems for electrical units, the Electromagnetic (or absolute) system of units (EMU) and the Electrostatic system of units (ESU).
In the 1820s Georg Ohm formulated Ohms Law which can be extended to relate power to current, electric potential (voltage) and resistance.^{ [44] }^{ [45] } During the following decades the realisation of a coherent system of units that incorporated the measurement of electromagnetic phenomena and Ohm's law was beset with problems – several different systems of units were devised.
Symbols used in this section

In the three CGS systems, the constants and and consequently and were dimensionless.^{[ clarification needed ]}
The electrical units of measure did not easily fit into the coherent system of mechanical units defined by the BAAS. Using dimensional analysis, the dimensions of voltage in the ESU system were identical to the dimensions of current in the EMU system, while resistance had dimensions of velocity in the EMU system, but the inverse of velocity in the ESU system.^{ [40] }
Maxwell and Boltzmann had produced theories describing the interrelational of temperature, pressure and volume of a gas on a microscopic scale but otherwise, in 1900, there was no understanding of the microscopic nature of temperature.^{ [55] }^{ [56] }
By the end of the nineteenth century, the fundamental macroscopic laws of thermodynamics had been formulated and although techniques existed to measure temperature using empirical techniques, the scientific understanding^{[ clarification needed ]} of the nature of temperature was minimal.
With increasing international adoption of the metre, the shortcomings of the mètre des Archives as a standard became ever more apparent. Countries which adopted the metre as a legal measure purchased standard metre bars that were intended to be equal in length to the mètre des Archives, but there was no systematic way of ensuring that the countries were actually working to the same standard. The meridianal definition, which had been intended to ensure international reproducibility, quickly proved so impractical that it was all but abandoned in favour of the artefact standards, but the mètre des Archives (and most of its copies) were "end standards": such standards (bars which are exactly one metre in length) are prone to wear with use, and different standard bars could be expected to wear at different rates.^{ [57] }
When, in 1867, it was proposed that a new international standard metre be created, the length was taken to be that of the mètre des Archives "in the state in which it shall be found".^{ [58] }^{ [59] } The International Conference on Geodesy in 1867 called for the creation of a new, international prototype metre^{ [58] }^{ [59] }^{ [Note 18] } and to arrange a system where national standards could be compared with it. The international prototype would also be a "line standard", that is the metre was defined as the distance between two lines marked on the bar, so avoiding the wear problems of end standards. The French government gave practical support to the creation of an International Metre Commission, which met in Paris in 1870 and again in 1872 with the participation of about thirty countries.^{ [58] }
On 20 May 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 states.^{ [20] }^{ [60] } This treaty established the following organisations to conduct international activities relating to a uniform system for measurements:
The international prototype metre and kilogram were both made from a 90% platinum, 10% iridium alloy which is exceptionally hard and which has good electrical and thermal conductivity properties. The prototype had a special Xshaped (Tresca) cross section to minimise the effects of torsional strain during length comparisons.^{ [20] } and the prototype kilograms were cylindrical in shape. The London firm Johnson Matthey delivered 30 prototype metres and 40 prototype kilograms. At the first meeting of the CGPM in 1889 bar No. 6 and cylinder No. X were accepted as the international prototypes. The remainder were either kept as BIPM working copies or distributed to member states as national prototypes.^{ [61] }
Following the Convention of the Metre, in 1889 the BIPM had custody of two artefacts – one to define length and the other to define mass. Other units of measure which did not rely on specific artefacts were controlled by other bodies. Although the definition of the kilogram remained unchanged throughout the twentieth century, the 3rd CGPM in 1901 clarified that the kilogram was a unit of mass, not of weight. The original batch of 40 prototypes (adopted in 1889) were supplemented from time to time with further prototypes for use by new signatories to the Metre Convention.^{ [62] }
In 1921 the Treaty of the Metre was extended to cover electrical units with the CGPM merging its work with that of the IEC.
The twentieth century history of measurement is marked by five periods: the 1901 definition of the coherent MKS system; the intervening 50 years of coexistence of the MKS, cgs and common systems of measures; the 1948 Practical system of units prototype of the SI; the introduction of the SI in 1960; and the evolution of the SI in the latter half century.
The need for an orthogonal electromagnetic unit to resolve the difficulties of defining such units in terms of length, mass and time was identified by Giorgi in 1901. This led to Giorgi presenting a paper in October 1901 to the congress of the Associazione Elettrotecnica Italiana (A.E.I.)^{ [63] } in which he showed that a coherent electromechanical system of units could be obtained by adding a fourth base unit of an electrical nature (ampere, volt or ohm) to the three base units proposed in the 1861 BAAS report. This gave the constants k_{e} and k_{m} physical dimensions and hence the electromechanical quantities ε_{0} and μ_{0} were also given physical dimensions.^{ [64] } His work also recognised the unifying concept that energy played in the establishment of a coherent, rational system of units with the joule as the unit of energy and the electrical units in the International system of units remaining unchanged.^{ [54] }^{:156} It took more than thirty years before Giorgi's work was accepted in practice by the IEC.
As the industrial era dawned in Britain as well as continental Europe and overseas, the cgs system of units as adopted by the British Association for the Advancement of Science in 1873 with its plethora of electrical units continued to be the dominant system of measurement, and would remain so for at least the next 60 years. The advantages were several: it had a variegated set of derived units which, while not quite coherent, were at least homologous; the MKS system lacked a defined unit of electromagnetism at all; the MKS units were inconveniently large for the sciences; customary systems of measures held sway in the United States, Britain and the British empire, and even to some extent in France the birthplace of the metric system, which inhibited adoption of any competing system. Finally, war, nationalism and other political forces inhibited development of the science favouring a coherent system of units.
At the 8th CGPM in 1933 the need to replace the "International" electrical units with "absolute" units was raised. The IEC proposal that Giorgi's 'system', denoted informally as MKSX, be adopted was accepted, but no decision was made as to which electrical unit should be the fourth base unit. In 1935 J E Sears^{ [65] }^{[ citation needed ]}, proposed that this should be the ampere, but World War II prevented this being formalised until 1946.
The first (and only) followup comparison of the national standards with the international prototype metre was carried out between 1921 and 1936,^{ [20] }^{ [59] } and indicated that the definition of the metre was preserved to within 0.2 µm.^{ [66] } During this followup comparison, the way in which the prototype metre should be measured was more clearly defined—the 1889 definition had defined the metre as being the length of the prototype at the temperature of melting ice, but in 1927 the 7th CGPM extended this definition to specify that the prototype metre shall be "supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 mm from each other".^{ [33] }^{:142–43, 148} The choice of 571 mm represents the Airy points of the prototype—the points at which the bending or droop of the bar is minimised.^{ [67] }
The 9th CGPM met in 1948, fifteen years after the 8th CGPM. In response to formal requests made by the International Union of Pure and Applied Physics and by the French government to establish a practical system of units of measure, the CGPM requested the CIPM to prepare recommendations for a single practical system of units of measurement, suitable for adoption by all countries adhering to the Metre Convention.^{ [68] } The CIPM's draft proposal, was an extensive revision and simplification of the metric unit definitions, symbols and terminology based on the MKS system of units.
In accordance with astronomical observations, the second was set as a fraction of the year 1900. The electromagnetic base unit as required by Giorgi was accepted as the ampere. After negotiations with the CIS and IUPAP, two further base units, the degree kelvin and the candela were also proposed as base units.^{ [69] } For the first time the CGPM made recommendations concerning derived units. At the same time the CGPM adopted conventions for the writing and printing of unit symbols and numbers and catalogued the symbols for the most important MKS and CGS units of measure.^{ [70] }
Until the advent of the atomic clock, the most reliable timekeeper available to mankind was the earth's rotation. It was natural therefore that the astronomers under the auspice of the International Astronomical Union (IAU) took the lead in maintaining the standards relating to time. During the twentieth century it became apparent that the earth's rotation was slowing down resulting in days becoming 1.4 milliseconds longer each century^{ [71] } – this was verified by comparing the calculated timings of eclipses of the sun with those observed in antiquity going back to Chinese records of 763 BC.^{ [72] } In 1956 the 10th CGPM instructed the CIPM to prepare a definition of the second; in 1958 the definition was published stating that the second (called an ephemeris second) would be calculated by extrapolation using earth's rotational speed in 1900.^{ [71] }
In accordance with Giorgi's proposals of 1901, the CIPM also recommended that the ampere be the base unit from which electromechanical units would be derived. The definitions for the ohm and volt that had previously been in use were discarded and these units became derived units based on the ampere. In 1946 the CIPM formally adopted a definition of the ampere based on the original EMU definition and redefined the ohm in terms of other base units.^{ [73] } The definitions for absolute electrical system based on the ampere was formalised in 1948.^{ [74] } The draft proposal units with these names are very close, but not identical to the International units.^{ [75] }
In the Celsius scale from the 18th century, temperature was expressed in degrees Celsius with the definition that ice melted at 0 °C and at standard atmospheric pressure, water boiled at 100 °C. A series of lookup tables defined temperature in terms of interrelated empirical measurements made using various devices. In 1948, definitions relating to temperature had to be clarified. The Celsius, as an angular measure, was adopted for general use in a number of countries, so in 1948 the General Conference on Weights and Measures (CGPM) recommended that the degree Celsius, as used for the measurement of temperature, be renamed the degree Celsius.^{ [76] } The SI Brochure (2006) notes that the gon is now a littleused alternative to the degree.^{ [77] }
At the 9th CGPM, the Celsius temperature scale was renamed the Celsius scale and the scale itself was fixed by defining the triple point of water as 0.01 °C,^{ [78] } though the CGPM left the formal definition of absolute zero until the 10th CGPM when the name "Kelvin" was assigned to the absolute temperature scale and triple point of water was defined as being 273.16 °K.^{ [79] }
Prior to 1937, the International Commission on Illumination (CIE from its French title, the Commission Internationale de l'Eclairage) in conjunction with the CIPM produced a standard for luminous intensity to replace the various national standards. This standard, the candela (cd) which was defined as "the brightness of the full radiator at the temperature of solidification of platinum is 60 new candles per square centimetre".^{ [80] } was ratified by the CGPM in 1948.
The newly accepted definition of the ampere allowed practical and useful coherent definitions of a set of electromagnetic derived units including farad, henry, watt, tesla, weber, volt, ohm, and coulomb. Two derived units, lux and lumen were based on the new candela, and one, degree Celsius, equivalent to the degree Kelvin. Five other miscellaneous derived units completed the draft proposal: radian, steradian, hertz, joule and newton.
In 1952 the CIPM proposed the use of wavelength of a specific light source as the standard for defining length, and in 1960 the CGPM accepted this proposal using radiation corresponding to a transition between specified energy levels of the krypton 86 atom as the new standard for the metre. The standard metre artefact was retired.
In 1960, Giorgi's proposals were adopted as the basis of the Système International d'Unités (International System of Units), the SI.^{ [33] }^{:109} This initial definition of the SI included six base units, the metre, kilogram, second, ampere, degree Kelvin and candela, and sixteen coherent derived units.^{ [81] }
The evolution of the SI after its publication in 1960 has seen the addition of a seventh base unit, the mole, and six more derived units, the pascal for pressure, the gray, sievert and becquerel for radiation, the siemens for electrical conductance, and katal for catalytic (enzymatic) activity. Several units have also been redefined in terms of physical constants.
Over the ensuing years, the BIPM developed and maintained crosscorrelations relating various measuring devices such as thermocouples, light spectra and the like to the equivalent temperatures.^{ [82] }
The mole was originally known as a gramatom or a grammolecule – the amount of a substance measured in grams divided by its atomic weight. Originally chemists and physicists had differing views regarding the definition of the atomic weight – both assigned a value of 16 atomic mass units (amu) to oxygen, but physicists defined oxygen in terms of the ^{16}O isotope whereas chemists assigned 16 amu to ^{16}O, ^{17}O and ^{18}O isotopes mixed in the proportion that they occur in nature. Finally an agreement between the International Union of Pure and Applied Physics ^{ [83] } (IUPAP) and the International Union of Pure and Applied Chemistry (IUPAC) brought this duality to an end in 1959/60, both parties agreeing to define the atomic weight of ^{12}C as being exactly 12 amu. This agreement was confirmed by ISO and in 1969 the CIPM recommended its inclusion in SI as a base unit. This was done in 1971 at the 14th CGPM.^{ [33] }^{:114–115}
The second major trend in the postmodern SI was the migration of unit definitions in terms of physical constants of nature.
In 1967, at the 13th CGPM the degree Kelvin (°K) was renamed the "kelvin" (K).^{ [84] }
Astronomers from the US Naval Observatory (USNO) and the National Physical Laboratory determined a relationship between the frequency of radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom and the estimated rate of rotation of the earth in 1900. Their atomic definition of the second was adopted in 1968 by the 13th CGPM.
By 1975, when the second had been defined in terms of a physical phenomenon rather than the earth's rotation, the CGPM authorised the CIPM to investigate the use of the speed of light as the basis for the definition of the metre. This proposal was accepted in 1983.^{ [85] }
The candela definition proved difficult to implement so in 1979, the definition was revised and the reference to the radiation source was replaced by defining the candela in terms of the power of a specified frequency of monochromatic yellowishgreen visible light,^{ [33] }^{: 115} which is close to the frequency where the human eye, when adapted to bright conditions, has greatest sensitivity.
After the metre was redefined in 1960, the kilogram remained the only SI base defined by a physical artefact. During the years that followed the definitions of the base units and particularly the mise en pratique^{ [87] } to realise these definitions have been refined.
The third periodic recalibration in 1988–1989 revealed that the average difference between the IPK and adjusted baseline for the national prototypes was 50 μg – in 1889 the baseline of the national prototypes had been adjusted so that the difference was zero. As the IPK is the definitive kilogram, there is no way of telling whether the IPK had been losing mass or the national prototypes had been gaining mass.^{ [86] }
During the course of the century, the various national prototypes of the kilogram were recalibrated against the International Prototype Kilogram (IPK) and therefore against each other. The initial 1889 startingvalue offsets of the national prototypes relative to the IPK were nulled,^{ [86] } with any subsequent mass changes being relative to the IPK.
In 2007 the CIPM and the CIE agreed a program of cooperation with the CIPM taking the lead in defining the use of units of measure and the CIE taking the lead in defining the behaviour of the human eye.^{ [88] }
Increasingly the use of the Boltzmann Relationship was used as the reference point and it appears likely that in 2015 the CGPM will redefine temperature in terms of the Boltzmann constant rather than the triple point of water.^{ [89] }
At its 23rd meeting (2007), the CGPM mandated the CIPM to investigate the use of natural constants as the basis for all units of measure rather than the artefacts that were then in use. At a meeting of the CCU held in a place called Reading, United Kingdom in September 2010, a resolution^{ [90] } and draft changes to the SI brochure that were to be presented to the next meeting of the CIPM in October 2010 were agreed to in principle.^{ [89] } The proposals that the CCU put forward were that:
Beginning in the early 1990s, the International Avogadro Project has been working to create a 1 kilogram, 94 mm, sphere made of a uniform silicon28 crystal. Because the sphere will have been created so precisely, it will be able to replace the IPK. Additionally, due to its precise construction, it may be the roundest object ever created.^{ [91] }
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.
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.
The General Conference on Weights and Measures is the supreme authority of the International Bureau of Weights and Measures, the intergovernmental 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 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 onethousandth of a cubic metre.
The metre or meter is the base unit of length in the International System of Units (SI). The SI unit symbol is m. The metre is defined as the length of the path travelled by light in vacuum in 1/299 792 458 of a second.
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 defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units and their physical quantities are the metre for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity, and the mole for amount of substance.
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 decimalbased 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 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.
France has a unique history of units of measurement due to the radical decision to invent and adopt the metric system after the French Revolution.
The grave was the original name of the kilogram, in an early version of the metric system between 1793 and 1795.
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 second, 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 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.
On voit que le projet de Mouton est, sans aucune différence de principe, celui qui a ét réalisé par notre Système métrique. [It can be seen that Mouton's proposal was, in principle, no different to the metric system as we know it.]
(pg 140) The originator of the metric system might be said to be Gabriel Mouton