History of the metric system
Encyclopedia
The origins of the metric system
date back to the sixteenth century when Simon Stevin
published details of his decimal
notation and the seventeenth century when John Wilkins
published a proposal for a decimal system of measurement based on natural units. The first practical realisation of the metric system came during the French Revolution
when the existing system of measure which had fallen into disrepute was replaced by a decimal system based on the kilogram
and the metre
. The metric system was, in the words of Condorcet
, "for all people for all time". 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 one thousandth of a cubic metre
. Reference copies for both units were manufactured and placed in the custody of the Academy of Sciences.
During the first half of the nineteenth century, the metric system was adopted by the scientific community. In the middle of the 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. This concept worked well with mechanics
, but attempts to describe electromagnetic force
s in terms of these units were beset with difficulties. By the end of the nineteenth century four principal variants of the metric system were in place for the measurement of electromagnetic phenomena - three based on the centimetre-gram-second system of units
(CGS system) and one on the metre-kilogram-second system of units (MKS system). 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 the fourth base unit.
Until 1875 the French government owned the prototype metre and kilogram, but in that year the Convention of the metre was signed and control of the standards relating to mass and length passed onto a trio of inter-government organisations, the senior of which was the General Conference on Weights and Measures
(in French the Conférence générale des poids et mesures or CGPM). During the first half of the twentieth century, the CGPM cooperated with a number of other organisations and by 1960 it had responsibility for defining temporal, electrical, thermal, molecular and luminar measurements while other international organisations continued their roles in how these units of measurement were used.
In 1960, the CGPM launched the International System of Units
(in French the Système international d'unités or SI) which had 6 base units - the metre, kilogram, second
, ampere
, degree kelvin
(subsequently renamed the "kelvin") and candela
and 22 derived units. The mole
was added as a seventh base unit in 1971. During this period, the metre was redefined in terms of the wavelength
of the waves from a particular light source and the second
was defined in terms of the frequency
of radiation
from another light source. By the end of the century work was well under way to redefine the ampere, kilogram, mole and kelvin in terms of the basic constants of physics
, thereby ensuring that all the base units were in theory available to everybody. It is expected that this work will be completed by 2015.
introduced standard units of measure for length and for mass throughout the Holy Roman Empire
. As the empire disintegrated these standards diverged and by the seventeenth century there were numerous units of measure within regions and different sizes having the same name across regions. The variations were promoted by local vested interests but hindered trade and taxation.
In 1586, the Flemish mathematician Simon Stevin
published a small pamphlet called De Thiende ("the tenth"). Decimal fractions had been employed for the extraction of square roots some five centuries before his time, but nobody used decimal numbers in daily life. He 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.
, first secretary of the Royal Society
of London in 1668 and backed up by Tito Livio Burattini
who promoted the "metro cattolico" from which the word metre was to be derived.
Wilkins' proposed a "seconds pendulum" (a pendulum
with a half-period of one second
) as the unit of length: such pendulums had recently been demonstrated by Christiaan Huygens, and their length is very close to one modern metre (as well as to length units which were then in use, such as the yard
). However, it was soon discovered that the length of a seconds pendulum varies from place to place: French astronomer Jean Richer
had measured the 0.3% difference in length between Cayenne (in French Guiana) and Paris.
, a French abbot and scientist, proposed a decimal system of measurement based on the circumference of the Earth. His suggestion was a unit, the milliare, be defined as a minute of arc along a meridian. He then suggested a system of sub-units, dividing successively by factors of ten into the centuria, decuria, virga, virgula, decima, centesima, and millesima. His ideas attracted interest at the time, and were supported by both Jean Picard
and Christiaan Huygens in 1673, and also studied at the Royal Society in London. In the same year, Gottfried Leibniz
independently made proposals similar to those of Mouton.
in 1215 decreed that "there shall be one unit of measure throughout the realm", France had a multitude of units of measure. It has been estimated that on the eve of the Revolution a quarter of a million different units of measure were in use in France; in many cases quantity associated with each unit of measure differed from town to town and even from trade to trade. Although certain standards, such as the pied du roi (the King's foot) had a degree of pre-eminence and were used by savant
s, many traders chose to use their own measuring devices giving scope for fraud and hindering commerce and industry.
By the mid eighteenth century the need for standardisation of weights and measures had become apparent - Spain aligned her units of measure with the royal units of France, and Peter the Great aligned the Russian units of measure with the English units.
In 1789 the French finances were in a perilous state and in May Louis XVI summonsed the Estates General
. On 14 July 1789, the mob stormed the Bastille
and in August 1789 the nobility surrendered their privileges, including the right to control local weights and measures. Louis XVI charged a group of experts including such notables as Lavoisier, Condorcet, Laplace, Borda and Legendre
to produce the system of measures that would replace the disparate system then in place.
There was also a wish that the units of measure should be for all people and for all time and therefore not dependant on an artifact owned by any one particular nation. Talleyrand, at the prompting of the savant Condorcet, approached the British and the Americans in the early 1790s with proposals of a joint effort to define the metre.
On 13 July 1790 Thomas Jefferson presented a document Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States to congress in which he advocated a decimal system that used traditional names for units (such as ten inches per foot). The report was considered but not adopted by Congress.
Talleyrand resurrected the idea of the seconds pendulum before the Constituent Assembly in 1790, suggesting that the new measure be defined at 45°N (a latitude that, in France, runs just north of Bordeaux and just south of Grenoble): despite the support of the Assembly, and of Great Britain and the newly independent United States, nothing came of Talleyrand's proposal.
The French Revolution and subsequent Napoleonic Wars marked the end of the Age of Enlightenment
. The forces of change that had been brewing manifest themselves across all of France, including the way in which units of measure should be defined. The savants of the day favoured the use of a system of units that were inter-related and which used a decimal basis.
The metric system of measure was first given a legal basis in 1795 by the French Revolution
ary government. Article 5 of the law of 18 Germinal, Year III (7 April 1795) defined five units of measure. The units and their preliminary values were:
Decimal multiples and submultiples of these units would be defined by Greek prefixes
- "myria", "kilo", "hecta" (100), "deka", "deci", "centi" and "milli". Using Cassini's
survey of 1744, a provisional value of 443.44 lignes was assigned to the metre which, in turn, defined the other units of measure.
The final value of the metre had to wait until 1799 when Delambre and Mechain presented the results of their survey between Dunkirk and Barcelona
which fixed the length of the metre at 443.296 lignes. The law 19 Frimaire An VIII (10 December 1799) defined the metre in terms of this value and the kilogram as being 18827.15 grains. These definitions enabled reference copies of the kilograms and metres to be constructed that were to used as standards for the next 90 years.
At the same time, a new decimal-based system for angular measurement was implemented. The right angle was divided into 100 grads
which in turn was divided in 100 centigrads. An arc on the earth’s surface formed by an angle of one centigrade was one kilometre. The use of the centigrade was adopted for general use in a number countries and in 1948 the General Conference on Weights and Measures
(CGPM) recommended that the degree centigrade (used for the measurement of temperature) be renamed the degree Celsius
.
The question of measurement reform was placed in the hands of the Academy of Sciences who appointed a commission chaired by Jean-Charles de Borda
. Borda could be said to have been a fanatic for decimalization: he had invented the "repeating circle", a surveying instrument which allowed a much-improved precision in the measurement of angles between landmarks, but insisted that it be calibrated in "grades
" ( of a quarter-circle) rather than degree
s, with 100 minutes to a grade and 100 seconds to a minute. For Borda, the seconds pendulum was a poor choice for a standard because the second (as a unit of time) was insufficiently decimal: he preferred a system of 10 hours to the day, 100 minutes to the hour and 100 seconds to the minute...
Instead, the commission – whose members included Lagrange, Laplace
, Monge
and Condorcet – decided that the new measure should be equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian
passing through Paris. Apart from the obvious nationalistic considerations, the Paris meridian
was also a sound choice for practical scientific reasons: a portion of the quadrant from Dunkerque to Barcelona (about 1000 km, or one-tenth of the total) could be surveyed with start- and end-points 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.
The task of surveying the meridian arc
fell to Pierre Méchain
and Jean-Baptiste Delambre, and armed with letters of authorisation signed by Louis XVI the task took more than six years (1792–98). In the meantime, the commission calculated a provisional value from older surveys of 443.44 ligne
s.
The project was split into two parts - the northern section of 742.7 km from the Belfry, Dunkirk to Rodez Cathederal
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.
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). 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. 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 cross-check their results and they then recomputed the kilometre. Their result came out at 0.144 lignes shorter than the provisional value, a difference of about 0.03%.
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 was selected and placed in the National Archives on 22 June 1799 (4 messidor An VII in the Republican calendar) as a permanent record of the result: this standard metre bar became known as the mètre des Archives.
The metric system
, that is the system of units based on the metre, was officially adopted in France on 10 December 1799 (19 frimaire An VIII) and became the sole legal system of weights and measures from 1801.
It soon became apparent that Méchain and Delambre's result (443.296 lignes) was slightly too short for the meridianal definition of the metre. Arago
and Biot
extended the survey to the island of Formentera in the western Mediterranean Sea in 1806–9, and found that one ten-millionth of the Earth's quadrant should be 443.31 lignes: later work increased the value to 443.39 lignes. The modern value, for the WGS 84 reference spheroid, is m or lignes.
Nevertheless, the mètre des Archives remained the legal and practical standard for the metre in France, even once it was known that it did not exactly correspond to the meridianal definition. 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".
s that was used for a few years until it was replaced by the kilogram
standard in 1799.
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”. Although this was the definition of the gram, the regulation of trade and commerce required a “practical realisation”: a single-piece, metallic reference standard that was one thousand times more massive that would be known as “grave” (symbol G). This mass unit, whose name is derived from the word “gravity”, defined by Lavoisier
and René Just Haüy
had been in use since 1793. Notwithstanding that the definition of the base unit of mass was the gramme (alternatively “gravet”), this new, practical realisation would ultimately become the base unit of mass. A provisional kilogram standard was made and work was commissioned to determine precisely how massive a cubic decimetre (later to be defined as equal to one litre
) of water was.
Although the decreed definition of the kilogram specified water at 0 °C — a highly stable temperature point — the scientists tasked with producing the new practical realisation chose to redefine the standard and perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C. 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. Four years later in 1799, an all-platinum standard, the “Kilogramme des Archives”, was fabricated with the objective that it would equal, as close as was scientifically feasible for the day, to the mass of cubic decimetre of water at 4 °C. The kilogram was defined to be equal to the mass of the Kilogramme des Archives and this standard stood for the next ninety years.
Note that the new metric system did not come into effect until after the French Revolution
, when the new revolutionary government captured the idea of the metric system. The decision of the Republican government to name this new unit the “kilogramme” had been mainly politically motivated, because the name “grave” was at that time considered politically incorrect as it resembled the aristocratic German title of the Graf
, an alternative name for the title of Count
that, like other nobility titles, was inconsistent with the new French Republic notion of equality (égalité)
. Accordingly, the name of the original, defined unit of mass, “gramme”, which was too small to serve as a practical realisation, was adopted and the new prefix “kilo” was appended 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.
had also adopted the metric system. There was considerable resistance to metrication in the United Kingdom and in the United States, though once the United Kingdom announced its metrication program in 1965, the Commonwealth
followed suite.
Napoleon himself ridiculed the metric system, but as an able administrator, recognised the value of a sound basis for a system of measurement and under the décret impérial du 12 février 1812 (imperial decree of 12 February 1812), a new system of measure - the mesure uselles or "customary measures" was introduced for use in small retail businesses - all government, legal and similar works still had to use the metric system and the metric system continued to be taught at all levels of education. The names of many units used during the ancien regime were reintroduced, but were redefined in terms of metric units. Thus the toise was defined as being two metres with six pied making up one toise, twelve pouce making up one pied and twelve lignes making up one pouce. Likewise the livre was defined as being 500 g, each livre comprising sixteen once and each once eight gros and the aune as 120 centimetres.
when it was part of the French Empire
. Under the Royal decree of 27 March 1817 (Koningklijk besluit van den 27 Maart 1817), the newly-formed Kingdom of the Netherlands
abandoned the mesures usuelles in favour of the "Dutch" metric system
(Nederlands metrisch stelsel) in which metric units were given the names of units of measure that were then in use. Examples include the ons (ounce) which was defined as being 100 g:
which has become a loose federation of kingdoms, principalities, free cities, bishoprics and other fiefdoms, each with its own system of measurement, though in most cases such system were loosely derived from the Carolignian system instituted by Charlemagne
a thousand years earlier.
During the Napoleonic era, there was a move among some of the German states to reform their systems of measurement using the prototype metre and kilogram as the basis of the new units. Baden
, in 1810, for example, redefined the ruthe (rods) as being 3.0 m exactly and defined the subunits of the ruthe as 1 ruthe = 10 fuβ (feet) = 100 zoll (inches) = 1,000 linie (lines) = 10,000 punkt (points) while the pfund was defined as being 500 g, divided into 30 loth, each of 16.67 g. Bavaria
, in its reform of 1811, trimmed the Bavarian pfund from 561.288 g to 560 g exactly, consisting of 32 loth, each of 17.5 g while the Prussia
n pfund remained at 467.711 g.
After the Congress of Vienna
there was a degree of commercial cooperation between the various German states resulting in the setting of the German Customs Union (Zollverein
). There were however still many barriers to trade until Bavaria
took the lead in establishing the General German Commercial Code in 1856. As part of the code the Zollverein introduce the Zollpfund (Customs Pound) which was defined to be exactly 500 g and which could be split into 30 'lot'. This unit was used for inter-state movement of goods, but was not applied in all states for internal use.
Although the Zollverein collapsed after the Austro-Prussian War
of 1866, the metric system became the official system of measurement in the newly formed German Empire
in 1871 and of Austria in 1875. The Zollpfund ceased to be legal in Germany after 1877.
, a North Italian republic set up by Napoleon in 1797 with its capital at Milan
first adopted a modified form of the metric system based in the braccio cisalpino (Cisalpine cubit) which was defined to be half a metre. In 1802 the Cisalpine Republic was renamed the Italian Republic
, with Napoleon as its head of state. The following year the Cisalpine system of measure was replaced by the metric system.
In 1806, the Italian Republic was replaced by the Kingdom of Italy
with Napoleon as its emperor. By 1812, all of Italy from Rome northwards was under the control of Napoleon, either as French Departments or as part of the Kingdom of Italy ensuring the metric system was in use throughout this region.
After the Congress of Vienna
, the various Italian states reverted to their original system of measurements, but in 1845 the Kingdom of Piedmont and Sardinia passed legislation to introduce the metric system within five years. By 1860, most of Italy had been unified under the King of Sardinia Victor Emmanuel II and under Law 132 of 28 July 28, 1861 the metric system became the official system of measurement throughout the kingdom. Numerous Tavole di ragguaglio (Conversion Tables) were displayed in shops until 31 December 1870.
monarchy in Spain in 1700, each of the regions of Spain retained their own system of measurement. The new Bourbon monarchy tried to centralise control and with it the system of measurement. There were debates regarding the desirability of retaining the Castilian
units of measure or, in the interests of harmonisation, adopting the French system. Although Spain assisted Machain in his meridian survey, the Government feared the French revolutionary movement and reinforced the Castilian units of measure to counter such movements. By 1849 however, it proved difficult to maintain the old system and in that year the metric system became the legal system of measure in Spain.
During the next eighty years a number of Parliamentary select committees recommended the adoption of the metric system each with a greater degree of urgency, but Parliament prevaricated. A Select Committee report of 1862 recommended compulsory metrication, but with an "Intermediate permissive phase", Parliament responded in 1864 by legalising metric units only for 'contracts and dealings'. Initially the United Kingdom declined to sign the Treaty of the Metre, but did so in 1883. Meanwhile British scientists and technologists were at the forefront of the metrication movement - it was the British Association for the Advancement of Science
that promoted the cgs system of units as a coherent system and it was the British firm Johnson Matthey
that was accepted by the CGPM in 1889 to cast the international prototype metre and kilogram.
In 1895 another Parliamentary select committee recommended the compulsory adoption of the metric system after a two-year permissive period, the 1897 Weights and Measures Act legalised the metric units for trade, but did not make them mandatory. A bill to make the metric system compulsory in order to enable British industrial base to fight off the challenge of the nascent German base passed through the House of Lords in 1904, but did not pass in the House of Commons before the next general election was called. Following opposition by the Lancashire cotton industry, a similar bill was defeated in 1907 in the House of Commons by 150 votes to 118.
It was only in 1965 that Britain formally commenced a program of metrication, a program that is not yet complete. The British metrication program signaled the start of metrication programs elsewhere in the Commonwealth
, though India had started her program before in 1959, six years before the United Kingdom. South Africa (then not a member of the Commonwealth) set up a Metrication Advisory Board in 1967, New Zealand set up its Metric Advisory Board in 1969, Australia passed the Metric Conversion Act in 1970 and Canada appointed a Metrication Commission in 1971. Metrication in Australia, New Zealand and South Africa was essentially complete within a decade while metrication in India and Canada is not complete. In addition the lakh
and crore
are still in widespread use in India. Most other Commonwealth countries adopted the metric system during the 1970s.
passed a bill making it lawful to use the metric system in the United States. The bill, which was permissive rather than mandatory in nature, defined the metric system in terms of customary units rather than with reference to the international prototype metre and kilogram. By 1893, the reference standards for customary units had become unreliable. Moreover, the United States, being a signatory of the Metre Convention was in possession of national prototype metres and kilograms that were calibrated against those in use elsewhere in the world. This led to the Mendenhall Order
which redefined the customary units by referring to the national metric prototypes, but used the conversion factors of the 1866 act. In 1896 a bill that would make the metric system mandatory in the United States was presented to Congress. Of the 29 people who gave evidence before the congressional committee who were considering the bill, 23 were in favour of the bill, but six were against. Four of the six dissenters represented manufacturing interests and the other two the United States Revenue service. The grounds cited were the cost and inconvenience of the change-over. Subsequent bills suffered a similar fate.
made the first absolute measurements of the Earth's magnetic field
using a decimal system based using the millimetre, gram, and second as the units of measure for length, mass, and time respectively, thereby implicitly making time a base unit of the metric system.
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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, to mechanical work. Energy became the unifying concept of nineteenth century science
, initially by bringing thermodynamics
and mechanics
together and electrical technology
and ultimately relativistic physics leading to Einstein's
equation
. The CGS unit of energy was the "erg
", but the SI unit of energy was named the "joule" in honour of Joule.
In 1861 a committee of the British Association for Advancement of Science (BAAS) including William Thomson (later Lord Kelvin)
, James Clerk Maxwell
and Joule among its members was tasked with investigating the "Standards of Electrical Resistance". In their first report (1862) 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) they introduced the concept of a coherent system of units whereby units of length, mass and time were identified as "fundemental 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.
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 (centimetre-gram-second) system of units
. The committee also recommended the names of "dyne
" and "erg
" for the CGS units of force and energy. The CGS system became the basis for scientific work for the next seventy years.
formulating law which can be extended to relate power to current, potential difference (voltage) and resistance. 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 - at least four different systems of units were devised. In the three CGS systems, the constants
and 
and consequently 
and 
were dimensionless.
the dimensions of charge as defined by the ESU system of units was identical to the dimensions of current as defined by the EMU system of units
while resistance had the same dimensions as velocity in the EMU system of units, but had the dimensions of the inverse of velocity in the ESU system of units.
From mid 1890s onwards Giovanni Giorgi
and Oliver Heaviside
corresponded with each other regarding these anomalous results. This led to Giorgi presenting a paper to the congress of the Associazione Elettrotecnica Italiana (A.E.I.) in October 1901 in which he showed that a coherent electro-mechanical 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 ke and km physical dimensions and hence the electrco-mechanical quantities ε0 and µ0 were also given physical dimensions.
It took more than thirty years before Giorgi's work was accepted in practice by international organisations and nearly another thirty years before they were incorporated as the basis of the Système International d'Unités (International System of Units), the SI.
, volt
, and farad
in honour of Georg Ohm
, Alessandro Volta
and Michael Faraday
respectively for the practical units based on the centimetre-gramme-second absolute system. This was supported by Thomson (Lord Kelvin) These names were later scaled for use in the Practical System. The concept of naming units of measure after noteworthy scientists was subsequently used for other units.
The International Conference on Geodesy in 1867 called for the creation of a new, international prototype metre 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.
On 20 May 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 states. This treaty established the following organisations to conduct international activities relating to a uniform system for measurements:
The international prototype metres and kilograms 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 X-shaped (Tresca
) cross section to minimise the effects of torsional strain during length comparisons. 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.
had yet to publish his theories of relativity. By the end of the century, a coherent system of units was in place under the control of the bodies set up by the Treaty of the Metre, the definition of the second relied on quantum theory, the definition of the metre relied on the theory of relativity and plans were being made to relegate the international prototype kilogram to the archives.
of the prototype - the points at which the bending or droop of the bar is minimized.
In 1887 Michelson
proposed the use of optical interferometers for the measurement of length, work which contributed to him being awarded the Nobel Prize
in 1907. 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. By 1975, when the second had been defined in terms of a physical phenomena rather than the earth's rotation and Einstein's assertion that the speed of light
was constant, 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.
, 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.
During the course of the century, the various national prototypes of the kilogram were recalibrated against the international prototype and therefore against each other. The initial 1889 starting-value offsets of the national prototypes relative to the IPK were nulled. and any subsequent mass changes being relative to the IPK. A technique for steam cleaning the prototypes to remove any contaminants was developed in 1946 as part of the second recalibration.
The third periodic recalibration in 1988-9 revealed that the average difference between International Prototype Kilogram 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.
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 would be calculated by extrapolation using earth's rotational speed in 1900. 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 cesium 133 atom and the estimated rate of rotation of the earth in 1900. Their value was adopted in 1968 by the 13th CGPM.
prevented this being formalised until 1946. The definitions for absolute electrical system became effective from 1 January 1948.
When, in 1948 the CGPM was charged with producing a coherent system of units of measure, definitions relating to temperature had to be clarified. At the 9th CGPM, the centigrade temperature scale was renamed the celsius temperature scale and the scale itself was fixed by defining the triple point of water as 0.01 °C, though the CGPM left the formal definition of absolute zero until the 10th GCPM when the name "degrees Kelvin" was assigned to the absolute temperature scale and triple point of water was defined as being 273.16 °K. In 1967, at the 13th GCPM the degree Kelvin (°K) was renamed the "kelvin" (K).
Over the ensuing years, the BIPM developed and maintained cross-correlations relating various measuring devices such as thermocouples, light spectra and the like to the equivalent temperatures. 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.
(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". was ratified by the GCPM in 1948 and in 1960 was adopted as an SI base unit. The definition proved difficult to implement so in 1967, the definition was revised and the reference to the radiation source was replaced by defining the candles in terms of the power of a specified wavelength of visible light.
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.
. 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 16O isotope whereas chemists assigned 16 amu to 16O, 17O and 18O isotopes mixed in the proportion that they occur in nature. Finally an agreement between the International Union of Pure and Applied Physics
(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 12C 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.
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. At the same time the CGPM formally adopted a recommendation for the writing and printing of unit symbols and of numbers. The recommendation also catalogued the recommended symbols for the most important MKS
and CGS
units of measure and for the first time the CGPM made recommendations concerning derived units.
The CIPM's draft proposal, which was an extensive revision and simplification of the metric unit definitions, symbols and terminology based on the MKS system of units, was put to the 10th CGPM in 1954. In accordance with Giorgi's proposals of 1901, the CIPM also recommended that the ampere be the base unit from which electromechanical 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 metre, ampere, second and kilogram. After negotiations with the CIS and IUPAP, two further base units, the degree kelvin and the candela were also proposed as base units. The full system and name "Système International d'Unités" were adopted at the 11th CGPM.
During the years that followed the definitions of the base units and particularly the mise en pratique to realise these definitions have been refined.
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 artifacts that were then in use. At a meeting of the CCU held in Reading, United Kingdom
in September 2010, a resolution 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. The proposals that the CCU put forward were:
The CIPM meeting of October 2010 found that "the conditions set by the General Conference at its 23rd meeting have not yet been fully met. For this reason the CIPM does not propose a revision of the SI at the present time"; however the CIPM presented a resolution for consideration at the 24th CGPM (17 - 21 October 2011) to agree the new definitions in principle, but not to implement them until the details have been finalised. This resolution was accepted by the conference and in addition the CGPM moved the date of the 25th meeting forward from 2015 to 2014.
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...
date back to the sixteenth century when Simon Stevin
Simon Stevin
Simon Stevin was a Flemish mathematician and military engineer. He was active in a great many areas of science and engineering, both theoretical and practical...
published details of his decimal
Decimal
The decimal numeral system has ten as its base. It is the numerical base most widely used by modern civilizations....
notation and the seventeenth century when John Wilkins
John Wilkins
John Wilkins FRS was an English clergyman, natural philosopher and author, as well as a founder of the Invisible College and one of the founders of the Royal Society, and Bishop of Chester from 1668 until his death....
published a proposal for a decimal system of measurement based on natural units. The first practical realisation of the metric system came during the French Revolution
French Revolution
The French Revolution , sometimes distinguished as the 'Great French Revolution' , was a period of radical social and political upheaval in France and Europe. The absolute monarchy that had ruled France for centuries collapsed in three years...
when the existing system of measure which had fallen into disrepute was replaced by a decimal system based on the kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
and the metre
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
. The metric system was, in the words of Condorcet
Marquis de Condorcet
Marie Jean Antoine Nicolas de Caritat, marquis de Condorcet , known as Nicolas de Condorcet, was a French philosopher, mathematician, and early political scientist whose Condorcet method in voting tally selects the candidate who would beat each of the other candidates in a run-off election...
, "for all people for all time". The unit of length, the metre was based on the dimensions of the earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and the unit of mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
, the kilogram, was based on the mass of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
having a volume
Volume
Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance or shape occupies or contains....
of one litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
or one thousandth of a cubic metre
Cubic metre
The cubic metre is the SI derived unit of volume. It is the volume of a cube with edges one metre in length. An alternative name, which allowed a different usage with metric prefixes, was the stère...
. Reference copies for both units were manufactured and placed in the custody of the Academy of Sciences.
During the first half of the nineteenth century, the metric system was adopted by the scientific community. In the middle of the century James Clerk Maxwell
James Clerk Maxwell
James Clerk Maxwell of Glenlair was a Scottish physicist and mathematician. His most prominent achievement was formulating classical electromagnetic theory. This united all previously unrelated observations, experiments and equations of electricity, magnetism and optics into a consistent theory...
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. This concept worked well with mechanics
Classical mechanics
In physics, classical mechanics is one of the two major sub-fields of mechanics, which is concerned with the set of physical laws describing the motion of bodies under the action of a system of forces...
, but attempts to describe electromagnetic force
Lorentz force
In physics, the Lorentz force is the force on a point charge due to electromagnetic fields. It is given by the following equation in terms of the electric and magnetic fields:...
s in terms of these units were beset with difficulties. By the end of the nineteenth century four principal variants of the metric system were in place for the measurement of electromagnetic phenomena - three based on the centimetre-gram-second system of units
Centimetre gram second system of units
The centimetre–gram–second system is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time...
(CGS system) and one on the metre-kilogram-second system of units (MKS system). This impasse was resolved by Giovanni Giorgi
Giovanni Giorgi
Giovanni Giorgi was an Italian electrical engineer who invented the Giorgi system of measurement, the precursor to the International System ....
who in 1901 proved that a coherent system that incorporated electromagnetic units had to have an electromagnetic unit as the fourth base unit.
Until 1875 the French government owned the prototype metre and kilogram, but in that year the Convention of the metre was signed and control of the standards relating to mass and length passed onto a trio of inter-government organisations, the senior of which was the General Conference on Weights and Measures
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...
(in French the Conférence générale des poids et mesures or CGPM). During the first half of the twentieth century, the CGPM cooperated with a number of other organisations and by 1960 it had responsibility for defining temporal, electrical, thermal, molecular and luminar measurements while other international organisations continued their roles in how these units of measurement were used.
In 1960, the CGPM launched the International System of Units
International System of Units
The International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
(in French the Système international d'unités or SI) which had 6 base units - the metre, kilogram, second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
, ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
, degree kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(subsequently renamed the "kelvin") and candela
Candela
The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one candela...
and 22 derived units. The mole
Mole (unit)
The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
was added as a seventh base unit in 1971. During this period, the metre was redefined in terms of the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the waves from a particular light source and the second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
was defined in terms of the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
from another light source. By the end of the century work was well under way to redefine the ampere, kilogram, mole and kelvin in terms of the basic constants of physics
Physical constant
A physical constant is a physical quantity that is generally believed to be both universal in nature and constant in time. It can be contrasted with a mathematical constant, which is a fixed numerical value but does not directly involve any physical measurement.There are many physical constants in...
, thereby ensuring that all the base units were in theory available to everybody. It is expected that this work will be completed by 2015.
Universal measure
In the early ninth century, CharlemagneCharlemagne
Charlemagne was King of the Franks from 768 and Emperor of the Romans from 800 to his death in 814. He expanded the Frankish kingdom into an empire that incorporated much of Western and Central Europe. During his reign, he conquered Italy and was crowned by Pope Leo III on 25 December 800...
introduced standard units of measure for length and for mass throughout the Holy Roman Empire
Holy Roman Empire
The Holy Roman Empire was a realm that existed from 962 to 1806 in Central Europe.It was ruled by the Holy Roman Emperor. Its character changed during the Middle Ages and the Early Modern period, when the power of the emperor gradually weakened in favour of the princes...
. As the empire disintegrated these standards diverged and by the seventeenth century there were numerous units of measure within regions and different sizes having the same name across regions. The variations were promoted by local vested interests but hindered trade and taxation.
In 1586, the Flemish mathematician Simon Stevin
Simon Stevin
Simon Stevin was a Flemish mathematician and military engineer. He was active in a great many areas of science and engineering, both theoretical and practical...
published a small pamphlet called De Thiende ("the tenth"). Decimal fractions had been employed for the extraction of square roots some five centuries before his time, but nobody used decimal numbers in daily life. He 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.
Wilkins
The concept of a "universal measure" based on a decimal system and natural phenomena was first made by John WilkinsJohn Wilkins
John Wilkins FRS was an English clergyman, natural philosopher and author, as well as a founder of the Invisible College and one of the founders of the Royal Society, and Bishop of Chester from 1668 until his death....
, first secretary of the Royal Society
Royal Society
The Royal Society of London for Improving Natural Knowledge, known simply as the Royal Society, is a learned society for science, and is possibly the oldest such society in existence. Founded in November 1660, it was granted a Royal Charter by King Charles II as the "Royal Society of London"...
of London in 1668 and backed up by Tito Livio Burattini
Tito Livio Burattini
Tito Livio Burattini was an inventor, architect, Egyptologist, scientist, instrument-maker, traveller, engineer, and nobleman. He was born in Agordo, Italy, and studied in Padua and Venice...
who promoted the "metro cattolico" from which the word metre was to be derived.
Wilkins' proposed a "seconds pendulum" (a pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...
with a half-period of one second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
) as the unit of length: such pendulums had recently been demonstrated by Christiaan Huygens, and their length is very close to one modern metre (as well as to length units which were then in use, such as the yard
Yard
A yard is a unit of length in several different systems including English units, Imperial units and United States customary units. It is equal to 3 feet or 36 inches...
). However, it was soon discovered that the length of a seconds pendulum varies from place to place: French astronomer Jean Richer
Jean Richer
Jean Richer was a French astronomer and assistant of Giovanni Domenico Cassini.Between 1671 and 1673 he traveled to Cayenne at the request of the French Academy of Sciences to observe Mars during its perigee...
had measured the 0.3% difference in length between Cayenne (in French Guiana) and Paris.
Mouton
In 1670, Gabriel MoutonGabriel Mouton
Gabriel Mouton was a French abbot and scientist. He was a doctor of theology from Lyon, but was also interested in mathematics and astronomy....
, a French abbot and scientist, proposed a decimal system of measurement based on the circumference of the Earth. His suggestion was a unit, the milliare, be defined as a minute of arc along a meridian. He then suggested a system of sub-units, dividing successively by factors of ten into the centuria, decuria, virga, virgula, decima, centesima, and millesima. His ideas attracted interest at the time, and were supported by both Jean Picard
Jean Picard
Jean-Felix Picard was a French astronomer and priest born in La Flèche, where he studied at the Jesuit Collège Royal Henry-Le-Grand. He was the first person to measure the size of the Earth to a reasonable degree of accuracy in a survey conducted in 1669–70, for which he is honored with a...
and Christiaan Huygens in 1673, and also studied at the Royal Society in London. In the same year, Gottfried Leibniz
Gottfried Leibniz
Gottfried Wilhelm Leibniz was a German philosopher and mathematician. He wrote in different languages, primarily in Latin , French and German ....
independently made proposals similar to those of Mouton.
Leading up to the French Revolution
Whereas in England the Magna CartaMagna Carta
Magna Carta is an English charter, originally issued in the year 1215 and reissued later in the 13th century in modified versions, which included the most direct challenges to the monarch's authority to date. The charter first passed into law in 1225...
in 1215 decreed that "there shall be one unit of measure throughout the realm", France had a multitude of units of measure. It has been estimated that on the eve of the Revolution a quarter of a million different units of measure were in use in France; in many cases quantity associated with each unit of measure differed from town to town and even from trade to trade. Although certain standards, such as the pied du roi (the King's foot) had a degree of pre-eminence and were used by savant
Savant syndrome
Savant syndrome , sometimes referred to as savantism, is a rare condition in which people with developmental disorders have one or more areas of expertise, ability, or brilliance that are in contrast with the individual's overall limitations...
s, many traders chose to use their own measuring devices giving scope for fraud and hindering commerce and industry.
By the mid eighteenth century the need for standardisation of weights and measures had become apparent - Spain aligned her units of measure with the royal units of France, and Peter the Great aligned the Russian units of measure with the English units.
In 1789 the French finances were in a perilous state and in May Louis XVI summonsed the Estates General
Estates-General of 1789
The Estates-General of 1789 was the first meeting since 1614 of the French Estates-General, a general assembly representing the French estates of the realm: the nobility, the Church, and the common people...
. On 14 July 1789, the mob stormed the Bastille
Bastille
The Bastille was a fortress in Paris, known formally as the Bastille Saint-Antoine. It played an important role in the internal conflicts of France and for most of its history was used as a state prison by the kings of France. The Bastille was built in response to the English threat to the city of...
and in August 1789 the nobility surrendered their privileges, including the right to control local weights and measures. Louis XVI charged a group of experts including such notables as Lavoisier, Condorcet, Laplace, Borda and Legendre
Legendre
Legendre, LeGendre or Le Gendre is a French surname. It may refer to:* Adrien-Marie Legendre , a French mathematician** Legendre , a lunar impact crater located near the eastern limb of the Moon...
to produce the system of measures that would replace the disparate system then in place.
There was also a wish that the units of measure should be for all people and for all time and therefore not dependant on an artifact owned by any one particular nation. Talleyrand, at the prompting of the savant Condorcet, approached the British and the Americans in the early 1790s with proposals of a joint effort to define the metre.
On 13 July 1790 Thomas Jefferson presented a document Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States to congress in which he advocated a decimal system that used traditional names for units (such as ten inches per foot). The report was considered but not adopted by Congress.
Talleyrand resurrected the idea of the seconds pendulum before the Constituent Assembly in 1790, suggesting that the new measure be defined at 45°N (a latitude that, in France, runs just north of Bordeaux and just south of Grenoble): despite the support of the Assembly, and of Great Britain and the newly independent United States, nothing came of Talleyrand's proposal.
The French Revolution and subsequent Napoleonic Wars marked the end of the Age of Enlightenment
Age of Enlightenment
The Age of Enlightenment was an elite cultural movement of intellectuals in 18th century Europe that sought to mobilize the power of reason in order to reform society and advance knowledge. It promoted intellectual interchange and opposed intolerance and abuses in church and state...
. The forces of change that had been brewing manifest themselves across all of France, including the way in which units of measure should be defined. The savants of the day favoured the use of a system of units that were inter-related and which used a decimal basis.
Revolutionary France (1795 - 1812)
Overtures made by the French government to the British and American governments for the establishment of a common system of weights and measures came to nothing and France decided to "go it alone".Decimal time (1793)
Decimal time was introduced in the decree of 5 October 1793 under which the day was divided into 10 "decimal hours", the "hour" into 100 " decimal minutes" and the "decimal minute" into 100 "decimal seconds". The "decimal hour" corresponded to 2 hr 24 min, the "decimal minute" to 1.44 min and the "decimal second" to 0.864 s. The revolutionary week was 10 days, but there were still twelve months in a year. The use of decimal time proved very unpopular, especially the ten day week and the calendar was officially discarded by Napoleon in 1806.Draft metric system (1795)
The implementation of decimal time proved an immense task and under the article 22 of the law of 18 Germinal, Year III (7 April 1795) , the use of decimal time was no longer mandatory. On 1 January 1806, France reverted to the traditional timekeeping.French Revolution
The French Revolution , sometimes distinguished as the 'Great French Revolution' , was a period of radical social and political upheaval in France and Europe. The absolute monarchy that had ruled France for centuries collapsed in three years...
ary government. Article 5 of the law of 18 Germinal, Year III (7 April 1795) defined five units of measure. The units and their preliminary values were:
- The metreMetreThe metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
, for length - defined as being one ten millionth of the distance between the North PoleNorth PoleThe North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
and the EquatorEquatorAn equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
through ParisParisParis is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region... - The areHectareThe hectare is a metric unit of area defined as 10,000 square metres , and primarily used in the measurement of land. In 1795, when the metric system was introduced, the are was defined as being 100 square metres and the hectare was thus 100 ares or 1/100 km2...
(100 m2) for area [of land] - The stèreStèreThe stere or stère is a unit of volume in the original metric system equal to one cubic metre. The name was coined from the Greek στερεός stereos 'solid' in 1793 France as a metric equivalent to the cord. The stère is typically used for measuring large quantities of firewood or other cut wood,...
(1 m3) for volume of firewood - The litreLitrepic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
(1 dm3) for volumes of liquid - The gramGramThe gram is a metric system unit of mass....
, for mass - defined as being the mass of one cubic centimetre of water
Decimal multiples and submultiples of these units would be defined by Greek prefixes
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...
- "myria", "kilo", "hecta" (100), "deka", "deci", "centi" and "milli". Using Cassini's
César-François Cassini de Thury
César-François Cassini de Thury , also called Cassini III or Cassini de Thury, was a French astronomer and cartographer.- Biography :...
survey of 1744, a provisional value of 443.44 lignes was assigned to the metre which, in turn, defined the other units of measure.
The final value of the metre had to wait until 1799 when Delambre and Mechain presented the results of their survey between Dunkirk and Barcelona
History of the metre
In the aftermath of the French Revolution , the old units of measure that were associated with the ancien régime were replaced by new units. The livre was replaced by the decimal franc, and a new unit of length was introduced which became known as the metre...
which fixed the length of the metre at 443.296 lignes. The law 19 Frimaire An VIII (10 December 1799) defined the metre in terms of this value and the kilogram as being 18827.15 grains. These definitions enabled reference copies of the kilograms and metres to be constructed that were to used as standards for the next 90 years.
At the same time, a new decimal-based system for angular measurement was implemented. The right angle was divided into 100 grads
Grad (angle)
The gradian is a unit of plane angle, equivalent to of a turn. It is also known as gon, grad, or grade . One grad equals of a degree or of a radian...
which in turn was divided in 100 centigrads. An arc on the earth’s surface formed by an angle of one centigrade was one kilometre. The use of the centigrade was adopted for general use in a number countries and in 1948 the General Conference on Weights and Measures
General Conference on Weights and Measures
The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures . It is one of the three organizations established to maintain the International System of Units under the terms of the Convention du Mètre of 1875...
(CGPM) recommended that the degree centigrade (used for the measurement of temperature) be renamed the degree Celsius
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
.
Meridianal definition
Jean-Charles de Borda
Jean-Charles, chevalier de Borda was a French mathematician, physicist, political scientist, and sailor.-Life history:...
. Borda could be said to have been a fanatic for decimalization: he had invented the "repeating circle", a surveying instrument which allowed a much-improved precision in the measurement of angles between landmarks, but insisted that it be calibrated in "grades
Grad (angle)
The gradian is a unit of plane angle, equivalent to of a turn. It is also known as gon, grad, or grade . One grad equals of a degree or of a radian...
" ( of a quarter-circle) rather than degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s, with 100 minutes to a grade and 100 seconds to a minute. For Borda, the seconds pendulum was a poor choice for a standard because the second (as a unit of time) was insufficiently decimal: he preferred a system of 10 hours to the day, 100 minutes to the hour and 100 seconds to the minute...
Instead, the commission – whose members included Lagrange, Laplace
Pierre-Simon Laplace
Pierre-Simon, marquis de Laplace was a French mathematician and astronomer whose work was pivotal to the development of mathematical astronomy and statistics. He summarized and extended the work of his predecessors in his five volume Mécanique Céleste...
, Monge
Gaspard Monge
Gaspard Monge, Comte de Péluse was a French mathematician, revolutionary, and was inventor of descriptive geometry. During the French Revolution, he was involved in the complete reorganization of the educational system, founding the École Polytechnique...
and Condorcet – decided that the new measure should be equal to one ten-millionth of the distance from the North Pole to the Equator (the quadrant of the Earth's circumference), measured along the meridian
Meridian (geography)
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations along it with a given longitude. The position of a point along the meridian is given by its latitude. Each meridian is perpendicular to all circles of latitude...
passing through Paris. Apart from the obvious nationalistic considerations, the Paris meridian
Paris Meridian
The Paris Meridian is a meridian line running through the Paris Observatory in Paris, France—now longitude 2°20′14.025″ east. It was a long-standing rival to Greenwich as the prime meridian of the world, as was the Meridian of Antwerp in Antwerp, Belgium....
was also a sound choice for practical scientific reasons: a portion of the quadrant from Dunkerque to Barcelona (about 1000 km, or one-tenth of the total) could be surveyed with start- and end-points 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.
The task of surveying the meridian arc
Meridian arc
In geodesy, a meridian arc measurement is a highly accurate determination of the distance between two points with the same longitude. Two or more such determinations at different locations then specify the shape of the reference ellipsoid which best approximates the shape of the geoid. This...
fell to Pierre Méchain
Pierre Méchain
Pierre François André Méchain was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep sky objects and comets.-Life:...
and Jean-Baptiste Delambre, and armed with letters of authorisation signed by Louis XVI the task took more than six years (1792–98). In the meantime, the commission calculated a provisional value from older surveys of 443.44 ligne
Ligne
The ligne is a unit of length that was in use prior to the French adoption of the metric system in the late 18th century, and is still used by French and Swiss wristwatch makers to measure the size of a watch movement.- Watchmakers' use :There are 12 lignes to one French inch...
s.
The project was split into two parts - the northern section of 742.7 km from the Belfry, Dunkirk to Rodez Cathederal
Rodez
Rodez is a town and commune in southern France, in the Aveyron department, of which it is the capital. Its inhabitants are called Ruthénois.-History:Existing from at least the 5th century BC, Rodez was founded by the Celts...
which was surveyed by Delambre and the southern section of 333.0 km from Rodez
Rodez
Rodez is a town and commune in southern France, in the Aveyron department, of which it is the capital. Its inhabitants are called Ruthénois.-History:Existing from at least the 5th century BC, Rodez was founded by the Celts...
to the Montjuïc Fortress
Montjuïc
Montjuïc is a hill located in Barcelona, Catalonia.-Etymology:Montjuïc is translated as 'Jew Hill' in medieval Catalan, or is perhaps related to the Latin phrase Mons Jovicus . The name is found in several locations in the Catalan Countries: the Catalan cities of Girona and Barcelona both have a...
, Barcelona
Barcelona
Barcelona is the second largest city in Spain after Madrid, and the capital of Catalonia, with a population of 1,621,537 within its administrative limits on a land area of...
which was surveyed by Méchain.
Delambre used a baseline of about 10 km in length along a straight road, located close to Melun
Melun
Melun is a commune in the Seine-et-Marne department in the Île-de-France region in north-central France. Located in the south-eastern suburbs of Paris, Melun is the capital of the department, as the seat of an arrondissement...
. In an operation taking six weeks, the baseline was accurately measured using four platinum rods, each of length two toise (about 3.9 m). Thereafter he used, where possible, the triangulation points used by Cassini
César-François Cassini de Thury
César-François Cassini de Thury , also called Cassini III or Cassini de Thury, was a French astronomer and cartographer.- Biography :...
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
Perpignan
-Sport:Perpignan is a rugby stronghold: their rugby union side, USA Perpignan, is a regular competitor in the Heineken Cup and seven times champion of the Top 14 , while their rugby league side plays in the engage Super League under the name Catalans Dragons.-Culture:Since 2004, every year in the...
area. Although Méchain's sector was half the length of Delambre, it included the Pyrenees
Pyrenees
The Pyrenees is a range of mountains in southwest Europe that forms a natural border between France and Spain...
and hitherto unsurveyed parts of Spain. After the two surveyors met, each computed the other's baseline in order to cross-check their results and they then recomputed the kilometre. Their result came out at 0.144 lignes shorter than the provisional value, a difference of about 0.03%.
Mètre des Archives
While Méchain and Delambre were completing their survey, the commission had ordered a series of platinumPlatinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
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 was selected and placed in the National Archives on 22 June 1799 (4 messidor An VII in the Republican calendar) as a permanent record of the result: this standard metre bar became known as the mètre des Archives.
The metric system
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...
, that is the system of units based on the metre, was officially adopted in France on 10 December 1799 (19 frimaire An VIII) and became the sole legal system of weights and measures from 1801.
It soon became apparent that Méchain and Delambre's result (443.296 lignes) was slightly too short for the meridianal definition of the metre. Arago
François Arago
François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...
and Biot
Jean-Baptiste Biot
Jean-Baptiste Biot was a French physicist, astronomer, and mathematician who established the reality of meteorites, made an early balloon flight, and studied the polarization of light.- Biography :...
extended the survey to the island of Formentera in the western Mediterranean Sea in 1806–9, and found that one ten-millionth of the Earth's quadrant should be 443.31 lignes: later work increased the value to 443.39 lignes. The modern value, for the WGS 84 reference spheroid, is m or lignes.
Nevertheless, the mètre des Archives remained the legal and practical standard for the metre in France, even once it was known that it did not exactly correspond to the meridianal definition. 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".
Kilogramme des Archives
A grave is a metallic reference standard of one thousand gramGram
The gram is a metric system unit of mass....
s that was used for a few years until it was replaced by the kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
standard in 1799.
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”. Although this was the definition of the gram, the regulation of trade and commerce required a “practical realisation”: a single-piece, metallic reference standard that was one thousand times more massive that would be known as “grave” (symbol G). This mass unit, whose name is derived from the word “gravity”, defined by Lavoisier
Antoine Lavoisier
Antoine-Laurent de Lavoisier , the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology...
and René Just Haüy
René Just Haüy
René Just Haüy – 3 June 1822 in Paris) was a French mineralogist, commonly styled the Abbé Haüy after he was made an honorary canon of Notre Dame. He is often referred to as the "Father of Modern Crystallography." -Biography:...
had been in use since 1793. Notwithstanding that the definition of the base unit of mass was the gramme (alternatively “gravet”), this new, practical realisation would ultimately become the base unit of mass. A provisional kilogram standard was made and work was commissioned to determine precisely how massive a cubic decimetre (later to be defined as equal to one litre
Litre
pic|200px|right|thumb|One litre is equivalent to this cubeEach side is 10 cm1 litre water = 1 kilogram water The litre is a metric system unit of volume equal to 1 cubic decimetre , to 1,000 cubic centimetres , and to 1/1,000 cubic metre...
) of water was.
Although the decreed definition of the kilogram specified water at 0 °C — a highly stable temperature point — the scientists tasked with producing the new practical realisation chose to redefine the standard and perform their measurements at the most stable density point: the temperature at which water reaches maximum density, which was measured at the time as 4 °C. 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. Four years later in 1799, an all-platinum standard, the “Kilogramme des Archives”, was fabricated with the objective that it would equal, as close as was scientifically feasible for the day, to the mass of cubic decimetre of water at 4 °C. The kilogram was defined to be equal to the mass of the Kilogramme des Archives and this standard stood for the next ninety years.
Note that the new metric system did not come into effect until after the French Revolution
French Revolution
The French Revolution , sometimes distinguished as the 'Great French Revolution' , was a period of radical social and political upheaval in France and Europe. The absolute monarchy that had ruled France for centuries collapsed in three years...
, when the new revolutionary government captured the idea of the metric system. The decision of the Republican government to name this new unit the “kilogramme” had been mainly politically motivated, because the name “grave” was at that time considered politically incorrect as it resembled the aristocratic German title of the Graf
Graf
Graf is a historical German noble title equal in rank to a count or a British earl...
, an alternative name for the title of Count
Count
A count or countess is an aristocratic nobleman in European countries. The word count came into English from the French comte, itself from Latin comes—in its accusative comitem—meaning "companion", and later "companion of the emperor, delegate of the emperor". The adjective form of the word is...
that, like other nobility titles, was inconsistent with the new French Republic notion of equality (égalité)
Liberté, égalité, fraternité
Liberté, égalité, fraternité, French for "Liberty, equality, fraternity ", is the national motto of France, and is a typical example of a tripartite motto. Although it finds its origins in the French Revolution, it was then only one motto among others and was not institutionalized until the Third...
. Accordingly, the name of the original, defined unit of mass, “gramme”, which was too small to serve as a practical realisation, was adopted and the new prefix “kilo” was appended to it to form the name “kilogramme”. Consequently, the kilogram is the only SI base unit
SI base unit
The International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...
that has an SI prefix
SI prefix
The International System of Units specifies a set of unit prefixes known as SI prefixes or metric prefixes. An SI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Each prefix has a unique symbol that is prepended to the unit symbol...
as part of its unit name.
Worldwide adoption of the metric system
During the nineteenth century the metric system proved a convenient political compromise during the unification processes in the Netherlands, Germany and Italy. Spain found it expedient in 1858 to follow the French example and within a decade Latin AmericaLatin America
Latin America is a region of the Americas where Romance languages – particularly Spanish and Portuguese, and variably French – are primarily spoken. Latin America has an area of approximately 21,069,500 km² , almost 3.9% of the Earth's surface or 14.1% of its land surface area...
had also adopted the metric system. There was considerable resistance to metrication in the United Kingdom and in the United States, though once the United Kingdom announced its metrication program in 1965, the Commonwealth
Commonwealth of Nations
The Commonwealth of Nations, normally referred to as the Commonwealth and formerly known as the British Commonwealth, is an intergovernmental organisation of fifty-four independent member states...
followed suite.
France: Mesures usuelles (1812 - 1839)
The introduction of the metric system into France in 1795 was done on a district by district basis with Paris being the first district, but it was, in terms of modern standards, poorly managed. Although thousands of pamphlets were distributed, the Agency of Weights and Measures who oversaw the introduction underestimated the work involved. Paris alone needed 500,000 metre sticks, yet one month after the metre became the sole legal unit of measure, they only had 25,000 in store. This, combined with other excesses of the Revolution and the high level of illiteracy made the metric system unpopular.Napoleon himself ridiculed the metric system, but as an able administrator, recognised the value of a sound basis for a system of measurement and under the décret impérial du 12 février 1812 (imperial decree of 12 February 1812), a new system of measure - the mesure uselles or "customary measures" was introduced for use in small retail businesses - all government, legal and similar works still had to use the metric system and the metric system continued to be taught at all levels of education. The names of many units used during the ancien regime were reintroduced, but were redefined in terms of metric units. Thus the toise was defined as being two metres with six pied making up one toise, twelve pouce making up one pied and twelve lignes making up one pouce. Likewise the livre was defined as being 500 g, each livre comprising sixteen once and each once eight gros and the aune as 120 centimetres.
The Dutch metric system
The Netherlands first used the metric system and then, in 1812, the mesures usuellesMesures usuelles
Mesures usuelles were a system of measurement introduced by Napoleon I in 1812 to act as compromise between the metric system and traditional measurements...
when it was part of the French Empire
French Empire
The term French Empire can refer to:* Two Empires of the French founded by members of the House of Bonaparte,** The First French Empire of Napoleon I and II ** The Second French Empire of Napoleon III...
. Under the Royal decree of 27 March 1817 (Koningklijk besluit van den 27 Maart 1817), the newly-formed Kingdom of the Netherlands
Kingdom of the Netherlands
The Kingdom of the Netherlands is a sovereign state and constitutional monarchy with territory in Western Europe and in the Caribbean. The four parts of the Kingdom—Aruba, Curaçao, the Netherlands, and Sint Maarten—are referred to as "countries", and participate on a basis of equality...
abandoned the mesures usuelles in favour of the "Dutch" metric system
Metric system
The metric system is an international decimalised system of measurement. France was first to adopt a metric system, in 1799, and a metric system is now the official system of measurement, used in almost every country in the world...
(Nederlands metrisch stelsel) in which metric units were given the names of units of measure that were then in use. Examples include the ons (ounce) which was defined as being 100 g:
The German Zollverein
At the outbreak of the French Revolution, much of modern-day Germany and Austria were part of the Holy Roman EmpireHoly Roman Empire
The Holy Roman Empire was a realm that existed from 962 to 1806 in Central Europe.It was ruled by the Holy Roman Emperor. Its character changed during the Middle Ages and the Early Modern period, when the power of the emperor gradually weakened in favour of the princes...
which has become a loose federation of kingdoms, principalities, free cities, bishoprics and other fiefdoms, each with its own system of measurement, though in most cases such system were loosely derived from the Carolignian system instituted by Charlemagne
Charlemagne
Charlemagne was King of the Franks from 768 and Emperor of the Romans from 800 to his death in 814. He expanded the Frankish kingdom into an empire that incorporated much of Western and Central Europe. During his reign, he conquered Italy and was crowned by Pope Leo III on 25 December 800...
a thousand years earlier.
During the Napoleonic era, there was a move among some of the German states to reform their systems of measurement using the prototype metre and kilogram as the basis of the new units. Baden
Baden
Baden is a historical state on the east bank of the Rhine in the southwest of Germany, now the western part of the Baden-Württemberg of Germany....
, in 1810, for example, redefined the ruthe (rods) as being 3.0 m exactly and defined the subunits of the ruthe as 1 ruthe = 10 fuβ (feet) = 100 zoll (inches) = 1,000 linie (lines) = 10,000 punkt (points) while the pfund was defined as being 500 g, divided into 30 loth, each of 16.67 g. Bavaria
Bavaria
Bavaria, formally the Free State of Bavaria is a state of Germany, located in the southeast of Germany. With an area of , it is the largest state by area, forming almost 20% of the total land area of Germany...
, in its reform of 1811, trimmed the Bavarian pfund from 561.288 g to 560 g exactly, consisting of 32 loth, each of 17.5 g while the Prussia
Prussia
Prussia was a German kingdom and historic state originating out of the Duchy of Prussia and the Margraviate of Brandenburg. For centuries, the House of Hohenzollern ruled Prussia, successfully expanding its size by way of an unusually well-organized and effective army. Prussia shaped the history...
n pfund remained at 467.711 g.
After the Congress of Vienna
Congress of Vienna
The Congress of Vienna was a conference of ambassadors of European states chaired by Klemens Wenzel von Metternich, and held in Vienna from September, 1814 to June, 1815. The objective of the Congress was to settle the many issues arising from the French Revolutionary Wars, the Napoleonic Wars,...
there was a degree of commercial cooperation between the various German states resulting in the setting of the German Customs Union (Zollverein
Zollverein
thumb|upright=1.2|The German Zollverein 1834–1919blue = Prussia in 1834 grey= Included region until 1866yellow= Excluded after 1866red = Borders of the German Union of 1828 pink= Relevant others until 1834...
). There were however still many barriers to trade until Bavaria
Bavaria
Bavaria, formally the Free State of Bavaria is a state of Germany, located in the southeast of Germany. With an area of , it is the largest state by area, forming almost 20% of the total land area of Germany...
took the lead in establishing the General German Commercial Code in 1856. As part of the code the Zollverein introduce the Zollpfund (Customs Pound) which was defined to be exactly 500 g and which could be split into 30 'lot'. This unit was used for inter-state movement of goods, but was not applied in all states for internal use.
Although the Zollverein collapsed after the Austro-Prussian War
Austro-Prussian War
The Austro-Prussian War was a war fought in 1866 between the German Confederation under the leadership of the Austrian Empire and its German allies on one side and the Kingdom of Prussia with its German allies and Italy on the...
of 1866, the metric system became the official system of measurement in the newly formed German Empire
German Empire
The German Empire refers to Germany during the "Second Reich" period from the unification of Germany and proclamation of Wilhelm I as German Emperor on 18 January 1871, to 1918, when it became a federal republic after defeat in World War I and the abdication of the Emperor, Wilhelm II.The German...
in 1871 and of Austria in 1875. The Zollpfund ceased to be legal in Germany after 1877.
Italy
The Cisalpine RepublicCisalpine Republic
The Cisalpine Republic was a French client republic in Northern Italy that lasted from 1797 to 1802.-Birth:After the Battle of Lodi in May 1796, Napoleon Bonaparte proceeded to organize two states: one to the south of the Po River, the Cispadane Republic, and one to the north, the Transpadane...
, a North Italian republic set up by Napoleon in 1797 with its capital at Milan
Milan
Milan is the second-largest city in Italy and the capital city of the region of Lombardy and of the province of Milan. The city proper has a population of about 1.3 million, while its urban area, roughly coinciding with its administrative province and the bordering Province of Monza and Brianza ,...
first adopted a modified form of the metric system based in the braccio cisalpino (Cisalpine cubit) which was defined to be half a metre. In 1802 the Cisalpine Republic was renamed the Italian Republic
Italian Republic (Napoleonic)
The Italian Republic was a short-lived republic located in Northern Italy. It was a vassal state of the First French Republic of Napoleon.-The republic:...
, with Napoleon as its head of state. The following year the Cisalpine system of measure was replaced by the metric system.
In 1806, the Italian Republic was replaced by the Kingdom of Italy
Kingdom of Italy (Napoleonic)
The Kingdom of Italy was a state founded in Northern Italy by Napoleon, fully influenced by revolutionary France, that ended with his defeat and fall.-Constitutional statutes:...
with Napoleon as its emperor. By 1812, all of Italy from Rome northwards was under the control of Napoleon, either as French Departments or as part of the Kingdom of Italy ensuring the metric system was in use throughout this region.
After the Congress of Vienna
Congress of Vienna
The Congress of Vienna was a conference of ambassadors of European states chaired by Klemens Wenzel von Metternich, and held in Vienna from September, 1814 to June, 1815. The objective of the Congress was to settle the many issues arising from the French Revolutionary Wars, the Napoleonic Wars,...
, the various Italian states reverted to their original system of measurements, but in 1845 the Kingdom of Piedmont and Sardinia passed legislation to introduce the metric system within five years. By 1860, most of Italy had been unified under the King of Sardinia Victor Emmanuel II and under Law 132 of 28 July 28, 1861 the metric system became the official system of measurement throughout the kingdom. Numerous Tavole di ragguaglio (Conversion Tables) were displayed in shops until 31 December 1870.
Spain
Until the ascent of the BourbonHouse of Bourbon
The House of Bourbon is a European royal house, a branch of the Capetian dynasty . Bourbon kings first ruled Navarre and France in the 16th century. By the 18th century, members of the Bourbon dynasty also held thrones in Spain, Naples, Sicily, and Parma...
monarchy in Spain in 1700, each of the regions of Spain retained their own system of measurement. The new Bourbon monarchy tried to centralise control and with it the system of measurement. There were debates regarding the desirability of retaining the Castilian
Castilian
Castilian may refer to:* Alternative name for the Spanish language .* Something related to the Crown of Castile, a former state in present-day Spain...
units of measure or, in the interests of harmonisation, adopting the French system. Although Spain assisted Machain in his meridian survey, the Government feared the French revolutionary movement and reinforced the Castilian units of measure to counter such movements. By 1849 however, it proved difficult to maintain the old system and in that year the metric system became the legal system of measure in Spain.
United Kingdom and the Commonwealth
In 1824 the Weights and Measures Act imposed one standard 'imperial' system of weights and measures on the British Empire. The effect of this act was to regularise existing British units of measure rather than to align them with the metric system..During the next eighty years a number of Parliamentary select committees recommended the adoption of the metric system each with a greater degree of urgency, but Parliament prevaricated. A Select Committee report of 1862 recommended compulsory metrication, but with an "Intermediate permissive phase", Parliament responded in 1864 by legalising metric units only for 'contracts and dealings'. Initially the United Kingdom declined to sign the Treaty of the Metre, but did so in 1883. Meanwhile British scientists and technologists were at the forefront of the metrication movement - it was the British Association for the Advancement of Science
British Association for the Advancement of Science
frame|right|"The BA" logoThe British Association for the Advancement of Science or the British Science Association, formerly known as the BA, is a learned society with the object of promoting science, directing general attention to scientific matters, and facilitating interaction between...
that promoted the cgs system of units as a coherent system and it was the British firm Johnson Matthey
Johnson Matthey
Johnson Matthey plc is multinational chemicals and precious metals company headquartered in London, United Kingdom.It is listed on the London Stock Exchange and is a constituent of the FTSE 100 Index.-History:...
that was accepted by the CGPM in 1889 to cast the international prototype metre and kilogram.
In 1895 another Parliamentary select committee recommended the compulsory adoption of the metric system after a two-year permissive period, the 1897 Weights and Measures Act legalised the metric units for trade, but did not make them mandatory. A bill to make the metric system compulsory in order to enable British industrial base to fight off the challenge of the nascent German base passed through the House of Lords in 1904, but did not pass in the House of Commons before the next general election was called. Following opposition by the Lancashire cotton industry, a similar bill was defeated in 1907 in the House of Commons by 150 votes to 118.
It was only in 1965 that Britain formally commenced a program of metrication, a program that is not yet complete. The British metrication program signaled the start of metrication programs elsewhere in the Commonwealth
Commonwealth of Nations
The Commonwealth of Nations, normally referred to as the Commonwealth and formerly known as the British Commonwealth, is an intergovernmental organisation of fifty-four independent member states...
, though India had started her program before in 1959, six years before the United Kingdom. South Africa (then not a member of the Commonwealth) set up a Metrication Advisory Board in 1967, New Zealand set up its Metric Advisory Board in 1969, Australia passed the Metric Conversion Act in 1970 and Canada appointed a Metrication Commission in 1971. Metrication in Australia, New Zealand and South Africa was essentially complete within a decade while metrication in India and Canada is not complete. In addition the lakh
Lakh
A lakh is a unit in the Indian numbering system equal to one hundred thousand . It is widely used both in official and other contexts in Pakistan, Bangladesh, India, Maldives, Nepal, Sri Lanka, Myanmar and is often used in Indian English.-Usage:...
and crore
Crore
A crore is a unit in the Indian number system equal to ten million , or 100 lakhs. It is widely used in India, Bangladesh, Nepal, and Pakistan....
are still in widespread use in India. Most other Commonwealth countries adopted the metric system during the 1970s.
United States
The United States Government acquired copies of the French metre and kilogram for reference purposes in 1805 and 1820 respectively. In 1866 the United States CongressUnited States Congress
The United States Congress is the bicameral legislature of the federal government of the United States, consisting of the Senate and the House of Representatives. The Congress meets in the United States Capitol in Washington, D.C....
passed a bill making it lawful to use the metric system in the United States. The bill, which was permissive rather than mandatory in nature, defined the metric system in terms of customary units rather than with reference to the international prototype metre and kilogram. By 1893, the reference standards for customary units had become unreliable. Moreover, the United States, being a signatory of the Metre Convention was in possession of national prototype metres and kilograms that were calibrated against those in use elsewhere in the world. This led to the Mendenhall Order
Mendenhall Order
The Mendenhall Order marked a decision to change the fundamental standards of length and mass of the United States from the customary standards based on those of England to metric standards. It was issued on April 5, 1893 by Thomas Corwin Mendenhall, superintendent of the U.S...
which redefined the customary units by referring to the national metric prototypes, but used the conversion factors of the 1866 act. In 1896 a bill that would make the metric system mandatory in the United States was presented to Congress. Of the 29 people who gave evidence before the congressional committee who were considering the bill, 23 were in favour of the bill, but six were against. Four of the six dissenters represented manufacturing interests and the other two the United States Revenue service. The grounds cited were the cost and inconvenience of the change-over. Subsequent bills suffered a similar fate.
Development of a coherent metric system
From its inception, the metric system was designed in such a manner that the various units of measure were linked to each other. At the start of the nineteenth century, length, mass and temperature were the only base units that had been standardised. The beginnings of a coherent system were in place with the units of area and volume linked to the unit of length, though at the time science did not understand the concepts of base units and derived units, nor how many physical quantities were inter-related.Time, work and energy
In 1832 Carl-Friedrich GaussCarl Friedrich Gauss
Johann Carl Friedrich Gauss was a German mathematician and scientist who contributed significantly to many fields, including number theory, statistics, analysis, differential geometry, geodesy, geophysics, electrostatics, astronomy and optics.Sometimes referred to as the Princeps mathematicorum...
made the first absolute measurements of the Earth's magnetic field
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
using a decimal system based using the millimetre, gram, and second as the units of measure for length, mass, and time respectively, thereby implicitly making time a base unit of the metric system.
James Prescott Joule
James Prescott Joule FRS was an English physicist and brewer, born in Salford, Lancashire. Joule studied the nature of heat, and discovered its relationship to mechanical work . This led to the theory of conservation of energy, which led to the development of the first law of thermodynamics. The...
first demonstrated a means of measuring the energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
transferred between different systems when work is done thereby relating Nicolas Clément's calorie
Calorie
The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use is archaic, having been replaced by the SI unit of energy, the joule...
, defined in 1824, to mechanical work. Energy became the unifying concept of nineteenth century science
Science
Science is a systematic enterprise that builds and organizes knowledge in the form of testable explanations and predictions about the universe...
, initially by bringing thermodynamics
Thermodynamics
Thermodynamics is a physical science that studies the effects on material bodies, and on radiation in regions of space, of transfer of heat and of work done on or by the bodies or radiation...
and mechanics
Mechanics
Mechanics is the branch of physics concerned with the behavior of physical bodies when subjected to forces or displacements, and the subsequent effects of the bodies on their environment....
together and electrical technology
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
and ultimately relativistic physics leading to Einstein's
Albert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
equation
. The CGS unit of energy was the "ergErg
An erg is the unit of energy and mechanical work in the centimetre-gram-second system of units, symbol "erg". Its name is derived from the Greek ergon, meaning "work"....
", but the SI unit of energy was named the "joule" in honour of Joule.
In 1861 a committee of the British Association for Advancement of Science (BAAS) including William Thomson (later Lord Kelvin)
William Thomson, 1st Baron Kelvin
William Thomson, 1st Baron Kelvin OM, GCVO, PC, PRS, PRSE, was a mathematical physicist and engineer. At the University of Glasgow he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging...
, James Clerk Maxwell
James Clerk Maxwell
James Clerk Maxwell of Glenlair was a Scottish physicist and mathematician. His most prominent achievement was formulating classical electromagnetic theory. This united all previously unrelated observations, experiments and equations of electricity, magnetism and optics into a consistent theory...
and Joule among its members was tasked with investigating the "Standards of Electrical Resistance". In their first report (1862) 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) they introduced the concept of a coherent system of units whereby units of length, mass and time were identified as "fundemental units" (now known as base units
SI base unit
The International System of Units defines seven units of measure as a basic set from which all other SI units are derived. These SI base units and their physical quantities are:* metre for length...
). All other units of measure could be derived (hence derived units
SI derived unit
The International System of Units specifies a set of seven base units from which all other units of measurement are formed, by products of the powers of base units. These other units are called SI derived units, for example, the SI derived unit of area is square metre , and of density is...
) from these base units. The metre, gram and second were chosen as base 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 (centimetre-gram-second) system of units
Centimetre gram second system of units
The centimetre–gram–second system is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time...
. The committee also recommended the names of "dyne
Dyne
In physics, the dyne is a unit of force specified in the centimetre-gram-second system of units, a predecessor of the modern SI. One dyne is equal to exactly 10 µN...
" and "erg
Erg
An erg is the unit of energy and mechanical work in the centimetre-gram-second system of units, symbol "erg". Its name is derived from the Greek ergon, meaning "work"....
" for the CGS units of force and energy. The CGS system became the basis for scientific work for the next seventy years.
Electrical units
In the 1820s Georg OhmGeorg Ohm
Georg Simon Ohm was a German physicist. As a high school teacher, Ohm began his research with the recently-invented electrochemical cell, invented by Italian Count Alessandro Volta. Using equipment of his own creation, Ohm determined that there is a direct proportionality between the potential...
formulating law which can be extended to relate power to current, potential difference (voltage) and resistance. 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 - at least four different systems of units were devised. In the three CGS systems, the constants
and and consequently and were dimensionless.Symbols used in this section
|
Electromagnetic system of units- In the 1820's André-Marie AmpèreAndré-Marie AmpèreAndré-Marie Ampère was a French physicist and mathematician who is generally regarded as one of the main discoverers of electromagnetism. The SI unit of measurement of electric current, the ampere, is named after him....
discovered a relationship between the force between two current-carrying conductors, now known as Ampere's lawAmpère's force lawIn magnetostatics, the force of attraction or repulsion between two current-carrying wires is often called Ampère's force law...
which can be written
where 
(SI units)
- In 1833 Gauss pointed out the possibility of equating this force with its mechanical equivalent. This proposal received further support from Wilhelm WeberWilhelm Eduard WeberWilhelm Eduard Weber was a German physicist and, together with Carl Friedrich Gauss, inventor of the first electromagnetic telegraph.-Early years:...
in 1851. The electromagnetic (or absolute) system of units was one of the two systems of units identified in the BAAS report of 1862 and defined in the refport of 1873. In this system unit for current is defined by setting the magnetic force constantAmpère's force lawIn magnetostatics, the force of attraction or repulsion between two current-carrying wires is often called Ampère's force law...
was set to unity and the unit for potential difference were defined to ensure the unit of power calculated by the relation 
is identical to the unit of power required to move a mass of one gram is moved a distance of one centimetre in one second when opposed by a force of one dyne. The electromagnetic units of measure were known as the abampere, the abvolt, the abcoulomb and so on.
- Electrostatic system of units
- In 1783 Coulomb discovered and published the relationship between the force exerted between two charged bodies. This relationship, now known as Coulomb's lawCoulomb's lawCoulomb's law or Coulomb's inverse-square law, is a law of physics describing the electrostatic interaction between electrically charged particles. It was first published in 1785 by French physicist Charles Augustin de Coulomb and was essential to the development of the theory of electromagnetism...
can be written
where 
(SI units)
- The electrostatic system was the second of the two systems of units identified in the 1862 BAAS report and defined in the report of 1873. In this system unit for charge is defined by setting the Coulomb force constant (
) to unity and the unit for potential difference were defined to ensure the unit of energy calculated by the relation 
is one erg. The electrostatic units of measure are now known as the statampere, the statvolt, the statcoulomb and so on.
- Gaussian system of units
- In 1888 Heinrich Hertz verified Maxwell's EquationsMaxwell's equationsMaxwell's equations are a set of partial differential equations that, together with the Lorentz force law, form the foundation of classical electrodynamics, classical optics, and electric circuits. These fields in turn underlie modern electrical and communications technologies.Maxwell's equations...
and in so doing realised that the CGS system of electromagnetic units to were related to the CGS system of electrostatic units by the relationship: - Using this relationship, he proposed merging the EMU and the ESU systems into one system using the EMU units for magnetic quantities (subsequently named the gaussGauss (unit)The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
and maxwellMaxwell (unit)The maxwell, abbreviated as Mx, is the compound derived CGS unit of magnetic flux. The unit was previously called a line. The unit name honours James Clerk Maxwell, who presented the unified theory of electromagnetism, and was established by the IEC in 1930.In a magnetic field of strength one...
) and ESU units elsewhere. He named this combined set of units "Gaussian unitsGaussian unitsGaussian units comprise a metric system of physical units. This system is the most common of the several electromagnetic unit systems based on cgs units. It is also called the Gaussian unit system, Gaussian-cgs units, or often just cgs units...
". This set of units has been recognised as being particularly useful in theoretical physics.
- Practical system of units
- The CGS units of measure used in scientific work were not practical when used in engineering leading to the development of the practical system of electric units. At the time that this system of units was proposed, the dimensions of electrical resistance was modelled as the ratio of length to time (ie a velocity). The most practical unit for resistance was multiples of - the base unit of length was the length of the Earth's quadrant, the base unit of time the second and the base unit of mass was , the latter having been chosen to allow conversion back to the CGS system. The names, but not the values, ampAmpereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
, voltVoltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
, faradFaradThe farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
and ohmOhmThe ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
were carried over from the CGS system. The system was adopted at the International Electrical Congress (IEC) in 1881. This system was formalised as the International System of Electrical and Magnetic UnitsInternational System of Electrical and Magnetic UnitsThe International System of Electrical and Magnetic Units is an obsolete system of units used solely for measuring electrical and magnetic quantities. It was introduced by the Fourth International Electrical Congress and modified in 1908...
at the 1893 congress of the IEC in Chicago where the volt, amp and ohm were formally defined. The SI units with these names are very close, but not identical to the "practical units".
A coherent system
The electrical units of measure did not easily fit into the coherent system using length, mass and time as its base units as proposed in the 1861 BAAS paper. Using dimensional analysisDimensional analysis
In physics and all science, dimensional analysis is a tool to find or check relations among physical quantities by using their dimensions. The dimension of a physical quantity is the combination of the basic physical dimensions which describe it; for example, speed has the dimension length per...
the dimensions of charge as defined by the ESU system of units was identical to the dimensions of current as defined by the EMU system of units
while resistance had the same dimensions as velocity in the EMU system of units, but had the dimensions of the inverse of velocity in the ESU system of units.From mid 1890s onwards Giovanni Giorgi
Giovanni Giorgi
Giovanni Giorgi was an Italian electrical engineer who invented the Giorgi system of measurement, the precursor to the International System ....
and Oliver Heaviside
Oliver Heaviside
Oliver Heaviside was a self-taught English electrical engineer, mathematician, and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques to the solution of differential equations , reformulated Maxwell's field equations in terms of electric and...
corresponded with each other regarding these anomalous results. This led to Giorgi presenting a paper to the congress of the Associazione Elettrotecnica Italiana (A.E.I.) in October 1901 in which he showed that a coherent electro-mechanical 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 ke and km physical dimensions and hence the electrco-mechanical quantities ε0 and µ0 were also given physical dimensions.
It took more than thirty years before Giorgi's work was accepted in practice by international organisations and nearly another thirty years before they were incorporated as the basis of the Système International d'Unités (International System of Units), the SI.
Naming the units of measure
In 1861, Charles Bright and Latimer Clark proposed the names of ohmOhm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
, volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
, and farad
Farad
The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
in honour of Georg Ohm
Georg Ohm
Georg Simon Ohm was a German physicist. As a high school teacher, Ohm began his research with the recently-invented electrochemical cell, invented by Italian Count Alessandro Volta. Using equipment of his own creation, Ohm determined that there is a direct proportionality between the potential...
, Alessandro Volta
Alessandro Volta
Count Alessandro Giuseppe Antonio Anastasio Gerolamo Umberto Volta was a Lombard physicist known especially for the invention of the battery in 1800.-Early life and works:...
and Michael Faraday
Michael Faraday
Michael Faraday, FRS was an English chemist and physicist who contributed to the fields of electromagnetism and electrochemistry....
respectively for the practical units based on the centimetre-gramme-second absolute system. This was supported by Thomson (Lord Kelvin) These names were later scaled for use in the Practical System. The concept of naming units of measure after noteworthy scientists was subsequently used for other units.
Convention of the metre
With increasing international adoption of the metre, the short-comings 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 is was all but abandoned in favour of the artifact 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.The International Conference on Geodesy in 1867 called for the creation of a new, international prototype metre 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.
On 20 May 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 states. This treaty established the following organisations to conduct international activities relating to a uniform system for measurements:
- Conférence générale des poids et mesures (CGPM), an intergovernmental conference of official delegates of member nations and the supreme authority for all actions;
- Comité international des poids et mesures (CIPM), consisting of selected scientists and metrologistMetrologistMetrologists perform metrology work involving precision measurement and comparison of physical quantities such as mass, length, time, force, speed, voltage and current. They calibrate precision equipment which measures these physical units. They may also certify that outside standards of such...
s, which prepares and executes the decisions of the CGPM and is responsible for the supervision of the International Bureau of Weights and Measures; - Bureau international des poids et mesures (BIPM), a permanent laboratory and world centre of scientific metrology, the activities of which include the establishment of the basic standards and scales of the principal physical quantities, maintenance of the international prototype standards and oversight of regular comparisons between the international prototype and the various national standards.
The international prototype metres and kilograms were both made from a 90% platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
, 10% iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
alloy which is exceptionally hard and which has good electrical and thermal conductivity properties. The prototype had a special X-shaped (Tresca
Tresca
Tresca may refer to:* Carlo Tresca , Italian-born American anarchist* Henri Tresca , French mechanical engineer...
) cross section to minimise the effects of torsional strain during length comparisons. and the prototype kilograms were cylindrical in shape. The London firm Johnson Matthey
Johnson Matthey
Johnson Matthey plc is multinational chemicals and precious metals company headquartered in London, United Kingdom.It is listed on the London Stock Exchange and is a constituent of the FTSE 100 Index.-History:...
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.
Twentieth century
At the beginning of the twentieth century, the BIPM had custody of two artifacts - one to define length and the other to define mass. Other units of measure which did not rely on specific artifacts were controlled by other bodies. In the scientific world, quantum theory was in its infantcy and EinsteinAlbert Einstein
Albert Einstein was a German-born theoretical physicist who developed the theory of general relativity, effecting a revolution in physics. For this achievement, Einstein is often regarded as the father of modern physics and one of the most prolific intellects in human history...
had yet to publish his theories of relativity. By the end of the century, a coherent system of units was in place under the control of the bodies set up by the Treaty of the Metre, the definition of the second relied on quantum theory, the definition of the metre relied on the theory of relativity and plans were being made to relegate the international prototype kilogram to the archives.
Metre
The first (and only) follow-up comparison of the national standards with the international prototype metre was carried out between 1921 and 1936, and indicated that the definition of the metre was preserved to with 0.2 µm. During this follow-up 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 definition of melting ice, but in 1927 the 7th CGPM extended this definition was 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". The choice of 571 mm represents the Airy pointsAiry Points
Airy Points are used for precision measurement to support a length standard in such a way as to minimise bending or droop. The points are symmetrically arranged around the centre of the length standard and are separated by a distance equal to...
of the prototype - the points at which the bending or droop of the bar is minimized.
In 1887 Michelson
Michelson
Michelson may refer to:* Albert Abraham Michelson American physicist* Robert C. Michelson American roboticist* Michelson discovered in 1991* Michelson on the moon* Michelson-Gale-Pearson experiment, science...
proposed the use of optical interferometers for the measurement of length, work which contributed to him being awarded the Nobel Prize
Nobel Prize
The Nobel Prizes are annual international awards bestowed by Scandinavian committees in recognition of cultural and scientific advances. The will of the Swedish chemist Alfred Nobel, the inventor of dynamite, established the prizes in 1895...
in 1907. 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. By 1975, when the second had been defined in terms of a physical phenomena rather than the earth's rotation and Einstein's assertion that the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
was constant, 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.
Kilogram
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 massMass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
, not of weight
Weight
In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...
. 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.
During the course of the century, the various national prototypes of the kilogram were recalibrated against the international prototype and therefore against each other. The initial 1889 starting-value offsets of the national prototypes relative to the IPK were nulled. and any subsequent mass changes being relative to the IPK. A technique for steam cleaning the prototypes to remove any contaminants was developed in 1946 as part of the second recalibration.
The third periodic recalibration in 1988-9 revealed that the average difference between International Prototype Kilogram 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.
Time
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 - 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.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 would be calculated by extrapolation using earth's rotational speed in 1900. Astronomers from the US Naval Observatory (USNO) and the National Physical Laboratory
National Physical Laboratory
National Physical Laboratory can refer to:*National Physical Laboratory *National Physical Laboratory of India...
determined a relationship between the frequency of radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom and the estimated rate of rotation of the earth in 1900. Their value was adopted in 1968 by the 13th CGPM.
Electrical units
In 1921 the Treaty of the Metre was extended to cover electrical units with the CGPM merging its work with that of the IEC. 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 proposal be adopted was accepted, but no decision was made as to which electrical unit should be the fourth base unit. In 1935 Sears proposed that this should be the ampere, but World War IIWorld War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
prevented this being formalised until 1946. The definitions for absolute electrical system became effective from 1 January 1948.
Temperature
At the start of the twentieth century, the fundamental macroscopic laws of thermodynamics had been formulated and although techniques existed to measure temperature using empirical techniques, the scientific understanding of the nature of temperature was minimal. Maxwell and Boltzmann had produced theories describing the inter-relational of temperature, pressure and volume of a gas on a microscopic scale but otherwise, in 1900, there was no understanding of the microscopic or quantum nature of temperature. Within the metric system, temperature was expressed in degrees Centigrade 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 inter-related empirical measurements made using various devices.When, in 1948 the CGPM was charged with producing a coherent system of units of measure, definitions relating to temperature had to be clarified. At the 9th CGPM, the centigrade temperature scale was renamed the celsius temperature scale and the scale itself was fixed by defining the triple point of water as 0.01 °C, though the CGPM left the formal definition of absolute zero until the 10th GCPM when the name "degrees Kelvin" was assigned to the absolute temperature scale and triple point of water was defined as being 273.16 °K. In 1967, at the 13th GCPM the degree Kelvin (°K) was renamed the "kelvin" (K).
Over the ensuing years, the BIPM developed and maintained cross-correlations relating various measuring devices such as thermocouples, light spectra and the like to the equivalent temperatures. 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.
Luminosity
Prior to 1937, the International Commission on IlluminationInternational Commission on Illumination
The International Commission on Illumination is the international authority on light, illumination, color, and color spaces...
(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
Candela
The candela is the SI base unit of luminous intensity; that is, power emitted by a light source in a particular direction, weighted by the luminosity function . A common candle emits light with a luminous intensity of roughly one 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". was ratified by the GCPM in 1948 and in 1960 was adopted as an SI base unit. The definition proved difficult to implement so in 1967, the definition was revised and the reference to the radiation source was replaced by defining the candles in terms of the power of a specified wavelength of visible light.
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.
Mole
The mole was originally known as a gram-atom or a gram-molecule - the amount of a substance measured in grams divided by its atomic weightAtomic weight
Atomic weight is a dimensionless physical quantity, the ratio of the average mass of atoms of an element to 1/12 of the mass of an atom of carbon-12...
. 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 16O isotope whereas chemists assigned 16 amu to 16O, 17O and 18O isotopes mixed in the proportion that they occur in nature. Finally an agreement between the International Union of Pure and Applied Physics
International Union of Pure and Applied Physics
The International Union of Pure and Applied Physics is an international non-governmental organization devoted to the advancement of physics...
(IUPAP) and the International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry
The International Union of Pure and Applied Chemistry is an international federation of National Adhering Organizations that represents chemists in individual countries. It is a member of the International Council for Science . The international headquarters of IUPAC is located in Zürich,...
(IUPAC) brought this duality to an end in 1959/60, both parties agreeing to define the atomic weight of 12C 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.
International System of Units (SI)
Mks system of units
The MKS system of units is a physical system of units that expresses any given measurement using fundamental units of the metre, kilogram, and/or second ....
and CGS
Centimetre gram second system of units
The centimetre–gram–second system is a metric system of physical units based on centimetre as the unit of length, gram as a unit of mass, and second as a unit of time...
units of measure and for the first time the CGPM made recommendations concerning derived units.
The CIPM's draft proposal, which was an extensive revision and simplification of the metric unit definitions, symbols and terminology based on the MKS system of units, was put to the 10th CGPM in 1954. In accordance with Giorgi's proposals of 1901, the CIPM also recommended that the ampere be the base unit from which electromechanical 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 metre, ampere, second and kilogram. After negotiations with the CIS and IUPAP, two further base units, the degree kelvin and the candela were also proposed as base units. The full system and name "Système International d'Unités" were adopted at the 11th CGPM.
During the years that followed the definitions of the base units and particularly the mise en pratique to realise these definitions have been refined.
The "New SI"
When the metre was redefined in 1960, the kilogram was the only SI base unit that relied on a specific artifact. Moreover, after the 1996-1998 recalibration a clear divergence between the various prototype kilograms was observed.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 artifacts that were then in use. At a meeting of the CCU held in Reading, United Kingdom
Reading, Berkshire
Reading is a large town and unitary authority area in England. It is located in the Thames Valley at the confluence of the River Thames and River Kennet, and on both the Great Western Main Line railway and the M4 motorway, some west of London....
in September 2010, a resolution 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. The proposals that the CCU put forward were:
- In addition to the speed of light, four constants of nature - Planck's constant, an elementary chargeElementary chargeThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
, Boltzmann constant and Avogadro's numberAvogadro's numberIn chemistry and physics, the Avogadro constant is defined as the ratio of the number of constituent particles N in a sample to the amount of substance n through the relationship NA = N/n. Thus, it is the proportionality factor that relates the molar mass of an entity, i.e...
be defined to have exact values. - The international prototype kilogram be retired
- The current definitions of the kilogram, ampereAmpereThe ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
, kelvinKelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
and moleMole (unit)The mole is a unit of measurement used in chemistry to express amounts of a chemical substance, defined as an amount of a substance that contains as many elementary entities as there are atoms in 12 grams of pure carbon-12 , the isotope of carbon with atomic weight 12. This corresponds to a value...
be revised. - The wording of the definitions of all the base units be tightened up
The CIPM meeting of October 2010 found that "the conditions set by the General Conference at its 23rd meeting have not yet been fully met. For this reason the CIPM does not propose a revision of the SI at the present time"; however the CIPM presented a resolution for consideration at the 24th CGPM (17 - 21 October 2011) to agree the new definitions in principle, but not to implement them until the details have been finalised. This resolution was accepted by the conference and in addition the CGPM moved the date of the 25th meeting forward from 2015 to 2014.