International Bureau Of Weights And Measures

The International Bureau of Weights and Measures (, BIPM) is an intergovernmental organisation, through which its 64 member-states act on measurement standards in areas including chemistry, ionising radiation, physical metrology, as well as the International System of Units (SI) and Coordinated Universal Time (UTC). It is headquartered in the Pavillon de Breteuil in Saint-Cloud, near Paris, France. The organisation has been referred to as IBWM (from its name in English) in older literature.

Function

The BIPM has the mandate to provide the basis for a single, coherent system of measurements throughout the world, traceable to the International System of Units (SI). This task takes many forms, from direct dissemination of units to coordination through international comparisons of national measurement standards (as in electricity and ionising radiation).

Following consultation, a draft version of the BIPM Work Programme is presented at each meeting of the General Conference for consideration with the BIPM budget. The final programme of work is determined by the CIPM in accordance with the budget agreed to by the CGPM.

Currently, the BIPM's main work includes:

* Making brochures that define the International System of Units.
* Scientific and technical activities carried out in its four departments: chemistry, ionising radiation, physical metrology, and time
* Liaison and coordination work, including providing the secretariat for the CIPM Consultative Committees and some of their Working Groups and for the CIPM MRA, and providing institutional liaison with the other bodies supporting the international quality infrastructure and other international bodies
* Capacity building and knowledge transfer programs to increase the effectiveness within the worldwide metrology community of those Member State and Associates with emerging metrology systems
* A resource centre providing a database and publications for international metrology

The BIPM is one of the twelve member organisations of the International Network on Quality Infrastructure (INetQI), which promotes and implements QI activities in metrology, accreditation, standardisation and conformity assessment.

The BIPM has an important role in maintaining accurate worldwide time of day. It combines, analyses, and averages the official atomic time standards of member nations around the world to create a single, official Coordinated Universal Time (UTC).

Structure

The BIPM is overseen by the International Committee for Weights and Measures (), a committee of eighteen members that meet normally in two sessions per year, which is in turn overseen by the General Conference on Weights and Measures () that meets in Paris usually once every four years, consisting of delegates of the governments of the Member States and observers from the Associates of the CGPM. These organs are also commonly referred to by their French initialisms.

History

The creation of the International Bureau of Weights and Measures followed the Metre Convention of 1875, after the Franco-Prussian War (1870–1871), at the initiative of the International Geodetic Association. This process began with the 1855 Paris Exposition, shortly after the Great Exhibition, when the need for international standardization of weights and measures became apparent. During the process of unification of Germany, geodesists called for the establishment of an International Bureau of Weights and Measures in Europe. These trends culminated in the 1889 General Conference on Weights and Measures, with the distribution of the metre and kilogram standards to the States parties to the Metre Convention.

When the metre was adopted as an international unit of length, it was well know that it no longer corresponded to its historical definition. Carlos Ibáñez e Ibáñez de Ibero, first president of both the International Geodetic Association and the International Committee for Weigths and Measures, took part to the remeasurement and extension of the arc measurement of Delambre and Méchain. At that time, mathematicians like Legendre and Gauss had developed new methods for processing data, including the least squares method which allowed to compare experimental data tainted with observational errors to a mathematical model. Moreover, the International Bureau of Weights and Measures would have a central role for international geodetic measurements as Charles Édouard Guillaume's discovery of invar minimized the impact of measurement inaccuracies due to temperature systematic errors.

Geodetic standards and the Expositions Universelles (1855 /1867)

In the 19th century, units of measurement were defined by primary standards, and unique artifacts made of different alloys with distinct coefficients of expansion were the legal basis of units of length. A wrought iron ruler, the Toise of Peru, also called ''Toise de l'Académie'', was the French primary standard of the toise, and the metre was officially defined by an artifact made of platinum kept in the National Archives. Besides the latter, another platinum and twelve iron standards of the metre were made by Étienne Lenoir in 1799. One of them became known as the Committee Meter in the United States and served as standard of length in the United States Coast Survey until 1890.
In 1855, the Dufour map (French: ''Carte Dufour''), the first topographic map of Switzerland for which the metre was adopted as the unit of length, won the gold medal at the Exposition Universelle. However, the baselines for this map were measured in 1834 with three toises long measuring rods calibrated on a toise made in 1821 by Jean Nicolas Fortin for Friedrich Georg Wilhelm von Struve.

The geodetic measuring device calibrated on the metre devised by Carlos Ibáñez e Ibáñez de Ibero and Frutos Saavedra Meneses, was displayed by Jean Brunner at the Exhibition. The four-metre-long Spanish measuring instrument, which became known as the Spanish Standard (French: ''Règle espagnole''), was compared with Borda's double-toise N° 1, which served as a comparison module for the measurement of all geodesic bases in France, and was also to be compared to the Ibáñez apparatus. These comparisons were essential, because of thermal expansion. Indeed, geodesists tried to accurately assess temperature of standards in the field in order to avoid temperature systematic errors.

The Earth measurements thus underscored the importance of scientific methods at a time when statistics were implemented in geodesy. As a leading scientist of his time, Carlos Ibáñez e Ibáñez de Ibero was one of the 81 initial members of the International Statistical Institute (ISI) and delegate of Spain to the first ISI session (now called World Statistic Congress) in Rome in 1887. On the sidelines of the Exposition Universelle (1855) and the second Congress of Statistics held in Paris, an association with a view to obtaining a uniform decimal system of measures, weights and currencies was created in 1855. Under the impetus of this association, a Committee for Weights and Measures and Monies (French: ''Comité des poids, mesures et monnaies'') would be created during the Exposition Universelle (1867) in Paris and would call for the international adoption of the metric system.

The metre and Struve Geodetic Arc (1816/1855)

In 1858, a Technical Commission was set up to continue cadastral surveying inaugurated under Muhammad Ali. This Commission suggested to buy geodetic devices which were ordered in France. Mohammed Sa'id Pasha entrusted to Ismail Mustafa al-Falaki the study of the precision apparatus calibrated against the metre intended to measure geodetic baselines and built by Jean Brunner in Paris. Ismail Mustafa had the task to carry out the experiments necessary for determining the expansion coefficients of the two platinum and brass bars, and to compare the Egyptian standard with a known standard. The Spanish standard designed by Carlos Ibáñez e Ibáñez de Ibero and Frutos Saavedra Meneses was chosen for this purpose, as it had served as a model for the construction of the Egyptian standard.

It was not until 1954 that the connection of the southerly extension of the Struve Geodetic Arc, a chain of survey triangulations stretching from Hammerfest in Norway to the Black Sea, with an arc running northwards from South Africa through Egypt would bring the course of a major meridian arc back to land where Eratosthenes had founded geodesy. The Struve Geodetic arc measurement extended on a period of forty years and initiated an international scientific collaboration between Russian Empire and the United Kingdoms of Sweden and Norway with the involvement of proeminent astronomers such as Friedrich Georg Wilhelm von Struve, Friedrich Wilhelm Bessel, Carl Friedrich Gauss and George Biddell Airy. A French scientific instrument maker, Jean Nicolas Fortin, made three direct copies of the Toise of Peru, one for Friedrich Georg Wilhelm von Struve, a second for Heinrich Christian Schumacher in 1821 and a third for Friedrich Wilhelm Bessel in 1823. In 1831, Henri-Prudence Gambey also realised a copy of the Toise of Peru which was kept at Altona Observatory in Hamburg.

According to geodesists, these standards were secondary standards deduced from the Toise of Peru. In continental Europe, except Spain, surveyors continued to use measuring instruments calibrated on the Toise of Peru. Among these, the toise of Bessel and the apparatus of Borda were respectively the main references for geodesy in Prussia and in France. These measuring devices consisted of bimetallic rulers in platinum and brass or iron and zinc fixed together at one extremity to assess the variations in length produced by any change in temperature. The combination of two bars made of two different metals allowed to take thermal expansion into account without measuring the temperature.

Metric act of 1866 and calls for an international standard unit of length

In 1866, Ferdinand Rudolph Hassler's use of the metre and the creation of the Office of Standard Weights and Measures as an office within the Coast Survey contributed to the introduction of the Metric Act of 1866 allowing the use of the metre in the United States, and preceded the choice of the metre as international scientific unit of length and the proposal by the 1867 General Conference of the European Arc Measurement (German: ''Europäische Gradmessung'') to establish the International Bureau of Weights and Measures. Moreover, it was asserted that the Toise of Peru, the standard of the toise constructed in 1735 for the French Geodesic Mission to the Equator, might be so much damaged that comparison with it would be worthless, while Bessel had questioned the accuracy of copies of this standard belonging to Altona and Koenigsberg Observatories, which he had compared to each other about 1840.

This assertion was particularly worrying, because when the primary Imperial yard standard had partially been destroyed in 1834, a new standard of reference was constructed using copies of the "Standard Yard, 1760", instead of the pendulum's length as provided for in the Weights and Measures Act 1824, because the pendulum method proved unreliable.

International Geodetic Association

The intimate relationships that necessarily existed between metrology and geodesy explain that the International Association of Geodesy, founded to combine the geodetic operations of different countries, in order to reach a new and more exact determination of the shape and dimensions of the Globe, prompted the project of reforming the foundations of the metric system, while expanding it and making it international. Not, as it was mistakenly assumed for a certain time, that the Association had the unscientific thought of modifying the length of the metre, in order to conform exactly to its historical definition according to the new values that would be found for the terrestrial meridian. But, busy combining the arcs measured in the different countries and connecting the neighbouring triangulations, geodesists encountered, as one of the main difficulties, the unfortunate uncertainty which reigned over the equations of the units of length used. Adolphe Hirsch, General Baeyer and Colonel Ibáñez decided, in order to make all the standards comparable, to propose to the Association to choose the metre for geodetic unit, and to create an international prototype metre differing as little as possible from the

In 1867 at the second General Conference of the International Association of Geodesy held in Berlin, the question of an international standard unit of length was discussed in order to combine the measurements made in different countries to determine the size and shape of the Earth. According to a preliminary proposal made in Neuchâtel the precedent year, the General Conference recommended the adoption of the metre in replacement of the toise of Bessel, the creation of an International Metre Commission, and the foundation of a World institute for the comparison of geodetic standards, the first step towards the creation of the International Bureau of Weights and Measures.

Saint Petersburg Academy

Ferdinand Rudolph Hassler's metrological and geodetic work also had a favourable response in Russia. In 1869, the Saint Petersburg Academy of Sciences sent to the French Academy of Sciences a report drafted by Otto Wilhelm von Struve, Heinrich von Wild, and Moritz von Jacobi, whose theorem has long supported the assumption of an ellipsoid with three unequal axes for the figure of the Earth, inviting his French counterpart to undertake joint action to ensure the universal use of the metric system in all scientific work. The French Academy of Sciences and the Bureau des Longitudes in Paris drew the attention of the French government to this issue. In November 1869, Napoleon III issued invitations to join the International Metre Commission.

The International Metre Commission (1870/1872)

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. There was much discussion within this commission, considering the opportunity either to keep as definitive the units represented by the standards of the Archives, or to return to the primitive definitions, and to correct the units to bring them closer to them. Since its origin, the metre has kept a double definition; it is both the ten-millionth part of the quarter meridian and the length represented by the ''Mètre des Archives''. The first is historical, the second is metrological. The first solution prevailed, in accordance with common sense and in accordance with the advice of the French Academy of Sciences. Abandoning the values represented by the standards, would have consecrated an extremely dangerous principle, that of the change of units to any progress of measurements; the Metric System would be perpetually threatened with change, that is to say with ruin. Thus the Commission called for the creation of a new international prototype metre which length would be as close as possible to that of the ''Mètre des Archives'' and the arrangement of a system where national standards could be compared with it.

At the session on 12 October 1872 of the Permanent Committee of the International Metre Commission, which was to become the International Committee for Weights and Measures, Carlos Ibáñez e Ibáñez de Ibero was elected president. On 19 April 1875, Ibáñez' presidency was confirmed and Adolphe Hirsch was elected secretary of the International Committee for Weights and Measures. On 6 May 1873 during the 6th session of the French section of the Metre Commission, Henri Étienne Sainte-Claire Deville cast a 20-kilogram platinum-iridium ingot from Matthey in his laboratory at the École normale supérieure (Paris). On 13 May 1874, 250 kilograms of platinum-iridium to be used for several national prototypes of the metre was cast at the Conservatoire national des arts et métiers. When a conflict broke out regarding the presence of impurities in the metre-alloy of 1874, a member of the Preparatory Committee since 1870 and president of the Permanent Committee of the International Metre Commission, Carlos Ibáñez e Ibáñez de Ibero intervened with the French Academy of Sciences to rally France to the project to create an International Bureau of Weights and Measures equipped with the scientific means necessary to redefine the units of the metric system according to the progress of sciences. In fact, the chemical analysis of the alloy produced in 1874 by the French section revealed contamination by ruthenium and iron which led the International Committee for Weights and Measures to reject, in 1877, the prototypes produced by the French section from the 1874 alloy. It also seemed at the time that the production of prototypes with an X profile was only possible through the extrusion process, which resulted in iron contamination. However, it soon turned out that the prototypes designed by Henri Tresca could be produced by milling.

The 1875 Metre Convention and the founding of the BIPM

The principal tasks facing the delegates at the 1875 conference was the replacement of the existing metre and kilogram artefacts that were held by the French Government and the setting up of an organization to administer the maintenance of standards around the globe. The conference did not concern itself with other units of measure. The conference had undertones of Franco-German political manoeuvring, particularly since the French had been humiliated by the Prussians during the war a few years previously. Although France lost control of the metric system, they ensured that it passed to international rather than German control and that the international headquarters were in Paris.

Two members of the Permanent Committee of the International Metre Commission, the German astronomer, Wilhelm Julius Foerster, director of the Berlin Observatory and director of the German Weights and Measures Service, and the Swiss geodesist of German origin, Adolphe Hirsch were also among the main architects of the Metre Convention. While the German astronomer Wilhelm Julius Foerster along with the Russian and Austrian representatives boycotted the Permanent Committee of the International Metre Commission in order to prompt the reunion of the Diplomatic Conference of the Metre and to promote the foundation of a permanent International Bureau of Weights and Measures, Adolphe Hirsch, delegate of Switzerland at this Diplomatic Conference in 1875, conformed to the opinion of Italy and Spain to create, in spite of French reluctance, the International Bureau of Weights and Measures in France as a permanent institution at the disadvantage of the ''Conservatoire national des arts et métiers''.

Spain notably supported France for this outcome and the first president of the International Committee for Weights and Measures, the Spanish geodesist, Carlos Ibáñez e Ibáñez de Ibero received the Grand Officer medal of the Légion d'Honneur for his diplomatic role on this issue and was awarded the Poncelet Prize for his scientific contribution to metrology. Indeed, as Carlos Ibáñez e Ibáñez de Ibero was collaborating with the French on the extension of the arc measurement of Delambre and Méchain since 1853, and was president of both the Permanent Committee of the International Metre Commission since 1872 and the Permanent Commission of the International Association of Geodesy since 1874, he was to play a pivotal role in reconciling French and German interests. The Metre Convention was signed on 20 May 1875 in Paris and the International Bureau of Weights and Measures was created under the supervision of the International Committee for Weights and Measures, presided by Carlos Ibáñez e ibáñez de Ibero.

The 1889 General Conference on Weights and Measures

In 1889, the General Conference on Weights and Measures, presided by Alfred Descloizeaux, met at the Pavillon de Breteuil, the seat of the International Bureau of Weights and Measures. It performed the first great deed dictated by the motto inscribed in the pediment of the splendid edifice that is the metric system: "''A tous les temps, à tous les peuples''" (For all times, to all peoples); and this deed consisted in the approval and distribution, among the governments of the states supporting the Metre Convention, of prototype standards of hitherto unknown precision intended to propagate the metric unit throughout the whole world.

For metrology the matter of expansibility was fundamental; as a matter of fact, the temperature measuring error related to the length measurement in proportion to the expansibility of the standard and the constantly renewed efforts of metrologists to protect their measuring instruments against the interfering influence of temperature revealed clearly the importance they attached to the expansion-induced errors. It was common knowledge, for instance, that effective measurements were possible only inside a building, the rooms of which were well protected against the changes in outside temperature, and the very presence of the observer created an interference against which it was often necessary to take strict precautions. Thus, the Contracting States also received a collection of thermometers whose accuracy made it possible to ensure that of length measurements. 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 comparison of the new prototypes of the metre with each other involved the development of special measuring equipment and the definition of a reproducible temperature scale. The BIPM's thermometry work led to the discovery of special alloys of iron–nickel, in particular invar, whose practically negligible coefficient of expansion made it possible to develop simpler baseline measurement methods, and for which its director, the Swiss physicist Charles-Edouard Guillaume, was granted the Nobel Prize in Physics in 1920. Guillaume's Nobel Prize marked the end of an era in which metrology was leaving the field of geodesy to become an autonomous scientific discipline able of redefining the metre through technological applications of physics. On the other hand, the fondation of the United States Coast and Geodetic Survey by Ferdinand Rudolph Hassler paved the way to the actual definition of the metre, with Charles Sanders Peirce being the first to experimentally link the metre to the wavelength of a spectral line. Albert Abraham Michelson soon took up the idea and improved it.

Additional roles for standardized time

In 1987 the International Bureau of Weights and Measures took over some of the work of the International Time Bureau when it was dissolved in 1987. The remaining tasks were assigned to the International Earth Rotation and Reference Systems Service (IERS), which replaced the International Polar Motion Service and the earth-rotation section of the International Time Bureau. Already in 1936, irregularities in the speed of Earth's rotation due to the unpredictable movement of air and water masses were discovered through the use of quartz clocks. They implied that the Earth's rotation was an imprecise way of determining time. In 1967, the second was redefined based on atomic clocks. resulting in the International Atomic Time (TAI). The International Bureau of Weights and Measures began to establish atomic clocks world-wide, numbering more than 450 currently.

The 2019 revision of the SI

The International System of Units (SI, abbreviated from the French ''Système international (d'unités)''), the modern form of the metric system was revised in 2019. It is the only system of measurement with an official status in nearly every country in the world. It comprises a coherent system of units of measurement starting with seven base units, which are the second (the unit of time with the symbol s), metre (length, m), kilogram (mass, kg), ampere (electric current, A), kelvin (thermodynamic temperature, K), mole (amount of substance, mol), and candela (luminous intensity, cd). Since 2019, the magnitudes of all SI units have been defined by declaring exact numerical values for seven ''defining constants'' when expressed in terms of their SI units. These defining constants are the hyperfine transition frequency of caesium Δ''ν''_Cs, the speed of light in vacuum ''c'', the Planck constant ''h'', the elementary charge ''e'', the Boltzmann constant ''k'', the Avogadro constant ''N''_A, and the luminous efficacy ''K''_cd.

Directors

Since its establishment, the directors of the BIPM have been:

See also

* Foundation of the International Bureau of Weights and Measures
* History of the metre
* Institute for Reference Materials and Measurements
* International Organization for Standardization
* '' Metrologia''
* National Institute of Standards and Technology
* Seconds pendulum
* World Metrology Day
* Versailles project on advanced materials and standards

Notes

References

External links

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{{DEFAULTSORT:International Bureau Of Weights And Measures
Intergovernmental organizations established by treaty
Metric system
Metrology organizations
Organizations based in Paris
Organizations established in 1875
Standards organizations in France