International Bureau of Weights and Measures
The International Bureau of Weights and Measures 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 and Coordinated Universal Time. It is headquartered in the Pavillon de Breteuil in Saint-Cloud, near Paris, France. The organisation has been referred to as IBWM 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. This task takes many forms, from direct dissemination of units to coordination through international comparisons of national measurement standards.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:
- Publishing definitive information about 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 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.
Structure
The BIPM has a supervisory board called 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, 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 standardisation 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 known 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 minimised 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 artefacts 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 artefact 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.File:Appareil_Ibáñez.jpg|thumb|313x313px|Ibáñez apparatus calibrated on the metric Spanish standard and used at Aarberg, in canton of Bern, Switzerland in 1880.|left
In 1855, the Dufour map, 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, 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. In order to maintain measurement traceability it was important to control the temperature during these intercomparisons in order to avoid 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 and delegate of Spain to the first ISI session in Rome in 1887. On the sidelines of the Exposition Universelle 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 would be created during the Exposition Universelle 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 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.