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# Physikalisch-Technische Bundesanstalt

Thematic toursGuardian of the unitsThe SI > The history of the SI
The history of the SI

The SI base units are interrelated in many ways. For example: the length, the time as well as the mass (through the definition of the force) enter into the definition of the current.

Up to a point not far from the mars, everything still was in order. But then the accident happened: Instead of entering a stable orbit, the Mars Climate Orbiter came too close to the red planet and burnt up in the atmosphere. This happened in September 1999. One immediately started a hectic search for the cause of this fault. The result was hardly believable: Different units had been used in the computations carried out by the two NASA control centres in Denver and Pasadena: one team had used metre and kilogram, the other foot and pound – and this more than a century after the United States and 16 other states had agreed to use exclusively the metric system and 40 years after the SI units (based on the metric system) had been introduced in almost all parts of the world.

An embarrassing incident – and an impressing example of how important it is that the whole world applies the same measure. "For the whole world, for all peoples" – this motto was coined during the French Revolution when the new unit of length, the metre, was created. The new unit became the basis of the international metric, decimal system of measures, which put an end to utter confusion as far as the units of measurement were concerned. Compliance with, and further development of, the metric system are supervised by the bodies of the Metre Convention. Its executive body, the General Conference on Weights and Measures, met for the first time in 1889. It approved prototypes of the metre and the kilogram and distributed them to the member states. The main purpose of the subsequent meetings was the creation of a new international system of units. In 1948, the 9th General Conference on Weights and Measures adopted a draft for such a system of units, which was originally based on six base units. All other units are related with these base units just through multiplication and division. The great advantage of this system: there are no conversion factors at all.

The 10th General Conference on Weights and Measures held in 1954 officially adopted the six base units: length (metre), mass (kilogram), time (second),electric current (ampere), thermodynamic (kelvin) and luminous intensity (candela).A seventh base unit, the amount of substance (mole),was added as late as 1973. Today it is usually stated in sixth place. This change in the historical order was made at the instigation of the International Bureau of Weights and Measures, (Bureau International des Poids et Mesures BIPM) to underline that the development of optics will possibly lead to a discussion about the candela and its status as a base unit. In 1960, the new system was given the name "Système International d’Unités", abbreviated SI. The 11th General Conference on Weights and Measures in 1960 decided that this abbreviation was to be used in all languages, and it adopted prefixes to form names for the decimal multiples and sub-multiples of units. The new system was introduced in the Federal Republic of Germany by the Units in Metrology Act (Units Act) dated July 2, 1969 and the decree implementing this Act dated July 5, 1970. Since January 1, 1978, application of the old units has been prohibited in Germany.

© Physikalisch-Technische Bundesanstalt, last update: 2015-01-26, Volker Großmann