Physikalisch-Technische Bundesanstalt

When you want to buy a pound of asparagus or a dozen tomatoes at the weekly market, you will still get the quantity you want. But the last old units and figures – which were officially abolished long ago – now slowly disappear from everyday life. They are relics from a time at which most varied units of measurement existed for one and the same quantity. "Each German province – however small it might be – has its own little quantum" – this old saying characterizes a situation which made trade between different places almost impossible. And the situation was not much different in the other states and provinces. France was the first country to put an end to this chaos: At the time of the French Revolution the metre (after the Greek word metron for measure) emerged as a novel unit of length which later became binding all over France.

The new unit with its decimal sub-multiples (with the prefixes milli-, centi-, kilo-, etc.) gradually won through also in the neighbouring countries. In 1870 it arrived in the German Reich. But its worldwide triumphant advance began as late as 1875 with the signing of the Metre Convention by which all signatories vowed that they would ensure the international unification and perfection of the metric system. For this purpose, they established several bodies: the International Committee for Weights and Measures (CIPM) and the General Conference on Weights and Measures (CGPM) which primarily conduct scientific investigations and formulate recommendations, as well as the International Bureau of Weights and Measures (Bureau International des Poids et Mesures BIPM) near Paris, which was declared the central guardian of the units. This body up to now maintains the international standards and prototypes for the physical quantities, which are regularly compared with their national counterparts in the individual countries (in Germany at PTB).

The Metre Convention has once and for all reformed metrology. In the place of the sovereign who once could frankly fix his own measure, it now was the state which was responsible and had to come to agreements with other states. Without this the further development of the industrialized world would virtually have been impossible – or at least completely different. Furthermore, now the foundations had been laid for the development of an international system of units, which in 1960 was created by the name "Système International d‘Unités" (SI) and is applied today in almost all states in the world: a system with seven base units from which all other units can be derived just through multiplication or division, without conversion factors. This simple structure made the SI well-known beyond its original goal: It was first intended only for science, technique and instruction but largely won through also in everyday economic activities.

Among the SI base units are the metre and the kilogram, which originally had been created as a unit derived from the metre. Likewise, the definition of the base unit of current, the ampere, is related to the metre. The metric system thus has entered the new system of units as an essential element and has been disseminated via the SI all over the world.