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Equal measure

125 years ago, the metre and the metric system started on their triumphal march


[es]The metre celebrates its birthday as the quantity after which a successful principle of metrology is named. On March 20, the international metric system was 125 years old. When that day in spring 1875 the representatives of 17 states signed an international agreement, they could not dream that its effects would be so far-reaching. But this international treaty - the Metre Convention - has decisively influenced the development of the industrialized world since. With this treaty the effective unification of the units of measurement began - a fundamental prerequisite for the perfect functioning of economy, trade and industry. The Metre Convention thus was an important step on the road toward the International System of Units (SI) which in 1960 made an end to the thousands of years of confusion as regards the units of measurement.

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 in the other states and provinces, the situation was not much different. France was the first country to put an end to this chaos: At the time of the French Revolution the metre (after the Greek work metron for measure) emerged as a novel unit of length which later became binding all over France.

The new unit with its decimal subdivision (with the prefixes milli-, centi-, kilo- etc.) gradually won also through in the neighbouring countries. In 1870 it arrived in the German Reich. But its worldwide triumphal march 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 various bodies: the International Committee for Weights and Measures (CIPM) and the General Conference for Weights and Measures (CGPM) which primarily conduct scientific investigations and formulate recommendations, as well as the International Bureau for Weights and Measures (BIPM) near Paris, which was declared the central keeper of the units. This body up to now maintains the international standards for the most important physical quantities and the international prototypes which are permanently compared with their national counterparts in the individual countries (in Germany at the Physikalisch-Technischen Bundesanstalt Braunschweig und Berlin, 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 the 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 were 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 of the world: a system with seven base units with which all other units are linked up only via multiplication by integers, i.e. without a complicated conversion being necessary. This simple structure made the SI well-known beyond its original goal: It was first intended only for science, technique and instruction but largely also won through in everyday economics.

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.