These days we take for granted that a meter in our own country is exactly equal to a meter at the other side of the globe. If you buy 5 meters of that beautiful cloth online in Canada, you (rightfully) assume that the Canadian meter is the same as the one in your country. If your energy supplier imports electricity from a neighboring country, it is taken that the ampere in both countries is the same. Straightforward as this may seem, it has only been so for a relatively short period of time. In the Middle Ages traveling salesmen had to beware, as the unit of length (for instance of the cloth he was selling) was determined by the length of the arm of the mayor of the village he was staying. As mayors come in sizes, so did the units of length. In the course of centuries, the first attempts were made to standardize units, though it is estimated that by the end of the eighteenth century in France alone approximately 250,000 different units were used.
One of the ideals of the French Revolution was to achieve measuring standards “all the time, for everyone”. This led to the metric system. In 1875, 17 countries signed The Metre Convention, in which they amongst others denoted physical platinum-iridium artifacts as the “world standards”, the prototypes, for the kilogram and the meter. These prototypes were famously stored in Sèvres, near Paris, at the newly founded Bureau International de Poids et Mesures (BIPM). In the course of nearly a century, the scope of BIPM was broadened to other units. In 1960 the Metric System was superseded by the Système International d’Unités, commonly known as the SI. Since then, several adaptations to the Si have been made. Currently the SI contains seven base units: the kilogram (kg), the meter (m), the second (s), the kelvin (K), the ampere (A), the mole (mol) and the candela (cd). From these seven base units, all other units (newton, pascal, joule, volt and many others) can be determined.
Learn about the SI units
Redefining the SI units
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