A metric system is a system of units for measurement developed in late 18th century France with decimal multipliers. In the early metric system there were two fundamental or base units, the metre and the kilogram, for length and mass. Other units were derived from these two fundamental units.
Multiples and submultiples of metric units are related by powers of ten; the names for these are formed with prefixes. This relationship is compatible with the decimal system of numbers and it contributes greatly to the convenience of metric units.
As the result of scientific progress, refinements, and different choices of base units, there have been a number attempts at creating metric systems. The modern (modern meaning post-1960) metric system is now widely used throughout the world and is called the International System of Units (SI).
Scientists, chiefly in France, had been advocating and discussing a decimal system of measurement based on natural units since at least 1640. It had some support by French monarchs. The first official adoption of such a system occured in France in 1791 after the French Revolution of 1789. The creators of this metric system tried to choose units that were logical and practical. The revolution gave an opportunity for drastic change with an official ideology of "pure reason". It was proposed as a considerable improvement over the inconsistent collection of customary units that existed before. The value of these units often depended on the region.
The adoption of the metric system in France was slow, but its desirability as an international system was recognised by geodesists and others. Since then a number of variations on the system evolved. Their use spread throughout the world, first to the non-English-speaking countries, and more recently to the English speaking countries.
On the 20th of May 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 states. This treaty established the following organisations to conduct international activities relating to a uniform system for measurements:
The original French system somewhat continued the tradition of having separate base units for geometrically related dimensions, i.e. metre for lengths, are (100 m²) for areas, stere (1 m³) for volumes and dry capacities and litre (1 dm³) for liquid capacities. The base unit of mass was the gram it also included only few prefixes,
Several national variants existed thereof with aliases for some common subdivisions. In general this entailed in redefinition of other units in use, e.g. 500-gram pounds or 10-kilometre miles. An example of these is mesures usuelles (or metrified English unit though never officially adopted). However it’s debatable whether such systems are true metric systems.
Early on in the history of the metric system various centimetre gram second system of units (CGS) had been in use. These units were particularly convenient in science and technology.
Later metric systems were based on the metre, kilogram and second (MKS) to improve the value of the units for practical applications. MKSC, metre-kilogram-second-coulomb systems and MKSA, metre-kilogram-second-ampere systems are extentions of these.
The International System of Units (Système international d'unités or SI) is the current international standard metric system and the system most widely used around the world. It is based on the metre, kilogram, second, ampere, kelvin, candela and mole.
The metre-tonne-second system of units (MTS) was based on the metre, tonne and second. It was invented in France and mostly used in the Soviet Union from 1933 to 1955.
Gravitational systems use the kilogram-force as a base unit of force, with mass measured in a unit known as the hyl, TME, mug or metric slug.
Several nations, notably the United States, typically use the spellings 'meter' and 'liter' instead of 'metre' and 'litre'. This is in keeping with standard American English spelling (for example, Americans also use 'center' rather than 'centre,' using the latter only rarely for its stylistic implications; see also American and British English differences). In addition, the official US spelling for the SI prefix 'deca' is 'deka'.
The US government has approved these spellings for official use. In scientific contexts only the symbols are used; since these are universally the same, the differences do not arise in practice in scientific use.
The unit 'gram' is also sometimes spelled 'gramme' in English-speaking countries other than the United States, though that is an older spelling and use is declining.
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