The history of the metric system began during the Age of Enlightenment with measures of length and weight derived from nature , along with their decimal multiples and fractions. The system became the standard of France and Europe within half a century. Other measures with unity ratios [ Note 1 ] were added, and the system went on to be adopted across the world. The first practical realisation of the metric system came in , during the French Revolution , after the existing system of measures had become impractical for trade, and was replaced by a decimal system based on the kilogram and the metre.
The basic units were taken from the natural world. The unit of length, the metre, was based on the dimensions of the Earth , and the unit of mass , the kilogram, was based on the mass of a volume of water of one litre a cubic decimetre. Reference copies for both units were manufactured in platinum and remained the standards of measure for the next 90 years.
After a period of reversion to the mesures usuelles due to unpopularity of the metric system, the metrication of France and much of Europe was complete by the s. In the middle of the 19th century, James Clerk Maxwell conceived a coherent system where a small number of units of measure were defined as base units , and all other units of measure, called derived units , were defined in terms of the base units.
Maxwell proposed three base units for length, mass and time. Advances in electromagnetism in the 19th century necessitated additional units to be defined, and multiple incompatible systems of such units came into use; none could be reconciled with the existing dimensional system.
The impasse was resolved by Giovanni Giorgi , who in proved that a coherent system that incorporated electromagnetic units required a fourth base unit, of electromagnetism. A seventh base unit, the mole , and six other derived units were added later in the 20th century. During this period, the metre was redefined in terms of the speed of light, and the second was redefined based on the microwave frequency of a caesium atomic clock.
