<P> The history of the metric system began in the Age of Enlightenment with simple notions of length and weight taken from natural ones, and decimal multiples and fractions of them . The system was so useful it became the standard of France and Europe in half a century . Other dimensions with unity ratios were added, and it went on to be adopted by the world . </P> <P> The first practical realisation of the metric system came in 1799, during the French Revolution, when the existing system of measures, which had become impractical for trade, was replaced by a decimal system based on the kilogram and the metre . In the pre-scientific era, 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 water having a volume of one litre or 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 as well as much of Europe was complete by mid-century . </P> <P> In the middle of the 19th century, James Clerk Maxwell put forward the concept of 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: length, mass and time . Advances in electromagnetism in the 19th century necessitated new units to be defined, and multiple incompatible systems of such units came into usage; none could be reconciled with the existing system of mechanical units . This impasse was resolved by Giovanni Giorgi, who in 1901 proved that a coherent system that incorporated electromagnetic units had to have an electromagnetic unit as a fourth base unit . </P>

Who ruled france when the metric system was established