The Metric System
Although some of the ideas incorporated into the metric system had been proposed as early as 1670, the system itself was not developed until the 1790s. Leaders of the French Revolution, hoping to bring unity to their country's chaotic variety of weights and measures, instructed the French Academy of Sciences to devise a new system. The report of the Academy was accepted by the government, but the new system did not become mandatory until 1840.
As devised by the Academy, the basic unit of the metric system was the meter (a name taken from the Greek metron, meaning “a measure”). The meter was defined as a ten millionth of the length of the meridian that passed through Paris from the North Pole to the equator. Other units, including those of capacity (volume) and mass (weight), were defined in terms of the meter. The gram was defined as the mass of one cubic centimeter of water at the temperature of its maximum density (4° C). The cubic decimeter, commonly called the liter, was the unit of capacity. The metric system was established as a base-10 system, with prefixes to indicate multiples and divisions of the basic units.
The political appeal of the French Revolution, the conquests of Napoleon, and the usefulness of the metric system itself helped it gain widespread adoption. The Metric Convention of 1875 established the International Bureau of Weights and Measures at Sèvres, France, to maintain standards and to make periodic revisions. By the mid-20th century most countries had adopted the metric system.
In 1960 the bureau adopted Le Système International d'Unités (SI). An expansion of the metric system, SI defines its base units in terms of physical processes. The meter, for example, is defined as 1,650,763.73 wavelengths in a vacuum of the orange-red line of the spectrum of krypton-86. The kilogram remains the only base unit still defined in terms of a physical prototype or object. Minor revisions and additions have since been made to SI.