The International System of Units (SI)
All systems of weights and measures, metric and non-metric, are linked through a network of international agreements supporting theInternational System of Units. The International System is called the SI, using the first two initials of its French name Système Internationald'Unités. The key agreement is the Treaty of the Meter (Convention du Mètre), signed in Paris on May 20, 1875. 48 nations have now signed this treaty, including all the major industrialized countries. The United States is a charter member of this metric club, having signed the original document back in 1875. The SI is maintained by a small agency in Paris, the International Bureau of Weights and Measures (BIPM, for Bureau International des Poids et Mesures), and it is updated every few years by an international conference, the General Conference on Weights and Measures (CGPM, forConférence Générale des Poids et Mesures), attended by representatives of all the industrial countries and international scientific and engineering organizations. The 24th CGPM met in 2011; the next meeting will be in 2014. As BIPM states on its web site, "The SI is not static but evolves to match the world's increasingly demanding requirements for measurement." At the heart of the SI is a short list of base units defined in an absolute way without referring to any other units. The base units are consistent with the part of the metric system called the MKS system. In all there are seven SI base units:
the meter for distance, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for intensity of light.
Other SI units, called SI derived units, are defined algebraically in terms of these fundamental units. For example, the SI unit of force, the newton, is defined to be the force that accelerates a mass of one kilogram at the rate of one meter per second per...