The international Bureau of Weights and Measures serve as a depository for the primary international standards and as a laboratory for certification and inter comparison of national standard copies. It is based in Sevres, France. In 1960, the SI system was built upon the metre-kilogram-second system, which has seven basic units. They are:
(Unit of length; symbol m), defined as the distance travelled by light in 1/299792458 second;
(Unit of weight; symbol kg), which equals 1000 grams as defined by the international prototype kilogram of platinum-iridium in the keeping of the international Bureau of Weights and Measures.
(Unit of time; symbol S), the duration of 9192631770 periods of radiation associated with a specified transition of the cesium-133 atom.
(Unit of electric current; symbol A), which is the current that, if maintained in two wires placed one meter apart in a vacuum would produce a force of 2*107 Newton per metre of length.
(Unit of luminous intensity; symbol cd), defined as the intensity in a given direction of a source emitting radiation of frequency 540*1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
(Unit of amount of substance; symbol mol), defined as containing as many elementary entities of a substance as there are atoms in 0.012 kilogram of Carbon-12.
(Unit of temperature; symbol K), which is 1/273.16 of the thermodynamic temperature of the triple point (equilibrium among the solid, liquid, and gaseous phases) of pure water.