![]() The unperturbed ground-state hyperfine splitting frequency of the cesium-133 atom Δ ν( 133Cs) hfs is exactly 9 192 631 770 hertz. New SI base quantities, defining constants, and definitions The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. ![]() The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12 the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles. The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 × 10 −7 newton per meter of length. The kilogram is the unit of mass it is equal to the mass of the international prototype of the kilogram. The meter is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second. The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. Present SI base quantities, base units, and definitions In the words of the Marquis de Condorcet, a new system of measurement “for all time, for all people” was born. Subsequently, in 1799, two platinum artifact standards for length and mass based on those definitions were deposited in the Archives de la République in Paris. 2 Definitions for the units of volume and mass followed, with the liter being 0.001 m 3 and the kilogram the mass of 1 liter of distilled water at 4 ☌. The meter came from a perceived constant of nature: one ten-millionth of the distance along Earth’s meridian through Paris from the North Pole to the equator. Following an idea proposed a century earlier by John Wilkins, 1 the new system of weights and measures took as its starting point a single universal measure-the meter-and used it to define length, volume, and mass. Although the present International System of Units (SI, from the French Système International d’Unités) was officially established in 1960, its origin goes back to the creation of the metric system during the French Revolution.
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