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SI Units | SI-units | UnitConversion
| SteamTable | StarchViscosity
| SieveComparison | TableIndex | International System of Base Units (The seven base units): 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 caesium-133 atom. The kilogram is defined as a mass equal to that of the International Prototype Kilogram - a platinum-iridium cylinder which in 1889 replaced the previous platinum protype matching the mass of 1 cubic decimetre of pure water at the temperature of its maximum density (4.0° C/39.2° F)). The metre was 1960 redefined as 1,650,763.73 wavelengths of the reddish-orange light emitted by the isotope krypton-86. The metre was again redefined in 1983 as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second. Since 1960 the temperature scale is based on the triple point of water. The triple point of a substance is the temperature and pressure at which its solid, liquid, and gas forms are in equilibrium. The temperature of 273.16 K (kelvins) was assigned to this point. The freezing point of water was designated as 273.15 K, equalling exactly 0° on the Celsius temperature scale. The Celsius scale has been redefined in terms of Kelvin temperatures, so that now absolute zero, 0 K, is at -273.15° C. As a consequence the freezing and boiling points of water are at present determined as 0° C and 99.974° C respectively. The ampere was defined as the constant electric current that, flowing in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed one metre apart in a vacuum, would produce between these conductors a force equal to 2 × 10-7 newton per metre of length. In 1971 the mole was defined as the amount of substance of a system that contains as many elementary entities—which may be molecules, atoms, ions, and so on—as there are atoms in 0.012 kilogram of carbon-12. This number, known as Avogadro’s Number, is about 6.022 × 1023. In 1979 the candela was redefined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012 hertz (cycles per second) and that has a radiant intensity of (1/638) watt per steradian. International System of Supplementary Units (Two supplementary units): The radian is the plane angle between two radii of a circle that cut off on the circumference an arc equal in length to the radius. The steradian is the solid angle that, having its vertex at the centre of a sphere, cuts off an area of the surface of the sphere equal to that of a square with sides of length equal to the radius of the sphere. The radian and the steradian are classed as derived units. Derived Units (Nine common units derived from SI base units)
Derived Units assigned special names (Units derived from SI base and supplementary units):
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