| SI-units | UnitConversion
| SteamTable | StarchViscosity
| SieveComparison | TableIndex |
| Time | Mass |
Length | Temperature
| Ampere | Mole | Light
| Radian | Steradian
| DerivedUnits | UnitPreFix
System of Base Units
(The seven base units):
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.
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)).
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.
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.
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.
1971 the mole was defined as the amount of substance of a system
that contains as many elementary entitieswhich may be molecules,
atoms, ions, and so onas there are atoms in 0.012 kilogram
of carbon-12. This number, known as Avogadros Number, is
about 6.022 × 1023.
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.
System of Supplementary Units (Two
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.
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
(Nine common units derived from SI base units)