SI Units
 SIunits  UnitConversion
 SteamTable  StarchViscosity
 SieveComparison  TableIndex 
 Time  Mass 
Length  Temperature
 Ampere  Mole  Light
 Radian  Steradian
 DerivedUnits  UnitPreFix

International
System of Base Units
(The seven base units):
Time
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 caesium133 atom.
Mass
The
kilogram is defined as a mass equal to that of the International
Prototype Kilogram  a platinumiridium 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)).
Length
The
metre was 1960 redefined as 1,650,763.73 wavelengths of the reddishorange
light emitted by the isotope krypton86. 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.
Temperature
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.
Ampere
The
ampere was defined as the constant electric current that, flowing
in two straight parallel conductors of infinite length, of negligible
circular crosssection, and placed one metre apart in a vacuum,
would produce between these conductors a force equal to 2 × 107 newton per metre of length.
Mole
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 carbon12. This number, known as Avogadro’s Number, is
about 6.022 × 10^{23}.
Light
In
1979 the candela was redefined as the luminous intensity, in a
given direction, of a source that emits monochromatic radiation
of frequency 540 × 10^{12} hertz (cycles per second) and that has a radiant intensity of (1/638)
watt per steradian.
International
System of Supplementary Units (Two
supplementary units):
Radian
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.
Steradian
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)