System of Base Units
(The seven base units):
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.
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°
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
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 supplementary units):
is the plane angle between two radii of a circle that cut off on
the circumference an arc equal in length to the radius.
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.
Units (Nine common units derived from SI base units)