** 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 caesium-133 atom.

**Mass
**

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)).

**
**Length

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.

**
**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 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.

**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 carbon-12. 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)