# I**nternational** **System** of **Units**

I**nternational** **System** of **Units**
*also called ***SI ****System**, French
**Système I****nternational** D'unités,
i**nternational** decimal **system** of weights and measures
derived from and extending the metric **system** of **units**.
Adopted by the 11th General Conference on Weights and Measures in 1960,
it is abbreviated SI in all languages.
Rapid advances in science and technology in the
19th and 20th centuries fostered the development of several overlapping
**system**s of **units** of measurements as scientists
improvised to meet the practical needs of their disciplines. The early i**nternational**
**system** devised to rectify this situation was called the metre-kilogram-second
(MKS) **system**. The General Conference on Weights and Measures
added three new **units** (among others) in 1948: a unit of force
(the newton),
defined as that force which gives to a mass of one kilogram an
acceleration of one metre per second per second; a unit of energy (the joule),
defined as the work done when the point of application of a newton is
displaced one metre in the direction of the force; and a unit of power
(the watt), which is
the power that in one second gives rise to energy of one joule. All
three **units** are named for eminent scientists.

The 1960 I**nternational** **System**
builds upon the MKS **system**. Its seven basic **units**, from
which other **units** are derived, are currently defined as
follows: for length, the metre, defined as
the distance traveled by light in a vacuum in 1/299,792,458 second; for
mass, the kilogram (about
2.2 pounds avoirdupois), which equals 1,000 grams as defined by the i**nternational**
prototype kilogram of platinum-iridium in the keeping of the I**nternational**
Bureau of Weights and Measures in Sèvres, France; for time, the second,
the duration of 9,192,631,770 periods of radiation associated with a
specified transition of the cesium-133 atom; for electric current, the ampere,
which is the current that, if maintained in two wires placed one metre
apart in a vacuum, would produce a force of 2 ´
10^{-7} newton per metre of length;
for luminous intensity, the candela, defined
as the intensity in a given direction of a source emitting radiation of
frequency 540 ´ 10^{12} hertz
and that has a radiant intensity in that direction of 1/683 watt per
steradian; for amount of substance, the mole,
defined as containing as many elementary entities of a substance as
there are atoms in 0.012 kilogram of carbon-12; and for thermodynamic
temperature, the kelvin (*see* absolute zero).

### Source:

"International System of Units." __Encyclopædia
Britannica__. 2004. Encyclopædia Britannica Online.

29 Jan. 2004 <http://search.eb.com/eb/article?eu=43567>.