In the past, the systems of units most commonly used were the English
, the English system is based on a single standard, the metric is subdivided into two interrelated standards:
and The CGS
The MKS system
, and Seconds
, while the CGS system
, and Seconds
Unfortunately, the use of more than one system of units in a world, would introduce unnecessary complications to the basic understanding of any technical data. The need for a standard set of units to be adopted by all nations has become increasingly obvious. In 1960, the General Conference adopted a system called Le Systeme International d'Unites (International System
, which has the international abbreviation SI
The International Bureau of Weights and Measures
located at Sevres, France, has been the host for the General Conference of Weights and Measures, attended by representatives from all nations of the world. Since then, SI has been adopted by the Institute of Electrical and Electronic Engineers
, Inc. (IEEE) in 1965 and by the United States of America Standards Institute in 1967 as a standard for all scientific and engineering literature.
was originally defined in 1790 to be 1/10,000,000 the
distance between the equator and either pole at sea level, a length preserved
on a platinum-iridium bar at the International Bureau of Weights and Measures at Sevres, France.
Meter is now defined with reference to the speed of light in a vacuum, which is 299,792,458 m/s.
Kilogram is defined as a mass equal to 1000 times the mass of one cubic centimeter of pure water at 4℃.
This standard is preserved in the form of a platinum-iridium cylinder in Sevres.
was originally defined as 1/86,400 of the mean solar
day. However, since Earth's rotation is slowing down by almost 1 second
every 10 years,
Second was redefined in 1967 as 9,192,631,770 periods of the electromagnetic radiation emitted by a particular transition of cesium atom.
This table shows interrelation between different systems of units.