The International System of Units (abbreviated *SI* from *systeme internationale* , the French version of the name) is a scientific method of expressing the magnitudes or quantities of important natural phenomena. There are seven base units in the system, from which other units are derived. This system was formerly called the meter-kilogram-second (MKS) system.

All SI units can be expressed in terms of standard multiple or fractional quantities, as well as directly. Multiple and fractional SI units are defined by prefix multipliers according to powers of 10 ranging from 10 ^{-24} to 10 ^{24} .

**SI base units:**

The meter (abbreviation, m) is the SI unit of displacement or length. One meter is the distance traveled by a ray of electromagnetic (EM) energy through a vacuum in 1/299,792,458 (3.33564095 x 10 ^{-9} ) second. The meter was originally defined as one ten-millionth (0.0000001 or 10 ^{-7} ) of the distance, as measured over the earth's surface in a great circle passing through Paris, France, from the geographic north pole to the equator.

The kilogram (abbreviation, kg) is the SI unit of mass. It is defined as the mass of a particular international prototype made of platinum-iridium and kept at the International Bureau of Weights and Measures. It was originally defined as the mass of one liter (10 ^{-3} cubic meter) of pure water.

The second (abbreviation, s or sec) is the SI unit of time. One second is the time that elapses during 9.192631770 x 10 ^{9} cycles of the radiation produced by the transition between two levels of Cesium 133. It is also the time required for an EM field to propagate 299,792,458 (2.99792458 x 10 ^{8} ) meters through a vacuum.

The kelvin (abbreviation K), also called the degree Kelvin (abbreviation, ^{o} K), is the SI unit of temperature. One Kelvin is 1/273.16 (3.6609 x 10 ^{-3} ) of the thermodynamic temperature of the triple point of pure water (H _{2} O).

The ampere (abbreviation, A) is the SI unit of electric current. One ampere is the current that would produce a force of 0.0000002 (2 x 10 ^{-7} ) newton between two straight, parallel, perfectly conducting wires having infinite length and zero diameter, separated by one meter in a vacuum. One ampere represents 6.24 x 10 ^{18} unit electric charge carriers, such as electrons, passing a specified fixed point in one second.

The candela (abbreviation, cd) is the SI unit of luminous intensity. It is the electromagnetic radiation, in a specified direction, that has an intensity of 1/683 (1.46 x 10 ^{-3} ) watt per steradian at a frequency of 540 terahertz (5.40 x 10 ^{14} hertz).

The mole (abbreviation, mol) is the SI unit of material quantity. One mole is the number of atoms in 0.012 kilogram of the most common isotope of elemental carbon (C-12). This is approximately 6.022169 x 10 ^{23} , and is also called the Avogadro constant.

SI derived units include the hertz , the newton , the pascal (unit of pressure or stress) , the ohm , the farad , the joule , the coulomb , the tesla , the lumen , the becquerel , the siemen , the volt , and the watt .

These units are included in our Table of Physical Units .

*This was last updated in*March 2011

*Posted by:*Margaret Rouse

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