The SI or International System of units has been established for many years and it forms the basis of most of the measurements that are used globally.

The International System of Units is officially termed "Système International d'Unités" and it was established in 1960 by the 11th General Conference on Weights and Measures (CGPM). Many of the older systems like the Imperial system used in the UK and many other places were not as easy to handle or use, and none were standardized across the world.

Within SI there are seven base units upon which all others are based. The base units include: mass, length, time, temperature, amount of substance, electric current, and luminous intensity.

## Table of the SI base units

SI Base Units | |||
---|---|---|---|

Physical quantity | Dimension Symbol | Unit name | Unit Symbol |

Mass | M | Kilogram | kg |

Length | L | Metre | m |

Time | T | Second | s |

Temperature | ° | Kelvin | k |

Amount of substance | N | Mole | mol |

Current | I | Ampere | A |

Luminous intensity | J | Candela | cd |

## SI unit definitions

In order that each of the SI units and quantities can be standardised across the globe, it is necessary to have exact definitions of each of them. While it is unlikely that these definitions of the SI units will be used in anything but a standards laboratory, they are often useful to see and know.

The metre is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 second.*Metre:*The kilogram is the unit of mass equal to the mass of the international prototype of kilogram.*Kilogram:*The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levers (F=4, m*Second:*_{F}=0 to F=3, m_{F}=0) of the ground state of the cesium 133 atom.The ampere is the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2x10*Ampere:*^{-7}Newton per meter of lengthThe Kelvin, unit of thermodynamic temperature is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.*Kelvin:*The mole is the mount of substance of a system which contains as many elementary entities as there are atoms in .012 kg of carbon 12 (about 6.022x10*Mole:*^{23}atoms). When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x10*Candela:*^{12}Hz and that has a radiant intensity in that direction of 1/683 watt per steradian.

## Supplementary SI Units

SI Supplementary Units | |||
---|---|---|---|

Physical quantity | Dimension Symbol | Unit name | Unit Symbol |

Plane angle | α | Radian | rad |

Solid Angle | ω | Steradian | sr |

## Rationale for seven SI base units

The SI or System International has the set of seven base units. These have been chosen to fulfil the requirements for science and technology measurements. The selection of seven base units is the responsibility of the International Committee of Weights and Measures (CIPM) which ahs defined and now maintains the SI.

The base units are to a certain extent an arbitrary choice. When the metric system was set up a three dimensional mechanical system was chosen with the metre kilogram and second as base units. This was later extended to a four dimensional system to include first the ampere, then the kelvin, candela and finally the mole were added. These additions brought the number of base SI units to seven.

The SI or International System of Units is well established although some countries still use other systems - for example the old Imperial system is still widely used in the USA, but generally SI units are used across the globe enabling a common system to be used, thereby reducing compatibility issues and errors in coversion between systems.