# Systems of units

Units may be defined as a part of a quantity by which comparison of the magnitude could be made.

There are two kinds of units:

- Fundamental units
- Derived units

The units for the fundamental for base quantities are called fundamental or base units. The units for all other physical quantities can be expressed as a combination of the base units. Such units obtained for the derived quantities are called derived units. A complete set of these units, both the base and derived units is known as the system of units.

** Systems of units: ** Several systems of units have been in use to describe measurements. The common systems are the CGS systems(Centimetre, Gram, Second); the FPS system (Foot, Pound, Second) which is the British system; the MKS system (Meter, Kilogram and Second) and the now internationally accepted which is known as International Systems of Units(came from French name “Le Systeme International d’ Unites”). abbreviated SI.

** SI Base Quantities And Units **

Base quantities | Units | Symbol is | Definitions |
---|---|---|---|

Length | meter | m | The meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. (1983) |

Mass | kilogram | kg | The kilogram is equal to the mass of international prototype of the kilogram (a platinum-irridium alloy cylinder) at international bureau of weights and measures, at Severs, near Paris, France. (1889) |

Time | second | s | The second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state to the Cesium-133 atom. (1967) |

Electric current | ampere | A | The ampere is that constant current, if maintained into straight parallel conductors of in finite length, of negligible circular cross-section, and placed 1 m apart in a vacuum, would produce between these conductors of force equal to 2×10^{-7} newton per metre of length.(1948) |

Thermodynamic temperature | Kelvin | K | The kelvin, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. (1967) |

Amount of substance | Mole | mol | The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.(1971) |

Luminous intensity | candela | cd | The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x 1012 Hertz and that has a radiant intensity in the direction of 1/683 watt per steradian. (1979) |