Systems of units


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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 platinumirridium 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 Cesium133 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 crosssection, and placed 1 m apart in a vacuum, would produce between these conductors of force equal to 2x10^{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 carbon12.(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) 
