SI Base Quantities and Units.

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    The System of units which is at present internationally accepted for measurement is the Systeme Internationale d'Unites (French for International System of Units)      abbreviated as SI. The SI, with standard scheme of symbols,units and abbreviations, was developed and recommended by General Conference an Weights and Measures in 1971 for international usage in scientific, technical, industrial and commercial work. Because SI units used decimal system, conversions within the system are quite simple and convenient.



BASE QUANTITY :  Length

SI UNITS :

Name:      metre

 Symbol:     m       

Definition: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.(1983)


BASE QUANTITY :   Mass

SI UNITS :

Name:     kilogram

Symbol:   kg      


Definition: The kilogram is equal to the mass of the international prototype of the kilogram (a platinum-iridium alloy cylinder) kept at international Bureau of Weights and Measures, at Sevres, near Paris, France. (1889) 



BASE QUANTITY :   Time

SI UNITS :  

Name:     second

Symbol:     s            

          

Definition: The second is the duration of 9,192,163,770 periods of   the radiation corresponding to the transition between the two hyperfine levels of the ground state of cesium-133 atom (1967).

 

BASE QUANTITY :   Electric Current

SI UNITS :   

Name:     Ampere

Symbol:    A       


Definition:  The ampere is that constant current which, if maintained in two straight parallel conductors of  infinite length, of negligible circula cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newton per metre of length.(1948)


BASE QUANTITY :   Thermodynamic Temperature

SI UNITS : 

Name:     kelvin

Symbol:     k    

     

Definition:  The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.(1967)


BASE QUANTITY :    Amount of Substance

SI UNITS : 

Name:      mole

Symbol:      mol   


Definition:  The mole is the amount of substance of a system, which contains as many elementary entities as there are atoms in 0.012 kilograms of carbon - 12. (1971)


BASE QUANTITY :   Luminous intensity

SI UNITS : 

Name:    candela

Symbol:      cd        

                    

Definition:  The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 1012   hertz and that has radiant intensity in that direction of 1/683 watt per steradian.(1979)

              

                                

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