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Base and Derived Unit of Measurement International System of Units (SI)


The International System of Units (in French Le System International d’Unites- bbreviated as SI) was established by the 11th General Conference on Weights and Measures

(CGPM from Conference Generate des Poids at Measures). The CGPM is an inter-governmental treaty organization created by a diplomatic treaty known as Meter Convention which was signed in Paris in 1875. The metric system was found to be more convenient as it was based on the decimal system. The fundamental units of metric system are gram for mass, the metre for length and the liter for the volume of fluids.

In 1960, the International Committee of Weights and Measures recommended the use of International System of Units, abbreviated as SI Units. SI units are largely a modification of metric system. The seven basic units in SI system are given in Table 2.1.

Table2.1. The Seven Basic SI Units

Physical  Quantity                   Abbreviation               Name of Unit                        Symbol

1. Length                                             l                                        metre                                      m

2. Mass                                                m                                    kilogram                                  kg

3. Tune                                                 t                                      second                                       s

4. Electric current                           I                                     ampere                                      A

5. Thermodynamic                          T                                     kelvin                                        K


6. Luminous                                      IV                            candela                       cd


’7.Amount of                                     n                                  mole                         mol



The SI units of some of the physical quantities are either too small or too large. To change the order of magnitude, these are expressed by using prefixes before the name of the base units.

The various prefixes used for this purpose are listed in Table.2.2.


    Table 2.2. Prefixes for Expressing the Decimal Fractions in the SI System

Multiple                                   Prefix                          Symbol

1024                                          yotta                            y

1021                                          zetta                            z

10I8                                          exa                               E

1015                                          peta                            p

1012                                          tera                              T

109                                           gig a                            G

106                                           mega                            M

103                                           kilo                              k

102                                           hector                          h

10                                            deka                            da

10-1                                          deci                             d

10-2                                          centi                           c

10-3                                          milli                            m

10-6                                          micro                           m

10-9                                         nano                            n

10-12                                         pi co                           p

10-15                                         fern to                        f

10-18                                         atto                              a

10-21                                         zepto                           z

10-24                                         yocto                          y


The idea of using prefixes is illustrated by an example below:

A hundredth of a meter, corresponding to 1 o-2, is a centimeter for which the symbol cm is used. Similarly for 103 metre we can use the term kilometer, symbolized as km.



In addition to the seven basic units given in Table 2.1, there are many derived units. These units are obtained by combination of basic units. For deriving these units, we can multiply or divide the symbols for units as if they are algebraic

(i) The volume of a box is determined by multiplying length, breadth and height i.e.,Volume = Length x Breadth x Height if the units of length are m, Volume = m x m x m = m3 Similarly,

(ii) Area= Length x Breadth= m x m = m2 (square metre)

(iii) Density = V o1 u me of sample =- m 3 =kg m-3 In the similar manner other units can also be built up. Some commonly used SI derived units are given in Table 2.3 for the purpose of reading reference.

Table 2.3. Some Common SI Derived Units