In expression R = p  l  if I = unity and a = unity then R = p
                             a
Hence resistance of material of unit length having unit cross-sectional area is defined as the resistivity of specific resistance of the material.
Specific resistance or resistivity of a material is also defined as the resistance between opposite faces of a unit cube of than material.

It is measured in ohm-meters or ohms per meter cube in m.k.s (or S I) system and in ohm-cm or ohms per cm cube in C.G.S. system

1 Ω - M = 100 Ω - cm

The reciprocal of resistance i.e. I  is called the conductance and is denoted by English letter G. It is
                                             R
defined as the inducement offered by the conductor to the flow of current and is measured in mhos ()

G = 1  =   1   = 1. a = k. a
      R     p  l     p  l         l
               R

where k = 1 and is known as conductivity of material. Hence conductivity is the reciprocal of the
               p
resistivity and is defined a the conductance between the two opposite faces of a unit cube. Its unit is mho per. meter( /m).

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