Resistivity or Specific Resistance

Experimentally, it is found that the resistance of a conductor is:

  1. Directly proportional to its length (l) or R l.
  2. Inversely proportional to its area of cross-section (A) or R 1/A.

Let’s combine these two conditions for the resistance as

R l / A


R = ρ × l / A 

where ρ is the constant of proportionality and known as the Resistivity or Specific Resistance of the conductor. 

Rearrange the above expression for ρ,

ρ = R × A / l 

  • Thus, the Resistivity of a conductor is defined as the resistance of the conductor of unit length and unit area of cross-section. 
  • In CGS system the unit of resistivity or specific resistance is ohm-cm (Ω-cm)
  • In SI unit of resistivity or specific resistance is ohm-meter (Ω-m).