Experimentally, it is found that the resistance of a conductor is:
- Directly proportional to its length (l) or
- Inversely proportional to its area of cross-section (A) or
Let’s combine these two conditions for the resistance as
R ∝ l / A
where ρ is the constant of proportionality and known as the or of the conductor.
Rearrange the above expression for ρ,
- Thus, the 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 .
- In SI unit of resistivity or specific resistance is