# Resistivity

**Resistivity or Specific Resistance**

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

*Directly proportional to its length (l) or***R***∝**l.**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

or

**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).**