Resistance of a conductor is directly proportional to the length of the conductor,
i.e., R ∝ l ………………….. (1)
Resistance of a conductor is inversely proportional to the cross-section area of the conductor.
i.e., R ∝ 1/A .........................(2)
From (1) and (2) R ∝ R ∝ l/A = R = pl/A
where ρ is a constant,
ρ is called specific resistance or resistivity.