- Resistance ‘R’ of a conductor is directly proportional to its length 7’ at constant temperature and cross sectional area.

R $\propto$ l (i) - Resistance ‘r’ of a conductor is inversely proportional to its cross section area at constant temperature and length.

R $\propto$ 1/A --------(ii)

From (1) and (2)

R $\propto$ 1/A

=====> R = p l/A

Where p is a proportionality constant and p is called specific resistance or resistivity.