1) Collect iron rods of equal lengths but different cross-section areas.
2) Make a circuit leaving gap between P and Q as shown in figure.
3) Connect one of the rods between P and Q and measure the current using ammeter and note values.
4) Repeat this with the other rods and note the corresponding values of current in each case and note them.
5) You will notice that the current flowing through the rod increases with increasing in the cross-section area of the rod.
6) Thus the resistance of the rod decreases with increasing the cross-section area.
7) From this we conclude that the resistance (R) of a conductor is inversely proportional to its cross-section area (A).
.-. R $\propto$ l/A (at constant temperature and length of the conductor)