1) Collect iron spokes of different lengths with same cross-sectional area.
2) Make a circuit leaving gap between P and Q as shown in figure.
3) Connect one of the iron spokes say 10 cm length between P and Q.
4) Measure the value of current using ammeter and note the value of current.
5) Repeat this experiment for other lengths say 20 cm, 30 cm, 40 cm of iron spokes and note the corresponding values of current in each case.
6) We notice that the value of current decreases with increasing in the length of the iron spoke.
7) Thus the resistance of iron spoke increases with increasing the length i.e., R l.
8) From this we conclude that the resistance (R) of a conductor is directly proportional to its length (l) for a constant area of cross section.