Conduct an activity to show that resistance of a conductor is proportional to the length of the conductor for constant cross-section area and temperature.

- Collect iron spokes of different lengths with same cross-sectional area.
- Make a circuit leaving gap between P and Q as shown in figure.
- Connect one of the iron spokes say 10 cm length between P and Q.
- Measure the value of current using ammeter and note the value of current.
- 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.
- We notice that the value of current decreases with increasing in the length of the iron spoke.
- Thus the resistance of iron spoke increases with increasing the length i.e., R l.
- From this we conclude that the resistance ® of a conductor is directly proportional to its length (l) for a constant area of cross section.