Measure the resistance of a bulb (filament) in open circuit with a multi-meter. Make a circuit with elements such as bulb, battry of 12V and key in siries. Close the key. Then again measure the resitance of the same bulb (filament) for every 30 seconds Record the observation in a proper table. What can you conclude from the above results ?

Explanation

When the bulb is connected in circuit and key is closed. The resistance decreases for every 30 seconds.
Conclusion :
1. The potential difference of a battery does not change even in open or closed circuit.
2. The resistance of a conductor decreases as temperature increases.