(a) When the battery is connected in a circuit, the voltage slowly decreases due to consumption of it. So, there is difference in voltage before using and after connecting.
(b) After connecting battery (12 V), key in ammeter and bulb as shown in figure we measure current (I) using the ammeter and voltage using multimeter or voltmeter.
Note these values in the below table.
Measure the resistance of the same bulb for every 30 seconds.
We conclude that the resistance is constant.