Resistance caused by the difficulty of ion movement in electrolyte
In circuit (a) of Fig 1, the switch is open and there is no current through the bulb, but the reading on the voltmeter is 12.1 V. In circuit (b), the switch is closed and there is a current through the bulb. The voltmeter shows the p.d. across the bulb and across the battery. The reading on the voltmeter is now slightly less, at 11.8V. In the first case, the only current is a few microamperes through the voltmeter. This current is negligible compared with the current in the bulb in the second circuit. Why does the voltage decrease? The difference in voltage can be explained by considering the internal resistance of the battery.