Batteries are always dead at the most unsuitable moment. What thing can replace battery (or chemical current source)?
Let’s take two metal plates: copper and lead (of course, thriftful manager has it). Let us call them (plates) electrodes. Lead — cathode (negative), and copper — anode (positive).
Turn on a voltmeter and look… Of course, there is no electricity.
If the electrodes are placed in a container and (sic!) water «Borjomi», is added, we get almost 0.5V! If we connect in series three of the containers with the electrodes, filled with «Borjomi», we will get almost a fully sound analog of «finger» battery.