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.
In our chemical current source the energy of chemical reactions energy is converted electrical. Between electrodes, placed in an electrolyte solution (in our case it is “Borjomi”), a potential difference is formed: an electromotive force appropriate to the free energy of oxidation-reduction reaction. On the cathode the deoxidizer is oxidized; formed free electrons move (creating a discharge current) through the external circuit to the anode. There electrons participate in the reduction reaction of the oxidant.
A bit of history. The first chemical current source had been invented by the Italian physicist Alessandro Volta in 1800. It was an element of Volta: a jar with salt water and the zinc and copper plates connected by a wire.
By the way, first batteries appeared in the mass sales in 1896.