Galvanic

a battery is two metals disagreeing about electrons.

drag either electrode through six metals and watch the needle jump the instant they disagree

Every metal holds its electrons with a different strength, measured as a standard reduction potential. Put two different metals in the same conductive solution and wire them together, and the one that holds electrons more weakly gives them up -- they flow through the wire toward the metal that wants them more, and that flow is current. The size of the gap between the two potentials is the voltage, and which metal loses electrons (the anode) versus gains them (the cathode) is decided entirely by the gap, not by which one you think of as "the battery." Force the current backward with an outside supply and the same reaction runs in reverse -- electroplating, which is corrosion happening on purpose.

Concepts: electricity, chemistry.

An interactive you play with one thumb, on Wundr.