The battery can be regarded as a container of dissolved agent (acid, base or salt) wherein two plates of different metals are partly immersed [footnote 1]. Instead of one metal plate, a graphite rod can also serve. Just as there is an exception to every rule, so it is in the electromagnetism. Coal is a special and unique case of a non-metal that is conductor of electricity. Consequently, it is also a good electrode in a battery. [ [footnote 1] According to the etymological meaning of the words, what we describe here should not be called a battery, but a cell (an electrochemical cell). A batter