The first person in Europe who observed the electrical effect produced by
dissimilar metals and who communicated his observations is believed to have been Luigi
Galvani. He reported on his observatons of "animal electricity." Though today we would say that he
misinterpreted his observations,
his experiments were widely discussed. Alessandro Volta extended Galvani's
experiments and exploited that which had been reported to produce the
first storage device for electrical energy: Volta's Pile (of dissimilar metals).
Today we would say that different metals have differing propensities to
oxidize from the metallic to the cationic or positively charged state. A reservoir of ions in aqueous
solution of one metal and in contact with the metal, when brought in contact
with a reservoir of ions of another metal, also in contact with that metal
will momentarily force an electrochemical potential in one direction or
the other. If the two metals should be connected by means of a wire, the
electrons produced by the oxidation of one metal will flow to the other metal
and be available for the reduction of the ions in solution of that metal.
This describes the principal of the electrochemical cell.
The symbol which describes a cell is shown at the left. The long and short lines represent the two poles or electrodes of the electrochemical cell. Those poles happen also to represent the two different metals. The positive pole is the one where reduction takes place, that is, the location or electrode which "soaks up" electrons or acts as an electron "sink". The electrons from the outside which are absorbed by this metal become available to the cations in solution producing a reduction to the metallic state. The cations are in contact with the electode surface and upon the absorption of enough electrons to become an uncharged metal atom, bonding occurs with the metallic surface and the atom is deposited becoming a part of the electrode. The negative pole or electrode is the one where electrons are being produced thanks to the oxidation of the metal, with the positive ions going into solution and leaving the electrons behind.
The electrical "pressure" or propensity for this two part process to take place is called "chemical potential" or perhaps "electrochemical potential." In honor of the work of Volta, we name that potential after him and call it "voltage" or "volts". Volts are sometimes referred to as electrical potential or electromotive force. A size D,C,B or A, AA, AAA or AAAA "battery" is actually a single cell and its electrical potential is rated at around 1.51 volts.
A curious feature of voltage, at least to this writer, is that it is additive, according to the number of cells which are connected to each other. Volta was the first to notice this with his pile. It is said that a large stack of cells produced enough electromotive force to give him a healthy shock when he grabbed the two wires from either end of the pile with both hands.
At the right is a symbol for a battery, made up of 8 cells. The voltage of that battery would be 8 x 1.51 or 12.08 volts.
The N21/23 battery is rated at 12 volts. Two of them are shown at the left, one which has been cut open to show the eight cells from which it is constructed.
The image at the right is a close-up of several cells. The three at the bottom have the positive side facing up and the four at the top have been turned over to show the negative side. The visible notches were inadvertently cut into the cells when the battery was cut open.