In the action of the cell, when the two plates are joined by a wire, it may be noticed that the zinc plate is consumed and that bubbles of hydrogen gas are formed on the surface of the copper plate.

Theory.  Just why or how chemical action in a voltaic cell results in the production of a negative charge on the consumed plate is not known.  Modern theory has it that when an acid is diluted in water the molecules of the acid are split up or dissociated into two oppositely charged atoms, or groups of atoms, one bearing a positive charge and the other a negative charge of electricity.  Such charged atoms or groups of atoms are called ions.  This separation of the molecules of a chemical compound into positively and negatively charged ions is called dissociation.

Thus, in the simple cell under consideration the sulphuric acid, by dissociation, splits up into hydrogen ions bearing positive charges, and SO_{4} ions bearing negative charges.  The solution as a whole is neutral in potential, having an equal number of equal and opposite charges.

[Illustration:  Fig. 60.  Simple Voltaic Cell]

It is known that when a metal is being dissolved by an acid, each atom of the metal which is torn off by the solution leaves the metal as a positively charged ion.  The carrying away of positive charges from a hitherto neutral body leaves that body with a negative charge.  Hence the zinc, or consumed plate, becomes negatively charged.

In the chemical attack of the sulphuric acid on the zinc, the positive hydrogen ions are liberated, due to the affinity of the negative SO_{4} ions for the positive zinc ions, this resulting in the formation of zinc sulphate in the solution.  Now the solution itself becomes positively charged, due to the positive charges leaving the zinc plate with the zinc ions, and the free positively charged hydrogen ions liberated in the solution as just described are repelled to the copper plate, carrying their positive charges thereto.  Hence the copper plate, or the unconsumed plate, becomes positively charged and also coated with hydrogen bubbles.

The plates or electrodes of a voltaic cell need not consist of zinc and copper, nor need the fluid, called the electrolyte, be of sulphuric acid; any two dissimilar elements immersed in an electrolyte that attacks one of them more readily than the other will form a voltaic cell.  In every such cell it will be found that one of the plates is consumed, and that on the other plate some element is deposited, this element being sometimes a gas and sometimes a solid.  The plate which is consumed is always the negative plate, and the one on which the element is deposited is always the positive, the current through the connecting wire always being, therefore, from the unconsumed to the consumed plate.  Thus, in the simple copper-zinc cell just considered, the zinc is consumed, the element hydrogen is deposited on the copper, and the current flow through the external circuit is from the copper to the zinc.