The Galvanic cell, also known as the voltaic cell, is part of a battery consisting of two metals joined by a salt bridge or a porous disc. It is a specialized type of electrochemical cell that uses chemical reactions to generate electromotive force and electrical current. Most batteries include several such cells.
In the late 18th century, Italian scientist Luigi Galvani discovered that he could make the leg muscles of a dead frog contract by connecting two different metals to each other and touching the leg with them at the same time. Galvani called his discovery "animal electricity," while his contemporaries called it galvanism. Today, the scientific field stemming from Galvani's research is known as bioelectromagnetism.
A Galvanic cell is composed of two half-cells, each with a metal electrode and a solution of a salt of the same metal. Zinc and copper are commonly used as the two metals. The solution contains a cation, or positively charged ion, of the metal, and an anion, a negatively charged ion, to counterbalance the charge of the cation. A redox, or oxidation-reduction, reaction takes place in each half-cell, producing electrical energy.
In one half-cell, the metal oxidizes, producing cations as a result; in the other cell, the cations in the solution gain electrons, losing their negative charge, and becoming stable metal molecules that collect on the electrode. If the circuit is connected, with wires leading from each electrode and making contact with each other directly or through some conductive material, there is a flow of electrons from the oxidizing half-cell into the reducing half-cell. The half-cell that releases cations is known as the anode, and the one that attracts cations is called the cathode.
The two half-cells in a Galvanic cell must be kept separate in order to preserve the composition of the salt solutions, so they are connected by a salt bridge or a porous plate. The salt bridge serves not only to separate the solutions, but also to allow the flow of anions from the reducing cell into the oxidizing cell to balance the flow of electrons. The voltage of the cell is the sum of the voltages of each half-cell. There must be an equal transfer of electrons from one half-cell to the other for the cell to function. It produces direct current (DC), or the flow of electricity in one direction only, as opposed to alternating current (AC), in which the flow moves in both directions.