By the oxidation of metallic zinc by weak sulphuric acid.
TYPES OF ELECTRIC CELLS OR “BATTERIES”
(1) Grove’s Cell.—Z. Zinc plate in dilute sulphuric acid; P. platinum plate in strong nitric acid. (2) Daniell’s Cell.—Z. Zinc rod in porous pot P containing dilute sulphuric acid; C. copper plate in outer vessel containing copper sulphate solution. (3) Leclanche Cell.—Zinc in sal-ammoniac solution; carbon slab in charcoal and manganese dioxide.
The electricity developed by the action of a single pair of plates immersed in acid water is very feeble: how can it be increased?
By increasing the number of the plates and the quantity of the liquid, we increase the intensity of the electricity developed.
Action Within a Voltaic Cell.—Let us try to see now how an electric current is set up in a simple voltaic cell, consisting of a zinc plate and a copper plate immersed in dilute acid. First we must understand the meaning of the word ion.
If we place a small quantity of salt in a vessel containing water, the salt dissolves, and the water becomes salt, not only at the bottom where the salt was placed, but throughout the whole vessel. This means that the particles of salt must be able to move through the water. Salt is a chemical compound of sodium and chlorine, and its molecules consist of atoms of both these substances. It is supposed that each salt molecule breaks up into two parts, one part being a sodium atom, and the other a chlorine atom, and further, that the sodium atom loses an electron, while the chlorine atom gains one. These atoms have the power of traveling about through the solution, and they are called ions, which means “wanderers.”
An ordinary atom is unable to wander about in this way, but it gains traveling power as soon as it is converted into an ion, by losing electrons if it be an atom of a metal, and by gaining electrons if it be an atom of a non-metal.
Returning to the voltaic cell, we may imagine that the atoms of the zinc which are immersed in the acid are trying to turn themselves into ions, so that they can travel through the solution. In order to do this each atom parts with two electrons, and these electrons try to attach themselves to the next atom. This atom, however, already has two electrons, and so in order to accept the newcomers it must pass on its own two. In this way electrons are passed on from atom to atom of the zinc, then along the connecting wire, and so to the copper plate. The atoms of zinc which have lost their electrons thus become ions, with power of movement. They leave the zinc plate immediately, and so the plate wastes away or dissolves. So we get a constant stream of electrons traveling along the wire connecting the two plates, and this constitutes an electric current.