Ques. What happens when the charging current is discontinued, and the two electrodes joined by an outside wire?
Ans. A small current will flow through the outside circuit, being due to the recomposition of the acid and water solution. The process is in a very definite sense a reversal of that by which the current is generated in a primary cell.
Hydrogen collected upon the negative plate, which was the cathode, so long as the primary battery was in circuit, is given off to the liquid immediately surrounding it, uniting with its particles of oxygen and causing the hydrogen, in combination with them, to unite with the particles of oxygen next adjacent. The process is continued until the opposite positive plate is reached, when the oxygen collected there is finally combined with the surplus hydrogen, going to it from the surrounding solution.
This chemical process causes the current to emerge from the positive plate, which was the anode, so long as the primary battery was in circuit. The current thus produced will continue until the recomposition of the gases is complete; then ceasing because these gases, as before stated, do not combine with the metal of the electrodes.
Types of Storage Battery.—There are three classes of storage cell which are commercially important:
1. Plante cells;
2. Faure cells;
3. Alkaline cells.
According to construction secondary cells may be classified as follows:
1. Lead sulphuric acid cells;
2. Lead copper cells;
3. Lead zinc cells;
4. Alkaline zincate cells.
The lead sulphuric acid type includes all those cells belonging to the Plante and Faure groups.
Lead copper cells consist of sheets of metal coated with lead oxide, serving as the positive electrode, and copper plates for the negative electrodes. These plates are immersed in a solution of copper sulphate. Cells belonging to this class are not employed in commercial practice, being useful only for laboratory experiments.
Lead zinc cells are similar to the preceding type, but differ by having zinc for the negative electrode, and zinc sulphate for the electrolyte. The voltage of these cells is slightly higher than that of the ordinary cell, and their capacity per unit of total weight is high, but they are apt to lose their charge on open circuit, besides they possess most of the disadvantages of the Plante cells.
Alkaline zincate cells have copper for the positive, and iron for the negative electrode. The electrolyte is composed of sodium, or potassium, zincate. Cells of this type are used to some extent for traction purposes.
In addition to the above there are some special forms of cell which do not belong to the four preceding types.
Ques. Describe the Plante type.
Ans. In the Plante type the lead is chemically attacked and finally converted into lead peroxide, probably after it has gone through several intermediate changes. The plates are all formed as positive plates first and then all that are intended for negative plates are reversed, the peroxide being changed into sponge lead.