We make use of dry cells in our torches, transistors and cameras. As soon as we press the switch of the torch the filament of the bulb gets heated up and it begins to glow. Similarly, when we switch on our transistors sound is produced. These devices get their energy through the electric current produced by the dry cells fitted in them. There are three main types of dry cells: carbon-zinc, alkaline and mercury.
A carbon-zinc dry cell consists of a cylindrical vessel made of zinc. A carbon rod with a brass cap is placed at the centre of this vessel. Zinc acts as the negative electrode and carbon as the positive electrode. A paste of ammonium chloride, zinc chloride, manganese dioxide and carbon is filled in the vessel around the carbon rod. The paste is further surrounded by another paste of plaster of Paris, ammonium chloride and zinc chloride. Plaster of Paris makes the paste hard. The zinc vessel is closed with a layer of pitch. Finally, the vessel is wrapped with a thin cardboard and we have a carbon-zinc dry cell. This cell is a lechlanche type of primary cell in which the electrolyte ammonium chloride is dissociated into positive and negative ions. These are attracted towards the electrodes and produce voltage. When they are connected with a wire, current begins to flow. In this reaction hydrogen gas is also produced and it is converted into water by manganese dioxide. These cells produce electricity till the whole of the manganese dioxide is used up to convert hydrogen into water. Manganese dioxide, after the reaction with hydrogen, gets converted into manganese oxide. After this conversion, no electricity is produced by the cell.
An alkaline dry cell battery is more powerful and lasts eight times more than a carbon-zinc cell. It also has a carbon electrode and a zinc casing electrode. The electrolyte is a strong alkali solution which has potassium hydroxide. Alkaline dry cells are used mainly for portable radios.
In a mercury dry cell, the voltage remains constant till the end of the life of the battery. A mercuric oxide electrode is used. The other electrode is the zinc casing. The electrolyte is potassium hydroxide.
Why these cells are called dry cells? It is simply because its electrolyte is not in a solution form but in a paste form. It would become useless the moment the electrolyte dries up. Dry cells are adversely affected by high temperatures. Hence they should be stored in a cool place, away from direct sunlight. It is better to take out the cells from the torchlight, camera or transistor if they are not being used because moisture leaks out from the cell and it swells up.