A brief introduction of dry cell is mentioned in the following box

● Construction:

1. It consists of a mercury coated cylindrical zinc vessel. A card board washer is placed at the bottom of the vessel.

2. The inside walls of the zinc vessel are lined with blotting paper.

3. There is a carbon rod with a brass cap which stands at the centre of the vessel. It acts as the positive pole.

4. The space between the lining and the carbon rod is packed with a paste of a mixture of a little water with ammonium chloride, manganese dioxide and graphite powder. It fills ^th height of the cell.

5. The top of the cell is covered with saw dust, sand and pitch to prevent the paste to become dry by evaporation. The top is provided with a pin-

hole to allow the gases to escape. Paper or plastic is wrapped round the cell.

● Working principle

1. Inside the cell, due to the chemical reaction between the zinc vessel and ammonium chloride, zinc vessel acquires negative potential with respect to the carbon rod.

2. In this cell carbon rod acts as a positive pole and zinc vessel as negative pole.

● Action

1. In the reaction between zinc and ammonium chloride, zinc atoms dissolve into the paste as Zn⁺⁺ ions by releasing their electrons. Due to accumulation of these excess electrons over the zinc vessel, it acquires negative potential with respect to the carbon rod.

2. If the two poles are joined by a conducting wire, the excess electrons from the zinc vessel move towards the carbon rod which produces a conventional current from the carbon rod to the zinc vessel. Electrons received by the carbon rod are captured by the ammonium ions (NH₄⁺) to produce ammonia and hydrogen.

3. Ammonia gas thus evolved at the carbon rod escapes through the upper pin-hole and MnO₂ oxidises hydrogen to water and hence polarization is prevented. This cell is not free from local action and the zinc vessel is gradually consumed. When the zinc is completely consumed the cell is rejected as useless.

In this way we get electrical energy at the cost of chemical energy from a dry cell.

Uses

Dry cells are widely use in toys, flashlights, portable radios, cameras, hearing aids, clocks, and many other devices in common use.