POLARISATION

If an ammeter is included in the external circuit of the simple cell, it indicates a gradual decrease in the current flowing. After some time the current may cease altogether. The decrease is due to the collection of hydrogen ubbles on the surface of Cu plate. The effect of this layer of hydrogen is two-fold.

It acts as an insulator, thus reducing the effective area of the Cu plate and thereby increasing the internal resistance of the cell.

The sticking layer of positive hydrogen ions on the Cu plate exerts a repulsive force on other hydrogen ions which are approaching the copper plate. Hence, the current is reduced. This phenomenon is called polarization and the cell which is in this condition is said to be polarized.

Moreover, the hydrogen and zinc set up between them an e.m.f. which is opposite to that set up between zinc and copper. This e.m.f. delivered by the cell diminishes to a very low value after a short time and hence the cell becomes useless for all practical purposes. The essential difference between various primary cells lies chiefly in the different methods employed to overcome this defect. The most widely used method is to surround the cathode by a solid or liquid depolarize which oxidizes the hydrogen as soon as it is liberated.

                                                              LOCAL ACTION

It is found that even when the voltaic cell is not supplying any load current, zinc goes on continuously dissolving in the electrolyte. This is due to the fact that some traces of impurities like iron and lead in the commercial zinc form tiny local cells which are short-circuited by the main body of zinc. The action of these parasitic cells cannot be controlled so that there is some wastage of zinc. This phenomenon is known as local action and can be prevented by amalgamating the zinc plate by rubbing mercury over the zinc plate. Mercury is supposed to cover the impurities and maintain a film of zinc dissolved in mercury.