Faults of a simple cell
When a simple cell is in use it is found that the current quickly falls to a very small value. This defect results from the formation of a layer of hydrogen bubbles on the copper plate and is called polarization. The hydrogen layer weakens the current for two reasons. First, the hydrogen layer sets up a “back e.m.f.” in the cell in opposition to that due to the copper and zinc; secondly. the gas partially insulates the plate, ~nd hence increases the internal resistance of the cell (page 411)
(2) Local action
If the zinc used in the simple cell is of the impure commercial variety bubbles of hydrogen will be seen coming off the zinc. This is called local action, and must not be confused with polarization, which is the name given to the formation of hydrogen bubbles on the copper plate.
Local action is caused by the presence in the zinc of small impurities such as iron or carbon which set up tiny local cells at the zinc surface. Bubbles of hydrogen are given off from the impurity and the surrounding zinc slowly dissolves in the acid. This serves no useful purpose and merely wastes the zinc. Fortunately, the trouble can easily be prevented by cleaning the zinc in sulphuric
acid and then rubbing a small globule of mercury over the surface with a small piece of cotton wool. The mercury dissolves pure zinc out of the plate and forms a bright coating of zinc amalgam all over the surface. Local action will not now occur, since the amalgam covers up the impurities and prevents them from coming into contact with the acid.