The simple cell has two major faults.
1. Polarization. The simple cell consists of a positive copper electrode and a negative zinc electrode suspended in dilute sulphuric acid. At the copper plate hydrogen ions recombine to form hydrogen molecules:
Hydrogen bubbles form on the copper plate, increasing the internal resistance of the cell and producing a back EMF, reducing the effective voltage between the zinc and copper electrodes. This formation of hydrogen bubbles is called polarization.
2. Local Action. If the zinc is impure, bubbles of hydrogen may be produced at the negative (zinc) electrode. Impurities such as iron or carbon in the zinc set up tiny cells in the surface of the zinc electrode. Bubbles of hydrogen are given off from the site of the impurity and the surrounding zinc slowly dissolves. Local action can easily be prevented by rubbing a globule of mercury over the surface of the zinc with a piece of cotton wool. The mercury dissolves zinc out of the plate and forms a zinc amalgam coating over the zinc and the impurities. Local action is prevented by isolating the impurities from the acid.