If the current is drawn for any appreciable length of time hydrogen gas will collect on the carbon cylinder and the cell will become polarized. When polarized it will not deliver much current.
Many methods have been devised for overcoming this difficulty, but even the best of them are only partially successful.
The usual method is to employ a chemical depolarizing agent. Figure 56 shows a Leclanche cell provided with a depolarizer.
The carbon is in the form of a plate placed in a porous cup made of earthenware and filled with manganese dioxide.
Chemists class manganese dioxide as an oxidizing agent, which means that it will furnish oxygen with comparative ease. Oxygen and hydrogen have a strong chemical affinity or attraction for each other.
Fig. 56.—A Leclanche Cell, showing the Porous Cup.
If the carbon plate is packed in manganese dioxide any hydrogen which tends to collect on the carbon and polarize the cell is immediately seized by the oxygen of the manganese dioxide and united with it to form water.
This form of Leclanche cell is called the disk type. It is capable of delivering a stronger current for a longer period of time than the carbon cylinder battery. The zinc is usually made in the form of a cylinder, and fits around the outside of the porous cup.
Dry Cells are used extensively nowadays for all open circuit work on account of their convenience and high efficiency.