The copper oxide acts both as the positive element and as a depolarizer, for the oxygen of the oxide immediately combines with any hydrogen tending to form on the plate.
This type of cell has some advantages but also many disadvantages, chief among which is the fact that the E. M. F. is very low. It is used principally for railway signal work, slot-machines, etc.
A Tomato-Can Battery using caustic soda as the exciting liquid is a simple form of home-made battery whose only disadvantage is the low voltage that it delivers.
Fig. 67.—A Tomato-Can Cell; Sectional View.
Fig. 68.—The Tomato-Can Cell Complete.
The cell is liable to polarization, but the large surface of its positive elements protects it to some extent.
The positive element and the outer vessel is a tomato can. Within it is a porous cup made out of blotting paper or unglazed earthenware such as a flower pot.
The space between the can and the porous cup is filled with fine scrap-iron such as borings and turnings. A zinc plate is placed in the porous cup.