and forms a compound known as sulphate of zinc. When the zinc is attacked by the acid, electrons are liberated which flow out of the zinc electron and into the copper where it combines with hydrogen atoms to form a gas molecule of hydrogen. During the formation of this sulphate some of the hydrogen contained in the acid solution is liberated in the form of bubbles. Some of these bubbles rise to the surface of the liquid and escape into the surrounding air, but others cling to the copper plate which gradually becomes covered with a film of hydrogen. Since hydrogen is a non-conductor of electricity, the amount of surface of the copper strip in contact with the solution gradually decreases as the accumulation of hydrogen gas increases, therefore the current output of the cell diminishes. Using an electrician’s term the cell is now said to be “run down” or dead.
Fig. 7
The so-called dry cell which is in use at the present time, works on the same principle. It is not really a “dry” cell, for the exciting solution is in paste form and if this paste were allowed to dry out the cell would be useless. The construction of an ordinary dry cell is as follows: A zinc cup both contains the material of which the cell is made and provides the negative electrode. A carbon electrode (positive) is placed in the center as a core, and the space about the carbon is filled with manganese dioxide and powdered carbon, this being separated form the zinc cup by a layer of sal-ammoniac paste held by porous blotting
