Electricity 209 



otherwise there will be no circuit. A small electric charge occurs 

 as a chemical reaction takes place between the zinc and the acid, 

 which releases energy that drives the current through the whole 

 circuit. 



Kinds of Electric Cells. This change from chemical to 

 electrical energy may be brought about by placing two kinds of 

 material, usually strips of metal, or one metal strip and one car- 

 bon strip, in a solution of some such substance as sal ammoniac 

 or sulphuric acid. If the two strips of material dissolve in the 

 solution at different rates, we may connect the tops of these 

 materials with a wire and thus get a current. We call such an 

 outfit a cell. A battery is composed of several cells connected 

 together in a group to give greater strength. 



The Simple Cell.-. Pour some water in a glass jar and add a 

 small amount of sulphuric acid. Then place a narrow strip of 

 zinc and a strip of copper in the jar, making sure that they do not 

 touch each other. Notice that many bubbles rise from the zinc 

 and almost none from the copper. This shows that the chemical 

 action is not the same on the two. Connect the tops of these 

 metals with a wire. Notice that the bubbles of gas appear on the 

 copper, instead of on the zinc. This is not because the greater 

 chemical action is on the copper, but because a complete path has 

 been formed by which the electric current can travel back to its 

 starting point. The bubbles of gas are really being formed on the 

 zinc as before, but they pass through the sulphuric acid and water 

 to the copper plate. When doing this they are so small that they 

 cannot be seen. We can see them on the copper plate because 

 they collect into large bubbles. Such a cell is known as a simple 

 cell and is the form that Volta, the discoverer of this method of 

 forming electricity, used in his study. 



There are many different types of these cells for various uses. 

 About the only ones we see in common use, however, are the 

 dry cell and the storage cell. 



The Dry Cell. The dry cell, very commonly used in doorbell 

 circuits, consists of a zinc cylinder, through the center of which 

 runs a carbon rod. Around the rod is packed a mixture of certain 

 chemicals. The cell is not really dry, for the chemicals are 



