210 



Our Surroundings 



Negative Bind ing Post 



five 

 Binding Post 



Sealing 

 Wax 6 



Pitch 



Chemicals 



A CROSS SECTION OF A 

 DRY CELL 



moistened and sealed airtight. The chemicals act on the zinc 

 much as the acid did in the simple cell. If the cell is connected 



in a circuit, a powerful current will 

 flow for a short time from the bind- 

 ing post at the top of the carbon rod, 

 through the circuit, to the binding post 

 at the top of the zinc cylinder. Such 

 cells are of little value for furnishing 

 a steady current for a long time. 



The Storage Cell. The storage cell 

 is a device for storing up electrical 

 energy in the form of chemical energy. 

 By passing a small electric current 

 through the cell .for a long period, 

 enough energy is stored in the cell to 

 give off a powerful current for a short 

 period. Storage cells usually are built 

 or connected in a group called a battery. 

 Some electric power plants have storage batteries in which 

 energy is stored from their dynamos during periods of low demand 

 for current. These batteries are then drawn on to add to the supply 

 of current at periods of high demand. Storage batteries are used 

 in automobiles for starting and lighting, in radio, and in country 

 homes for private lighting plants. 



A storage cell can be prepared by placing two strips of lead 

 in sulphuric acid that has been diluted by adding to it about 

 five times its volume of water. These strips of lead, when con- 

 nected up as in the simple cell, will show no electric current 

 because the two plates are of the same kind of material and, 

 the action of the sulphuric acid on the two being the same, no 

 potential difference is set up by the chemical action. If we pass 

 an electric current through this cell by connecting it to a dynamo 

 or to a group of cells, we notice that the colors of the two strips 

 of lead change. In a short time one of them turns to a dark 

 red and the other becomes a darker gray. One of these plates has 

 been changed over to a new substance called lead peroxide, 

 while the other has become porous. This process is called charging. 



