Transportation on Land 333 



in series. This gives a total electric pressure equal to the 

 sum of the electric pressures of the separate cells. Such an 

 arrangement is shown in Fig. 1 1 1 (A ) . In (B) the same grouping 

 of cells is shown by symbols that are commonly used, the zinc 

 being indicated by a heavy line and the carbon by a light line. 



FIG. 111. A, a battery of cells arranged in series. B, the symbols 

 commonly used to show such connections. 



The secondary cell, or storage cell as it is generally called, is 

 utilized in all sorts of motor vehicles and in many other ways. 

 For some vehicles it provides the motive power, for others it is 

 used for ignition, lighting, starting, and so on. Through recent 

 improvements its use has been greatly extended in many new 

 fields. Trucks and delivery wagons, electric motor cars, street 

 cars, light and power plants for the purpose of maintaining un- 

 interrupted service, telephone and telegraph, private lighting 

 plants for homes, and a thousand other appliances make use of 

 secondary cells. The essential facts in its construction and 

 operation should be studied experimentally. 



Exercise. For class study and demonstration a secondary cell 

 of the lead type is readily made. Fasten two pieces of lead plate to 

 a piece of dry wood about 2" x f" X 8", and connect the leads with 

 copper wires. Place the lead plates in a glass jar of about two quart 

 capacity and fill jar nearly full of a 15 per cent solution of sulfuric 

 acid. Connect wires with three or more dry batteries or a direct 

 current generator of six to ten volts capacity, as shown in diagram 

 Fig. 112. Connect up a small electric bell as in the diagram with a 

 switch or merely broken wires that may be closed when needed. 



When the apparatus is set up, close switch No. 1, to charge the 

 cell. Observe the changes that occur. The electric current passing 

 along the wire and plate marked (+) combines with the sulfuric acid 

 solution and the lead to produce a visible change on the surface of the 



