210 SIMPLE ELECTRIC DEVICES 



which it can be taken from place to place. This cell (Fig. 74) 

 consists of a zinc cup, within which is a carbon rod; the space 

 between the cup and rod is packed with a moist paste containing 

 certain chemicals. The moist paste takes the place of the liquids 

 used in other cells. 



A battery of cells. The electromotive force of one cell 

 may not give a current strong enough to ring a doorbell or to 

 operate a telephone. But by using a number of cells, called a 

 battery, the current may be increased to almost any desired 

 strength. If three cells are arranged as in Figure 75, so that 



the copper of one cell is con- 

 nected with the zinc of an- 

 other cell, the voltage of the 

 battery will be three times as 

 great as the voltage of a single 

 cell. If four cells are arranged 

 in the same way, the voltage 



FIG. 75- ~ A battery of three cells. of ^ ^^ {& four times as 



great as the voltage of a single cell ; when five cells are combined, 

 the voltage is five times as great. 



Heat. Any one who handles electric wires knows that 

 they are more or less heated by the currents which flow through 

 them. If three cells are arranged as in Figure 75 and the 

 connecting wire is coarse, the heating of the wire is scarcely 

 noticeable ; but if a shorter wire of the same kind is used, the 

 heat produced is slightly greater ; and if the coarse wire is re- 

 placed by a short, fine wire, the heating of the wire becomes 

 very marked. We are accustomed to say that a wire offers 

 resistance to the flow of a current ; that is, whenever a current 

 meets resistance, heat is produced in much the same way as 

 when mechanical motion meets an obstacle and spends its energy 

 in friction. The flow of electricity along a wire can be compared 

 to the flow of water through pipes : a small pipe offers a greater 



