208 SIMPLE ELECTRIC DEVICES 



are immersed in the acid, minute bubbles of gas rise from 

 the zinc strip and it begins to waste away slowly. The 

 solution gradually dissolves the zinc and at the same time gives 

 up some of the hydrogen which it contains; but it has little 

 or no effect on the copper, since there is no visible change in the 

 copper strip. 



If, now, the strips are connected by means of metal wires, 

 the zinc wastes away rapidly, numerous bubbles of hydrogen 

 pass over to the copper strip and collect on it, and a current 

 of electricity flows through the connecting wires. Evidently, 

 the source of the current is the chemical action between the 

 zinc and the liquid. 



Mere inspection of the connecting wire will not enable us to 

 determine that a current is flowing, but there are various ways in 

 which the current makes itself evident. If the ends of the 

 wires attached to the strips are brought in contact with each 

 other and then separated, a faint spark passes, and if the ends 

 are placed on the tongue, a twinge is felt. 



Experiments which grew out of the voltaic cell. Since 

 chemical action on the zinc is the source of the current, it would 

 seem reasonable to expect a current if the cell consisted of two 

 zinc plates instead of one zinc plate and one copper plate. But 

 when the copper strip is replaced by a zinc strip so that the cell 

 consists of two similar plates, no current flows between them. 

 In this case, chemical action is expended in heat rather than in 

 the production of electricity and the liquid becomes hot. But 

 if carbon and zinc are used, a current is again produced, the 

 zinc dissolving away as before and bubbles collecting on the 

 carbon plate. By experiment it has been found that many 

 different metals may be employed in the construction of an elec- 

 tric cell ; for example, current may be obtained from a cell made 

 with a zinc plate and a platinum plate, or from a cell made with 

 a lead plate and a copper plate. Then, too, some other chemi- 



