174 



APPLIED SCIENCE 



O 



The watt is a very small unit, so that the kilowatt (1000 

 watts) is generally used. Electricity is measured by the 

 number of kilowatts used per hour. To illustrate: If an 

 electric generator gives 14 kw. for 9 hrs., it produces 126 

 kilowatt-hours of work. 



207. Simple Voltaic Cell. The voltaic cell (Fig. 74) 

 consists of a strip of zinc and a strip of copper in a glass jar 



(T\ nearly full of sulphuric acid, supported 



[Pi side by side without touching each 



| _ _ other. These two metal strips are 



^S :ft =fcs * connected by a copper wire. Electric 

 current will flow from the copper to 

 the zinc. The copper is called the 

 positive pole and the zinc the nega- 

 tive pole of the cell. The current 

 may be detected by placing the free 

 ends of the copper wire on the tip of 



FIG. 74. Simple Voltaic the tongue. A slight stinging sensa- 

 C^pll 



tion will be felt, thus proving the 



presence of an electric current. 



208. Battery Cells. When electricity is desired for 

 bells, burglar alarms, etc., it is obtained from battery cells. 

 The electricity is generated by chemical means. There are 

 many forms, each of which has its advantages and disad- 

 vantages. The four types most commonly used are described 

 below. 



The Ledanche cell consists of a glass jar containing a solution of 

 sal ammoniac (ammonium chloride) with a zinc rod for one pole 

 and a carbon plate in a block of compressed manganese dioxide 

 for the other. The purpose of the manganese dioxide is to prevent 



