ELECTRICITY 



pletely covered with a saturated solution of copper sulphate. 

 Then a large, irregular zinc electrode is suspended from the 

 top of the jar, and is completely covered with dilute sulphuric 

 acid which does not mix with the copper sulphate, but floats 

 on the top of it like oil on water. The hydrogen formed by 

 the chemical action of the dilute sulphuric acid on the zinc 

 moves toward the copper electrode, as in the simple voltaic 

 cell. It does not reach the electrode, however, because, when 

 it comes in contact with the copper sulphate, it changes places 

 with the copper there, setting it free, but itself entering into 

 the solution. The copper freed from the copper sulphate 

 solution travels to the copper electrode, and is deposited 

 on it in a clean, smooth layer. Instead of a deposit of 

 hydrogen there is a deposit of copper, and falling off in 

 current is prevented. 



The gravity cell is cheap, easy to construct, and of constant 

 strength, and is in almost universal use in telegraphic work. 



Practically all small railroad sta- 

 tions and local telegraph offices 

 use these cells. 



286. Dry Cells. The gravity 

 cell, while cheap and effective, is 

 inconvenient for general use, owing 

 to the fact that it cannot be easily 

 transported, and the dry cell has 

 largely supplanted all others, be- 

 cause of the ease with which it 

 can be taken from place to place. 

 This cell 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. 



FIG. 199. A dry cell. 



