ELECTRICAL APPLIANCES AND MACHINES 257 



Carbon 



MODIFIED LECLANCHE 

 CELL 



zinc sulphate remains on the top. The zinc plate dis- 

 solves in the zinc sulphate. The zinc of the zinc sulphate 

 replaces the copper of the copper sulphate where the 

 two solutions touch. The liberated copper atoms are 

 thus given electrical charges which 

 they carry to the copper plate at 

 the bottom of the cell and they 

 adhere to the plate when the charge 

 is given up. The copper sulphate 

 crystals in the bottom are dissolv- 

 ing to keep up the strength of the 

 solution. Polarization cannot occur 

 in this cell as no gas is liberated. 



This type of cell is used extensively in telegraphy, where 

 the circuit is kept closed nearly all the time. 



179. The Dry Cell. The dry cell is a form of the 

 Leclanche cell. It is made by taking a cylindrical zinc 

 cup, which serves as the negative pole, and putting into 

 the middle of the cup a carbon rod for the positive pole 

 and then filling the intervening space with a spongy or 

 paste-like substance containing zinc oxide, sal ammoniac, 

 zinc chloride, plaster of Paris, and water. 

 Sometimes powdered manganese dioxide is 

 mixed with the paste, and the top is sealed 

 to prevent evaporation. If the cell is given 

 continuous use, it polarizes in a short time 

 and then needs rest or time for the hydrogen 

 to be oxidized by the manganese dioxide, or 

 to escape through a small hole left in the top. 

 The dry cell is portable and is used, very widely where 

 a continuous current is not needed. It is convenient 

 for door bells and is used for producing the electric 

 spark which ignites the gas in gas engines. The com- 





DRY CELL 



