MAGNETISM AND ELECTRICITY 



175 



polarization of the cell, that is, the collecting of bubbles of hydrogen 

 on the plate. Polarization diminishes the voltage by increasing the 

 resistance. As the manganese dioxide is in a powdered form it 

 hardens slowly, and if too large a current is taken from the cell, 

 polarization takes place. The advantage of this type of cell lies 

 in its freedom from local action and in the fact that it can be used 

 for a long time without deterioration. 



The Daniell cell consists of a zinc sulphate solution and a copper 

 plate in a copper sulphate solution. They are separated by a porous 

 cup to prevent undue mixing. As no hydrogen is developed in 

 this cell, there can be no polarization. When the circuit is left 

 open the copper coats the zinc and impairs its efficiency. 



The gravity cell consists of a copper sheet placed in the bottom of a 

 glass. Crystals of copper sulphate are placed over the plate and 

 water is added until the jar is nearly full. A zinc plate is suspended 

 at the top of the jar and sulphuric acid added to start the cell. The 

 sulphuric acid acts on the zinc forming zinc sulphate. The zinc 

 sulphate is so much lighter than copper sulphate that, so long as the 

 cell is kept on a closed circuit, the solution mixes but slightly. 



The bichromate cell is used for operating small cells or motors, 

 and consists of a zinc and carbon plate in a solution of chromic acid 

 (mixture of bichromate and sulphuric 

 acid). When the cell is not used, the zinc 

 must be removed from the liquid, so that 

 the chromic acid will not attack it. 



209. Dry Cells. Dry cells (Fig. 

 75) are not actually dry. They con- 

 tain the same ingredients as the 

 Leclanche cell, but instead of con- 



FlG ' 75 -~ Dr y 



taining a fluid electrolyte they have the solution absorbed 

 in a plastic mass of manganese dioxide and plaster of Paris 

 or other inactive substances. 



The great advantage of the dry cell lies in the fact that 

 the liquid will not spill out under any conditions and there 

 are no vapors arising from the cell, as it is almost invariably 



