STORAGE BATTERIES. 251 



storage battery is high, it is important that it should have proper 

 care. 



The electrolyte should be made of very pure sulphuric acid. 

 The least trace of platinum (from the platinum still used in the 

 manufacture of the acid) is very harmful. In general a metallic 

 salt of any kind is objectionable, inasmuch as the foreign metal 

 is deposited on the grids and gives rise to local voltaic action. 

 Also the presence of lead dissolving acids such as nitric or acetic 

 is objectionable, as is also the presence of chlorides. The effect 

 of these impurities is to accelerate the corrosion of the solid lead 

 of the grid converting it into active material. 



High concentration of the electrolyte reduces the internal re- 

 sistance of the cell, and it facilitates to some extent the diffusion 

 of acid into and out of the active material. On the other hand, 

 high concentration of the electrolyte tends to cause trouble from 

 sulphatation as explained later. Therefore a cell which is likely 

 to stand for a long time unused should have a low density elec- 

 trolyte, and a cell that is cared for properly may have a higher 

 density electrolyte. The usual densities range from 1.21 to 1.24 

 charged, and from 1.185 to 1.195 discharged. 



The over-concentrated acid at the surface of the grids during 

 charge tends to flow to the bottom of the cell and the under- 



o 



concentrated acid at the surface of the grids during discharge 

 tends to flow to the top of the cell. In large and especially in 

 deep cells the electrolyte should be continuously circulated by 

 pumping a small stream of air to the bottom of the cell. 



When it is desired to test the electrolyte it should be thoroughly 

 stirred by blowing air through it. 



Water must be occasionally added to make up for evaporation. 

 This should be poured to the bottom of the cell through a long 

 rubber tube attached to a funnel. The electrolyte should always 

 cover the grids. 



The grids should be frequently inspected with a view to the 

 discovery of any sulphatation, or buckling, and a thin wooden 

 stick should be occasionally passed between the grids to make 



