MAGNETISM AND ELECTRICITY 201 



sulphate in addition to the current produced. This form 

 of cell is of little practical value, however, because the 

 little bubbles of hydrogen collect in such numbers on 

 the carbon or copper that they weaken and finally stop 

 the passage of the current. A cell in such a condition is 

 said to be polarized* If bichromate of potassium is dis- 

 solved in the sulphuric acid, it will unite with the hydrogen 

 as fast. as it forms at the zinc and thus prevent it from 

 reaching the carbon or copper and stopping the current. 



Another form of celt in which polarization is com- 

 pletely avoided is the gravity or crowfoot type of the 

 Daniell cell. This cell is commonly 

 used on telegraph lines. A group of 

 copper plates is placed in the bottom 

 of the jar and surrounded with crys- 

 tals of copper sulphate (Figure 171). 

 A zinc plate is hung near the top, 

 and the jar is then filled with a very 

 dilute solution of sulphuric acid. 

 Hydrogen is formed at the zinc plate, 

 but on its way to the copper it is FlG - m - 



used up, and copper from the copper sulphate is deposited 

 on the copper plate in its stead. The copper sulphate 

 solution, being heavier than the acid solution, remains 

 near the bottom of the jar. 



One of the most common and convenient types of cells 

 is the dry cell. In this cell the zinc serves a double pur- 

 pose. It forms one plate of the cell and also is the con- 

 tainer for the chemicals which fill the space between the 

 carbon center and the zinc. These chemicals are sal 

 ammoniac, manganese dioxide, and charcoal, mixed with 

 water enough to make a paste. The cell is sealed to pre- 

 vent the evaporation of the water. 



