THE GRAVITY CELL 



309 



thus produced slowly diminishes in strength and, after a 

 short time, becomes feeble. Examination of the cell shows 

 that the copper plate is covered with hydrogen bubbles. If 

 these bubbles are brushed away by means of a rod or stick, 

 the current rises to its former strength, but as the bubbles 

 again gather on the + electrode the current strength di- 

 minishes, and when the bubbles form a thick film on the 

 copper plate, the current is too weak to be of any practical 

 value. The film of bubbles weakens the current because it 

 practically substitutes a hydrogen plate for a copper plate, 

 and we saw in Section 282 that a change in any one of 

 the materials of which a cell is composed changes the 

 current. 



This weakening of the current can be prevented mechani- 

 cally by brushing away the bubbles as soon as they are formed ; 

 or chemically, by surrounding the copper plate with a sub- 

 stance which will combine with the free hydrogen and pre- 

 vent it from passing onward to 

 the copper plate. 



In practically all cells, the 

 chemical method is used in pref- 

 erence to the mechanical one. 

 The numerous types of cells in 

 daily use differ chiefly in the de- 

 vices employed for preventing 

 the formation of hydrogen bub- 

 bles, or for disposing of them when 

 formed. One of the best-known 

 cells in which weakening of the 

 current is prevented by chemical 

 means is the so-called gravity cell. 



285. The Gravity Cell. A large, irregular copper elec- 

 trode is placed in the bottom of a jar (Fig. 198), and com- 



FlG. 198. The gravity cell. 



