20 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



CHARGING. 

 Positive grid.* PbS0 4 + 2H 2 O + S0 4 = 2H,SO 4 + PbO, 



V Direction of current through the cell 

 * (positive to negative grid). 



Negative grid.* PbSO 4 + H, = H 2 SO 4 + Pb 



10. Open-circuit cells and closed-circuit cells. A voltaic cell 

 in which the local action is very slight does not deteriorate 

 appreciably when it is not called upon to deliver current. Such 

 a cell may be left standing on open circuit in readiness for use at 

 any moment to supply current for any purpose such as to ring an 

 electric bell. All that is necessary is to provide a device for 

 closing the circuit when it is desired to obtain current from the 

 cell, and then the circuit should be opened in order to avoid 

 deterioration of the cell by the continued flow of current. A 

 voltaic cell which is adapted to this kind of use is called an open- 

 circuit cell and perhaps the best form of open-circuit cell is the 

 ordinary dry cell. 



When an ordinary dry cell is called upon to give a steady cur- 

 rent the electromotive force f falls off rapidly on account of what 

 is called polarization, and the current decreases accordingly. A 

 voltaic cell which is capable of delivering a fairly large steady 

 current is called a closed-circuit cell. The gravity cell is one of 

 the best types of closed-circuit cell. The chromic acid cell is also 

 frequently used for delivering current more or less steadily. The 

 Edison -LaLande cell is a fairly good open-circuit cell and it is 

 satisfactory also for closed-circuit work. 



PROBLEMS. 



1. The anode of an electrolytic cell consists of a copper rod 

 3 centimeters in diameter, and the cathode consists of a hollow 



* It is the usual practice among electrical engineers to call that terminal of an 

 electric generator out of which current flows, the positive terminal, and that terminal 

 into which current flows, the negative terminal. In conformity with this usage, that 

 electrode of a storage cell which is cathode during discharge is called the positi 've grid 

 and the other the negative grid. The positive grids are of a pale salmon color and 

 the negative grids are a neutral gray. 



| See Art. 22. 



