ELEMENTS OF ELECTRICAL ENGINEERING. 



CHAPTER VIII. STORAGE BATTERIES. 



139. A primary battery which is free from local action gives a 

 current of 1.5 amperes for 50 hours. Calculate the number of 

 grams of zinc consumed. Ans. 91 grams. 



Note. The electrochemical equivalent of zinc is 0.000337 gram per ampere per 

 second. 



140. A Grenet cell consumes 12.5 grams of zinc during the 

 time that the current from the cell is depositing 8 grams of silver 

 in an electrolytic cell containing a solution of silver nitrate. What 

 portion of the zinc is consumed by local action? Ans. 80.7 per 

 cent. 



Note. The electrochemical equivalent of silver is 0.001118 gram per ampere per 

 second. See Note to problem 139. 



141. The charge and discharge curves, shown in Fig. 143, 

 Chapter VIII., apply to the case in which an 80 ampere-hour cell 

 is charged for 8 hours and 5 minutes by a steady current of 10 

 amperes, and discharged for 8 hours by a steady current of 10 

 amperes. Calculate : (a) the energy in watt-hours delivered to 

 the cell in charging, (U) the energy in watt-hours delivered by the 

 cell in discharging ; and (c) find the efficiency of the cell. Ans. 

 (a) 185.7 watt-hours; (^) i 59-4 watt -hours ; (c) 85.3 per cent. 



142. The total weight of all the grids of a 120 ampere-hour 

 storage cell (discharged) is 40 pounds, and one-third of this 

 weight is active material, (a) Calculate from electrochemical data 

 the weight of PbSO 4 that is converted into PbO 2 on the positive 

 grids, and the weight of PbSO 4 that is converted into Pb on the 

 negative grids when the cell is charged, (fr) What part of the so- 

 called active material is really active? Ans. (a) 1.49 pounds of 

 PbSO 4 on the positive grids ; 1.49 pounds of PbSO 4 on the nega- 

 tive grids ; ($) 22.4 per cent. 



Note. One ampere in one hour liberates 37.4 milligrams of hydrogen from 

 H 2 SO 4 . The weight of PbSO 4 may be calculated from the weight of the hydrogen 

 by the ordinary methods of chemical arithmetic. The atomic weights are as follows : 

 H i, O = 16, S = 32, and Pb 207. 



