STORAGE BATTERIES. 257 



the battery, are not completely discharged. Therefore, when the 

 battery is recharged, the end-cells are placed in circuit at the 

 start and cut out one by one when they become fully charged, as 

 indicated, for example, by the copious evolution of gas. 



An important detail in the carrying out of the end-cell method 

 of voltage control is the design of the switch for connecting and 

 disconnecting the end-cells without interrupting the delivery of 

 current, and without momentarily short-circuiting the individual 

 cells. The essential features of this end-cell switch are shown in 

 Fig. 147. The terminals of the end-cells are brought out to a 



Main 



tn Lywwvsq. 



Main 



Fig. 147. 



series of contact blocks, c c c c, and the movable contact consists 

 of two pieces, a and b, which together bridge across between the 

 blocks, cc, when they are moved along and short-circuit a cell 

 through a resistance, R, which is large enough to limit the short- 

 circuit current to about the value of the normal discharge current 

 of the cells. 



The shunt booster. In the use of a storage battery as above 

 described, a voltage greatly in excess of the normal station volt- 

 age is required in charging, especially when the battery approaches 

 complete charge. Thus the 61 cells, required to give HO volts 

 at 1.8 volts per cell, require about 2.5 volts each, or a total of 

 17 



