256 ELEMENTS OF ELECTRICAL ENGINEERING. 



stant this method of control is fairly satisfactory, for, in this case, 

 the resistance has to be adjusted only as the battery voltage falls 

 off. When the lamp load fluctuates, however, the rheostat re- 

 quires constant attention, inasmuch as the voltage drop, AY, in 

 the rheostat may change suddenly with a sudden change of load. 



Control of voltage by counter-electromotive-force cells. When 

 current flows through a low-resistance electrolytic cell consisting 

 of plain lead plates in dilute sulphuric acid, the voltage drop 

 through the cell varies from about 2. 1 to 2.4 volts according to 

 the value of the current. The excess voltage of a discharging 

 storage battery may be taken up by causing the current to flow 

 through a number of such cells connected in series, the number 

 being reduced as the battery voltage decreases. The advantage 

 of this arrangement is that the voltage which is lost in these con- 

 trolling cells does not vary greatly with the current. This 

 method of voltage control is seldom used in practice. It has no 

 advantage over the rheostat method when the load is constant, 

 and the end-cell method is usually preferred when the load is 

 variable. 



Control of voltage by end-cells. This method of control will be 

 explained by giving an actual example of a battery to be used for 

 delivering current at no volts. The lowest permissible voltage 

 at the end of the discharge is usually taken to be 1.8 volts per 

 cell. Therefore the number of cells required to give a minimum 

 of no volts is no -T- 1.8, which is equal to 61. The highest 

 voltage is about 2.15 volts per cell at the very beginning of the 

 discharge (see Fig. 143), and 51 cells are therefore required at 

 the very beginning of the discharge to give 1 10 volts. There- 

 fore, the entire battery being fully charged, 5 1 cells are used at 

 the beginning of the discharge, and as the voltage of the battery 

 falls off the number of cells is increased, by connecting-in addi- 

 tional cells at one end of the set, until, when the battery reaches 

 the limit of discharge, all of the 61 cells are in service. Under 

 these conditions it is evident that the end-cells, which are in ser- 

 vice only a portion of the time during the delivery of current by 



