606 



ELECTRICAL EQUIPMENT 



entirely upon the internal resistance of the electrolyte, for exam- 

 ple, at double the normal operating voltage, or 600 volts per cell, 

 the current discharge is about 600 amperes for a brief time. 



When a cell is connected permanently to the circuit, two con- 

 ditions of voltage are involved, which may be distinguished as the 

 temporary critical voltage and the permanent critical voltage. 

 For example, if each cell has 300 volts applied to it constantly, 

 and the voltage is suddenly raised to, say, 325 volts, there will be 

 a considerable rush of current until the film thickness has been in- 

 creased to withstand the extra 25 volts; this usually requiring 



400 



6240 



2 160 



o 



80 







Amperes 



FIG. 387. Volt-ampere Characteristic Curve of an Aluminum Cell on Alter- 

 nating Current. 



several seconds. In this case 325 volts is the temporary critical 

 value of the cell. Similar action will occur at any potential up to 

 about the permanent critical voltage, or the voltage at which the 

 film cannot further thicken and therefore allows a free flow of 

 current. If the voltage is again reduced to 300, the excess thick- 

 ness of the film will be gradually dissolved, and if it varies period- 

 ically between two values, each of which is less than the perma- 

 nent critical value, the temporary critical voltage will be higher. 

 This feature is of great importance as it provides a means of dis- 

 charging abnormal surges the instant the pressure rises above the 

 impressed value. 



The number of cells for a circuit is so chosen that the maxi- 

 mum voltage per cell will be approximately 300 volts, or always 

 less than the permanent critical voltage. 



Besides the valve action already described there is another 



