If the voltage be again reduced to 300 the excess thickness of film will be gradually dissolved, and if it vary periodically between two values, each of which is less than the permanent critical value, the temporary critical voltage will be the higher value. This feature is of great importance as it provides a means of discharging abnormal surges, the instant the pressure rises above the impressed value.
Fig. 2,391.—Volt ampere characteristic curve of a General Electric aluminum (electrolytic) cell on alternating current. The permanent critical voltage is between 335 and 360 volts. With alternating current, the cell acts as a fairly good condenser, and there is not only the leakage through the film, but also a capacity current flowing into the cell. The phase of this current, then, is nearly 90 degrees ahead of the pressure and represents a very low energy factor.
Ques. How is the number of cells required for a given circuit determined?
Ans. The number required for a given operating voltage is determined by allowing about 250 to 300 volts per cell.
Ques. In putting cells in commission how is the electrolyte introduced?
Ans. It is poured into the aluminum trays and the overflow drawn off at the bottom of the tank.
Ques. Describe the further operations in putting cells in commission.
Ans. After putting in the electrolyte it is allowed to stand for a few days until part has evaporated, then the oil is poured over the surface to prevent further evaporation.