Ques. What is the critical voltage?
Ans. The voltage at which the current begins to flow freely.
FIG. 2,386 to 2,390.—Parts of General Electric 15,000 volt aluminum lightning arrester, not including horn gaps, etc.
Up to a certain voltage the cell allows an exceedingly low current to flow, but at a higher voltage the current flow is limited only by the internal resistance of the cell, which is very low. A close analogy to this action is found in the well known safety valve of the steam boiler, by which the steam is confined until the pressure rises above a given value, when it is released. On the aluminum plates there are myriads of minute safety valves, so that, if the electric pressure rise above the critical voltage, the discharge takes place equally over the entire surface. It is important to distinguish between the valve action of this hydroxide film and the failure of any dielectric substance.
Ques. When a cell is connected permanently to the circuit what two conditions are involved?
Ans. The temporary critical voltage and the permanent critical voltage.
For instance, if the cell have 300 volts applied to it constantly, and the pressure be suddenly increased to, say 325 volts, there will be a considerable rush of current until the film thickness has been increased to withstand the extra 25 volts; this usually requires several seconds. In this case 325 volts is the temporary critical voltage of the cell.
Similar action will occur at any pressure 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.