The Action and Effect of Moisture in a Dielectric Field

In studying the subject of dielectric loss in electric cables the author has become convinced that the moisture content of the dielectric is the dominant factor determining the a-c. resistance. Evershead´s explanation of the action of moisture in a fibrous dielectric seems plausible but leads to the conclusion that moisture causes a decrease of a-c. resistance with increasing voltage, whereas the experience of the author is that with a fairly well dried dielectric a-c. resistance is independent of voltage, and that decreasing the moisture content still further gives higher and higher a-c. resistance, with no limit in sight. It seems obvious, therefore, that Evershead has not fully covered the subject. In order to get a picture of the action of moisture in a dielectric field the author has assumed a simply hypothetical case and tried to follow it to its logical conclusions. He assumed a pure dielectric of a homogeneous and plastic nature between parallel electrodes and subject to electric stress. He then mentally placed a very small globule of conducting moisture in the dielectric and watched the action. Under constant potential stress the moisture elongated into a thread-like filament until it bridged the dielectric. But under alternating stress the moisture globule, if sufficiently small stretched out only a short distance and then no further, no matter how high the voltage. This showed how the a-c. resistance could be independent of the voltage and yet depend upon moisture.