Abstract :
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. Following up the analysis the author was surprised to find that such a dielectric, containing particles of moisture would show absorption and residual charge and many other characteristics of actual insulation. He also found that certain unusual test data, obtained while testing cables, could be explained as due to a breaking up of the moisture filaments by evaporation. The author does not claim that the paper that follows meets the complexity of actual insulation, but rather that it adds to - ur conception of the importance of moisture in its effect upon a dielectric field.
