The paper describes investigations of the variations of discharge-extinction voltage which occur when a dielectric sample containing a single air-filled cavity is subjected to a 50c/s testing voltage greater than that required to initiate discharges in the cavity. A particular aim of this work was to determine the causes of the phenomenon of self-extinction, whereby, under certain conditions, such discharges may spontaneously extinguish, even though the voltage applied to the sample may be several times the initial discharge-inception value. Automatic test equipment was used to measure and record the discharge-extinction voltages of samples under test at regular intervals, so permitting the uninterrupted study of the variations over long periods. The majority of the tests were made on polythene samples, but self-extinction of discharges has been observed with cavities in a number of other dielectrics. The conditions under which discharges in cavities spontaneously extinguish are described, and it is shown that the effect may be attributed to an increase in the conductivity of the cavity walls, induced by the discharges themselves.
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