The effect of corona impinging on thin insulating materials, i.e. cellulose acetate and varnish cloth, is studied by exposing the material to the discharge in a plane-parallel air-gap, and determining the changes in power factor, permittivity and weight which are produced. The time elapsing before the material is electrically punctured, and the increase of acid content, are also measured. Subsidiary tests such as a comparison of the effect of direct and indirect corona, the differences arising when pure oxygen and pure nitrogen are employed separately as the discharge medium, and visual and photographic studies of the discharge and the measurements of corona onset and extinction voltages are also described. The results are discussed, and it is concluded that direct corona results in a general form of deterioration owing to the absorption and subsequent chemical action of ozone and nitrogen oxides by the material, and at the same time a highly localized or microscopic deterioration which is a consequence of electronic and/or ionic bombardment of the surface and which accounts for the ult i mate electrical breakdown. The two forms of deterioration are not related. These conclusions lead to suggestions for tests for assessment of the corona resistance of materials and rule out the possibility of any single test being in itself sufficient for this purpose.
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