Abstract :
Various shapes of test sample have been recommended for determining the electric strength of ceramics. The form described by the British Standards Institution (BS 1598: 1949) for a proof test for electric strength would normally be chosen, but this type always fails by surface flashover rather than by puncture through the material. It was therefore decided to compare the electric-strength values obtained from five sample shapes (including a thinner version of the BS sample), all prepared from one batch of h.v. porcelain, produced by the same forming process and fired together. The thickness of the porcelain, the surface finish and the electrode material were varied, but other factors influencing the results were standardised. All samples were inspected for faults visually, by a fluorescent-penetrant method under ultraviolet light, and in some cases by X radiography. The fluorescent-penetrant method was very quick and easily applied, and more successful in detecting flaws than X ray inspection. On the bases of the electric-strength results, their consistencies of determined values, and the ease of manufacture and test procedure, an 80mm-diameter disc with a spherical depression in one face, using fired-on silver electrodes on a ground surface, was found to be most satisfactory.
