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Development and evaluation of corona measurements have contributed greatly toward solution of the problem of determining satisfactory voltage stress levels in capacitors. Until recent years, short-time voltage breakdown tests were the only quick means of determining required dielectric thicknesses. However, practical operating voltage stress levels were established more by service experience than by short-time measurements. The time consumed in obtaining the service experience on new materials or designs made advances in design practice slow and uncertain. During the last 15 years it has become evident how closely the life of a capacitor in service is linked with the ionization voltage level of its dielectric. New measurement techniques have been developed, by means of which insulation ionization levels can be quickly and quantitatively established. Results of ionization tests have been correlated with long-time life tests and service experience. As a result, it now is possible to use the ionization test as a valuable guide in selecting improved dielectric materials, and in designing to meet new operating conditions. While this paper refers specifically to the very thin dielectrics used in capacitors, the techniques described can be applied also to evaluate electrical insulations for other devices.