Open atmosphere and dry transformer oil as high-voltage insulators

I. Cause of the Great Dielectric Strength of Air Films on the Surface of a Conductor at High-potential a. By the method that employs a conductor of circular section mounted in the air at the center of a hollow conducting cylinder, the electric stresses at the conductor surface required to start corona were observed and reported to the Institute.∗ These observations are recharted, using kilovolts per inch, in lieu of coulombs per inch-cube, for the stresses, so as to locate the single curve, drawn in Figs. 1a and 1b. These data apply to the normal indoor atmosphere at a temperature of 70 deg. fahr., barometer of 29.5 in. (750 mm.) and an elevation of 850 ft. (259 m.) above sea level. The galvanized sheet iron cylinder was new and clean, 3 ft. (91.4 cm.) long, 15 in. (38 cm.) diameter and open at both ends. Approximate sine-wave, 133-cycle, high-voltage alternating e.m.fs. applied the electric stresses between the conductors and the cylinder. The maximum values of these e.m.fs. were checked by needle spark-gaps. The conductors were clean brass rods for the one quarter inch (6.35 mm.) and larger diameters, and clean copper wires for the smaller diameters. The work was done indoors and at night to facilitate visual observation of the complete corona-start. The size of the room employed was approximately 40 by 40 by 15 ft. (12.2 by 12.2 by 4.5 m.), and the air in it reasonably dust free due to ordinary settling. The ions and radioactivity present in the air were not observed because their existence and importance in corona formation were not understood at the time.