The electric strength of air — IV

The investigation of the influence of pressure on corona formation described in the second paper of this series is extended with larger apparatus so as to include a wider range of pressure and diameter of conductor. The results show that the influence of pressure in this extended range are in substantial agreement with the relation expressed in the formula suggested by Peek. A few observations were also made on the influence of temperature within the range 5 deg. to 55 deg. cent., and these observations are also in accord with the empirical relation mentioned. Observations with carbon dioxide as a gas indicate that the critical corona intensity is independent of the absolute density of the gas, but depends on the number and spacing of the molecules in accord with the theory of secondary ionization. A short discussion of the results in the light of the theory of ionization by collision is also given.