The electric strength of air. — II

In a former paper¹ with the title of the present one the author described a series of investigations of the conditions under which the air breaks down in the neighborhood of clean, round wires subjected to high voltage. A principal feature of that paper was the description of a method for observing with a close degree of accuracy the critical or corona voltage for various sizes of wire when centred in cylinders forming the opposite side of the source of voltage. There has been a great diversity in the values of critical voltage as given by other observers, who for the most part have used the appearance of the visible corona and the readings of instruments in the primary circuits of transformers as indications of the voltage at which the air breaks down. The method referred to was developed as the result of a conviction that the laws governing the loss between high-tension lines could not be satisfactorily determined without a study and knowledge of the fundamental phenomena. So far therefore these investigations have been concerned only with the conditions under which the air actually breaks down causing a large increase in conductivity and power loss. The results of the former paper show among other things that when corrected for wave form, temperature and pressure the electric intensity at the surface of a clean, round conductor, corresponding to the voltage at which corona starts and loss sets in is a constant for each size of wire. This value of surface intensity varies with the temperature and pressure and is that corresponding to the maximum value of the voltage wave. It is different for different sizes of wire but is independent of the material of the wire, of the moisture content, and of the amount of free ionization in the air. In the present paper some further facts bearing on the fundamental relation between diameter and critical surface intensity are given, and a series of investigations of the influence of stranding a conductor, of variations of atmospheric pressure, and of frequency on the critical electric intensity are also described.