Abstract :
The breakdown strength of air has been determined for alternating electric fields of frequencies between 100 and 300 Mc/s. Parallel plane gaps up to 1 mm wide have been investigated. A description is given of the apparatus employed, together with details of a dielectric voltmeter which has enabled the voltage across the breakdown gap to be measured accurately. A discontinuity is shown to exist in the relation between the breakdown stress and the gap width. The breakdown stress for small gap widths appears to follow a curve representing values some 10¿15% below those obtained with direct current. At a critical gap width, which is dependent primarily on the frequency of the applied electric field, there is a sudden decrease in the electric stress at which breakdown occurs, to a constant value, which for atmospheric pressure is 29 kV/cm. The critical gap width is found to vary slightly with gas pressure, and may be increased by the application of a small d.c. electric field across the gap. An examination of the results shows that under transition conditions Paschen´s law is not applicable. A theory is put forward on the basis that breakdown may be initiated in one of three different ways, each of which may be represented by a curve that separately obeys Paschen´s law.
