From experimental investigations on ionization currents and high-voltage sparking in uniform electric fields it has generally been assumed that the growth of pre-breakdown currents followed a simple exponential law involving no secondary ionization, and that therefore to account for critical breakdown it was necessary to assume the sudden introduction of a new ionization mechanism not related to the growth of initial pre-breakdown currents. These data are critically analyzed, and it is shown that they do not in fact preclude the existence of important secondary ionization, as was assumed. The experimental conditions which must be satisfied in order to reveal and measure any such secondary ionization are discussed, and a high-voltage ionization chamber designed and built to satisfy these conditions is described. Measurements of pre-breakdown ionization currents between parallel plates in air under conditions which include those corresponding to a 1 cm gap in the atmosphere ( E / p = 40; pd = 800; V 8 = 30 kV) showed that, while the growth of the currents was simply exponential over the greater part of the sparking distance, in the important region near breakdown itself the currents did, in fact, increase at the faster rate represented by I = I 0 exp ( αd )/[1 — ( ω / α ) {exp ( αd )—1}]. This indicates the existence of a secondary mechanism throughout the complete breakdown process. Furthermore, the measurement of the ionization coefficients α and ( ω / α ) in the early stages of the growth of a small (≃ 10 -14 A) photo-electric current can predict, within the experimental error, the static potential at which the gas breaks down, thus indicating that the mechanism of the spark itself was the same as that which led to the growth of the photo-current even in its early stages.
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