Measurements of the formative time lags in homogeneous fields for different gases up to
pd
values of 3000 mmHg cm have been made. For overvolted gaps in dry air and oxygen a discontinuity is found in the logarithmic plot of overvoltage against formative time. This is considered to be evidence of a transition from a Townsend- to a streamer-type of discharge. No definite sign of this change has been found in hydrogen, nitrogen and pure argon. Additional evidence that the discontinuity is the transition from one initiating mechanism to another is obtained from still photographs of the sparks. These indicate that at low values of overvoltage the discharge consists of a diffuse glow and spark channel while the streamer initiated spark has a narrow constricted glow. The position of the bright constriction or knob in the gap varies with overvoltage and its position agrees with the value predicted by the Meek criterion
αx
crit.
= 20. The value of overvoltage at which the transition occurs decreases with
pd
and is about 5 % at
pd
= 1000 mm Hg cm and 1-5 % at
pd
= 3000 mmHg cm. By extrapolation it seems unlikely that the streamer mechanism will occur at minimum break-down for
pd
values less than 7000 mmHg cm. Oscillographic studies have shown that the disappearance of the glow to arc step is not always a satisfactory Townsend to streamer criterion. Spectra of the sparks in air show a continuum associated with the constriction in the streamer together with bright atomic lines and weakening of the band spectra.
