The initial electron avalanche created in the high nonuniform fields near an anode leaves behind it a space charge of positive ions that enhance the field farther away from the anode and, hence, produce the necessary conditions for successive electron avalanches to produce the streamer. A mathematical model of these processes has been derived and the minimum anode voltage required to satisfy these equations has been computed for different geometrical parameters. This streamer onset voltage agrees well with measured values. The conditions for streamer onset are found to exist whenever the average gradient of the field is higher than a certain value specified by the initial avalanche length.

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The Lichtenberg figure technique for the study of the streamer mechanism in the spark transition in positive point to negative plane geometry has been extended to longer gaps. In this extension the phenomena were investigated over a wide range of variables, some of the results of which, including the nature of the figures, the potential of the streamer tips, and properties of negative streamers, are being published elsewhere. This study is confined to the observation of the growth and attenuation of branching in relation to the appearance of the commonly observed solitary spark channel. Featured in longer gaps is the very extensive branching before midgap with decline as the cathode is approached. The luminous primary-secondary streamer sequence observed by the two-photomultiplier technique is an instrumentally conditioned phenomenon resulting from branching. There is no such sequence otherwise indicated by these observations. The same instrumental effect deriving from the branching accounts for discrepancies in the earlier reported values of streamer tip velocities.

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Absrracr-'Ibe electric field ad potential d i s t r i h t i o n in the gap between acylindricalrodhavingahemisphericPltipadaninfioiteplanepe~r to the cylinder axif was determined afiog a charge simulation techniqw. This metbod ~ssomes a charge at the center of the hemisphere and a W t e number of semi-infinite axial charges io the cylindrical portioa of the rod electrode. Bormdary cmditkm in the cylindrical ad spherical portiols enabled the formulation of simoltnneols quatiom whose digital solution yielded of the potential and both field components anywhere in the gap with au acmracy of about 2 percent. the assnmed llanped charges. digital computer progrsm provided V d w s