The discharge space between a positive cylindrical electrode (in the diameter range 0.3185-3.175 cm) and a coaxial outer electrode of diameter 58.1 cm has been irradiated by a radioactive beta -source to obtain a stable glow discharge in which streamers are absent. Onset electric gradient, voltage-current characteristics and electric gradient measurements at the anode surface in the presence of corona have been obtained. A modified form of the Townsend voltage-current characteristic has been derived which is in good agreement with the measured values for all conductor diameters. The surface gradient is found to be constant at about 5% below the onset value after establishment of the glow discharge, and to be independent of further voltage increase. Calculations suggest that this is consistent with a positive-ion mobility of 1.8*-4m2V-1s-1.

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A theoretical determination based upon a critical charge criterion and ionization processes is presented of the striking distance r s at which the upward leader is initiated from a struck object. The striking distance is shown to be leader-current dependent in the form r s ∝ i 2/3 . Comparisons are made with earlier, more numerically based models, which used an alternative equivalent-radius approach. The altitude dependence of the striking distance is also investigated. Since pressure, absolute humidity and temperature decrease with increasing altitude, the variation of streamer properties with altitude can be inferred from laboratory experiments. Calculations on this basis show that the striking distance increases significantly with increasing altitude, causing a corresponding increase in the risk of lightning strikes for given structures in mountain or aviation environments.

The work shows the crucial need for a careful choice of boundary conditions, particularly near the corona electrode, and revises the boundary conditions usually employed in classical methods. Commonly adopted assumptions which result in the corona radius and corona voltage drop being taken as constant have been shown to be inappropriate, since they lead to ambiguity in the mobility values. Values of positive-ion mobility based upon all three measured parameters have been calculated, using various models for the corona region, and these show a decrease in mobility with increasing humidity. This mobility is consistent with the field equations, and no dependence was found upon field strength or ion aging. The models yield an estimate of the dimensions and voltage drop for the corona ionisation region and the value of the field at the corona electrode.

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