Observation of corona phenomena at very high direct voltages

Corona currents of negative polarity flowing from a variety of earthed electrodes of small radii of curvature have been measured up to direct voltages of 1000 kV. The upper, high-voltage electrode has been either a sphere of 2 m diameter or a plane of 130 cm diameter (with curved edges). Over a current range extending from 0.5 ¿¿A to 2500 ¿¿A the current rises very nearly according to the fairly well established equation i = A(V¿¿V0)V where V0 is the threshold voltage for corona development, an equation applicable to corona in small gaps. Gaps of 25 cm up to 4 m have been studied. The corona is emitted in a series of pulses, generally known as Trichel pulses (1938), each probably lasting less than 1 ¿¿sec; the number rises almost linearly with current at least up to a rate of 1 pulse per ¿¿s, the maximum rate recorded at 600 kV across a 2.5 m gap. Their amplitude diminishes slightly with increasing voltage, falling from 600 kV to about half their value at 60 kV. Corona currents from earthed points 4 m below a 500 kV DC transmission line can amount to several microamperes.