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A laboratory investigation into the radio interference arising from electrical discharges on porcelain pin insulators and string insulator units (cap-and-pin type) under high, direct-voltage conditions is described. The effects of the magnitude and polarity of the applied voltage, the surface conditions of the insulators, and the relative humidity of the air, have been studied and phenomena compared with those observed under alternating-voltage conditions. Measurements have been made in the frequency range 0.15Â¿30 MHz, and details of the techniques adopted are given. It has been shown that radio interference is much lower with direct voltages than with alternating voltages of the same peak value.