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Predischarge current and breakdown measurements of solid PlexiglasÂ® (PMMA) insulators in vacuum and nitrogen gas gaps, excited by 60 Hz ac voltages, are reported. Results show that for bridged vacuum gaps, the predischarge current is in the form of pulses. In addition to electron emission from the cathode, secondary emission from the insulator surface is postulated to play a significant role in determining the characteristics of the current pulses. It is reported that for bridged nitrogen filled gaps no partial discharges prior to breakdown were noted, except for the case of introduced artificial voids. Insulating spacers in the form of circular cylinders and conical frustrums of different angles have been studied. The breakdown characteristics are found to depend strongly on the shape of the insulating spacer and also on the surface condition. For any given gas pressure, the breakdown strength was found to decrease with decreasing angle of the cone frustrum at its base. A rough insulator surface exhibited considerable discharge track resistance compared to a smooth surface. For cylindrical spacers, the predischarge and breakdown characteristics are found to be strongly influenced by the shape and position of the introduced voids. The effect of gas pressure on the pre-discharge and breakdown characteristics are reported.