The electrical breakdown in borosilicate glass cells of plane parallel plate geometry with external electrodes was studied under static and 60‐cycle alternating potential using Linde spectroscopically pure grade Ar gas introduced under Alpert vacuum conditions at 7, 15, and 35 mm. Oscilloscopic studies at fast and slow sweep as well as measurement of charge, wall potential and of apparent leakage charge through the glass using a large series condenser with Keithly electrometer and recorder were made. Three differing modes of breakdown appear as potential increases. These are associated with space‐charge distribution and density. The two modes at lower potentials are Townsend discharges operating by photoelectric processes at the cathode. The third mode has streamer‐like characteristics and a rise time equal to one avalanche crossing time. All breakdowns terminate through counter potentials from charge accumulation on the glass electrodes. The fairly complete data lead to a clear understanding of the observed breakdown patterns with 60‐cycle alternating potential. The electrical properties of the glass involved are discussed in terms of recent findings.