Electrical essay: Spark-gap in wonderland

ALICE walked over to a workbench in the laboratory upon which there was some apparatus with an open laboratory notebook beside it. The apparatus consisted of a glass tube whose internal diameter was about four centimeters, containing two rather snugly fitting plane electrodes, clearing the walls by about a millimeter, and separated from each other by two or three centimeters. Wires came up to the tube, and there was a confusion of switches, meters, and gauges. Alice read in the notebook: With one megohm in series to limit the current I applied 3,000 volts direct current to the spark-gap. No breakdown occurred. On lowering the voltage to about 2,000 volts, the gap suddenly broke down, glowing, with a current of 0.2 milliampere flowing through it. On raising the voltage, the gap is suddenly cleared, giving zero current. This I repeated many times. The gap spacing was 2.7 centimeters and the gas pressure was 0.8 millimeter of mercury.