Mode and current paths for a discharge in a vacuum switch placed in a magnetic field were investigated experimentally and analytically to find a possible method for measuring the pressure in the switch. A low‐pressure gas discharge is generated in the switch, and discharge properties show evidence that the arc shield, which is electrically floating, is an essential element. Simulators of the vacuum switch were assembled to determine the effects of the shield on the discharge. Pressure dependence and current distributions in various electric‐field configurations in the simulators were measured and analyzed. The discharge in the vacuum switch is identified as a series discharge of an inverted magnetron and of a magnetron through the shield as a common electrode. The mode for each elemental discharge is the same as that employed by cold‐cathode vacuum gauges. In the inverted magnetron, the current flows from the switch‐contact periphery at positive high voltage radially to the shield. In the magnetron, it flows from the shield end radially to the rod of a switch electrode, which is electrically grounded.