Complex sentence: The starting of Townsend discharge sets the upper limit to the blocking voltage a glow discharge gas-filled tube can withstand this limit is the Townsend discharge breakdown voltage also called ignition voltage of the tube.
Split into simpler sentences: The starting of Townsend discharge sets the upper limit to the blocking voltage a glow discharge gas-filled tube can withstand. This limit is the Townsend discharge breakdown voltage, also called ignition voltage of the tube.