text: 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.
  The occurrence of Townsend discharge, leading to glow discharge breakdown shapes the current–voltage characteristic of a gas-discharge tube such as a neon lamp in a way such that it has a negative differential resistance region of the S-type. The negative resistance can be used to generate electrical oscillations and waveforms, as in the relaxation oscillator whose schematic is shown in the picture on the right. The sawtooth shaped oscillation generated has frequency
  :where
  :* is the glow discharge breakdown voltage,
  :* is the Townsend discharge breakdown voltage,
  :*,  and  are respectively the capacitance, the resistance and the supply voltage of the circuit.
  :Since temperature and time stability of the characteristics of gas diodes and neon lamps is low, and also the statistical dispersion of breakdown voltages is high, the above formula can only give a qualitative indication of what the real frequency of oscillation is.