A neutral gas behaves like an insulator at not-too-high voltages. In reality, however, some gas particles are always ionized due to thermal collisions or interactions with radioactive radiation, so that free ions and electrons are present. In a sufficiently large volume, the so-called Debye volume, there are therefore always equal numbers of positive and negative charge carriers.
If electrodes are placed into such a gas volume and an electric field is applied, the current increases proportionally with the voltage, meaning Ohm’s law holds quite well. At higher voltages, however, a saturation occurs. The current then remains almost constant over a wider voltage range. This saturation can be explained by the limited number of available charge carriers and the relatively constant generation of new electron-ion pairs.
Beyond a certain voltage, the electric field strength becomes so large that electrons released from the negatively charged electrode can ionize additional gas molecules. This voltage is called the breakdown voltage. At this point, a gas discharge occurs, which manifests as an arc and is comparable to lightning during a thunderstorm.
As long as the gas discharge is maintained by external ionization processes, it is called a non-self-sustaining discharge. If, however, the field strength is so large that charge carriers multiply in an avalanche-like process, the conductivity increases with current, and a self-sustaining discharge is formed. In this case, the current can grow arbitrarily large, potentially destroying the apparatus unless limited by other means, such as a series resistor.
Gas discharges are used technically in arc welding and in carbon arc lamps, which serve as intense light sources. However, the ionization of air molecules also produces ozone, which can lead to poisoning if inhaled for extended periods.