 Electrical discharge

An electrical discharge transfers electric charge through a medium or gap. A gas discharge can begin when accelerated charge carriers produce enough further ionization to sustain an electric current. An electron avalanche describes multiplication by successive ionizing collisions; the supply and loss processes determine whether it becomes a sustained conducting state.

A motivating gas-breakdown experiment varies pressure while keeping the gas, electrode separation and electrode geometry specified. The required electric potential difference does not generally decrease monotonically with pressure. At very low density, an electron encounters too few molecules to sustain multiplication; at higher density, collisions can interrupt acceleration before sufficient ionizing energy is acquired. The resulting dependence on pressure and gap is summarized, within its assumptions, by the Paschen law.^[ref]

The experimental apparatus therefore needs electrodes with a known separation, pressure control and measurement, and a current-limited electrical source with current and potential measurements. Geometry and electrode surfaces must be recorded along with the gas conditions. A measured onset threshold and the later current–potential characteristic answer different questions; the existence of a glowing plasma is not itself a measurement of its onset threshold.