= 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>.<Paschenentladung: Isolation und Plasmaentladung by TU Berlin>[A15 experiment description, ionization avalanche and the competing low- and high-pressure limitations]{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.
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