A gas-discharge tube conducts electric current through a gas made partially ionized by an applied electric field. Initial free electrons gain energy between collisions and can cause further ionization and excitation, producing a conducting discharge rather than ordinary conduction through a metal.
The behavior depends on gas composition, pressure, electrode spacing and the external circuit. Excited species can emit light, making a gas-discharge lamp; other designs use the discharge to switch current. A current-limiting external element often matters because discharge voltage need not increase with current like an ordinary resistor.
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