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Assembly of electrodes at either end of an insulated tube filled with gas
tubes producing visible discharges or used for switching purposes

A gas filled tube, also commonly known as a discharge tube or formerly as a Plücker tube, is an arrangement of electrodes in a gas within an insulating, temperature resistant envelope. Gas filled tubes exploit phenomena related to electric discharge in gases, and operate by ionizing the gas with an applied voltage sufficient to cause electrical conduction by the underlying phenomena of the Townsend discharge. A gas discharge lamp is an electric light using a gas filled tube; these include fluorescent lamps, metal halide lamps, sodium vapor lamps, and neon lights. Specialized gas filled tubes such as krytrons, thyratrons, and ignitrons are used as switching devices in electric devices. The voltage required to initiate and sustain discharge is dependent on the pressure and composition of the fill gas and geometry of the tube. Although the envelope is typically glass, power tubes often use ceramics, and military tubes often use glass lined metal. Both hot cathode and cold cathode type devices are encountered.

Hydrogen is used in tubes used for very fast switching, e. g. some thyratrons, dekatrons, and krytrons, where very steep edges are required. The build up and recovery times of hydrogen are much shorter than in other gases.

Deuterium is used in ultraviolet lamps for ultraviolet spectroscopy, in neutron generator tubes, and in special tubes (e. g. crossatron). It has higher breakdown voltage than hydrogen. In fast switching tubes it is used instead of hydrogen where high voltage operation is required. The electrodes undergo damage by high velocity ions. The neutral atoms of the gas slow the ions down by collisions, and reduce the energy transferred to the electrodes by the ion impact. Gases with high molecular weight, e. g. xenon, protect the electrodes better than lighter ones, e. g. neon. Helium is used in helium–neon lasers and in some thyratrons rated for high currents and high voltages. Helium provides about as short deionization time as hydrogen, but can withstand lower voltage, so it is used much less often. Neon has low ignition voltage and is frequently used in low voltage tubes. Discharge in neon emits relatively bright red light; neon filled switching tubes therefore also act as indicators, shining red when switched on. This is exploited in the decatron tubes, which act as both counters and displays. Its red light is exploited in neon signage. Used in fluorescent tubes with high power and short length, e. g. industrial lighting tubes. Has higher voltage drop in comparison with argon and krypton. Its low atomic mass provides only a little protection to the electrodes against accelerated ions; additional screening wires or plates can be used for prolonging the anode lifetime. In fluorescent tubes it is used in combination with mercury. Radon, despite being a noble gas, is dangerously radioactive and its most stable isotope has a half life of less than four days. Consequently, it is not commonly used in electronic devices. Penning mixtures are used where lower ionization voltage is required, e. g. in the neon lamps, Geiger–Müller tubes and other gas filled particle detectors. A classical combination is about 98–99.5% of neon with 0.5–2% of argon, used in, e. g. neon bulbs and in monochrome plasma displays.

The gas in the tube has to be kept pure to maintain the desired properties; even small amount of impurities can dramatically change the tube values. The presence of non inert gases generally increases the breakdown and burning voltages. The presence of impurities can be observed by changes in the glow color of the gas. Air leaking into the tube introduces oxygen, which is highly electronegative and inhibits the production of electron avalanches. This makes the discharge look pale, milky, or reddish. Traces of mercury vapors glow bluish, obscuring the original gas color. Magnesium vapor colors the discharge green. To prevent outgassing of the tube components during operation, a bake out is required before filling with gas and sealing. Thorough degassing is required for high quality tubes; even as little as 10−8 torr (≈1 μPa) of oxygen is sufficient for covering the electrodes with monomolecular oxide layer in few hours. Non inert gases can be removed by suitable getters. For mercury containing tubes, getters that do not form amalgams with mercury (e. g. zirconium, but not barium) have to be used. Cathode sputtering may be used intentionally for gettering non inert gases; some reference tubes use molybdenum cathodes for this purpose.

Fluorescent lighting, CFL lamps, mercury and sodium discharge lamps and HID lamps are all gas filled tubes used for lighting. Neon lamps and neon signage (most of which is not neon based these days) are also low pressure gas filled tubes. Specialized historic low pressure gas filled tube devices include the Nixie tube (used to display numerals) and the Decatron (used to count or divide pulses, with display as a secondary function). Xenon flash lamps are gas filled tubes used in cameras and strobe lights to produce bright flashes of light. The recently developed sulfur lamps are also gas filled tubes when hot.

Some important examples include the thyratron, krytron, and ignitron tubes, which are used to switch high voltage currents. A specialized type of gas filled tube called a Gas Discharge Tube (GDT) is fabricated for use as surge protectors, to limit voltage surges in electrical and electronic circuits.

The Schmitt trigger effect of the negative differential resistance region can be exploited to realize timers, relaxation oscillators and digital circuits with neon lamps, trigger tubes, relay tubes, dekatrons and nixie tubes. Thyratrons can also be used as triodes by operating them below their ignition voltage, allowing them to amplify analog signals as a self quenching superregenerative detector in radio control receivers.

Hot cathode, gas discharge noise diodes were available in normal radio tube glass envelopes for frequencies up to UHF, and as long, thin glass tubes with a normal bayonet light bulb mount for the filament and an anode top cap, for SHF frequencies and diagonal insertion into a waveguide. They were filled with a pure inert gas such as neon because mixtures made the output temperature dependent. Their burning voltage was under 200 V, but they needed optical priming by an incandescent 2 watt lamp and a voltage surge in the 5 kV range for ignition. One miniature thyratron found an additional use as a noise source, when operated as a diode in a transverse magnetic field.

In the mid 20th century, voltage regulator tubes were commonly used.

Cathode sputtering is taken advantage of in the Time Totalizer, a metal vapor coulometer based elapsed time meter where the sputtered metal is deposited on a collector element whose resistance therefore decreases slowly.