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.
A thyratron is a gas-filled electronic switching tube in which a control electrode triggers a conducting discharge between cathode and anode. The control signal governs the onset of conduction, allowing a comparatively small trigger to switch a larger supplied pulse of electric current.
Once the discharge is established, the control electrode does not act like a continuously adjustable linear gain control. The external circuit must bring the current down sufficiently for the gas to recover its nonconducting state. Trigger timing, recovery and the load circuit therefore determine repeated-pulse behavior. This distinguishes a thyratron switch from a linear electronic amplifier.
A mercury-vapor lamp excites Mercury atoms in an electrical discharge and collects the light emitted as they relax. Its output contains characteristic spectral lines; the lamp's pressure and construction affect their widths and the balance between lines and background.
A spectroscope separates those lines, making the lamp useful for wavelength references and optical excitation. Some emitted ultraviolet radiation can excite a phosphor, which converts it into a broader visible output. The observed spectrum then depends on the discharge, envelope transmission and any conversion coating; it is not simply the isolated-atom spectrum at every operating condition.
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