Definition
An electron tube whose envelope contains a small amount of an inert gas, such as argon, neon, xenon, or mercury vapor, instead of being fully evacuated. When voltage is applied across the electrodes, the gas ionizes and conducts current, allowing the tube to function as a rectifier, voltage regulator, indicator lamp, or switching device.
Plain English
A sealed glass tube with a little gas inside instead of a vacuum. When electricity is applied, the gas glows and lets current pass, which makes the tube useful as a switch, regulator, or indicator light.
Context Anchor
Seen in descriptions of older aircraft electrical, radio, instrument, or power-supply equipment.
Why Pilots Care
Gives early, reliable warning of engine fire or dangerous overheating so the crew can take immediate action.
Analogy
A small neon indicator lamp is a familiar kind of gas-filled tube: the gas inside glows and conducts electricity when the voltage is high enough.
Intuition Check
Do not read “gas-filled tube” as a hose or pipe carrying gas. Here it means a sealed electrical part that uses gas inside the tube to control the flow of electricity.
Example Sentence 1
The voltage regulator in the older trainer used a gas-filled tube that glowed faintly whenever the system was powered up.
Example Sentence 2
Maintenance replaced the gas-filled tube after it failed the pressure test on the overheat detection loop.