Definition
A cathode in an electron tube that emits electrons when heated by a separate, electrically isolated heater element rather than by passing current through the cathode itself. The heater warms a sleeve coated with electron-emitting material, which then releases electrons into the tube without being part of the heater circuit.
Plain English
A part inside an electron tube that gives off electrons when it gets hot. Instead of heating it by running electricity directly through it, a small separate heater warms it from the inside, like a heating element inside a metal cup.
Context Anchor
Seen in aircraft electronics and maintenance discussions involving older radios, tube-type equipment, or electronic components.
Derivation
Cathode comes from the Greek 'kathodos,' meaning 'a way down' — the path electrons take as they leave the heated surface. 'Indirectly heated' simply means the heat reaches the cathode through a separate part, not through the cathode itself.
Why Pilots Care
A pilot normally will not operate this part directly, but understanding the term helps when reading older aircraft equipment manuals or maintenance write-ups for tube-type avionics.
Analogy
Think of a kettle warmed by a heating element inside it: the element gets electricity, the kettle wall gets hot, but the two are not the same circuit. The cathode is the kettle wall; the heater is the element.
Intuition Check
Do not read “indirectly heated” as meaning “barely heated” or “not really heated.” It means the cathode is heated by a separate part, not by being the heater itself.
Example Sentence 1
The radio's electron tube uses an indirectly heated cathode, so the heater circuit can run on AC without affecting the signal.
Example Sentence 2
Technicians servicing legacy aircraft radios check the indirectly heated cathode inside vacuum tubes for proper electron emission.