Definition
In a vacuum-tube circuit, the low-voltage battery used to heat the filament (or cathode heater) of the tube so that it can emit electrons. It supplies filament current only and is separate from the higher-voltage batteries that power the rest of the tube circuit.
Plain English
The small battery whose only job is to warm up the heating wire inside an old-style vacuum tube so the tube can start working. It does not run the tube's signal side -- just the heater.
Context Anchor
Seen mainly in older aircraft electrical-system, radio, and restoration references, especially where vacuum-tube equipment is described.
Derivation
In early radio design, engineers labeled the separate batteries in a tube circuit by letter. The 'A' battery powered the filament, the 'B' battery supplied the plate (anode) voltage, and the 'C' battery biased the grid. The letters simply marked them in order of function, and the names stuck.
Why Pilots Care
Most modern aircraft no longer use vacuum tubes, but pilots and technicians working with vintage aircraft, warbirds, or older avionics may still see references to A, B, and C batteries in wiring diagrams and manuals. Knowing what each one powered helps when troubleshooting or restoring legacy gear.
Intuition Check
Do not read the “A” as meaning “aircraft battery” or “best battery.” Here, “A” is simply the old radio-circuit label for the battery that powered the tube heaters.
Example Sentence 1
When restoring the wartime radio, the technician confirmed the A-battery was supplying the correct filament voltage before applying plate power.
Example Sentence 2
In a restored 1950s aircraft, the A-battery provided six volts to keep the tube filaments warm.