In early radio and electronics, the A battery provided the low voltage and high current necessary to heat the cathode filament inside a vacuum tube, causing it to emit electrons. It was used alongside B and C batteries, which provided the plate and grid voltages respectively. These were typically lead-acid or large dry cells.
Heard on the job
I found an old schematic that calls for a 6-volt A battery to run the tube heaters.
Insider tip
The letter designations (A, B, C) for tube batteries were mostly abandoned as AC power supplies replaced them, though the terminology survives in historic radio restoration.