I know I've seen the symbol before, it's something like a circle with an open triangle facing in the direction of current flow with a line through the middle. I'm specifically thinking of the transducers in electric guitar pickups. I've seen the symbol before but now I can't find it.
Motional Pickup Transducer
Use only if a more specific symbol is not applicable, e.g., tachometer generator, microphone, motor, loudspeaker, etc.
2.11.1 General, electrical output