What's that, an output jack? Yes, that is correct. Because it has a steel body, I thought it would be fun to install a magnetic pickup inside it, to pick up the vibrations of the whole soundboard. That way I can still use acoustic strings, and get the benefits of that when it's unplugged.
The pickup is made from an old relay, or solenoid of some sort, that I found on a lighting circuit when rummaging through a skip one day. I wired that directly to the output jack, and attached it to the neck, just under the soundboard.
It works pretty well, and has a rather unique gain adjustment system.
The pickup doesn't actually have any magnets on it. It does pick up a fairly clean signal as is, but I can add volume and distortion by placing magnets on the outside, directly over where the pickup is. It's a lot of fun playing around with magnets of different strengths to see what effects they have. Feedback is surprisingly controllable.