Compass bell. The compass is capable of remarkable accuracy. I've compared it against my orienteering compass and iPhone compass.

But as folks above noted, even on my aluminum frame bike the steel handlebar on which the compass was mounted threw off the reading just enough to be useless. I suppose if I was determined to use a conventional compass I could swap to an aluminum handlebar. My other bikes have steel frames, too much for accuracy with a conventional compass mounted on the bike.

Now that I've been using phone apps for a couple of years I'm not sure I'd go back to paper maps and a compass unless I planned to do some serious backwoods exploring. One app I've used quite a bit, Cyclemeter, can be set with audio feedback to tell me my direction, among other info, every minute or at longer intervals of time and distance. Very handy when I'm exploring unfamiliar routes and don't want to pause to consult a map app. The direction audio feedback at intervals is handy at night or heavily overcast days when I can't get a quick fix on my heading.