The only place I wouldn't use a compass would be in the presence of a lot of electric current, such as standing under a high voltage power line. I just finished building an experiment that puts current through an 1/8" brass rod 6' long (with means to position it anywhere from 2" to 24" off the table top). Then a simple compass is placed underneath it. You would be surprised how the current effects the direction the needle points to.
Last edited by gnoahhh; 04/28/16.
