Gary's account of two bent wires is the method I was taught. The water co in rutherford co tn uses this method to find water lines. If you try it the wires will cross when you walk under a power line or over ground that has been disturbed such as when a trench has been dug and filled in. To make a pair of "witchers" take two coat hangers cut 18 in of wire from each. Bend them into an L shape 6 in on the short side. Hold the wires on the short side very loosely one in each fist. Arms at your side, elbows bent at 90 deg so that your forarms are parallel to the ground. The long side of the wire should also be parallel to the ground and be pointed in a forward direction. the wires should be able to turn freely in your hand when you pass through a magnetic feild it will cause the wires to point toward each other. running water, buried metal and power lines produce magnetic feilds.
Give it a try
Satan is not involved unless he is the originator of magnetism.