Just as magnetic influences can have an adverse effect on the accuracy of your compass's readings, so too can electrical influences. Electric cables, tools, radios, speakers, lights or even hairdryers may yield undesirable effects because of the current passing through them. As with magnetic influences, your goal is to eliminate or minimize such interference.

Before permanently affixing your compass, test the circuitry for every conceivable problem. Switch on and off all electrical loads. You're going to do this individually–one object at a time–for anything controlled by a switch located on the instrument panel or near your compass. Fire the ignition. You want all the electrical instruments operating, and don't overlook the auxiliary generator, if you have one. You are actually going to make two full sets of checks, one on each of two cardinal headings that are 90 degrees apart–North or South and East or West.

The way it works is that with your compass anchored temporarily in place in its proposed permanent location, you are going to check to see if the compass card moves as the various objects are turned on and off.

You should also work with the wiring, too, twisting together two wires on a circuit to block the creation of a magnetic field. Twist the wires for your compass light and pair any wires beneath the compass and the instrument panel (also by twisting). Which precise wires are paired is in inconsequential, only the fact of grouping is important to the elimination of electromagnetic influence. Here again, you'll watch the effect on the card for each manipulation.

By the end of testing, you should have eliminated or reduced most influences or have repositioned your compass. There may be some instances in which you'll decide you can work with two separate deviations for a given object–one for when it's on and another for when it's off. It's a value judgment.

- Linda Hoff