We rely on information reported from past UFO sightings. Reports that detail major electrical and magnetic disturbances. See bibliography at the end of this article.
Various electromagnetic disturbances have been reported along with UFO sightings. For instance, a running car engine will stumble, cough, and stall. Magnetic compasses have been known to swing wildly around and around during UFO sightings. If this information is accurate, we can use these side effects to make a UFO detector.
Using the magnetic anomaly side effect is not a new idea. There are a number of simple plans around the Internet that use a magnet suspended on a conductive wire through a ring terminal, see Figure 3. When something causes the magnet to move, the conductive wire touches the ring terminal completing an electric signaling circuit that sounds an alarm or causes a light to turn on.
A simple but far more sensitive UFO detector, employing the same principle, can be made from a standard magnetic compass/sensor. You place the compass down, away from any magnetic fields, electric fields and ferrous materials. Let the compass needle settle down and point to Earth's magnetic north. The compass-detector is now set, you sit and watch the compass needle. If the needle starts jumping around and swinging wildly you may have detected a magnetic anomaly that could signal a UFO event. However, just because the needle started jumping around doesn't always mean it's detecting a UFO. You first have to rule out a number of common reasons that could cause the compass needle to act erratically.
UFO Detector Kit Page