Electricity & Magnetism


People often think that feedback only occurs when a microphone gets near a speaker. Feedback is also a very important mechanical and biological process that is often overlooked in museum exhibits. Whenever one reaches for an object, visual and tactile feedback help the muscles make mid-course corrections so that the hand arrives on target. This exhibit demonstrates feedback by suspending a steel earth globe a small distance beneath an electromagnet. It does this by allowing the globe to interrupt a light beam that is shining on a photocell. When the photocell senses more light the globe has fallen too low and the circuitry turns up the current to the electromagnet. If the photocell sees less light the globe has risen too high and the current to the electromagnet is turned down.