|MadSci Network: Engineering|
You have heard correctly. Fluorescent bulbs can be lit with electrical fields, even when the bulb is not attached to anything. Alternating current works best. The energy needed to light the bulbs with direct current could be dangerous. The alternating current develops a waveform in space. If the waveform is energetic enough, the mercury vapor in the bulb will ionize, forming a bright light. The bigger the bulb, the higher the current required. Long bulbs work better than round bulbs. There are two other places you can see this effect. The first is somewhat dangerous and hard to reach. If you were to stand under a high power transmission line with a fluorescent bulb pointing straight up, most of the time, the bulb will light. The light comes from strong magnetic fields that build up around electrical lines and encircle the lines, which themselves, cause an electrical field that has its center at the wire. The long fluorescent bulb crosses a physical distance that is long enough to create a large enough voltage difference between the ends of the bulbs to light the bulb. Some people have used this to demonstrate the harmful effect of alternating current, but not everyone believes that low frequency alternating current is physicaly harmful to living creatures. The second way to demonstrate the use of an electrical field to light a fluorescent bulb is to use your home microwave oven. This is also potentially harmful, since the fluorescent bulb will get very hot very fast, and the bulb may burst. The contents of such a bulb are not healthy. I have done the experiment in a laboratory in a hood and I recommend that you do the same should you decide to do it. First, buy a circular fluorescent bulb that will fit in your microwave oven. The bigger the wattage the better. I recommend 200 watts and nothing less than 100 watts. Put the bulb in a Pyrex or microwave safe glass container that is then covered should the bulb break. Set the microwave to turn on for two to five seconds. While the oven is on, the bulb will glow brighty. You may notice that the bulb is especially bright in some spots and almost dark in others. If that is the case, you are seeing the presence of the actual field pattern in the oven in three dimensions. Practice safe science! Dr. Ed Peterson
Try the links in the MadSci Library for more information on Engineering.