MadSci Network: Engineering

Re: Can floresent bulbs be lit by the proxcemity of electricity?

Date: Tue Jan 2 22:48:09 2001
Posted By: Edward Peterson, Staff, Chemical Engineering, S&B Engineers and Constructors
Area of science: Engineering
ID: 977893751.Eg

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

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