MadSci Network: Physics

Re: How can fluorecent tubes glow even when they are not plugged in?

Date: Mon Oct 18 10:52:14 1999
Posted By: Adrian Popa, Directors Office, Hughes Research Laboratories
Area of science: Physics
ID: 939960726.Ph


Fluorescent lamps are made from a glass tube which has an inner surface 
coated with a fluorescent compound. A fluorescent compound absorbs short 
wavelength radiation and re-emits it as radiation with a longer wavelength 
in a very short time. The lamp is filled with a mixture of mercury (Hg) 
vapor and argon (Ar) gas with an overall pressure of about 10mbar. 
Electrical conduction within the gas occurs by means of a small fraction (a 
few percent) of ionized Hg atoms and their corresponding electrons. The 
ionized Hg radiates energy in the ultraviolet region of the electromagnetic 
spectrum, which in turn excites the fluorescent compound to give off white 
light. The Hg atoms are ionized by an electrical field that typically is 
supplied by a battery or by the electrical power system.

Compared to many other gasses, Hg gas is rather easily ionized and the 
electric field required to ionize the gas can come from a number of sources. 
Besides the electrical power system, two other common sources of electric 
fields are those produced by static electricity and electric fields produced 
by radio transmitters. Human bodies can be charged to several thousand volts 
just by walking on a nylon carpet and most of us have seen sparks when we 
touch grounded objects. Because the fluorescent lamp's glass tube is also a 
non-conductor of electricity, it takes many seconds or minutes for a static 
electrical charge to bleed off of our bodies through the lamp causing a 
slight glow produced from the ionized Hg gas. If you touch one end of the 
lamp to a grounded water pipe and hold the other end, the lamp will glow 
brightly for a shorter period of time. 

Some households are close to powerful radio transmitter stations or 
high voltage electrical power lines and their fluorescent lamps will glow 
brightly with out any electrical connection to the mains. In this case the 
lamps Hg atoms absorb electrical energy directly from the electromagnetic 
radio or power line fields. Radio amateurs often check their transmitting 
antennas for power flow by placing fluorescent tubes along the transmission 
lines near the antenna. If the antenna is not efficiently connected to the 
transmitter ("impedance matched"), it is possible to see equally spaced dark 
bands along the fluorescent tube every one half wavelength of the radio 
transmitter frequency where the strength of the electrical fields are at 
minimum. A well tuned antenna will glow brighty all along the transmission 
line without any dark bands (voltage minimums).

Best regards, Your Mad Scientist
Adrian Popa

Current Queue | Current Queue for Physics | Physics archives

Try the links in the MadSci Library for more information on Physics.

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network,
© 1995-1999. All rights reserved.