|MadSci Network: Physics|
Greetings: 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
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