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Name: Jenny
Status: other
Grade: other
Location: N/A
Country: N/A
Date: 6/28/2005

Why does an ordinary fluorescent tube glow when held in an high voltage electric field (for example, near a Tesla coil)? Do you think there is enough mercury vapour present in the tube and that the tube's filaments are warm enough at room temperature for electrical discharge to occur?

Enough energy is present from that electric field to excite the gasses contained in the tube. Ordinarily, those gasses are excited by the electrodes at either end of the tube to produce light. Under the influence of RF fields though, those gasses are excited without the need for the electrodes, and produce light without needing to be "plugged in".

Ryan Belscamper

In a Tesla coil field, the filament does not need to be hot. A fluorescent tube is a very "soft", easily-conductive medium to that field. And a Tesla coil invisibly reaches out through space more strongly than you are imagining.

The resonant frequency is at least 1000 times higher than 60Hz, so if a plasma gets started, it cannot quench itself during each zero-crossing of the sine-wave, as can happen with 60Hz. It can quench itself between "zaps" of the Tesla coil, but apparently that does not matter much.

If the terminals are touching something, the voltage is high enough to make sparks in air. Initiating plasma in a partial vacuum such as the Mercury/Argon vapor in the tube is much easier. Then electrons can leave the electrodes just by high electric field and ion bombardment. Filament heat is not needed.

The Tesla-coil's voltage is very high and the current is low, so the impedance of this power supply is exceptionally high. The frequency is high, so the impedance of parasitic capacitances across empty space is lower than what we are usually familiar with, almost as low as this supply.

So a large fraction of the Tesla-coil's energy can travel as capacitive induction, charge displaced in empty space by the electric field. Then our electric current is so pervasive that metal terminals are not really needed. An all-glass tube with any low-pressure gas sealed inside would glow too. A fluorescent tube with metal electrode-ends might draw stronger sparks to the ends and light up brighter than my all-glass tube at those moments, but I am not sure of even that.

Tesla-coil energy has notably different behaviors from standard wired electricity.

If it is high-voltage DC, it does not cross space so easily. Your fluorescent tube would not light up without a complete circuit, and my all-glass tube would be useless. DC current would require conductor-contact, or visible sparks, at both ends.

Jim Swenson

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