Fluorescent Tube Glow in High Potential Field
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".
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
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
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
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
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
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.
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Update: June 2012