John, and all interested,
In an experiment to determine whether or not a lit fluorescent tube can act as
a capacitor when surrounded with a conductive material, I did the following
experiment.
A relaxation oscillator was constructed; +V of about 110VDC from a 'bandolier'
of 9V batteries put together was supplied through a 1 Megohm resistor to an
NE-2 bulb and capacitor in parallel. The free ends of the capacitor and NE-2
went to the batteries' minus terminal. On the oscilloscope, a nice relaxation
oscillation was observed, varying of course as the capacitor's value was
changed.
A fluorescent light bulb, GE type F15T8-CW, cool white, was used in the test
setup which I will now describe. A 12" wide band of aluminum foil was wrapped
around the glass of the tube, forming a capacitor plate. The idea is that the
ionized gas within the tube would form the second capacitor plate, with the
tube's glass envelope acting as the dielectric.
One end of the 18" long tube was connected to a 6V lantern battery. This caused
the filament at the tube's end to reach a warm orange glow. Half-wave rectified
mains 120VAC was smoothed by a 330uF 250VDC electrolytic capacitor, and the
resulting ~170VDC was applied across the tube via a 15kohm limiting resistor.
When a piezo striker from a barbecue was struck next to the tube, the tube
would glow to moderate brightness. Disconnecting either the rectified mains
voltage or the filament would kill the plasma, extinguishing the tube.
With the tube glowing and the NE-2 relaxation oscillator running, displaying a
clear oscillation on the scope screen, I switched out the capacitor, connecting
the leads that led to it to the tube; one lead to the plasma, the 'grounded'
lead to the foil wrapping. Relaxation oscillation then restarted immediately,
giving a period of 1div @ 2mSec/div, or 500cps. To test whether this was a
spurious effect, I disconnected the mains supply to 15kohm limiter connection,
extinguishing the tube; the relaxation oscillator stopped immediately. Once
restarted, the oscillation returned, same period.
I will try using different current limiters to see if the amount of excitation
of the plasma has any effect on the tube's capacitance.
Judging by substituting known values of capacitors, the fluorescent tube + foil
had an "on-capacitance" of approximately 0.0015uF
--Kyle
