hmmm, that seems reasonable. I wonder if a metal-metal version would have a similar capacitance?
I might get a multimeter, play with some foil and wire and try to see if that capacitance is any below the metal-metal version. At any rate that it has capacitance is good and means that IMO it's only a question of how to get the effect not if you can. On Sun, Jun 21, 2009 at 6:35 AM, Kyle Mcallister <[email protected]>wrote: > > 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 > > > > >
