Date: Mon, 18 Feb 2002 10:31:13 -0600

Author: "Karl I. Trappe"

Subject: Re: VDG/Tesla coil


My experience, and explanation, is similar to Dick Berg's.

I have always explained that the body provides the coupling to
ground. It is either across the surface of the body because of the
high frequency nature of the tesla coil (resistive coupling to
"ground"), or it is through the body's capacitance to ground. Either
way, the body is at a lower potential than the closest electrode to
the VDG, and the fluorescence of the lamp is substantially better
with the body coupling the other end to ground. Karl

>Indeed there is a bit of discharge if the tube is just hung out on the hot
>ring with nothing attached to the other end. The current through the
>tube ultimately goes out the other end and sprays around in the air.
>On the other hand, when you put your hand onto the tube it gets much
>brighter, and you can "play it like a trombone." The brighter part is
>between the high voltage and your hand. I would think that this
>indicates that the current likes to have somewhere to go other than up the
>tube. Your hand acts as one plate of the capacitor, drawing current from
>you or through the air surrounding you.
>What is the logical error here?
>Dick Berg
>On Fri, 15 Feb 2002, Steve Anderson wrote:
>> Reply to: Re: VDG/Tesla coil
>> Geoff Nunes wrote:
>> >Not in a situation like this. Replace the bulb with something
>> >conceptually simpler: a capacitor. Connect one lead to the Telsa coil
>> >and leave the other floating free. That free end will have a fairly
>> >good sized oscillating charge on it. The bulb is like that. The coil
>> >shoves a bunch of charge to the far end of the tube and then yanks it
>> >all back, generating some light along the way. No connection to "earth"
>> >required.
>> Thank
>> s Geoff, that summarizes it well, now it is a capacitance demo, too!
>> The frequency and intensity of the discharge depends on where down
>> the tube you hold it. Shorter length, faster sparks with less
>>pop. Holding a longer length down the tube gives slower, bigger,
>>brighter strikes.
>> The discharge is between your hand and the point on the bulb
>>closest to sphere.
>> I insulate the metal on both ends (heat shrink caps tied with tye-wrap)
>> and touch the tube about 6-8 inches in. You can play it like a trombone.
>> There is also distinct tug and then repel feeling, when it fires.
>> That hypothetical plate capacitor would probably physically ring a
>>lot with that free end.
>> s.anderson
>> CSU Sonoma
>Dr. Richard E. Berg, Director Phone: (301) 405-5994
>Lecture-Demonstration Facility FAX: (301) 314-9525
>Department of Physics e-mail
>University of Maryland
>College Park, MD 20742-4111
>If you think education is expensive, try ignorance.
> Derek Bok,
> Former President, Harvard University

Dr. Karl I. Trappe Desk (512) 471-4152
Lecture Demonstration Office Office (512) 471-5411
Physics Department, Mail Stop C-1600 Home (512) 264-1616
The University of Texas at Austin
Austin, Texas 78712-1081