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RE: [TCML] Tesla's topload
Ok, so after a careful analysis of Tesla's patent #1,119,732, I think
I've determined the true reason for Tesla's use of the half spherical
metal plates.
He hints around the usability of separate elements with a small radius
of curvature that are spaced close together:
"I employ a terminal of relatively small capacity, which I charge to as
high a pressure as practicable. To accomplish this I have found it
imperative to so construct the elevated conductor so that its outer
surface, on which the electrical charge chiefly accumulates, has itself
a large radius of curvature, or is composed of separate elements which,
irrespective of their own radius of curvature, are arranged in close
proximity to each other, and so, that the that the outside ideal surface
enveloping them is of a large radius."
And thus the small diameter ring toroid and other designs are given the
ok.
But he doesn't give the specific reason for using the half spheres.
He does later though, while describing the diagram.
He describes the 3rd coil, and then a connecting metal cylinder that
connects the 3rd coil to the elevated terminal, and specifies that the
diameter of that cylinder should be "of a radius much larger than that
of the half spherical elements"
So what I am gathering here is that the half spherical plates, even
though they are a part of a larger outer surface, will still tend to
have their own electrical field shape that, relative to their small
radius of curvature, is sharper than that of the metal connecting
cylinder, and thus a streamer is more likely to break out from them than
it is from the cylindrical connector.
It ties in with his desire to avoid the "inconceivable violence" that he
mentions will take place should the points of maximum pressure be
shifted below the elevated terminal.
Although I am curious...if we are to use a toroid constructed of the
smaller diameter rings...is the capacitance the same as a equally sized
solid surface toroid? If so, it this only true at high voltage? He was
really pushing the use of the smallest possible capacity and charging it
to the maximum potential.
Does a multi ring toroid have less capacitance but still have the same
potential before breakout occurs?
I'm not an expert on capacitance, but I'm assuming that the surface area
of the electrodes, the dielectric constant of the insulating material,
and the thickness of the dielectric determine the overall capacitance.
Do the rules change when we're dealing with high voltage?
I'm trying to visualize it all in my head...I'm sure you genius peeps
out there can fill in the gaps.
Have a great weekend all!
Shannon Weinhold
"An inexpensive instrument, not bigger than a watch, will enable its
bearer to hear anything, on sea or land, music or song, the speech of a
political leader, the address of an eminent man of science, or the
sermon of an eloquent clergyman, delivered in some other place, however
distant. In the same manner any picture, character, drawing, or print
can be transferred from one to another place."
~Nikola Tesla - "The Future of the Wireless Art" 1908
(how did he know?)
-----Original Message-----
From: Derek, Extreme Electronics [mailto:tesla@xxxxxxxxxxxxxxxxxxxxxxxx]
Sent: Thursday, April 07, 2011 8:58 AM
To: Tesla Coil Mailing List
Subject: Re: [TCML] Tesla's topload
Carl,
If we assume that corona as composed of small streamers and "The
appearance of streamers will increase the capacitance". Then surely the
corona will increase the capacitance. I also think that as a finite
element toroid appears to have roughly the same capacitance as a
similarly sized traditional one the field must account for the extra
charge handling capacity. As I said earlier I suspect this is a small
effect and possibly very small on a regular toroid. But on a finite
element toroid or one designed with bumps maybe the effect is larger..
I totally agree regards the breakdown voltage of a smaller radius, and I
don't think irregular toroids will be the next great improvement in
tesla coiling, but I'd like to prove that there was some change in
capacitance and of what scale it was..
RE: Streamer loading IIRC Steve Connor and Terry Friz did some tests a
while ago regarding the capacitance and loading of streamers, have a
search in the TCML archives or on 4hv for the details, Terrys rule of
thumb for streamer loading was 220k + (1pF/foot).
Cheers
Derek
On 07/04/2011 15:50, Carl Noggle wrote:
> Hey---
>
> The capacitance of any object is the effect of the electric field
> between the object and the rest of the universe. If we visualize the
> EF of a toroid, the nature of the surface will be seen to have little
> influence on the whole EF. A rough surface will cause the toroid to
> light up with corona at a lower voltage, since the EF writ small at
> any part of the toroid is what determines the breakdown voltage at
> that point or area.
>
> Since the breakdoown field for air is about 25 or 30 kV/cm (depending
> on altitude), a smooth sphere will have a breakdown voltage of 30kV
> times the radius. A toroid will have a breakdown voltage of about
> twice that of the sphere if the "tire" radius is the same as the
> sphere. That is, a clean, smooth toroid with a 5cm "tire" radius will
> have a breakdown voltage of 30 x 2 x 5, or 300 kV. Any small
> imperfection will reduce the breakdown voltage, usually drastically,
> as the local EF at the point is much higher than the EF at the smooth
> surface of the toroid. Therefore a corona will form at that point at
> a lower voltage. However, this effect will not change the toroid
> capacitance.
>
> The appearance of streamers will increase the capacitance. The plasma
> channels of these streamers probably have relative high resistivity,
> so this adds energy dissipation in the sparks (which is why they tend
> to rise). The advantage of this is that it lowers the Q of the
> secondary tuned circuit, so that the tuning of the coil becomes less
> critical. This is why TCs usually seem to work pretty well over a 10%
> or 20% primary tuning range.
>
> Has anybody actually measured this change in Q, or does anybody know
> the resisitivity of the spark channels? These things would be very
> interesting to know. The sparks in the main switch gap have very low
> resistivity, and like lightning their resistivity may be lower than
> copper or silver, so the primary Q should be quite high.
>
> ---Carl
>
>
>
>
>
>
>> Phil,Dave, All,
>>
>> My gut feeling is that the effect is quite small, especially with
>> smooth top loads (spheres/toroids). Because of this, any breakout
>> loading will overwhelm the additional field created capacitance.
>>
>> As it happens, the coil that I am building at the moment is a
>> classic SSTC with a non-smooth toroid design. As it utilises base
>> current feedback, I'm hoping that running the coil with no visible
>> breakout, I will see a resonant frequency change as the bus voltage
>> is increased, indicating a change in the capacitance of the top-load.
>> This should prove or disprove the theory.
>>
>> Also as Tesla coils are an AC system, it would also suggest that
>> the capacitance of the top load would vary over the period of the
>> driving frequency, with zero crossing having minimum capacitance and
>> peak voltage points having more. This should be seen in the shape of
>> the field waveform measured in open air near the coil. A comparison
>> to a similar sized but more conventional topload would help to
>> eliminate any driver/coil related waveform shape irregularities.
>>
>> Cheers
>> Derek
>>
>>
>>
>> On 06/04/2011 23:14, Phil Tuck wrote:
>>> Derek, et al
>>> If it did then people would have noticed an anomaly in the tuning
>>> figures by now I think. We know that best performance generally
>>> occurs when the primary is set to resonant lower than the secondary
>>> because of the streamer loading and this normally is by an amount of
>>> around 1.5pF per foot. However, if what you say were right, then
>>> would we not have noticed additional factors at play on the bigger
>>> coils that could not be accounted for purely by streamer loading
>>> alone?
>>>
>>> Regards
>>> Phil Tuck
>>>
>>> www.hvtesla.com
>>>
>>>
>>>
>>> -----Original Message-----
>>> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
>>> Behalf Of Derek, Extreme Electronics
>>> Sent: 06 April 2011 05:49
>>> To: Tesla Coil Mailing List
>>> Subject: Re: [TCML] Tesla's topload
>>>
>>> This does raise an interesting question, does the capacitance of the
>>> topload increase as the voltage on the topload increases because the
>>> field around the topload adds to the effective external dimensions
>>> of the topload, so increasing capacitance ?
>>>
>>> Derek
>>>
>>>
>>> On 06/04/2011 01:48, bartb wrote:
>>>> I dunno, it would be interesting to model: say of sphere's with
>>>> diameters equal to a corresponding toroid minor diameter, but with
>>>> the spheres spaced apart so that 1/2 of the diameter of each sphere
>>>> intersects the other creating large bumps of sorts. Unfortunately
>>>> not something Javatc can do as the topload sphere's would have to
>>>> be offset from center (not something it can do now). Possibly Paul
>>>> or Antonio's software could do a quick and dirty comparison.
>>>>
>>>> Take care,
>>>> Bart
>>>>
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