Re: Catching up, + help!

 * Original msg to: usa-tesla-at-usa-dot-net

Quoting James Peters <jpeters-at-enterprise.powerup-dot-com.au>:

> Hi Richard!,

> Long time no chat! Hows it been going? 

Moving again. Hopefully it will be the last time in the
foreseeable future. All of that iron, copper, and oil...
I easily have over a ton of Tesla equipment, easily. I threw 
away hundreds of pounds of stuff two years ago when I moved
last. I am getting a house with a full garage and basement. 
Knowing I would have room for larger setups and more storage,
I must have accumulated back the same poundage that I scrapped
two years ago. Only the quality of the junk has improved with the
turnover. I will be back to burning barns again in short order.

> Anyway, back to the point. I haven't used my coil for some time
> as I blew my last capacitor. I'd like to make a bottle cap 
> until I have enough money to pay for some polyethylene. Some 
> questions:

> 1. What is your personal opinion on bottle caps? (in terms of 
> quality, loss etc)

As far as quality goes, they suck. When you factor in the cost,
time, and availablity of construction materials, their image
improves. I know of more than one coiler who used a brine/glass
capacitor to get them sparking on their first serious project. 

> 2. How does glass compare to mylar as a dielectric? (in rf 
> applications) I don't have access to these figures.

As far as RF dissipation factors are concerned, even Mylar, as
poor as it is, beats bottle or plate glass. There are specialty
glasses that have acceptable dissipation/thermal properties in RF
applications, but you will not find bottles molded from it. If
you have access to PYREX containers (i.e. flasks) this would be
an improvement over bottle or plate glass when RF losses,
dielectric constant, and dielectric strength are considered.

> I think I will make the caps out of HDPE bottles wired in 
> series. 

This is a better dielectric when looking at dissipation factors,
but because the dielectric constant of plastic is so much lower
than glass (2 vs 7), the size of the capacitor grows pretty
large, nearly four times so. As the size increases you come
across another problem in brine capacitors; the plate mass and
surface area go way up. This mass is electrically part of the
tank circuit, it's like having many pounds of conductor wired in
that is not doing anything. The large surface area of brine means
high corona losses and much lower Q factors. It is hard to get a
good trade off with brine caps. This is not to say it is
impossible with a good design and the right dielectric.

> suggestions?

A good brine capacitor works fine for a coiler who is looking for
some spark pending a capacitor upgrade. It is better to build and
run a brine cap than not to fire at all for lack of a capacitor.
Since they are very cheap and easy to build, they should not be
ignored by the novice-intermediate coiler. 

I was just out at the convenience store this evening getting
washer fluid for the car. Having just written a short paper on a
brine/bottle capacitor yesterday, I noticed some 1 pint long neck
bottles of beer sold as singles. A green ST PAULI GIRL came home
with me. I think I could make a nice PORTABLE salt-water/glass
capacitor with these bottles inside of a 5 gallon plastic bucket.
With everything else that is going on now I don't have time to
mix up a batch of brine and measure the capacitance of the
bottle, but judging by eye, it should work pretty well. I should
think you could get a .01 at 15 kvac rms in a five gallon bucket.

> I finally have a toroid. It was free! It is a commercial 
> aluminium one that comes off the top of a van de graff 
> generator. This should combine the advantages of a sphere and 
> toroid discharge terminal as it is dome-  shaped. I got it from
> the physics department at school. ideas, suggestions? 
> Oh yeah, it's a 16" outside diameter.

Use it. If the sparks tend to leave to discharger upwards rather
than to the side, then make a true toroid. I know the commer-
cially made units look spiffy in the fit & finish dept., but the
homemade toroids work every bit as well for pennies on the

> I now have an industrial compressor. I think I'll make an air 
> blast gap. 

Upgrade your capacitance before you invest time building an air
blast gap. Air blast and other fast quenching gaps eat second
rate capacitors for breakfast. 

> Thanks for your time and trouble, Now and in the past!

No problem!

Richard Quick

... If all else fails... Throw another megavolt across it!
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