Re: Capacitors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gregory Hunter by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ghunter31014-at-yahoo-dot-com>

Hi John,

No, your idea does not sound stupid--quite the
opposite. It would be very useful to have "on the fly"
tuning capability, so one could peak out the spark
length while firing, if only there was a way to do it
safely! I like your slide cap idea with two concentric
tubes. I have often contemplated similar ideas. This
could be implemented cheaply with regular hard copper
water pipe from a hardware store. The smaller tube
could be sheathed in PE tubing, which is also sold by
the foot at many hardware stores. You wouldn't get
much capacitance--maybe only a few pF for a reasonable
sized cap, but perhaps enough to act as a trimmer if
you already had the coil tuned pretty close using the
primary tap. Now if you could find a way to tube the
cap from a safe distance--perhaps a loooong handle
made from a length of PVC pipe?

BTW, others have used an un-coupled, outboard tuning
coil in series with the primary coil for this purpose.
A few turns of Cu tube wound into a helix, and
slightly compressed or stretched like slinky to fine
tune the tank circuit, for example. For "hot" tuning,
again--you'd need a way to adjust this tuning coil
from a safe distance.

Best regards,


--- Tesla list <tesla-at-pupman-dot-com> wrote:
> Original poster: "John Richardson by way of Terry
> Fritz <twftesla-at-qwest-dot-net>" <jprich-at-up-dot-net>
> 
> Hi Everyone,
>  
> Initial disclaimer:  This may sound stupid!  That
> said, here goes.
>  
> I know that old radio tuning capacitors work with an
> air gap and sets of
> interfacing plates, and that makes them useless for
> our use.
> But, would it be feasible, or more importantly,
> useful, to have the ability to
> vary capacitance real time while operating?  I can
> see the advantages in
> regards to utilizing different toroids and power
> sources.  Anyhow, this is what
> I'm thinking:
>  
> Two brass or copper tubes, one slightly smaller than
> the other, so that one has
> the ability to slide in and out of the larger one. 
> Over the smaller of the
> tubes, a dielectric of some type, maybe super thin
> walled lexan(?)  One of the
> tubes would remain stationary, and plastic
> all-thread would be attached to the
> mobile portion via an end cap with corresponding
> threads, so that as the rod is
> turned the one "plate" is allowed to move within the
> other.  The whole assembly
> could be oil immersed, with the "crank" coming out
> of the top of the container
> to eliminate sealing problems (as opposed to exiting
> on the container side). 
> What would the efficiency of a design such as this
> be?  What are the
> possibilities of finding a decent dielectric, and
> how thick would it need to
> be?
>  
> Thanks for entertaining my ramblings.
>  
> John Richardson
>  
> 
> 
> 


=====
Gregory R. Hunter

http://hot-streamer-dot-com/greg