Capacitors

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

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