One more thought for the morning...
A fixed plate capacitor could be made variable by immersing it in a
combination of (pure) water and silicone. Those fluids won't mix. And
by controlling the level of the interface between the fluids at the
plates the capacitance could be varied over about a 40-1 range, as the
dielectric constant varies between 2 and 80. This arrangement could be
totally sealed -- no rotating shaft to worry about.
Unfortunately this adds one more constraint -- operating position. And,
of course, this contrivance would most certainly not be suitable for
mobile applications! :-)
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