>>>A question: are variable capacitors actually in use? They seem not
nearly so common as variable resistors.
Variable capacitors tend to be very "physical," therefore very expensive and
a general nightmare to work with. So in an RC type circuit where you can
vary either one, the resistor ends up being a MUCH simpler proposition. It
sure would be handy to have variable caps for some applications though, if
they were as easy to get & use as pots.
Variable capacitors (at least all the ones I've seen) tend to be rows of
semi-circular plates mounted on a spindle, and that hangs over another set
of fixed plates in such a way that the two dovetail alternately. Turning the
spindle moves the variable plates in or out of the others, sort of like
moving nuclear control rods up or down, if you'll buy that analogy.
The problem is that to get any real capacitance, you need a giant set of
plates. I had one of these in an ooooold radio and the thing weighed about
15 pounds for that one part! It was huge! Yet I think it was a relatively
small capacitance.