"John A" wrote in
:

You cannot buy variable capacitors with these values, but you could
make up or purchase an arrangement of switches and fixed capacitors
quite cheaply which a) would be the next best thing and b) would
directly address your "how do you measure the capacitance" question.
Commercially they are called Capacitance Substitution Boxes or
Capacitance DecadeBoxes . They're not infinitely variable, of course,
but are finitely practical!


Nice. I'm not sure that's what the OP wanted, (more likely a single
continuous control of something), but if this switched-cap box were built
to 1 nF resolution, for $40 extra or so, you can add a variable capacitor
shown on the page David linked to: http://www.stormwise.com/page3.htm

That way you can have any infinitely variable value, just not in one sweep.