[Arran]

Then there is femtofarad, ten to the -15, 1/1000 of the picofarad. Radios dontdeal with that but some devices do. I have a friend who measures those.

And no one needs Attofarads.

Mike;
  A femtofarad sounds like something that one would use in UHF or microwave devices. where a straight lenth of wire is considered a coil, but that's just a guess. Bob Andersen once found a 1/2 picofarad/mmf capacitor inside a TV tuner, someone took a very small ceramic cap and then ground part of it away to get the value.
Regards
Arran