Curiosity forced me to look around a little more for info on the 640.
The first link includes some info on the 640 with pictures. The
vibrating capacitor thing looks like a sort of vacuum tube. I thought
maybe it was custom made by or for Keithley, but it seems to actually
have been an off-shelf commercial product. The second link shows
pictures of what it likely is - XL7900, made by Philips back then. The
third link is to the data sheet that explains a lot.
https://doc.xdevs.com/doc/_Metrology/femto_ampere_current_source.pdf
http://lampes-et-tubes.info/sp/sp003.php?l=e
http://frank.pocnet.net/sheets/009/x/XL7900.pdf
I also found some info on the Varian/Cary 401 vibrating reed type
electrometer. It seems it was very common and used in a lot of
applications from the late 1960s on. From sparse technical info and
pictures, it appears to be actually built up from parts, into an
interesting machine. I could not find any manuals (without signing up
for some kind of subscription at document websites) or detailed info.
Ed
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