[volt-nuts] Vibrating reed electrometers

[ed breya]

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