)The HP 4140A instruments do well to below 10^-12 A.
Thanks, Win, that's very kind of you. I actually have a few instruments tha t can do that. The best is probably a box I made long ago with some Russian 100G glass resistors and an LM660AN CMOS amp in PDIP, but the one I reach for is a Keithley 610C, which has a 10 fA FS range--completely useless, but still.
My sentimental fave is a Keithley 410, which takes a couple of hours' warmu p before it's 100 fA FS scale is useful, but then it's from 1963 (when I wa s 3) and uses a tube.
Then there are a few many-digit DMMS whose lowest voltage ranges make prett y decent picoammeters.
Cheers
Phil Hobbs