The Model 261 Picoampere Source is a secondary standard for calibration of picoammeters and electrometers. It is a "passive" source, consisting of a selectable 0 to 10V voltage in series with a specially selected and tested hi-meg resistor.
Keithley 261 Specs
||10-14 A (10-11 A full range) to 1.1 x 10-4 A, positive or negative, in eight decade ranges
||0 to 11 V in 0.01 V steps
||3 significant figures from 10-12 to 1.1 x 10-4 A
||105 to 1012 ohms in decade steps
||Low to ground; > 109 ohms shunted by 0.001 µf