I saw the other day a Keithley 2636B SMU which has 0.1fA resolution @ 10uV in a "normal" Keithley DMM box size. Keithley also have some systems with external amps that go down even further, to tens of atto-Amp regions. Even the old HP resolves down to 1fA. It made me curious. Dear list: What's the basic principle of such measurements?