Agilent Technologies B1500A User Manual page 118

Installation
Connecting Measurement Devices
Guarding
Guarding reduces the leakage current between the measurement points and
instrument. This is important when you measure low current.
The following figure shows the theory of guarding. The buffer amplifier (1) keeps
the potential of the guard conductor at the same potential as the force conductor, so
current does not flow between the force and guard conductors. Therefore, the
current measured by SMU is same as current at measurement point because no
current is leaked.
3- 44 Agilent B1500A User's Guide, Edition 10