So... I'd like to understand guarding for measuring high resistances, e.g. with the electrometer, where there is a convenient guard voltage available, and delivered to my test chamber on the inner shield. I'm looking through Low Level Measurements Handbook, 7th edition, at figure 2-16b. It shows a resistance measurement test chamber, with the guard voltage tied to a base plate, and two leaky standoff insulators holding the resistor under test.
If the base plate is at HI force, then there is no potential difference at the first standoff, and no current through it.
But the second standoff is sitting on the HI force voltage at the base and the top of it is at LO force /ground. This seems like a potential difference that would drive a current through the second standoff leakage (upward) and join the measurement current through DUT. The second standoff leakage resistance is effectively in parallel with the DUT, is it not? 4-wire would fix it, but with this 2-wire connection, it seems wildly wrong to me.
And then, a similar but opposite situation exists if the base plate is at ground.
I always first I assume I'm wrong. What am I missing? Is that the right way to configure a simple 2-wire high-value resistance test chamber?