Posted Tue, 05 Dec 2023 18:40:17 GMT by Balmer, Robert
I am trying to measure low level (pA) electrostatic current with me 6517B  electrometer. The electrostatic current is generated by a plate moving in liquid petroleum. The electrometer is mounted on the moving plate so no cables flex during the motion (see attached illustration). Obviously there is a varying capacitance in this arrangement. When the experiment is run without a liquid present between the plates the 6517B shows (detects) a varying current, but there should be no current in this situation. Is the electrometer producing a false reading that results from the varying capacitance?
Current is the rate of flow of charge, the relation for the charge in the capacitor is: Q = CV.  If we differentiate this relation with respect to time, we get a relationship for current (i) through the capacitor. i = dQ/dt = d(CV)/dt = C(dV/dt) + V(dC/dt).
So, if there is a voltage across a capacitor and the capacitance changes (e.g., the distance between the capacitor’s plate’s changes) then a current will be induced.

Is this what is happening?

Posted Sun, 10 Dec 2023 14:51:23 GMT by C, Andrea
Have you evaluated the 6517B response to the motion/vibration?  I suggest to remove any cable at triax connector and put metal cap on triax input to shield it.  Use foil if you do not have metal triax cap.  Run your test.  Is 6517B unaware of motion?

But pertaining to your question, in amps mode, the 6517B is essentially a dead short between HI and LO.  For static situation, if charge is stored on a capacitor and a voltage results, when you connect amps measure….big whoosh of charge, and voltage depleted.
I would not expect appreciable voltage potential to develop while amp meter is attached.  The voltage burden spec tells you the max expected value.

is the setup well shielded?


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