Robert Balmer

I have a charged liquid stream flowing into an insulated metal container. My Keithley 6517B is connected to the container to measure  a pA current produced by the charged liquid stream. The stream is constant so the current should be constant, but the electrometer shows the current decreasing in time. Does the electrometer discharge the container as it measures the current? Does it discharge the current faster than the liquid stream can replenish it? Why else would the electrometer show a decreasing current?

By the way, the technical support "ticket" on the Tektronix website doesn't work!!!
 

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?