Suppose I have a capacitor and I want to observe its charge decay over time. How can I do that without affecting its discharge rate through measurement?
AFAIK a typical voltmeter runs current through a known resistance to determine voltage, but in the process this would discharge the capacitor being measured. With increasing complexity one could reduce the current required to make an accurate measurement, and then reduce the frequency of measurements, but in the limit the measurements will still drain some voltage.
In the hydraulic analogy it's possible to measure the pressure (voltage) by putting a spring gauge on a piston impinged by the two sides of the reservoir. No water flows from one side to the other, but we get a constant reading of the pressure.
So is there a meter, mechanism, or circuit that can do that for voltage on a capacitor or other power supply?