Let's say I had the following laboratory set-up:
Two conductive plates with an electrical potential $\phi$ between them, and a fluctuating potential $\phi$ (created by some arrangement of fluctuating charges) outside the plates. How could I stop the potential outside $\phi'$ the plates from affecting the potential $\phi$ inside the plates?
My educated guesses:
A simple battery. A battery holds a constant voltage, but the relationship between voltage and $\phi$ is complicated: Are the $V$ in electronics and the $\phi$ in physics the same?
A battery in parallel with a diode with a forward voltage equal to the desired $\phi$ between the plates. It's a diode's job to hold a desired voltage between two parts of a circuit. Again it depends on the relation between voltage and $\phi$.
Some device to keep the charge (q) on the plates the same. No idea what'd do that though.