Interelectrode capacitance

Any two pieces of conducting material, when they are brought near each other, will act as a capacitor. Often, this interelectrode capacitance is so small that it can be neglected. It rarely amounts to more than a few picofarads. In ac circuits and at audio frequencies, interelectrode capacitance is not usually significant.
But it can cause problems at radio frequencies. The chances for trouble increase as the frequency increases. The most common phenomena are feedback, and a change in the frequency characteristics of a circuit.
Interelectrode capacitance is minimized by keeping wire leads as short as possible. It can also be reduced by using shielded cables and by enclosing circuits in metal housings if interaction might produce trouble. This is why, if you’ve ever opened up a sophisticated communications radio, you might have seen numerous metal enclosures inside the main box.