The Faraday Cage

The potential of a conducting surface is the same ever-where. The electrons on a conducting surface are free to move, so they move in such a way as to make the potential the same everywhere on the surface. If the surface is bent so that it encloses a space, then since the potential is everywhere the same on the surface, there will be no potential difference along any line joining points on the surface and no electric field inside the space. This is the principle behind a Faraday cage.