Faraday cages

A Faraday cage is an enclosure of conductive material that blocks electromagnetic fields.

The mesh or solid metal shell distributes electrical charge around its exterior, shielding the interior from outside interference—or preventing signals from escaping.

Microwave ovens use the principle to contain radiation; the mesh in their doors is a Faraday cage.

Sensitive electronics are tested inside them, and tinfoil-hat conspiracy theorists sometimes invoke them: He lined his room with aluminum foil, trying to build a Faraday cage against government surveillance.