Faraday cage

A Faraday cage, also called a Faraday shield, is an enclosure made from conducting material. When an external electric field is applied, mobile charges in the conductor redistribute until the field inside an ideal closed conductor in electrostatic equilibrium is zero. The rearranged charges reside on the conductor's surfaces; for a real enclosure, construction details and openings affect how well the interior is shielded. A cage therefore redirects or attenuates fields rather than making electromagnetic energy disappear.

Conducting enclosures can also reduce electromagnetic interference from changing fields and radio-frequency waves. Their performance is not universal: it depends on the frequencies involved, the conductivity and thickness of the enclosure, seams, grounding and the size and shape of apertures. A mesh can function as a shield when its openings are sufficiently small for the waves of concern, but a cage should not be assumed to block every frequency or field equally.

A common classroom demonstration places an electroscope or other field indicator inside a conducting enclosure and compares its response with one outside when charge is applied. Sensitive laboratories use shielded rooms or enclosures to reduce unwanted electromagnetic signals in measurements. Conductive vehicle bodies can also help protect occupants from lightning by carrying much of the external charge over the vehicle's exterior, although safety depends on remaining inside the enclosed body and avoiding contact with conductive parts connected to the outside.

The device is named for the English scientist Michael Faraday. In 1836, Faraday demonstrated electrostatic shielding by entering a metal-lined enclosure and showing that its interior was protected from external electrical influence. The experiment helped establish how charge redistributes on conductors and how a conducting enclosure can isolate its interior from electrostatic fields.