A Faraday cage (or Faraday shield) is an enclosure made of electrically conducting materials—such as solid sheet metal or a continuous metallic wire mesh—designed to block external static electric fields and non-static electromagnetic radiation from penetrating its interior volume. Invented in 1836 by English scientist Michael Faraday, this device embodies the principle of electrostatic shielding. Faraday famously constructed a twelve-foot metal-coated wooden room, mounted it on glass insulators, and demonstrated that when intense high-voltage electrical sparks were discharged against the outside of the room, sensitive electroscopes placed inside detected no trace of electrical charge or disturbance.
The physics governing a Faraday cage differs depending on whether it is shielding against static electric fields or dynamic electromagnetic waves. In the case of a static or slow-moving electric field, the shielding mechanism is explained by Gauss's Law and the fundamental behavior of free conduction electrons in conductors. When an external electric field is applied to a conductive enclosure, the mobile electrons within the metal rapidly redistribute across the exterior surface: electrons migrate toward the positively charged external side, leaving a deficit of electrons (net positive charge) on the opposite side. This induced surface charge distribution creates an internal electric field that precisely cancels the external field throughout the interior cavity, ensuring the net electric field inside remains strictly zero (E=0).
When shielding against dynamic electromagnetic waves—such as radio waves, microwaves, or electromagnetic pulses (EMPs)—the cage operates through reflection and absorption governed by the "skin effect." High-frequency electromagnetic radiation induces eddy currents in the conductive skin of the cage, which reflect the incident radiation while remaining energy is attenuated as it penetrates into the metal's skin depth. A solid metal sheet is not strictly necessary; a wire mesh functions effectively provided that the mesh openings are significantly smaller than the wavelength of the radiation being blocked. In modern society, Faraday cages are applied everywhere: in copper-shielded hospital MRI rooms, microwave oven door screens, lightning-safe aircraft fuselages, and cyber-forensic bags that prevent remote data wiping of seized smartphones.