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Originally Posted by jonobro
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Faraday cage or
Faraday shield is an enclosure used to block
electromagnetic fields. A Faraday shield may be formed by a continuous covering of
conductive material or in the case of a Faraday cage, by a mesh of such materials. Faraday cages are named after the English scientist
Michael Faraday, who invented them in 1836.
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Video of a Faraday cage shielding a man from electricity
A Faraday cage operates because an external electrical field causes the
electric charges within the cage's conducting material to be distributed such that they cancel the field's effect in the cage's interior. This phenomenon is used to protect sensitive
electronic equipment from external
radio frequency interference (RFI). Faraday cages are also used to enclose devices that produce RFI, such as
radio transmitters, to prevent their radio waves from interfering with other nearby equipment. They are also used to protect people and equipment against actual electric currents such as
lightning strikes and
electrostatic discharges, since the enclosing cage conducts current around the outside of the enclosed space and none passes through the interior.
Faraday cages cannot block stable or slowly varying magnetic fields, such as the
Earth's magnetic field (a
compass will still work inside). To a large degree, though, they shield the interior from external
electromagnetic radiation if the conductor is thick enough and any holes are significantly smaller than the
wavelength of the radiation. For example, certain
computer forensic test procedures of electronic systems that require an environment free of
electromagnetic interference can be carried out within a screened room. These rooms are spaces that are completely enclosed by one or more layers of a fine metal mesh or perforated sheet metal. The metal layers are grounded to dissipate any electric currents generated from external or internal electromagnetic fields, and thus they block a large amount of the electromagnetic interference. See also
electromagnetic shielding. They provide less attenuation from outgoing transmissions versus incoming: they can shield
EMP waves from natural phenomena very effectively, but a tracking device, especially in upper frequencies, may be able to penetrate from within the cage (e.g., some cell phones operate at various radio frequencies so while one cell phone may not work, another one will).
A common misconception is that a Faraday cage provides full blockage or attenuation; this is not true. The reception or transmission of
radio waves, a form of
electromagnetic radiation, to or from an
antenna within a Faraday cage is heavily attenuated or blocked by the cage; however, a Faraday cage has varied attenuation depending on wave form, frequency or distance from receiver/transmitter, and receiver/transmitter power. Near-field high-powered frequency transmissions like HF RFID are more likely to penetrate. Solid cages generally attenuate fields over a broader range of frequencies than mesh cages.