A Faraday cage is an enclosure with no apertures (holes, slits, windows or doors) made of a perfectly conducting material. No electric fields are produced within the Faraday cage by the incidence of external fields upon it or by currents flowing on the perfect conductor; that is, the perfectly conducting enclosure is a perfect electromagnetic shield. If no electrical energy sources are within the Faraday cage, then there will be no electric fields within, since none can penetrate the conducting enclosure.
Since a perfect conductor is an idealization unavailable in nature, perfect Faraday cages do not exist. However, extremely good Faraday cages are constructed for electromagnetic experiments; they are commonly called "screen rooms." Early (and still common) screen rooms were made of copper mesh screen with a somewhat tighter mesh than typical aluminum window screen. Screen rooms are also made of welded sheet aluminum or sheet steel. For all screen rooms, special attention is given to electromagnetic sealing of closed doors and to metallic penetrations for power or communication. Screen rooms are typically designed to shield the enclosed volume from low-energy, high-frequency transmitted electromagnetic waves (e. g., AM/FM radio waves). Since mobile (cellular) phones use electromagnetic waves for transmission, they would be inoperative in such an enclosure... resembling the results of driving through a tunnel with a cell phone...(with partial thanks to Wikepedia).
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