Wikipedia
A Faraday cage or Faraday shield is an enclosure used in order to block electric fields. It is formed by conductive material or by a mesh of such materials. Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836. 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 static 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 also provide less attenuation from outgoing transmissions versus incoming, they can shield EMP waves from natural phenomena very effectively and a tracking device, especially in upper frequencies may be able to penetrate from within the cage. i.e., 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 steel cages generally provide better attenuation over mesh cages.