A Faraday cage is a metallic enclosure that prevents the entry or escape of an electromagnetic field (EM field). An ideal Faraday cage consists of an unbroken, perfectly conducting shell. This ideal cannot be achieved in practice, but can be approached by using fine-mesh copper screening. For best performance, the cage should be directly connected to an earth ground.
Faraday cages are used in electronic labs where stray EM fields must be kept out. This is important in the testing of sensitive wireless receiving equipment. In addition, a Faraday cage can prevent the escape of the EM fields emitted by a cathode-ray-tube (CRT) computer monitor. Such fields can be intercepted and translated to allow hackers to remotely view on-screen data in real time without the need for wires, cables or cameras. This practice, known as van Eck phreaking, can also be used by government officials to view the computer activities of known criminals and certain criminal suspects.
A heavy-duty Faraday cage can protect anything inside against direct lightning strikes. When properly connected to an earth ground, the cage conducts the high current harmlessly to ground, and keeps the EM pulse from affecting personnel or hardware inside.
The Faraday Cage in this photo is located at the Boston Museum of Science. It protects the employee who is tasked with demonstrating the world's largest air-insulated Van de Graaff generator in the museum's Theater of Electricity.