The concept of a Faraday cage is logically attributed to Michael Faraday, an 19th Century pioneer in the field of electromagnetic energy. Faraday studied the work of earlier scientists such as Benjamin Franklin and theorized that electromagnetic waves naturally flowed around the surface of conductive materials, not through them. For example, if a metal box containing a mouse were placed directly in the path of an electrical current, the electricity would flow over the box but not into the compartment with the mouse. The mouse would not be electrocuted. Such a box would be considered a Faraday cage.
The important concept to remember is that a Faraday cage acts as a shield against the effects of electromagnetic energy. When a car is struck by lightning, the metal frame becomes a Faraday cage and draws the electricity away from the passengers inside. A microwave oven's door has a screen which prevents electromagnetic energy from escaping into the room. Electronic parts which generate radio frequencies are often protected by Faraday cages called RF shields. Even a concrete building reinforced with lead or rebar can be considered a Faraday cage.
Few consumers of electronic products would ever ask the sales clerk for a Faraday cage, but designers and engineers understand the importance of electromagnetic shielding very well. Whenever sensitive electronic parts are used in machinery, some form of shielding is generally in place, whether it be the machine's metal shell, a capsule or a grounding wire. If the electronic parts generate electromagnetic energy of their own, a Faraday cage must be used to shield users from excessive exposure. This is why cell phone use is often discouraged in hospitals or other public places with electronic equipment. Unshielded equipment may be exposed to the microwave energy created by cell phones or other radio transmitters.
