Faraday Cage


The Faraday cage is a practical application of the effect demonstrated in school as the Faraday ice-pail experiment. If an ice pail, or any other hollow conductor is given a charge on its inside, then the charge will spread all over the outside surface of the conductor in such a way as to produce no electric field inside. The Van de Graaff generator uses this effect to produce such a large potential on the outside surface of its dome. In large Van de Graaff generators it is possible to have a lab or office actually inside the dome, and even have people there, while the machine is on! The idea that a hollow metal conductor will protect the interior from electric fields can be used using Faraday screens, cans or cages.

A Faraday can is nothing more complicated that a metal biscuit barrel for example. If an electronic watch is placed inside such a container, and then a spark is drawn to the container either from a Van de Graaff generator or induction coil, then after the demonstration, the watch will be unharmed.

A more impressive demonstration of the Faraday cage effect is that of an aircraft being struck by lightning. This happens frequently, but does not harm the plane or passengers. The metal body of the aircraft protects the interior. For the same reason, and if it were not for the highly flammable nature of petrol, a car would be a very safe place to be in a thunderstorm.



To demonstrate the Faraday cage effect we have constructed a human-size Faraday cage which we allows someone to stand inside while the discharge from a Tesla coil is directed towards it. To increase the dramatic content of the demonstration, the person inside is asked to hold a fluorescent tube, which does not light, while similar tubes balanced against the sides of the cage do light when the discharge is enabled. Our cage measures 1m square and is 2m tall. It is built from 1cm pitch steel mesh. The mesh is secured to a strong 'speedframe' framework, and riveted all round the frame except for the door section which is hinged on, and has two retainer catches to ensure the door stays shut while the demonstration is taking place.