Faraday cage

This phenomenon can be used to protect sensitive electronic equipment (for example RF receivers) from external radio frequency interference (RFI) often during testing or alignment of the device.

Faraday cages are also used to protect people and equipment against electric currents such as lightning strikes and electrostatic discharges, because the cage conducts electrical current around the outside of the enclosed space and none passes through the interior.

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.

In 1754, Jean-Antoine Nollet published an account of the cage effect in his Leçons de physique expérimentale.

[2] In 1755, Benjamin Franklin observed the effect by lowering an uncharged cork ball suspended on a silk thread through an opening in an electrically charged metal can.

He used an electroscope to show that there was no electric charge present on the inside of the room walls.

The redistributed charges greatly reduce the voltage within the surface, to an extent depending on the capacitance; however, full cancellation does not occur.

The field line paths in this inside space (to the endpoint negative charges) are dependent on the shape of the inner containment walls.

See Faraday's ice pail experiment, for example, for more details on electric field lines and the decoupling of the outside from the inside.

A good example of the effectiveness of a Faraday shield can be obtained from considerations of skin depth.

Whereas continuous shields essentially attenuate all wavelengths whose skin depth in the hull material is less than the thickness of the hull, the holes in a cage may permit shorter wavelengths to pass through or set up "evanescent fields" (oscillating fields that do not propagate as EM waves) just beyond the surface.