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Like this article? We recommend What is a Faraday Cage and How Could It Help?

What is a Faraday Cage and How Could It Help?

In 1836, Michael Faraday observed that the charge on a charged conductor resided only on its exterior and had no influence on anything enclosed within it. To demonstrate this fact, he built a room coated with metal foil and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room. He used an electroscope to show that there was no electric charge present on the inside of the room's walls. This concept can be applied to assuage some of the effect of an EMP.

A Faraday cage is best understood as an approximation to an ideal hollow conductor. Externally applied electric fields produce forces on the charge carriers (usually electrons) within the conductor, generating a current that rearranges the charges. Once the charges have rearranged so as to cancel the applied field inside, the current stops.

If a charge is placed inside an ungrounded Faraday cage, the internal face of the cage will be charged to prevent the existence of a field inside the body of the cage. However, this charging of the inner face would re-distribute the charges in the body of the cage. For all intents and purposes, the cage will generate the same electric field it would generate if it were simply charged by the charge placed inside. If the cage is grounded, however, the excess charges will go to the ground instead of the outer face, so the inner face and the inner charge will cancel each other out and the rest of the cage would remain neutral. This phenomenon is used, for example, to protect electronic equipment from lightning strikes and other electrostatic discharges. The scan room of a Magnetic Resonance Imaging (MRI) machine is designed as a Faraday cage. This prevents external RF (radio frequency) signals from being added to data collected from the patient, which would affect the resulting image.

A single atmospheric nuclear detonation can cause an electromagnetic pulse equal to 100,000 volts per square centimeter on the ground. A single detonation could fry every unprotected computer chip from coast to coast. A Faraday cage can provide a measure of protection, but there are considerations. Objects inside the cage must be electrically isolated from the walls and from the outside. Moreover, any electric signal coming from the outside world would need to be isolated before it passes into the Faraday cage. You may have the perfect Faraday cage, but if you have power lines coming into it, you defeat your purpose.

One simple and low-budget way to protect small electronic devices is to construct your own Faraday cage using two cardboard boxes, with one fitting tightly inside the other. The outer box is covered with aluminum foil, and a grounding wire is then taped to the foil. At the end of the ground wire, attach a small alligator clip. The item to be protected is placed inside the inner box (which acts as insulation from the outer box—remember, it can’t touch the wall itself). Any EMP hitting the foil and is bled away by the ground wire. There are many variations on this design, but this one illustrates the principle. If you do a web search on “Faraday cage and EMP,” all kinds of things pop up.

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