How Faraday Cages Work

Although the Faraday Cage is named after Michael Faraday, it was Benjamin Franklin who helped inspire many of the ideas behind Faraday cages. Franklin, of course, spent part of his illustrious career flying kites in thunderstorms in attempts to attract lightning and thus was already somewhat acquainted with the vagaries and concepts of electricity.

In 1755, Franklin began toying with electricity in new ways. He electrified a silver pint can and lowered an uncharged cork ball attached to a non-conductive silk thread into it until the cork touched the bottom of the can. “The cork was not attracted to the inside of the cann as it would have been to the outside, and though it touched the bottom, yet, when drawn out, it was not found to be electrified by that touch, as it would have been by touching the outside. The fact is singular,” Franklin wrote in a letter to a colleague.

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He was mystified by the interplay of electricity and the charged and uncharged objects and admitted as much: “You require the reason; I do not know it. Perhaps you may discover it, and then you will be so good as to communicate it to me.”

Decades later, an English physicist and chemist named Michael Faraday made other pertinent observations — namely, he realized that an electrical conductor (such as a metal cage), when charged, exhibited that charge only on its surface. It had no effect on the interior of the conductor.

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Faraday reaffirmed this observation by lining a room with metal foil and then charging the foil with the use of an electrostatic generator. He placed an electroscope (a device that detects electrical charges) inside the room, and, as he anticipated, the scope indicated that there was no charge within the room. The charge just moved along the surface of the foil and didn’t penetrate the room at all.

Faraday further examined this phenomenon with his famous ice pail experiment. In this test, he basically duplicated Franklin’s idea by lowering a charged brass ball into a metal cup. As expected, his results were the same as Franklin’s.

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This concept has all sorts of amazing applications, but here’s one that’s relevant to anyone who’s ever been in an airplane. Imagine flying in an airplane that’s suddenly struck by lightning. This isn’t a rare occurrence — it happens regularly, yet the plane and its passengers aren’t affected. That’s because the aluminum hull of the plane creates a Faraday cage. The charge from the lightning passes harmlessly over the surface of the plane without damaging the equipment or people inside.

It’s not shocking, really. It’s just science. So, how does this clever kind of cage design really work?


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