Faraday cage

A Faraday cage is a conducting enclosure — solid metal or a fine mesh — that shields its interior from external electric fields. Any external field induces charges on the cage's outer surface that arrange themselves to cancel the field inside. Inside a good Faraday cage, the electric field is essentially zero no matter what is happening outside.

The shielding works because conductors are equipotentials in electrostatic equilibrium. If you try to impose an external field, the conduction electrons rearrange themselves until the field inside the conductor — and inside the cavity it encloses — is zero. The rearrangement takes picoseconds; from then on, the interior is protected.

Faraday cages are everywhere in modern life. Microwave oven doors use a metal mesh with holes smaller than the 12 cm microwave wavelength; car bodies and airplane fuselages protect passengers from lightning strikes; MRI machine rooms are wrapped in copper to keep radio interference out; coaxial cables shield signals inside a grounded braid. The electromagnetic isolation rooms used for sensitive measurements and national-security work are industrial Faraday cages built from welded copper sheet.

Michael Faraday built the first experimental Faraday cage in 1836: a room-sized wooden frame sheathed in metal foil. He sat inside it with the most sensitive electroscope he could build while his assistants drove enormous sparks onto the outside. The electroscope never twitched. Faraday wrote in his notebook, "I went into the cube and lived in it, but although I used lighted candles, electrometers, and all other tests of electrical states, I could not find the least influence upon them."