## How to restrain a wild Wi-Fi signal

Say you wanted to protect your Wi-Fi network from surrounding buildings. The most obvious way to do this would be to secure the devices on your network using the wireless security protocol of choice. A very effective, but more extreme, way to do this would be to secure the building itself by making it act as a Faraday cage, shielding the radio frequency waves used by Wi-Fi.

Making a large building into a Faraday cage involves encasing the building in a thin layer of conductive material or metal mesh. In physics, a Faraday cage or Faraday shield -- named for the British physicist Michael Faraday, who discovered the phenomenon in the 19th century and built the first iteration in 1836 -- is an ingenious application of Gauss' law.

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Gauss' Law establishes the relation between electric flux flowing out of a closed surface and the electrical charge enclosed inside the surface.  Basically, in a hollow object that can conduct electricity, such as an aluminum sphere, charge will (ideally) distribute itself evenly over the surface of the sphere, and there won't be an electric field inside the sphere. This has the effect of blocking EMFs (elecromagnetic fields) and shielding radio frequency waves.

Faraday cages are far more common entities than you might realize. For instance, cars and airplanes can act like Faraday cages. That's why a lightning bolt striking an airplane is unlikely to take the airplane down; the aircraft will distribute the electric charge over its entire surface, protecting what's inside. You've probably experienced the phenomenon in even more mundane circumstances -- losing your cell-phone coverage inside an office building, or radio reception as you pass through a tunnel. And you're likely to "assemble" a Faraday cage on a regular basis: The metal screen built into the glass of your microwave oven lets the microwave act as a Faraday cage when the door is closed.

But constructing a functional, building-size Faraday cage might be another matter. It would likely be prohibitively expensive (though that didn't stop the Pentagon from doing so some years ago).  Not only would it be costly, but other equipment is likely to be hindered by the effect. For instance, cell phones will get poor reception or most likely not work at all in a building tricked out to be a Faraday cage. Resale values might also be affected for obvious reasons.

For a small space, though, such a structure might make perfect sense. Other expensive and goofy techniques to achieve similar ends, such as "Wi-Fi-proof wallpaper" and "Wi-Fi paint" with metallic ingredients, have been rolled out over the past few years, but unless you live in a cave, you'd probably also need to replace your windows with special radio-absorbing glass.

Small installations of RF shielding don't have to be expensive, and the basic concept of a Faraday cage can be extended to all kinds of small everyday objects.  Leather wallets sandwiched with a conductive RF-shielding layer can prevent RFID scanners from reading personal information implanted in everything from RFID-enabled access control cards to some credit cards; they're widely available for as little as \$15. For those favoring a more DIY route, several Web sites have information on how to make an RFID-blocking wallet with duct tape and aluminum  foil.

Who'd want such a thing? Quite possibly, you. U.S. passports are now set to include RFID chips, and future editions of U.S. currency might be RFID-enabled too. Expect further expansion of the burgeoning market for RFID-blocking wallets, passport holders and other paraphernalia for the paranoid and/or privacy-savvy consumer, from companies such as Emvelope and DIFRWear, which sports the motto "Faraday caged apparel." A sure sign of growing consumer interest? According to the Digital World Tokyo blog, a Japanese company is offering Faraday-cage card holders featuring the visage of that fierce data protector... Hello Kitty.

Dayal is a freelance writer in New York.

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