Faraday cages
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.
Gauss’ Law establishes the relation between electric flux flowing out of a 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 oven 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.
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. There are some companies such as Emvelope and DIFRWear, which sport the motto “Faraday caged apparel.”A sure sign of growing consumer interest?
法拉第籠
如果要保護你的Wi-Fi網，免受周圍樓宇的干擾，做到這點最顯而易見的方法就是利用選擇的無線安全協議來確保網上設備的安全。而另一個非常有效、但比較極端的方法是使大樓像一個法拉第籠，屏蔽掉Wi-Fi使用的無線電波，也能確保大樓的安全。
要使一幢大樓成為一個法拉第籠，需要將大樓用一層導電的材料或導電網包裹起來。在物理上，以英國物理學家米歇爾·法拉第命名的法拉第籠或法拉第屏蔽是高斯定律聰明絕頂的應用，法拉第在19世紀發現這個現象，在1836年首次重現了這個現象。
高斯定律建立了流出一個導電表面的電通量之間的關系�；旧�，在一個能導電的空心物體，如鋁球體中，電荷（在理想的情況下）是均勻分布在球體的表面，在球體的里面是沒有電場的。這就有阻斷電磁場、屏蔽無線電波的效應。
法拉第籠是較為常見的事物，遠遠比你所知的要多。例如，汽車和飛機就能起到法拉第籠的作用。這就是為什么閃電擊中飛機，飛機不會掉下去，飛機將電荷分布到整個飛機的表面，保護了內部。你也可能在更普通的情況下經歷過這個現象——在辦公大樓內你的手機沒了信號或者在經過隧道時收音機收不到廣播。你還可以在正常的基礎上“裝配”一個法拉第籠：微波爐玻璃中的金屬屏讓微波爐門關上時就構成了一個法拉第籠。
但是，構建一個功能性的、樓宇大小的法拉第籠卻是另一回事。代價可能高得令人望而卻步（雖然幾年前這種高代價沒有阻止五角大樓這么做）。不僅費錢，而且這個效應可能還會影響到其他的設備。例如，在搞成法拉第籠的大樓里，手機接收很差、甚至幾乎不能用。
在小的空間中，這樣的結構可能十分完美。在過去幾年里推出了其他一些昂貴且笨拙的技術獲得類似的效果，如帶有金屬成分的“防Wi-Fi墻紙”和“Wi-Fi油漆”等，但除非你生活在山洞里，否則你還是需要特殊的吸收無線電波的玻璃代替普通的玻璃窗。
不過安裝小型的射頻屏蔽不必花大價錢，法拉第籠的基本概念可以延伸到各種各樣的小型常用品。中間夾有導電的射頻屏蔽層的皮質錢包可以防止RFID（電子標簽）掃描儀閱讀保存在RFID的進入控制卡和某些信用卡等上的個人信息。它們只需花上15美元，所以能廣泛使用。對于那些喜愛自己動手的人來說，有些網站就有如何利用導電帶和鋁箔制作阻斷RFID的錢包的信息。
誰需要這樣的東西？很有可能就是你。美國的護照現在就包含了RFID芯片，未來版的美國貨幣也可能含有RFID。防止RFID掃描的錢包、護照夾以及其他一些多疑和（或）特別關注保護隱私的消費者用的隨身物品的新興市場，將進一步得到發展�，F有這樣的一些公司，如Emvelepe公司和DIFRWear公司，鼓吹“法拉第籠屏蔽服飾”理念。這是不是越來越多的消費者對此感興趣的可靠信號？
文章來源于領測軟件測試網 http://www.kjueaiud.com/
