计算机英语：法拉第笼

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 aclearcase/" target="_blank" >ccess 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?

法拉第笼

　　如果要保护你的Wi-Fi网，免受周围楼宇的干扰，做到这点最显而易见的方法就是利用选择的无线安全协议来确保网上设备的安全。而另一个非常有效、但比较极端的方法是使大楼像一个法拉第笼，屏蔽掉Wi-Fi使用的无线电波，也能确保大楼的安全。

　　要使一幢大楼成为一个法拉第笼，需要将大楼用一层导电的材料或导电网包裹起来。在物理上，以英国物理学家米歇尔·法拉第命名的法拉第笼或法拉第屏蔽是高斯定律聪明绝顶的应用，法拉第在19世纪发现这个现象，在1836年首次重现了这个现象。

　　高斯定律建立了流出一个导电表面的电通量之间的关系。基本上，在一个能导电的空心物体，如铝球体中，电荷（在理想的情况下）是均匀分布在球体的表面，在球体的里面是没有电场的。这就有阻断电磁场、屏蔽无线电波的效应。

　　法拉第笼是较为常见的事物，远远比你所知的要多。例如，汽车和飞机就能起到法拉第笼的作用。这就是为什么闪电击中飞机，飞机不会掉下去，飞机将电荷分布到整个飞机的表面，保护了内部。你也可能在更普通的情况下经历过这个现象——在办公大楼内你的手机没了信号或者在经过隧道时收音机收不到广播。你还可以在正常的基础上“装配”一个法拉第笼：微波炉玻璃中的金属屏让微波炉门关上时就构成了一个法拉第笼。

　　但是，构建一个功能性的、楼宇大小的法拉第笼却是另一回事。代价可能高得令人望而却步（虽然几年前这种高代价没有阻止五角大楼这么做）。不仅费钱，而且这个效应可能还会影响到其他的设备。例如，在搞成法拉第笼的大楼里，手机接收很差、甚至几乎不能用。

　　在小的空间中，这样的结构可能十分完美。在过去几年里推出了其他一些昂贵且笨拙的技术获得类似的效果，如带有金属成分的“防Wi-Fi墙纸”和“Wi-Fi油漆”等，但除非你生活在山洞里，否则你还是需要特殊的吸收无线电波的玻璃代替普通的玻璃窗。

　　不过安装小型的射频屏蔽不必花大价钱，法拉第笼的基本概念可以延伸到各种各样的小型常用品。中间夹有导电的射频屏蔽层的皮质钱包可以防止RFID（电子标签）扫描仪阅读保存在RFID的进入控制卡和某些信用卡等上的个人信息。它们只需花上15美元，所以能广泛使用。对于那些喜爱自己动手的人来说，有些网站就有如何利用导电带和铝箔制作阻断RFID的钱包的信息。

　　谁需要这样的东西？很有可能就是你。美国的护照现在就包含了RFID芯片，未来版的美国货币也可能含有RFID。防止RFID扫描的钱包、护照夹以及其他一些多疑和（或）特别关注保护隐私的消费者用的随身物品的新兴市场，将进一步得到发展。现有这样的一些公司，如Emvelepe公司和DIFRWear公司，鼓吹“法拉第笼屏蔽服饰”理念。这是不是越来越多的消费者对此感兴趣的可靠信号？

原文转自：http://www.ltesting.net