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.

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.

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.

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?

In order to understand how Faraday cages work, you need a basic understanding of how electricity operates in conductors. The process is simple: Metal objects, such as an aluminum mesh, are conductors, and have electrons (negatively charged particles) that move around in them. When no electrical charge is present, the conductor has roughly the same number of commingling positive and negative particles.

If an external object with an electrical charge approaches the conductor, the positive and negative particles separate. Electrons with a charge opposite that of the external charge are drawn to that external object. Electrons with the same charge as the external object are repelled and move away from that object. This redistribution of charges is called electrostatic induction.

With the external charged object present, the positive and negative particles wind up on opposite sides of the conductor. The result is an opposing electric field that cancels out the field of the external object's charge inside the metal conductor. The net electric charge inside the aluminum mesh, then, is zero.

And here's the real kicker: Although there's no charge inside the conductor, the opposing electric field does have an important effect — it shields the interior from exterior static electric charges and also from electromagnetic radiation, like radio waves and microwaves. Therein lies the true value of Faraday cages.

The effectiveness of this shielding varies depending on the cage's construction. Variations in the conductivity of different metals, such as copper or aluminum, affect the cage's function. The size of the holes in the screen or mesh also changes the cage's capabilities and can be adjusted depending on the frequency and wavelength of the electromagnetic radiation you want to exclude from the interior of the cage.

Faraday cages sometimes go by other names. They can be called Faraday shields, RF (radio frequency) cages, or EMF (electromotive force) cages.

No matter what you call them, Faraday cages are most often used in scientific labs, either in experiments or in product development.

People use Faraday cages for a wide array of purposes — sometimes in esoteric lab settings, other times in common products. Your car, for example, works basically as a Faraday cage. It's this effect, not the rubber tires, that protects you in case of a nearby lightning strike.

A lot of buildings act as Faraday cages, too, if only by accident. With their plaster or concrete walls strewn with metal rebar or wire mesh, they often wreak havoc with wireless internet networks and cellphone signals.

But the shielding effect most often benefits humankind. Microwave ovens reverse the effect, trapping waves within a cage and quickly cooking your food. Screened TV cables help to maintain a crisp, clear image by reducing interference.

Power utility linemen often wear specially made suits that exploit the Faraday cage concept. Within these suits, the linemen can work on high-voltage power lines with a much-reduced risk of electrocution.

Governments can protect vital telecommunications equipment from lightning strikes and other electromagnetic interference by building Faraday cages around them. Science labs at universities and corporations employ advanced Faraday cages to completely exclude all external electric charges and electromagnetic radiation to create a totally neutral testing environment for all sorts of experiments and product development.

Swing by a hospital and you'll find Faraday cages in the form of MRI (magnetic resonance scanning) rooms. MRI scans rely on powerful magnetic fields to create medically useful scans of the human body. MRI rooms must be shielded to prevent stray electromagnetic fields from affecting a patient's diagnostic images.

There are plenty of political and military uses for Faraday cages, too. Politicians may opt to discuss sensitive matters only in shielded rooms that can block out eavesdropping technologies. All modern armed forces depend on electronics for communications and weapons systems, but there's a catch — these systems are vulnerable to aggressive EMPs (electromagnetic pulses), which can be a result of a solar storm or even man-made EMP attacks. To safeguard critical systems, militaries sometimes use shielded bunkers and vehicles.

It's for this same reason that Faraday cages are a fond subject in the survivalist subculture. These people, who preach self-sufficiency and mistrust of governmental response in the face of human-caused or natural disasters, believe in shielding all important electronics using homemade Faraday cages. In the event that an apocalyptic cataclysm strikes, they'll still have their shortwave radios and other high-tech tools that could be lifesavers.

Even if you're not particularly concerned with doomsday scenarios, Faraday cages likely play a role in your life every day. These cages harness a basic principle of physics and help people all over the planet put those principles to use — for safety, luxury, convenience and to help further evermore exciting technological advances.