Unveiling the secrets of the Faraday Cage: how do they really work?

Non-static or current electricity is where electrons are moving within a conductor. Faraday cages can protect their contents, or indeed occupants, from feeling the effects of both. They can be made from a continuous covering of conductive material or a fine mesh. Faraday cages are named after their inventor, the English Scientist Michael Faraday. He devised them in 1836.

They range in design and size from simple chain-link fences to delicate-looking fine metallic meshes. Regardless of their exact appearance, all Faraday cages take electrostatic charges, or even certain types of electromagnetic radiation, and distribute them around the exterior of the cage.

When was the Faraday Cage invented?

In the 1800s, Michael Faraday had been putting his considerable intellect into investigating electricity. He soon realized that an electrical conductor (like a metal cage), when charged, appeared to exhibit that charge on its surface only. 

It appeared to not affect the interior of the conductor at all. He set out to demonstrate this on a larger scale and, in 1836, devised an ambitious experiment. 

During the now-legendary experiment, Michael Faraday lined a room in metal foil. He then allowed high-voltage discharges from an electrostatic generator to strike the outside of the room. 

He then used a unique device called an electroscope (a device that detects electrical charges) to conclusively prove his hypothesis. As he had suspected, the room was utterly devoid of electrical charge.

He also confirmed that only the outer surface of the metal foil conducted any current at all. Faraday later reaffirmed his observations with another famous experiment - his ice pail experiment. During this experiment, he duplicated an earlier investigation by Benjamin Franklin. 

Michael lowered a charged brass ball into a metal cup. As anticipated, the experiment confirmed Franklin's earlier observations and his own. 

Benjamin Franklin deserves some credit too

Although this kind of apparatus bears Michael Faraday's name today, Benjamin Franklin should be recognized for his contributions almost 90 years before.  In 1755, Mr. Franklin observed a similar phenomenon. He lowered an uncharged cork ball, on a silk thread, through an opening in an electrically charged metal can.

He observed that "the cork was not attracted to the inside of the can 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 (charged) by that touch, as it would have been by touching the outside. The fact is singular."

He also showed that the cork was affected by the electrostatic charge of the can by dangling it near the car's exterior. The cork ball was immediately drawn toward the can's surface. This, as you might expect, mystified Franklin at the time. He even admitted his confusion to a colleague in a letter.

"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." While discovering the effect years before Faraday, Franklin would never fully develop a reason for his curious observations. That would be left to the great Michael Faraday decades later.

How do Faraday Cages work?

Put, Faraday Cages distribute electrostatic charge around their exterior. They, therefore, act as a shield to anything within them. In this respect, they are a hollow conductor whereby the electromagnetic charge remains on the external surface of the cage only. 

But in reality, like many things, it is a little more complicated than that. Unless you are familiar with the concept of electricity and conductors you might want to brush up on that first before moving on. This video offers a great little refresher on the subject.

In essence, conductors have a reservoir of free-moving electrons to conduct electricity. When there is no electrical charge present, the conductor has, more or less, the same number of commingling positive and negative particles throughout it. Suppose an external electrical charged object approaches the cage, the positive (nuclei) and free negative (electron) particles in the conductor suddenly separate. 

If the approaching object is positively charged, free-moving electrons swarm toward it. This leaves the rest of the cage's material relatively devoid of negatively charged electrons giving it a positive charge. If the approaching object is negatively charged, the opposite occurs, and electrons are repelled, but the net effect is the same, just in reverse.

This process is called electrostatic induction and creates an opposing electrical field to the external object. This process effectively cancels out the external electrical field throughout the entire cage. This phenomenon insulates the cage's interior from the external electrical field.

What are Faraday Cages used for?

As you can imagine, these cages are pretty handy in various applications. You've likely been in one very recently, indeed. The most famous examples are automobiles and airplanes. An aircraft's and car's fuselages act as Faraday Cages for their occupants.

While less of an issue for cars, lighting strikes are pretty common in the air. Thanks to the plane's aluminum exterior, when this occurs, the plane's delicate avionics and priceless passengers are left entirely unscathed. Incredibly fittingly, MRI scanning rooms are effective imitations of Faraday's famous 1836 experiment. They need to be built like this to prevent external radio frequency signals from being added to the data from the MRI machine.  

If they were allowed to penetrate the room, it could affect the resulting images. Despite this, operators are usually trained to detect RF interference in the unlikely event that the Faraday Cage is damaged.

Microwave ovens are another notable example of everyday uses of Faraday Cages. However, unlike other applications, they are designed to work in reverse and keep the microwave radiation within the oven. 

You can see part of the cage on the microwave oven's transparent window. Many buildings are also accidental Faraday cages, as it turns out. Large metal rebar or wire mesh use can wreak havoc with wireless internet networks and cellphone signals.

The military and other organizations use another exciting application of Faraday cages. Faraday cages protect vital IT and other electrical equipment from EMP attacks and lightning strikes.

They are also widely used when eavesdropping devices must be blocked. Politicians and other high-level meetings often opt to discuss sensitive matters in unique Faraday cage design shielded rooms. 

Are Faraday Cages 100% effective?

Faraday cage effectiveness is defined by the cage's design, size, and choice of construction materials. If of a mesh-type construction, they will shield their interiors if the conductor is thick enough and the holes in the mesh are smaller than the wavelength of the radiation in question.

Yet as impressive as Faraday cages and shields are, they are far from perfect. Overall, they do not provide 100% insulation from electromagnetic waves. While longer wavelengths, like radio waves, tend to be heavily attenuated or blocked by the cage, near-field high-powered frequency transmissions like HF RFID can usually penetrate the shield.

That being said, solid cage constructions, as opposed to mesh forms, tend to provide better shielding over a broader range of frequencies. Microwave ovens are a prime example that Faraday cages are not 100% effective as EM shields. Most do not block all the microwave radiation from leaking from the device.

But this is nothing actually to be worried about. Not only is the radiation not ionizing, but microwave ovens undergo extensive testing before being released for general sale. The FDA, for example, allows for a small amount of leakage from microwave ovens. This is currently set to 5 mW/cm2. 

And that is your lot for today.

Faraday cages have come a long way since Michael Faraday's early experiments. Today, they are used in various industries to protect sensitive equipment, safeguard data, and ensure the safety of critical systems.