How Does a Faraday Cage Work

Faraday cages are simply incredible things. But just how do they work?
Christopher McFadden

There are high chances that you've spent a significant amount of your time in a Faraday Cage at some point in your life. You might even have one in your kitchen. Surprised?

First invented in 1836 by venerable English scientist Michael Faraday, they have become ubiquitous in our modern technological world. From keeping you safe in the air to keeping state secrets, they are simple yet very powerful tools.

In the following article, we'll take a quick look a just what they are and have a look at how they actually work.

What is a Faraday Cage?

A Faraday Cage, sometimes known as a Faraday Shield, is an enclosure that is used to shield things from electromagnetic fields (both static and non-static).

Static electricity is that where the charges are at rest, hence the name. They, in effect, accumulate on the surface of a particular insulator. 

Non-static or current electricity is where electrons are moving within a conductor. Faraday cages are able to protect their contents, or indeed occupants, from feeling the effects of both.

They can be made from a continuous covering of conductive material or from a fine mesh of conductive material.

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 to the investigation of 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 have no effect on 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. 

Faraday Cages Michael Faraday
The great Michael Faraday. Source: Royal Institution of London

He then used a special device called an electroscope (a device that detects electrical charges) to conclusively prove his hypothesis As he had suspected the room was completely 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 experiment of Benjamin Franklin. 

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

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Benjamin Franklin deserves some credit too

Although today this kind of apparatus bears Michael Faraday's name, 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.

Faraday Cage Benjamin Franklin
Source: The White House Historical Association/Wikimedia Commons

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 was also able to show that the cork was affected by the electrostatic charge of the can by dangling it near the can's exterior. The cork ball was immediately drawn towards 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."

Whilst he discovered 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 simply, Faraday Cages distribute electrostatic charge around their exterior. They, therefore, act as a shield to anything within them.

They are, in this respect, a form of 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 that allow them 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.

If 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 towards 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 it creates an opposing electrical field to that of the external object. 

This process effectively cancels out the external electrical field throughout the entire cage. It is this phenomenon that 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 a variety of applications. It is likely you've been in one very recently indeed.

The most famous examples are automobiles and airplanes. Both an aircraft's and car's fuselages act as Faraday Cages for their occupants.

Whilst less of an issue for cars, in the air lighting strikes are quite a common occurrence. Thanks to the planes aluminum exterior, when this does occur both the planes delicate avionics and priceless passengers are left completely unscathed. 

Incredibly fittingly, MRI scanning rooms are effectively 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 seriously 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 actually see part of the cage on the microwave oven's transparent window. 

Many buildings are also accidental Faraday cages, as it turns out. Large use of metal rebar or wire mesh can wreak havoc with wireless internet networks and cellphone signals.

Another interesting application of Faraday cages is used by the military and other organizations. Faraday cages are often used to protect vital IT and other electrical equipment from EMP attacks and lightning strikes.

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

Faraday cages in power plant
Faraday shield at a power plant in Heimbach, Germany. Source: Frank Vincentz/Wikimedia Commons

Are Faraday Cages 100% effective?

Faraday cage effectiveness is defined by the cages 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 amazing as Faraday cages and shields are, they are far from perfect. They, on the whole, do not provide 100% insulation from electromagnetic waves.

Whilst longer wavelengths, like radio waves, tend to be heavily attenuated or blocked by the cage, near-field high-powered frequency transmissions like HF RFID are usually able to penetrate the shield.

That being said, solid cage constructions, as oppose to mesh forms, do tend to provide a better amount of shielding over a broader range of frequencies. 

Microwave ovens are a prime example of the fact that Faraday cages are not 100% effective as EM shields. Most do not block all the microwave radiation from leaking from the device.

Faraday Cages Microwave
Microwaves are Faraday Cages in reverse. Source: Apoltix/Wikimedia commons

But this is nothing to actually 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, allow for a small amount of leakage from microwave ovens. This is currently set to 5 mW/cm2

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