22 Oct What is a Faraday Cage?
In a world surround by technology, pretty much everything we use or consume is powered by electricity, daily necessities included. Even the device you’re currently reading from generates some form of electric charge!
As resourceful as it is however, electricity can have some negative impacts on the technical tools we often take for granted.
For example a camera HD/SDHC Card if exposed to Electromagnetic radiation can be damaged and unusable, even before being inserted into the camera. This is why the SD Card cases you see them come in are made of plastic – an insulator which doesn’t easily allow the flow of electricity to pass through them.
However, this is different to a Faraday cage.
So, what is a Faraday cage?
A Faraday cage or shield is characterised as a sealed enclosure created by materials which are electrically conductive.
Usually the outer layer will block external electric fields both static and non-static protecting the item or device inside.
An electric field is a force field which surrounds a charged particle for example an electron or proton.
Made from any closed shape these cages can either be made purely from conductive material, or made from non-conductive material and then wrapped in conductive materials to protect a variety of electronic equipment from electrostatic discharges.
Often, a fine metallic mesh will be used to create the Faraday shield.
How do they work?
Microwaves, Radio’s, Mobiles, TV’s, Laptops and Tablets are a few of the devices we interact with daily which emit Electromagnetic radiation. Most of us are unaware of the fields that surround us.
These electric fields however can be disruptive (take hospitals for example). These fields can be blocked however, through use of a Faraday cage. They are even effective against mobile signals and wifi.
A story that captured the public attention (making it into mainstream media) is of the bar owner who created a Faraday cage around his premises. The rationale? To force visitors to socialise with each other! Read more by clicking here.
A detailed explanation
How electricity works in conductors is the key to understanding how a Faraday cage works.
Metals have negatively charged particles or electrons in them. Without electrical charge the conductor will have the same amount of positive and negative particles.
When an external object with electrical charge approaches the conductor (the metal) the particles separate. Any electrons with an opposite charge to the external object are drawn to that object, but, any electrons with the same charge are repelled and move away from the object.
The negative and positive particles then end up on the opposite sides of the conductor, the following process is a redistribution of charges, which, results in an opposing electrical field that cancels out the field of the external object.
The opposing field shields the interior from exterior static charges therefore protecting the item inside.
So in summary, a Faraday cage, being a conductor, works in the same way. The cage will distribute charges around the cage’s exterior and will cancel out radiation or charges from within the cages interior.
Although technically invented by Michael Faraday, Ben Franklin was the first to play a part in the discovery of Faraday cages.
Franklin’s experiment took place in 1755 where he electrified a silver can and lowered an uncharged cork ball attached to a silk thread (non-conductive).
He found that when he lowered the ball into the can the can had no effect on the cork ball but when he withdrew it and dangled it near the edge of the can he observed that the cork was immediately drawn to the electrified surface.
Fast forward to 1836, Physicist Michael Faraday makes other observations, such as electrical conductors when charged only charge on the surface.
He further tested and confirmed his findings by lining a room with metal foil and then charging the foil with an electrostatic generator. He then used an electroscope inside of the room which indicated there was no charge present just as he expected. The charge only penetrated the surface of the foil.
The experiment which really elevated the idea though was his famous ice pail experiment.
In essence, the experiment was a follow on from Franklin’s idea where he lowered a charged brass ball into a metal pail (usually used to hold ice, which is where the name comes from). The results were the same as Franklin’s – the electric charge inside the conductive shell impels an equal charge and the charge then resides entirely on the surface.
The ice pail experiment was the first precise quantitative experiment on electrostatic charge, which later resulted in the invention of the Faraday cage.
Commonly asked questions
There are a number of common questions asked when discussing Faraday cages and their performance.
How thick should the conductive layer be?
Surprisingly, the conductive layer can be quite thin. This is because the current flows primarily on the skin of a conductor. The conductive layer needs to be greater than its skin depth to provide excellent shielding where the absorption loss is large.
What type of conductor can be used?
The conductivity of any metal will be good enough to allow the carriers to realign and cancel the external fields. Silver is the best conductor, but you probably wouldn’t want to use it purely for the purpose of a conductive material (mainly due to the consideration of its price).
Should the cage be grounded?
Grounding a cage means to connect it to an earth-referenced source of charge. This however has very little effect on the field levels seen inside of the cage. Primarily, grounding helps to keep the cage from becoming charged, but realistically this is the purpose of a cage anyway. In effect, an ungrounded cage will protect the contents just as well as a grounded cage.
Applications & uses
Faraday cages often go by other names. They can be called Faraday shields, RF (radio frequency) cages, or EMF (electromotive force) cages.
They are most often found in science labs, used in experiments or help with product development. They do have a wider range of uses however.
Power utility linemen will wear suits that utilise the Faraday cage concept. This allows them to work on high-voltage power lines with a reduced risk of electrocution.
Hospitals also use this technology. An MRI scanning room will always enforce a ‘switch your mobile off’ sign and you’ll often find that in those parts of the hospital (even the waiting rooms) you’ll either have no mobile signal or there will interference with the signal.
This is because MRI scans are dependent on powerful magnetic fields to create medical scans of the human body. MRI rooms have to be shielded to prevent stray electromagnetic fields from affecting and altering a patient’s diagnostic image.
In addition, Government’s protect vital telecommunications equipment from lightning strikes and other interference by using Faraday shields.
As the UK leading designer & manufacturer of electro static discharge (ESD) safe and anti-static packaging GWP Conductive offer a range of packaging, storage and handling products that utilise Faraday cages.
The well known Corstat coated fibreboard for example, is effectively a carbon coating which allows the creation of the Faraday cage. Corriplast and SSI Schaefer products, both have carbon impregnated directly into the Polypropylene material during manufacture.
It must be noted however that simply using a conductive material will not create a Farady cage – the structural design of the packaging also plays a key role.
Despite this, there are a wide range of boxes, layer pads, dividers, totes, cases, foam inserts and other materials handling products available. These are predominantly used to protect static sensitive, devices, tools or components from harmful exposure to electromagnetic fields.
And if you cannot find a box, bin or tote that is exactly what your application requires, you can also specify completely bespoke designs and sizes.
If you think that packaging that creates a Faraday cage could reduce or eliminate damage to your products or components, please use the link below to get in touch. Alternatively, you can see the full range of anti static packaging products available by clicking below.
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