Electromagnetic fields
Occasional and limited presence of natural electromagnetic fields on this earth is well known but in the present electronic based civilization, we are surrounded all over and every time with a large amount of man made electromagnetic radiations/fields. These fields/radiations along with their tremendous advantages and industrial applications no doubt have really revalorized the living of human beings but their presence and use have also its limitations and disadvantages on human beings and on the safety of electronic devices themselves. Electricity is the lifeblood of many aspects of our world. Without volts and amps, many of our technological innovations would cease to exist. Even our bodies wouldn’t function without an electrical charge zipping through our cells. But what electricity gives, electricity can take away. Although this form of energy is vital to so much of our lives, it’s one of those things that are only good in the right amounts. Too much electricity can electrocute people. Likewise, it can kill our modern electronics and machines. But thanks to Michael Faraday, the brilliant 19th-century scientist, and one of his namesake inventions, the Faraday cage, we humans have developed plenty of ways to control electricity and make it safer for our computers, cars and other inventions — and for us, too. Therefore, in order to protect each and every thing from the side effects of electromagnetic fields, scientists and engineers have stressed on the use of Faraday cages and their (Faraday cages) applications are expanding with the ever growing use of electromagnetic fields.
Faraday cage
Faraday cages are named after the English scientist Michael Faraday, who invented them in 1836 but Benjamin Franklin also made a great contribution to “Faraday Cage” development and application. A Faraday shield may be formed by a continuous covering of conductive material or in the case of a Faraday cage, by a mesh of such materials. A Faraday cage or shield is an enclosure used to block electromagnetic fields. Faraday noticed that the conductor charge (on a charged conductor) did not influence anything that was enclosed within; the charge resided only on the exterior. Faraday constructed a room, coated the entire room with metal foil, and used an electrostatic generator to create high-voltage discharges that stroke the outside of his metal foil-coated room. He found no electric charge on the inside walls. Faraday used an electroscope to prove this.
How it works
A Faraday cage operates because an external electrical field causes the electric charges within the cage’s conducting material to be distributed such that they cancel the field’s effect in the cage’s interior. Faraday cages are Faraday shields which have holes in the conductor and are more complex to analyze. Whereas continuous shields essentially attenuate all frequencies above the skin depth, the holes in a cage can permit higher frequencies to diffract through them or set up evanescent waves a short distance inside the surface. The higher the frequencies, the better they pass through a mesh of given size. Thus to work well at high frequencies the holes in the cage must be smaller than the wavelength of the incident EM wave, and Faraday cages may be thought of as band block devices.
How to build a Faraday cage
We can build our own Faraday cages. There are also other numerous examples of the Faraday cage. Here is how to construct our own Faraday cage.
Material:
- Two cardboard boxes – one cardboard box should fit tightly inside the other
- Aluminum foil
- 6 to 10 mm black polyethylene sheeting
- Grounding wire (wire that connects metal components in a circuit to the ground)
- An Alligator clip (also called spring clip / crocodile clip)
- Cellophane tape
Procedure:
- Place the smaller cardboard box inside the bigger one
- Cover the external box completely with aluminum foil
- Attach a grounding wire to the aluminum foil using the cellophane tape. Attach the crocodile clip to the end of the grounding wire
- Wrap the covered box with the black polyethylene sheeting
- Use tape to prevent the foil from ripping
- Put the item into the smaller box
Applications
The Faraday cage can be used to prevent the passage of electromagnetic waves and electric fields, either containing them in or excluding them from its interior space. Take notice that Faraday cages also shield the interior from external electromagnetic radiation, besides from electrical fields, if the conductor or wall is thick enough and any holes are significantly smaller than the radiation’s wavelength. This shielding effect is used to protect electronic equipment from lightning strikes and other electrostatic discharges, for RF (radio frequency) shielding and for shielded telecommunication cables. Practical Faraday cages can be made of a conducting mesh instead of a solid conductor.
- A microwave oven utilizes a Faraday cage, which can be partly seen covering the transparent window, to contain the electromagnetic energy within the oven and to shield the exterior from radiation.
- Any ordinary shopping bag, pocketed garment, pouch or backpack with an inner lining of multiple layers of aluminium foil can act as Faraday bags. If electronically tagged items are carried in such bags, they skip detection by security panels at store exits or electronic toll collection devices. This property, unfortunately, can be used unethically by shoplifters and dishonest persons wanting to avoid toll tax.
- Automobile and airplane passenger compartments are essentially Faraday cages, protecting passengers from electric charges, such as lightning.
- Elevators and other rooms with metallic conducting frames and walls simulate a Faraday cage effect, leading to a loss of signal and “dead zones” for users of cellular phones, radios, and other electronic devices that require external electromagnetic signals.
- Even the electronics designed by some of the world’s best brands can be susceptible to electromagnetic noise. Such disturbances affect their functionality and efficiency considerably. A Faraday cage is used as a protective shield against the electromagnetic radiation coming from the external environment, or it prevents electromagnetic energy radiated from the internal components from escaping the cage. These protective shields are used in different kinds of electronic and electrical equipment.
- Faraday bags are often used in digital forensics to prevent remote wiping and alteration of criminal digital evidence.
- Faraday cages are routinely used in analytical chemistry to reduce noise while making sensitive measurements.
- Faraday cages in the shape of wallets act as good RFID blockers for ATM cards, driver’s license, credit cards and passports. These articles of daily use commonly have RFID chips in them which can be maliciously intercepted or scanned to track down one’s personal particulars.
- Faraday cages used as part of a broader effort to provide emission security for computers.
- Plastic bags that are impregnated with metal are used to enclose electronic toll collection devices during shipment to the customer, so that a toll charge is not registered if the delivery truck carrying the item passes through a toll booth.
- Properly designed conductive clothing can also form a protective Faraday cage. Some electrical linemen wear Faraday suits, which allow them to work on live, high-voltage power lines without risk of electrocution. The suit prevents electric current from flowing through the body, and has no theoretical voltage limit.
- Proximity to high tension wires, power lines, cell towers and dozens of electronic devices makes any house an abode of electromagnetic radiation, which, according to some technologists could disturb an individual’s sleep pattern. Though its contribution to causing cancer is debatable, still, many people prefer to include a metallic mesh in their building material, to keep their homes safe from any electromagnetic disturbance. EMF bed canopy sheets fulfill the same purpose.
- The scan room of a magnetic resonance imaging (MRI) machine is designed as a Faraday cage. This prevents external RF (radio frequency) signals from being added to data collected from the patient, which would affect the resulting image. Radiographers are trained to identify the characteristic artifacts created on images should the Faraday cage be damaged during a thunderstorm.
- The shield of a screened cable, such as USB cables or the coaxial cable used for cable television, protects the internal conductors from external electrical noise and prevents the RF signals from leaking out.
- The metallic body of a car behaves like a portable Faraday cage and can effectively protect you against lightning while driving. However, it should be borne in mind that convertibles lack a metallic roof, compromising on the Faraday cage effect.
Limitations
- A common misconception is that a Faraday cage provides full blockage or attenuation; this is not true. The reception or transmission of radio waves, a form of electromagnetic radiation, to or from an antenna within a Faraday cage is heavily attenuated or blocked by the cage. However, a Faraday cage has varied attenuation depending on wave form, frequency or distance from receiver/transmitter, and receiver/transmitter power. Near-field high-powered frequency transmissions like HF RFID are more likely to penetrate. Solid cages generally provide better attenuation than mesh cages.
- To a large degree, though, they shield the interior from external electromagnetic radiation if the conductor is thick enough and any holes are significantly smaller than the wavelength of the radiation.
- Faraday cages cannot block static or slowly varying magnetic fields, such as the Earth’s magnetic field.
- They provide less attenuation from outgoing transmissions versus incoming: they can shield EMP waves from natural phenomena very effectively, but a tracking device, especially in upper frequencies, may be able to penetrate from within the cage (e.g., some cell phones operate at various radio frequencies so while one cell phone may not work, another one will).
Author
Dr. S. S. Verma, Department of Physics, S.L.I.E.T., Longowal, Distt.-Sangrur (Punjab)-148106.
