faraday cage



We explain what a Faraday cage is, its history, how it works and how it is built. Also, examples from everyday life.

Many roofs function like a Faraday cage to isolate the interior from electricity.

What is a Faraday cage?

A Faraday cage is a container lined with materials electricity conductors (such as metal plates or mesh) that works as a shield against the effects of a electric field coming from abroad.

Many elements that we use in everyday life apply the Faraday cage principle, for example: cables, microwave ovens, cars and airplanes. Its shape and size can vary, as well as the materials it is coated with.

History of the Faraday cage

In 1836 Michael Faraday carried out the experiments that allowed him to build the insulating cage.

The Faraday cage is named after its inventor, the British physicist Michael Faraday (1791-1867), who observed that a conductive material showed the effects of an electrical discharge only on the outside. This seemed to indicate that the charges on the conductor were distributed in such a way as to cancel out the internal electric fields.

To prove this, in 1836, Faraday covered the walls of a room with aluminum sheets. Using an electrostatic generator, he applied discharges of high voltage outside of said room. And with an electroscope (a device that detects the presence of electric charges in a body) was able to verify that inside the room the electric field was zero.

Thanks to this and many other experiments, Faraday occupies a prominent place among those scientists who made it possible for the electricity have the practical uses we know today.

How does a Faraday cage work?

When an electric field is applied to a container that has been coated with aluminum or metal mesh, the container functions as an electrical conductor that is polarized.

When polarized, the conductor becomes positively charged in the direction in which the external electromagnetic field moves and, at the same time, it becomes negatively charged in the opposite direction, generating an electric field equal in magnitude but opposite to the electromagnetic field that has been applied.

The sum of both fields, inside said container or Faraday cage, will be equal to zero. That is, conductive materials, in the presence of external electric fields, always order their charges on their surface in such a way that the internal electric field is zero.

How to make a Faraday cage?

Fully wrapping a phone in aluminum will block its signal.

Making a Faraday cage is simple: it only involves enclosing a certain space within a conductive material. The necessary materials are quite accessible: metal screens, aluminum foil, boxes or even a steel trash can.

Before proceeding, we must take into account the following:

  • If we are going to use wire mesh or grids (such as those used for chicken coops), the holes in that conductor must be smaller than the length signal to be blocked.
  • The interior space must be completely insulated, without cracks.
  • The thickness of the conductor to be used will depend on the frequency to be blocked.

There are many ways to make a Faraday cage, but with this simple experiment we can check its characteristic shielding effect:

  • Make a cylinder with metal mesh and an aluminum platform.
  • Place a tuned radio on the platform, and then mount the wire mesh cylinder on the platform. You will immediately notice how the radio transmission stops. The electromagnetic waves that the radio should receive are interrupted by the placement of the mesh.
  • Take two cell phones and verify that they can make and receive calls without difficulty. Then wrap one of the phones in a sheet of aluminum foil. When making a call to this phone you will notice that the signal is blocked.

Examples of the Faraday cage

The principle according to which Faraday cages work can be observed in many examples from everyday life:

  • When we notice that our cell phones do not work in an elevator or inside a building made of metal grids, we are facing a manifestation of the principle of the Faraday cage.
  • Our microwave ovens are equipped with Faraday cages to prevent their waves escape abroad and have some harmful effect on our Health.
  • The special suits of electrical technicians who repair high voltage lines.
  • When driving a car during a thunderstorm, it is recommended to stay inside the vehicle, as its bodies will function like a Faraday cage in the face of lightning.
  • Metal foils or mesh are also placed on the walls of MRI laboratories where MRIs are performed, to prevent waves from escaping and protect the health of operators.
  • Cyberattacks have generated a significant supply of products to inhibit the electromagnetic waves sent by eventual hackers. Various Business They offer accessories to make our devices invisible in the field of wireless connections: covers for car keys, backpacks, envelopes, wallets or briefcases manufactured under the principle of the Faraday cage.
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