Hands On Activity: Demonstrate the Faraday Cage Principle

 Michael Faraday delivering a Christmas Lecture in 1856
Michael Faraday was one of the great scientists in history. Some historians of science refer to him as the greatest experimentalist in the history of science. It was largely due to his efforts that electricity became a viable technology.

He developed the first dynamo in the form of a copper disk rotated between the poles of a permanent magnet, the precursor of modern dynamos and generators. From his discovery of electromagnetic induction (1831) stemmed a vast development of electrical machinery for industry. In 1825, Faraday discovered the compound benzene. In addition to other contributions he did research on electrolysis, formulating its two laws. He laid the foundations of the classical field theory, later was fully developed by J. C. Maxwell.

In his work on static electricity, Faraday demonstrated that the charge only resided on the exterior of a charged conductor, and exterior charge had no influence on anything enclosed within a conductor. This is because the exterior charges redistribute such that the interior fields due to them cancel (the Faraday cage principle).

To demonstrate his ideas, Faraday built in 1836 a room (the Faraday cage), coated with metal foil, and allowed high-voltage discharges from an electrostatic generator to strike the outside of the room. He used an electroscope to show that there was no excess electric charge on the inside of the room's walls and volume.

How does a Faraday Cage work?

In the upper picture an external electrical field is created (blue lines) in the direction from plus to minus.

Immediately after this field is applied, the electrical charges inside the cage walls are rearranged in the way depicted in the lower image since opposite electrical charges attract each other. As a result, is created another electrical field (red lines) between the cage walls in the opposite direction to the original field and with equal magnitude, and then, these two fields are cancelling each other and the electrical charge inside the cage would remain neutral.

Another explanation for the Faraday cage effect can be derived from Gauss's law that states that if there is no charge in a closed surface then the net flow of electric field from the surface must be zero.

Faraday cage is sometimes referred to as “Faraday's Ice Pail” because Faraday also used for his experiments a metal ice bucket as the receiver for the electroscope.

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. However, this reduces the cage's effectiveness as an RF (Radio Frequency) shield.

Some real-world structures, such as automobiles, behave approximately like a Faraday cage. For example: If lightning hits near a car, it does not affect the people sitting in the car.

Some traditional architectural materials act as Faraday shields in practice. These include plaster with wire mesh, and rebar concrete. These will impact the use of cordless phones and wireless networks inside buildings and houses.

The shield of a screened cable, such as the coax used for cable television, protects the internal conductors from electrical noise.

A shopping bag lined with aluminum foil acts as a Faraday cage. It is often used by shoplifters to steal RFID tagged items (don't try it!).

For heavy duty tasks is recommended to ground the shield in order to receive better results.

First, build a simple electroscope according to the following image and instructions.

Prepare a hook from a paperclip or a wire. Cut two small aluminum foil strips (leaves) (1 x 3 cm) and hang them, not opened up, on the hook, as shown in the picture.

Prepare a cardboard square, big enough to cover the glass or jar, and push the hook through it and place them on the glass as shown in the picture.

Test your electroscope by touching the hook terminal with a charged straw or comb - the aluminum leaves will diverge since same electrical charge reaches the leaves and they will repel. Now, touch the terminal with your finger and the aluminum foil strips will be closed together again because the terminal is grounded and the charge is transferred through the human body into the earth and the leaves are now neutral.

• Cut an aluminium foil square large enough to wrap the electroscope from the bottom to the cardboard lid.
• Lay the square on the table.
• Place the electroscope on the middle of the square and wrap the glass or jar up to the cardboard leaving the terminal of the electroscope free and also leave some space to look at the aluminium leaves inside the glass.
• Discharge the electroscope by touching the free terminal with your finger and ensure that the leaves are closed together. Charge an object and bring it close to the terminal (hook) of the electroscope - the leaves diverge (the electroscope is not shielded yet since the terminal is still free).
• Discharge the electroscope again with your finger and after the leaves are closed envelope also the free terminal of the electroscope with aluminium foil and bring again the charged object close to the terminal again - this time the leaves remain close together since the metal foil surrounding the electroscope constitute an electrostatic shield screen..

This experiment proves that external charges have not any effect on the shielded (Faraday caged) electroscope.

More quick experiments:
• Place a radio set or a cell phone inside a build Faraday cage or microwave and see if they work.
More Faraday Cage Experiments and Background
Faraday’s "Ice Pail" Experiment - The Open Door Web Site
Faraday's Ice Pail Experiment - Catharine H. Colwell
Faraday's Ice Pail - J. B. Calvert
Easy Electrostatics Experiments - Michel Maussion, University of Nantes, France
Faraday cage - Holland Shielding Systems
EMI Shielding EMC/RFI Solutions - Holland Shielding Systems
Faraday Cages, TEMPEST equipment - Holland Shielding Systems

Science fair projects and experiments:
Static Electricity Science Fair Projects and Experiments

Michael Faraday biographies and general resources:
Michael Faraday - The Royal Institution
Michael Faraday - Eric W. Weisstein