Not What Michael Had in Mind

When Faraday built his first cage, he probably didn't have thievery on the brain. But cops often catch shoplifters lining bags with aluminum foil, which interferes with the antitheft RFID tags affixed to pricey products in retail stores.

Electrostatic for the People

In order to understand how Faraday cages work, you need a basic understanding of how electricity operates in conductors. The process is simple: Metal objects, such as an aluminum mesh, are conductors, and have electrons (negatively charged particles) that move around in them. When no electrical charge is present, the conductor has roughly the same number of commingling positive and negative particles.

If an external object with an electrical charge approaches the conductor, the positive and negative particles separate. Electrons with a charge opposite that of the external charge are drawn to that external object. Electrons with the same charge as the external object are repelled and move away from that object. This redistribution of charges is called electrostatic induction.

With the external charged object present, the positive and negative particles wind up on opposite sides of the conductor. The result is an opposing electric field that cancels out the field of the external object's charge inside the metal conductor. The net electric charge inside the aluminum mesh, then, is zero.

And here's the real kicker. Although there's no charge inside the conductor, the opposing electric field does have an important effect-- it shields the interior from exterior static electric charges and also from electromagnetic radiation, like radio waves and microwaves. Therein lies the true value of Faraday cages.

The effectiveness of this shielding varies depending on the cage's construction. Variations in the conductivity of different metals, such as copper or aluminum, affect the cage's function. The size of the holes in the screen or mesh also changes the cage's capabilities and can be adjusted depending on the frequency and wavelength of the electromagnetic radiation you want to exclude from the interior of the cage.

Faraday cages sometimes go by other names. They can be called Faraday shields, RF (radio frequency) cages, or EMF (electromotive force) cages.

No matter what you call them, Faraday cages are most often used in scientific labs, either in experiments or in product development. On the next page, you'll discover exactly how engineers put these ingenious shields to the test.