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Faraday stated that the charge on a charged conductor resided only on its exterior, and had no influence on anything enclosed within it. To demonstrate this fact he built a room 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.
This shielding effect is used to eliminate electric fields within a volume, for example to protect electronic equipment from lightning strikes and other electrostatic discharges (ESDs).
The same effect was predicted earlier by Francesco Beccaria (1716 - 1781) at the University of Turin, a student of Benjamin Franklin's work, who stated that "all electricity goes up to the free surface of the bodies without diffusing in their interior substance". Later, the Belgian physicist Louis Melsens (1814-1886) applied the principle to lightning conductors.
The Faraday cage is sometimes known as a Faraday shield. The latter term is also used more generally for any kind of electrostatic shielding .
Consider an idealized hollow electrical conductor such as an empty sphere or box.
If the outside of the cage is an idealized conductor, it will form an equipotential surface, that is to say, its surface will have the same electrical potential at every point. If there is no electrical charge inside the box, then by Gauss' law and the divergence theoremMultivariate calculus Theorems In vector calculus, the divergence theorem also known as Gauss' theorem or Ostrogradsky-Gauss theorem is a result that links the divergence of a vector field to the value of surface integrals of the flow defined by the field, there should be no electrostatic field inside the equipotential surface, regardless of what the field is outside the box.
Since the electrostatic field equations are linear, this means that even if there are charges in the box to generate a field, they will still not be affected by any fields outside the box.
Faraday cages are often put to a dual purpose: to block electric fields, as explained above, and to block electromagnetic radiation. The latter application is known as RF shieldingRF shielding is the protection of sensitive electrical equipment from external radiofrequency ( RF) electromagnetic radiation by enclosing it in a conducting material. RF shielding is a refinement of the principle of the Faraday cage, which protects equip.
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 shield.
Some real-world structures, such as automobiles, behave approximately like a Faraday cage. That's why:
Some United StatesThe United States of America also referred to as the United States U. America ¹ or the States is a federal republic in central North America, stretching from the Atlantic in the east to the Pacific Ocean in the west. It shares land borders with Canada in national securityHouses of Parliament, London, England. These heavy (and ugly) blocks of concrete will prevent a vehicle being rammed into the building National security describes the measures taken by a state to ensure its survival and security. Measures taken to ensure buildings are contained in Faraday cages, intended to act as a TEMPESTTEMPEST is a U. government code word for a set of standards for limiting electric or electromagnetic radiation emanations from electronic equipment such as microchips, monitors, or printers. It is a counter-intelligence measure aimed at the prevention of shield, and possibly also as a mitigation against EMP.
Some traditional architectural materials act as Faraday shields in practice. These include plaster with wire mesh, and rebarRebar is common steel reinforcing bar, an important component of reinforced concrete structures. It is usually formed from mild steel, and is given ridges for better adhesion to the concrete. Tied Rebar Concrete is a material that is very strong in compre concrete. These will impact the use of cordless phones and wireless networks inside buildings and houses.