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Electromagnetic protection is the attempt to prevent electromagnetic interference in an electronic device. Electromagnetic waves, which have both an electric and magnetic component, can cause some devices to malfunction in a variety of ways. Metal walls can often be used to block the electrostatic waves from an electric field. Magnetic waves, on the other hand, cannot be blocked and must be diverted around an object. Certain materials can redirect magnetic field lines around a device for this aspect of electromagnetic protection.
The first part of electromagnetic protection is electrostatic protection. Protection against external electric fields can be achieved with a metal box, often known as a Faraday cage. A Faraday cage, named after the English physicist Michael Faraday, is a solid or mesh enclosure made of a conducting material. When an external electric field reaches the walls of a Faraday cage, electrons that act as charge carriers will redistribute themselves to compensate for the field. If the walls of the cage are grounded, or connected to an external conducting path, electric currents in the walls will dissipate.
A Faraday cage can also block electric fields originating inside it. In fact, a typical microwave is an example of such an application. The structure of a microwave is made of a conducting material, while the door is usually a metal mesh screen. Since the holes in the screen are smaller than the wavelength of microwaves—generally defined as between one millimeter and one meter—the microwaves do not escape the enclosure. For this reason, removing the metal screen from a microwave door is not recommended.
Though a Faraday cage can provide much electromagnetic protection, it cannot block static magnetic fields—the other component of an electromagnetic wave. While electrostatic waves are generated from stationary charges, magnetic fields are produced by moving charges. An electric current is a collection of moving charges, so magnetic waves are often caused by nearby electric currents. Both constant and slowly-varying magnetic fields can be a problem for some electronic devices.
One solution for electromagnetic protection is to use a shield that reroutes magnetic field lines. Unlike electric field lines, magnetic field lines must always return to their point of origin. Therefore, magnetic shielding does not attempt to stop magnetic field lines; rather, it attempts to divert them around an object. Materials that can be easily magnetized make good magnetic shields. Mu-metal—an alloy consisting primarily of nickel—is a very effective, but expensive, material.