Computers are high-speed digital devices. That means their internal signals are high-frequency electric pulses. In 1886 the German physicist Heinrich Rudolph Hertz demonstrated that rapid variations in electric current could be emitted into space in the form of radio waves. When these radio waves interfere with the operation of nearby electronic devices, it's referred to as Electromagnetic Interference or EMI.
The high-frequency digital circuitry in PC's can cause EMI that interferes with the operation of nearby electronic devices. For this reason the Federal Communication Commission (FCC) regulates emissions in the United States. PCs are required by the FCC to be certified as Class B devices. This certification limits the amount of EMI that a PC can produce.
PCs can also be affected by EMI from other electronic devices. Potential sources of EMI are any type of radio transmitter, or any device with a coil or solenoid, or transformer, such as a doorbell, toaster ovens, electric blanket, electric bug zapper, heating pad, and may other electric devices.
Radiation spreads out as it travels away from it's source. Following Newton's Inverse Square Law the intensity of radiation drops off rapidly with distance. Therefore the main method of protecting PC's and other electronic devices from EMI is to separate them in distance. If the EMI from an electronic device has sufficient energy and is close enough to another device, they will cause interfere with that device. Devices that emit EMI should be kept a distance from PCs, peripherals and media.
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