EMI (electromagnetic interference) is the unwanted effects in the electrical system due to electromagnetic radiation and electromagnetic conduction. Electromagnetic radiation and electromagnetic conduction are differentiated by the way an EM field propagates. Conducted EMI is caused by the physical contact of the conductors as opposed to radiated EMI which is caused by induction (without physical contact of the conductors). Electromagnetic disturbances in the EM field of a conductor will no longer be confined to the surface of the conductor and will radiate away from it. This persists in all conductors and mutual inductance between two radiated electromagnetic fields will result in EMI.
Due to this EMI, the electromagnetic field around the conductor is no longer evenly distributed and causes skin effect, proximity effect, hysteresis losses, transients, voltage drops, electromagnetic disturbances, EMP/HEMP, eddy current losses, harmonic distortion, and reduction in the permeability of the material.
EMI can be conductive and/or radiative. Its behavior is dependent on the frequency of operation and cannot be controlled at higher frequencies. For lower frequencies, EMI is caused by conduction and, for higher frequencies, by radiation. For example, skin effect is due to the conductive EMI and proximity effect is due to the radiative EMI.
The worst part of a high frequency electromagnetic signal is that it makes every conductor an antenna, in the sense that they can generate and absorb electromagnetic fields. In the case of a PCB (printed circuit board), which consists of capacitors and semiconductor devices which are soldered to the bread board, the capacitors and soldering act like antennas, generating and absorbing electromagnetic fields. The chips on these boards are so close to each other that the chances of conducted and radiated EMI are significant. Bread boards are designed in such a way that the case of the board is connected to the ground and the radiated EMI is diverted to ground. Technological advancements have drastically reduced the size of chipboards and electronics; however, this means they are also much more sensitive to EMI.
The most common solution to EMI is electromagnetic shielding. However, EMI shielding is expensive and has negative consequences. Another method to reduce EMI is to twist wires; however many facilities have tens of thousands of feet of wire, so this is not practical.
A common example of radiated EMI is a cable TV wire and the TV. If you unhook the cable from the TV and place it in front of the plug, video can still be seen on the TV. This is due to electromagnetic signals capable of traveling through the air from cable to TV.