Electromagnetic coupling is a phenomenon common to electrical wiring and circuits where an electromagnetic field in one results in a electrical charge in another. It is often referred to as inductive coupling because the process occurs due to electrical inductance, where a transferring of electromagnetic properties from one location to another occurs without physical contact taking place. In order for electromagnetic coupling to take place, there must be a change in the electromagnetic field that is generating it. For this reason, direct current (DC) devices do not produce the effect, but it is common in alternating current (AC) circuits. The principle of electromagnetic coupling was discovered by Michael Faraday and Joseph Henry in 1831, and is known as Faraday's Law.
When an AC current in a circuit or wire induces a voltage in another wire, it is usually due to the fact that they are both in close proximity to each other, such as in the electrical windings that transformers have. This is not always true, however, and coupling at a distance that is unintentional, called cross talk, can occur with radio and telephone transmissions as well. Intentional electromagnetic coupling is the principle that transformers are based upon, where current can be stepped up or stepped down in voltage in a secondary wire winding based on the current level in a primary winding on the device.
Since electromagnetic radiation is a dual condition in nature where electromagnetic waves are composed of both electrical and magnetic properties, couplings are also of two types. An electrical coupling results when a positive or negative charge density in a wire or circuit changes, and this repels like charges in another circuit wire. The process of repelling like charges in nearby wire causes them to move within the wire, and this is the definition of what electrical current is. This form of current flow is often referred to as charge coupling or capacitance coupling.
Magnetic coupling is the flip side of this effect. As a current flows in a wire, it generates a magnetic field. With AC current, this magnetic field will fluctuate and cause a changing magnetic field in coupled circuits or wires. Magnetic fields are directly perpendicular to electric fields in electromagnetic coupling, so altering a magnetic field in one circuit can alter the current flow in another.
The principle of electromagnetic coupling is what all modern electric motors, relays, and transformers are built upon. Electrical generators also utilize it, as do a wide variety of communications-related devices, from citizen's band (CB) radios to televisions and wireless door locks for buildings and automobiles. It can also be detrimental to how a circuit functions and cause interference in telecommunications. In this case, it's often referred to as electromagnetic interference (EMI). Not all EMI is unintentional, however, as it can be used as a form of carrier wave to enhance signal strength as well.