External magnetic fields interact and interfere with electrical currents, which are used by electronic devices to transmit and receive data. This can cause data to be lost, or for erroneous data to be created. Furthermore, electricity flowing through a conductor creates its own magnetic field, and when the amount or frequency of the flowing electricity changes then the magnetic field will change. Due to a force called 'induction,' the changing magnetic field can then negatively affect the electrical current that is creating it.

 

To solve these problems, electrical engineers test electrical systems for their electromagnetic compatibility, or 'EMC.' The goal of EMC is not to create an electrical system that is invulnerable to all electromagnetic problems; it is rather to create an electrical system that is resistant to all of the magnetic problems that it will be subjected to in its environment, or as much as possible. EMC principles include shielding cables and grounding them.

 

When electrical currents get distorted by external forces, they tend to be affected in an opposite manner when their polarity gets reversed. In other words, wires with positive energy are affected by distortions in the opposite way to wires with negative energy are. Thanks to this principle, electrical systems with 'balanced lines' (two wires transmitting the same information, one with positive energy and the other with negative) are able to reduce the amount of 'noise' (random emissions, as opposed to 'signal' which means important emissions) that gets picked up by the receiver. In a balanced line system, the positive and negative versions of the signal are both recorded, the distortions found in both are compared, and then they are digitally removed. This is called 'differential signaling.'

 

A problem with balanced lines is that when the source of noise is closer to one side of the cable the distortion is greater on that one side of the signal and therefore won't be able to be fully removed. 'Twisted pair' cabling solves this problem. Twisted pair cables are twisted, so that the interference that the cable gets subjected to does not affect one side more than the other.