More precisely
Brian Twisted pair wiring provides a form of interference self-cancelling for either signal or power circuits. Any electric current through a wire produces a magnetic field. This field can couple to other devices, either electronic (e.g., radio, instruments, autopliot, etc.) or magnetic (e.g., Compass) For this self-cancellation to work, one wire carries the signal current in one direction, and the other wire carries the equal "return" current in the other. The "return" wire often is the signal ground line. As long as an equal signal current flows in both directions, any electromagnetic radiation from these wires is self-cancelling. The fields produced by both wires are equal but oppositely polarized, and cancel each other out. Twisting the wires just ensures a better, more uniform cancellation effect. This also works to protect the local circuit from external interference. You can inadvertantly negate this positive self-cancellation effect by locally grounding the signal at both ends instead of letting the return current flow through that line to a single common ground point. --Ron
