A phantom circuit can be created when electrical wires for one circuit are also configured to conduct signals for another. It is often used for telephone systems and electrical engineering, as well as to power audio recording and broadcast microphones. Phantom circuits are typically dependent on the operation of an audio transformer called a repeat coil, which often includes different lengths of wire wound around an iron, nickel, or other type of core. These coils are usually part of a simplex circuit normally consisting of cables connected to the main electrical line and to the wiring of the telephone equipment. Two simplex circuits can be combined to form a third; this additional circuit is also called a phantom group.
When one winding in the repeat coil, also called a center-tapped transformer, is exposed to an Alternating Current (AC), the other wire usually has an equally strong signal. The connection of telephone lines with repeat coils is therefore suitable because voice and ringing signals are typically AC powered. Repeat coils are located at either side of the phantom circuit, while connections between the wires can be made at the ends or the middle of the circuits. A current applied at the midpoint is usually divided equally among each wire, if the electrical resistance of each is the same.
Two currents on the same line typically travel in different directions and can cancel each other out. Telephone signals that are connected to a simplex electrical circuit also typically don’t get carried to the endpoints of the other lines. Other capabilities, like Morse code functions, can be added to the phantom circuit without degrading telephone function, especially if the Earth is used as a conductor. Frequent maintenance of these circuits is usually important, because electrical interference in the lines can make voice signals unintelligible.
If more repeat coils are used to form more connections, multiple phantom circuits can be created. The technology was first used in the early 20th century. Telephone companies often tried to maximize how many circuits there could be on long-distance lines without having to build extra equipment.
A phantom circuit can also be applied to Direct Current (DC) signals in telecommunications, while more modern carrier telephone systems often use it as well. The phantom circuit configuration normally enables a telephone company to get a cable plant to handle the most traffic. This potentially saves revenue, because operators often need less equipment and can add more subscribers to each line.