A phantom circuit is created when electrical wires for one circuit also conduct signals for another, often used in telephone systems and electrical engineering. It relies on a repeater coil, and multiple circuits can be combined to form a ghost group. The technology was first used in the early 20th century and is still used in modern carrier telephone systems. It saves on revenue and allows for more subscribers on each line.
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. Ghost circuits typically depend on the operation of an audio transformer called a repeater coil, which often includes several lengths of wire wrapped around an iron, nickel, or other core. Such coils are usually part of a simplex circuit usually consisting of wires connected to the main power line and telephone equipment wiring. Two simplex circuits can be combined to form a third; this additional circuit is also called a ghost group.
When one winding in the repeater 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 repeater coils is therefore suitable because voice and call signals are typically powered by alternating current. Repeater coils are located on either side of the phantom loop, while connections between wires can be made at the ends or in the middle of the loops. A current applied at the midpoint is usually divided equally between 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 connected to a simplex electrical circuit are also typically not carried at the ends of other lines. Other capabilities, such as Morse code functions, can be added to the phantom circuit without degrading the telephone function, especially if Earth is used as a conductor. Frequent maintenance of these circuits is usually important, as electrical interference in the lines can render speech signals unintelligible.
If multiple repeating coils are used to form multiple connections, multiple ghost loops can be created. The technology was first used in the early 20th century. Telephone companies have often sought to maximize the number of circuits there could be on long-distance lines without having to build additional equipment.
A phantom circuit can also be applied to direct current (DC) signals in telecommunications, while more modern carrier telephone systems often use it. The phantom circuit configuration normally allows a telephone company to obtain a cable facility to handle most of the traffic. This potentially saves on revenue, because operators often need less equipment and can add more subscribers to each line.
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