h. If faults on the line make one wire of greater resistance than the other, balance can be obtained by inserting a
rheostat in the low-resistance side, and varying it until the resistances of both sides are equal.
i. A phantom circuit provides a means for obtaining an extra channel over two full-metallic circuits. The
additional channel thus obtained is called the phantom circuit. It may be used either for telephone or telegraph. The two
full-metallic channels are called side circuits, and the entire combination is called a phantom group.
j. Efficient operation of a phantom group depends on the use of six repeating coils which must be perfectly
balanced. This prevents interference between the current in the phantom circuit and the currents in the side circuits.
k. Cables used in the construction of phantom circuits may be loaded by the use of specially designed loading
coils. These reduce attenuation and improve the performance of the phantom group.
l. A simplexed-phantom circuit extends the application of a phantom group by providing an additional telegraph
channel. This channel is a simplex circuit superimposed on the phantom circuit of the group.
m. Perfect balance in the repeating coils and the lines of the side circuits is required also in a simplexed-phantom
circuit.
n. Simultaneous telephone and telegraph communications also is provided in the composite circuit. This circuit
permits four additional telegraph channels to be superimposed on a phantom group without mutual interference. Its
operation is based on the use of capacitors in series with the lines of the side circuits, and coils in series with the telegraph
instruments that are bridged across the lines of the side circuits. The condensers and coils behave like high-pass and low-
pass filters, respectively.
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