less capacity to ground than do other pairs. This capacity is added by connecting the wires on one end of a short length of
a twisted pair to the cable pair and leaving the wires on the other end of the short length unconnected.
d. Use of repeating coils. In remote or rural areas, or in emergency installations for military uses, one-wire
ground-return telephone circuits are often used. Since transposition is impossible on such circuits, they are much more
susceptible to inductive interference from adjacent circuits. Even when connected directly to full metallic two-wire
circuits, there is usually an objectionable amount of noise interference, since one side of the two-wire line must be
grounded, creating an unbalance to ground. However, if the one-wire line is connected to the two-wire line through a
repeating coil which isolates the two circuits, the two-wire line need not be grounded, and it operates as a balanced line.
Ground-return circuits usually are replaced by two-wire circuits as soon as possible in order to avoid excessive
interference.
e. Noise filters. Battery chargers and similar apparatus used to maintain batteries in common-battery systems
are often the cause of hum, because the output voltage from these devices contains large amounts of energy at random
frequencies. Filtering the output voltage by means of low-pass filters, which consist of series choke coils and shunt
electrolytic condensers of fairly large capacity, removes the higher frequencies that lie in the voice-frequency range and,
therefore, prevents noise interference from this source.
FIGURE 63. Reduction of Crosstalk by
Transportation.
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