This chapter covers single-line telephony and some of the elementary methods of obtaining an extra voice
channel over a telephone line as used in early long distance telephony. These methods, referred to as multiplexing, are in
use today in commercial and military systems, although they gradually are being replaced by more advanced techniques.
Single-line telephone circuits may be full-metallic circuits or ground-return circuits, depending on the type of
physical connection provided between the two telephone sets.
a. Full-metallic telephone circuit. This circuit is one in which two conductors are used to interconnect the
telephone sets (chs. 4 and 5). It has the advantage of permitting transposition of wires as a means of overcoming
inductive interference. For this reason, full-metallic circuits are used almost exclusively in military applications.
telephone circuit. Figure 64
telephone circuit. Only one
wire is used to connect
telephone sets T1 and T2; the
other terminal of each set is
returned to ground.
FIGURE 64. Ground Return Telephone Circuit.
(1) One advantage of this type of circuit is that it is more economical to construct than a full-metallic circuit,
since it requires only half as much wire. Other advantages are the relative ease with which the circuit may be installed,
and its lower line resistance (if the ground connections are made carefully). Lower line resistance means lower
attenuation and more efficient transmission. Because of these advantages, ground-return telephone circuits still are used
in rural areas, especially those where interference from power lines is not an important factor.