Each conductor is a tinned solid-copper wire, covered with rubber or latex insulation. The insulation on one wire of each
pair is white and the insulation on the other wires is colored-coded for identification, a different color insulation being
used for each wire--red, yellow, green, blue, and black. The outer covering is made of buna (a synthetic rubber), which
encloses, in addition to the ten conductors, five strands of jute twine, inserted as filler and for mechanical strength.
(3) Toll cable (not illustrated) differs from the two types just described primarily in that its outermost covering is
made of lead. In general, it is used in permanent installations for long-distance transmission, and either may be strung
overhead on poles, or installed underground. The conductors usually are twisted pairs of annealed copper wire, insulated
either by spirally wound paper tape or by a covering formed on the wires from paper pulp. The wire sizes customarily
used are #19 and #16 AWG, although smaller wires sometimes are used for short distances.
c. Field-wire lines. Field wires consist of simple pairs of insulated wire twisted together. Field-wire lines are
used in military applications for emergency and temporary installations. They are used primarily for short lines, because
of their high transmission loss. The military designations for the field wires most commonly used are W-110-B, WD-
14/TT, and WD-1/TT (the latter type illustrated in fig. 51). Each of the two conductors has seven strands, of which four
are copper and three are steel. Each seven strand conductor is covered with polyethylene insulation and an outer
protective covering of nylon.
d. Talking range of lines. Although field wires
as a rule are used as single pairs, two pairs occasionally
are used in order to extend the talking range (fig. 52).
The two wires of one pair are connected to form one
conductor of the line, and the two wires of the other pair
are connected to form the other conductor of the line. In
FIGURE 52. Arrangement of Twin Pair