For emergency operation, the two conductors of an ordinary field wire can be connected and used as a single wire in order
to extend the talking range of a telephone circuit.
(2) The disadvantages of ground-return circuits include their susceptibility to inductive interference from power
lines, variations in operation which may result from differences in ground potential at different points, and the possibility
of additional noise produced by faulty ground connections. Although ground-return circuits are not suited particularly to
telephone communication, they have considerable application in military telegraphy, and also may be used for ringing
circuits in telephony.
A simplex circuit is one in which a ground-return telephone or telegraph circuit is superimposed on a full-
metallic circuit in order to obtain an extra channel. Of course, provision must be made for preventing interaction or
interference between the two circuits. The principle of operation of a simplex circuit is explained later in this paragraph.
a. Repeating coils. In order to obtain the required isolation of the simplex and metallic circuits, repeating coils
are used. A, figure 65, illustrates one type of coil used in switchboards of newer types because it is small, light, and
efficient. The two upper terminals on the frame, marked SWITCHBOARD, are connected to the switchboard line
terminals. The two outside bottom terminals, marked LINE, are connected to the incoming trunk line. The center bottom
terminal, marked TELEG, is connected to one terminal of the telephone or telegraph set that is being operated on a
simplex circuit. The symbol used to represent this type of repeating coil in schematic diagrams is shown in B. In this
symbol, the switchboard terminals are designated by SB1 and SB2 the line terminal by L1 and L2 and the telephone or
telegraph set by T. Repeating coils used with simplex circuits are highly efficient transformers of 1-to-1 ratio. The
primary is identical with the secondary, each consisting of two balanced windings in series. The resistance of each of
these four windings is 21 ohms. The only physical difference between them is that the secondary has a center terminal, T,
connected to the junction of its two windings. Repeating coils installed at a switchboard are mounted either above or
below the terminal strip. They are mounted so that they are accessible for maintenance, but they are protected from
moisture or accidental injury. Figure 66 shows the position of a repeating coil in a line terminating and simplex panel.