.

Simplex and composite circuits are arrangements of wires whereby
telephony and telegraphy can take place at the same time over the same
wires without interference.

[Illustration: Fig. 460. Phantom Circuit]

=Phantom.= In Fig. 460 four wires join two offices. _RR_ are repeating
coils, designed for efficient transforming of both talking and ringing
currents. The devices marked _A_ in this and the following figures are
air-gap arresters. Currents from the telephones connected to either
physical pair of wires pass, at any instant, in opposite directions in
the two wires of the pair. The phantom circuit uses one of the physical
pairs as a _wire_ of its line. It does this by tapping the middle point
of the line side of each of the repeating coils. The impedance of the
repeating-coil winding is lowered because, all the windings being on
the same core, the phantom line currents pass from the middle to the
outer connections so as to neutralize each other's influence. The
currents of the phantom circuit, unlike those of the physical circuits,
are _in the same direction_ in both wires of a pair at any instant.
Their potentials, therefore, are equal and simultaneous.

A phantom circuit is formed most simply when both physical lines end in
the same two offices. If one physical line is longer than the other, a
phantom circuit may be formed as in Fig. 461, wherein the repeating coil
is inserted in the longer line where it passes through a terminal
station of the shorter.

[Illustration: Fig. 461. Phantom from Two Physical Circuits of Unequal
Length]

[Illustration: Fig. 463. Two Phantoms Joined by Physical Circuit]

A circuit may be built up by adding a physical circuit to a phantom. A
circuit may be made up of two or more phantom circuits, joined by
physical ones. In Fig. 462 a phantom circuit is extended by the use of a
physical circuit, while in Fig. 463, two phantom circuits are joined by
placing between them a physical circuit.

[Illustration: Fig. 462. Phantom Extended by Physical Circuit]

_Transpositions._ In phantom circuits formed merely by inserting
repeating coils in physical circuits and doing nothing else, an exact
balance of the sides of the phantom circuit is lacking. The resistances,
insulations, and capacities to earth of the sides may be equal, but the
exposures to adjacent telephone and telegraph circuits and to power
circuits will not be equal unless the phantom circuits are transposed.

To transpo