th
that of phantom telephone circuits. The potentials placed on the
telephone line by the telegraph operations are equal and simultaneous.
They cause no current to flow _around_ the telephone loop, only _along_
it. If all qualities of the loop are balanced, the telephones will not
overhear the telegraph impulses. In the figure, _AA_ are arresters, as
before, _GG_ are Morse relays; a 2-microfarad condenser is shunted
around the contact of each Morse key _F_ to quench the noises due to the
sudden changes on opening the keys between dots and dashes.

[Illustration: Fig. 465. Simplex Telegraph Circuit]

A simplex arrangement even more simple substitutes impedance coils for
the repeating coils of Fig. 465. The operation of the Morse circuit is
the same. An advantage of such a circuit, as shown in Fig. 466, is that
the telephone circuit does not suffer from the two repeating-coil losses
in series. A disadvantage is, that in ringing on such a line with a
grounded generator, the Morse relays are caused to chatter.

[Illustration: Fig. 466. Simplex Telegraph Circuit]

The circuit of Fig. 465 may be made to fit the condition of a through
telephone line and a way telegraph station. The midway Morse apparatus
of Fig. 467 is looped in by a combination of impedance coils and
condensers. The plans of Figs. 465 and 466 here are combined, with the
further idea of stopping direct and passing alternating currents, as is
so well accomplished by the use of condensers.

[Illustration: Fig. 467. Simplex Circuit with Waystation]

[Illustration: Fig. 468. Composite Circuit]

=Composite.= Composite circuits depend on another principle than that of
producing equal and simultaneous potentials on the two wires of the
telephone loop. The opposition of impedance coils to alternating
currents and of condensers to direct currents are the fundamentals. The
early work in this art was done by Van Rysselberghe, of Belgium. In Fig.
468, one telephone circuit forms two Morse circuits, two wires carrying
three services. Each Morse circuit will be seen to include, serially,
two 50-ohm impedance coils, and to have shunts through condensers to
ground. The 50-ohm coils are connected differentially, offering low
consequent impedance to Morse impulses, whose frequency of interruption
is not great. As the impedance coils are large, have cores of
considerable length, and are wound with two separate though serially
connected windings each, their impedance to vo