telegraph mechanism of a
station by another condenser, and thus keeping out of the line circuit
changes in current values which would be heard in the telephones if
violent, and might be inaudible if otherwise.

[Illustration: Fig. 473. Railway Composite Circuit]

[Illustration: FRONT OF LONG-DISTANCE POWER BOARD U.S. Telephone
Company, Cleveland, Ohio. _The Dean Electric Co._]

A further interesting element is the very heavy shunting of the
telephone receiver by means of an inductive coil. This shunt is applied
for by-path purposes so that heavy disturbing currents may be kept out
of the receiver while a sufficient amount of voice current is diverted
through the receiver. It is well to have the inductance of this shunt
made adjustable by providing a movable iron core for the shunt winding.
When the core is drawn out of the coil, its impedance is diminished
because the inductance is diminished. This reduces the amount of
disturbing noise in the receiver. The core should be withdrawn as little
as the amount of disturbance permits, as this also diminishes the
loudness of the received speech.

Because the signaling over lines equipped with this form of composite
working results in the ringing of a bell by means of local current, it
is of particular advantage in cases where the bell needs to ring loudly.
Switch stations, crossings, and similar places where the attendant is
not constantly near the telephone can be equipped with this type of
composite apparatus and it so offers a valuable substitute for regular
railway telegraph equipment, with which the attendant may not be
familiar. The success of the local bell-ringing arrangement, however,
depends on accurate relay adjustment and on the maintenance of a primary
battery. The drain on the ringing battery is greater than on the talking
battery.

A good substitute for the bell signal on railway composite circuits is a
telephone receiver responding directly to high-frequency currents over
the line. The receiver is designed specially for the purpose and is
known as a "howler." Its signal can be easily heard through a large
room. The condenser in series with it is of small capacity, limiting the
drain upon the line. Usually the howler is detached by the switch hook
during conversation from a station.

_Railway Composite Set._ The circuit of a set utilizing such an
arrangement together with other details of a complete railway composite
set is shown in Fig. 474. The drawing