is arranged thus, in the hope of
simplifying the understanding of its principles. It will be seen that
the induction coil serves as an interrupter as well as for transmission.
All of the contacts are shown in the position they have during
conversation. The letters _Hc1_, _Hc2_, etc., and _Kc1_, _Kc2_, etc.,
refer to hook contacts and key contacts, respectively, of the numbers
given. The arrangements of the hook and key springs are shown at the
right of the figure. _RR_ represent impedance coils connected serially
in the line and placed at terminal stations. The composite telephone
sets are bridged from the line to ground at any points between the
terminal impedance coils.

The direct currents of telegraphy are prevented from passing to ground
through the telephone set during conversation by the 2-microfarad
condenser which is in series with the receiver. They are prevented from
passing to ground through the telephone set when the receiver is on the
hook by a .05 microfarad condenser in series with the howler. The
alternating currents of speech and interrupter signaling are kept from
passing to ground at terminals by the impedance coils.

Signals are sent from the set by pressing the key _K_. This operates the
vibrator by closing contacts _Kc6_ and _Kc7_. The howler is cut off and
the receiver is short-circuited by the same operation of the key. The
impedance of the coil _I_ is changed by moving its adjustable core.

[Illustration: Fig. 474. Railway Composite Set]

=Applications.= A chief use of composite and simplex circuits is for
ticket wire purposes. These are circuits over which long-distance
operators instruct each other as to connecting and disconnecting lines,
the routing of calls, and the making of appointments. One such wire will
care for all the business of many long-distance trunks. The public also
absorbs the telegraph product of telephone lines. Such telegraph service
is leased to brokers, manufacturers, merchants, and newspapers. Railway
companies use portable telephone adjuncts to telegraph circuits on
trains for service from stations not able to support telegraph
attendants, and in a limited degree for the dispatching of trains.
Telephone train dispatching, however, merits better equipment than a
railway composite system affords.

CHAPTER XL

TELEPHONE TRAIN DISPATCHING[A]

It has been only within the past three few that the telephone has begun
to replace the telegraph for handling train