ontact
corresponding to that station.

If, now, a given station key is pressed in the master sender, the
telegraph-line relays will again operate when the master-clock arm
reaches that point, sending out another impulse of battery over the
line. The selector contact at the waystation is closed at this moment;
therefore, the closing of the relay contact operates the ringing relay
through a local circuit, as shown. The ringing relay is immediately
locked through its own contact, thus maintaining the bell circuit closed
until it is opened by the key and the ringing is stopped.

As the master-clock arm passes the last point on the contact dial, the
current flows through the restoring relay operating the restoring magnet
which releases all the keys. A push button is provided by means of which
the keys may be manually released, if desired. This is used in case the
dispatcher presses a key by mistake. Retardation coils and variable
resistances are provided at the waystation just as with the other
selector systems which have been described and for the same reasons.

The circuits of the operator's telephone equipment shown in Fig. 495,
are also bridged across the line. This apparatus is of high impedance
and of a special design adapted to railroad service. There may be any
number of telephones listening in upon a railroad train wire at the same
time, and often a dispatcher calls in five or six at once to give
orders. These conditions have necessitated the special circuit
arrangement shown in Fig. 495.

[Illustration: Fig. 495. Telephone Circuits]

The receivers used at the waystations are of high impedance and are
normally connected, through the hook switch, directly across the line in
series with a condenser. When the operator, at a waystation wishes to
talk, however, he presses the key shown. This puts the receiver across
the line in series with the retardation coil and in parallel with the
secondary of the induction coil. It closes the transmitter battery
circuit at the same time through the primary of the induction coil.

The retardation coil is for the purpose of preventing excessive side
tone, and it also increases the impedance of the receiver circuit, which
is a shunt on the induction coil. This latter coil, however, is of a
special design which permits just enough current to flow through the
receiver to allow the dispatcher to interrupt a waystation operator when
he is talking.

The key used to close the transmi