t will be noticed that the ringer and the relay coil 6 at the first station are
bridged across the sides of the line leading to the central office. In like
manner the bell and the relay magnets are bridged across the two limbs of
the line leading into each succeeding station, but this bridge at each of the
stations beyond Station A is ineffective because the line extension R_{x} is
open at the next station nearest the central office.
In order to ring Station A it is only necessary to send out ringing current
from the central office. This current is in such direction as not to cause the
operation of the relay, although it passes through the coil 6. If, on the other
hand, it is desired to ring Station B, a preliminary impulse would be sent
over the metallic circuit from the central office, which impulse would be of
such direction as to operate the relay at Station A, but not to operate the bell
at that station. The operation of the relay at Station A causes the spring 2
of this relay to engage the spring 3, thus extending the line on to the second
station. After the spring 2 at Station A has been forced into contact with
the spring 3, it is caught by the latch of the spring 5 and held mechanically.
When the impulse from the central office ceases, the spring 1 resumes its
normal position, thus breaking the bridge circuit through the bell at that
3 AMERICAN SCHOOL OF CORRESPONDENCE
station. It is apparent now that the action of coil 6 at Station A has made
the relay powerless to perform any further action, and at the same time the
line has been extended on to the second station. A second similar impulse
from the central office will cause the relay at Station B to extend the line on
to Station C, and at the same time break the circuit through the operating
coil and the bell at Station B. In this way any station may be picked out by
sending the proper number of impulses to operate the line relays of all the
stations between the station desired and the central office, and having picked
out a station it is only necessary to send out ringing current, which current
is in such direction as to ring the bell but not to operate the relay magnet at
that station.
In Fig. 197, a four-station line, such as is shown in Fig. 196, is illustrated, but
the condition shown in this is that existing when two preliminary impulses
have been sent over the line, which caused the line relays at Station A and
Station B to be operated. The bell at Station C is, therefore, the only one
susceptible to ringing current from the central office.
Since only one bell and one relay are in circuit at any one time, it is obvious
that all of the current that passes over the line is effective in operating a sin-
gle bell or relay only. There is no splitting up of the current among a large
number of bells as in the bridging system of operating step-by-step devices,
which method sometimes so greatly reduces the effective current for each
bell that it is with great difficulty made to respond. All the energy available
is applied directly to the piece of apparatus at the time it is being operated.
This has a tendency toward greater surety of action, and the adjustment of
the various pieces of apparatus may be made with less delicacy than is re-
quired where many pieces of apparatus, each having considerable work to
do, must necessarily be operated in multiple.
The method of unlatching the relays has been briefly referred to. After a
