Charles Thom.

Cyclopedia of Telephony & Telegraphy Vol. 1 A General Reference Work on Telephony, etc. etc online

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Online LibraryCharles ThomCyclopedia of Telephony & Telegraphy Vol. 1 A General Reference Work on Telephony, etc. etc → online text (page 23 of 33)
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systems now before the public will, therefore, be described in some
detail.

Poole System. The Poole system is a lock-out system pure and simple,
its devices being in the nature of a lock-out attachment for
selective-signaling lines, either of the polarity or of the harmonic
type wherein common-battery transmission is employed. It will be here
described as employed in connection with an ordinary harmonic-ringing
system.

In Fig. 188 there is shown a four-station party line equipped with
Poole lock-out devices, it being assumed that the ringers at each
station are harmonic and that the keys at the central office are the
ordinary keys adapted to impress the proper frequency on the line for
ringing any one of the stations. In addition to the ordinary talking
and ringing apparatus at each subscriber's station, there is a relay
of special form and also a push-button key.

[Illustration: Fig. 188. Poole Lock-Out System]

Each of the relays has two windings, one of high resistance and the
other of low resistance. Remembering that the system to which this
device is applied is always a common-battery system, and that,
therefore, the normal condition of the line will be one in which there
is a difference of potential between the two limbs, it will be evident
that whenever any subscriber on a line that is not in use raises his
receiver from its hook, a circuit will be established from the upper
contact of the hook through the lever of the hook to the
high-resistance winding _1_ of the relay and thence to the other side
of the line by way of wire _6_. This will result in current passing
through the high-resistance winding of the relay and the relay will
pull up its armature. As soon as it does so it establishes two other
circuits by the closure of the relay armature against the contacts _4_
and _5_.

The closing of the contact _4_ establishes a circuit from the upper
side of the line through the upper contact of the switch hook, thence
through the contacts of the push button _3_, thence through the
low-resistance winding _2_ of the relay to the terminal _4_, thence
through the relay armature and the transmitter to the lower side of
the line. This low-resistance path across the line serves to hold the
relay armature attracted and also to furnish current to the
transmitter for talking. The establishment of this low-resistance path
across the line does another important thing, however; it practically
short-circuits the line with respect to all the high-resistance relay
windings, and thus prevents any of the other high-resistance relay
windings from receiving enough current to actuate them, should the
subscriber at any other station remove his receiver from the hook in
an attempt to listen in or to make a call while the line is in use. As
a subscriber can only establish the proper conditions for talking and
listening by the attraction of this relay armature at his station, it
is obvious that unless he can cause the pulling up of his relay
armature he can not place himself in communication with the line.

The second thing that is accomplished by the pulling up of the relay
armature is the closure of the contacts _5_, and that completes the
talking circuit through the condenser and receiver across the line in
an obvious fashion. The result of this arrangement is that it is the
first party who raises his receiver from its hook who is enabled to
successfully establish a connection with the line, all subsequent
efforts, by other subscribers, failing to do so because of the fact
that the line is short-circuited by the path through the
low-resistance winding and the transmitter of the station that is
already connected with the line.

A little target is moved by the action of the relay so that a visual
indication is given to the subscriber in making a call to show whether
or not he is successful in getting the use of the line. If the relay
operates and he secures control of the line, the target indicates the
fact by its movement, while if someone else is using the line and the
relay does not operate, the target, by its failure to move, indicates
that fact.

When one party desires to converse with another on the same line, he
depresses the button _3_ at his station until after the called party
has been rung and has responded. This holds the circuit of his
low-resistance winding open, and thus prevents the lock-out from
becoming effective until the called party is connected with the line.
The relay armature of the calling party does not fall back with the
establishment of the low-resistance path at the called station,
because, even though shunted, it still receives sufficient current to
hold its armature in its attracted position. After the called party
has responded, the button at the calling station is released and both
low-resistance holding coils act in multiple.

[Illustration: ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate
Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines.
Siemens-Halske Equipment. Note Horizontal Disposal of Multiple Jack
Field.]

No induction coil is used in this system and the impedance of the
holding coil is such that incoming voice currents flow through the
condenser and the receiver, which, by reference to the figure, will be
seen to be in shunt with the holding coil. The holding coil is in
series with the local transmitter, thus making a circuit similar to
that of the Kellogg common-battery talking circuit already discussed.

A possible defect in the use of this system is one that has been common
to a great many other lock-out systems, depending for their operation
on the same general plan of action. This appears when the instruments
are used on a comparatively long line. Since the locking-out of all the
instruments that are not in use by the one that is in use depends on
the low-resistance shunt that is placed across the line by the
instrument that is in use, it is obvious that, in the case of a long
line, the resistance of the line wire will enter into the problem in
such a way as to tend to defeat the locking-out function in some cases.
Thus, where the first instrument to use the line is at the remote end
of the line, the shunting effect that this instrument can exert with
respect to another instrument near the central office is that due to
the resistance of the line plus the resistance of the holding coil at
the end instrument. The resistance of the line wire may be so high as
to still allow a sufficient current to flow through the high-resistance
coil at the nearer station to allow its operation, even though the more
remote instrument is already in use.

Coming now to a consideration of the complete selective-signaling
lock-out systems, wherein the selection of the party and the locking
out of the others are both inherent features, a single example of the
step-by-step, and of the broken-line selective lock-out systems will
be discussed.

Step-by-Step System. The so-called K.B. system, manufactured by the
Dayton Telephone Lock-out Manufacturing Company of Dayton, Ohio,
operates on the step-by-step principle. The essential feature of the
subscriber's telephone equipment in this system is the step-by-step
actuating mechanism which performs also the functions of a relay. This
device consists of an electromagnet having two cores, with a permanent
polarizing magnet therebetween, the arrangement in this respect being
the same as in an ordinary polarized bell. The armature of this magnet
works a rocker arm, which, besides stepping the selector segment
around, also, under certain conditions, closes the bell circuit and
the talking circuit, as will be described.

[Illustration: Fig. 189. K.B. Lock-Out System]

Referring first to Fig. 189, which shows in simplified form a
four-station K.B. lock-out line, the electromagnet is shown at _1_ and
the rocker arm at _2_. The ratchet _3_ in this case is not a complete
wheel but rather a segment thereof, and it is provided with a series
of notches of different depths. It is obvious that the depth of the
notches will determine the degree of movement which the upper end of
the rocker arm may have toward the left, this being dependent on the
extent to which the pawl _6_ is permitted to enter into the segment.
The first or normal notch, _i.e._, the top notch, is always of such a
depth that it will allow the rocker-arm lever _2_ to engage the
contact lever _4_, but will not permit the rocker arm to swing far
enough to the left to cause that contact to engage the bell contact
_5_. As will be shown later, the condition for the talking circuit to
be closed is that the rocker arm _2_ shall rest against the contact
_4_; and from this we see that the normal notch of each of the
segments _3_ is of such a depth as to allow the talking circuit at
each station to be closed. The next notch, _i.e._, the second one in
each disk, is always shallow, as are all of the other notches except
one. A deep notch is placed on each disk anywhere from the third to
the next to the last on the segment. This deep notch is called the
_selective notch_, and it is the one that allows of contact being made
with the ringer circuit of that station when the pawl _6_ drops into
it. The position of this notch differs on all of the segments on a
line, and obviously, therefore, the ringer circuit at any station may
be closed to the exclusion of all the others by stepping all of the
segments in unison until the deep notch on the segment of the desired
station lies opposite to the pawl _6_, which will permit the rocker
arm _2_ to swing so far to the left as to close not only the circuit
between _2_ and _4_, but also between _2_, _4_, and _5_. In this
position the talking and the ringing circuits are both closed.

The position of the deepest notch, _i.e._, the selective notch, on the
circumference of the segment at any station depends upon the number of
that station; thus, the segment of Station 4 will have a deep notch in
the sixth position; the segment for Station 9 will have a deep notch
in the eleventh position; the segment for any station will have a deep
notch in the position corresponding to the number of that station plus
two.

From what has been said, therefore, it is evident that the first, or
normal, notch on each segment is of such a depth as to allow the
moving pawl _6_ to fall to such a depth in the segment as to permit
the rocker arm _2_ to close the talking circuit only. All of the other
notches, except one, are comparatively shallow, and while they permit
the moving pawl _6_ under the influence of the rocker arm _2_ to move
the segment _3_, yet they do not permit the rocker arm _2_ to move so
far to the left as to close even the talking circuit. The exception is
the deep notch, or selective notch, which is of such depth as to
permit the pawl _6_ to fall so far into the segment as to allow the
rocker arm _2_ to close both the talking and the ringing circuits.
Besides the moving pawl _6_ there is a detent pawl _7_. This always
holds the segment _3_ in the position to which it has been last moved
by the moving pawl _6_.

The actuating magnet _1_, as has been stated, is polarized and when
energized by currents in one direction, the rocker arm moves the pawl
_6_ so as to step the segment one notch. When this relay is energized
by current in the opposite direction, the operation is such that both
the moving pawl _6_ and the detent pawl _7_ will be pulled away from
the segment, thus allowing the segment to return to its normal position
by gravity. This is accomplished by the following mechanism: An
armature stop is pivoted upon the face of the rocker arm so as to swing
in a plane parallel to the pole faces of the relay, and is adapted,
when the relay is actuated by selective impulses of one polarity, to be
pulled towards one of the pole faces where it acts, through impact with
a plate attached to the pole face of the relay, as a limiting means
for the motion of the rocker arm when the rocker arm is actuated by the
magnet. When, however, the relay is energized by current in the
opposite direction, as on a releasing impulse, the armature stop swings
upon its pivot towards the opposite pole face, in which position the
lug on the end of the armature stop registers with a hole in the plate
on the relay, thus allowing the full motion of the rocker arm when it
is attracted by the magnet. This motion of the rocker arm withdraws the
detent pawl from engagement with the segment as well as the moving
pawl, and thereby permits the segment to return to its normal position.
As will be seen from Fig. 189, each of the relay magnets _1_ is
permanently bridged across the two limbs of the line.

Each station is provided with a push button, not shown, by means of
which the subscriber who makes a call may prevent the rocker arm of
his instrument from being actuated while selective impulses are being
sent over the line. The purpose of this is to enable one party to make
a call for another on the same line, depressing his push button while
the operator is selecting and ringing the called party. The segment at
his own station, therefore, remains in its normal position, in which
position, as we have already seen, his talking circuit is closed; all
of the other segments are, however, stepped up until the ringing and
talking circuits of the desired station are in proper position, at
which time ringing current is sent over the line. The segments in Fig.
189, except at Station C, are shown as having been stepped up to the
sixth position, which corresponds to the ringing position of the
fourth station, or Station D. The condition shown in this figure
corresponds to that in which the subscriber at Station C originated
the call and pressed his button, thus retaining his own segment in its
normal position so that the talking circuits would be established with
Station D.

When the line is in normal position any subscriber may call central by
his magneto generator, not shown in Fig. 189, which will operate the
drop at central, but will not operate any of the subscribers' bells,
because all bell circuits are normally open. When a subscriber desires
connection with another line, the operator sends an impulse back on
the line which steps up and locks out all instruments except that of
the calling subscriber.

[Illustration: Fig. 190. K.B. Lock-Out Station]

A complete K.B. lock-out telephone is shown in Fig. 190. This is the
type of instrument that is usually furnished when new equipment is
ordered. If, however, it is desired to use the K.B. system in
connection with telephones of the ordinary bridging type that are
already in service, the lock-out and selective mechanism, which is
shown on the upper inner face of the door in Fig. 190, is furnished
separately in a box that may be mounted close to the regular telephone
and connected thereto by suitable wires, as shown in Fig. 191. It is
seen that this instrument employs a local battery for talking and also
a magneto generator for calling the central office.

The central-office equipment consists of a dial connected with an
impulse wheel, together with suitable keys by which the various
circuits may be manipulated. This dial and its associated mechanism
may be mounted in the regular switchboard cabinet, or it may be
furnished in a separate box and mounted alongside of the cabinet in
either of the positions shown at _1_ or _2_ of Fig. 192.

In order to send the proper number of impulses to the line to call a
given party, the operator places her finger in the hole in the dial
that bears the number corresponding to the station wanted and rotates
the dial until the finger is brought into engagement with the fixed
stop shown at the bottom of the dial in Fig. 192. The dial is then
allowed to return by the action of a spring to its normal position,
and in doing so it operates a switch within the box to make and break
the battery circuit the proper number of times.

_Operation._ A complete description of the operation may now be had in
connection with Fig. 193, which is similar to Fig. 189, but contains
the details of the calling arrangement at the central office and also
of the talking circuits at the various subscribers' stations.

[Illustration: Fig. 191. K.B. Lock-Out Station]

Referring to the central-office apparatus the usual ringing key is
shown, the inside contacts of which lead to the listening key and to
the operator's telephone set as in ordinary switchboard practice.
Between the outside contact of this ringing key and the ringing
generator there is interposed a pair of contact springs _8-8_ and
another pair _9-9_. The contact springs _8_ are adapted to be moved
backward and forward by the impulse wheel which is directly controlled
by the dial under the manipulation of the operator. When these springs
_8_ are in their normal position, the ringing circuit is continued
through the release-key springs _9_ to the ringing generator. These
springs _8_ occupy their normal position only when the dial is in its
normal position, this being due to the notch _10_ in the contact wheel.
At all other times, _i.e._, while the impulse wheel is out of its
normal position, the springs _8-8_ are either depressed so as to engage
the lower battery contacts, or else held in an intermediate position so
as to engage neither the battery contacts nor the generator contacts.

[Illustration: Fig. 192. Calling Apparatus K.B. System]

When it is desired to call a given station, the operator pulls the
subscriber's number on the dial and holds the ringing key closed,
allowing the dial to return to normal. This connects the impulse
battery to the subscriber's line as many times as is required to move
the subscriber's sectors to the proper position, and in such direction
as to cause the stepping movement of the various relays. As the
impulse wheel comes to its normal position, the springs _8_,
associated with it, again engage their upper contacts, by virtue of
the notch _10_ in the impulse wheel, and this establishes the
connection between the ringing generator and the subscriber's line,
the ringing key being still held closed. The pulling of the
transmitter dial and holding the ringing key closed, therefore, not
only sends the stepping impulses to line, but also follows it by the
ringing current. The sending of five impulses to line moves all of the
sectors to the sixth notch, and this corresponds to the position
necessary to make the fourth station operative. Such a condition is
shown in Fig. 193, it being assumed that the subscriber at Station C
originated the call and pressed his own button so as to prevent his
sector from being moved out of its normal position. As a result of
this, the talking circuit at Station C is left closed, and the talking
and the ringing circuit of Station D, the called station, are closed,
while both the talking and the ringing circuits of all the other
stations are left open. Station D may, therefore, be rung and may
communicate with Station C, while all of the other stations on the
line are locked out, because of the fact that both their talking and
ringing circuits are left open.

[Illustration: Fig. 193. Circuit K.B. System]

When conversation is ended, the operator is notified by the usual
clearing-out signal, and she then depresses the release button, which
brings the springs _9_ out of engagement with the generator contact
but into engagement with the battery contact in such relation as to
send a battery current on the line in the reverse direction from that
sent out by the impulse wheel. This sends current through all of the
relays in such direction as to withdraw both the moving and the
holding pawls from the segments and thus allow all of the segments to
return to their normal positions. Of course, in thus establishing the
release current, it is necessary for the operator to depress the
ringing key as well as the release key.

A one-half microfarad condenser is placed in the receiver circuit at
each station so that the line will not be tied up should some
subscriber inadvertently leave his receiver off its hook. This permits
the passage of voice currents, but not of the direct currents used in
stepping the relays or in releasing them.

The circuit of Fig. 193 is somewhat simplified from that in actual
practice, and it should be remembered that the hook switch, which is
not shown in this figure, controls in the usual way the continuity of
the receiver and the transmitter circuits as well as of the generator
circuits, the generator being attached to the line as in an ordinary
telephone.

Broken-Line System. The broken-line method of accomplishing
selective signaling and locking-out on telephone party lines is due to
Homer Roberts and his associates.

[Illustration: Fig. 194. Roberts Latching Relay]

To understand just how the principles illustrated in Figs. 186 and 187
are put into effect, it will be necessary to understand the latching
relay shown diagrammatically in its two possible positions in Fig. 194,
and in perspective in Fig. 195. Referring to Fig. 194, the left-hand
cut of which shows the line relay in its normal position, it is seen
that the framework of the device resembles that of an ordinary
polarized ringer. Under the influence of current in one direction
flowing through the left-hand coil, the armature of this device
depresses the hard rubber stud _4_, and the springs _1_, _2_, and _3_
are forced downwardly until the spring _2_ has passed under the latch
carried on the spring _5_. When the operating current through the coil
_6_ ceases, the pressure of the armature on the spring _1_ is relieved,
allowing this spring to resume its normal position and spring _3_ to
engage with spring _2_. The spring _2_ cannot rise, since it is held by
the latch _5_, and the condition shown in the right-hand cut of Fig.
194 exists. It will be seen that the spring _2_ has in this operation
carried out just the same function as the switch lever performed as
described in connection with Figs. 186 and 187. An analysis of this
action will show that the normal contact between the springs _1_ and
_2_, which contact controls the circuit through the relay coil and the
bell, is not broken until the coil _6_ is de-energized, which means
that the magnet is effective until it has accomplished its work. It is
impossible, therefore, for this relay to cut itself out of circuit
before it has caused the spring _2_ to engage under the latch _5_. If
current of the proper direction were sent through the coil _7_ of the
relay, the opposite end of the armature would be pulled down and the
hard rubber stud at the left-hand end of the armature would bear
against the bent portion of the spring _5_ in such manner as to cause
the latch of this spring to release the spring _2_ and thus allow the
relay to assume its normal, or unlatched, position.

A good idea of the mechanical construction of this relay may be
obtained from Fig. 195. The entire selecting function of the Roberts
system is performed by this simple piece of apparatus at each station.

[Illustration: Fig. 195. Roberts Latching Relay]

The diagram of Fig. 196 shows, in simplified form, a four-station
line, the circuits being given more in detail than in the diagrams of
Chapter XVI.

It 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.

[Illustration: Fig. 196. Simplified Circuits of Roberts System]

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



Online LibraryCharles ThomCyclopedia of Telephony & Telegraphy Vol. 1 A General Reference Work on Telephony, etc. etc → online text (page 23 of 33)