Almost wherever automatic telephony is to be found—and
its use is extensive and rapidly growing—the so-called Strowger
system is employed. It is so named because it is the outgrowth of
the work of Almon B. Strowger, an early inventor in the automatic
telephone art. That the system should bear the name of Strowger,
however, gives too great prominence to his work and too little to that
of the engineers of the Automatic Electric Company under the leadership
of Alexander E. Keith.
Principles of Selecting Switch. The underlying features of
this automatic system have already been referred to in the abstract.
A better grasp of its principles may, however, be had by considering
a concrete example of its most important piece of apparatus—the
selecting switch. The bare skeleton of such a switch, sufficient
only to illustrate the salient point in its mode of operation, is shown
in Fig. 380. The essential elements of this are a vertical shaft capable
of both longitudinal and rotary motion; a pawl and ratchet
mechanism actuated by a magnet for moving the shaft vertically
a step at a time; another pawl and ratchet mechanism actuated by
another magnet for rotating the shaft a step at a time; an arm carrying
wiper contacts on its outer end, mounted on and moving with
the shaft; and a bank of contacts arranged on the inner surface of a
section of a cylinder adapted to be engaged by the wiper contacts on
this movable arm.
These various elements are indicated in the merest outline
and with much distortion in Fig. 380, which is intended to illustrate
the principles of operation rather than the details as they actually
are in the system. In the upper left-hand corner of this figure, the
magnet shown will, if energized by impulses of current, attract and
release its armature and, in doing so, cause the pawl controlled by
this magnet to move the vertical shaft of the switch up a step at a time,
as many steps as there are impulses of current. The vertical movement
of this shaft will carry the wiper arm, attached to the lower end
of the shaft, up the same number of steps and, in doing so, will bring
the contacts of this wiper arm opposite, but not engaging, the corresponding
row of stationary contacts in the semi-cylindrical bank.
Likewise, through the ratchet cylinder on the intermediate portion
of the shaft, the magnet shown at the right-hand portion of this figure
will, when energized by a succession of electrical impulses, rotate
the shaft a step at a time, as many steps as there are impulses.
This will thus cause the contacts of the wiper arm to move over the
successive contacts in the row opposite to which the wiper had been
carried in its vertical movement.
At the lower left-hand corner of this figure, there is shown a
pair of keys either one of which, when operated, will complete the
circuit of the magnet to which it is connected, this circuit including
a common battery. In a certain rough way this pair of key switches
in the lower left-hand corner of the drawing may be taken as representing
the call-transmitting apparatus at the subscriber's station,
and the two wires extending therefrom may be taken as representing
the line wires connecting that subscriber's station to the central
office; but the student must avoid interpreting them as actual
representations of the subscriber's station calling apparatus or the
subscriber's line since their counterparts are not to be found in the
system as it really exists. Here again accuracy has been sacrificed
for ease in setting forth a feature of operation.
Still referring to Fig. 380, it will be seen that the bank contacts
consist of ten rows, each having ten pairs of contacts. Assume
again, for the sake of simplicity, that the exchange under consideration
has one hundred subscribers and that each pair of bank contacts
represents the terminals of one subscriber's line. Assume
further that the key switches in the lower left-hand corner of the
figure are being manipulated by a subscriber at that station and
that he wishes to obtain a connection with line No. 67. By pressing
and releasing the left-hand key six times, he will cause six separate
impulses of current to flow through the upper left-hand magnet and
this will cause the switch shaft to move up six steps and bring the
wiper arm opposite the sixth row of bank contacts. If he now
presses and releases his right-hand key seven times, he will, through
the action of the right-hand magnet, rotate the shaft seven steps,
thus bringing the wipers into contact with the seventh contact of the
sixth row and thus into contact with the desired line. As the wiper
contacts on the switch arm form the terminals of the calling subscriber's
line, it will be apparent that the calling subscriber is now connected
through his switch with the line of subscriber No. 67.
As stated, each of the pairs of bank contacts are connected with
the line of a subscriber; the line, Fig. 380, is shown so connected to
the forty-first pair of contacts, that is to the first contact in the fourth
row. The selecting switch shown in Fig. 380 would be for the sole
use of the subscriber on the line No. 41. Each of the other subscribers
would have a similar switch for his own exclusive use. Since any
of the switches must be capable of reaching line No. 67, for instance,
when moved up six rows and around seven, it follows that the
sixty-seventh pair of contacts in each bank of the entire one hundred switches
must also be connected together and to line No. 67. The same is,
of course, true of all the contacts corresponding to any other number.
Multiple connections are thus involved between the corresponding
contacts of the banks, in much the same way as in the corresponding
jacks in the multiple of a manual switchboard. As a result of this
multiple connection of the bank contacts, any subscriber may move
the wiper arm of his selecting switch into connection with the line of
any other subscriber.
The "Up-and-Around" Movement. The elemental idea to be
grasped by the discussion so far, is the so-called "up-and-around"
method of action of the selecting switches employed in this system.
This preliminary discussion may be carried a step further
by saying that the arrangement is such that when a subscriber
presses both his keys and grounds both of the limbs of his line,
such a condition is brought about as will cause all holding pawls
to be withdrawn from the shaft, and thus allow it to return to its
normal position with respect to both its vertical and rotary movements.
No attempt has been made in Fig. 380 to show how this is
accomplished.
Function of Line Switch. Such a system as has been briefly
outlined in the foregoing would require a separate selecting switch
for each subscriber's line and would be limited to use in exchanges
having not more than one hundred lines. In the modern system of
the Automatic Electric Company, the requirement that each subscriber
shall have a selective switch, individual to his own line,
has been eliminated by introducing what is called an individual
line switch by means of which any one of a group of subscribers'
lines, making a call, automatically appropriates one of a smaller
group of selecting switches and makes it its own only while the connection
exists.
Subdivision of Subscribers' Lines. The limitation as to the
size of the exchange has been overcome, without increasing the number
of bank contacts in any selecting switch, by dividing the subscribers'
lines into groups of one hundred and causing selecting
switches automatically to extend the calling subscriber's line first into
a group of groups corresponding, for instance, to the thousand
containing the called subscriber's line, and then into the particular
group containing the line, and lastly, to connect with the individual
line in that group.
Underlying Feature of Trunking System. It will be remembered
that in the chapter on fundamental principles of automatic systems,
it was stated that the subscriber, by means of the signal transmitter
at his station, was made to govern the action of the central-office
apparatus in the selection of a proper group of trunks; and the group
being selected, the central-office apparatus was made to act automatically
to pick out and connect with the first idle trunk of such
group. This selection by the subscriber of a group followed by
the automatic selection from among that group forms the basis of
the trunking system. It is impossible, by means of any simple
diagram, to show a complete scheme of trunking employed, but
Fig. 381 will give a fundamental conception of it. This figure
shows how a single calling line, indicated at the bottom, may find
access into any particular line in an office having a capacity for ten
thousand.
Names of Selecting Switches. Selecting switches of the "up-and-around"
type are the means by which the calling line selects
and connects with the trunk lines required in building up the connection,
and finally selects and connects with the line of the called
subscriber. Where such a switch is employed for the purpose of
selecting a trunk, it is called a selector switch. It is a first selector
when it serves to pick out a major group of lines, i. e., a group containing
a particular thousand lines or, in a multi-office system, a
group represented by a complete central office. It is a second selector
when it serves to make the next subdivision of groups; a third
selector if further subdivision of groups is necessary; and finally it is
a connector when it is employed to pick out and connect with the
particular line in the final group of one hundred lines to which the
connection has been brought by the selectors. In a single office of
10,000-line capacity, therefore, we would have first and second selectors
and connectors, the first selectors picking out the thousands,
the second selectors the hundreds, and the connectors the individual
line. In a multi-office system we may have first, second, and third
selectors and connectors, the first selector picking out the office, the
second selector the thousands in that office, the third selector the
hundreds, and the connector the individual lines.
The Line Switch. In addition to the selectors and connectors
there are line switches, which are comparatively simple, one individual
to each line. Each of these has the function, purely automatic,
of always connecting a line, as soon as a call is originated on it, to
some one of a smaller group of first selectors available to that line.
This idea may be better grasped when it is understood that, in the
earlier systems of the Automatic Electric Company, there was a
first selector permanently associated with each line. By the addition
of the comparatively simple line switch, a saving of about ninety
per cent of the first selectors was effected, since the number of first
selectors was thereby reduced from a number equal to the number
of lines in a group to a number equal to the number of simultaneous
connections resulting from calls originating in that group. In other
words, by the line switch, the number of first selectors is determined
by the traffic rather than by the number of lines.
Scheme of Trunking. With this understanding as to the names and
broader functions of the things involved, Fig. 381 may now be understood.
The line switch of the single line, as indicated here, has only
the power of selection among three trunks, but it is to be understood
that in actual practice, it would have access to a greater number,
usually ten. So, also, throughout this diagram we have shown the
apparatus and trunks arranged in groups of three instead of in groups
of ten, only the first three thousands groups being indicated and the
first three hundreds groups in each thousand. Again only three
levels instead of ten are indicated for each selecting switch, it being
understood that in the diagram the various levels are represented by
concentric arcs of circles, and the trunk contacts by dots on these
arcs.
Line-Switch Action. When the subscriber, whose line is shown
at the bottom of the figure, begins to make a call, the line switch
acts to connect his line with one of the first selector trunks available
to it. This selection is entirely preliminary and, except to start
it, is in no way under the control of the calling subscriber. The
calling line now has under its control a first selector which, for
the time being, becomes individual to it. Let it be assumed that
the line switch found the first of the first selector trunks already appropriated
by some other switch, but that the second one of these
trunks was found idle. This trunk being appropriated by the line
switch places the center one of the first selectors shown under the
control of the subscriber's line. This first selector then acts in response
to the first set of selective impulses sent out by his signal
transmitter.
First Selector Action. We will assume that the calling subscriber
desires to connect with No. 3213. The first movement of the subscriber's
signal transmitter will send, therefore, three impulses over
the line. These impulses will act on the vertical magnet of the first
selector switch to move it up three steps. On this "level" of the
contact bank of this switch all of the contacts will represent second
selector trunks leading to the third thousand group. The other
ends of these trunks will terminate in the wipers and also in the controlling
magnets of second selectors serving this thousand. This
function on the part of the first selector controlled by the act of the
subscriber will have thus selected a group of trunks leading to the
third thousand, but the subscriber has nothing to do with which one
of the trunks of this group will actually be used. Immediately
following the vertical movement of the first selector switch the rotary
movement of this switch will start and will continue until the wipers
of that switch have found contacts of an idle trunk leading to a second
selector. Assuming that the first trunk was the one found
idle, the first selector wipers would pause on the first pair of contacts
in the third level of its bank, and the trunk chosen may be seen leading
from that contact off to the group of second selectors belonging to
the third thousand. For clearness, the chosen trunks in this assumed
connection are shown heavier than the others.
Second Selector Action. The next movement of the dial by the
subscriber in establishing his desired connection will send two impulses,
it being desired to choose the second hundred in the third
thousand. The first selector will have become inoperative before
this second series of impulses is sent and, therefore, only the second
selector will respond. Its vertical magnet acting under the influence
of these two impulses will step up its wiper contacts opposite the
second row of bank contacts, and the subscriber will thus have
chosen the group of trunks leading to the second hundred in the
third thousand. Here, again, the automatic operation of picking
out the first idle one of this chosen group of trunks will take place
without the volition of the subscriber, and it will be assumed that
the first two trunks on this level of the second selector were found
already engaged and that the third was therefore chosen. The
connection continues, as indicated by heavy lines in Fig. 381, to the
third one of the connectors in the second hundred of the third thousand.
Any one of these connectors would have accomplished the
purpose but this is assumed to be the first one found idle by the
second selector.
Connector Action. The third movement of the subscriber's dial
will send but one impulse, this corresponding to the first group of
ten in the second hundred in the third thousand. This impulse will
move the connector shaft up to the first level of bank contacts; and
from now on the action of the connector differs radically from that
of the selectors. The connector is not searching for an idle trunk in
the group but for a particular line and, therefore, having chosen the
group of ten lines in the desired hundred, the connector switch waits
for further guidance from the subscriber. This comes in the form
of the final set of impulses sent by the subscriber's signal transmitter
which, in this case, will be three in number, corresponding
to the final digit in the number of the called subscriber. This series
of impulses will control the rotary movement of the connector wipers
which will move along the first level and stop on the third one. The
process is seen to be one of successive selection, first of a large group,
then of a smaller, again of a smaller, and finally of an individual.
If the line is found not busy, the connection between the two
subscribers is complete and the called subscriber's bell will be rung.
If it is found busy, however, the connector will refuse to connect and
will drop back to its normal position, sending a busy signal back
to the calling subscriber. The details of ringing and the busy-back
operation may only be understood by a discussion of drawings, subsequently
to be referred to.
Two-Wire and Three-Wire Systems. In most of the systems
of the Automatic Electric Company in use today the impulses by
which the subscriber controls the central-office apparatus flow over
one side of the line or the other and return by ground. The metallic
circuit is used for talking and for ringing the called subscriber's
bell, while ground return circuits, on one side of the line or the other,
are used for sending all the switch controlling impulses.
Recently this company has perfected a system wherein no
ground is required at the subscriber's station and no ground return
path is used for any purpose between the subscriber and the central
office. This later system is known as the "two-wire" system, and in
contra-distinction to it, the earlier and most used system has been
referred to as the "three-wire." It is not meant by this that the line
circuits actually have three wires but that each line employs three
conductors, the two wires of the line and the earth. The three-wire
system will be referred to and described in detail, and from it
the principles of the two-wire system will be readily understood.
Subscriber's Station Apparatus. The detailed operation of the
three-wire system may be best understood by considering the subscriber's
station apparatus first. The general appearance of the
wall set is shown in Fig. 382, and of the desk set in Fig. 383. These
instruments embody the usual talking and call-receiving apparatus
of a common-battery telephone and in addition to this, the signal
transmitter, which is the thing especially to be considered now. The
diagrammatic illustration of the signal transmitter and of the relation
that its parts bear to the other elements of the telephone set is shown
in Fig. 384. It has already been stated that the subscriber manipulates
the signal transmitter by rotating the dial on the face of the
instrument. A clearer idea of this dial and of the finger stop for it
may be obtained from Figs. 382 and 383.
Operation. To make a call for a given number the subscriber
removes his receiver from its hook, then places his forefinger in the
hole opposite the number corresponding to the first digit of the desired
number. By means of the grip thus secured, he rotates the
dial until its movement is stopped by the impact of the finger against
the stop. The dial is then released and in its return movement it
sends the number of impulses corresponding to the first digit in the
called number. A similar movement is made for each digit.
In Fig. 384 is given a phantom view of the dial, in order to show
more clearly the relation of the mechanical parts and contacts controlled
by it. For a correct idea of its mechanical action it must be
understood that the shaft 1, the lever 2, and the interrupter segment
3 are all rigidly fastened to the dial and move with it. A coiled
spring always tends to move the dial and these associated parts
back to their normal positions when released by the subscriber, and
a centrifugal governor, not shown, limits the speed of the return movement.
The subscriber's hook switch is mechanically interlocked with
the dial so as to prevent the dial being moved from its normal position
until the hook is in its raised position. This interlocking function
involves also the pivoted dog 4. Normally the lower end of this
dog lies in the path of the pin 5 carried on the lever 2, and thus
the shaft, dial, and segment are prevented from any considerable movement
when the receiver is on the hook. However, when the receiver
is removed from its hook, the upwardly projecting arm from the
hook engages a projection on the dog 4 and moves the dog out of the
path of the pin 5. Thus the dial is free to be rotated by the subscriber.
The pin 6 is mounted in a stationary position and serves
to limit the backward movement of the dial by the lever 2 striking
against it.
Ground Springs:—Five groups of contact springs must be
considered, some of which are controlled wholly by the position
of the switch hook, others jointly by the position of the switch hook
and the dial, others by the movement of the dial itself, and still others
by the pressure of the subscriber's finger on a button. The first of
these groups consists of the springs 7 and 8, the function of which
is to control the continuity of the ground connection at the subscriber's
station. The arrangement of these two springs is such that the
ground connection will be broken until the subscriber's receiver is
removed from its hook. As soon as the receiver is raised, these
springs come together in an obvious manner, the dog 4 being lifted
out of the way by the action of the hook. The ledge on the lower
portion of the spring 7 serves as a rest for the insulated arm of the
dog 4 to prevent this dog, which is spring actuated, from returning
and locking the dial until after the receiver has been hung up.
Bell and Transmitter Springs:—The second group is that embracing
the springs 9, 10, 11, and 12. The springs 10 and 11
are controlled by the lower projection from the switch hook, the
spring 11 engaging the spring 12 only when the hook is down.
The spring 10 engages the spring 9 only when the hook lever
is up and not then unless the dial is in its normal position. While
the hook is raised, therefore, the springs 9 and 10 break contact
whenever the dial is moved and make contact again when it returns
to its normal position. The springs 11 and 12 control the circuit
through the subscriber's bell while the springs 9 and 10 control the
continuity of the circuit from one side of the line to the other so as
to isolate the limbs from each other while the signal transmitter
is sending its impulses to the central office.
Impulse Springs:—The third group embraces springs 13, 14,
and 15 and these are the ones by which the central-office switches
are controlled in building up a connection.
Something of the prevailing nomenclature which has grown
up about the automatic system may be introduced at this point.
The movements of the selecting switches at the central office are
referred to as vertical and rotary for obvious reasons. On account
of this the magnet which causes the vertical movement is referred to as
the vertical magnet and that which accomplishes the rotary movement
as the rotary magnet. It happens that in all cases the selecting
impulses sent by the subscriber's station, corresponding respectively
to the number of digits in the called subscriber's number, are sent
over one side of the line and in nearly all cases these selecting impulses
actuate the vertical movements of the selecting switches.
For this reason the particular limb of the line over which the selecting
impulses are sent is called the vertical limb. The other limb of the
line is the one over which the single impulse is sent after each group
of selecting impulses, and it is this impulse in every case which causes
the selector switch to start rotating in its hunt for an idle trunk.
This side of the line is, therefore, called rotary. For the same reasons
the impulses over the vertical side of the line are called vertical
impulses and those over the rotary side, rotary impulses. The naming
of the limbs of the line and of the current impulses vertical and
rotary may appear odd but it is, to say the least, convenient and expressive.
Coming back to the functions of the third group of springs,
13, 14, and 15, 15 may be called the vertical spring since it sends
vertical impulses; 13, the rotary spring since it sends rotary impulses;
and 14, the ground spring since, when the hook is up, it is connected
with the ground.
On the segment 3 there are ten projections or cams 16 which,
when the dial is moved, engage a projection of the spring 15. When
the dial is being pulled by the subscriber's finger, these cams engage
the spring 15 in such a way as to move it away from the ground
spring and no electrical contact is made. On the return of the dial,
however, these cams engage the projection on the spring 15 in the
opposite way and the passing of each cam forces this vertical spring
into engagement with the ground spring. It will readily be seen,
therefore, by a consideration of the spacing of these cams on the
segment and the finger holes in the dial that the number of cams
which pass the vertical spring 15 will correspond to the number on
the hole used by the subscriber in moving the dial.
Near the upper right-hand corner of the segment 3, as shown
in Fig. 384, there is another projection or cam 17, the function of
which is to engage the rotary spring 13 and press it into contact with
the ground spring. Thus, the first thing that happens in the movement
of the dial is for the projection 17 to ride over the hump on the
rotary spring and press the contact once into engagement with the
ground spring; and likewise, the last thing that happens on the return
movement of the dial is for the rotary spring to be connected
once to the ground spring after the last vertical impulse has been
sent.
If both the rotary and vertical sides of the line are connected
with the live side of the central-office battery, it follows that every
contact between the vertical and the ground spring or between the
rotary and the ground spring will allow an impulse of current to flow
over the vertical or the rotary side of the line.
We may summarize the action of these impulse springs by
saying that whenever the dial is moved from its normal position,
there is, at the beginning of this movement, a single rotary impulse
over the rotary side of the line; and that while the dial returns,
there is a series of vertical impulses over the vertical side of the line;
and just before the dial reaches its normal position, after the sending
of the last vertical impulse, there is another impulse over the rotary
side of the line.
The mechanical arrangements of the interrupter segment 3 and
its associated parts have been greatly distorted in Fig. 384 in order
to make clear their mode of operation. This drawing has been
worked out with great care, with this in mind, at a sacrifice of accuracy
in regard to the actual structural details.
Ringing Springs:—The fourth group of springs in the subscriber's
telephone is the ringing group and embraces the springs
18, 19, and 20. The springs 19 and 20 are normally closed and
maintain the continuity of the talking circuit. When, however,
the button attached to the spring 19—which button may be seen
projecting from the instrument shown in Fig. 382, and from the base
of the one shown in Fig. 383—is pressed, the continuity of the talking
circuit is interrupted and the vertical side of the line is connected
with the ground. It is by this operation, after the connection has
been made with the desired subscriber's line, that the central-office
apparatus acts to send ringing current out on that line.
Release Springs:—The fifth set of springs is the one shown at
the left-hand side of Fig. 384, embracing springs 21, 22, and 23. The
long curved spring 21 is engaged by the projecting lug on the switch
hook when it rises so as to press this spring away from the other
two. On the return movement of the hook, however, this spring
is pressed to the left so as to bring all three of them into contact,
and this, it will be seen, grounds both limbs of the line at the subscriber's
station. This combination cannot be effected by any of
the other springs at any stage of their operation, and it is the one
which results in the energization of such a combination of relays
and magnets at the central office as will release all parts involved in
the connection and allow them to return to their normal positions
ready for another call.
Salient Points. If the following things are borne in mind about
the operation of the subscriber's station apparatus, an understanding
of the central-office operations will be facilitated. First, the selective
impulses always flow over the vertical side of the line; they are
always preceded and always followed by a single impulse over the
rotary side of the line. The ringing button grounds the vertical side
of the line and the release springs ground both sides of the line
simultaneously.
The Line Switch. The first thing to be considered in connection
with the central-office apparatus is the line switch. This, it will
be remembered, is the device introduced into each subscriber's line
at the central office for the purpose of effecting a reduction of the
number of first selectors required at the central office, and also for
bringing about certain important functional results in connection
with trunking between central and sub-offices. The function of the
line switch in connection with the subscriber's line, however, is
purely that of reducing the number of first selectors.
The line switches of one hundred lines are all associated to
form a single unit of apparatus, which, besides the individual line
switches, includes certain other apparatus common to those lines.
Such a group of one hundred line switches and associated common
apparatus is called a line-switch unit, or frequently, a Keith unit.
Confusion is likely to arise in the mind of the reader between the
individual line switch and the line-switch unit, and to avoid this we
will refer to the piece of apparatus individual to the line as the line
switch, and to the complete unit formed of one hundred of these devices
as a line-switch unit.
Line and Trunk Contacts. Each line switch has its own bank
of contacts arranged in the arc of a circle, and in this same arc are
also placed the contacts of each of the ten individual trunks which
it is possible for that line to appropriate. The contacts individual to
the subscriber's line in the line switch are all multipled together,
the arrangement being such that if a wedge or plunger is inserted at
any point, the line contacts will be squeezed out of their normal position
so as to engage the contacts of the trunk corresponding to the
particular position in the arc at which the wedge or plunger is inserted.
A small plunger individual to each line is so arranged that
it may be thrust in between the contact springs in the line-switch
bank in such manner as to connect any one of the trunks with the
line terminals represented in that row, the particular trunk so connected
depending on the portion of the arc toward which the
plunger is pointed at the time it is thrust in the contacts.
These banks of lines and trunk contacts are horizontally arranged,
and piled in vertical columns of twenty-five line switches each. The
ten trunk contacts are multipled vertically through the line-switch
banks, so that the same ten trunks are available to each of the
twenty-five lines. We thus have, in effect, an old style, Western
Union, cross-bar switchboard, the line contacts being represented
in horizontal rows and the trunk contacts in vertical rows, the connection
between any line and any trunk being completed by inserting
a plunger at the point of intersection of the horizontal and the vertical
rows corresponding to that line and trunk.
Trunk Selection. The plungers by which the lines and trunks
are connected are, as has been said, individual to the line, and all
of the twenty-five plungers in a vertical row are mounted in such
manner as to be normally held in the same vertical plane, and this
vertical plane is made to oscillate back and forth by an oscillating
shaft so as always to point the plungers toward a vertical row of
trunk contacts that represent a trunk that is not in use at the time.
The to-and-fro movement of this oscillating shaft, called the master
bar, is controlled by a master switch and the function of this master
switch is always to keep the plungers pointed toward the row of
contacts of an idle trunk. The thrusting movement of the individual
plungers into the contact bank is controlled by magnets
individual to the line and under control of the subscriber in
initiating a call. As soon as the plunger of a line has been thus
thrust into the contact bank so as to connect the terminals of that
line with a given trunk, the plunger is no longer controlled by the
master bar and remains stationary. The master bar then at once
moves all of the other plungers that are not in use so that they will
point to the terminals of another trunk that is not in use. The
plungers of all the line switches in a group of twenty-five are, therefore,
subject to the oscillating movements of the master bar when
the line is not connected to a first selector trunk. As soon as a call
is originated on a line, the corresponding plunger is forced into the
bank and is held stationary in maintaining the connection to a first
selector trunk, and all of the other plungers not so engaged, move on
so as to be ready to engage another idle trunk.
Trunk Ratio. The assignment of ten trunks to twenty-five
lines would be a greater ratio of trunks than ordinary traffic conditions
require. This ratio of trunks to lines is, however, readily
varied by multipling the trunk contacts of several twenty-five line
groups together. Thus, ten trunks may be made available to one
hundred subscribers' lines by multipling the trunks of four twenty-five
line switch groups together. In this case the four master bars
corresponding to the four groups of twenty-five line switches are all
mechanically connected together so as to move in unison under the
control of a single master switch. If more than ten and less than
twenty-one trunks are assigned to one hundred lines, then each set
of ten trunks is multipled to the trunk contacts of fifty line switches,
the two master bars of these switches being connected together and
controlled by a common master switch.
Structure of Line Switch. The details of the parts of a line
switch that are individual to the line are shown in Fig. 385, the line
and trunk contact bank being shown in the lower portion of this
figure and also in a separate view in the detached figure at the right.
A detailed group of several such line switches with the oscillating
master bar is shown in Fig. 386. This figure shows quite clearly
the relative arrangement of the line and trunk contact banks, the
plungers for each bank, and the master bar.
In practice, four groups of twenty-five line switches each are
mounted on a single framework and the group of one hundred line
switches, together with certain other portions of the apparatus that
will be referred to later, form a line-switch unit. A front view of
such a unit is shown in Fig. 387. In order to give access to all portions
of the wiring and apparatus, the framework supporting each
column of fifty line switches is hinged so as to open up the interior
of the device as a whole. A line-switch unit thus opened out is shown
in Fig. 388.
Circuit Operation. The mode of operation of the line switch
may be best understood in connection with Fig. 389, which shows
in a schematic way the parts of a line switch that are individual to a
subscriber's line, and also those that are common to a group of fifty
or one hundred lines. Those portions of Fig. 389 which are individual
to the line are shown below the dotted line extending across
the page. The task of understanding the line switch will be made
somewhat easier if Figs. 385 and 389 are considered together. The
individual parts of the line switch are shown in the same relation to
each other in these two figures with the exception that the bank of
line and trunk springs in the lower right-hand corner of Fig. 389
have been turned around edgewise so as to make an understanding
of their circuit connections possible.
The vertical and rotary sides of the subscriber's line are shown
entering at the lower left-hand corner of this figure, and they pass
to the springs of the contact bank. Immediately adjacent to these
springs are the trunk contacts from which the vertical and the rotary
limbs of the first selector trunk proceed. The plunger is indicated
at 1, it being in the form of a wheel of insulating material.
It is carried on the rod 2 pivoted on a lever 3, which, in turn, is pivoted
at 4 in a stationary portion of the framework. A spring 5, secured
to the underside of the lever 3 and projecting to the left beyond the
pivot 4 of this lever, serves always to press the right-hand portion of
the lever 3 forward in such direction as to tend to thrust it into the
contact bank. The plunger is normally held out of the contact
bank by means of the latch 6 carried on the armature 7 of the trip
magnet. When the trip magnet is energized it pulls the armature
7 to the left and thus releases the plunger and allows it to enter the
contact bank.
The master bar is shown at 8, and a feather on this bar engages
a notch in the segment attached to the rear end of the plunger rod 2.
This master bar is common to all of the plunger rods and by its oscillatory
movement, under the influence of the master switch, it always
keeps all of the idle plunger bars pointed toward the contacts
of an idle trunk. As soon, however, as the trip magnet is operated
to cause the insertion of a plunger into the contact bank, the feather
on the master bar is disengaged by the notch in the segment of the
plunger rod, and the plunger rod is, therefore, no longer subject to
the oscillating movement of the master bar.
When the release magnet is energized, it attracts its armature
9 and this lifts the armature 7 of the trip magnet so that the latch 6
rides on top of the left-hand end of the lever 3. Then, when the release
magnet is de-energized, the spring 5, which was put under
tension by the latch, moves the entire structure of levers back
to its normal position, withdrawing the plunger from the bank of
contacts. The notch on the edge of the segment of the plunger
rod, when thus released, will probably not strike the feather on the
master bar, and the plunger rod will thus not come under the control
of the master bar until the master bar has moved, in its oscillation,
so that the feather registers with the notch, after which this bar
will move with all the others.
If, while the plunger is waiting to be picked up by the master
bar, the same subscriber should call again, his line will be connected
with the same trunk as before. There is no danger in this, however,
that the trunk will be found busy, because the master bar will not
have occupied a position which would make it possible for any of
the lines to appropriate this trunk during the intervening time.
Master Switch. Associated with each master bar there is a
master switch which determines the position in which the master
bar shall stop in order that the idle plungers may be pointed
always to the contacts of an idle trunk. The arm 10 of this
switch is attached to the master bar and oscillates with it and
serves to connect the segment 11 successively with the contacts 12,
which are connected respectively to the third, or release wire of
each first selector trunk. In the figure the arm 10 is shown resting
on the sixth contact of the switch and this sixth contact is connected
to a spring 13 in the line-switch contact bank that has not yet been
referred to. As soon as the plunger is inserted into the contact
bank, the spring 14 will be pressed into engagement with the spring
13, and this spring 14 is connected with the live side of the battery
through the release magnet winding.
The contact strip 11 on the master switch is thus connected
through the release magnet to the battery and from this current
flows through the left-hand winding of the master-switch relay.
This energizes this relay and causes the closure of the circuit of
the locking magnet which magnet unlocks the master bar to permit
its further rotation. The unlocking of the master bar brings the
spring 15 into engagement with 16 and thus energizes the master
magnet, the armature of which vibrates back and forth after the
manner of an electric-bell armature, and steps the wheel 17 around.
The wheel 17 is mechanically connected to the master bar so that
each complete revolution of the wheel will cause one complete oscillation
of the master bar. The master bar will thus be moved so as to
cause all the idle plungers to sweep through an arc and this movement
will stop as soon as the master-switch arm 10 connects the
arc 11 with one of the contacts 12 that is not connected to the live
side of the battery through the springs 13 and 14 of some other line
switch. It is by this means that the plungers of the line switches
are always kept pointing at the contacts of an idle trunk. The way
in which this feature has been worked out must demand admiration
and accounts for the marvelous quickness of this line switch. The
fact that the plungers are pointed in the right direction before the
time comes for their use, leaves only the simple thrusting motion of
the plunger to accomplish the desired connection immediately upon
the initiation of a call by the subscriber.
Locking Segment. It will be understood that the locking segment
18 and the master-switch contact finger 10 are both rigidly connected
with the master bar 8 and move with it, the locking segment
18 serving always to determine accurately the angular position at
which the master bar and the master-switch arm are brought to rest.
Bridge Cut-Off. One important feature of automatic switching,
particularly as exemplified in the system of the Automatic Electric
Company, is the disconnection, after its use, of each operating
magnet of each piece of apparatus involved in making a connection.
Since these operating magnets are always bridged across the line at
the time of their operation and then cut off after they have performed
their function, this feature may be referred to as the bridge cut-off.
Guarding Functions. Still another feature of importance is
the means for guarding a line or a piece of apparatus that has already
been appropriated or made busy, so that it will not be appropriated
or connected with for use in some other connection. For this latter
purpose contacts and wires are associated with each piece of apparatus,
which are multipled to similar contacts on other pieces of apparatus
in much the same way and for a similar purpose that the
test thimbles in a multiple switchboard are multipled together.
Such wires and contacts in the Automatic Electric Company's apparatus
are called private wires and contacts.
The bridge cut-off and guarding functions are provided for in
the line switch by a bridge cut-off relay shown in Fig. 389 and also
in Fig. 385, it being the upper one of the individual line relays in
each of those figures. This bridge cut-off relay is operated as soon
as the plunger of the line is thrust into the bank; the contacts 19 and
20, closed by the plunger, serving to complete the circuit of this relay.
To make clear the bridge cut-off feature it will be noted that the trip
magnet of a line switch is connected in a circuit traced from the
rotary side of the line through the contacts 21 and 22 of the bridge
cut-off relay, thence through the coil of the trip magnet to the common
wire leading to the spring 23 of the master-bar locking device
and thence to the live side of the battery. Obviously, therefore, as
soon as the bridge cut-off relay operates, the trip magnet becomes
inoperative and can cause no further action of the line switch because
its circuit is broken between the springs 21 and 22.
The private or guarding feature is taken care of by the action
of the plunger in closing contacts 19 and 20, since the private wire
leading to the bridge cut-off relay is, as has already been stated, connected
to ground when these contacts are closed. This private wire
leads off and is multipled to the private contacts on all the connectors
that have the ability to reach this line, and the fact that this wire
is grounded by the line switch as soon as it becomes busy, establishes
such conditions at all of the connectors that they will refuse
to connect with this line as long as it is busy, in a way that will be
pointed out later on.
Relation of Line Switch and Connectors. The vertical and
rotary wires of the subscriber's line are shown leading off to the
connector banks at the left-hand side of Fig. 389, and one side of this
connection passes through the contacts 24 and 25 of the bridge cut-off
relay on the line switch. It is through this path that a connection
from some other line through a connector to this line is established
and it is seen that this path is held open until the bridge cut-off relay
of the line switch is operated. For such a connection to this line the
bridge cut-off relay of the line switch is operated over the private
wire leading from the connector, and the operation of the bridge cut-off
relay at this time serves to render inoperative the line switch, so
that it will not perform its usual functions should the called subscriber
start to make a call after his line had been seized.
Summary of Line-Switch Operation. To summarize the operation
of a line switch when a call is originated on its line, the
first movement of the calling subscriber's dial will ground the rotary
side of the line and operate the trip magnet. This will cause
the plunger to be inserted into the bank, and thus extend the line to
the first selector trunk through the closing of the right-hand set of
springs shown in the lower right-hand corner of Fig. 389. The
insertion of the plunger will also connect the battery through the
left-hand winding of the master-switch relay and, by the sequence of
operations which follows, cause the master bar to move all of the idle
plungers so as to again point them to an idle trunk. The closure of
contacts 19 and 20 by the plunger causes the operation of the bridge
cut-off relay which opens the circuit of the trip magnet, rendering it
inoperative; and also establishes ground potential on all the private
wire contacts of that line in the banks of the connectors, so as
to guard the line and its associated apparatus against intrusion by
others. The line is cut through, therefore, to a first selector and all
of the line-switch apparatus is completely cut off from the talking
circuit.
It must be remembered that all of the actions of the line switch,
which it has taken so long to describe, occur practically instantaneously
and as a result of the first part of the first movement of the subscriber's
dial. The line switch has done its work and "gone out of
business" before the selective impulses of the first digit begin to
take place.
Selecting Switches. The first selector is now in control of the
calling subscriber. The circuits and elements of the first selector
switch are shown in Fig. 390. The general mechanical structure
of the first selectors, second selectors, and connectors, is the same
and may be referred to briefly here. Fig. 391 shows a rear view of a
first selector; Fig. 392, a side view of a second selector; and Fig.
393, a front view of a connector. The arrangement of the vertical
and rotary magnets, of the selector shafts, and of the contact banks
are identical in all three of these pieces of apparatus and all these
switches work on the "up-and-around principle" referred to in connection
with Fig. 380. It is thought that with the general structure
shown in Figs. 391, 392, and 393 in mind, the actual operation may
be understood much more readily from Fig. 390.
Four magnets—the vertical, the rotary, the private, and the release—produce
the switching movements of the machine. These
magnets are controlled by various combinations brought upon the
circuits by three relays—the vertical, the rotary, and the back release.
The fourth relay shown, called the off-normal, is purely for
signaling purposes, as will be described.
Side Switch. Another important element of the selecting
switches is the so-called side switch which might better be called a
pilot switch—but we are not responsible for its name. This side
switch has for its function the changing of the control of the subscriber's
line to successive portions of the selector mechanism, rendering
inoperative those portions that have already performed their
functions and that, therefore, are no longer needed. This switch
may be seen best in Fig. 392 just above the upper bank of contacts.
It is shown in Fig. 390 greatly distorted mechanically so as to better
illustrate its electrical functions.
The contact levers 1, 2, 3, and 4 of the side switch are carried
upon the arm 5 which is pivoted at 6. All of these contact levers,
therefore, move about 6 as an axis. The side switch has three positions
and it is shown, in Fig. 390, in the first one of these. When
the private magnet armature is attracted and released once, the escapement
carried by it permits the spring 7 to move the arm 5 so as
to bring the wipers of the side switch into its second position; the
second pulling up and release of the private magnet armature will
cause the movement of the side switch wipers into the third position.
It is to be noted that the escapement
which releases the side
switch arm may be moved either
by the private or by the rotary
magnet, since the armature of the
latter has a finger which engages
the private magnet armature.
Functions of Side Switch.
The functions of the side switch
may be briefly outlined in connection
with the first selector, as
an example. In the first position
it extends the control of the subscriber's
signal transmitter through
the first selector trunk and line
relays to the vertical and private
magnets so that these magnets
will be responsive to the selecting
impulses corresponding to the first
digit. In its second position it
brings about such a condition of
affairs that the rotary magnet will
be brought into play and automatically
move the wipers over
the bank contacts in search of an
idle trunk. In its third position,
both the vertical and rotary relays
are cut off and the line is cut
straight through to the second selector
trunk, and only those parts
of the first selector apparatus are left in an operative state which
have to do with the private or guarding circuits and with the release.
Similar functions are performed by the side switch in connection
with the other selecting switches.
Release Mechanism. Another one of the features of the switch
that needs to be considered before a detailed understanding of its
operation may be had, is the mechanical relation of the holding and
the release dog. This dog is shown at 8 and, in the language of
the art, is called the double dog.
As will be seen, it has two retaining
fingers, one adapted to engage
the vertical ratchet and the
other, the rotary ratchet on the
selector shaft. This double dog
is pivoted at 9 and is interlinked
in a peculiar way with the armature
of the vertical magnet, the
armature of the release magnet,
and the arm of the side switch.
The function of this double dog
is to hold the shaft in whatever
vertical position it is moved by
the vertical magnet and then,
when the rotary magnet begins
to operate, to hold the shaft in
its proper angular position. It
will be noted that the fixed dog
10 is ineffective when the shaft
is in its normal angular position.
But as soon as the shaft is rotated,
this fixed dog 10 becomes the
real holding pawl so far as the
vertical movement is concerned.
The double dog 8 is normally
held out of engagement with the
vertical and the rotary ratchets
by virtue of the link connection,
shown at 11, between the release
magnet armature and the rear
end of the double dog. On the previous release of the switch the
attraction of the release magnet armature permitted the link 11 to
hook over the end of the dog 8 and thus, on its return movement,
to pull this dog out of engagement with its ratchets. This movement
also resulted in pushing on the link 12 which is pivoted to the
side switch arm 5, and thus the return movement of the release
magnet is made to restore the side switch to its normal position.
In order that the double dog may
be made effective when it is required,
and in order that the side
switch may be free to move under
the influence of the private magnet,
the double dog is released
from its connection with the release
magnet armature by the first
movement of the vertical magnet
in a manner which is clear from
the drawing.
First Selector Operation. In
discussing the details of operation
of the various selectors it will be
found convenient to divide the
discussion according to the position
of the side switch. This will
bring about a logical arrangement
because it is really the side switch
which determines by its position
the sequence of operation.
First Position of Side Switch.
This is the position shown in Fig.
390, and is the normal position.
The vertical and the rotary lines
extending from the calling subscriber
are continued by the levers
1 and 2 of the side switch through
the vertical and the rotary relay
coils, respectively, to the live side
of battery. The lever 4 of the
side switch in this position connects to ground the circuit leading
from the line switch through the release trunk, and the winding
of the off-normal relay. This winding is thus put in series
with the release magnet of the line switch, but on account of high
resistance of the off-normal relay no operation of the release magnet
is caused. This will, however, permit such current to flow through
the release circuit as will energize the sensitive off-normal relay
and cause it to attract its armature and light the off-normal lamp.
If this lamp remains lighted more than a brief period of time, it
will attract notice and will indicate that the corresponding selector
has been appropriated by a line switch and that for some reason the
selector has gone no further. This lamp, therefore, is an aid in
preventing the continuance of this abnormal condition.
The first thing that happens after the line switch has connected
the calling subscriber with the first selector is a succession of impulses
over the vertical side of the line, this being the set of impulses
corresponding in number to the thousands digit or to the office, if
there is more than one. It will be understood that here we are considering
a single office of ten-thousand-line capacity or thereabouts,
and that, therefore, this first set of impulses corresponds to the
thousands digit in the called subscriber's line. Each one of these impulses
will flow from the battery through the vertical relay and each
movement of this relay armature will close the circuit of the vertical
magnet and cause the shaft of the selector to be stepped up to the
proper level. Immediately following the first series of selecting
impulses from the subscriber's station, a single impulse follows
over the rotary side of the line. This gives the rotary relay armature
one impulse and this in turn closes the circuit of the private
magnet once. The single movement of the private magnet armature
allows the escapement finger on the arm 5 to move one step and
this brings the side switch contacts into the second position.
Second Position of Side Switch. In this position lever 4 of the
side switch places a ground on the wire leading through the rotary
magnet to a source of interrupted battery current. The impulses
which thus flow through the rotary magnet occur at a frequency
dependent upon the battery interrupter and this is at a rate of approximately
fifteen impulses per second. The rotary magnet will
step the selector shaft rapidly around until something occurs to stop
these impulses. This something is the finding by the private wiper
of an ungrounded private contact in the bank, since all of the contacts
corresponding to busy trunks are grounded, as will be explained.
The action of the private magnet enters into this operation in
the following way: A circuit may be traced from the battery through
the private magnet to the third side switch wiper when in its second
position, thence through the back release relay to the private
wiper. If the wiper is at the time on the private bank contact of a
busy trunk, it will find that contact grounded and the private magnet
will be energized. The energizing of this magnet will not, however,
cause the release of the side switch. It must be energized and
de-energized. The private magnet armature will, therefore, be operated
by the finger of the rotary magnet armature on the first rotary
step. The private magnet will be energized and hold its armature
operated if the private wiper finds a ground on the first bank contact
and will stay energized as long as the private wiper is passing
over private contacts of busy trunks. Its armature will not be
allowed to fall back during the passage of the wiper from one trunk
to another, because during that interval the finger of the rotary
magnet will hold it operated. As soon, however, as the private
wiper reaches the private bank contact of an idle trunk, no ground
will be found and the circuit of the private magnet will be left open.
When the impulse through the rotary magnet ceases, the private
magnet armature will fall back and the side switch will be released to
its third position.
Third Position of Side Switch. The first thing to be noted in this
position is that the calling line is cut straight through to the second
selector trunk, the connection being clean with no magnets bridged
across or tapped off. The third wiper of the side switch, when in its
third position, is grounded and this connects the release wire of the
second selector trunk, on which the switch wipers rest, through the
private wiper, the winding of the back release magnet, and the third
wiper of the side switch to ground. This establishes a path for the
subsequent release current through the back release magnet; and, of
equal importance, it places a ground on the private bank contact of
that trunk so that the private wiper of any other switch will be prevented
from stopping on the contacts of this trunk in the same manner
that the wiper of this switch was prevented from stopping on other
trunks that were already in use.
The fourth lever on the side switch, when in its third position,
serves merely to close the circuit of the rotary off-normal lamp. This
lamp is for the purpose of calling attention to any first selector switch
that has been brought into connection with some second selector
trunk and which, for some reason, has failed in its release. These
off-normal lamps are so arranged that they may be switched off
manually to avoid burning them during the hours of heaviest
traffic. At night they afford a ready means of testing for switches
that have been left off-normal, since the manual switches controlling
these lamps may then be closed, and any lamps which burn will show
that the switches corresponding to them are off-normal. Simple
tests then suffice to show whether they are properly or improperly
in their off-normal position.
Release of the First Selector. As will be shown later, the normal
way of releasing the switches is from the connector back over the release
wire. It is sufficient to say at this point that when the proper
time for release comes, an impulse of current will come back over
the second selector trunk release wire through the private wiper,
to the back release relay magnet, and thence to ground through the
third wiper of the side switch which is in its third position. It may
be asked why the back release magnet was not energized during the
previous operations described, when current passed through it.
The reason for this is that in those previous operations the private
magnet was always included in series in the circuit and on account of
the high resistance of the private magnet, sufficient current did not
pass through the back release magnet to energize it.
When the back release relay is energized, it closes the circuit
of the release magnet and thus, through the link 11, draws the double
dog away from its engagement with the shaft ratchets and at the same
time, through the link 12, restores the side switch to its normal position.
Whenever the release magnet is operated it acts as a relay to close a
pair of contacts associated with it and thus to momentarily ground
the release wire of the first selector trunk extending back to the line
switch. Referring to Fig. 389, it will be seen that this path leads
through the contacts 13 and 14 and the release magnet to the battery.
It is by this means that the line switch is released, the release
impulse being relayed back from the first selector.
Second Selector Operation. For the purpose of considering the
action of the second selector, we will go back to the point where the
first selector had connected with a second selector trunk and where
its side switch had moved into its third position. In this condition,
it will be remembered, the trunk line was cut through to a second
selector trunk and all first selector apparatus cleared from the
talking circuit.
The second selector chosen is one corresponding to the thousands
group as determined by the first digit of the called subscriber's
number. The circuits of a second selector are shown in Fig. 394
and it must be borne in mind that the mechanical arrangements for
producing the vertical and the rotary movement of the shaft and
for operating the side switch are practically the same as those of the
first selector. As in the first selector, the sequence of operation
is controlled by the successive positions of the side switch, the
first position permitting the selection of the hundreds corresponding
to the vertical impulses, the second position allowing the selector
to search for an idle trunk in that hundred, and the third position
cutting the trunk through and clearing the circuit of obstructing
apparatus.
First Position of Side Switch. The first thing that happens
when the subscriber begins to move his dial in the transmission of
the second series of selecting impulses is the sending of a preliminary
impulse over the rotary side of the line. This, in the case of the
second selector, energizes the rotary relay which, in turn, energizes
the private magnet; but the private magnet in the case of the second
selector can do nothing toward the release of the side switch because
the projection 5', on the side switch arm 5, meets a projection on the
rear of the selector shaft which thus prevents the movement of the
side switch arm 5 until the selector shaft has been moved out of
its normal position.
Immediately after the establishment of the connection to the
selector, the second set of selecting impulses comes in over the vertical
wire from the subscriber's station. These impulses, corresponding
in number to the hundreds digit, will energize the vertical
relay and cause it, in turn, to energize the vertical magnet, stepping
up the selector shaft to the level corresponding to the hundred sought.
The single rotary impulse, which follows just before the subscriber's
dial reaches its normal position, will energize the rotary relay of the
second selector. This, in turn, energizes the private magnet which
makes a single movement of its armature and allows the escapement
finger on the side switch arm to move one step and bring the side
switch contacts into the second position.
Second Position of Side Switch. No detailed discussion of this is
necessary, since, with the side switch in its second position, the actions
which occur in causing the wipers of the second selector to seek and connect
with an idle trunk line, are exactly the same as in the case of the
first selector. When the second selector wipers finally reach a resting
place on the bank contacts, the private magnet armature, operated
during the hunting process, is released and the side switch is thus
shifted into the third position.
Third Position of Side Switch. The moving of the side switch
into its final position brings about the same state of affairs with respect
to the second selector that already exists with respect to the
first selector. The trunk line is cut straight through and all bridge
circuits or by-paths from it are cut off. The same guarding conditions
are established to prevent other lines or other pieces of apparatus
from making connections that will interfere with the one being established,
and the same provisions are made for working the back
release when the proper impulse comes from the connector, and for
passing this back release impulse on to the first selector in the same
way that the first selector passes it on to the line switch. The line
of the calling subscriber has now been extended to a connector, and
that connector is one of a group—usually ten—which alone has the
ability to reach the particular hundred lines containing the line of
the desired subscriber. The selection has, therefore, been narrowed
down from one in ten thousand to one in one hundred.
The Connector—Its Functions. It has already been stated
that the connector is of the same general type of apparatus as the
first and the second selectors. Unlike the first and the second
selectors, however, the connector is required to make a double selection
under the guidance of the subscriber. The first selector makes
a single selection of a group under the guidance of the subscriber
and then an automatic selection in that group not controlled by the
subscriber. So it is with the second selector. The connector, however,
makes a selection of a group of ten under the guidance of the
subscriber and then, again under the guidance of the subscriber,
it picks out a particular one of that group.
The connector also has other functions in relation to the ringing
of the called subscriber and the giving of a busy signal to the calling
subscriber in case the line wanted is found busy. It has still other
functions in that the talking current, which is finally supplied to
connected subscribers, is supplied through paths furnished by it.
Location of the Connectors. Connectors are the only ones of
the selecting switches that are in any sense individual to the subscribers'
lines. None of them is individual to a subscriber's line, but it
may be said that a group of ten connectors is individual to a group of
one hundred subscribers' lines. Since each group of one hundred
lines has a group of connectors of its own and since each one hundred
lines also has a line-switch unit of its own, and since the lines of
this group must be multipled through the bank contacts of the
connectors of this individual group and through the bank contacts
of the line switches of this particular unit, it follows that on account
of the wiring problems involved there is good reason for mounting
the connectors in close proximity to the line switches representing
the same group of lines. Some help in the grasping of this thought
may result if it be remembered that the line switch is, so to speak,
the point of entry of a call and that the connector is the point of exit,
and, in order to reduce the amount of wiring and to economize space,
the point of exit and the point of entry are made as close together
as possible.
The relative locations and grouping of the line switches and
connectors are clearly shown in Fig. 395, which is a rear view of the
same line-switch unit that was illustrated in Figs. 387 and 388.
Operation of the Connector. The circuits of the connector
are shown in Fig. 396. In addition to the features that have been
pointed out in the first and the second selectors, all of which are
to be found, with some modifications, perhaps, in the connector,
there must be considered the features in the connector of busy-signal
operation, of ringing the called subscriber, of battery supply to
both subscribers, and of the trunk release operation. These may
be best understood by tracing through the operations of the connector
from the time it is picked up by a second selector until the connection
is finally completed, or until the busy signal has been given in case
completion was found impossible. As in the first and the second
selectors, the sequence of operations is determined by the position
of the side switch.
First Position of Side Switch. The connector in a ten-thousand-line
system is the recipient of the impulses resulting from the third
and fourth movements of the subscriber's dial. Considering the
third movement of the subscriber's dial, the first impulse resulting
from it comes over the rotary side of the line and results in the rotary
relay attracting its armature once. This results in a single impulse
through the private magnet which, however, does nothing because the
projection 5' strikes against a projection on the selector shaft. These
two projections interfere only when the selector shaft is in its normal
position. Then follows the series of impulses from the subscriber's
station corresponding to the tens digit in the called subscriber's
number. These pass over the vertical side of the line and through
the vertical relay, energizing that relay a corresponding number
of times.
The vertical magnet, as in the case of the first and the second
selectors, is included in the circuit controlled by the vertical relay
and this results in the connector shaft being stepped up to the level
corresponding to the particular tens group containing the called subscriber's
number. It will be noted that the impulses from the vertical
side of the line, which cause this selection, pass through one winding
13 of the calling battery supply relay. This relay is operated by
these vertical selecting impulses, but in this position of the side switch
the closure of its local circuits accomplishes nothing.
Immediately after the tens group of selecting impulses over the
vertical side of the line, there follows a single rotary impulse from the
subscriber's station which, as in the case of the first and the second
selectors, energizes the rotary relay and causes it to give one impulse
to the private magnet. This impulse is now able, since the shaft
has moved from its normal position, to release the side switch arm
one notch, and the side switch, therefore, moves into its second position.
Second Position of Side Switch. It is principally in this second
position of the side switch that the connector selecting function differs
from that of the first and the second selector. There is no trunk
to be hunted, but rather the rotary movement of the connector wipers
must be made in response to the impulses, from the subscriber's
station, which correspond to the units digit in the selected number.
The first impulse corresponding to the fourth movement of the subscriber's
dial is a rotary one, and, as usual, it passes through the rotary
relay winding and this, in turn, gives an impulse to the private
magnet. The private magnet at this time has already released the
side switch arm to its second position, but it is unable to release it
further because of a feather on the wiper shaft—which projects just
far enough to engage the lug 5', when the shaft is in its normal
angular position—thus preventing the side switch arm from moving farther
than its second position.
Then follows over the vertical side of the line the last set of
selecting impulses corresponding to the units digit. This, as before,
energizes the vertical relay, but in the second position of the side
switch, it is to be noted, that the vertical relay no longer controls the
vertical magnet; the side switch has shifted the control of the vertical
relay to the rotary magnet. The rotary magnet is, therefore, energized
a number of times corresponding to the last digit in the called
number and the wipers of the connectors are thus brought to the contacts
of the line sought—their final goal. At this point many things
may happen, and the things that do happen depend on whether
the called subscriber's line is idle or busy.
Called-Line Busy:—It will first be assumed that the called
line is busy. The testing operation at the connectors occurs in the
second position of the side switch. If the called line is busy, it will
be either because it is connected to by some other connector or because
it has itself made a call. In the former case the private contacts
of that line in the banks of all the connectors serving that
hundreds group of lines will be grounded through the private wiper
of some other connector. That this is so, may be seen by tracing
the circuit from the private wiper on the shaft to the third side switch
wiper which is grounded in the third position; the other connector
that has already engaged the line will, of course, have its side switch
in its final, or third position. Again, if the line called is busy, because
a call has already been made from this line to some other
line, the private contacts on the connectors corresponding to the line
will be grounded, as will be seen by tracing from the private bank
contacts, which are shown in Fig. 396, through the private wire to the
line switch, which is shown in Fig. 389, and from thence to ground
through the springs 19 and 20, which are brought together when the
line switch is operated.
In any event, therefore, the determining condition of a busy
line is that its private bank contacts on all connectors of its group
shall be grounded. Under the present assumed condition, therefore,
the connector wipers, which have been brought to the bank contacts
of the desired line, will find a ground at the private bank contact.
The connector shaft stops for an instant on the contacts of this busy
line and immediately there follows over the rotary side of the line
the inevitable single rotary impulse. This energizes the rotary relay
and this, as usual, energizes the private magnet. Remembering now
that the connector side switch is in its second position and that the
private wiper of the connector has found a ground, we may trace
back from the private wiper through the third side switch wiper to
its second contact; thence through the contact springs 14 and 15,
closed by the private magnet; thence through the release magnet;
thence through the contact springs 16 and 17 of the calling battery
supply relay to the live side of the battery. This calling battery
supply relay will, at this time, have its core energized because the
coil 18 is in series with the rotary relay coil which, as just stated,
was energized by the last rotary impulse. This series of operations
has led to the energizing of the release magnet, and, as a result, the
double dog of the connector is pulled out of the connector shaft
ratchets and the shaft and the side switch are restored to their
normal position.
Busy-Back Signal:—The connector has dropped back to
normal in all respects. The calling subscriber, not knowing this,
presses his ringing button. This grounds the vertical side of the line
at his station and operates the vertical relay at the connector. This
steps the shaft of the connector up one step and causes the closure
of the contacts 19 and 20 at the top of the connector shaft. This
establishes a connection to a circuit carrying periodically interrupted
battery current on which an inductive hum is placed. This circuit
may be traced from this source through the springs 20 and 19 to
the first wiper of the side switch, thence through the normally closed
contacts of the ringing relay to the rotary side of the line, and the
varying potential to which this path is subjected produces an inductive
flow back to the calling subscriber's telephone, and gives
him the necessary signal which consists of a hum or buzzing noise
with which all users of automatic systems soon become familiar.
Release on Busy Connection:—The connector, since its last
release, has been stepped up one notch and must again be released.
When the subscriber hangs up his receiver after receiving the busy
signal, he grounds both sides of his line momentarily by the
action of the springs 21, 22, and 23 of Fig. 384. This operates the
rotary and the vertical relays on the connector simultaneously and
brings together for the first time the springs 21 and 22 of Fig. 396.
This establishes a connection from the battery through the springs
16 and 17 on the calling battery supply relay, thence through the
release magnet of the connector, thence through the springs 22 and
21 of the vertical and the rotary relay, thence through the release
trunk back to the second selector. From here the circuit passes
through the private wiper of that selector and the back release relay
to ground through the third side switch wiper which is in the third
position. Considering this circuit in respect to its action on the connector
it is obvious that it energizes the release magnet on the connector
which restores the connector to normal as before. At the
second selector this circuit passed through the back release relay, which
closed a circuit through the release magnet and through the back release
relay contacts, thence back over the second selector release trunk
to the back release relay of the first selector, and through the third
wiper of the side switch on that selector to ground, since that side
switch also is in its third position. The current through this circuit
energizes the release magnet of the second selector and restores it to
its normal position and also energizes the back release relay of the
first selector. This, in turn, closes the circuit from the battery
through the release magnet of the first selector and contacts of the
back release relay to ground. This works the release magnet of
the first selector and restores that selector to normal. The contacts
on the first selector release magnet, shown in Fig. 390, are closed by
the action of the release magnet and this closes the path from ground
back through the first selector release wire, and through the contacts
13 and 14 of the line switch, through the line switch release magnet
to battery, and this restores the line switch to normal.
The reason for the term back release will now be apparent. The
release operation at the connector is relayed back to the second selector;
that of the second selector back to the first selector; and that
of the first selector back to the line switch. Until this plan was
adopted, the release magnet of each selector and connector involved
in a connection was left bridged across the talking circuit so as to
be available for release; and it sometimes occurred that a first selector
would be released before a second selector or connector, which
latter switches would thus be left off-normal until rescued by an
attendant. The back release plan makes it impossible for the connection
necessary for the release of a switch to be torn down until
the release is actually accomplished.
Called Line Found Idle:—It will be remembered that, before
the digression necessary to trace through the operations occurring
upon the finding of a busy line, the connector wipers had been
brought, by the influence of the calling subscriber's impulses, into
engagement with the contacts of the desired line; that the connector
side switch was in its second position; and that the final rotary impulse
following the last series of selecting impulses had not been
sent. The condition now to be assumed is that the called subscriber's
line is free and the private wiper, therefore, has found and rests on an
ungrounded private bank contact. The final rotary impulse which
immediately follows will operate the rotary relay and this, in turn,
will operate the private magnet. This happened under the assumed
condition that the line was busy, but in that case the release magnet
was also operated at the same time and restored all conditions to
normal. Under the present condition the operation of the private
magnet will perform its usual function and move the side switch
of the connector into its third position.
Third Position of Side Switch. When the side switch of the
connector moves to its third position, it, as usual, cuts the talking
circuit straight through from the vertical and the rotary sides of the
trunk leading from the previous selector to the outgoing terminal
of the subscriber's line, which may be traced upon Fig. 396 back
through the line switch, shown in Fig. 389. Several things are to be
noted about the talking circuit so established: First, the inclusion
of the condensers in the vertical and the rotary sides of the connector
circuit. The purpose of this will be referred to later. Second,
the inclusion in this circuit at the connector of a pair of normally
closed contacts in the ringing relay. It may be said in passing that
the ringing relay corresponds exactly in function to a ringing key
in a manual switchboard. Third, the talking circuit leading from
the connector to the called subscriber's line passes on one side through
the springs 24 and 25 of the bridge cut-off relay of the line switch,
which is shown in Fig. 389. These springs are normally open and
would prevent the completion of the talking circuit but for the fact
that the bridge cut-off relay of the line switch is energized over the
private wire leading to the connector bank and then through the
connector wiper to the third side switch wiper which, at this time, is
in its third position. The talking circuit is thus complete. The
operation of this bridge cut-off relay on the line switch has not only
completed the talking circuit but it has also opened the circuit of
the trip magnet of the line switch so as to prevent the operation of
the trip magnet by the subscriber on that line in case he should attempt
to make a call during the interval between the time when his
line was connected with by the connector and the time when he answers
the call.
The third wiper of the connector side switch when moved into
its third position, puts the ground on all of the private bank contacts
of the line chosen and thus guards that line against connection by
others, as already described. It also operates the bridge cut-off
relay of the line switch as just mentioned.
The fourth wiper of the side switch, when moved into its third
position, establishes such a connection as will place the ringing relay
under the control of the vertical relay. This may be seen by
tracing from ground to the vertical relay springs 23 and 24,
thence through the normally closed upper pair of contacts on the
private magnet, thence through the fourth wiper on the side switch
to its third contact, thence through the ringing relay magnet, and
through the springs 16 and 17 of the calling battery supply relay
and to battery. The calling battery supply relay winding being in
series with the vertical relay winding, the two operate together and
close the two normally open points in the ringing relay circuit. This
ringing relay acts as an ordinary ringing key and connects the generator
to the called subscriber's line in an obvious manner, at the
same time opening the talking circuit back of the ringing relay in
order to prevent the ringing current chattering the relays in the circuit
back of it. All that remains now is for the called subscriber
to respond. When he does he closes the metallic circuit of the line
through his talking apparatus.
Battery Supply to Connected Subscriber. Throughout the
whole process of building up a connection, it will be remembered
that both sides of the calling line are connected through the respective
vertical and rotary relays involved in building up the connection
with the live side of the battery. At the time when the connection
is finally established and the called subscriber rung, both
sides of the calling line are connected through various relay windings
to the live side of the battery. Such a condition leaves both sides of
the line at the same potential and, therefore, there is no tendency
for current to flow through the calling subscriber's talking apparatus,
even though it is connected across the circuit of the line. It remains,
therefore, to be seen how these conditions are so changed
after the building up of a connection as to supply the calling subscriber
with talking current.
The calling subscriber can get no current until the called subscriber
responds. When the connection is first made with the called
subscriber's line, battery connection to his line is made from the live
side of battery through the normally closed contacts of the calling
battery supply relay, thence through the winding 25 of the called
battery supply relay to the vertical side of the called line. The
grounded side of the battery is connected to the rotary side of his
line through the third wiper of the connector and the coil 26 of the
called battery supply relay. As a result, this subscriber receives
proper talking current through the coils 25 and 26, and this relay
is operated by the flow of this current. The operation of this called
battery supply relay merely shifts the connection of the rotary side
of the calling subscriber's line from its normal battery connection,
to ground, and thus the battery is placed straight across the calling
subscriber's line so as to supply talking current. This supply circuit
to the calling subscriber may be traced from the live side of
the battery through the winding 13 of the calling battery supply relay
and the winding of the vertical relay to the vertical side of the line,
and from the grounded side of battery through the third side switch
wiper in its third position to the now closed pair of contacts in the
called battery supply relay through the coil 18 of the calling battery
supply relay and the coil of the rotary relay to the rotary side of
the line.
It will be noted that the system of battery supply is that of the
standard condenser and retardation coil scheme largely employed
in manual practice; and that aside from the coils through which
the battery current is supplied to the connected subscribers, there are
no taps from, or bridges across, the two sides of the talking circuit.
Release after Conversation. It remains now only to secure
the disconnection of the subscribers after they are through talking.
When the calling subscriber hangs up, the whole disconnection
is brought about, all of the apparatus, including connector,
selectors, and line switch, returning to normal. This is done by
the back release system and is accomplished in almost the same
way as has already been described in connection with the disconnect
after an unsuccessful call. There is this difference, however:
after an unsuccessful call when the line called for was found busy,
the release was made while the connector side switch was in its
normal position. In the present case, the release must be made
with the connector side switch in its third position and with the
talking battery bridged across the metallic circuit rather than connected
between each limb of the line and ground. It must be
remembered that the calling battery supply relay, while traversed
by current during the conversation, is not magnetically energized
because, with the current flowing through the metallic circuit of the
line, the two windings exert a differential effect. As soon, however,
as the calling subscriber hangs up his receiver, this differential
action ceases, due to the grounding of both sides of the line at the subscriber's
station. This relay, therefore, operates and cuts off battery
from the called battery supply relay and this, in turn, releases
its armature and thus changes the connection of the rotary side of the
calling line from ground to live side of the battery. The normal
condition of the battery connection now being restored, both the
vertical and the rotary relays at the connector become operated, due
to the ground on both sides of the line at the subscriber's station, and
this, as we have seen, is the condition which brings about the operation
of the connector release magnet, and the relaying back of the
disconnect impulse successively through the selectors to the line
switch.
Multi-Office System. In exchanges involving more than one
office, the same general principles and mode of operation already
outlined apply. If the total number of subscribers in the multi-office
exchange is to be less than ten thousand, then four digit numbers
suffice, and the first movement of the dial may be made to select
the office into which the connection is to go, the subscribers' lines
being so numbered with respect to the offices that each office will
contain only certain thousands. The choosing of the thousand by
the calling subscriber, therefore, takes care in itself of the choice of
offices. Where, however, a multi-office exchange is to provide for
connections among a greater number of lines than ten thousand and
less than one hundred thousand, then it will take five movements of
the dial to make the selection—the five movements corresponding
either to the five digits in a number or to the name of an office, as
indicated on the dial, and the four digits of a smaller number. The
lines may all carry five digit numbers or, what is considered better
practice, may be designated by an office name followed by a four
digit number. In this latter case the numbers of the subscribers'
lines will in each case be contained in one or more of the tens of thousands
groups, no number having more than four digits. And the
first movement of the dial, whether the name or number plan be
adopted, will select an office; or, looking at it another way, will select
a group of ten thousand and this being done, the next four successive
movements of the dial will select the numbers in that ten
thousand in just the some way that has been already described.
Certain difficulties arise, however, in multi-office working
due to the fact that the three-wire trunks between offices would
in most cases be objectionable. As long as the trunks extend
between the various groups of apparatus in the same office, it is cheaper
to provide three wires for each of them than it is to make any additional
complication in the apparatus. Where the trunking is done
between offices, however, the system may be so modified as to work
over two wire inter-office trunks.
The Trunk Repeater. The purpose of the trunk repeater is
to enable the inter-office trunking to be done over two wires. It
may be said that the trunk repeater is a device placed in the outgoing
trunk circuit at the office in which a call originates, which will do
over the two wires of the trunk leading from it to the distant office
just the same thing that the subscriber's signal transmitter does
over the two wires of the subscriber's lines. It has certain other
functions in regard to feeding the battery for talking purposes back
to the calling subscriber's line, taking the place in this respect of the
calling battery feed relay in the connector in a single office exchange.
The circuits of a trunk repeater are shown in Fig. 397. In considering
it, it must be understood that the three wires entering the
figure at the left are the vertical, rotary, and release wires of a second
selector trunk leading from the first selector banks in the same
office. The two wires leading from the right of the figure are those
extending to the distant office, and terminate there in second selectors.
The vertical and the rotary sides of this trunk as shown at
the left will receive the impulses from the subscriber's station coming
through the line switch and the first selector, as usual. The
vertical impulses will pass through the winding of the vertical relay
and through the winding 1 of the calling battery supply relay and
thence to battery, the same as on a connector. These impulses
will work the armatures of both of these relays in unison. The
movements of the vertical relay armature in response to these impulses
will cause corresponding impulses to flow over a circuit which
may be traced from ground, through the springs 3 and 2 of the vertical
relay, the springs 4 and 5 of the bridged relay 6 and thence to the
vertical side of the trunk and to the distant office, where it passes into
a second selector and through its vertical relay to battery. Thus
the vertical impulses are passed on over the two-wire trunk to the
second selector at the distant office. It becomes necessary, however,
to prevent these impulses from passing back through the winding
of the bridge relay 6 and this is done by means of the sluggish
relay 7. This relay receives local battery impulses in unison with
those sent over the trunk by the vertical relay, these being supplied
from the battery at the local office through the contacts 8 and 9 of
the calling battery supply relay, which works in unison with the
vertical relay. These rapidly recurring impulses are too fast for
the sluggish relay 7 to follow. And this relay merely pulls up its
armature and cuts off both sides of the trunk leading back to the first
selector. The rotary impulses are repeated to the rotary side of
the two-wire trunk in a similar way.
Considering now the operation of the trunk repeater in the reverse
direction, the action of the bridging relay 6 is of vital importance.
Normally both sides of trunk line are connected to the live
side of the battery and, therefore, there is no difference of potential
between them and no tendency to operate the bridged relay. When
the connection has been fully established to the subscriber at the
distant office, and that subscriber has responded, the action of his
battery supply relay will, as before stated, change the connection
of the rotary side of the line from battery to ground, and thus bridge
the battery at the distant exchange across the trunk. This action
will pull up the bridged relay 6 at the trunk repeater and will perform
exactly the same function with respect to the connection of the battery
with the calling subscriber's line. In other words, it will change
the connection of the rotary side of the calling line from battery to
ground, thus establishing the necessary difference in potential to give
the calling subscriber the necessary current for transmission purposes.
The disconnect feature is about the same as already described.
When the calling subscriber hangs up his receiver both the vertical
and rotary relays of the trunk repeater operate, which places the
ground on both sides of the two-wire trunk to the distant office,
which is the condition for releasing all of the apparatus there.
For the purpose of convenience the simplified diagram of Fig.
398 has been prepared, which shows the complete connection from
a calling subscriber to a called subscriber in a multi-office exchange,
wherein the first movement of the dial is employed to establish the
connection to the proper office and the four succeeding movements
to make a selection among ten thousand lines in that office. This
circuit, therefore, employs at the first office the line switch, the
first selector, and the trunk repeater; and at the second office the
second selector, third selector, connector, and line switch.
The third selector is omitted from Fig. 398, but this will cause
no confusion, since it is exactly like the second selector. The circuits
shown are exactly like those previously described but in drawing
them the main idea has been to simplify the connections to the
greatest possible extent at a sacrifice in the clearness with which
the mechanical inter-relation of parts is shown. No correct understanding
of the circuits of an automatic system is possible without
a clear idea of the mechanical functions performed by the different
parts, and, therefore, we have described what are apparently the
more complex circuit drawings first.
It is believed that the student, in attempting
to gain an understanding
of this marvel of mechanical and electrical
intricacy, will find his task less
burdensome if he will refer freely to
both the simplified circuit drawing of
Fig. 398 and the more complex ones
preceding it. By doing so he will
often be enabled to clear up a doubtful
circuit point from the simpler diagram
and a doubtful mechanical point
from those diagrams which represent
more clearly the mechanical relation of
parts.
Automatic Sub-Offices. Obviously, the system of trunking employed
in automatic exchanges lends itself with great facility to the
subdivision of an exchange into a large number of comparatively
small office districts and the establishment of branch offices or sub-offices
at the centers of these districts.
The trunking between large offices has already been described.
An attractive feature of the automatic system is the establishment
of so-called sub-stations or sub-offices. Where there is, in an outlying
district, a distinct group of subscribers whose lines may readily
be centered at a common point within that district and where the
number of such subscribers and lines is insufficient to establish a
fully equipped office, it is possible to
establish a so-called sub-station or sub-office
connected with the main office
of that district by trunk lines. At
this sub-office there are placed only
line switches and connectors. When a
call is originated on one of these sub-office
lines, the line switch acts instantly
to connect that line with one
of the trunks leading to the main
office of that district, at which this
trunk terminates in a first selector.
From there on, the connection is the
same as that in a system in which
no sub-offices are employed. Calls
coming into this sub-office over trunk
lines from the main office are received on the connectors at the
sub-office and the connection is made with the sub-office line by
the connector in the usual manner. This arrangement, it is seen,
amounts merely to a stretching of the connector trunks for a given
group of lines so that they will reach out from a main office to a sub-office,
it being more economical to lengthen the smaller number of
trunks and by so doing to decrease in length the larger number of
subscribers' lines.
The Rotary Connector. For certain purposes it becomes
desirable in automatic work to employ a special form of connector
which will have in itself a certain ability to make automatic selection
of one of a group of previously chosen trunks in much the same
manner as the first and second selectors automatically choose the
first idle one of a group of trunks.
Such a use is demanded in private branch-exchange working where
a given business establishment, for instance, has a plurality of lines
connecting its own private switchboard with the central office. The
directory number of all these lines is, for convenience, made the same,
and it is important, therefore, that when a person attempts to make
a connection with this establishment, he will not fail to get his connection
simply because the first one of these lines happens to be busy.
For such use a given horizontal row of connector terminals or a part
of such a row is assigned to the lines leading to the private branch
exchange and the connector is so modified as to have a certain "discretionary"
power of its own. As a result, when the common number
of all these lines is called, the connector will choose the first one,
if it is not already engaged by some other connector, but if it is, it
will pass on to the next, and so on until an idle one is found. It is
only when the connector has hunted through the entire group of
lines and found them all busy that it will refuse to connect and will
give the busy signal to the calling subscriber.
Party Lines. The description of this system as given above
has been confined entirely to direct line working; however, party
lines may be and are frequently employed.
The circuits and apparatus used with direct lines are, with slight
modifications, applicable to use with party lines.
The harmonic method of ringing is employed and the stations
are so arranged with respect to the connectors that those requiring
the same frequency for ringing the bells are in groups served by
the same set of connectors.
The party lines are operated on the principle commonly
known in manual practice as the jack per station arrangement.
Each party line will, therefore, have sets of terminals appearing in
separate hundreds; the connectors associated with each of these
hundreds being so arranged as to impress the proper frequency of
ringing current on the line.
From the subscribers' standpoint the operation is the same as
for direct lines, as the particular hundreds digit of a number serves
to select one of a group of connectors
capable of connecting the
proper ringing current to the line.
To avoid confusion, which
would be caused by a subscriber
on a party line attempting to make
a call when the line is already in
use by some other subscriber, the
subscribers' stations are so arranged
that when the line is in
use all other stations on the line
are locked out.
The Two-Wire Automatic System. The two-wire system that has
recently been introduced by the Automatic Electric Company brings
about the very important result of accomplishing all of the automatic
switching over metallic circuit lines without the use of ground
or common returns. The system is thus relieved of the disturbing
influences to which the three-wire system is sometimes subjected,
due to differences in earth potential between various portions of
the system, which may add to or subtract from the battery potential
and alter the net potential available between two distant points.
The introduction of this system has also made possible certain other
incidental features of advantage, one of which is a great simplification
and reduction in size of the subscriber's station signal-transmitting
apparatus.
With the doing away of the ground as a return circuit, it becomes
impossible to send vertical impulses over one side of the line and to
follow them by single rotary impulses over the other side of the line.
Yet it becomes necessary to distinguish between the pure selective
impulses and those impulses which dictate a change of function at
the central office. The plan has, therefore, been adopted of accomplishing
the selection in each case by short and rapidly recurring
impulses and of accomplishing those functions formerly brought
about by the single impulse over the rotary side of the line by a pause
between the respective series of
selective impulses. This is accomplished
at the central office
by replacing the vertical and the
rotary relays of the three-wire
system by a quick-acting and a
sluggish relay, respectively; the
quick-acting relay performing
the functions previously carried
out by the vertical relay, and
the sluggish relay acting only
during the pauses between the
successive series of quick impulses
to do the things formerly
done by the rotary relay. This
has resulted in a delightful simplification
of subscriber's apparatus,
since it is now necessary
only to provide a device which
will connect the two sides of the
line together the required number of times in quick succession and
then allow a pause with the circuit closed while the subscriber is
getting ready to transmit another set of impulses corresponding to
another digit. The calling device has no mechanical function co-acting
with any of the other parts of the telephone and may be
considered as a separate mechanical device electrically connected
with the line. The transmitting device is not much larger than
a large watch and a good idea of it may be had from Fig. 399,
which shows the latest wall set, and Fig. 400, which shows the latest
desk set of the Automatic Electric Company. We regret the fact
that this company has made the request that the complete details
of their two-wire system be not published at this time.
ToC
The present day development of the "talking wire" has
annihilated both time and space, and has enabled men
thousands of miles apart to get into almost instant
communication. The user of the telephone and the telegraph
forgets the tremendousness of the feat in the simplicity of its
accomplishment; but the man who has made the feat possible
knows that its very simplicity is due to the complexity of the
principles and appliances involved; and he realizes his need of
a practical, working understanding of each principle and its
application. The Cyclopedia of Telephony and Telegraphy presents
a comprehensive and authoritative treatment of the whole
art of the electrical transmission of intelligence.
