Cyclopedia of Telephony and Telegraphy Volume Ii Part 9

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 ill.u.s.trate 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 ill.u.s.trate 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.

[Ill.u.s.tration: Fig. 380. Principles of Automatic Switch]

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. a.s.sume 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. a.s.sume 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 a.s.sociated 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 a.s.sumed 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.

[Ill.u.s.tration: DEAN HARMONIC CONVERTER Dry Cell Type for Magneto Exchange. _The Dean Electric Co._]

[Ill.u.s.tration: Fig. 381. Scheme of Trunking]

_First Selector Action._ We will a.s.sume 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. a.s.suming 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 a.s.sumed 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 a.s.sumed 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 a.s.sumed 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.

[Ill.u.s.tration: Fig. 382. Automatic Wall Set]

[Ill.u.s.tration: Fig. 383. Automatic Desk Stand]

=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 ill.u.s.tration 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.

[Ill.u.s.tration: Fig. 384. Circuits of Telephone Set]

_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 a.s.sociated 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 b.u.t.ton. 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 pa.s.sing of each cam forces this vertical spring into engagement with the ground spring. It will readily be seen, therefore, by a consideration of the s.p.a.cing of these cams on the segment and the finger holes in the dial that the number of cams which pa.s.s 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 a.s.sociated 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 b.u.t.ton attached to the spring _19_--which b.u.t.ton 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 b.u.t.ton 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.