Volume Ii Part 9 (2/2)
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.
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