Volume Ii Part 11 (1/2)
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 pa.s.sed 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 pa.s.s 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 a.s.sociated 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.
[Ill.u.s.tration: Fig. 394. Circuits of Second Selector]
_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 s.h.i.+fted 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 pa.s.sing this back release impulse on to the first selector in the same way that the first selector pa.s.ses 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 s.p.a.ce, 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 ill.u.s.trated in Figs. 387 and 388.
[Ill.u.s.tration: GAS ENGINE AND POWER BOARD Citizens' Telephone Co., Racine, Wis. _The Dean Electric Co._]
=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.
[Ill.u.s.tration: Fig. 395. Connector Side of Line-Switch Unit]
[Ill.u.s.tration: Fig. 396. Circuits of Connector]
_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 pa.s.s 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, pa.s.s 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 princ.i.p.ally 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 pa.s.ses 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 s.h.i.+fted 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.