Cyclopedia of Telephony and Telegraphy Volume I Part 23

While all of the commercial harmonic systems on the market are limited to four or five frequencies, it does not follow that a greater number than four or five stations may not be selectively rung. Double these numbers may be placed on a party line and selectively actuated, if the first set of four or five is bridged across the line and the second set of four or five is connected between one limb of the line and ground. The first set of these is selectively rung, as already described, by sending the ringing currents over the metallic circuit, while the second set may be likewise selectively rung by sending the ringing currents over one limb of the line with a ground return. This method is frequently employed with success on country lines, where it is desired to place a greater number of instruments on a line than four or five.

Step-by-Step Method. A very large number of step-by-step systems have been proposed and reduced to practice, but as yet they have not met with great success in commercial telephone work, and are nowhere near as commonly used as are the polarity and harmonic systems.

_Principles_. An idea of the general features of the step-by-step systems may be had by conceiving at each station on the line a ratchet wheel, having a pawl adapted to drive it one step at a time, this pawl being a.s.sociated with the armature of an electromagnet which receives current impulses from the line circuit. There is thus one of these driving magnets at each station, each bridged across the line so that when a single impulse of current is sent out from the central office all of the ratchet wheels will be moved one step. Another impulse will move all of the ratchet wheels another step, and so on throughout any desired number of impulses. The ratchet wheels, therefore, are all stepped in unison.

Let us further conceive that all of these ratchet wheels are provided with a notch or a hole or a projection, alike in all respects at all stations save in the position which this notch or hole or projection occupies on the wheel. The thing to get clear in this part of the conception is that all of these notches, holes, or projections are alike on all of the wheels, but they occupy a different position on the wheel for each one of the stations.

Consider further that the bell circuit at each of the stations is normally open, but that in each case it is adapted to be closed when the notch, hole, or projection is brought to a certain point by the revolution of the wheel.

Let us conceive further that this distinguis.h.i.+ng notch, hole, or projection is so arranged on the wheel of the first station as to close the bell circuit when one impulse has been sent, that that on the second station will close the bell circuit after the second impulse has been sent, and so on throughout the entire number of stations. It will, therefore, be apparent that the bell circuits at the various stations will, as the wheels are rotated in unison, be closed one after the other. In order to call a given station, therefore, it is only necessary to rotate all of the wheels in unison, by sending out the proper stepping impulses until they all occupy such a position that the one at the desired station is in such position as to close the bell circuit at that station. Since all of the notches, holes, or projections are arranged to close the bell circuits at their respective stations at different times, it follows that when the bell circuit at the desired station is closed those at all of the other stations will be open. If, therefore, after the proper number of stepping impulses has been sent to the line to close the bell circuit of the desired station, ringing current be applied to the line, it is obvious that the bell of that one station will be rung to the exclusion of all others. It is, of course, necessary that provision be made whereby the magnets which furnish the energy for stepping the wheels will not be energized by the ringing current. This is accomplished in one of several ways, the most common of which is to have the stepping magnets polarized or biased in one direction and the bells at the various stations oppositely biased, so that the ringing current will not affect the stepping magnet and the stepping current will not affect the ringer magnets.

After a conversation is finished, the line may be restored to its normal position in one of several ways. Usually so-called release magnets are employed, for operating on the releasing device at each station. These, when energized, will withdraw the holding pawls from the ratchets and allow them all to return to their normal positions.

Sometimes these release magnets are operated by a long impulse of current, being made too sluggish in their action to respond to the quick-stepping impulses; sometimes the release magnets are tapped from one limb of the line to ground, so as not to be affected by the stepping or ringing currents sent over the metallic circuit; and sometimes other expedients are used for obtaining the release of the ratchets at the proper time, a large amount of ingenuity having been spent to this end.

As practically all step-by-step party-line systems in commercial use have also certain other features intended to a.s.sure privacy of conversation to the users, and, therefore, come under the general heading of lock-out party-line systems, the discussion of commercial examples of these systems will be left for the next chapter, which is devoted to such lock-out systems.

Broken-Line Method. The broken-line system, like the step-by-step system, is also essentially a lock-out system and for that reason only its general features, by which the selective ringing is accomplished, will be dealt with here.

_Principles_. In this system there are no tuned bells, no positively and negatively polarized bells bridged to ground on each side of the line, and no step-by-step devices in the ordinary sense, by which selective signaling has ordinarily been accomplished on party lines.

Instead of this, each instrument on the line is exclusively brought into operative relation with the line, and then removed from such operative relation until the subscriber wanted is connected, at which time all of the other instruments are locked out and the line is not enc.u.mbered by any bridge circuits at any of the instruments that are not engaged in the conversation. Furthermore, in the selecting of a subscriber or the ringing of his bell there is no splitting up of current among the magnets at the various stations as in ordinary practice, but the operating current goes straight to the station desired and to that station alone where its entire strength is available for performing its proper work.

In order to make the system clear it may be stated at the outset that one side of the metallic circuit line is continued as in ordinary practice, pa.s.sing through all of the stations as a continuous conductor. The other side of the line, however, is divided into sections, its continuity being broken at each of the subscriber's stations. Fig. 186 is intended to show in the simplest possible way how the circuit of the line may be extended from station to station in such manner that only the ringer of one station is in circuit at a time. The two sides of the line are shown in this figure, and it will be seen that limb _L_ extends from the central office on the left to the last station on the right without a break. The limb _R_, however, extends to the first station, at which point it is cut off from the extension _R_{x}_ by the open contacts of a switch. For the purpose of simplicity this switch is shown as an ordinary hand switch, but as a matter of fact it is a part of a relay, the operating coil of which is shown at _6_, just above it, in series with the ringer.

[Ill.u.s.tration: Fig. 186. Principle of Broken-Line System]

Obviously, if a proper ringing current is sent over the metallic circuit from the central office, only the bell at Station A will operate, since the bells at the other stations are not in the circuit.

If by any means the switch lever _2_ at Station A were moved out of engagement with contact _1_ and into engagement with contact _3_, it is obvious that the bell of Station A would no longer be in circuit, but the limb _R_ of the line would be continued to the extension _R_{x}_ and the bell of Station B would be in circuit. Any current then sent over the circuit of the line from the central office would ring the bell of this station. In Fig. 187 the switches of both Station A and Station B have been thus operated, and Station C is thus placed in circuit. Inspection of this figure will show that the bells of Station A, Station B, and Station D are all cut out of circuit, and that, therefore, no current from the central office can affect them.

This general scheme of selection is a new-comer in the field, and for certain cla.s.ses of work it is of undoubted promise.

[Ill.u.s.tration: Fig. 187. Principle of Broken-Line System]

CHAPTER XVII

LOCK-OUT PARTY-LINE SYSTEMS

The party-line problem in rural districts is somewhat different from that within urban limits. In the latter cases, owing to the closer grouping of the subscribers, it is not now generally considered desirable, even from the standpoint of economy, to place more than four subscribers on a single line. For such a line selective ringing is simple, both from the standpoint of apparatus and operation; and moreover owing to the small number of stations on a line, and the small amount of traffic to and from such subscribers as usually take party-line service, the interference between parties on the same line is not a very serious matter.

For rural districts, particularly those tributary to small towns, these conditions do not exist. Owing to the remoteness of the stations from each other it is not feasible from the standpoint of line cost to limit the number of stations to four. A much greater number of stations is employed and the confusion resulting is distressing not only to the subscribers themselves but also to the management of the company. There exists then the need of a party-line system which will give the limited user in rural districts a service, at least approaching that which he would get if served by an individual line.

The princ.i.p.al investment necessary to provide facilities for telephone service is that required to produce the telephone line. In many cases the cost of instruments and apparatus is small in comparison with the cost of the line. By far the greater number of subscribers in rural districts are those who use their instruments a comparatively small number of times a day, and to maintain an expensive telephone line for the exclusive use of one such subscriber who will use it but a few minutes each day is on its face an economic waste. As a result, where individual line service is practiced exclusively one of two things must be true: either the average subscriber pays more for his service than he should, or else the operating company sells the service for less than it costs, or at best for an insufficient profit. Both of these conditions are unnatural and cannot be permanent.

The party-line method of giving service, by which a single line is made to serve a number of subscribers, offers a solution to this difficulty, but the ordinary non-selective or even selective party line has many undesirable features if the attempt is made to place on it such a large number of stations as is considered economically necessary in rural work. These undesirable features work to the detriment of both the user of the telephone and the operating company.

Many attempts have been made to overcome these disadvantages of the party line in spa.r.s.ely settled communities, by producing what are commonly called lock-out systems. These, as their name implies, employ such an arrangement of parts that when the line is in use by any two parties, all other parties are locked out from the circuit and cannot gain access to it until the parties who are using it are through.

System after system for accomplis.h.i.+ng this purpose has been announced but for the most part these have involved such a degree of complexity and have introduced so many undesirable features as to seriously affect the smooth operation of the system and the reliability of the service.

We believe, however, in spite of numerous failures, that the lock-out selective-signaling party line has a real field of usefulness and that operating companies as well as manufacturing companies are beginning to appreciate this need, and as a result that the relief of the rural subscriber from the almost intolerable service he has often had to endure is at hand. A few of the most promising lock-out party-line systems now before the public will, therefore, be described in some detail.

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

In Fig. 188 there is shown a four-station party line equipped with Poole lock-out devices, it being a.s.sumed that the ringers at each station are harmonic and that the keys at the central office are the ordinary keys adapted to impress the proper frequency on the line for ringing any one of the stations. In addition to the ordinary talking and ringing apparatus at each subscriber's station, there is a relay of special form and also a push-b.u.t.ton key.

[Ill.u.s.tration: Fig. 188. Poole Lock-Out System]

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

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

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

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

When one party desires to converse with another on the same line, he depresses the b.u.t.ton _3_ at his station until after the called party has been rung and has responded. This holds the circuit of his low-resistance winding open, and thus prevents the lock-out from becoming effective until the called party is connected with the line.

The relay armature of the calling party does not fall back with the establishment of the low-resistance path at the called station, because, even though shunted, it still receives sufficient current to hold its armature in its attracted position. After the called party has responded, the b.u.t.ton at the calling station is released and both low-resistance holding coils act in multiple.

[Ill.u.s.tration: ONE WING OF OPERATING ROOM, BERLIN, GERMANY Ultimate Capacity 24,000 Subscribers' Lines and 2,100 Trunk Lines.

Siemens-Halske Equipment. Note Horizontal Disposal of Multiple Jack Field.]

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

A possible defect in the use of this system is one that has been common to a great many other lock-out systems, depending for their operation on the same general plan of action. This appears when the instruments are used on a comparatively long line. Since the locking-out of all the instruments that are not in use by the one that is in use depends on the low-resistance shunt that is placed across the line by the instrument that is in use, it is obvious that, in the case of a long line, the resistance of the line wire will enter into the problem in such a way as to tend to defeat the locking-out function in some cases.

Thus, where the first instrument to use the line is at the remote end of the line, the shunting effect that this instrument can exert with respect to another instrument near the central office is that due to the resistance of the line plus the resistance of the holding coil at the end instrument. The resistance of the line wire may be so high as to still allow a sufficient current to flow through the high-resistance coil at the nearer station to allow its operation, even though the more remote instrument is already in use.

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

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

[Ill.u.s.tration: Fig. 189. K.B. Lock-Out System]

Referring first to Fig. 189, which shows in simplified form a four-station K.B. lock-out line, the electromagnet is shown at _1_ and the rocker arm at _2_. The ratchet _3_ in this case is not a complete wheel but rather a segment thereof, and it is provided with a series of notches of different depths. It is obvious that the depth of the notches will determine the degree of movement which the upper end of the rocker arm may have toward the left, this being dependent on the extent to which the pawl _6_ is permitted to enter into the segment.

The first or normal notch, _i.e._, the top notch, is always of such a depth that it will allow the rocker-arm lever _2_ to engage the contact lever _4_, but will not permit the rocker arm to swing far enough to the left to cause that contact to engage the bell contact _5_. As will be shown later, the condition for the talking circuit to be closed is that the rocker arm _2_ shall rest against the contact _4_; and from this we see that the normal notch of each of the segments _3_ is of such a depth as to allow the talking circuit at each station to be closed. The next notch, _i.e._, the second one in each disk, is always shallow, as are all of the other notches except one. A deep notch is placed on each disk anywhere from the third to the next to the last on the segment. This deep notch is called the _selective notch_, and it is the one that allows of contact being made with the ringer circuit of that station when the pawl _6_ drops into it. The position of this notch differs on all of the segments on a line, and obviously, therefore, the ringer circuit at any station may be closed to the exclusion of all the others by stepping all of the segments in unison until the deep notch on the segment of the desired station lies opposite to the pawl _6_, which will permit the rocker arm _2_ to swing so far to the left as to close not only the circuit between _2_ and _4_, but also between _2_, _4_, and _5_. In this position the talking and the ringing circuits are both closed.

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

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

Besides the moving pawl _6_ there is a detent pawl _7_. This always holds the segment _3_ in the position to which it has been last moved by the moving pawl _6_.

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

As will be seen from Fig. 189, each of the relay magnets _1_ is permanently bridged across the two limbs of the line.

Each station is provided with a push b.u.t.ton, not shown, by means of which the subscriber who makes a call may prevent the rocker arm of his instrument from being actuated while selective impulses are being sent over the line. The purpose of this is to enable one party to make a call for another on the same line, depressing his push b.u.t.ton while the operator is selecting and ringing the called party. The segment at his own station, therefore, remains in its normal position, in which position, as we have already seen, his talking circuit is closed; all of the other segments are, however, stepped up until the ringing and talking circuits of the desired station are in proper position, at which time ringing current is sent over the line. The segments in Fig.

189, except at Station C, are shown as having been stepped up to the sixth position, which corresponds to the ringing position of the fourth station, or Station D. The condition shown in this figure corresponds to that in which the subscriber at Station C originated the call and pressed his b.u.t.ton, thus retaining his own segment in its normal position so that the talking circuits would be established with Station D.

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