A difficulty encountered on non-selective bridging party lines, which at first seems amusing rather than serious, but which nevertheless is often a vexatious trouble, is that due to the propensity of some people to "listen in" on the line on hearing calls intended for other than their own stations. People whose ethical standards would not permit them to listen at, or peep through, a keyhole, often engage in this telephonic eavesdropping.
Frequently, not only one but many subscribers will respond to a call intended for others and will listen to the ensuing conversation. This is disadvantageous in several respects: It destroys the privacy of conversation between any two parties; it subjects the local batteries to an unnecessary and useless drain; and it greatly impairs the ringing efficiency of the line. The reason for this interference with ringing is that the presence of the low-resistance receivers across the line allows the current sent out by any of the generators to pa.s.s in large measure through the receivers, thus depriving the ringers, which are of comparatively high resistance and impedance, of the energy necessary to operate them. As a result of this it is frequently impossible for one party to repeat the call for another because, during the interval between the first and second call, a number of parties remove their receivers from their hooks in order to listen.
Ring-off or clearing-out signals are likewise interfered with.
[Ill.u.s.tration: Fig. 169. Circuits of Bridging Station]
A partial remedy for this interference with ringing, due to eavesdropping, is to introduce a low-capacity condenser into the receiver circuit at each station, as shown in Fig. 169. This does not seriously interfere with the speech transmission since the condensers will readily transmit the high-frequency voice currents. Such condensers, however, have not sufficient capacity to enable them readily to transmit the low-frequency ringing currents and hence these are forced, in large measure, to pa.s.s through the bells for which they are intended rather than leaking through the low-resistance receiver paths.
The best condenser for this use is of about 1/2-microfarad capacity, which is ample for voice-transmitting purposes, while it serves to effectively bar the major portion of the generator currents. A higher capacity condenser would carry the generator currents much more readily and thus defeat the purpose for which it was intended.
In order that the requisite impedance may be given to the ringers employed for bridging party lines, it is customary to make the cores rather long and of somewhat larger diameter than in series ringers and at the same time to wind the coils with rather fine wire so as to secure the requisite number of turns. Bridging bells are ordinarily wound to a resistance of 1,000 or 1,600 ohms, these two figures having become standard practice. It is not, however, the high resistance so much as the high impedance that is striven for in bridging bells; it is the number of turns that is of princ.i.p.al importance.
As has already been stated, the generators used for bridging lines are made capable of giving a greater current output than is necessary in series instruments, and for this purpose they are usually provided with at least four, and usually five, bar magnets. The armature is made correspondingly long and is wound, as a rule, with about No. 33 wire.
Sometimes where a bridged party line terminates in a central-office switchboard it is desired to so operate the line that the subscribers shall not be able to call up each other, but shall, instead, be able to signal only the central-office operator, who, in turn, will be enabled to call the party desired, designating his station by a suitable code ring. One common way to do this is to use biased bells instead of the ordinary polarized bells. In order that the bells may not be rung by the subscribers" generators, these generators are made of the direct-current type and these are so a.s.sociated with the line that the currents which they send out will be in the wrong direction to actuate the bells. On the other hand, the central-office generator is of direct-current type and is a.s.sociated with the line in the right direction to energize the bells. Thus any subscriber on the line may call the central office by merely turning his generator crank, which action will not ring the bells of the subscribers on the line. The operator will then be able to receive the call and in turn send out currents of the proper direction to ring all the bells and, by code, call the desired party to the telephone.
[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]
Signal Code. The code by which stations are designated on non-selective party lines usually consists in combinations of long and short rings similar to the dots and dashes in the Morse code. Thus, one short ring may indicate Station No. 1; two short rings Station No.
2; and so on up to, say, five short rings, indicating Station No. 5.
It is not good practice to employ more than five successive short rings because of the confusion which often arises in people"s minds as to the number of rings that they hear. When, therefore, the number of stations to be rung by code exceeds five, it is better to employ combinations of long and short rings, and a good way is to adopt a partial decimal system, omitting the numbers higher than five in each ten, and employing long rings to indicate the tens digits and short rings to indicate the units digit, Table X.
TABLE X
Signal Code +--------------+---------------+--------------+---------------+STATION NUMBERRINGSTATION NUMBERRING11 short121 long, 2 short22 short131 long, 3 short33 short141 long, 4 short44 short151 long, 5 short55 short212 long, 1 short111 long, 1 short222 long, 2 short+--------------+---------------+--------------+---------------+
Other arrangements are often employed and by almost any of them a great variety of readily distinguishable signals may be secured. The patrons of such lines learn to distinguish, with comparatively few errors, between the calls intended for them and those intended for others, but frequently they do not observe the distinction, as has already been pointed out.
Limitations. With good telephones the limit as to the number of stations that it is possible to operate upon a single line is usually due more to limitations in ringing than in talking. As the number of stations is increased indefinitely a condition will be reached at which the generators will not be able to generate sufficient current to ring all of the bells, and this condition is likely to occur before the talking efficiency is seriously impaired by the number of bridges across the line.
Neither of these considerations, however, should determine the maximum number of stations to be placed on a line. The proper limit as to the number of stations is not the number that can be rung by a single generator, or the number with which it is possible to transmit speech properly, but rather the number of stations that may be employed without causing undue interference between the various parties who may desire to use the line. Overloaded party lines cause much annoyance, not only for the reason that the subscribers are often not able to use the line when they want it, but also, in non-selective lines, because of the incessant ringing of the bells, and the liability of confusion in the interpretation of the signaling code, which of course becomes more complex as the number of stations increases.
The amount of business that is done over a telephone line is usually referred to as the "traffic." It will be understood, however, in considering party-line working that the number of calls per day or per hour, or per shorter unit, is not the true measure of the traffic and, therefore, not the true measure of the amount of possible interference between the various subscribers on the line.
An almost equally great factor is the average length of the conversation. In city lines, that is, in lines in city exchanges, the conversation is usually short and averages perhaps two minutes in duration. In country lines, however, serving people in rural districts, who have poor facilities for seeing each other, particularly during the winter time, the conversations will average very much longer. In rural communities the people often do much of their visiting by telephone, and conversations of half an hour in length are not unusual. It is obvious that under such conditions a party line having a great many stations will be subject to very grave interference between the parties, people desiring to use the line for business purposes often being compelled to wait an undue time before they may secure the use of the line.
It is obvious, therefore, that the amount of traffic on the line, whether due to many short conversations or to a comparatively few long ones, is the main factor that should determine the number of stations that, economically, may be placed on a line. The facilities also for building lines enter as a factor in this respect, since it is obvious that in comparatively poor communities the money may not be forthcoming to build as many lines as are needed to properly take care of the traffic. A compromise is, therefore, often necessary, and the only rule that may be safely laid down is to place as few parties on a given line as conditions will admit.
No definite limit may be set to apply to all conditions but it may be safely stated that under ordinary circ.u.mstances no more than ten stations should be placed on a non-selective line. Twenty stations are, however, common, and sometimes forty and even fifty have been connected to a single line. In such cases the confusion which results, even if the talking and the ringing efficiency are tolerable, makes the service over such overloaded lines unsatisfactory to all concerned.
CHAPTER XVI
SELECTIVE PARTY-LINE SYSTEMS
The problem which confronts one in the production of a system of selective ringing on party lines is that of causing the bell of any chosen one of the several parties on a circuit to respond to a signal sent out from the central office without sounding any of the other bells. This, of course, must be accomplished without interfering with the regular functions of the telephone line and apparatus. By this is meant that the subscribers must be able to call the central office and to signal for disconnection when desired, and also that the a.s.sociation of the selective-signaling devices with the line shall not interfere with the transmission of speech over the line. A great many ways of accomplishing selective ringing on party lines have been proposed, and a large number of them have been used. All of these ways may be cla.s.sified under four different cla.s.ses according to the underlying principle involved.
Cla.s.sification. (_1_) _Polarity_ systems are so called because they depend for their operation on the use of bells or other responsive devices so polarized that they will respond to one direction of current only. These bells or other devices are so arranged in connection with the line that the one to be rung will be traversed by current in the proper direction to actuate it, while all of the others will either not be traversed by any current at all, or by current in the wrong direction to cause their operation.
(_2_) The _harmonic_ systems have for their underlying principle the fact that a pendulum or elastic reed, so supported as to be capable of vibrating freely, will have one particular rate of vibration which it may easily be made to a.s.sume. This pendulum or reed is placed under the influence of an electromagnet a.s.sociated with the line, and owing to the fact that it will vibrate easily at one particular rate of vibration and with extreme difficulty at any other rate, it is clear that for current impulses of a frequency corresponding to its natural rate the reed will take up the vibration, while for other frequencies it will fail to respond.
Selection on party lines by means of this system is provided for by tuning all of the reeds on the line at different rates of vibration and is accomplished by sending out on the line ringing currents of proper frequency to ring the desired bell. The current-generating devices for ringing these bells are capable of sending out different frequencies corresponding respectively to the rates of vibration of each of the vibrating reed tongues. To select any one station, therefore, the current frequency corresponding to the rate of vibration of the reed tongue at that station is sent and this, being out of tune with the reed tongues at all of the other stations, operates the tongue of the desired station, but fails to operate those at all of the other stations.
(_3_) In the _step-by-step_ system the bells on the line are normally not in operative relation with the line and the bell of the desired party on the line is made responsive by sending over the line a certain number of impulses preliminary to ringing it. These impulses move step-by-step mechanisms at each of the stations in unison, the arrangement being such that the bells at the several stations are each made operative after the sending of a certain number of preliminary impulses, this number being different for all the stations.
(_4_) The _broken-line_ systems are new in telephony and for certain fields of work look promising. In these the line circuit is normally broken up into sections, the first section terminating at the first station out from the central office, the second section at the second station, and so on. When the line is in its normal or inactive condition only the bell at the first station is so connected with the line circuit as to enable it to be rung, the line being open beyond.
Sending a single preliminary impulse will, however, operate a switching device so as to disconnect the bell at the first station and to connect the line through to the second station. This may be carried out, by sending the proper number of preliminary impulses, so as to build up the line circuit to the desired station, after which the sending of the ringing current will cause the bell to ring at that station only.
Polarity Method. The polarity method of selective signaling on party lines is probably the most extensively used. The standard selective system of the American Telephone and Telegraph Company operates on this principle.
_Two-Party Line._ It is obvious that selection may be had between two parties on a single metallic-circuit line without the use of biased bells or current of different polarities. Thus, one limb of a metallic circuit may be used as one grounded line to ring the bell at one of the stations, and the other limb of the metallic circuit may be used as another grounded line to ring the bell of the other station; and the two limbs may be used together as a metallic circuit for talking purposes as usual.
This is shown in Fig. 170, where the ringing keys at the central office are diagrammatically shown in the left-hand portion of the figure as _K_^{1} and _K_^{2}. The operation of these keys will be more fully pointed out in a subsequent chapter, but a correct understanding will be had if it be remembered that the circuits are normally maintained by these keys in the position shown. When, however, either one of the keys is operated, the two long springs may be considered as pressed apart so as to disengage the normal contacts between the springs and to engage the two outer contacts, with which they are shown in the cut to be disengaged. The two outer contacts are connected respectively to an ordinary alternating-current ringing generator and to ground, but the connection is reversed on the two keys.
[Ill.u.s.tration: Fig. 170. Simple Two-Party Line Selection]
At Station A the ordinary talking set is shown in simplified form, consisting merely of a receiver, transmitter, and hook switch in a single bridge circuit across the line. An ordinary polarized bell is shown connected in series with a condenser between the lower limb of the line and ground. At Station B the same talking circuit is shown, but the polarized bell and condenser are bridged between the upper limb of the line and ground.
If the operator desires to call Station A, she will press key _K_^{1} which will ground the upper side of the line and connect the lower side of the line with the generator _G_^{1}, and this, obviously, will cause the bell at Station A to ring. The bell at Station B will not ring because it is not in the circuit. If, on the other hand, the operator desires to ring the bell at Station B, she will depress key _K_^{2}, which will allow the current from generator _G_^{2} to pa.s.s over the upper side of the line through the bell and condenser at Station B and return by the path through the ground. The object of grounding the opposite sides of the keys at the central office is to prevent cross-ringing, that is, ringing the wrong bell. Were the keys not grounded this might occur when a ringing current was being sent out while the receiver at one of the stations was off its hook; the ringing current from, say, generator _G_^{1} then pa.s.sing not only through the bell at Station A as intended, but also through the bell at Station B by way of the bridge path through the receiver that happened to be connected across the line. With the ringing keys grounded as shown, it is obvious that this will not occur, since the path for the ringing current through the wrong bell will always be shunted by a direct path to ground on the same side of the line.
In such a two-party-line selective system the two generators _G_^{1} and _G_^{2} may be the same generator and may be of the ordinary alternating-current type. The bells likewise may be of the ordinary alternating-current type.
The two-party selective line just described virtually employs two separate circuits for ringing. Now each of these circuits alone may be employed to accomplish selective ringing between two stations by using two biased bells oppositely polarized, and employing pulsating ringing currents of one direction or the other according to which bell it is desired to ring. One side of a circuit so equipped is shown in Fig.
171. In this the two biased bells are at Station A and Station B, these being bridged to ground in each case and adapted to respond only to positive and negative impulses respectively. At the central office the two keys _K_^{1} and _K_^{2} are shown. A single alternating-current generator _G_ is shown, having its brush _1_ grounded and brush _2_ connected to a commutator disk _3_ mounted on the generator shaft so as to revolve therewith. One-half of the periphery of this disk is of insulating material so that the brushes _4_ and _5_, which bear against the disk, will be alternately connected with the disk and, therefore, with the brush _2_ of the generator. Now the brush _2_, being one terminal of an alternating-current machine, is alternately positive and negative, and the arrangement of the commutator is such that the disk, which is always at the potential of the brush _2_, will be connected to the brush _5_ only while it is positively charged and with the brush _4_ only while it is negatively charged. As a result, brush _5_ has a succession of positive impulses and brush _4_ a succession of negative ones. Obviously, therefore, when key _K_^{1} is depressed only the bell at Station A will be rung, and likewise the depression of key _K_^{2} will result only in the ringing of the bell at Station B.
[Ill.u.s.tration: Fig. 171. Principle of Selection by Polarity]
_Four-Party Line._ From the two foregoing two-party line systems it is evident that a four-party line system may be readily obtained, that is, by employing two oppositely polarized biased bells on each side of the metallic circuit. The selection of any of the four bells may be obtained, choosing between the pairs connected, respectively, with the two limbs of the line, by choosing the limb on which the current is to be sent, and choosing between the two bells of the pair on that side of the line by choosing which polarity of current to send.
Such a four-party line system is shown in Fig. 172. In this the generators are not shown, but the wires leading from the four keys are shown marked plus or minus, according to the terminal of the generator to which they are supposed to be connected. Likewise the two bells connected with the lower side of the line are marked positive and negative, as are the two bells connected with the upper side of the line. From the foregoing description of Figs. 170 and 171, it is clear that if key _K_^{1} is pressed the bell at Station A will be rung, and that bell only, since the bells at Station C and Station _D_ are not in the circuit and the positive current sent over the lower side of the line is not of the proper polarity to ring the bell at Station B.
The system shown in Fig. 172 is subject to one rather grave defect. In subsequent chapters it will be pointed out that in common-battery systems the display of the line signal at the central office is affected by any one of the subscribers merely taking his receiver off its hook and thus establishing a connection between the two limbs of the metallic circuit. Such common-battery systems should have the two limbs of the line, normally, entirely insulated from each other. It is seen that this is not the case in the system just described, since there is a conducting path from one limb of the line through the two bells on that side to ground, and thence through the other pair of bells to the other limb of the line. This means that unless the resistance of the bell windings is made very high, the path of the signaling circuit will be of sufficiently low resistance to actuate the line signal at the central office.
[Ill.u.s.tration: Fig. 172. Four-Party Polarity Selection]
It is not feasible to overcome this objection by the use of condensers in series with the bells, as was done in the system shown in Fig. 170, since the bells are necessarily biased and such bells, as may readily be seen, will not work properly through condensers, since the placing of a condenser in their circuit means that the current which pa.s.ses through the bell is alternating rather than pulsating, although the original source may have been of pulsating nature only.
[Ill.u.s.tration: Fig 173. Standard Polarity System]
The remedy for this difficulty, therefore, has been to place in series with each bell a very high non-inductive resistance of about 15,000 or 20,000 ohms, and also to make the windings of the bells of comparatively high resistance, usually about 2,500 ohms. Even with this precaution there is a considerable leakage of the central-office battery current from one side of the line to the other through the two paths to ground in series. This method of selective signaling has, therefore, been more frequently used with magneto systems. An endeavor to apply this principle to common-battery systems without the objections noted above has led to the adoption of a modification, wherein a relay at each station normally holds the ground connection open. This is shown in Fig. 173 and is the standard four-party line ringing circuit employed by the American Telephone and Telegraph Company and their licensees.
In this system the biased bells are normally disconnected from the line, and, therefore, the leakage path through them from one side of the line to the other does not exist. At each station there is a relay winding adapted to be operated by the ringing current bridged across the line in series with a condenser. As a result, when ringing current is sent out on the line all of the relays, _i.e._, one at each station, are energized and attract their armatures. This establishes the connection of all the bells to line and really brings about temporarily a condition equivalent to that of Fig. 172. As a result, the sending of a positive current on the lower line with a ground return will cause the operation of the bell at Station A. It will not ring the bell at Station B because of the wrong polarity. It will not ring the bells of Station C and Station D because they are in the circuit between the other side of the line and ground. As soon as the ringing current ceases all of the relays release their armatures and disconnect all the bells from the line.
By this very simple device the trouble, due to marginal working of the line signal, is done away with, since normally there is no leakage from one side of the line to the other on account of the presence of the condensers in the bridge at each station.