CHAPTER XXVII

TRUNKING IN MULTI-OFFICE SYSTEMS

It has been stated that a single exchange may involve a number of offices, in which case it is termed a multi-office exchange. In a multi-office exchange, switchboards are necessary at each office in which the subscribers' lines of the corresponding office district terminate. Means for intercommunication between the subscribers in one office and those in any other office are afforded by inter-office trunks extended between each office and each of the other offices.

If the character of the community is such that each of the offices has so few lines as to make the simple switchboard suffice for its local connections, then the trunking between the offices may be carried out in exactly the same way as explained between the various simple switchboards in a transfer system, the only difference being that the trunks are long enough to reach from one office to another instead of being short and entirely local to a single office. Such a condition of affairs would only be found in cases where several small communities were grouped closely enough together to make them operate as a single exchange district, and that is rather unusual.

The subject of inter-office trunking so far as manual switchboards are concerned is, therefore, confined mainly to trunking between a number of offices each equipped with a manual multiple switchboard.

=Necessity for Multi-Office Exchanges.= Before taking up the details of the methods and circuits employed in trunking in multi-office systems, it may be well to discuss briefly why the multi-office exchange is a necessity, and why it would not be just as well to serve all of the subscribers in a large city from a single huge switchboard in which all of the subscribers' lines would terminate. It cannot be denied, when other things are equal, that it is better to have only one operator involved in any connection which means less labor and less liability of error.

The reasons, however, why this is not feasible in really large exchanges are several. The main one is that of the larger investment required. Considering the investment first from the standpoint of the subscriber's line, it is quite clear that the average length of subscriber's line will be very much greater in a given community if all of the lines are run to a single office, than will be the case if the exchange district is divided into smaller office districts and the lines run merely from the subscribers to the nearest office. There is a direct and very large gain in this respect, in the multi-office system over the single office system in large cities, but this is not a net gain, since there is an offsetting investment necessary in the trunk lines between the offices, which of course are separate from the subscribers' lines.

Approaching the matter from the standpoint of switchboard construction and operation, another strong reason becomes apparent for the employment of more than one office in large exchange districts. Both the difficulties of operation and the expense of construction and maintenance increase very rapidly when switchboards grow beyond a certain rather well-defined limit. Obviously, the limitation of the multiple switchboard as to size involves the number of multiple jacks that it is feasible to place on a section. Multiple switchboards have been constructed in this country in which the sections had a capacity of 18,000 jacks. Schemes have been proposed and put into effect with varying success, for doubling and quadrupling the capacity of multiple switchboards, one of these being the so-called divided multiple board devised by the late Milo G. Kellogg, and once used in Cleveland, Ohio, and St. Louis, Missouri. Each of these boards had an ultimate capacity of 24,000 lines, and each has been replaced by a "straight" multiple board of smaller capacity. In general, the present practice in America does not sanction the building of multiple boards of more than about 10,000 lines capacity, and as an example of this it may be cited that the largest standard section manufactured for the Bell companies has an ultimate capacity of 9,600 lines.

European engineers have shown a tendency towards the opposite practice, and an example of the extreme in this case is the multiple switchboard manufactured by the Ericsson Company, and installed in Stockholm, in which the jacks have been reduced to such small dimensions as to permit an ultimate capacity of 60,000 lines.

The reasons governing the decision of American engineers in establishing the practice of employing no multiple switchboards of greater capacity than about 10,000 lines, briefly outlined, are as follows: The building of switchboards with larger capacity, while perfectly possible, makes necessary either a very small jack or some added complexity, such as that of the divided multiple switchboard, either of which is considered objectionable. Extremely small jacks and large multiples introduce difficulties as to the durability of the jacks and the plugs, and also they tend to slow down the work of operators and to introduce errors. They also introduce the necessity of a smaller gauge of wire through the multiple than it has been found desirable to employ. Considered from the standpoint of expense, it is evident that as a multiple switchboard increases in number of lines, its size increases in two dimensions, _i. e._, in length of board and height of section, and this element of expense, therefore, is a function of the square of the number of lines.

The matter of insurance, both with respect to the risk as to property loss and the risk as to breakdown of the service, also points distinctly in the direction of a plurality of offices rather than one. Both from the standpoint of risk against fire and other hazards, which might damage the physical property, and of risk against interruption to service due to a breakdown of the switchboard itself, or a failure of its sources of current, or an accident to the cable approaches, the single office practice is like putting all one's eggs in one basket.

Another factor that has contributed to the adoption of smaller switchboard capacities is the fact that in the very large cities even a 40,000 line multiple switchboard would still not remove the necessity of multi-office exchanges with the consequent certainty that a large proportion of the calls would have to be trunked anyway.

Undoubtedly, one of the reasons for the difference between American and European practice is the better results that American operating companies have been able to secure in the handling of calls at the incoming end of trunks. This is due, no doubt, in part to the differences in social and economic conditions under which exchanges are operated in this country and abroad, and also in part to the characteristics of the English tongue when compared to some of the other tongues in the matter of ease with which numbers may be spoken. In America it has been found possible to so perfect the operation of trunking under proper operating conditions and with good equipment as to relieve multi-office practice of many of the disadvantages which have been urged against it.

=Classification.= Broadly speaking there are two general methods that may be employed in trunking between exchanges. The first and simplest of these methods is to employ so-called _two-way trunks_. These, as their name indicates, may be used for completing connections between offices in either direction, that is, whether the call originates at one end or the other. The other way is by the use of _one-way trunks_, wherein each trunk carries traffic in one direction only. Where such is the case, one end of the trunk is always used for connecting with the calling subscriber's line and is termed the _outgoing_ end, and the other end is always used in completing the connection with the called subscriber's line, and is referred to as the _incoming_ end. Traffic in the other direction is handled by another set of trunks differing from the first set only in that their outgoing and incoming ends are reversed.

As has already been pointed out, a system of trunks employing two-way trunks is called a _single-track system_, and a system involving two sets of one-way trunks is called a _double-track system_. It is to be noted that the terms outgoing and incoming, as applied to the ends of trunks and also as applied to traffic, always refer to the direction in which the trunk handles traffic or the direction in which the traffic is flowing with respect to the particular office under consideration at the time. Thus an _incoming trunk_ at one office is an _outgoing trunk_ at the other.

_Two-Way Trunks._ Two-way trunks are nearly always employed where the traffic is very small and they are nearly always operated by having the _A_-operator plug directly into the jack at her end of the trunk and displaying a signal at the other end by ringing over the trunk as she would over an ordinary subscriber's line. The operator at the distant exchange answers as she would on an ordinary line, by plugging into the jack of that trunk, and receives her orders over the trunk either from the originating operator or from the subscriber, and then completes the connection with the called subscriber. Such trunks are often referred to as "ring-down" trunks, and their equipment consists in a drop and jack at each end. In case there is a multiple board at either or both of the offices, then the equipment at each end of the trunk would consist of a drop and answering jack, together with the full quota of multiple jacks. It is readily seen that this mode of operation is slow, as the work that each operator has to do is the same as that in connecting two local subscribers, plus the time that it takes for the operators to communicate with each other over the trunk.

_One-Way Trunks._ Where one-way trunks are employed in the double-track system, the trunks, assuming that they connect multiple boards, are provided with multiple jacks only at their outgoing ends, so that any operator may reach them for an outgoing connection, and at their incoming ends they terminate each in a single plug and in suitable signals and ringing keys, the purpose of which will be explained later. Over such trunks there is no verbal communication between the operators, the instructions passing between the operators over separate order-wire circuits. This is done in order that the trunk may be available as much as possible for actual conversation between the subscribers. It may be stated at this point that the duration of the period from the time when a trunk is appropriated by the operators for the making of a certain connection until the time when the trunk is finally released and made available for another connection is called the _holding time_, and this holding time includes not only the period while the subscribers are in actual conversation over it, but also the periods while the operators are making the connection and afterwards while they are taking it down. It may be said, therefore, that the purpose of employing separate order wires for communication between the operators is to make the holding time on the trunks as small as possible and, therefore, for the purpose of enabling a given trunk to take part in as many connections in a given time as possible.

In outline the operation of a one-way trunk between common-battery, manual, multiple switchboards is, with modifications that will be pointed out afterwards, as follows: When a subscriber's line signal is displayed at one office, the operator in attendance at that position answers and finding that the call is for a subscriber in another office, she presses an order-wire key and thereby connects her telephone set directly with that of a _B_-operator at the proper other office. Unless she finds that other operators are talking over the order wire, she merely states the number of the called subscriber, and the _B_-operator whose telephone set is permanently connected with that order wire merely repeats the number of the called subscriber and follows this by designating the number of the trunk which the _A_-operator is to employ in making the connection. The _A_-operator, thereupon, immediately and without testing, inserts the calling plug of the pair used in answering the call into the trunk jack designated by the _B_-operator; the _B_-operator simultaneously tests the multiple jack of the called subscriber and, if she finds it not busy, inserts the plug of the designated trunk into the multiple jack of the called subscriber and rings his bell by pressing the ringing key associated with the trunk cord used. The work on the part of the _A_-operator in connecting with the outgoing end of the trunk and on the part of the _B_-operator in connecting the incoming end of the trunk with the line goes on simultaneously, and it makes no difference which of these operators completes the connection first.

It is the common practice of the Bell operating companies in this country to employ what is called automatic or machine ringing in connection with the _B_-operator's work. When the _B_-operator presses the ringing key associated with the incoming trunk cord, she pays no further attention to it, and she has no supervisory lamp to inform her as to whether or not the subscriber has answered. The ringing key is held down, after its depression by the operator, either by an electromagnet or by a magnet-controlled latch, and the ringing of the subscriber's bell continues at periodic intervals as controlled by the ringing commutator associated with the ringing machine. When the subscriber answers, however, the closure of his line circuit results in such an operation of the magnet associated with the ringing key as to release the ringing key and thus to automatically discontinue the ringing current.

When a connection is established between two subscribers through such a trunk the supervision of the connection falls entirely upon the _A_-operator who established it. This means that the calling supervisory lamp at the _A_-operator's position is controlled over the trunk from the station of the called subscriber, the answering supervisory lamp being, of course, under the control of the calling subscriber as in the case of a local connection. It is, therefore, the _A_-operator who always initiates the taking down of a trunk connection, and when, in response to the lighting of the two lamps, she withdraws her calling plug from the trunk jack, the supervisory lamp associated with the incoming end of the trunk at the other office is lighted, and the _B_-operator obeys it by pulling down the plug.

If, upon testing the multiple jack of the called subscriber's line, the _B_-operator finds the line to be busy, she at once inserts the trunk plug into a so-called "busy-back" jack, which is merely a jack whose terminals are permanently connected to a circuit that is intermittently opened and closed, and which also has impressed upon it an alternating current of such a nature as to produce the familiar "buzz-buzz" in a telephone receiver. The opening and closing of this circuit causes the calling supervisory lamp of the _A_-operator to flash at periodic intervals just as if the called subscriber had raised and lowered his receiver, but more regularly. This is the indication to the _A_-operator that the line called for is busy. The buzzing sound is repeated back through the cord circuit of the _A_-operator to the calling subscriber and is a notification to him that the line is busy.

Sometimes, as is practiced in New York City, for instance, the buzzing feature is omitted, and the only indication that the calling subscriber receives that the called-for line is busy is being told so by the _A_-operator. This may be considered a special feature and it is employed in New York because there the custom exists of telling a calling subscriber, when the line he has called for has been found busy, that the party will be secured for him and that he, the calling subscriber, will be called, if he desires.

A modification of this busy-back feature that has been employed in Boston, and perhaps in other places, is to associate with the busy-back jack at the _B_-operator's position a phonograph which, like a parrot, keeps repeating "Line busy--please call again." Where this is done the calling subscriber, _if he understands what the phonograph says_, is supposed to hang up his receiver, at which time the _A_-operator takes down the connection and the _B_-operator follows in response to the notification of her supervisory lamp. The phonograph busy-back scheme, while ingenious, has not been a success and has generally been abandoned.

As a rule the independent operating companies in this country have not employed automatic ringing, and in this case the _B_-operators have been required to operate their ringing keys and to watch for the response of the called subscriber. In order to arrange for this, another supervisory lamp, termed the _ringing lamp_, is associated with each incoming trunk plug, the going out of this lamp being a notification to the _B_-operator to discontinue ringing.

=Western Electric Trunk Circuits.= The principles involved in inter-office trunking with automatic ringing, are well illustrated in the trunk circuit employed by the Western Electric Company in connection with its No. 1 relay boards. The dotted dividing line through the center of Fig. 371 represents the separating space between two offices. The calling subscriber's line in the first office is shown at the extreme left and the called subscriber's line in the second office is shown at the extreme right. Both of these lines are standard multiple switchboard lines of the form already discussed. The equipment illustrated in the first office is that of an _A_-board, the cord circuit shown being that of the regular _A_-operator. The outgoing trunk jacks connecting with the trunk leading to the other office are, it will be understood, multipled through the _A_-sections of the board and contain no relay equipment, but the test rings are connected to ground through a resistance coil _1_, which takes the place of the cut-off relay winding of a regular line so far as test conditions and supervisory relay operation are concerned. The equipment illustrated in the second office is that of a _B_-board, it being understood that the called subscriber's line is multipled through both the _A_- and _B_-boards at that office. The part of the equipment that is at this point unfamiliar to the reader is, therefore, the cord circuit at the _B_-operator's board. This includes, broadly speaking, the means: (1) for furnishing battery current to the called subscriber; (2) for accomplishing the ringing of the called subscriber and for automatically stopping the ringing when he shall respond; (3) for performing the ordinary switching functions in connection with the relays of the called subscriber's line in just the same way that an _A_-operator's cord carries out these functions; and (4) for causing the operation of the calling supervisory relay of the _A_-operator's cord circuit in just the same manner, under control of the connected called subscriber, as if that subscriber's line had been connected directly to the _A_-operator's cord circuit.

The operation of these devices in the _B_-operator's cord circuit may be best understood by following the establishment of the connection. Assuming that the calling subscriber in the first office desires a connection with the subscriber's line shown in the second office, and that the _A_-operator at the first office has answered the call, she will then communicate by order wire with the _B_-operator at the second office, stating the number of the called subscriber and receiving from that operator in return the number of the trunk to be employed. The two operators will then proceed simultaneously to establish the connection, the _A_-operator inserting the calling plug into the outgoing trunk jack, and the _B_-operator inserting the trunk plug into the multiple jack of the called subscriber's line after testing. We will assume at first that the called subscriber's line is found idle and that both of the operators complete their respective portions of the work at the same time and we will consider first the condition of the calling supervisory relay at the _A_-operator's position.

The circuit of the calling supervisory lamp will have been closed through the resistance coil _1_ connected with the outgoing trunk jacks and the lamp will be lighted because, as will be shown, it is not yet shunted out by the operation of its associated supervisory relay. Tracing the circuit of the calling supervisory relay of the _A_-operator's circuit, it will be found to pass from the live side of the battery to the ring side of the trunk circuit through one winding of the repeating coil of the _B_-operator's cord; beyond this the circuit is open, since no path exists through the condenser _2_ bridged across the trunk circuit or through the normally open contacts of the relay _3_ connected in the talking circuit of the trunk. The association of this relay _3_ with the repeating coil and the battery of the trunk is seen to be just the same as that of a supervisory relay in the _A_-operator's cord, and it is clear, therefore, that this relay _3_ will not be energized until the called subscriber has responded. When it is energized it will complete the path to ground through the _A_-operator's calling supervisory relay and operate to shunt out the _A_-operator's calling supervisory lamp in just the same manner as if the _A_-operator's calling plug had been connected directly with the line of the calling subscriber. In other words, the called subscriber in the second office controls the relay _3_, which, in turn, controls the calling supervisory relay of the _A_-operator, which, in turn, shunts out its lamp.

The connection being completed between the two subscribers, the _B_-operator depresses one or the other of the ringing keys _5_ or _6_, according to which party on the line is called, assuming that it is a two-party line. It will be noticed that the springs of these ringing keys are not serially arranged in the talking circuit, but the cutting off of the trunk circuit back of the ringing keys is accomplished by the set of springs shown just at the left of the ringing keys, which set of springs _7_ is operated whenever either one of the ringing keys is depressed. An auxiliary pair of contacts, shown just below the group of springs _7_, is also operated mechanically whenever either one of the ringing keys is depressed, and this serves to close one of two normally open points in the circuit of the ringing-key holding magnet _8_. This holding magnet _8_ is so arranged with respect to the contacts of the ringing key that whenever any one of them is depressed by the operator, it will be held depressed as long as the magnet is energized just the same as if the operator kept her finger on the key. The other normally open point in the circuit of the holding magnet _8_ is at the lower pair of contacts of the test and holding relay _9_. This relay is operated whenever the trunk plug is inserted in the jack of a called line, regardless of the position of the subscriber's equipment on that line. The circuit may be traced from the live side of the battery through the trunk disconnect lamp _4_, coil _9_, sleeve strand of cord, and to ground through the cut-off relay of the line. The insertion of the trunk plug into the jack thus leaves the completion of the holding-magnet circuit dependent only upon the auxiliary contact on the ringing key, and, therefore, as soon as the operator presses either one of these keys, the clutch magnet is energized and the key is held down, so that ringing current continues to flow at regular intervals to the called subscriber's station.

The ringing current issues from the generator _10_, but the supply circuit from it is periodically interrupted by the commutator _11_ geared to the ringing-machine shaft. This periodically interrupted ringing current passes to the ringing-key contacts through the coil of the ringing cut-off relay _12_, and thence to the subscriber's line. The ringing current is, however, insufficient to cause the operation of this relay _12_ as long as the high resistance and impedance of the subscriber's bell and condenser are in the circuit. It is, however, sufficiently sensitive to be operated by this ringing current when the subscriber responds and thus substitutes the comparatively low resistance and impedance path of his talking apparatus for the previous path through his bell. The pulling up of the ringing cut-off relay _12_ breaks a third normally closed contact in the circuit of the holding coil _8_, de-energizing that coil and releasing the ringing key, thus cutting off ringing current. There is a third brush on the commutator _11_ connected with the live side of the central battery, and this is merely for the purpose of assuring the energizing of the ringing cut-off relay _12_, should the subscriber respond during the interval while the commutator _11_ held the ringing current cut off. The relay _12_ may thus be energized either from the battery, if the subscriber responds during a period of silence of his ringer, or from the generator _10_, if the subscriber responds during a period while his bell is sounding; in either case the ringing current will be promptly cut off by the release of the ringing key.

The trunk operator's "disconnect lamp" is shown at _4_, and it is to be remembered that this lamp is lighted only when the _A_-operator takes down the connection at her end, and also that this lamp is entirely out of the control of the subscribers, the conditions which determine its illumination being dependent on the positions of the operators' plugs at the two ends of the trunk. With both plugs up, the lamp _4_ will receive current, but will be shunted to prevent its illumination. The path over which it receives this current may be traced from battery through the lamp _4_, thence through the coil of the relay _9_ and the cut-off relay of the called subscriber's line. This current would be sufficient to illuminate the lamp, but the lamp is shunted by a circuit which may be traced from the live side of battery through the contact of the relay _13_, closed at the time, and through the coil of the trunk cut-off relay coil _14_. The resistance of this coil is so proportioned to the other parts of the circuit as to prevent the illumination of the lamp just exactly as in the case of the shunting resistances of the lamps in the _A_-operator's cord. It will be seen, therefore, that the supply of current to the trunk disconnect lamp is dependent on the trunk plug being inserted into the jack of the subscriber's line and that the shunting out of this lamp is dependent on the energization of the relay _13_. This relay _13_ is energized as long as the _A_-operator's plug is inserted into the outgoing trunk jack, the path of the energizing circuit being traced from the live side of the battery at the second office through the right-hand winding of this relay, thence over the tip side of the trunk to ground at the first office. From this it follows that as long as both plugs are up, the disconnect lamp will receive current but will be shunted out, and as soon as the _A_-operator pulls down the connection, the relay _13_ will be de-energized and will thus remove the shunt from about the lamp, allowing its illumination. The left-hand winding of the relay _13_ performs no operating function, but is merely to maintain the balance of the talking circuit, it being bridged during the connection from the ring side of the trunk to ground in order to balance the bridge connection of the right-hand coil from the live side of battery to the tip side of the trunk circuit.

The relay _14_, already referred to as forming a shunt for the trunk disconnect lamp, has for its function the keeping of the talking circuit through the trunk open until such time as the relay _13_ operates, this being purely an insurance against unnecessary ringing of a subscriber in case the _A_-operator should by mistake plug into the wrong trunk. It is not, therefore, until the _A_-operator has plugged into the trunk and the relay _13_ has been operated to cause the energization of the relay _14_ that the ringing of the called subscriber can occur, regardless of what the _B_-operator may have done.

The relay _9_ has an additional function to that of helping to control the circuit of the ringing-key holding magnet. This is the holding of the test circuit complete until the operator has tested and made a connection and then automatically opening it. The test circuit of the _B_-operator's trunk may be traced, at the time of testing, from the thimble of the multiple jack under test, through the tip of the cord, thence through the uppermost pair of contacts of the relay _9_ to ground through a winding of the _B_-operator's induction coil. After the test has been made and the plug inserted, the relay _9_, which is operated by the insertion of the plug, acts to open this test circuit and at the same time complete the tip side of the cord circuit.

In the upper portion of Fig. 371 the order-wire connections, by which the _A_-operator and the _B_-operator communicate, are indicated. It must be remembered in connection with these that the _A_-operator only has control of this connection, the _B_-operator being compelled necessarily to hear whatever the _A_-operators have to say when the _A_-operators come in on the circuit.

The incoming trunk circuit employed by the Western Electric Company for four-party line ringing is shown in Fig. 372, it being necessarily somewhat modified from that shown in Fig. 371, which is adapted for two-party line ringing only. In addition to the provision of the four-party line ringing keys, by which positive or negative pulsating current is received over either limb of the line, and to the provision of the regular alternating current ringing key for ringing on single party lines, it is necessary in the ringing cut-off relay to provide for keeping the alternating and the pulsating ringing currents entirely separate. For this reason, the ringing cut-off relay _12_ is provided with two windings, that at the right being in the path of the alternating ringing currents that are supplied to the alternating current key, and that at the left being in the ground return path for all of the pulsating ringing currents supplied to the pulsating keys. With this explanation it is believed that this circuit will be understood from what has been said in connection with Fig. 371. The operation of the holding coil _8_ is the same in each case, the holding magnet in Fig. 372 serving to hold depressed any one of the five ringing keys that may have been used in calling the subscriber.

The standard four-party line, trunk ringing key of the Western Electric Company is shown in Fig. 373. In this the various keys operate not by pressure but rather by being pulled by the finger of the operator in such a way as to subject the key shaft to a twisting movement. The holding magnet lies on the side opposite to that shown in the figure and extends along the full length of the set of keys, each key shaft being provided with an armature which is held by this magnet until the magnet is de-energized by the action of the ringing cut-off relay.

The standard trunk relays employed by the Western Electric Company in connection with the circuits just described are shown in Figs. 374 and 375. In each case the dust-cap or shield is also shown. The relay of Fig. 374 is similar to the regular cut-off relay and is the one used for relays _9_ and _14_ of Figs. 371 and 372. The relay of Fig. 375 is somewhat similar to the subscriber's line relay in that it has a tilting armature, and is the one used at _13_ in Figs. 371 and 372. The trunk relay _3_ in Figs. 371 and 372 is the same as the _A_-operator's supervisory relays already discussed.

It has been stated that under certain circumstances _B_-operator's trunk circuits devoid of ringing keys, and consequently of all keys, may be employed. This, so far as the practice of the Bell companies is concerned, is true only in offices where there are no party lines, or where, as in many of the Chicago offices, the party lines are worked on the "jack per station" basis. In "jack per station" working, the selection of the station on a party line is determined by the jack on which the plug is put, rather than by a ringing key, and hence the keyless trunk may be employed.

A keyless trunk as used in New York is shown in Fig. 376. This has no manually operated keys whatever, and the relay _17_, when it is operated, establishes connection between the ringing generator and the conductors of the trunk plug. The relays _3_, _13_, and _12_ operate in a manner identical with those bearing corresponding numbers in Fig. 371. As soon as the trunk operator plugs into the multiple jack of the called subscriber, the relay _16_ will operate for the same reason that the relay _9_ operated in connection with Fig. 371. The trunk disconnect lamp will receive current, but if the operator has already established connection with the other end of the trunk, this lamp will not be lighted because shunted by the relay _17_, due to the pulling up of the armature of the relay _13_. The relay _15_ plays no part in the operation so far described, because of the fact that its winding is short-circuited by its own contacts and those of relay _12_, when the latter is not energized. As a result of the operation of the relay _17_, ringing current is sent to line, the supply circuit including the coil of the relay _12_. As soon as the subscriber responds to this ringing current, the armature of the relay _12_ is pulled up, thus breaking the shunt about the relay _15_, which, therefore, starts to operate in series with the relay _17_, but as its armatures assume their attracted position, the relay _17_ is cut out of the circuit, the coil of the relay _15_ being substituted for that of the relay _17_ in the shunt path around the lamp _4_. The relay _17_ falls back and cuts off the ringing current. The relay _15_ now occupies the place with respect to the shunt around the lamp _4_ that the relay _17_ formerly did, the continuity of this shunt being determined by the energization of the relay _13_. When the _A_-operator at the distant exchange withdraws the calling plug from the trunk jack, this relay _13_ becomes de-energized, breaking the shunt about the lamp _4_ and permitting the display of that lamp as a signal to the operator to take down the connection. It may be asked why the falling back of relay _15_ will not again energize relay _17_ and thus cause a false ring on the called subscriber. This will not occur because both the relays _15_ and _17_ depend for their energization on the closure of the contacts of the relay _13_, and when this falls back the relay _17_ cannot again be energized even though the relay _15_ assumes its normal position.

=Kellogg Trunk Circuits.= The provision for proper working of trunk circuits in connection with the two-wire multiple switchboards is not an altogether easy matter, owing particularly to the smaller number of wires available in the plug circuits. It has been worked out in a highly ingenious way, however, by the Kellogg Company, and a diagram of their incoming trunk circuit, together with the associated circuits involved in an inter-office connection, is shown in Fig. 377.

This figure illustrates a connection from a regular two-wire multiple subscriber's line in one office, through an _A_-operator's cord circuit there, to the outgoing trunk jacks at that office, thence through the incoming trunk circuit at the other office to the regular two-wire multiple subscriber's line at that second office. The portion of this diagram to be particularly considered is that of the _B_-operator's cord circuit. The trunk circuit terminates in the multipled outgoing trunk jacks at the first office, the trunk extending between offices consisting, of course, of but two wires. We will first consider the control of the calling supervisory lamp in the _A_-operator's cord circuit, it being remembered that this control must be from the called subscriber's station. It will be noticed that the left-hand armature of the relay _1_ serves normally to bridge the winding of relay _2_ across the cord circuit around the condenser _3_. When, however, the relay _1_ pulls up, the coil of relay _4_ is substituted in this bridge connection across the trunk. The relay _2_ has a very high resistance winding--about 15,000 ohms--and this resistance is so great that the tip supervisory relay of the _A_-operator's cord will not pull up through it. As a result, when this relay is bridged across the trunk circuit, the tip relay on the calling side of the _A_-operator's cord circuit is de-energized, just as if the trunk circuit were open, and this results in the lighting of the _A_-operator's calling supervisory lamp. The winding of the relay _4_, however, is of low resistance--about 50 ohms--and when this is substituted for the high-resistance winding of the relay _2_, the tip relay on the calling side of the _A_-operator's cord is energized, resulting in the extinguishing of the calling supervisory lamp. The illumination of the _A_-operator's calling supervisory lamp depends, therefore, on whether the high-resistance relay _2_, or the low-resistance relay _4_, is bridged across the trunk, and this depends on whether the relay _1_ is energized or not. The relay _1_, being bridged from the tip side of the trunk circuit to ground and serving as the means of supply of battery current to the called subscriber, is operated whenever the called subscriber's receiver is removed from its hook. Therefore, the called subscriber's hook controls the operation of this relay _1_, which, in turn, controls the conditions which cause the illumination or darkness of the calling supervisory lamp at the distant office.

Assuming that the _A_-operator has received and answered a call, and has communicated with the _B_-operator, telling her the number of the called subscriber, and has received, in turn, the number of the trunk to be used, and that both operators have put up the connection, then it will be clear from what has been said that the calling supervisory lamp of the _A_-operator will be lighted until the called subscriber removes his receiver from its hook, because the tip relay in the _A_-operator's cord circuit will not pull up through the 15,000-ohm resistance winding of the relay _2_. As soon as the subscriber responds, however, the relay _1_ will be operated by the current which supplies his transmitter. This will substitute the low-resistance winding of the relay _4_ for the high-resistance winding of the relay _2_, and this will permit the energizing of the tip supervisory relay of the _A_-operator and put out the calling supervisory lamp at her position. As in the Western Electric circuit, therefore, the control of the _A_-operator's calling supervisory lamp is from the called subscriber's station and is relayed back over the trunk to the originating office.

In this circuit, manual instead of automatic ringing is employed, therefore, unlike the Western Electric circuit, means must be provided for notifying the B-operator when the calling subscriber has answered. This is done by placing at the _B_-operator's position a ringing lamp associated with each trunk cord, which is illuminated when the _B_-operator places the plug of the incoming trunk into the multiple jack of the subscriber's line, and remains illuminated until the subscriber has answered. This is accomplished in the following manner: when the operator plugs into the jack of the line called, relay _5_ is energized but is immediately de-energized by the disconnecting of the circuit of this relay from the sleeve conductor of the cord when the ringing key is depressed, the selection of the ringing key being determined by the particular party on the line desired. These ringing keys have associated with them a set of springs _9_, which springs are operated when any one of the ringing keys is depressed. Thus, with a ringing key depressed and the relay _5_ de-energized, the ringing lamp will be illuminated by means of a circuit as follows: from the live side of the battery, through the ringing lamp _12_, through the back contact and armature of the relay _6_, through the armature and contact of relay _4_, then through the armature and front contact of relay _2_--which at this time is the relay bridged across the trunk and, therefore, energized--and thence through the back contact and armature of relay _5_ to ground. When the subscriber removes his receiver from the hook, the relay _1_ will become energized as previously described, and will, therefore, operate relay _6_ to break the circuit of the ringing lamp. The circuit thus established by the operation of relay _1_ is as follows: from the live side of battery, through the winding of relay _6_, through the armature and contact of relay _1_, through the armature and contact of relay _4_, through the armature and front contact of relay _2_, thence through the armature and back contact of relay _5_ to ground. As soon as the _B_-operator notes that the ringing lamp has gone out, she knows that no further ringing is required on that line, thus allowing the operation of relay _5_ and accomplishing the locking out of the ringing lamp during the remainder of that connection. The relay _6_, after having once pulled up, remains locked up through the rear contact of the left-hand armature of relay _5_ and ground, until the plug is removed from the jack.

At the end of the conversation, when the _A_-operator has disconnected her cord circuit on the illumination of the supervisory signals, both relays _2_ and _4_ will be in an unoperated condition and will provide a circuit for illuminating the disconnect lamp associated with the _B_-operator's cord. This circuit may be traced as follows: from battery through the disconnect lamp, through the armatures and contacts of relays _2_ and _4_, thence through the front contact and armature of relay _5_ to ground, thus illuminating the disconnect lamp. The ringing lamp will not be re-illuminated at this time, due to the fact that it has been previously locked out by relay _6_. The operator then removes the plug from the jack of the line called, and the apparatus in the trunk circuit is restored to normal condition.

In the circuit shown only keys are provided for ringing two parties. This circuit, however, is not confined to the use of two-party lines, but may be extended to four parties by simply duplicating the ringing keys and by connecting them with the proper current for selectively ringing the other stations.

The method of determining as to whether the called line is free or busy is similar to that previously described for the _A_-operator's cord circuit when making a local connection, and differs only in the fact that in the case of the trunk cord the test circuit is controlled through the contacts of a relay, whereas in the case of the _A_-operator's cord, the test circuit was controlled through the contacts of the listening key. The function of the resistance _10_ and the battery connected thereto is the same as has been previously described.

The general make-up of trunking switchboard sections is not greatly different from that of the ordinary switchboard sections where no trunking is involved. In small exchanges where ring-down trunks are employed, the trunk line equipment is merely added to the regular jack and drop equipment of the switchboard. In common-battery multiple switchboards the _A_-boards differ in no respect from the standard single office multiple boards, except that immediately above the answering jacks and below the multiple there are arranged in suitable numbers the jacks of the outgoing trunks.

Where the offices are comparatively small, the incoming trunk portions of the _B_-boards are usually merely a continuance of the _A_-boards, the subscriber's multiple being continuous with and differing in no respect from that on the _A_-sections. Instead of the usual pairs of _A_-operators' plugs, cords, and supervisory equipment, there are on the key and plug shelves of these _B_-sections the incoming trunk plugs and their associated equipment.

In large offices it is customary to make the _B_-board entirely separate from the _A_-board, although the general characteristics of construction remain the same. The reason for separate _A_- and _B_-switchboards in large exchanges is to provide for independent growth of each without the growth of either interfering with the other.

A portion of an incoming trunk, or _B_-board, is shown in Fig. 378. The multiple is as usual, and, of course, there are no outgoing trunk jacks nor regular cord pairs. Instead the key and plug shelves are provided with the incoming-trunk plug equipments, thirty of these being about the usual quota assigned to each operator's position.

In multi-office exchanges, employing many central offices, such, for instance, as those in New York or Chicago, it is frequently found that nearly all of the calls that originate in one office are for subscribers whose lines terminate in some other office. In other words, the number of calls that have to be trunked to other offices is greatly in excess of the number of calls that may be handled through the multiple of the _A_-board in which they originate. It is not infrequent to have the percentage of trunked calls run as high as 75 per cent of the total number of calls originating in any one office, and in some of the offices in the larger cities this percentage runs higher than 90 per cent.

This fact has brought up for consideration the problem as to whether, when the nature of the traffic is such that only a very small portion of the calls can be handled in the office where they originate, it is worth while to employ the multiple terminals for the subscribers' lines on the _A_-boards. In other words, if so great a proportion as 90 per cent of the calls have to be trunked any way, is it worth while to provide the great expense of a full multiple on all the sections of the _A_-board in order to make it possible to handle the remaining 10 per cent of the calls directly by the _A_-operators?

As a result of this consideration it has been generally conceded that where such a very great percentage of trunking was necessary, the full multiple of the subscribers' lines on each section was not warranted, and what is known as the partial multiple board has come into existence in large manual exchanges. In these the regular subscribers' multiple is entirely omitted from the _A_-board, all subscribers' calls being handled through outgoing trunk jacks connected by trunks to _B_-boards in the same as well as other offices. In these partial multiple _A_-boards, the answering jacks are multipled a few times, usually twice, so that calls on each line may be answered from more than one position. This multipling of answering jacks does not in any way take the place of the regular multipling in full multiple boards, since in no case are the calls completed through the multiple jacks. It is done merely for the purpose of contributing to team work between the operators.

A portion of such a partial multiple _A_-board is shown in Fig. 379. This view shows slightly more than one section, and the regular answering jacks and lamps may be seen at the bottom of the jack space just above the plugs. Above these are placed the outgoing trunk jacks, those that are in use being indicated with white designation strips. Above the outgoing trunk jacks are placed the multiples of the answering jacks, these not being provided with lamps.

The partial multiple _A_-section of Fig. 379 is a portion of the switchboard equipment of the same office to which the trunking section shown in Fig. 378 belongs. That this is a large multiple board may be gathered from the number of multiple jacks in the trunking section, 8,400 being installed with room for 10,500. That the board is a portion of an equipment belonging to an exchange of enormous proportions may be gathered from the number of outgoing trunk jacks shown in the _A_-board, and in the great number of order-wire keys shown between each of the sets of regular cord-circuit keys. The switchboards illustrated in these two figures are those of one of the large offices of the New York Telephone Company on Manhattan Island, and the photographs were taken especially for this work by the Western Electric Company.

=Cable Color Code.= A great part of the wiring of switchboards requires to be done with insulated wires grouped into cables. In the wiring of manual switchboards as described in the seven preceding chapters, and of automatic and automanual systems and of private branch-exchange and intercommunicating systems described in succeeding chapters, cables formed as follows are widely used:

Tinned soft copper wires, usually of No. 22 or No. 24 B. & S. gauge, are insulated, first with two coverings of silk, then with one covering of cotton. The outer (cotton) insulation of each wire is made of white or of dyed threads. If dyed, the color either is solid red, black, blue, orange, green, brown, or slate, or it is striped, by combining one of those colors with white or a remaining color. The object of coloring the wires is to enable them to be identified by sight instead of by electrical testing.

Wires so insulated are twisted into pairs, choosing the colors of the "line" and "mate" according to a predetermined plan. An assortment of these pairs then is laid up spirally to form the cable core, over which are placed certain wrappings and an outer braid. A widely used form of switchboard cable has paper and lead foil wrappings over the core, and the outer cotton braid finally is treated with a fire-resisting paint.

STANDARD COLOR CODE FOR CABLES

+---------------+-------------------------------------------------+ | | MATE | | LINE WIRE +-------+-------+-------+-----------+-------------+ | | White | Red | Black | Red-White | Black-White | +---------------+-------+-------+-------+-----------+-------------+ | Blue | 1 | 21 | 41 | 61 | 81 | | Orange | 2 | 22 | 42 | 62 | 82 | | Green | 3 | 23 | 43 | 63 | 83 | | Brown | 4 | 24 | 44 | 64 | 84 | | Slate | 5 | 25 | 45 | 65 | 85 | | Blue-White | 6 | 26 | 46 | 66 | 86 | | Blue-Orange | 7 | 27 | 47 | 67 | 87 | | Blue-Green | 8 | 28 | 48 | 68 | 88 | | Blue-Brown | 9 | 29 | 49 | 69 | 89 | | Blue-Slate | 10 | 30 | 50 | 70 | 90 | | Orange-White | 11 | 31 | 51 | 71 | 91 | | Orange-Green | 12 | 32 | 52 | 72 | 92 | | Orange-Brown | 13 | 33 | 53 | 73 | 93 | | Orange-Slate | 14 | 34 | 54 | 74 | 94 | | Green-White | 15 | 35 | 55 | 75 | 95 | | Green-Brown | 16 | 36 | 56 | 76 | 96 | | Green-Slate | 17 | 37 | 57 | 77 | 97 | | Brown-White | 18 | 38 | 58 | 78 | 98 | | Brown-Slate | 19 | 39 | 59 | 79 | 99 | | Slate-White | 20 | 40 | 60 | 80 | 100 | +---------------+-------+-------+-------+-----------+-------------+

The numerals represent the pair numbers in the cable.

The wires of spare pairs usually are designated by solid red with white mate for first spare pair, and solid black with white mate for second spare pair. Individual spare wires usually are colored red-white for first individual spare, and black-white for second individual spare.