Cyclopedia of Telephony and Telegraphy Volume Ii Part 6

The Kellogg Company employs the type of relay of which the magnetic circuit was ill.u.s.trated in Fig. 95. In its multiple boards it commonly mounts the line and cut-off relays together, as shown in Fig. 366. A single, soft iron sh.e.l.l is used to cover both of these, thus serving as a dust shield and also as a magnetic shield to prevent cross-talk between adjacent relays--an important feature, since it will be remembered the cut-off relays are left permanently connected with the talking circuit. Fig. 367, which shows a strip of twenty such pairs of relays, from five of which the covers have been removed, is an excellent detail view of the general practice in this respect; obviously, a very large number of such relays may be mounted in a comparatively small s.p.a.ce. The mounting strip shown in this cut is of heavy rolled iron and is provided with openings through which the connection terminals--shown more clearly in Fig. 366--project. On the back of this mounting strip all the wiring is done and much of this wiring--that connecting adjacent terminals on the back of the relay strip--is made by means of thin copper wires without insulation, the wires being so short as to support themselves without danger of crossing with other wires. When these wires are adjacent to ground or battery wires they may be protected by sleeving, so as to prevent crosses.

[Ill.u.s.tration: Fig. 368. Monarch Relay]

An interesting feature in relay construction is found in the relay of the Monarch Telephone Manufacturing Company shown in Figs. 368 and 369.

The a.s.sembled relay and its mounting strip and cap are shown in Fig.

368. This relay is so constructed that by the lifting of a single latch not only the armature but the coil may be bodily removed, as shown in Fig. 369, in which the latch is shown in its raised position. As seen, the armature has an L-shaped projection which serves to operate the contact springs lying on the iron plate above the coil. The simplicity of this device is attractive, and it is of convenience not only from the standpoint of easy repairs but also from the standpoint of factory a.s.sembly, since by manufacturing standard coils with different characters of windings and standard groups of springs, it is possible to produce without special manufacture almost any combination of relay.

[Ill.u.s.tration: Fig. 369. Monarch Relay]

=a.s.sembly.= The arrangement of the key and jack equipment in complete multiple switchboard sections is clearly shown in Fig. 370, which shows a single three-position section of one of the small multiple switchboards of the Kellogg Switchboard and Supply Company. The arrangement of keys and plugs on the key shelf is substantially the same as in simple common-battery boards. As in the simple switchboards the supervisory lamps are usually mounted on the hinged key shelf immediately in the rear of the listening and ringing keys and with such s.p.a.cing as to lie immediately in front of the plugs to which they correspond. The reason for mounting the supervisory lamps on the key shelf is to make them easy of access in case of the necessity of lamp renewals or repairs on the wiring. The s.p.a.ce at the bottom of the vertical panels, containing the jacks, is left blank, as this s.p.a.ce is obstructed by the standing plugs in front of it. Above the plugs, however, are seen the alternate strips of line lamps and answering jacks, the lamps in each case being directly below the corresponding answering jacks. Above the line lamps and answering jacks in the two positions at the right there are blank strips into which additional line lamps and jacks may be placed in case the future needs of the system demand it. The s.p.a.ce above these is the multiple jack s.p.a.ce, and it is evident from the small number of multiple jacks in this little switchboard that the present equipment of the board is small. It is also evident from the amount of blank s.p.a.ce left for future installations of multiple jacks that a considerable growth is expected. Thus, while there are but four banks of 100 multiple jacks, or 400 in all, there is room in the multiple for 300 banks of 100 multiple jacks, or 3,000 in all.

The method of grouping the jacks in banks of 100 and of providing for their future growth is clearly indicated in this figure. The next section at the right of the one shown would contain a duplicate set of multiple jacks and also an additional equipment of answering jacks and lamps.

[Ill.u.s.tration: A MULTIPLE MANUAL SWITCHING BOARD FOR TOLL CONNECTIONS IN AN AUTOMATIC SYSTEM Multiple Jacks are Provided for Each Line through Which Toll Connections are Handled Directly.]

[Ill.u.s.tration: Fig. 370. Small Multiple Board Section]

For ordinary local service no operator would sit at the left-hand position of the section shown, that being the end position, since the operator there would not be able easily to reach the extreme right-hand portion of the third position and would have nothing to reach at her left. This end position in this particular board ill.u.s.trated is provided with toll-line equipment, a practice not uncommon in small multiple boards. To prevent confusion let us a.s.sume that the multiple jack s.p.a.ce contains its full equipment of 3,000 jacks on each section. The operator in the center position of the section shown could easily reach any one of the jacks on that section. The operator at the third position could reach any jack on the second and third position of her section, but could not well reach multiple jacks in the first position. She would, however, have a duplicate of the multiple jacks in this first position in the section at her right, _i. e._, in the fourth position, and it makes no difference on what portion of the switchboard she plugs into the multiple so long as she plugs into a jack of the right line.

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.

=Cla.s.sification.= 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, a.s.suming 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 pa.s.sing 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 a.s.sociated 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 a.s.sociated 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 a.s.sociated with the ringing machine. When the subscriber answers, however, the closure of his line circuit results in such an operation of the magnet a.s.sociated 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 a.s.sociated 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 a.s.sociate 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 a.s.sociated 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 ill.u.s.trated 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 s.p.a.ce 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 ill.u.s.trated 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 ill.u.s.trated 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.

[Ill.u.s.tration: Fig. 371. Inter-Office Connection--Western Electric System]

The operation of these devices in the _B_-operator's cord circuit may be best understood by following the establishment of the connection.

a.s.suming 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 a.s.sume 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 a.s.sociated supervisory relay.

Tracing the circuit of the calling supervisory relay of the _A_-operator's circuit, it will be found to pa.s.s 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 a.s.sociation 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, a.s.suming 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 pa.s.ses 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 subst.i.tutes 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 a.s.suring 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.

[Ill.u.s.tration: Fig. 372. Incoming Trunk Circuit]