Fig. 371. Inter-Office Connection—Western Electric SystemView full size illustration.
The operation of these devices in theB-operator's cord circuitmay 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 theA-operator at the first office has answered the call, she will then communicate by order wire with theB-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, theA-operator inserting the calling plug into the outgoing trunk jack, and theB-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 theA-operator's position.
The circuit of the calling supervisory lamp will have been closed through the resistance coil1connected 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 theA-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 theB-operator's cord; beyond this the circuit is open, since no path exists through the condenser2bridged across the trunk circuit or through the normally open contacts of the relay3connected in the talking circuit of the trunk. The association of this relay3with the repeating coil and the battery of the trunk is seen to be just the same as that of a supervisory relay in theA-operator's cord, and it is clear, therefore, that this relay3will not be energized until the called subscriber has responded. When it is energized it will complete the path to ground through theA-operator's calling supervisory relay and operate to shunt out theA-operator's calling supervisory lamp in just the same manner as if theA-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 relay3, which, in turn, controls the calling supervisory relay of theA-operator, which, in turn, shunts out its lamp.
The connection being completed between the two subscribers, theB-operator depresses one or the other of the ringing keys5or6, 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 springs7is operated whenever either one of the ringing keys is depressed. An auxiliary pair of contacts, shown just below the group of springs7, 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 magnet8. This holding magnet8is 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 magnet8is at the lower pair of contacts of the test and holding relay9. 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 lamp4, coil9, 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 generator10, but the supply circuit from it is periodically interrupted by the commutator11geared to the ringing-machine shaft. This periodically interrupted ringing current passes to the ringing-key contacts through the coil of the ringing cut-off relay12, and thence to the subscriber's line. The ringing current is, however, insufficient to cause the operation of this relay12as 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 relay12breaks a third normally closed contact in the circuit of the holding coil8, de-energizing that coil and releasing the ringing key, thus cutting off ringing current. There is a third brush on the commutator11connected with the live side of the central battery, and this is merely for the purpose of assuring the energizing of the ringing cut-off relay12, should the subscriber respond during the interval while the commutator11held the ringing current cut off. The relay12may thus be energized either from the battery, if the subscriber responds during a period of silence of his ringer, or from the generator10, 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 at4, and it is to be remembered that this lamp is lighted only when theA-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 lamp4will 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 lamp4, thence through the coil of the relay9and 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 relay13, closed at the time, and through the coil of the trunk cut-off relay coil14. 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 theA-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 relay13. This relay13is energized as long as theA-operator's plug is inserted into the outgoingtrunk 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 theA-operator pulls down the connection, the relay13will be de-energized and will thus remove the shunt from about the lamp, allowing its illumination. The left-hand winding of the relay13performs 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 relay14, 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 relay13operates, this being purely an insurance against unnecessary ringing of a subscriber in case theA-operator should by mistake plug into the wrong trunk. It is not, therefore, until theA-operator has plugged into the trunk and the relay13has been operated to cause the energization of the relay14that the ringing of the called subscriber can occur, regardless of what theB-operator may have done.
The relay9has 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 theB-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 relay9to ground through a winding of theB-operator's induction coil. After the test has been made and the plug inserted, the relay9, 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 theA-operator and theB-operator communicate, are indicated. It must be remembered in connection with these that theA-operator only has control of this connection, theB-operatorbeing compelled necessarily to hear whatever theA-operators have to say when theA-operators come in on the circuit.
Fig. 372. Incoming Trunk CircuitView full size illustration.
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-offrelay12is 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 coil8is 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.
Fig. 373. Western Electric Trunk Ringing KeyView full size illustration.
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 twistingmovement. 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.
Fig. 374. Trunk RelayView full size illustration.
Fig. 375. Trunk RelayView full size illustration.
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 relays9and14of 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 at13in Figs. 371 and 372. The trunk relay3in Figs. 371 and 372 is the same as theA-operator's supervisory relays already discussed.
It has been stated that under certain circumstancesB-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.
Fig. 376. Keyless TrunkView full size illustration.
A keyless trunk as used in New York is shown in Fig. 376. This has no manually operated keys whatever, and the relay17, when it is operated, establishes connection between the ringing generator and the conductors of the trunk plug. The relays3,13, and12operate in a manner identical with those bearing correspondingnumbers in Fig. 371. As soon as the trunk operator plugs into the multiple jack of the called subscriber, the relay16will operate for the same reason that the relay9operated 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 relay17, due to the pulling up of the armature of the relay13. The relay15plays 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 relay12, when the latter is not energized. As a result of the operation of the relay17, ringing current is sent to line, the supply circuit including the coil of the relay12. As soon as the subscriber responds to this ringing current, the armature of the relay12is pulled up, thus breaking the shunt about the relay15, which, therefore, starts to operate in series with the relay17, but as its armatures assume their attracted position, the relay17is cut out of the circuit, the coil of the relay15being substituted for that of the relay17in the shunt path around the lamp4. The relay17falls back and cuts off the ringing current. The relay15now occupies the place with respect to the shunt around the lamp4that the relay17formerly did, the continuity of this shunt being determined by the energization of the relay13. When theA-operator at the distant exchange withdraws the calling plug from the trunk jack, this relay13becomes de-energized, breaking the shunt about the lamp4and 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 relay15will not again energize relay17and thus cause a false ring on the called subscriber. This will not occur because both the relays15and17depend for their energization on the closure of the contacts of the relay13, and when this falls back the relay17cannot again be energized even though the relay15assumes 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.
Fig. 377. Inter-Office Connection—Kellogg SystemView full size illustration.
This figure illustrates a connection from a regular two-wire multiple subscriber's line in one office, through anA-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 theB-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 theA-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 relay1serves normally to bridge the winding of relay2across the cord circuit around the condenser3. When, however, the relay1pulls up, the coil of relay4is substituted in this bridge connection across the trunk. The relay2has a very high resistance winding—about 15,000 ohms—and this resistance is so great that the tip supervisory relay of theA-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 theA-operator's cord circuit is de-energized, just as if the trunk circuit were open, and this results in the lighting of theA-operator's calling supervisory lamp. The winding of the relay4, however, is of low resistance—about 50 ohms—and when this is substituted for the high-resistance winding of the relay2, the tip relay on the calling side of theA-operator's cord is energized, resulting in the extinguishing of the calling supervisory lamp. The illumination of theA-operator's calling supervisory lamp depends, therefore, on whether the high-resistance relay2, or the low-resistance relay4, is bridged across the trunk, and this depends on whether the relay1is energized or not. The relay1, 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 relay1, which, in turn, controls the conditions which cause the illumination or darkness of the calling supervisory lamp at the distant office.
Assuming that theA-operator has received and answered a call, and has communicated with theB-operator, telling her the numberof 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 theA-operator will be lighted until the called subscriber removes his receiver from its hook, because the tip relay in theA-operator's cord circuit will not pull up through the 15,000-ohm resistance winding of the relay2. As soon as the subscriber responds, however, the relay1will be operated by the current which supplies his transmitter. This will substitute the low-resistance winding of the relay4for the high-resistance winding of the relay2, and this will permit the energizing of the tip supervisory relay of theA-operator and put out the calling supervisory lamp at her position. As in the Western Electric circuit, therefore, the control of theA-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 theB-operator's position a ringing lamp associated with each trunk cord, which is illuminated when theB-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, relay5is 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 springs9, which springs are operated when any one of the ringing keys is depressed. Thus, with a ringing key depressed and the relay5de-energized, the ringing lamp will be illuminated by means of a circuit as follows: from the live side of the battery, through the ringing lamp12, through the back contact and armature of the relay6, through the armature and contact of relay4, then through the armature and front contact of relay2—which at this time is the relay bridged across the trunk and, therefore, energized—and thence through theback contact and armature of relay5to ground. When the subscriber removes his receiver from the hook, the relay1will become energized as previously described, and will, therefore, operate relay6to break the circuit of the ringing lamp. The circuit thus established by the operation of relay1is as follows: from the live side of battery, through the winding of relay6, through the armature and contact of relay1, through the armature and contact of relay4, through the armature and front contact of relay2, thence through the armature and back contact of relay5to ground. As soon as theB-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 relay5and accomplishing the locking out of the ringing lamp during the remainder of that connection. The relay6, after having once pulled up, remains locked up through the rear contact of the left-hand armature of relay5and ground, until the plug is removed from the jack.
At the end of the conversation, when theA-operator has disconnected her cord circuit on the illumination of the supervisory signals, both relays2and4will be in an unoperated condition and will provide a circuit for illuminating the disconnect lamp associated with theB-operator's cord. This circuit may be traced as follows: from battery through the disconnect lamp, through the armatures and contacts of relays2and4, thence through the front contact and armature of relay5to 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 relay6. 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 theA-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 controlledthrough the contacts of a relay, whereas in the case of theA-operator's cord, the test circuit was controlled through the contacts of the listening key. The function of the resistance10and 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 theA-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 theB-boards are usually merely a continuance of theA-boards, the subscriber's multiple being continuous with and differing in no respect from that on theA-sections. Instead of the usual pairs ofA-operators' plugs, cords, and supervisory equipment, there are on the key and plug shelves of theseB-sections the incoming trunk plugs and their associated equipment.
In large offices it is customary to make theB-board entirely separate from theA-board, although the general characteristics of construction remain the same. The reason for separateA- andB-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, orB-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 theA-board in which they originate. It isnot 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.
Fig. 378. Section of Trunk SwitchboardView full size illustration.
Fig. 379. Section of Partial Multiple SwitchboardView full size illustration.
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 theA-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 theA-board in order to make it possible to handle the remaining 10 per cent of the calls directly by theA-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 theA-board, all subscribers' calls being handled through outgoing trunk jacks connected by trunks toB-boards in the same as well as other offices. In these partial multipleA-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 multipleA-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 multipleA-section of Fig. 379 is a portion of the switchboard equipment of the same office to which the trunking sectionshown 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 theA-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 CABLESLINE WIREMATEWhiteRedBlackRed-WhiteBlack-WhiteBlue121416181Orange222426282Green323436383Brown424446484Slate525456585Blue-White626466686Blue-Orange727476787Blue-Green828486888Blue-Brown929496989Blue-Slate1030507090Orange-White1131517191Orange-Green1232527292Orange-Brown1333537393Orange-Slate1434547494Green-White1535557595Green-Brown1636567696Green-Slate1737577797Brown-White1838587898Brown-Slate1939597999Slate-White20406080100The 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.
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
LINE WIREMATEWhiteRedBlackRed-WhiteBlack-WhiteBlue121416181Orange222426282Green323436383Brown424446484Slate525456585Blue-White626466686Blue-Orange727476787Blue-Green828486888Blue-Brown929496989Blue-Slate1030507090Orange-White1131517191Orange-Green1232527292Orange-Brown1333537393Orange-Slate1434547494Green-White1535557595Green-Brown1636567696Green-Slate1737577797Brown-White1838587898Brown-Slate1939597999Slate-White20406080100
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.
ToC
Definition.The term automatic, as applied to telephone systems, has come to refer to those systems in which machines at the central office, under the guidance of the subscribers, do the work that is done by operators in manual systems. In all automatic telephone systems, the work of connecting and disconnecting the lines, of ringing the called subscriber, even though he must be selected from among those on a party line, of refusing to connect with a line that is already in use, and informing the calling subscriber that such line is busy, of making connections to trunk lines and through them to lines in other offices and doing the same sort of things there, of counting and recording the successful calls made by a subscriber, rejecting the unsuccessful, and nearly all the thousand and one other acts necessary in telephone service, are performed without the presence of any guiding intelligence at the central office.
The fundamental object of the automatic system is to do away with the central-office operator. In order that each subscriber may control the making of his own connections there is added to his station equipment a call transmitting device by the manipulation of which he causes the central-office mechanisms to establish the connections he desires.
We think that the automatic system is one of the most astonishing developments of human ingenuity. The workers in this development are worthy of particular notice. From occupying a position in popular regard in common with long-haired men and short-haired women they have recently appeared as sane, reasonable men with the courage of their convictions and, better yet, with the ability to make their convictions come true. The scoffers have remained to pray.
Arguments Against Automatic Idea.Naturally there has been a bitter fight against the automatic. Those who have opposed it have contended:
First: that it is too complicated and, therefore, could be neither reliable or economical.
Second: that it is too expensive, and that the necessary first cost could not be justified.
Third: that it is too inflexible and could not adapt itself to special kinds of service.
Fourth: that it is all wrong from the subscribers' point of view as the public will not tolerate "doing its own operating."
Complexity.This first objection as to complexity, and consequent alleged unreliability and lack of economy should be carefully analyzed. It too often happens that a new invention is cast into outer darkness by those whose opinions carry weight by such words as "it cannot work; it is too complicated." Fortunately for the world, the patience and fortitude which men must possess before they can produce meritorious, though intricate inventions, are usually sufficient to prevent their being crushed by any such offhand condemnation, and the test of time and service is allowed to become the real criterion.
It would be difficult to find an art that has gone forward as rapidly as telephony. Within its short life of a little over thirty years it has grown from the phase of trifling with a mere toy to an affair of momentous importance to civilization. There has been a tendency, particularly marked during recent years, toward greater complexity; and probably every complicated new system or piece of apparatus has been roundly condemned, by those versed in the art as it was, as being unable to survive on account of its complication.
To illustrate: A prominent telephone man, in arguing against the nickel-in-the-slot method of charging for telephone service once said, partly in jest, "The Lord never intended telephone service to be given in that way." This, while a little off the point, is akin to the sweeping aside of new telephone systems on the sole ground that they are complicated. These are not real reasons, but rather convenient ways of disposing of vexing problems with a minimum amount of labor. Important questions lying at the very root of the development of a great industry may not be put aside permanently in this offhand way. The Lord has never, so far as we know, indicated just what his intentions were in the matter of nickel service; and noone has ever shown yet just what degree of complexity will prevent a telephone system from working.
It is safe to say that, if other things are equal, the simpler a machine is, the better; but simplicity, though desirable, is not all-important. Complexity is warranted if it can show enough advantages.
If one takes a narrow view of the development of things mechanical and electrical, he will say that the trend is toward simplicity. The mechanic in designing a machine to perform certain functions tries to make it as simple as possible. He designs and re-designs, making one part do the work of two and contriving schemes for reducing the complexity of action and form of each remaining part. His whole trend is away from complication, and this is as it should be. Other things being equal, the simpler the better. A broad view, however, will show that the arts are becoming more and more complicated. Take the implements of the art of writing: The typewriter is vastly more complicated than the pen, whether of steel or quill, yet most of the writing of today is done on the typewriter, and is done better and more economically. The art of printing affords even more striking examples.
In telephony, while every effort has been made to simplify the component parts of the system, the system itself has ever developed from the simple toward the complex. The adoption of the multiple switchboard, of automatic ringing, of selective ringing on party lines, of measured-service appliances, and of automatic systems have all constituted steps in this direction. The adoption of more complicated devices and systems in telephony has nearly always followed a demand for the performance by the machinery of the system of additional or different functions. As in animal and plant life, so in mechanics—the higher the organism functionally the more complex it becomes physically.
Greater intricacy in apparatus and in methods is warranted when it is found desirable to make the machine perform added functions. Once the functions are determined upon, then the whole trend of the development of the machine for carrying them out should be toward simplicity. When the machine has reached its highest stage of development some one proposes that it be required to do something that has hitherto been done manually, or by a separate machine,or not at all. With this added function a vast added complication may come, after which, if it develops that the new function may with economy be performed by the machine, the process of simplification again begins, the whole design finally taking on an indefinable elegance which appears only when each part is so made as to be best adapted in composition, form, and strength to the work it is to perform.
Achievements in the past teach us that a machine may be made to do almost anything automatically if only the time, patience, skill, and money be brought to bear. This is also true of a telephone system. The primal question to decide is, what functions the system is to perform within itself, automatically, and what is to be done manually or with manual aid. Sometimes great complications are brought into the system in an attempt to do something which may very easily and cheaply be done by hand. Cases might be pointed out in which fortunes and life-works have been wasted in perfecting machines for which there was no real economic need. It is needless to cite cases where the reverse is true. The matter of wisely choosing the functions of the system is of fundamental importance. In choosing these the question of complication is only one of many factors to be considered.
One of the strongest arguments against intricacy in telephone apparatus is its greater initial cost, its greater cost of maintenance, and its liability to get out of order. Greater complexity of apparatus usually means greater first cost, but it does not necessarily mean greater cost of up-keep or lessened reliability. A dollar watch is more simple than an expensive one. The one, however, does its work passably and is thrown away in a year or so; the other does its work marvelously well and may last generations, being handed down from father to son. Merely reducing the number of parts in a machine does not necessarily mean greater reliability. Frequently the attempt to make one part do several diverse things results in such a sacrifice in the simplicity of action of that part as to cause undue strain, or wear, or unreliable action. Better results may be attained by adding parts, so that each may have a comparatively simple thing to do.
The stage of development of an art is a factor in determining the degree of complexity that may be allowed in the machinery of thatart. A linotype machine, if constructed by miracle several hundred years ago, would have been of no value to the printer's art then. The skill was not available to operate and maintain it, nor was the need of the public sufficiently developed to make it of use. Similarly the automatic telephone exchange would have been of little value thirty years ago. The knowledge of telephone men was not sufficiently developed to maintain it, telephone users were not sufficiently numerous to warrant it, and the public was not sufficiently trained to use it. Industries, like human beings, must learn to creep before they can walk.
Another factor which must be considered in determining the allowable degree of complexity in a telephone system is the character of the labor available to care for and manage it. Usually the conditions which make for unskilled labor also lend themselves to the use of comparatively simple systems. Thus, in a small village remote from large cities the complexity inherent in a common-battery multiple switchboard would be objectionable. The village would probably not afford a man adequately skilled to care for it, and the size of the exchange would not warrant the expense of keeping such a man. Fortunately no such switchboard is needed. A far simpler device, the plain magneto switchboard—so simple that the girl who manipulates it may also often care for its troubles—is admirably adapted to the purpose. So it is with the automatic telephone system; even its most enthusiastic advocate would be foolish indeed to contend that for all places and purposes it was superior to the manual.
These remarks are far from being intended as a plea for complex telephone apparatus and systems; every device, every machine, and every system should be of the simplest possible nature consistent with the functions it has to perform. They are rather a protest against the broadcast condemnation of complex apparatus and systems just because they are complicated, and without regard to other factors. Such condemnation is detrimental to the progress of telephony. Where would the printing art be today if the linotype, the cylinder press, and other modern printing machinery of marvelous intricacy had been put aside on account of the fact that they were more complicated than the printing machinery of our forefathers?
That the automatic telephone system is complex, exceedingly complex, cannot be denied, but experience has amply proven thatits complexity does not prevent it from giving reliable service, nor from being maintained at a reasonable cost.
Expense.The second argument against the automatic—that it is too expensive—is one that must be analyzed before it means anything. It is true that for small and medium-sized exchanges the total first cost of the central office and subscribers' station equipment, is greater than that for manual exchanges of corresponding sizes. The prices at which various sizes of automatic exchange equipments may be purchased vary, however, almost in direct proportion to the number of lines, whereas in manual equipment the price per line increases very rapidly as the number of lines increases. From this it follows that for very large exchanges the cost of automatic apparatus becomes as low, and may be even lower than for manual. Roughly speaking the cost of telephones and central-office equipment for small exchanges is about twice as great for automatic as for manual, and for very large exchanges, of about 10,000 lines, the cost of the two for switchboards and telephones is about equal.
For all except the largest exchanges, therefore, the greater first cost of automatic apparatus must be put down as one of the factors to be weighed in making the choice between automatic and manual, this factor being less and less objectionable as the size of the equipment increases and finally disappearing altogether for very large equipments. Greater first cost is, of course, warranted if the fixed charges on the greater investment are more than offset by the economy resulting. The automatic screw machine, for instance, costs many times more than the hand screw machine, but it has largely displaced the hand machine nevertheless.
Flexibility.The third argument against the automatic telephone system—its flexibility—is one that only time and experience has been able to answer. Enough time has elapsed and enough experience has been gained, however, to disprove the validity of this argument. In fact, the great flexibility of the automatic system has been one of its surprising developments. No sooner has the statement been made that the automatic system could not do a certain thing than forthwith it has done it. It was once quite clear that the automatic system was not practicable for party-line selective ringing; yet today many automatic systems are working successfully with this feature; the selection between the parties on a line beingaccomplished with just as great certainty as in manual systems. Again it has seemed quite obvious that the automatic system could not hope to cope with the reverting call problem,i. e., enabling a subscriber on a party line to call back to reach another subscriber on the same line; yet today the automatic system may do this in a way that is perhaps even more satisfactory than the way in which it is done in multiple manual switchboards. It is true that the automatic system has not done away with the toll operator and it probably never will be advantageous to require it to do so for the simple reason that the work of the toll operator in recording the connections and in bringing together the subscribers is a matter that requires not only accuracy but judgment, and the latter, of course, no machine can supply. It is probable also that the private branch-exchange operator will survive in automatic systems. This is not because the automatic system cannot readily perform the switching duties, but the private branch-exchange operator has other duties than the mere building up and taking down of connections. She is, as it were, a door-keeper guarding the telephone door of a business establishment; like the toll operator she must be possessed of judgment and of courtesy in large degree, neither of which can be supplied by machinery.
In respect to toll service and private branch-exchange service where, as just stated, operators are required on account of the nature of the service, the automatic system has again shown its adaptability and flexibility. It has shown its capability of working in harmony with manual switchboards, of whatever nature, and there is a growing tendency to apply automatic devices and automatic principles of operation to manual switchboards, whether toll or private branch or other kinds, even though the services of an operator are required, the idea being to do by machinery that portion of the work which a machine is able to do better or more economically than a human being.
Attitude of Public.The attitude of the public toward the automatic is one that is still open to discussion; at least there is still much discussion on it. A few years ago it did seem reasonable to suppose that the general telephone user would prefer to get his connection by merely asking for it rather than to make it himself by "spelling" it out on the dial of his telephone instrument. We have studied thispoint carefully in a good many different communities and it is our opinion that the public finds no fault with being required to make its own connections. To our minds it is proven beyond question that either the method employed in the automatic or that in the manual system is satisfactory to the public as long as good service results, and it is beyond question that the public may get this with either.
Subscriber's Station Equipment.The added complexity of the mechanism at the subscriber's station is in our opinion the most valid objection that can be urged against the automatic system as it exists today. This objection has, however, been much reduced by the greater simplicity and greater excellence of material and workmanship that is employed in the controlling devices in modern automatic systems. However, the automatic system must always suffer in comparison with the manual in respect of simplicity of the subscriber's equipment. The simplest conceivable thing to meet all of the requirements of telephone service at a subscriber's station is the modern common-battery manual telephone. The automatic telephone differs from this only in the addition of the mechanism for enabling the subscriber to control the central-office apparatus in the making of calls. From the standpoint of maintenance, simplicity at the subscriber's station is, of course, to be striven for since the proper care of complex devices scattered all over a community is a much more serious matter than where the devices are centered at one point, as in the central office. Nevertheless, as pointed out, complexity is not fatal, and it is possible, as has been proven, to so design and construct the required apparatus in connection with the subscribers' telephones as to make them subject to an amount of trouble that is not serious.
Comparative Costs.A comparison of the total costs of owning, operating, and maintaining manual and automatic systems usually results in favor of the automatic, except in small exchanges. This seems to be the consensus of opinion among those who have studied the matter deeply. Although the automatic usually requires a larger investment, and consequently a larger annual charge for interest and depreciation, assuming the same rates for each case, and although the automatic requires a somewhat higher degree of skill to maintain it and to keep it working properly than the manual, the elimination of operators or the reduction in their number and theconsequent saving of salaries and contributory expenses together with other items of saving that will be mentioned serves to throw the balance in favor of the automatic.
The ease with which the automatic system lends itself to inter-office trunking makes feasible a greater subdivision of exchange districts into office districts and particularly makes it economical, where such would not be warranted in manual working. All this tends toward a reduction in average length of subscribers' lines and it seems probable that this possibility will be worked upon in the future, more than it has been in the past, to effect a considerable saving in the cost of the wire plant, which is the part of a telephone plant that shows least and costs most.
Automatic vs. Manual.Taking it all in all the question of automatic versus manual may not and can not be disposed of by a consideration of any single one of the alleged features of superiority or inferiority of either. Each must be looked at as a practical way of giving telephone service, and a decision can be reached only by a careful weighing of all the factors which contribute to economy, reliability, and general desirability from the standpoint of the public. Public sentiment must neither be overlooked nor taken lightly, since, in the final analysis, it is the public that must be satisfied.
Methods of Operation.In all of the automatic telephone systems that have achieved any success whatever, the selection of the desired subscriber's line by the calling subscriber is accomplished by means of step-by-step mechanism at the central office, controlled by impulses sent or caused to be sent by the acts of the subscriber.
Strowger System.In the so-called Strowger system, manufactured by the Automatic Electric Company of Chicago, the subscriber, in calling, manipulates a dial by which the central-office switching mechanism is made to build up the connection he wants. The dial is moved as many times as there are digits in the called subscriber's number and each movement sends a series of impulses to the central office corresponding in number respectively to the digits in the called subscriber's number. During each pause, except the last one, between these series of impulses, the central-office mechanism operates to shift the control of the calling subscriber's line from one set of switching apparatus at the central office to another.
In case a four-digit number is being selected first, the movementof the dial by the calling subscriber will correspond to the thousands digit of the number being called, and the resulting movement of the central-office apparatus will continue the calling subscriber's line through a trunk to a piece of apparatus capable of further extending his line toward the line terminals of the thousand subscribers whose numbers begin with the digit chosen. The next movement of the dial corresponding to the hundreds digit of the called number will operate this piece of apparatus to again extend the calling subscriber's line through another trunk to apparatus representing the particular hundred in which the called subscriber's number is. The third movement of the dial corresponding to the tens digit will pick out the group of ten containing the called subscriber's line, and the fourth movement corresponding to the units digit will pick out and connect with the particular line called.
Lorimer System.In the Lorimer automatic system invented by the Lorimer Brothers, and now being manufactured by the Canadian Machine Telephone Company of Toronto, Canada, the subscriber sets up the number he desires complete by moving four levers on his telephone so that the desired number appears visibly before him. He then turns a handle and the central-office apparatus, under the control of the electrical conditions thus set up by the subscriber, establishes the connection. In this system, unlike the Strowger system, the controlling impulses are not caused by the movement of the subscriber's apparatus in returning to its normal position after being set by the subscriber. Instead, the conditions established at the subscriber's station by the subscriber in setting up the desired number, merely determine the point in the series of impulses corresponding to each digit at which the stepping impulses local to the central office shall cease, and in this way the proper number of impulses in the series corresponding to each digit is determined.
Magnet- vs. Power-Driven Switches.These two systems differ radically in another respect. In the Strowger system it is the electrical impulses initiated at the subscriber's apparatus that actually cause the movement of the switching parts at the central office, these impulses energizing electromagnets which move the central-office switching devices a step at a time the desired number of steps. In the Lorimer system the switches are all power-driven and the impulses under the control of the subscriber's instrument merely serveto control the application of this power to the various switching mechanisms. These details will be more fully dealt with in subsequent chapters.
Multiple vs. Trunking.It has been shown in the preceding portion of this work that the tendency in manual switchboard practice has been away from trunking between the various sections or positions of a board, and toward the multiple idea of operating, wherein each operator is able to complete the connection with any line in the same office without resorting to trunks or to the aid of other operators. Strangely enough the reverse has been true in the development of the automatic system. As long as the inventors tried to follow the most successful practice in manual working, failure resulted. The automatic systems of today are essentially trunking systems and while they all involve multiple connections in greater or less degree, all of them depend fundamentally upon the extending of the calling line by separate lengths until it finally reaches and connects with the called line.
Grouping of Subscribers.In this connection we wish to point out here two very essential features without which, so far as we are aware, no automatic telephone system has been able to operate successfully. The first of these is the division of the total number of lines in any office of the exchange into comparatively small groups and the employment of correspondingly small switch units for each group. Many of the early automatic systems that were proposed involved the idea of having each switch capable in itself of making connection with any line in the entire office. As long as the number of lines was small—one hundred or thereabouts—this might be all right, but where the lines number in the thousands, it is readily seen that the switches would be of prohibitive size and cost.
Trunking between Groups.This feature made necessary the employment of trunk connections between groups. By means of these the lines are extended a step at a time, first entering a large group of groups, containing the desired subscriber; then entering the smaller group containing that subscriber; and lastly entering into connection with the line itself. The carrying out of this idea was greatly complicated by the necessity of providing for many simultaneous connections through the switchboard. It was comparatively easy to accomplish the extension of one line through a seriesof links or trunks to another line, but it was not so easy to do this and still leave it possible for any other line to pick out and connect with any other idle line without interference with the first connection. A number of parallel paths must be provided for each possible connection. Groups of trunks are, therefore, provided instead of single trunks between common points to be connected. The subscriber who operates his instrument in making a call knows nothing of this and it is, of course, impossible for him to give any thought to the matter as to which one of the possible paths he shall choose. It was by a realization of these facts that the failures of the past were turned into the successes of the present. The subscriber by setting his signal transmitter was made to govern the action of the central-office apparatus in the selection of the propergroupof trunks. The group being selected, the central-office apparatus was made to act at onceautomaticallyto pick out and connect withthe first idle trunk of such group. Thus, we may saythat the subscriber by the act performed on his signal transmitter, voluntarily chooses the group of trunks, and immediately thereafter the central-office apparatus without the volition of the subscriber picks out the first idle one of this group of trunks so chosen. This fundamental idea, so far as we are aware, underlies all of the successful automatic telephone-exchange systems. It provides for the possibility of many simultaneous connections through the switchboard, and it provides against the simultaneous appropriation of the same path by two or more calling subscribers and thus assures against interference in the choice of the paths.
Outline of Action.In order to illustrate this point we may briefly outline the action of the Strowger automatic system in the making of a connection. Assume that the calling subscriber desires a connection with a subscriber whose line bears the number 9,567. The subscriber in making the call will, by the first movement of his dial, transmit nine impulses over his line. This will cause the selective apparatus at the central office, that is at the time associated with the calling subscriber's line, to move its selecting fingers opposite a group of terminals representing the ends of a group of trunk lines leading to apparatus employed in connecting with the ninth thousand of the subscribers' lines.
While the calling subscriber is getting ready to transmit thenext digit, the automatic apparatus, without his volition, starts to pick out the first idle one of the group of trunks so chosen. Having found this it connects with it and the calling subscriber's line is thus extended to another selective apparatus capable of performing the same sort of function in choosing the proper hundreds group.
In the next movement of his dial the calling subscriber will send five impulses. This will cause the last chosen selective switch to move its selective fingers opposite a group of terminals representing the ends of a group of trunks each leading to a switch that is capable of making connection with any one of the lines in the fifth hundred of the ninth thousand. Again during the pause by the subscriber, the switch that chose this group of trunks will start automatically to pick out and connect with the first idle one of them, and will thus extend the line to a selective switch that is capable of reaching the desired line, since it has access to all of the lines in the chosen hundred. The third movement of the dial sends six impulses and this causes this last chosen switch to move opposite the sixth group of ten terminals, so that there has now been chosen the nine hundred and fifty-sixth group of ten lines. The final movement of the dial sends seven impulses and the last mentioned switch connects with the seventh line terminal in the group of ten previously chosen and the connection is complete, assuming that the called line was not already engaged. If it had been found busy, the final switch would have been prevented from connecting with it by the electrical condition of certain of its contacts and the busy signal would have been transmitted back to the calling subscriber.
Fundamental Idea.This idea of subdividing the subscribers' lines in an automatic exchange, of providing different groups of trunks so arranged as to afford by combination a number of possible parallel paths between any two lines, of having the calling subscriber select, by the manipulation of his instrument, the proper group of trunks any one of which might be used to establish the connection he desires, and of having the central-office apparatus act automatically to choose and connect with an idle one in this chosen group, should be firmly grasped. It appears, as we have said, in every successful automatic system capable of serving more than one small group of lines, and until it was evolved automatic telephony was not a success.
Testing.As each trunk is chosen and connected with, conditions are established, by means not unlike the busy test in multiple manual switchboards, which will guard that trunk and its associated apparatus against appropriation by any other line or apparatus as long as it is held in use. Likewise, as soon as any subscriber's line is put into use, either by virtue of a call being originated on it, or by virtue of its being connected with as a called line, conditions are automatically established which guard it against being connected with any other line as long as it is busy. These guarding conditions of both trunks and lines, as in the manual board, are established by making certain contacts, associated with the trunks or lines, assume a certain electrical condition when busy that is different from their electrical condition when idle; but unlike the manual switchboard this different electrical condition does not act to cause a click in any one's ear, but rather to energize or de-energize certain electromagnets which will establish or fail to establish the connection according to whether it is proper or improper to do so.
Local and Inter-Office Trunks.The groups of trunks that are used in building up connections between subscribers' lines may be local to the central office, or they may extend between different offices. The action of the two kinds of trunks, local or inter-office, is broadly the same.
ToC