The incoming trunk circuit employed by the Western Electric Company for four-party line ringing is shown in Fig. 372, it being necessarily somewhat modified from that shown in Fig. 371, which is adapted for two-party line ringing only. In addition to the provision of the four-party line ringing keys, by which positive or negative pulsating current is received over either limb of the line, and to the provision of the regular alternating current ringing key for ringing on single party lines, it is necessary in the ringing cut-off relay to provide for keeping the alternating and the pulsating ringing currents entirely separate. For this reason, the ringing cut-off relay _12_ is provided with two windings, that at the right being in the path of the alternating ringing currents that are supplied to the alternating current key, and that at the left being in the ground return path for all of the pulsating ringing currents supplied to the pulsating keys.
With this explanation it is believed that this circuit will be understood from what has been said in connection with Fig. 371. The operation of the holding coil _8_ is the same in each case, the holding magnet in Fig. 372 serving to hold depressed any one of the five ringing keys that may have been used in calling the subscriber.
[Ill.u.s.tration: AUTOMATIC EQUIPMENT, MAIN OFFICE, BERKELEY, CALIFORNIA A Feature of Interest Here is That the Cement Floor is Treated with a Filler and Painted, with No Other Covering.]
[Ill.u.s.tration: Fig. 373. Western Electric Trunk Ringing Key]
The standard four-party line, trunk ringing key of the Western Electric Company is shown in Fig. 373. In this the various keys operate not by pressure but rather by being pulled by the finger of the operator in such a way as to subject the key shaft to a twisting movement. The holding magnet lies on the side opposite to that shown in the figure and extends along the full length of the set of keys, each key shaft being provided with an armature which is held by this magnet until the magnet is de-energized by the action of the ringing cut-off relay.
[Ill.u.s.tration: Fig. 374. Trunk Relay]
[Ill.u.s.tration: Fig. 375. Trunk Relay]
The standard trunk relays employed by the Western Electric Company in connection with the circuits just described are shown in Figs. 374 and 375. In each case the dust-cap or shield is also shown. The relay of Fig. 374 is similar to the regular cut-off relay and is the one used for relays _9_ and _14_ of Figs. 371 and 372. The relay of Fig. 375 is somewhat similar to the subscriber"s line relay in that it has a tilting armature, and is the one used at _13_ in Figs. 371 and 372. The trunk relay _3_ in Figs. 371 and 372 is the same as the _A_-operator"s supervisory relays already discussed.
It has been stated that under certain circ.u.mstances _B_-operator"s trunk circuits devoid of ringing keys, and consequently of all keys, may be employed. This, so far as the practice of the Bell companies is concerned, is true only in offices where there are no party lines, or where, as in many of the Chicago offices, the party lines are worked on the "jack per station" basis. In "jack per station" working, the selection of the station on a party line is determined by the jack on which the plug is put, rather than by a ringing key, and hence the keyless trunk may be employed.
[Ill.u.s.tration: Fig. 376. Keyless Trunk]
A keyless trunk as used in New York is shown in Fig. 376. This has no manually operated keys whatever, and the relay _17_, when it is operated, establishes connection between the ringing generator and the conductors of the trunk plug. The relays _3_, _13_, and _12_ operate in a manner identical with those bearing corresponding numbers in Fig.
371. As soon as the trunk operator plugs into the multiple jack of the called subscriber, the relay _16_ will operate for the same reason that the relay _9_ operated in connection with Fig. 371. The trunk disconnect lamp will receive current, but if the operator has already established connection with the other end of the trunk, this lamp will not be lighted because shunted by the relay _17_, due to the pulling up of the armature of the relay _13_. The relay _15_ plays no part in the operation so far described, because of the fact that its winding is short-circuited by its own contacts and those of relay _12_, when the latter is not energized. As a result of the operation of the relay _17_, ringing current is sent to line, the supply circuit including the coil of the relay _12_. As soon as the subscriber responds to this ringing current, the armature of the relay _12_ is pulled up, thus breaking the shunt about the relay _15_, which, therefore, starts to operate in series with the relay _17_, but as its armatures a.s.sume their attracted position, the relay _17_ is cut out of the circuit, the coil of the relay _15_ being subst.i.tuted for that of the relay _17_ in the shunt path around the lamp _4_. The relay _17_ falls back and cuts off the ringing current. The relay _15_ now occupies the place with respect to the shunt around the lamp _4_ that the relay _17_ formerly did, the continuity of this shunt being determined by the energization of the relay _13_. When the _A_-operator at the distant exchange withdraws the calling plug from the trunk jack, this relay _13_ becomes de-energized, breaking the shunt about the lamp _4_ and permitting the display of that lamp as a signal to the operator to take down the connection. It may be asked why the falling back of relay _15_ will not again energize relay _17_ and thus cause a false ring on the called subscriber. This will not occur because both the relays _15_ and _17_ depend for their energization on the closure of the contacts of the relay _13_, and when this falls back the relay _17_ cannot again be energized even though the relay _15_ a.s.sumes 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 a.s.sociated circuits involved in an inter-office connection, is shown in Fig. 377.
[Ill.u.s.tration: Fig. 377. Inter-Office Connection--Kellogg System]
This figure ill.u.s.trates a connection from a regular two-wire multiple subscriber"s line in one office, through an _A_-operator"s cord circuit there, to the outgoing trunk jacks at that office, thence through the incoming trunk circuit at the other office to the regular two-wire multiple subscriber"s line at that second office. The portion of this diagram to be particularly considered is that of the _B_-operator"s cord circuit. The trunk circuit terminates in the multipled outgoing trunk jacks at the first office, the trunk extending between offices consisting, of course, of but two wires. We will first consider the control of the calling supervisory lamp in the _A_-operator"s cord circuit, it being remembered that this control must be from the called subscriber"s station. It will be noticed that the left-hand armature of the relay _1_ serves normally to bridge the winding of relay _2_ across the cord circuit around the condenser _3_. When, however, the relay _1_ pulls up, the coil of relay _4_ is subst.i.tuted in this bridge connection across the trunk. The relay _2_ has a very high resistance winding--about 15,000 ohms--and this resistance is so great that the tip supervisory relay of the _A_-operator"s cord will not pull up through it. As a result, when this relay is bridged across the trunk circuit, the tip relay on the calling side of the _A_-operator"s cord circuit is de-energized, just as if the trunk circuit were open, and this results in the lighting of the _A_-operator"s calling supervisory lamp. The winding of the relay _4_, however, is of low resistance--about 50 ohms--and when this is subst.i.tuted for the high-resistance winding of the relay _2_, the tip relay on the calling side of the _A_-operator"s cord is energized, resulting in the extinguishing of the calling supervisory lamp. The illumination of the _A_-operator"s calling supervisory lamp depends, therefore, on whether the high-resistance relay _2_, or the low-resistance relay _4_, is bridged across the trunk, and this depends on whether the relay _1_ is energized or not. The relay _1_, being bridged from the tip side of the trunk circuit to ground and serving as the means of supply of battery current to the called subscriber, is operated whenever the called subscriber"s receiver is removed from its hook. Therefore, the called subscriber"s hook controls the operation of this relay _1_, which, in turn, controls the conditions which cause the illumination or darkness of the calling supervisory lamp at the distant office.
a.s.suming that the _A_-operator has received and answered a call, and has communicated with the _B_-operator, telling her the number of the called subscriber, and has received, in turn, the number of the trunk to be used, and that both operators have put up the connection, then it will be clear from what has been said that the calling supervisory lamp of the _A_-operator will be lighted until the called subscriber removes his receiver from its hook, because the tip relay in the _A_-operator"s cord circuit will not pull up through the 15,000-ohm resistance winding of the relay _2_. As soon as the subscriber responds, however, the relay _1_ will be operated by the current which supplies his transmitter. This will subst.i.tute the low-resistance winding of the relay _4_ for the high-resistance winding of the relay _2_, and this will permit the energizing of the tip supervisory relay of the _A_-operator and put out the calling supervisory lamp at her position. As in the Western Electric circuit, therefore, the control of the _A_-operator"s calling supervisory lamp is from the called subscriber"s station and is relayed back over the trunk to the originating office.
In this circuit, manual instead of automatic ringing is employed, therefore, unlike the Western Electric circuit, means must be provided for notifying the B-operator when the calling subscriber has answered.
This is done by placing at the _B_-operator"s position a ringing lamp a.s.sociated with each trunk cord, which is illuminated when the _B_-operator places the plug of the incoming trunk into the multiple jack of the subscriber"s line, and remains illuminated until the subscriber has answered. This is accomplished in the following manner: when the operator plugs into the jack of the line called, relay _5_ is energized but is immediately de-energized by the disconnecting of the circuit of this relay from the sleeve conductor of the cord when the ringing key is depressed, the selection of the ringing key being determined by the particular party on the line desired. These ringing keys have a.s.sociated with them a set of springs _9_, which springs are operated when any one of the ringing keys is depressed. Thus, with a ringing key depressed and the relay _5_ de-energized, the ringing lamp will be illuminated by means of a circuit as follows: from the live side of the battery, through the ringing lamp _12_, through the back contact and armature of the relay _6_, through the armature and contact of relay _4_, then through the armature and front contact of relay _2_--which at this time is the relay bridged across the trunk and, therefore, energized--and thence through the back contact and armature of relay _5_ to ground. When the subscriber removes his receiver from the hook, the relay _1_ will become energized as previously described, and will, therefore, operate relay _6_ to break the circuit of the ringing lamp.
The circuit thus established by the operation of relay _1_ is as follows: from the live side of battery, through the winding of relay _6_, through the armature and contact of relay _1_, through the armature and contact of relay _4_, through the armature and front contact of relay _2_, thence through the armature and back contact of relay _5_ to ground. As soon as the _B_-operator notes that the ringing lamp has gone out, she knows that no further ringing is required on that line, thus allowing the operation of relay _5_ and accomplishing the locking out of the ringing lamp during the remainder of that connection. The relay _6_, after having once pulled up, remains locked up through the rear contact of the left-hand armature of relay _5_ and ground, until the plug is removed from the jack.
At the end of the conversation, when the _A_-operator has disconnected her cord circuit on the illumination of the supervisory signals, both relays _2_ and _4_ will be in an unoperated condition and will provide a circuit for illuminating the disconnect lamp a.s.sociated with the _B_-operator"s cord. This circuit may be traced as follows: from battery through the disconnect lamp, through the armatures and contacts of relays _2_ and _4_, thence through the front contact and armature of relay _5_ to ground, thus illuminating the disconnect lamp. The ringing lamp will not be re-illuminated at this time, due to the fact that it has been previously locked out by relay _6_. The operator then removes the plug from the jack of the line called, and the apparatus in the trunk circuit is restored to normal condition.
In the circuit shown only keys are provided for ringing two parties.
This circuit, however, is not confined to the use of two-party lines, but may be extended to four parties by simply duplicating the ringing keys and by connecting them with the proper current for selectively ringing the other stations.
The method of determining as to whether the called line is free or busy is similar to that previously described for the _A_-operator"s cord circuit when making a local connection, and differs only in the fact that in the case of the trunk cord the test circuit is controlled through the contacts of a relay, whereas in the case of the _A_-operator"s cord, the test circuit was controlled through the contacts of the listening key. The function of the resistance _10_ and the battery connected thereto is the same as has been previously described.
The general make-up of trunking switchboard sections is not greatly different from that of the ordinary switchboard sections where no trunking is involved. In small exchanges where ring-down trunks are employed, the trunk line equipment is merely added to the regular jack and drop equipment of the switchboard. In common-battery multiple switchboards the _A_-boards differ in no respect from the standard single office multiple boards, except that immediately above the answering jacks and below the multiple there are arranged in suitable numbers the jacks of the outgoing trunks.
Where the offices are comparatively small, the incoming trunk portions of the _B_-boards are usually merely a continuance of the _A_-boards, the subscriber"s multiple being continuous with and differing in no respect from that on the _A_-sections. Instead of the usual pairs of _A_-operators" plugs, cords, and supervisory equipment, there are on the key and plug shelves of these _B_-sections the incoming trunk plugs and their a.s.sociated equipment.
In large offices it is customary to make the _B_-board entirely separate from the _A_-board, although the general characteristics of construction remain the same. The reason for separate _A_- and _B_-switchboards in large exchanges is to provide for independent growth of each without the growth of either interfering with the other.
A portion of an incoming trunk, or _B_-board, is shown in Fig. 378. The multiple is as usual, and, of course, there are no outgoing trunk jacks nor regular cord pairs. Instead the key and plug shelves are provided with the incoming-trunk plug equipments, thirty of these being about the usual quota a.s.signed to each operator"s position.
In multi-office exchanges, employing many central offices, such, for instance, as those in New York or Chicago, it is frequently found that nearly all of the calls that originate in one office are for subscribers whose lines terminate in some other office. In other words, the number of calls that have to be trunked to other offices is greatly in excess of the number of calls that may be handled through the multiple of the _A_-board in which they originate. It is not infrequent to have the percentage of trunked calls run as high as 75 per cent of the total number of calls originating in any one office, and in some of the offices in the larger cities this percentage runs higher than 90 per cent.
[Ill.u.s.tration: Fig. 378. Section of Trunk Switchboard]
[Ill.u.s.tration: Fig. 379. Section of Partial Multiple Switchboard]
This fact has brought up for consideration the problem as to whether, when the nature of the traffic is such that only a very small portion of the calls can be handled in the office where they originate, it is worth while to employ the multiple terminals for the subscribers" lines on the _A_-boards. In other words, if so great a proportion as 90 per cent of the calls have to be trunked any way, is it worth while to provide the great expense of a full multiple on all the sections of the _A_-board in order to make it possible to handle the remaining 10 per cent of the calls directly by the _A_-operators?
As a result of this consideration it has been generally conceded that where such a very great percentage of trunking was necessary, the full multiple of the subscribers" lines on each section was not warranted, and what is known as the partial multiple board has come into existence in large manual exchanges. In these the regular subscribers" multiple is entirely omitted from the _A_-board, all subscribers" calls being handled through outgoing trunk jacks connected by trunks to _B_-boards in the same as well as other offices. In these partial multiple _A_-boards, the answering jacks are multipled a few times, usually twice, so that calls on each line may be answered from more than one position. This multipling of answering jacks does not in any way take the place of the regular multipling in full multiple boards, since in no case are the calls completed through the multiple jacks. It is done merely for the purpose of contributing to team work between the operators.
A portion of such a partial multiple _A_-board is shown in Fig. 379.
This view shows slightly more than one section, and the regular answering jacks and lamps may be seen at the bottom of the jack s.p.a.ce just above the plugs. Above these are placed the outgoing trunk jacks, those that are in use being indicated with white designation strips.
Above the outgoing trunk jacks are placed the multiples of the answering jacks, these not being provided with lamps.
The partial multiple _A_-section of Fig. 379 is a portion of the switchboard equipment of the same office to which the trunking section shown in Fig. 378 belongs. That this is a large multiple board may be gathered from the number of multiple jacks in the trunking section, 8,400 being installed with room for 10,500. That the board is a portion of an equipment belonging to an exchange of enormous proportions may be gathered from the number of outgoing trunk jacks shown in the _A_-board, and in the great number of order-wire keys shown between each of the sets of regular cord-circuit keys. The switchboards ill.u.s.trated 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 a.s.sortment 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
+---------------+-------------------------------------------------+MATELINE WIRE +-------+-------+-------+-----------+-------------+WhiteRedBlackRed-WhiteBlack-White+---------------+-------+-------+-------+-----------+-------------+Blue121416181Orange222426282Green323436383Brown424446484Slate525456585Blue-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.
CHAPTER XXVIII
FUNDAMENTAL CONSIDERATIONS OF AUTOMATIC SYSTEMS
=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.