Fig. 398. Connection between a Calling and a Called Subscriber in an Automatic SystemView full size illustration.
Automatic Sub-Offices.Obviously, the system of trunking employed in automatic exchanges lends itself with great facility to the subdivision of an exchange into a large number of comparatively small office districts and the establishment of branch offices or sub-offices at the centers of these districts.
The trunking between large offices has already been described. An attractive feature of the automatic system is the establishment of so-called sub-stations or sub-offices. Where there is, in an outlying district, a distinct group of subscribers whose lines may readily be centered at a common point within that district and where the number of such subscribers and lines is insufficient to establish a fully equipped office, it is possible to establish a so-called sub-station or sub-office connected with the main office of that district by trunk lines. At this sub-office there are placed only line switches and connectors. When a call is originated on one of these sub-office lines, the line switch acts instantly to connect that line with one of the trunks leading to the main office of that district, at which this trunk terminates in a first selector. From there on, the connection is the same as that in a system in which no sub-offices are employed. Calls coming into this sub-office over trunklines from the main office are received on the connectors at the sub-office and the connection is made with the sub-office line by the connector in the usual manner. This arrangement, it is seen, amounts merely to a stretching of the connector trunks for a given group of lines so that they will reach out from a main office to a sub-office, it being more economical to lengthen the smaller number of trunks and by so doing to decrease in length the larger number of subscribers' lines.
The Rotary Connector.For certain purposes it becomes desirable in automatic work to employ a special form of connector which will have in itself a certain ability to make automatic selection of one of a group of previously chosen trunks in much the same manner as the first and second selectors automatically choose the first idle one of a group of trunks.
Such a use is demanded in private branch-exchange working where a given business establishment, for instance, has a plurality of lines connecting its own private switchboard with the central office. The directory number of all these lines is, for convenience, made the same, and it is important, therefore, that when a person attempts to make a connection with this establishment, he will not fail to get his connection simply because the first one of these lines happens to be busy. For such use a given horizontal row of connector terminals or a part of such a row is assigned to the lines leading to the private branch exchange and the connector is so modified as to have a certain "discretionary" power of its own. As a result, when the common number of all these lines is called, the connector will choose the first one, if it is not already engaged by some other connector, but if it is, it will pass on to the next, and so on until an idle one is found. It is only when the connector has hunted through the entire group of lines and found them all busy that it will refuse to connect and will give the busy signal to the calling subscriber.
Party Lines.The description of this system as given above has been confined entirely to direct line working; however, party lines may be and are frequently employed.
The circuits and apparatus used with direct lines are, with slight modifications, applicable to use with party lines.
The harmonic method of ringing is employed and the stations are so arranged with respect to the connectors that those requiringthe same frequency for ringing the bells are in groups served by the same set of connectors.
The party lines are operated on the principle commonly known in manual practice as the jack per station arrangement. Each party line will, therefore, have sets of terminals appearing in separate hundreds; the connectors associated with each of these hundreds being so arranged as to impress the proper frequency of ringing current on the line.
From the subscribers' standpoint the operation is the same as for direct lines, as the particular hundreds digit of a number serves to select one of a group of connectors capable of connecting the proper ringing current to the line.
To avoid confusion, which would be caused by a subscriber on a party line attempting to make a call when the line is already in use by some other subscriber, the subscribers' stations are so arranged that when the line is in use all other stations on the line are locked out.
Fig. 399. Wall Set for Two-Wire SystemView full size illustration.
The Two-Wire Automatic System.The two-wire system that has recently been introduced by the Automatic Electric Company brings about the very important result of accomplishing all of the automatic switching over metallic circuit lines without the use of ground or common returns. The system is thus relieved of the disturbing influences to which the three-wire system is sometimes subjected, due to differences in earth potential between various portions of the system, which may add to or subtract from the battery potential and alter the net potential available between two distant points. The introduction of this system has also made possible certain other incidental features of advantage, one of which is a great simplification and reduction in size of the subscriber's station signal-transmitting apparatus.
With the doing away of the ground as a return circuit, it becomes impossible to send vertical impulses over one side of the line and to follow them by single rotary impulses over the other side of the line.Yet it becomes necessary to distinguish between the pure selective impulses and those impulses which dictate a change of function at the central office. The plan has, therefore, been adopted of accomplishing the selection in each case by short and rapidly recurring impulses and of accomplishing those functions formerly brought about by the single impulse over the rotary side of the line by a pause between the respective series of selective impulses. This is accomplished at the central office by replacing the vertical and the rotary relays of the three-wire system by a quick-acting and a sluggish relay, respectively; the quick-acting relay performing the functions previously carried out by the vertical relay, and the sluggish relay acting only during the pauses between the successive series of quick impulses to do the things formerly done by the rotary relay. This has resulted in a delightful simplification of subscriber's apparatus, since it is now necessary only to provide a device which will connect the two sides of the line together the required number of times in quick succession and then allow a pause with the circuit closed while the subscriber is getting ready to transmit another set of impulses corresponding to another digit. The calling device has no mechanical function co-acting with any of the other parts of the telephone and may be considered as a separate mechanical device electrically connected with the line. The transmitting device is not much larger than a large watch and a good idea of it may be had from Fig. 399, which shows the latest wall set, and Fig. 400, which shows the latest desk set of the Automatic Electric Company. We regret the fact that this company has made the request that the complete details of their two-wire system be not published at this time.
Fig. 400. Desk Stand for Two-Wire SystemView full size illustration.
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The Lorimer automatic telephone system has not been commercially used in this country but is in commercial operation in a few places in Canada. It is interesting from several points of view. It was invented, built, and installed by the Lorimer Brothers—Hoyt, George William, and Egbert—of Brantford, Ontario. These young men without previous telephonic training and, according to their statements, without ever having seen the inside of a telephone office, conceived and developed this system and put it in practical operation. With the struggles and efforts of these young men in accomplishing this feat we have some familiarity, and it impresses us as one of the most remarkable inventive achievements that has come to our attention, regardless of whatever the merits or demerits of the system may be.
The Lorimer system is interesting also from the fact that, in most cases, it represents the mechanical rather than the electrical way of doing things. The switches are power driven and electrically controlled rather than electrically driven and electrically controlled, as in the system of the Automatic Electric Company.
The subscriber's station apparatus consists of the usual receiver, speech transmitter, call bell, and hook switch, and in addition a signal transmitter arranged to be manipulated by the subscriber so as to control the operation of the central-office apparatus in connecting with any desired line in the system.
The central-office apparatus is designed throughout upon the principle of switching by means of power-driven switches which are under the control of the signal transmitters of the calling subscriber's station. The switches employed in making a connection are all so arranged with respect to constantly rotating shafts that the movable member of such switches may be connected to the shafts by means of electromagnets controlled directly or indirectly by relays, which, in turn, are brought under the control of the signal transmitters.
The circuits are so designed in many instances that the changes necessary for the different steps are brought about by the movement of the switches themselves, thus permitting the use of circuits which are rather simple. The switches employed are all of a rotary type; the co-ordinate selection, which is accomplished in the Automatic Electric Company's system by a vertical and rotary movement, being brought about in this system by the independent rotation of two switches.
Subscriber's Station Equipment.A subscriber's desk-stand set, except the call bell, is shown in Fig. 401, and a wall set complete in Fig. 402. In both of these illustrations may be seen the familiar transmitter, receiver, and hook switch, and in the wall set, the call bell. The portion of these telephone sets which is unfamiliar at present is the part which is enclosed in the enlarged base of the desk stand and the protruding device below the speech transmitter in the wall set—the signal transmitter referred to earlier in the chapter. The small push button and small plate through which the number may be seen directly below the transmitter in Fig. 402, are for the purpose of registering calls.
Fig. 401. Lorimer Automatic Desk StandView full size illustration.
The signal transmitter is a device whose function is to record mechanically the number of the subscriber's station with which connection is desired, and to transmit that record to the central office by a system of electrical impulses over the line conductors. Instead of operating by its own initiative, the signal transmitter is adapted to respond to central-office control in transmitting electrically the number which has been recorded mechanically upon it.
The signal transmitter shown removed from the base of the desk stand at the left in Fig. 403 comprises in part four sets of contact pins having ten pins in each set, one set for each of the digits of a four-digit number. There are also several additional contact pins for signaling and auxiliary controlling purposes. All of these contact pins are arranged upon the circumference of a circle and a movablebrush mounted upon a shaft at the center of the circle is adapted to be rotated by a clock spring and to make contact with each of the pins successively. The call is started, after the number desired has been set on the dial, by giving the crank at the right of the signal transmitter a complete turn and thus winding the spring. The shaft carrying the signal transmitter brush carries also an escapement wheel, the pallet of which is directly controlled by an electromagnet.
Fig. 402. Lorimer Automatic Wall SetView full size illustration.
The four dials with the numerals printed on them are attached to four levers, respectively, and are moved by their levers opposite windows, near the top of the casing. Through each of these windows a single numeral may be seen on the corresponding one of the dials. The dials may be adjusted so that the four numerals seen will read from left to right to correspond to the number of the line with which connection is desired.
The setting of the dials so that the number desired shows at the small circular opening results in connecting the earth or a common return conductor to one pin of each set of ten pins, the pin grounded in each set depending upon the numerical value of the digit for which the dial is set.
The circle of contact pins is set in an insulating disk, the signal transmitting brush operates upon the pins on one side of the disk, and electrical fingers attached to the dials operate upon the pins on the other side of the disk. The escapement wheel is a single toothed disk attached directly to the shaft which carries the signal brush and its pallet is attached rigidly to the magnet armature.
Fig. 403. Desk Stand with Signal Transmitter RemovedView full size illustration.
Once a call has been turned in, the entire subscriber's station equipment is locked beyond power of the subscriber to tamper with it in any way, rendering it impossible either to defeat the call which has been started or to prevent the subscriber's station as a whole from returning completely to normal position and thus restoring itself for regular service. The key shown just below the signal transmitter in the case of the desk stand, and at the right in the wall set, is for the purpose of operating a relay at the central office which, in turn, connects ringing current to the line of the subscriber with which connection has been made, and thus actuates the call bell.
As the number set up at the signal transmitter remains in full view until reset for some other number, it is easily checked by inspection and also lessens the labor involved in making a second call for the same line, which is frequently necessary when the line is found busy the first time called.
Central-Office Apparatus.The subscriber's lines are divided into groups of one hundred lines each at the central office, each group being served by a single unit of central-office apparatus.In a central-office unit there is "sectional apparatus" which appears but once for the unit of one hundred lines; "divisional apparatus" which appears a number of times for each unit, depending upon the traffic; and "line apparatus" which appears one hundred times for each unit or once for each line.
The sectional apparatus comprises devices whose duties are, first, to detect a calling line, and second, to assign to the calling line a set of idle divisional apparatus which serves to perform the necessary switching functions and complete the connection.
The sets of divisional apparatus, or, as called in this system, "divisions," are common to a section and are employed in a manner similar to the connecting cords of a manual switchboard. The number of these divisions provided for each section is, therefore, determined by the number of simultaneous connections resulting from calls originating in the section. It has been the custom in building this apparatus to provide each section with seven divisions or connective elements.
The line apparatus comprises one relay, having a single winding, and two pairs of contacts operated by its armature. This device is substantially the well known cut-off relay almost universally employed in common-battery systems. The fixed multiple contacts of the lines in the switching banks of the connecting apparatus are considered as pertaining to the various pieces of apparatus on which they are found rather than to their respective lines. A good idea may be obtained of the arrangement of the sectional and divisional apparatus by referring to Fig. 404, which is one unit of a thousand-line equipment. The apparatus in the vertical row at the extreme left of the illustration is the sectional apparatus, while the remaining seven vertical rows of apparatus are the divisions.
The Section.The sectional apparatus for each unit consists of three separate devices called for convenience adecimal indicator, adivision starter, and adecimal-register controller. All of these devices are normally motionless when idle. The energization of the decimal indicator, in response to the inauguration of a call at a subscriber's station, results immediately in an action of the division starter which starts a division to connect with the line calling. It results also in the starting of the decimal-register controller, the remaining unit of sectional apparatus.
It is thus seen that upon the starting of a call by a subscriber, all of the sectional apparatus belonging to his one hundred lines immediately becomes active, the division starter acting to start a division, the decimal indicator becoming energized to indicate the tens group in which the call has appeared, and the decimal-register controller becoming active to adjust the decimal register of the division assigned by the division starter. The division starter having assigned a division for the exclusive use of this particular call, passes to a position from which it may start a similar idle division when the next call is received. The decimal register controller makes its half revolution for the call and comes to rest, awaiting a subsequent call, and the decimal indicator continues energized but only momentarily, since it is released by the action of the cut-off relay when the call is taken in charge by the divisional connective devices.
Calls may follow each other rapidly, the connective devices being entirely independent of each other after having been assigned to the respective calling lines. As has been described, the decimal indicator starts the division starter and the decimal-register controller in quick succession. The division starter, shown at the extreme bottom of the left-hand row of Fig. 404, is a cylinder switch of the same general type as used throughout this system. In it the terminals of a switch in each division appear as fixed contact points in a circle over which move the brushes of the division starter.
The decimal-register controller has the duties of transmitting to the divisional apparatus a series of current impulses corresponding in number to the numerical value of the tens digit of the calling line. This is effected by providing before a movable brush ten contacts from which the brush may receive current. These contacts are normally not connected to battery, so that the brush in passing over them does not receive current from them; however, when the brush has reached the contact corresponding in number to the tens digit of the calling line, a relay associated with the decimal-register controller charges the contacts with the potential of the main battery, and each of the remaining contacts passed over by the brush sends a current impulse to a device designed to indicate on the division selected for the call the tens digit of the calling line.
The Connective Division.The connective division, seven of which are shown in Fig. 404, is an assemblage of switches comprising,as a whole, a set suitable for a complete connection from calling to called subscriber. Each connective division in the unit illustrated is completely equipped to care for a called number of three digits,i. e., each division will connect its calling line with any one of one thousand lines which may be called. By a system of interconnecting between divisions, each division may be equipped with interconnecting apparatus so as to make it possible to complete a call with any one of ten thousand lines. Each connecting division of a ten-thousand-line exchange comprises six major switches. Of the six major switches, one is termed asecondary connector, another aninterconnector, and the four remaining are termed theprimary portionof the division.
Fig. 404. Unit of Switching ApparatusView full size illustration.
Before taking up the operation of the switches, the mechanical nature of the switches themselves will be described. The switches are built with a contact bank cylindrical in form and with internal movable brushes traveling in a rotary manner in circular paths upon horizontal rows of contacts fixed in the cylindrical banks. For driving these brushes a constantly rotating main power-driven shaft isprovided. Between each shaft and the rotating brushes of each major switch is an electric clutch, which, by the movement of an armature, causes the brushes of the switch to partake of the motion of the shaft and by the return of the armature to come again to rest. The motion of the brushes of the major switches, or cylinder switches, as they are frequently called because of their form, is constantly in the same direction. They have a normal position upon a set of the cylinder contacts. They leave their normal position and take any predetermined position as controlled by the magnets of the clutch, and, having served the transient purpose, they return to their normal position by traversing the remainder of their complete revolution and stopping in their position of rest or idleness.
The mechanical construction of each of the cylinder switches is such that it may disengage its clutch and bring its brushes to rest only with the brushes in some one of a number of predetermined positions. The locations of the brushes in these positions of rest, or "stop" positions, as they are called, may differ with the different cylinder switches, according to the nature of the duty required of the switch, and the total number of stop positions also may vary. The primary and secondary connectors, the interconnector selectors, and the interconnectors each have eleven stop positions; the rotary switch has eight stop positions; the signal-transmitter controller has but two.
In the six cylinder switches making up a connective division and required for any conversation, in a ten-thousand-line exchange some of the switches are set to positions which are determined by the control of the calling subscriber and represent by their selective positions the value of some digit of the calling or called subscriber's number. Others are switches controlling the call in its progress and controlling the switches responsive to the call. These latter switches take positions independent of the numbers.
In addition to the major switches, there are upon each division four minor switches termedregisters. Each consists of an arc of fixed contacts accompanied by a set of brushes which sweep over the contacts. Instead of being driven by an electromagnet, the register brushes are placed under tension of a spring which tends at all times to draw them forward. They are then restrained by an escapement device similar to a pallet escapement in a clock, the pallet being controlledby the register's magnets. When a series of impulses are received by the register magnets, the pallet is actuated a corresponding number of times and the register brushes are permitted to move forward under tension of their powerful propelling spring. Each register is associated with a major switch, and the register brushes are engaged by a cam upon the associated major switch, and are restored to normal position against the tension of their propelling spring, the force of restoration being obtained from the main shaft.
The electrical clutches which connect and disconnect the movable brushes of the major switches from the main driving shaft are controlled in all instances by circuits local to the central office. In some instances these circuits include relay contacts and are controlled by a relay. In other instances they are formed solely through switch contacts. In all cases the control, when from a distance, is received upon relays suitable for being controlled by the small currents which are adapted to flow over long lines. In all instances the power for moving a brush is derived from the main shaft and only the control of the movement is derived from electromagnets, relays, or other electric sources. In many instances the clutch circuit is closed through contacts of its own switch and, therefore, may be closed only when its switch is in some predetermined position. All of the switches are mechanically powerful and designed particularly to sustain the wear of long-continued and oft-repeated usage. This is true also of the moving parts which carry the brushes and of the journals sustaining those parts.
The Switches of the Connective Division.The six major switches of the connecting division are as follows:
The Primary Connector:—The function of this switch is to connect the conductors of the calling line with the switching devices of the connective division. Associated with this switch is a register termed thedecimal register. The one hundred lines of the section are terminated in fixed multiple contacts in the cylinder switch of the primary connector. The calling line is selected and connected with by adjusting the decimal register to a position corresponding to the calling line's tens digit and adjusting the brushes of the cylinder switch to a position corresponding to the calling line's unit digit.
The Rotary Switch:—This is a master switch, or pilot switch,consisting of a cylinder switch without register. Its duty is the control of other switches and the completion of circuits formed in part through other switches. It is the pilot switch and the switch of initiative and control for the entire connective division.
Signal-Transmitter Controller:—The primary function of this switch is the generation of signaling impulses of two classes. Impulses of the first class pass over central-office circuits only and are effective upon magnets of the divers major and minor switches; impulses of the second class pass over a line conductor of the calling line and are effective upon the signal transmitter at the subscriber's station. The impulses sent out over the line to the subscriber's station cause the brush to pass over the contacts and thereby indicate the numerical values of the various digits set by the dials. This switch also enters in an important manner into the circuits involved in the testing of the called line for the busy condition. It is controlled by the rotary switch.
Interconnector Selector:—In an exchange using four digits in the numbers, the register of the interconnector selector is adjusted in each call to a position corresponding to the numerical value of the thousands digit of the called number. The cylinder switch then acts to select an idle trunk. The switch is controlled by the rotary switch in connection with the signal transmitter controller.
Interconnector:—This switch is similar to the interconnector selector in design and in function. It is a cylinder switch with register. The register is adjusted in each call to a position corresponding to the numerical value of the hundreds digit of the number called and the cylinder switch then operates to select an idle trunk. The switch is controlled by the rotary switch in connection with the signal transmitter controller.
Secondary Connector:—This switch contains in its cylinder bank of contacts the multiple points of one hundred subscribers' lines and its function is to connect the conductors of the called line to the conductors of the connective division. This is accomplished by adjusting the register to correspond to the value of the tens digit of the line desired and by adjusting the cylinder brushes to correspond to the value of the units digit of the line. The switch is controlled by the rotary switch in connection with the signal-transmitter controller.
Operation.A brief description of the progress of a call from its institution to the complete connection and subsequent disconnection begins with the adjustment of the dial indicators of the telephone set and the turning of the crank of the signal transmitter one revolution. This act, performed by the calling subscriber, connects one of the line conductors to earth. Immediately the decimal indicator associated with the section in which the calling line terminates is energized and starts the division starter. The division starter instantly starts the rotary switch of an idle division. The rotary switch now starts the decimal-register controller and connects to it the decimal register of the primary connector of the division selected.
All of the above acts in the central office occur practically simultaneously. The impulses generated by the controller are effective upon the decimal register of the started division and, therefore, adjust that register to a position corresponding to the tens value of the calling line.
The rotary switch now disconnects the tens register and starts the cylinder brushes of the primary connector which automatically stop when they encounter the calling line. At this instant the cut-off relay of the line is energized and the decimal indicator is released. The call now is clear of all sectional apparatus and another call may come through immediately, being assigned in charge of another idle division.
The total time in which any call is in charge of the sectional apparatus,i. e., the total time from the grounding of the line conductor at the sub-station until the line has been connected with by the primary connector of some division of that section and the sectional apparatus has been released by the operation of the cut-off relay, approximates two-fifths of a second.
The next operation initiated by the rotary switch is the starting of the signal-transmitter controller of the connective division, which, in turn, adjusts the register of the interconnector selector to a position corresponding to the thousands digit of the number of the called line as indicated by the signal transmitter at the calling station. This selects an interconnector serving the lines of the selected thousand.
This initial selection being completed the rotary switch readjusts the circuits of the connective division in such manner that in thefurther progress of the signal-transmitter controller, its impulses will be effective upon the register of the selected interconnector. In this manner, the register of the interconnector, which may be upon the same connective division as the rotary switch handling the call, or which may be the interconnector of some other division, as determined by the number of the called subscriber, is adjusted to a position corresponding to the second or hundreds digit of the number called. The cylinder switch of the interconnector then selects and appropriates an idle trunk extending to a secondary connector upon some connective division serving the hundred selected.
The rotary switch again shifts the circuits of the connective division in such manner that the signal-transmitter controller is effective upon the secondary connector, both register and cylinder, and adjusts the register and cylinder, respectively, with their brushes in contact with the tens and units digits, respectively, of the number of the called line.
The conductors of the called line now are connected through the secondary connector, the interconnector, and the interconnector selector to the rotary switch; the conductors of the calling line are connected through the primary connector to the rotary switch; thus completely connecting the lines except at the rotary switch. To effect the connecting together of the two lines, both rotary switch and signal-transmitter controller must pass forward into their next positions, the connection when thus effected being made through conductors containing a repeating coil and main battery connection for supplying talking current to the two lines and containing also ringing and supervisory relays.
The called line is tested to determine if busy during the short interval in which the rotary switch takes a short step to connect the calling and the called lines. In this step of the rotary switch the busy-test relay is connected to the guard wire or busy-test wire of the called line, and if that line be busy, the relay interferes with the control exercised by the rotary switch upon the signal-transmitter controller, and the controller is prevented from taking the step required to connect the line. Thus, when a busy line is encountered, the final step of the rotary switch is taken to set up the conversation conditions, but the signal-transmitter controller does not take its final step; by this failure of the signal-transmitter controller due tothe action of the busy-test relay, the calling line is not connected to the called line but is connected to a busy-back tone generator instead.
Whether the line encountered be busy or idle, the connective division remains in its condition as then adjusted until the subscriber hangs his receiver upon the hook switch to obtain disconnection. The ringing of the bell of the called station is done directly by the calling subscriber in pressing the ringing key.
The disconnection is effected, when the receiver of the calling line is hung up, by the supervisory relay in the central office, whose winding is included in the line circuit, and whose contacts act directly to start the rotary switch. In disconnecting, the rotary switch starts the primary and the secondary connectors and thus instantly releases both the calling and the called lines. Thereafter the rotary switch in passing from position to position restores switch after switch of the connective division to normal and finally itself returns to normal in preparation for its assignment to service in answering a subsequent call.
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Two systems of telephony are now in common use in this country—the manual system and the automatic. With the growth of the automatic, and the gradually ripening conviction, which is now fully matured in the minds of most telephone engineers, that automatic switching is practical, there has been a growing tendency toward doing automatically many of the things that had previously been done manually. One of the results of this tendency has been the production of theautomanualsystem, the invention of Edward E. Clement, an engineer and patent attorney, of Washington, D. C. In connection with Mr. Clement's name, as inventor, must be mentioned that of Charles H. North, whose excellent work as a designer and manufacturer has contributed much toward the present excellence of this highly interesting system.
Characteristics of System.The name "automanual" is coined from the two words, automatic and manual, and is intended to suggest the idea that the system partakes in part of the features of the automatic system and in part of those of the manual system.
We regret that neither space nor the professional relation which we have had with the development of this system will permit us to make public an extended and detailed description of its apparatus and circuits. Only the general features of the system may, therefore, be dealt with.
The underlying idea of the automanual system is to relieve the subscriber of all work in connection with the building up of his connection, except the asking for it; to complicate the subscriber's station equipment in no way, it being left the same as in the common-battery manual system; to do away with manual apparatus, such as jacks, cords and plugs, at the central office, and to substitute for it automatic switching apparatus which will be guided in its movements,not by the subscriber, but by a very much smaller number of operators than would be necessary to manipulate a manual switchboard.
General Features of Operation.A broad view of the operation of the system is this. The subscriber desiring to make a call takes down his receiver, and this causes a lamp to light in front of an operator. The operator presses a button and is in telephonic communication with the subscriber. Receiving the number desired, the operator sets it up on a keyboard in just about the same way that a typist will set up the letters of a short word on a typewriting machine. The setting up of the number on the keyboard being accomplished, the proper condition of control of the associated automatic apparatus at the central office is established and the operator has no further connection with the call. The automatic switching apparatus guided by the conditions set up on the operator's keyboard proceeds to make the proper selection of trunks and to establish the proper connections through them to build up a talking circuit between the calling subscriber and the called and to ring the called subscriber's bell, or, if his line is found busy, the apparatus refuses to connect with it and sends a busy signal back to the calling subscriber. The operator performs no work in disconnecting the subscribers, that being automatically taken care of when they hang up their receivers at the close of the conversation.
From the foregoing it will be seen that there is this fundamental difference between the automatic and the automanual—the automatic system dispenses entirely with the central-office operator for all ordinary switching functions; the automanual employs operators but attempts to so facilitate their work that they may handle very many more calls than would be possible in a manual system, and at the same time secures the advantages of secrecy which the automatic system secures to its subscribers.
Subscriber's Apparatus.One of the main points in the controversy concerning automaticversusmanual systems is whether or not it is desirable to have the subscriber ask for his connection or to have him make certain simple movements with his fingers which will lead to his securing it. The developers of the automanual system have taken the position that the most desirable way, so far as the subscriber is concerned, is to let him ask for it. It is probablethat this point will not be a deciding one in the choice of future systems, since it already seems to be proven that the subscribers in automatic systems are willing to go through the necessary movements to mechanically set up the call. The advantage which the automanual system shares with the manual, however, in the greater simplicity of its subscriber's station apparatus, cannot be gainsaid.
Fig. 405. Operators' Key TablesView full size illustration.
Fig. 406. Top View of Key TableView full size illustration.
Operator's Equipment.The general form of the operator's equipment is shown in Fig. 405. A closer view of the top of one of the key tables is shown in Fig. 406. As will be seen, the equipment on each operator's position consists of three separate sets of push-button keys closely resembling in external appearance the keys of a typewriter or adding machine. Immediately above each set of keys are the signal lamps belonging to that set.
The operator's keys are arranged in strips of ten, placedacrossrather thanlengthwiseon the key shelf. One of these strips is shown in Fig. 407. There are as many strips of keys in each set as there are digits in the subscribers' numbers,i. e., three in a system having a capacity of less than one thousand; four in a system of less than ten thousand; and so on. In addition to the number keys of each set is a partial row of keys, including what is called astarting keyand also keys for making the party-line selection.
Fig. 407. Strip of Selecting KeysView full size illustration.
Fig. 408. Wiring of Key ShelfView full size illustration.
The simplicity of the operator's key equipment is one of its attractive features. Fig. 408 shows one of the key shelves opened so as to expose to view all of the apparatus and wiring that is placed before the operator. The reason for providing more than one keyset on each operator's position is, that after a call has been set up on one key set, a few seconds is required before the automatic apparatus controlled by the key set can do its work and release the key set ready for another call. The provision of more than one key set makes it possible for the operator to start setting up another call on another key set without waiting for the first to be released by the automatic apparatus.
Fig. 409. Switch Room of Automanual Central OfficeView full size illustration.
Automatic Switching Equipment.A general view of the arrangement of automatic switches in an exchange established by the North Electric Company at Ashtabula, Ohio, is shown in Fig. 409. The desk in the foreground is that of the wire chief. This automatic apparatus consists largely of relays and automatic selecting switches.The switches are of the step-by-step type, having vertical and rotary movements, and an idea of one of them, minus its contact banks, is given in Fig. 410. The control of the automatic switches by the operator's key sets is through the medium of a power-driven, impulse-sending machine. From this machine impulses are taken corresponding to the numbers of the keys depressed.
Fig. 410. Selecting SwitchView full size illustration.
Automatic Distribution of Calls.A feature of great interest in this system is the manner in which the incoming calls are distributed among the operators. From each key set an operator's trunk is extended to what is called a secondary selector switch, through which it may be connected to a primary selector trunk and calling line. When a subscriber calls by taking down his receiver, his line relay pulls up and causes a primary selector switch to connect his line with an idle local trunk or link circuit, at the same time starting up a secondary selector switch which immediately connects the primary trunk and the calling line to an operator's idle key set. If an operator is at the time engaged in setting up a call on a key set, or if that key set is still acting to control the sending of impulses to the automatic switches, it may be said to be busy, and it is not selected by this preliminary selecting apparatus in response to an incoming call. As soon, however, as the necessary impulses have been taken from the key set by the automatic apparatus, that key set is released and is again ready to receive a call. In this waythe calls come before each operator only as that operator is able and ready to receive them.
Setting up a Connection.As soon as the key-set lamp lights, in response to such an incoming call, the operator presses a listening button, receives the number from the subscriber, and depresses the corresponding number buttons on that key set, thereby determining the numbers in each of the series of impulses to be sent to the selector and the connector switches to make the desired connection. The operator repeats this number to the calling subscriber as she sets it up, and then presses the starting button, whereupon her work is done so far as that call is concerned. If, upon repeating the call to the subscriber, the operator finds that she is in error, she may change the number set up at any time before she has pressed the starting button.
Building up a Connection.The keys so set up determine the number of impulses that will be transmitted by the impulse-sending machine to the selector and the connector switches. These switches, impelled by these impulses, establish the connection if the line called for is not already connected to. If a party-line station is called for, the proper station on it will be selectively rung as determined by the party-line key depressed by the operator. If the line is found busy, the connector switch refuses to make the connection and places a busy-back signal on the calling line.
Speed in Handling Calls.This necessarily brief outline gives an idea only of the more striking features of the automanual system. A study of the rapidity with which calls may be handled in actual practice shows remarkable results as compared with manual methods of operating. The operators set up the number keys corresponding to a called number with the same rapidity that the keys of a typewriter are pressed in spelling a word. In fact, even greater speed is possible, since it is noticed that the operators frequently will depress all of the keys of a number at once, as by a single striking movement of the fingers. The rapidity with which this is done defies accurate timing by a stop watch in the hands of an expert. It is practically true, therefore, that the time consumed by the operator in handling any one call is that which is taken in getting the number from the subscriber and in repeating it back to him.
TABLE XI
Total Time Consumed by Operator in Handling Calls on Automanual System