US2237742A - Telephone system - Google Patents

Description

April 8 1941., A. A. LUNDsTRoM TELEPHONE SYSTEM Filed Nov. 28, 1939 13 Sheets-Sheet 1 ATTORNEY A. A. LUNDSTROM TELEPHONE SYSTEM April 8, 1941.

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TELEPHONE SYSTEM Filed Nov. 28, 1939 13 Sheets-Sheet 10 NMS /NVE/VTO AALU/VDSTROM TELEPHONE SYSTEM Filed Nov. 28, 1939 13 Sheets-Sheet 11 April 8, 1941. A. A. LUNDSTRM- TELEPHONE SYSTEM Filed Nov. 28, 1939 13 Sheets-Sheet 12 MON .00nd \Iy my. `Q NON hmm /NVENTOR By A.,4./.u/vasmou AT7'ORNEV April 8, 1941 A. A. LUNDsTRoM 2,237,742

TELEPHONE SYSTEM Filed Nov. 28, 1939 l5 Sheets-Sheet 15 /Nl/EN To@ A. A. LUNDSTROM y l ATTORNEY Patented Apr. 8, 1941 TELEPHONE SYSTEM Alexis A. Lunastmm, am ons, N. s., saigner to ,Bell Telephone laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 28, 1939, Serial No. 306,466

25 Claims.

invention relates to telephone systems and `more particularly to one in which the designation oi a wanted subscriber is transmitted by currents of different frequencies generated at the calling station by a device operatively responsive to the manipulation of a key-set. In certain automatic telephone systems the subscriber station is provided with a rotary dial, and when the establishment of a connection is desired, this dial is operated a number of times to produce trains oi impulses which are transmitted to the exchange over the line wires and there received either on a succession of step-by-step selectors which are directvely operated in response to the impulses to reach the called subscribers line or upon so-called senders which function to directlvely set the selectors under the control of its own pulses, produced in response to those produced at the calling station but recorded in the sender.

Both of these methods of establishing auto matic connections, while satisfactory from an operating standpoint, are, nevertheless, susceptible ol distinct improvements, on the one hand from the calling subscribers viewpoint who, in dialing as many as nine different trains of impulses for certain types of connections, may, in waiting for the dial to return home during the production oi each train of impulses, forget how many digits have already been dialled and, therefore, make it necessary for him to start the establishment oi the connection all over again thus incurring loss of time; and on the other hand also from the operating companys viewpoint which, in dial-operated systems and because the quantity ci central office equipment is, among other things, a function of the time taken to dial a number, must provide more equipment than would otherwise be necessary if the time taken to transmit a wanted designation could be appreclably reduced below that required by the use oi the dial.

According to my invention a calling subscriber communicates the ofilce and number cfa wanted subscriber by means which control the production of alternating or pulsating currents of various irequencies, said means preferably compris lng a set of depressible keys any one of which, when depressed, causes alternating currents of particular frequency or frequencies to be produced for transmission over the line wires. These alternating current may be generated at the callers instrument as, for example, by means of a number of reeds which, in response to the depression of a key, are set into vibration within a magnetic field in the combination required to produce the frequencies corresponding to the key depressed.

According to one feature of my invention the subscribers station is arranged, with respect to the receiving equipment at the central cnice to which the line extends, to avoid the production of currents other than the signal frequencies produced by the current generating instrument at the subscribers station. In operating with signal frequency transmission it has been found desirable to use alternating currents having frequencies lying within the voice range, and in the embodiment of the present invention in which the current generating device is located at the subscriber's station and is equipped with reeds that have natural periods within lthis range, the equipment at the central oflice which responds to the currents produced by these reeds cannot distinguish between these currents and. say, other currents within the voice range which may be produced by sounds that impinge upon the telephone transmitter during the act of operating the caller's sending instrument. To avoid the production of these false currents it is necessary to so dispose the reed generator across the line during the transmission of the signals that practically no line current passes through the transmitter during the generation of the signal frequencies. According to this feature of my invention a rectifier network is provided for the transmitter and the reed generator whereby, on signaling, the network presents a high loss to any line current that would flow to or from the transmitter but little or no impedance to the flow of the alternating line current through the coil of the instrument that produces the signal frequencies, On the other hand, when the connection has been set up, the direction of the line cur 4e rent is reversed so lthat, in this case, the rectifier network presents little or no impedance to the flow of direct or alternating line current through the transmitter and a high impedance to the flow of alternating current through the coil of the sending instrument.

According to another feature of my invention a sender is provided at the central office in which the alternating currents produced by the reed generator and transmitted over the line wires control the operation of as many channel detectors as there are signal frequencies, each detector being responsive substantially to a single frequency, to cause the operation of a relay. The combination of two relays operated for a single digit is then employedto record on settable regis- 'I'he setting of the registers is'then utilized in the known manner to cause the operation of switches for selectively reaching the wanted line.

Yet according to another feature of my invention 4the intensity of the dial tone transmitted to the calling station is controlled by the distance of the station from the home oi'iice in order that the intensity of the tone to the several stations at diil'erent distances from the omce shall be uniform.

According to another feature of my invention a delay circuit is provided whereby the channel detectors are not made responsive to the incoming signals for a short interval subsequent to their arrival, in order that a portion of the energy delivered to them may be dissipated before the detectors are made available for responsive operations. The reason for this is to make sure that relays which will be operated by the 2o activation of the individual detectors will not be so by a false response of any detector to the transient disturbance set up in all the channel filters by any sudden arrival of the signals at their inputs due to the depression of a key at the calling instrument. In this way the relays will not be subject to false operations from transients of durations shorter than a predetermined interval.

Yet according to another feature of my invention this delay circuit is adapted to paralyze or otherwise render inactive any of the detector channels if the input signal is of greater intensity than a predetermined value, and, also, to keep`the detectors paralyzed until the disturbance has passed below the Just operate level of the delay circuits for a predetermined interval of time. If the disturbance then goes above this z Fig. 4 is the outgoing part of the line-nder switch including the sender connector:

Fig. 5 shows the private branch exchange selector PBX, a final connector C and wanted station B; while Figs. 6 to 13, inclusive disclose the sender.

Considering nrst Fig. 1, it will be seen that the subscribers station A is equipped with a reed plucking current generating device III. A typ- 1d] ical instrumentality of this kind is that shown level it must, of course, persist for longer than another predetermined interval to render the detector channel eiIective.

Yet another feature of my invention is the use of a special tuned feedback arrangement in the detector channel which renders the operation of the relay controlledv thereby more sensitive to the incoming frequency and renders the release of the relay more quickly than otherwise when the signal current is removed. This makes for positive relay action which expedites and renders reliable those functions of the sender that depend upon the operation and release of the channel relays.

These and other features of my invention Iwill be readily ascertained from the following detailed description of the invention, appended claims and attached drawings in which:

Fig. 1 is a schematic representation of the invention as embodied in a connection between the calling subscriber's station A and the called subscribers station B over a train of switches selectively positioned by a sender responsive to the signaling frequencies generated at the calling station;

Fig. 1A shows the arrangement of Figs. 2 to 13, inclusive to disclose the complete invention:

Fig. 2 shows a subscribers station circuit equipped with a reed current generating device and rectler network for controlling current through the station transmitter and effectively switching from talking to reed generator circuit and vice-versa for conversation or signaling respectively;

Fig. 3 is a combined line circuit and the line connecting side of a line-finder switch of a private branch exchange;

in Patent 2,147,710, issued to R. F. Mllina on February 21, 1939. Briefly, this instrument comprises flve reeds, tuned, respectively, to as many diiferent frequencies and mounted to modulate the field of a permanent magnet which, in turn, causes a voltage to be induced in a common pick-up winding connect to the line. Any two of the reeds are plucked simultaneously by the depression of one of the ten key buttons and are thus caused to vibrate at their natural period to produce alternating currents determined by their diiferent and respective periodicities. It is understood, however, that the alternating current generating device located at the subscribers premises is not limited to the one above-mentioned and briefly described but may be any suitable instrument for producing 8181181 frequencies either singly or combined with means responsive to the operation of a key 0r similar device.

In order to disclose the invention in its broadest aspects and thus bring out its nexibility with respect to its adaptation to telephone systems employing diiferent types of equipment and used for establishing different kinds of connections, said invention is applied by way of illustration to the establishment of telephone connections over a system involving a calling sulmcriber's line located in a private branch exchange which is provided with mechanical switching facilities that are selectively operated te connect with trunks radiating in the several directions through which desired connections may be established in accordance with the number transmitted from the calling station. The switching structures herein used for illustrating the operation of the invention are of the well known step-by-step or Strowger type and reference may be had to pages 53 to 67, inclusive. of the second edition of Automatic Telephony by Smith and Campbell for a detailed description of the operation of the circuits associated with these switches. Only those portions of the circuits of the selector and connector switches are shown as are required for a clear and complete description of this invention, the omitted portions of these circuits being indicated by broken lines. It should be understood, however, and the description of the operation of the invention will amply show, that the invention may Just as readily be applied to any other type of switching structure as, for instance, the so-called panel or cross-bar type, and the modification of the controlling circuits for adapting the invention to these or any other type of switches may be easily made by anyone skilled in the art.

Referring now more particularly to Fig. 1. there are shown a calling subscribers station A equipped, as said before, with a reed current generating device 20|, the associated line finder LF which includes the line circuit w, omitted from Fig. 1 but shown in detail in Fig. 3, a. sender connector SC, a private branch exchange selector switch PBX, intermediate selector switches, indicated but not shown, a connector switch C and a called subscriber-s station B. Directly belovi the sender connector are a plurality of senders designated by the first sender and last sender. In brief, the invention functions as follows:

When the subscriber A lifts the receiver from the cradle, the line finder LF is operated to move its brushes to the terminals of the line, after which an idle sender is connected to the line finder by the operation of a relay 405 of whatever sender happens to be free, the senders being disposed in a preferred order for distributive connections among the several groups of line finders serving the different groups of lines. The linefinder switch LF', however, is one connecting element of a switch structure which comprises the line-finder switch proper having access to calling line terminals, the sender connector element 405 by which an idle sender is connected to the calling line via the brushes of the lineilnder switch and a selector switch structure that has access, through its brushes, to the different trunl; groups leading to the several points of destination.

New when the sender becomes connected to the line, the private branch exchange switch becomes operatively associated with the sender and it transmits dial tone" thereto just as it would transmit it to the calling subscriber to notify him that he may proceed to dial if it were connected directly to the subscribers line. In my invention, however, the switch PBX, instead of being selectively positioned by impulses produced at the calling station as it would be in a dial operated system, will be selectively positioned by impulses produced by the sender, and since the sender can produce these switch-stepping impulses only in response to the call designation as keyed by the subscriber and recorded in the sender, dial tone cannot be transmitted from the selector directly to the calling station, but from the -sender to the station after the sender has become operatively connected to the selector. The tone which is transmitted by the selector PBX to the sender operates equipment therein which produces another tone that is then transmitted over the line to the calling station.

In response to this tone, the subscriber operates device to transmit the wanted designation by depressing a keyfor each letter or digit representing the wanted number, and the depression of each of the keys will cause two of the reeds to vibrate at a definite frequency, each thereby producing a current of that frequency. 'I'hese currents are transmitted over the line into the sender which is provided with a frequency detector channel for each signal frequency. The particular channels tuned to the transmitted frequencies are operated, after a small time iterval to allow for the dissipation of transients, and cause the operation of a relay in each voi! the responding channels. Obviously the-"successive depression of different keys will vproduce a successive combination of different frequencies which will cause the operation of a different combination of detector channels? and relays operated in response thereto. if.'

In combination with the five detector channels there is afiiliated a special delay system network. This system has the function vof connecting the channel filters to the input side of the line after a partof the signal energy has dissipated itself and, some time later, to sensitize the channel detector units. The purpose of this is, as said before, to avoid in the signal frequency the effect of components of other frequencies present which, unless kept out of a particular signal band, will tend to decrease the frequency discriminating qualities of the channel filters by setting up -an additional frequency spectrum. The additional delay thereafter, before the channel detectors are sensitized, is necessary in order to allow the spectrum resulting from suddenly applying the signals to the filters at the end of the first delay to subside. This permits the filters to realize a good portion of their steady state frequency discriminating characteristics and thus enable the detectors to respond to larger variations in loss due to loop differences and other causes without obtaining false operations in adjoining channels.

The delay circuit serves three other functions in addition to those mentioned. The first is to block any signal of less than a predetermined duration; the second is to block any signal which has an intensity of a certain predetermined value above the just operate level of the channels and the third is to block the operation of the channels until the signal passes below the just operate level of the delay network to a predetermined lower level. If the signal then goes above the :lust operate level after dropping below the predetermined lower level, it must persist for a period longer than the interval of time controlling the second function mentioned above before the channels will be activated.

The combination of relays operated in response to the incoming combination of two frequencies that corresponds to the depression of a key is recorded in a. group of register relays which are locked to preserve the record, and every time the channel relays operate in response to succeeding combinations of frequencies, a corresponding record is locked in another and succeeding register, each digit of the called line designation thus being recorded on a separate register, there being provided in the sender as many registers as the maximum number of digits that may be required to set up a connection to any part of the telephone area.

Once the called number is recorded on the sev-v eral registers, the sender proceeds to use the registrations successively to produce the number of impulse trains required to directively position all the selectors necessary to reach the wanted line. According to Fig. 1 the subscriber will have to key one digit for setting the private branch exchange selector PBX, and the number of levels to which the brushes of this selector will be caused to step by the sender will depend upon the direction of the call; lthe destination from the private branch exchange outlet coni trolling, in any case, the number of additional digits required. If, for instance, the call is to terminate before the manual board of another Aprivate branch exchange, one digit will suffice to directively set the private branch exchange switch PBX on the bank level containing the trunks that lead to this board. On the other hand, if the connection is for a called subscriber B, reached through an automatic central oflice. a maximum of nine digits maybe required, that is, one digit for directively setting the Aprivate branch exchange switch PBX to the outlet level for reaching the office selectors, three digits for the designation of the wanted office in order that the oflice selectors may be positioned for connection, to a trunk extending to the wanted oiiice, four digits for the directive setting of the selectors in the oiiice for reaching the terminals of the wanted line and an additional digit for a possible party line or ten thousand digit designation i'or the wanted line. When the appropriate number of selectors has been positioned by the corresponding number oi pulse trains produced by the sender, the sender provides a suitable signal, in the form of a circuit for operating relay 408, for closing the connection through, after which the called line B is rung in the known manner and the sender is disconnected from the line by the release of relay 405, in readiness to set up another call.

Referring now more particularly to Figs. 2 to 13, the complete understanding of the invention may be obtained from a consideration of the circuit operations which follow the initiation of a call and the transmission from the subscribers station of the wanted subscribers number.

Assuming that line A is located in the second level of terminals in the lower bank 320 of the two banks of terminals 320 and 330 accessible to the brushes of line nder LF and that a call is initiated from said line by the removal of the handset 200 from its cradle. As a result, ott- nornormal switchhook contacts 203, 204 and 203 are closed and a circuit is completed for the subscribers line relay 302 which extends from battery through the right winding of said relay, No. l. contacts of cut-01T relay 303, line conductor 2I0, swtchhook contacts 204, one arm of the rectifier bridge consisting of rectiers 201 and 206, primary winding ofthe induction coil 202, swtchhook contacts 209, line conductor 2`II, No. 3 contacts of cut-off relay 303, left winding of line relay 302 to battery. Relay 302 operates and through its No. 2 contacts closes a circuit for operating group relay 306 while through its No. 1 contacts and conductor 304 it connects the winding of cut-off relay 303 to the sleeve terminal of the line in bank 340. Relay 306 operates and disconnects ground from conductor 301 so as to mark the second commutator segment of all of the line-nders having access to the group of lines containing line A, and connects ground to start conductor 308. Assuming the line finder LF to be the rst idle nder in the group, the ground on conductor 308 is extended through the normal No. 1 contacts of relay 3I3 to the winding o relay 3I0. Relay 3l@ operates, thereby closing a circuit for energizing relays 3II and 3I2, the circuit for the latter relay being traced from battery through its winding, through the brush and normal segment of commutator 3I4, to ground at the No. 2 contacts oi' relay 3I0 and the circuit for the former being traced from battery through resistance 329, No. 4

contacts of relay 3 I 9, through the normally closed contacts of rotary magnet 3Ill, winding of relay 3l I, back contact of the vertical magnet 3I3, and through the No. 2 back contacts of relay 3I2, to ground at the No. 2 contacts of relay 3I0. When relay 3I2 operates, the circuit for operating relay 3H may be traced through the No 2 front contact of relay 3I2 (instead of through the No. 2 back contact) and thence through the brush and normal segment of commutator 3M, to ground at the left front contact of relay 3I0. Relay 3H is Vlocked operated through its front contacts to ground on the No. 2 contact of relay 3m, independent of relay 3I2 and independent of the commutator 3I4, while relay 3I2 is locked operated through its No. 2 front contact and the front contacts of relay 3I I to the ground at the No. 2 contacts of relay 3II).

With both relays 3II and 3I2 operated, a circuit is closed for operating the vertical magnet 75 3 I 3, said circuit being traced from battery through its winding, the No. 1 front contacts of relay 3I2, the outer set of contacts of relay 3I I, to ground at the No. 2 contacts of relay 3I0. The operation of vertical magnet 3I3 raises the shaft and all multiple and commutator brushes up to the iirst level of line terminals and opens the circuit through the winding of relay 3II in consequence of which said relay releases and opens the circuit through the Winding of magnet 3 I 3, which releases. Now if the terminals of the calling line were located in the first level of the terrnlnal bank, relay 3 I 2 would release when its locking circuit is opened by the release of relay 3l I since its own operating circuit, opened when the brush of commutator 3I4 was advanced from its normal position, would not be closed on the first segment of commutator due to the disconnection of ground from this segment by the operation of relay 306. It has been assumed, however, that the terminals of the calling line are not located in the first level. Therefore relay 3 I2 will be held operated through the brush and iirst segment of commutator 3I4, to ground at the back contact of the group relay associated with the lines which do connect to terminals in the first level. Relay 3II is then reoperated through the No. 2 contacts of relay 3I2, to ground on the iirst off-normal segment of commutator 3I4. When relay 3II reoperates, the circuit for energizing the vertical magnet 3I3 is closed once more and the reoperatlon of the magnet accordingly steps the shaft and brushes up to the second level. With magnet 3 I 3 operated, however, relay 3II again releases, whereupon the circuit through the Winding of the magnet is again opened and relay 3I2 is unlocked. Since the ground normally connected to the second segment of commutator 3|@ has been disconnected therefrom by the operation of relay 306, relay 3I2 releases to prevent the further operation of magnet 3I3. The calling line being connected to bank 320 to which brushes 32| and 322 have access, it is not necessary to operate the brush switching relay 324; and since relay 306 was operated through its left-hand winding, no ground is connected to conductor 309 to cause the operation of relay 325 over the circuit that would be completed through its right contacts and the brush accessible to the segments of commutator 3I5 reserved `for the lines located in the upper terminal bank 330.

As soon as the shaft was moved upward from its normal position, the vertical off-normal springs 3I6 were operated to connect ground to conductor 325l and to prepare for the operation of the rotary magnet 3Il. The release of relay 3I2, after the brushes have been advanced to the calling level, again closes a circuit to ground on the No. 2 contacts for operating relay 3l l, and the reoperation of this relay closes a circuit for operating the rotary magnet 3H, said circuit extending from battery through the winding of the magnet, the No. e and No. 5 contacts of the vertical oi-normal springs 3I6, the No. 1 back contact of relay 352, the front contacts of relay 3H to ground at the No. 2 contacts of relay 3M. Magnet 3H operates and advances the lower brushes 32E and 322 to the first set of terminals in the selected level, opens the circuit for operating relay 3H and connects the winding of this relay through the front contacts of magnet 3I7, the lower winding of relay 3I8, and the No. 1 normal contacts of relay 324 to the test brush 323. if the calling une is connected to the erst set of terminals in the selected level, the circuit through the winding of relay 3II is extended over brush 323, over the sleeve conductor 304 of the calling line, through the No. 1 contacts of the operated 1ine relay 302, through the winding of the cut-off relay 303 to battery, thereby holding relay 3I| operated to prevent the further advance of the brushes. If the calling line is connected to some other set of terminals in the selected level, then relay 3|| is not held operated and its release will cause the release of magnet 3H. When this magnet releases, relay 3II reoperates, causing thereby the reoperation of magnet 3|1. Relay 3|I and magnet 3|1 are, ln this manner, alternately operated and released until the test brush 323 engages the sleeve terminal to which conductor 304 is connected, at which time relay 3|I is held operated in series with the winding of cut-off relay 303 over the path previously traced. The cut-off relay 303 operates and relay 3|3 is sufficiently energized by the current through its lower winding to close a locking circuit from battery through its upper winding and No. 2 front contacts, through the front contacts of magnet 3H, the winding of relay 3l I, the back contact of magnet 3|3, and through the front contacts of relay 3| I to ground at the No. 2 contacts of relay 3|0. Relay 3|8, with the aid ofy the current ilowing through its locking winding is now completely operated to extend a connection from line A through brushes 32| and 322, the No. 2 and No. 4 back contacts of relay 324, and the No. 1 and No. 4 contacts of relay 3|3, over conductors 35| and 352, No. 3 and No. 4 back contacts of relay 408, both windings of relay 40 I both windings of relay 403, the upper windings and the lower windings oi both relays being serially connected, respectively, with the upper winding of relay 403 bridged by rectifier 442 and lower winding bridged by rectifier 443, resistance 401 to battery, and resistance 409 to ground. Relays 40| and 403 operate to perform functions which will be shortly described except to note at this time that relay 40| closes an obvious circuit for relay 402 which, in operating, applies ground over its No. 3 contacts to the holding conductor 353 which functions to hold relay 3|0 in an operated condition over a locking circuit through the No. 3 contacts of relay 3|9, No. 2 contacts of relay 3|8, upper winding of said relay to battery and, further, to hold operated the brush transfer relay 324, if operated, over a locking circuit extending from ground on said conductor, No. 2 and 3 contacts of vertical olf-normal springs 3|6, No. 3 contacts oi' relay 324, winding of said relay to battery. It further closes a circuit for relay 4|3 over the contacts of relay 4|2 which operates but performs no function at this time. Relay 3|9 operates in a circuit through its left winding, the No. 3 front contacts of relay 3|8 and the No. 1 and 2 contacts of vertical off-normal springs 3|6, locks to ground on conductor 306 should there be another calling line in the same level in either bank to keep the group relay 306 operated from the contacts of the line relay of said other line, extends this vrovnd over its No. 1 contacts to conductor 328 which is the start conductor in the next line-finder switch of the group that will be taken into use and, at its No. 4 contacts, disconnects battery through resistance 329 from the winding of relay 3| I, and opens the circuit of relay 3|0 which releases and closes a circuit path for the left winding of relay 3I9 through its No. 1 back contacts to off-normal ground on conductor 321. The purpose of this circuit is, of course, to insure that if another call is initiated in the group during the interval when the line finder LF is releasing and, therefore, when its brushes have not been completely reset, the releasing line finder will not be re.. seized by the reclosure of the circuit for relay 3|0 and the consequent operation of vertical magnet 3| 3; for, in such an event. the brushes may be arrested from returning to normal and restarted in their upward movement from'some position other than normal. Therefore by holding relay 3|!! operated to ground on the vertical oir-normal contacts 3|6 until the brushes reach normal, any call initiated prior to this time Will have the start ground on conductor 308 extended. to conductor 328 and thence to relay 3I0 oi.' whatever line-finder switch in the group is normal and selectable.

When relay 40| operates, it closes an obvious circuit for relay 402 which operates as already described. Relay 402 is made slow-release so that any momentary accidental depression of the switchhook which may cause the release of relay 40|, said last mentioned relay being held over the line loop will not, at the same time, cause the release of relay 402 and thereby disturb the circuits established through its contacts. Relay 402, on operating, connects battery through its No. 1 contacts to the Winding of a relay in the group of relays 404 to 404 of the first idle sender available to the group of line finders serving line A, characterized by the normal position of the sender busy relay 406 to 406".

Each line finder, depending upon the trafc density of the send-er group, has a number of sender connectors as an integral'part of its circuit organisation, each sender connector comprising a relay 405, sender busy relay 406 and a sender selecting relay 404, each connector giving accessibility to one sender. The designations of the relays just mentioned refer to the first connector in Fig. 4 which -is also nrst in the order of selection. while all intermediate connectorrs have similarly designated relays that are primed (those of the last connector being double primed) to indi-cate identity of function in the different connectors.

Now when a sender is made busy through operative association with aline ilnder in any group or through the removal of the sender from service, a circuit is closed for the upper Winding of relay 406 in every sender connector through which the busy sender may be avail-able. Postponi-ng for the moment the tracing of the current path involved for this relay, it will be observed that if relay 406 operates as a result of the sender connect-able to the rst sender connector being made busy, said relay will lock in a path extending from battery through its lower winding and lower front contacts, lower back cont-acts of all intermediate sender busy relays 406' to 406" which are normal because their associated senders are free, conductor 420 to ground on the No. 2 contacts of relay 402. Similar locking circuits will, of course, be avail-able to the other sender -busy relays in the event that the associated senders are not free.

If the senders available to the sender connectors forming an integral part of the line finder LF are all free, then, when ground is connected to the No. l contacts of relay 402, the sender accessible through the rst sender connector will be selected for operative association with said line finder and this is accomplished by operating relay 404 over a circuit path which extends from battery on the No. 1 contacts of relayl 402,

conductor 429, upper back contacts of relay 406, winding of relay 404, lower back contacts of said relay, through the back contacts of other relays 404', intermediate .the first sender-connector and the last, to ground on relay 404" of 'the last sender-connector. On the other hand, if all senders available to these connectors are busy except the intermediate sender and, therefore, all rel'ays 406 to 406" are operated and locked except the intermediate relay 406', then the sender accessible through the intermediate sender-connector will be selected by the operation of relay 404' over a circuit which extends from battery on conductor 429, upper front contacts of relay 406, upper fron-t contacts of relays 406' of rall senderconnectors between the rst and the intermediate connector, upper back contacts of relay 406', winding of relay 404', bottom back contacts of relays 404', bottom back contacts of relays 404 of all sender-connectors between the intermediate connectors and the last connector to ground on the bottom back contact of relay 404" In the same manner if all senders are busy except the one accessible through .the last sender-connector, the above circuit will be traced through the top front contacts of all relays 406 to 406', top back cont-acts of the last relay 406", winding of relay 404" to ground on the bottom back contacts of said relay. Thus it is seen that a chain preference lock-out circuit is provided whereby the line nder LF is caused to be connected to the idle sender available through the first senderconnector which is not busy, a condition which is denoted by the non-operated condition of the sender busy relay 408 of the sender available' through that channel.

For the purpose of continuing with the description of the operation of the system, however, it may be assumed that the sender connecte-ble to the line finder LF through the first senderconnect-or is idle and that, therefore, as a result, relay 404 operates in the manner above described,

Upon the oper-ation of relay 404 a circuit is completed for relay 405 which extends from ground on the top back contact of relay 404", serially through the top back contacts of all other relays 404' in each of the connectors, which relays are non-operated by the opening of their respective circuit paths through the top contacts of their corresponding sender-busy relays 406 as soon as these relays operate in response to the non-availability of their respective senders, top front cont-acts of relay 404, Winding of relay 405 to battery. Relay 405 operates to perform functions noted hereinafter.

With relay 405 operated, ground is connected over its No. l contacts to conductor 4|4 and thereby completes a circuit for relay |300 which operates to perform a number of functions to be noted hereinaf-ter. Relay 405 over its No. 2 contacts connects the ring conductor 422 to conductor 4|5 (via conductor 422') which, in turn,

joins conductor 4|6 through the No. 4 contactsof relay |200 when operated; over its No. 3 oont-acts relay 405 extends the tip conductor 423 to conductor 4|6 (via conductor 423'); over its No. I

contacts it extends the other line conductor 352 over conductor 4|9 to the other side of said resistance bridge to form the fourth arm thereof; over its No. 7 contacts it connects the winding of relay 408 to conductor 420 and thence to conductor via the No.{2 contacts of relay 938 and conductor, thence to the No. 2 contacts of relay |309 which, when operated, cause relay 408 to operate as described hereinafter; over its No. 8 contact-s it opens the operating circuit of its associ-ated sender busy relay 406 which, however, will not release at this time since it is locked over its lower winding and contacts to ground on the No. 2 contacts of relay 402 as already described; while over its No. 9 contacts it connects battery through resistance 401 to conductor 430 which extend-s to the grid of vacuum tube 6|2.

In the present embodiment of the invention, lt is assumed lthat the connection is to be completed to subscribers line B located in an automatic office equipped with step-by-step switches of the Strowger type, the A or first digit office selector of which can be reached from the terminal banks of .the PBX selector shown at the left of Fig. 5 to the windings of whose line relay 5|5 the tip and ring conductors 423 and 422 are respectively extended. Now when relay 405 operates and these two conductors are joined to conductors 4|6 and 4| 5, respectively, as above described, a current path is completed for relay 5| 5 which extends from battery, lower winding of relay 5|5, No. 5 back contacts of relay 5|4, conductor 422, conductor 422', No. 2 contacts of relay 405, conductor 4|5, primary winding of transformer 628, conductor 621, No. 2 normally made contacts of relay I2', conductor 4|6, No. 3 contacts of relay 405, conductor 423', conductor 423, No. 1 back contacts of relay 5|4, upper winding of relay 5|5, Nos. 1 and 2 contacts of 10th Rot. step rotary olf-normal springs 524, secondary winding of transformer 522 to ground. Relay 5|5 operates and closes an obvious circuit for relay 5|6 which also operates and, in doing so, applies ground to hold conductor 5|3.

Inasmuch as the primary winding of transformer 522 is connected to a source of dial tone, said tone is induced over the operating circuit of relay 5|5, above traced, including the primary winding of transformer 628. This tone, therefore, is further induced into the secondary winding of said transformer, resulting in the operation of detector-amplifier 6|3. This detectoramplifer may be of any suitable type wherein an energy supply is activated to operate relay 600 upon the induction of the tone current in the secondary winding of transformer 628, and the operation of relay 600 closes an obvious circuit for relay 6|4 which operates and connects generator 603 to the network B20. The current produced by the generator 603 is transmitted through the network 620, and through the filter 6|0, but the quantity of energy that gets through will depend upon thyrite resistance elements 601 and 608 bridged across the network, the shunting effect of which is controlled by the quantity of current flowing through the anode circuit of vacuum tube 6|2 of which said elements form a part.

It will be remembered that the operation of the sender-connector relay 405 extendedthe line conductors 35| and 352 to conductors 4|8 and 4|9, respectively, and that these, in turn, are connected to the fourth arm of the resistance bridge SI5. Now the two vertical terminals of this bridge are connected to the primary winding of transformer 62| while its two horizontal points are connected to the primary winding of transformer 6 I 9.

Referring again to relay 403 which, it will be recalled, operated over the line loop, the flow of current through resistance 401 causes a pctential drop measured by this resistance, and the current flowing therethrough, that is applied to conductor 430 which, as said before, is connected to the grid or control element of vacuum tube 6l2. This tube is of the heater" type having a thyrite resistance element 606 in series with its cathode. Prior to the application of the potential drop through resistance 401 to the grid of the tube, lthere is a slight current flowing through its anode circuit which extends from battery, thyrite 606, the cathode (the filament of the tube being heated through a series battery connection with the filament of vacuum tube SII), anode of tube SI2, the two thyrite elements 601 and 600 to ground. Thyrite is selected to supply the fixed negative bias on tube SI2 because it has the electrical characteristic that potential drop across itself increases very slowly with increase in current through itself. Therefore, variation in anode current of tube 6I'2 will not materially affect the negative potential drop across thyrite 606 and it will remain almost fixed. If there is no potential drop or a very slight potential drop in resistance 401 due to the factthat, for instance,

the line loop is long, the grid bias is strongly negative because of the application to the cathode of the negative potential drop through the -thyrite element 606. The current flowing through the anode circuit is, under these circumstances, very small. Conversely, ifthe potential drop from resistance 401 applied to the grid is large as would be the case, for instance, when the loop is small, the resulting grid bias due to the sum of the positive potential drop across resistance 401 and the negative potential drop across thyrite B06 would be less negative. In consequence, the anode current would be large. In other words, the voltage drop across thyrite element 606 works ln opposition to the voltage applied to the grid from the loop so that, on long loops, the total grid bias will be highly negative to permit very little current to flow through the anode circuit of the tube while on a short loops the total negative grid bias will be small to permit a very large current to flow through the anode circuit. Consequently y the potential drop applied to the grid of tube E l2 and the quantity of current which then flows through its `anode circuit, above traced, depends upon the length of the loop, including the resistance of conductors 2I0 and 2H that extend l between the subscribers station A and the terminals on the line-finder bank 320 at the telephone office. Whence it follows that the current flowing through the anode circuit of vacuum tube BIZ, being dependent upon the value of the positive potential on the gridof the tube, is a maximum for zero line loop and a minimum for the longest line loop. The anode current, however,`

circulates through the thyrite elements 601 and 608, which have the property of decreasing in resistance with an increase in current flowing therethrough. For the small current through the anode circuit produced by a long line loop, the current through the thyrite elements is slight,

the resistance thereof remains comparatively high and the shunting effect of these elements, bridged across the network 620 is very small. Hence for a long line loop, by which is meant the ohmic distance of the calling subscribers station A from the oflice in which the line terminates,

most of the energy from the generator 603 will be transmitted directly into the filter 6|0, without any appreciable quantity of it passing through the shunting thyrite bridge. Onthe other hand, when the loop is small or of negligible resistance, the current through the anode circuit is large, the thyrite elemerts decrease their resistance in response -to the par sage of the current and the effect is to produce a shunt path across the network 620 -that will cause most of the energy from the generator 603 to flow through the shunt circuit with little of it passing into the filter 6 I0.

The reason for the above arrangement is to regulate the quantity of dial or keying tone current transmitted back to the calling station in accordance with the length of the loop so that the intensity of the tone will be practically uniform for all stations regardless of their distance from the oiiice. In dial telephone systems in which the designation of the wanted line is transmitted by means of direct current impulses, no such case need be exercised about the tone intensity at the station because the tone used is usually of low frequency, preferably within the voice range, say around 500 to 600 cycles. 'Since the line loss increases with the frequency used, the loss in the intensity of the tone at this frequency (500 to 600 cycles per second) is small for different loo-ps and the same energy level, therefore, may be applied to all lines regardless of their distance from the oice. But in the present telephone system in which the designation of the wanted line is transmitted by means of combination of frequencies within the voice range, the frequency of the dial or keying tone used cannot be within the same range as the signal frequencies because the keying tone transmission network is coupled to the channel detectors responsive to these signal frequencies and such detectors therefore, may be falsely operated in response to the dial current. To avoid any such false operations, the keying tone frequency is chosen well above the range used for signaling from the calling station. Since the line loss is greater with the higher frequency than it is with the frequency used in dial systems. it is evident that if the tone level ls suitable for the most remote station it will be too loud for the nearest station which fact makes it necessary to make the quantity of tone transmitted over the line depend upon the length of the loop as above described. Thus when the line is long the quantity of keying tone current transmitted thereover will have to be sufficient to provide for ohmic and attenuation losses over it whereas, when the line. is short and the electrical constants `making for such losses are not present, the quantity of tone current is reduced in proportion to allow for the production of the same intensity of sound at the station receiver.

The keying tone is produced by the generator 603 which generates the odd harmonics of some base frequency say, for example, 20 cycles, and the pass-band filter 6I0 has an upper and lcwer frequency cut-ofi which defines the harmonics to be produced by the generator 603. Thus if the lter is designed to pass all frequencies between an upper limit of 1480 cycles and a lower limit of 1400 cycles. the generator should be adapted to produce the '11st or 73rd harmonic of the 20 cycles and either or both of these harmonies would be transmitted through the iilter and transformer SIS to the horizontal terminals

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