US2224477A - Telephone system - Google Patents

Description

6 Sheets-Sheet 1 Dec. 10, 1940- R. H. HERRICK TELEPHONE SYSTEM Filed Feb. 24, 1959 @u Po N@ @NQ w` Roswell H Herr/ck BY WZL fmZ/ ATTORNEYS Dec. 10, 1940. R, H. HERRICK TELEPHONE SYSTEM Filed Feb. 24, 1939 6 Sheets-Sheet 2 xbm@ NR .QQN ,wauw k, ..C w3@ m m Nm n @Nw Em SEQ w w# \I|. S M N l S om Smm |L 36 l SNE LNG 25 2 m @6mm R www1 h @C 3S @QQ SG $05 @QQ @www @www www@ @www @SQ w @NND ATTORNEYS Dec. 10, 1940. R. H. HERRlcK TELEPHONE SYSTEM 6 Sheets-Sheet 5 Filed Feb. 24, 1939 NNN,

Roswell fi Herr/'Ck ATTORNEYS Dec. 10, 1940. R. H. HERRICK TELEPHONE SYSTEM Filed Feb. 24, 1959 6 Sheets-Sheet 4 INVENTOR Roswell H He BY iard/1?, f5/Lv@ rr/ck ATTORNEYS DeC 10, 1940- R. H. HERRICK 2.224,477

TELEPHONE SYSTEM Filed Feb. 24, 1939 6 Sheets-Sheet 5 INVENTOR Romae h. HQfr/'G/f BY im JLM@ ATTORNEYS Dec. `10, 1940. R. H. HERRlcK TELEPHONE SYSTEM e sheets-shea e Filed Feb. 24,' 1939 @www m .mi

L @Q5 www@ 1N VENTOR. Rosale/ H Herr/Ck ATTGRNEYS QQ@ X S IQWV ,.IL @En es@ @EQU [JIJbw du ,SS www# S $56 Patented Dec. 10, 1940 UNITED STATES PATENT vOFI-ICE TELEPHONE SYSTEM Application February 24, 1939, Serial No. 258,264

30 Claims.

The present invention relates to improvements in telephone apparatus of the general character disclosed and claimed in applicants copending application Serial No. 207,433, led May 12, 1938, now Patent No. 2,187,902, granted Jan. 23, 1940. More particularly, thisy invention relates to improved telephone apparatus for providing simplified and more direct calling facilities between the telephone substation of an important executive or oicial and the telephone substations of subordinates who are frequently called from the executive telephone substation.

As explained in the above referred to patent, it sometimes occurs that one party located at a given substation may have occasion to call the telephones of a particular group of other parties at frequent intervals. This is particularly true of an executive who relies upon telephone facilities to secure information from his immediate subordinates. In a situation of this character, the delay incident to the operation of a manual system or the labor incident to the operation of the calling device in automatic systems may prove burdensome to the party initiating the calls. In order to simplify the calling operation, the improved direct call system disclosed in the above-mentioned copending application was developed. This system is so arranged that calls to a number of telephones may be initiated at the special substation either by using a conventional dial operated calling device or by the simple expedient of selectively actuating calling keys individually corresponding to the telephones which are frequently called. In the disclosed arrangement, there is provided an automatic telephone system including an exchange at which the plurality of lines of the system terminate. VOne of the lines of the system extends to a special or executive substation of the character mentioned above. Automatic switching equipment of the impulse responsive type is provided in the exchange for establishing connections between the lines extending to the exchange, the arrangement being such that the automatic switching equipment is controllable from the executive substation or by way 0f any of the lines extending to the exchange, to establish communication connections between the executive substation and the lines. There is Ialso provided, in the exchange, special automatic switching means having access only to the group of subordinate lines to which calls are frequently initiated at the executive substation. This special automatic switching means is individual to the executive substation and is controllable only from this substation to establish a second communication connection between the substation and a second of the lines extending to the exchange. I'his second communication connection may be set up in the absence of a connection of the first type as mentioned above, or may be set up while a connection of the rst type exists and without releasing the same. More specifically, there is provided, at the executive substation, a plurality of key operated calling switches which individually correspond to the subordinate lines forming the group accessible to the automatic switching means men.- tioned above, and control apparatus operative in response to the operation of any one of the calling switches for causing the special switching means to extend a connection from the executive substation to the particular one of the subordinate lines corresponding to the operated calling switch. For the purpose of communicating over a connection of the rst type, there is provided, at the special or executive substation, the usual transmitter and receiver preferably arranged in the form of the handset, which two elements are normally connected and arranged to be used in conversing over a connection of the rst type. Auxiliary transmitting and receiving means are also provided at the special substation which are normally connected and arranged to be used in communicating over a connection of the second type. Switching apparatus is provided for operatively connecting the auxiliary transmitting and receiving means to be used in communicating over either of the two types of connections briey described above. More particularly, a pair of relays are provided which are under the control of manually operable answer and transfer switches located at the substation and are operative selectively to connect the auxiliary transmitting and receiving means to be used in communicating over an established connection of either type. The arrangement is such that the two relays are `iointly operative to transfer the auxiliary transmitting and receiving means from a connection of one type to a connection of the other type. 'I'he answer switch includes contacts for completing the operating circuits of both relays, and the transfer switch includes contacts for interrupting the operating circuit of the transfer relay without deenergizing the other relay, thus providing an arrangement wherein the desired transfer of the auxiliary transmitting and receiving means may be effected by selective operation of the answer and transfer switches. In one embodiment of the apparatus, there is provided a relay which is operative in response to the removal of theV substation receiver from its supporting hook or cradle to connect the handset receiver to be used in lieu of the auxiliary receiving means communicating over a connection of the second type, namely, a direct line connection. While perfectly satisfactory in operation, the arrangement is such that control of the lastmentioned switching relay is achieved only through modification of the subset to include an extra set of hook switch springs, thus rendering a standard substation instrument unsuitable for use.

It is an object of the present invention to provide in a system of the character described above an improved and exceedingly simple substation circuit arrangement wherein either of two sets of transmitting and receiving apparatus, one set of which comprises a standard subset, may selectively be connected to an established communication connection involving the substation.

It is a further object of the invention to provide in a system of the character described an improved substation circuit arrangement wherein either of two sets of transmitting and receiving apparatus one set of which comprises a standard telephone subset, may selectively be connected to 'either of two established connections extending to the substation.

It is another object of the invention to provide in a system of the character described, wherein two communication channels may simultaneously be established between the executive substation and two different lines of the system, an improved arrangement whereby the channel which is not in use may effectively be short-circuited, thereby to prevent cross talk between the two channels.

It is a further object of the invention to provide an improved arrangement controllable from the executive substation for marking the line over which incoming calls to the executive substation are routed las busy in the numerical switches having access to this line without busying any of the eoupment in the exchange.

'Ihe invention is illustrated as being incorporated in an automatic telephone system of the general arrangement outlined above. In accordance with one feature of the present invention, the arrangement provided at the executive substation for selectively connecting the regular substation subset and the auxiliary transmitting and receiving means to an established connection involving the executive substation comprises a first relay having a winding serially included in the regular substation line over which incoming calls are routed to the substation. and a second relay which functions to switch the connection from one set of transmitting and receiving apparatus to the other. The first relay is arranged to be energized when a loop circuit is established by way of the above-mentioned line. which occurs when the receiver of the subset is removed from its supporting element. The operating circuit for the second relay is jointly controlled by the rst relay, the special automatic switching equipment and the transfer switching apparatus, mentioned above. More particularly, with either the special switching equipment or the transfer switching apparatus operated, the operating circuit for the second relay is completed in response to operation of the first relay. This second relay not only functions to perform the line switching operations noted,

but, in addition, functions to complete a holding circuit serially including its own winding and the winding of the first relay, and renders the auxiliary receiving means inoperative by short-circuiting the input channel thereto. The indicated holding circuit is interrupted to release the two relays and restore the normal circuit arrangement when the receiver of the subset is replaced upon its supporting element.

In accordance with another feature of the invention, the above-mentioned arrangement for rendering inactive the one of the two established communication channels extending to the executive substation which is not in use, comprises apparatus controllable by the transfer means for short-circuiting the inactive channel. More particularly, the transfer relay, which responds to operation of the transfer switch to select the desired channel is provided with contacts for effectively short-circuiting each of the two channels when not in use.

In accordance with a further feature of the invention, the apparatus for guarding the executive substation against incoming calls routed over the regular substation line comprises apparatus controllable from the substation for busying the line to the automatic switching equipment having access thereto. More particularly, a path, jointly controlled by the transfer and answer switches, is provided for impressing a line busying potential upon the private conductor of the line, thereby to prevent seizure of the line by the automatic switches. By virtue of this arrangement none of the automatic switching equipment is tied up or held out of service as a result of busying of the line.

the particular arrangement of the apparatus whereby the above and additional operating features are attained.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, Iboth as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification, taken in connection with the accompanying drawings, in which Figures 1 to 6, inclusive, taken together, illustrate a telephone system having incorporated therein the above-outlined features of the present invention, and Fig. 7 is a diagram illustrating the manner of assembling the other figures of the drawings to form a unified system.

Referring now more particularly to Figs. 1 to 6 of the drawings, the equipment there illustrated is, with the exception of the substation apparatus shown in Fig. 1 and the substation apparatus 600 shown in Fig. 6, included in an automatic exchange of the type frequently used to provide telephone facilities between the offices of a large manufacturing organization. This type of exchange is commonly known as a private automatic exchange. Extending to the exchange and terminating in automatic switching equipment diagrammatically illustrated at 320 are a plurality of substation lines, three of which are indicated at 32|, 322 and 323. The ends of the lines remote from the exchange terminate in substations of the automatic type, that associated with the line 32| being diagrammatically illustrated at B and that associated with the line 322 being indicated A. The line 323 extends to a special or executive substation of the character mentioned above and includes two line conductors C321 and C328 and a private or control conductor C323. The automatic equipment 320 comprises the usual subscriber-controlled automatic switches for setting up connections between the telephone lines of the system. More particularly, these switches are, in accordance with conventional practice, divided into two classes, namely, non-numerical switches, such, for example, as line finder or line switches, and numerical switches, such, for example, as selector and connector switches, which latter switches are responsive to impulses transmitted thereto over calling ones of the telephone lines and function to perform the usual line group, subgroup and unit selecting operations. 'I'he connector switches at which the lines terminate are, in accordance with conventional practice, equipped to perform the usual auxiliary functions of testing a selected called line to determine the idle or busy condition thereof, returning busy tone over the calling line in the event a selected called line is found busy, and projecting ringing current over a selected called line to actuate the signal device at the substation in the event the called line is idle at the time it is tested. Further in accordance with conventional practice, the lines extending to the automatic switching equipment 320 are, when busy, guarded against seizure by the connector switches having access thereto through the application of ground or line busying potential to the control conductors thereof. For example, when the line 323 is busy. ground potential is impressed upon the control conductor C320 so that none of the connector switches having access to this line may seize the line.

The substation apparatus 600 and the apparatus provided at each of the substations A, B, etc., may be of any desired commercial construction and arrangement. In brief, this apparatus may comprise the usual handset type of transmitter and receiver, a cradle or hook comprising supporting means for the receiver, a cradle or hook switch, a ringer, and an impulsing device for transmitting switch control impulses by way of the associated lines to the automatic switching equipment 320 located in the line terminatingr exchange. Preferably,y the elements of each substation circuit are connected and arranged to form the well-known anti-side-tone circuit, whereby operation of the substation transmitter does not cause operation of the substation receiver.

-As pointed out above, the present invention relates to the provision of improved apparatus for increasing the utility of the available telephone facilities at the executive substation where the substation apparatus 600 is located. With the apparatus thus far described, communication connections may be established between the executive substation and any of the other substa-V tions associated with lines terminating at the automatic switching equipment 320 in the usual manner. More particularly, on regular calls initiated at the executive substation, the calling device or dial operated impulsing device included in the apparatus 600 is actuated in the usual manner causing impulse trains to be transmitted to the automatic switching equipment 320. An auxiliary dial operated calling device 350 may also be provided ii' desired, this device being required for initiating calls under certain conditions of operation described in detail hereinafter. The automatic switching equipment responds to impulse trains transmitted thereto by routing the connection to the desired called line. All calls incoming to the executive substation are routed by way of the regular automatic switching equipment 320 provided in the exchange and the line 323 to the substation apparatus 800, the signal device or ringer 64| embodied in this apparatus being energized in response `to ringing current projected over the line 323 following seizure of the line by one of the connector switches included in the automatic switching equipment 320.

For the purpose of routing calls from the executive substation directly to certain of the frequently called subordinate lines terminating at the automatic switching equipment 320 without operating a calling device of the type conventionally used in automatic systems and without the delay incident to the operations required to set up a connection in a manual system, there is provided automatic switching means comprising a rotary switch 3 I 0 having bank contacts at which the group of subordinate lines terminate. This switch is a standard twenty-five point rotary switch and comprises four sets of bank contacts 3|| to 3M, inclusive, wipers 3|5 to 3|B, inclusive, respectively associated with the enumerated contact sets, and means comprising a motor magnet 3I9 for driving the enumerated wipers over the contacts oi' their respective associated sets. w As indicated, the contacts in the two sets 3|3 and 3M terminate the line conductors of the respective subordinate lines which are directly accessible to the executive substation, and the contacts in the set 3|2 terminate the private or control conductors of the subordinate lines. The contacts of the fourth contact set 3| I terminate conductors extending by way of the cable 295 to the contact springs of a group of coding relays shown in Fig. 2. These conductors individually correspond to the subordinate lines and are included in marking circuits by means of which operation of the switch 3|0 is controlled. Since the control apparatus described hereinafter is only varranged to provide direct calling facilities to twenty subordinate lines only twenty of the contacts of each contact set embodied in the switch 3|0 are used, the remaining contacts being left blank. switch 3|0 is arranged to be controlled by the group of coding relays shown in Fig. 2 and by the control Aapparatus 4|5 illustrated in Figs. 4 and 5. More particularly, the coding relays shown in Fig. 2 and indicated at R200, R2|0, R220, R230. R240, R250 and R260, respectively, are arranged to be energized in combinations of three in response to the selective operation of any one of the plurality of calling switches illustrated in Fig. l. These switches, which are indicated at |00 to |09, respectively, individually correspond to the lines forming the group of subordinate lines directly accessible to the executive substation. Each of the enumerated calling switchesis of the key operated type and is operable from a normal position to either of two off-normal positions individually corresponding to two subordinate lines. Each of the switches is of the well-known locking type, such that, when once operated to an olfnormal position, it remains in its operated posi tion until manually restored to normal. The conn the above-mentioned marking paths extending to one of the contacts in the set 3H of the rotary switch 3|0, thereby to identify the called line desired by the calling party.

The line wipers 3 I 1 and 3 8 of the rotary switch 3|0 are arranged to be connected'by way of a link line comprising the conductors C335 and C339 to a repeating transformer 500 through operation of the control apparatus indicated generally at 4|5. This apparatus comprises a start relay R450 which is arranged to be energized in response to any one of the calling switches shown in Fig. 1 and functions to initiate the operation of the rotary switch 3|0 to select a desired called line. Operation of the rotary switch is controlled by the three relays R420, R430 and R440, of which the relays R430 and R440 directly control the stepping operation of the switch 3|0 and the relay R420 is a test relay l or arresting the operation of the rotary switch when its wipers are operated to a position corresponding to a desired called line. The relay equipment also includes a pair of relays R400 and R4I0 which function to test the condition of a selected called line to determine whether it is idle or busy. More particularly, the slow-torelease relay R4I0 is a busy relay which is operative to return busy tone to the executive substation in the event a selected called line is found busy, and the slow-to-operate relay R400 is operative in the event a selected called line is found idle and is arranged to control the ringing relay R560, shown in Fig. 5. The busy relay R4|0 also functions as a device for impressing a background signal on an established connection involving a busy subordinate line called from the executive substation. The apparatus 4|5 also includes a ring cutoi relay R550 and a back bridge or called party answer relay R5|0, which relays are arranged to perform the functions conventionally required of relays of this character.

As indicated above, auxiliary transmitting and receiving means are provided at the executive substation for use in communicating over connections established through operation of the auxiliary or special switching means just described. The auxiliary transmitting means comprises a microphone |35, and the auxiliary receiving means comprises a sound reproducing device in the form of a loud speaker, diagrammatically illustrated at |36, connected in tandem with a signal amplifier including the vacuum tube 620. The auxiliary transmitting and receiving means are interconnected with an anti-side-tone impedance network indicated generally at 630 to form a substation circuit of the well-known antiside-tone type having line terminals 643 and 644 arranged to be connected by way of a link line including the conductors C515 and C516 to one winding of the repeating transformer 500 or by way of a second link line comprising the conductors C296 and C291 to one winding of a second repeating transformer 300, shown in Fig. 3. More particularly, the anti-side-tone impedance network 630 comprises an induction coil or transformer 6I0 including five windings 6| I to 6|5, in-

clusive, the last-mentioned of which is coupled to the primary winding 6 I6 of an input transformer 6|8 having its secondary winding 46|`| connected across the non-adjustable terminals of a voltage dividing resistor 6|9. The adjustable terminals of the resistor 6|9 are coupled to the control grid 62| and the cathode 622 of the tube 620. The tube 620, which is of the well-known pentode type and includes a directly heated cathode, is promary winding 628 of an output transformer 621 connected in series with a source of anode potential between the anode 623 and the cathode 622. For the purpose of biasing the control grid 62| negatively with respect to the cathode 622, there is provided a resistor G25 which is commonly included in the output circuit of the tube and the energizing circuit for the 'cathode 622. The anode current source is by-passed for signal frequency currents by a condenser 626. The secondary winding 629 of the transformer 621 is connected to impress the signal voltage developed thereacross upon the input terminals of the loud speaker |36 through a volumeA control potentiometer 4|. This potentiometer includes an adjustable contact carrying arm |45 so arranged that adjustment of the arm varies the magnitude of the signal voltage impressed upon the input introducing distortion into the reproduced output of the loud speaker |36.

The anti-side-tone impedance network 630 includes, in addition to the transformer 6|0 a pair of line balancing circuits which are arranged selectively to be included in the network and are individually effective, under different operating conditions, to balance the circuit so that, during operation of the microphone |35, no substantial portion of the developed signal voltage is impressed on the input circuit of the amplifier tube 620. The first of the two balancing networks comprises an adjustable inductance element 63| connected in series with an adjustable resistor 632, and the second network similarly comprises an adjustable inductance element 63| connected in series with an adjustable resistor 632'. As shown; the transmitter microphone |35 is included in a substantially closed transmission circuit comprising this microphone, a low impedance condenser |39, and the primary winding |38 of a coupling transformer |31. .This transformer is provided with a secondary winding |40, one terminal of which is connected by way of the conductor C|46 to the junction point between the two transformer windings 6II and 6|2 and the other terminal of which is connected by way of the conductor C|41 to the junction point between the two transformer windings 6|3 and 6|4. The two junction points noted form the conjugate terminals of the anti-side-tone bridge circuit, which is effectively formed when the auxiliary substation equipment is connected for. use in communicating over an established connection. More particularly, signal currents traversing the secondary winding |40 of the transformer |31 during operation of the microphone |35 divide at the two junction points noted, portions of the currents traversing a path comprising the two windings 6|| and 6|3 and one of the two balancing circuits, described above, and other portions of the currents' traversing the two windings 6|2 and 6|4 and the established loop circuit extending to the repeating transformer 500 or the repeating transformer 300. By suitably proportioning the impedance values of the elements respectively included in the two balancing circuits, the currents traversing the windings 6|3 and 6|4 are rendered substantially equal in magnitude and opposite in phase at all frequencies within the audio range irrespective of which of the two repeating transformers 300 and 500 is included in the established connection. Similarly, when the impedance values of the elements included in the balancing circuits are proportioned in the manner indicated, the signal currents respectively traversing the two windings 6|| and 6|2 are rendered substantially equal in magnitude and opposite in phase at all frequencies within the audio range. Thus, during operation of the transmitter microphone |36, the currents traversing the windings 6H, 6|2, 6|3 and 6|4 cause substantially equal and opposite signal voltages to be induced in the secondary winding 3|5, so that substantially no signal energy is delivered to the input circuit of the vacuum tube 620. By virtue of this arrangement, suppression of the side tone is achieved.

For the purpose of utilizing the auxiliary transmitting and receiving means to answer calls incoming to the executive substation and routed by way of the substation line 323, and for the further purpose of permitting the auxiliary transmitting and receiving means selectively to be connected for use in communicating over either of two simultaneously established connections, each extending to the executive substation, there are provided an answer switch III, a transfer switch ||0, the repeating transformer 300, the link line comprising the two conductors C296 and C231, and two control relays R210 and R520. 'I'he key operated switches and ||I are of the wellknown locking type. 'Ihe transfer switch ||0 in combination with the start relay R450 also forms a part of apparatus, controllable from the executive substation for marking the line 323 Aas busy in the bank contacts of the connector switches having access thereto. Both of the two relays 40 R210 and R520 are arranged to be energized in response to the operation of the answer switch III, the first thereof being operative to condition the signal amplier comprising the tube 620 for operation, and alsoto complete an auxiliary 45 loop circuit extending to a distant subordinate substation and terminated at the windings 303 and 304 of the repeating transformer 300. Arranged to be serially included in an established auxiliary loop circuit is an adjustable resistor 306 50 for conducting direct current and by-passed for signal frequency currents by a condenser 305. The relay R210 conditions the signal amplifier for operation by supplying anode and cathode energizing voltages to the tube 620 over circuits traced hereinafter. In this regard, it is pointed out that the tube 620 is of the well-known C33 type, which type of tube requires only a low anode voltage to provide reasonably good amplification. Hence, the exchange battery is used to supply 60 anode and screen voltages to this tube. The exterminals 643 and 644, from a connection withr the winding50| of the repeating transformer 500 to a connection with the winding 30| of the 75 repeating transformer 300. This relay also functions selectively to include the two balancing circuits, described previously, in the anti-slde-tone impedance network 630 and to short-circuit the communication channel not in use, all in the manner pointed out more in detail hereinafter. In order to remind a user of the' executive substation equipment that the answer switch I I is in its operated or ofi-normal position, there is provided a signal lamp |26 which is arranged to be energized in response to the operation of the switch |I| to its off-normal position.

For the purpose of preventing an established connection extending to one of the subordinate substations from being reestablished after the usual release operation is performed at the subordinate substation and before the. connection is released at the executive substation, there are included in the control apparatus 4i5 two relays R530 and R540, the first of which is of the slowto-operate type, and the last-mentioned of which is of the slow-to-release type. More particularly, current for energizing the transmitter microphone |35 is normally supplied to this microphone over a circuit including the winding of R540. When, however, a release operation is performed at a subordinate substation involved in an established connection extending to the executive substation, the relay R540 cooperates with the back bridge relay R5|0 to cause the operation of the relay R530, and the last-mentioned relay functions to open the circuit for supplying energizing current to the transmitter microphone |35 and to maintain this circuit open so long as the connection is not released at the executive substation.

For the purpose of rendering the inclusion of the regular substation transmitter and receiver in an established direct call connection under the control of the hook or cradle switch conventionally included in the regular substation appa.-V ratus 600, the three relays R610, R600 and R630 are provided. The arrangement is such that, when the transmitter and receiver hand set is being supported upon its hook or cradle, the loud speaker |36 and the microphone |35 are utilized as the transmitting and receiving means, respectively, and, when the substation hand set is removed from its supporting hook or cradle, the transmitter and receiver embodied therein are substituted for the elements |36 and |35 as the receiving and transmitting means to be used.

In order to provide secretary calling facilities there is also provided a call buzzer 642 which is arranged to be energized in response to operation of a non-locking call key |42 over a circuit including one conductor CMS of the two leads coupling the loud speaker |36 to the output transformer 621. It will be noted that a circuit including the windings of the loud speaker |36, the conductor C|40 and the transformer winding 626 is connected in parallel with the conductor C|49 comprising a part of the energizing circuit for the buzzer 642. The impedance of this parallel circuit is, however, so high as compared with the impedance of the conductor CI49, that it is traversed only a negligible portion of the current for energizing the buzzer. Hence, operation of the buzzer does not cause any substantial reproduction of the pulsating direct current lby the loud speaker.

Although the apparatus shown in detail in Figs.

1 to 6, inclusive. may be arranged in any desired l manner, it is contemplated that only the apparatus shown in Fig. 1 and the substation apparatus 600 shall be located in the office of the executive for whose use it is provided. It is further contemplated that the remaining apparatus illustrated in the drawings with the exception of the buzzer 642 shall be conveniently located in the exchange terminating the various lines-of the system, a suitable multiconductor cable being provided for interconnecting the circuit elements to provide the circuit arrangement depicted in the drawings and described above.

Referring now more particularly to the operation of the special switching equipment to establish a connection to one of the regular automatic substations and assuming that the substation A is being called, the call is initiated by operating the calling switch |00 to the particular oft-normal position corresponding to the called substation. More particularly, if the substation A is being called, the key switch |00 is operated from its normal position to its lower off-normal position to close the springs of the three sets I I4, ||5 and I I6. As indicated above, three of the coding relays illustrated in Fig. 2 respond to the operation of any one of the key switches to an olfnormal position to provide a marking path for marking the particular called line in the contact level 3|| of the rotary switch 3|0. In the case under consideration, when the springs ||4, ||5 and ||6 are closed, circuits are completed for energizing the. three relays R200, R2|0 and R220, which circuits commonly include the winding of the start relay R450. 'I'he operating circuits for the three relays noted may be considered as a single circuit extending from battery at the lowi er terminal of the winding of R450 by way of this winding to C293, where it divides, one branch extending by way of the winding of R220, C|32 and the springs ||4 to the grounded conductor C|21; a second branch extending by way of R2I0, C|33 and the springs ||6 to the grounded conductor C|21; and thethird branch extending by way 0f R200, CI34 and the springs ||5 to the grounded conductor C|21. When energized over the above-traced circuits, the four relays R200, R2l0, R220 and R450 operate and the rst three thereof function to complete a marking path identifying the line 322 as the line being called. This marking path extends from ground at RA20| by way of RA2| I, RA22| and C211 to the twentyfifth contact of the contact set 3| I. Dueto the above-described arrangement of the coding relays, none of the other marking paths individually corresponding to the other lines directly accessible to the special Vequipment are completed.

As a further example of the manner in which the coding relays operate in response to selective operation of the calling switches, it is pointed-out that, if the switch |00 is operated to close the contact springs II2, ||3 and ||1, a series-parallel circuit is completed for energizing the three coding relays R200, R2|0 and R230, and the start relay R450. In this case, the two relays R200 and R2I0 are energized over circuits substantially similar to the branch circuits traced above y contact of the set 3| Again, 1f the tenth caning switch vme is operated to its lower olf-normal position, the springs I2I, |22 and |23 are closed to complete a seriesparallel circuit for energizing the three relays R200.. R240 and R260 in series with the relay R450. In this case the relay R200 is energized over a branch circuit extending by way of C|34 and the springs |23 to the grounded conductor C|21; the relay R240 is energized over a branch circuit extending by way of C|30 and the springs |22 to the grounded conductor C|21; and the relay R260 is energized over a branch circuit extending by way of C|28 and the springs |2| to the grounded conductor C|21. Upon operating, the three relays complete a path extending from ground at RA26| and by way of RA246, RA203 and C219 for impressing marking ground on the nineteenth contact of the contact set 3II. It will be apparent from the foregoing description that by selectively operating the calling switches, the coding relays may be caused to operate in different combinations of three, thereby selectively to complete the twenty marking paths extending to the respective contacts -of the conv tact set 3| I.

In any case when three of the coding relays are energized the start relay R450 is also energized and, upon operating, completes, at RA452 Vand RA453, respectively, paths commonly including the grounded armature RA559 and C410 for applying ground to the tone apparatus and ringing machine start conductors C401 and C408., thereby to initiate the operation of the tone apparatus and the ringing apparatus in the usual manner. AtvRA454, the relay R450 applies ground to the amplifier start conductor C294, thereby to condition the amplifier comprising the tube 620 for operation. More particularly, operation of the relay R450 results in the connection of the positive terminal of the exchange battery to C294, so that anode potential is impressed through the primary winding 628 of the output transformer 621 on the anode 623 of the vacuum tube 620. Operation of the relay R450 also results in the completion of a circuit for energizing the cathode 622 of the tube 620, this circuit extending from the grounded positive terminal of the exchange battery at RA454 by way of C294, the voltage dropping resistor 624, the cathode 622 and the biasing resistor 625 to the negative .terminal of the exchange battery. It will be noted that positive screen potential is applied to the screen electrode of the tube 620 directly from the positive terminal of the exchange battery over the path including the conductor C294. At RA455, the relay R450 prepares a path, traced hereinafter for applying ground potential to the private conductor C329 of the line 323.

At RA45I, the relay R450 completes a circuit for energizing the slow-to-operate relay R430, this circuit extending from ground at RA45| by way of RA422, RC423, C336, the armature 3|9, C331 and the winding of R430 to battery. The relay R430 operates, after an interval, and, at RA43|, prepares a circuit, traced hereinafter, for energizing the relay R420 when the wipers of the rotary switch 3| 0 are stepped to the position corresponding to the selected called line 322. At RA432 and RC433, the relay R430 also prepares a test circuit, traced hereinafter, for

energizing the busy relay R4|0 in the event the F selected called line is busy at the time the switch 3|0 is operated to the position corresponding to the called line. At RA435, the relay R430 completes a circuit for energizing the relay R440, this circuit extending from ground at RA45| by way of RA422, RC423, C336, the armature 3|0', C331, RA435 and the winding'of R440 to battery.

When energized over the circuit Vlust traced, the relay R440 operates to complete, at RA44I, a circuit for energizing the motor magnet 3|! of the switch 3|0, this circuit extending from ground at the grounded contact RC421 by way of RA426, RA44I, C338, and the winding of the magnet 3I9 to battery. Upon operating, the magnet 3|9 steps the wipers 3I4 to 3|0, inclusive, of the switch 3|0 one step and interrupts, at its associated armature 3I0', the above-traced operating circuits for the two relays R430 and R440. The relay R440 immediately restores to interrupt, at RA44|, the operating circuit as traced above for the magnet 3 I 9, whereupon this magnet restores its associated armature 3|9 to recomplete the operating circuits for the two relays R430 and R440, respectively. When the operating circuit for R440 is again completed, this relay operates to recomplete the above-traced circuit for energizing the magnet 3|9, causing this magnet to again step the enumerated Wipers of the switch 3|0 a second step and to again interrupt the operating circuits for the two relays R430 and R440. The above-described interaction between the relay'R440 and the magnet 3|9 continues until such time as the wipers of the switch 3|0 are stepped to the position corresponding to the called line 322, or until the wiper 3|5 is moved into engagement with the contact marked with ground over the abovetraced marking path. During the interaction between'the relay R440 and the magnet 3|9, the relay R430, due to its slow-to-release characteristic, stays operated.

When the wiper 3|5 of the switch 3|0 is 0perated into engagement with its associated twenty-iifth contact, the above-mentioned circuit is completed for energizing the relay R420, this circuit extending from the grounded conductor C211 by way of the twenty-fifth contact of the set 3|I, the wiper 3|5, C333, RA43I and the lower winding of R420 to battery. When its lower winding is energized over the circuit, just traced, the relay R420 immediately operates to interrupt, at RA426 and RC421, the abovetraced circuit for energizing the magnet 3|9, thereby to arrest the interaction between this magnet and the relay R440 and to prevent further movement of the wipers embodied in the rotary switch 3|0. At RA425, the relay R420 completes a holding circuit for itsself, this holding circuit extending from the grounded conductor C333, by way of RA425 and the upper winding of R420 to battery. At RA422 and RC423, the relay R420 interrupts a point in 4 the circuits over which R430 and R440 are being held energized, causing these two relays to restore. At RA422 and RC424, the relay R420 prepares a holding circuit, traced hereinafter, for R560.

At RA42 I, the relay R420 completes the abovementioned circuit for testing the called line 322 to determine the idle or busy condition thereof. In this regard and as previously explained, the automatic switching equipment provided in the exchange 320 is conventionally arranged so that, when the line 322 is busy, ground potential is present upon the control conductor C324 thereof, whereas, if the line is idle, battery potential appears on this control conductor. Assuming the selected called line 322 is busy, the busy relay R4I0 is energized over the above-mentioned test circuit, which circuit extends by Way of the control conductor C324, the twenty-fifth contact of the set 3|2. the wiper 3|6, C334, RA42I, RA432, RC433 and the winding of- R4|0 to battery. Upon operating, the busy relay R4|0 interrupts, at RA4I I, a point in the circuit, traced hereinafter, for energizing the slow-to-operate control relay R400. At RA4I2, the relay R4I0 completes a holding circuit for itself, this circuit extending by way of the conductor C334, RA42 I, RA4|2 and the winding of R4I0 to battery.

The relay R4|0, upon operating, functions to complete a circuit for impressing busy tone voltage on the input circuit of the signal amplifier comprising the vacuum tube 620., Specifically, the path of the busy tone current extends from the ungrounded terminal of the busy tone source, not shown, by way of the busy tone conductor C406,the condenser 404, RA4I3, C465, RC529, RA520, C516, RA616, the transformer windings 6|2 and 6I I, the inductance element 63|, the resistor 632, C512, RC522, RA52I, C514, the transformer windings 3I3 and 6|4. RA61I, C515, RA524, RC525 and the upper half of the Winding 50| to the grounded terminal of the busy tone source at the grounded center tap 502 of the winding 50|. This current causes an induced voltage to be developed in the transformer winding 6|5 which is impressed through the coupling transformer 6I8 between the input electrodes of the tube 620, and the amplified output energyvis delivered through the transformer 621 and the volume control potentiometer |4| to the loud speaker |36 for reproduction. The reproduced busy tone signal indicates to the calling party that the desired connection cannot immediately be established.

Presumably, the substation at which the illustrated special equipment is located is that of an important ollicial or executive, such that calls originating at this substation and' intended for the various subordinate lines are of suflicient importance to warrant direct attention on the part of the called parties. For this reason, the busy relay R4| 0 is so arranged that it functions to impress a background signal on a selected subordinate line which is tested and found busy. More particularly, when the relay R4|0 operates, it completes a path for impressing the voltage of the ringing current source upon the previously established loop circuit of which the called linc forms a part. In the case under consideration, the path of the ringing current extends from the ungrounded terminal of the ringing current source, not shown, by Way of the ringing current conductor C405, the condenser 402, RA4I4, C339, the wiper 3I6, the twenty-fifth contact of the set 3|4, the line conductor C326, the receiver at the called substation A, the line conductor C325, and

through one Winding of the back bridge relay conventionally provided in the operated connector switch of the switch train leading to the line 322, to the grounded terminal of the ringing current source. The condenser 402 included in this path is selected of a value such that the current traversing the above-traced path is insuflicient to produce a signal `which will interfere with the ensuing conversation over the established connectionincluding the line 322, but is of suflicient strength to indicate that the substation A is being called from the executive substation.

Following the operation of the busy relay R4I 0, no further operation' of the apparatus occurs until the partially established connection is re- 40 terrupted causing this relay to restore.

leased. To release the connection it is only necessaryto restore the operated key switch to its normal position, thereby to interrupt the above-traced operating circuits for the four relays R200, R2|0, R220 and R450, causing all of these relays to restore. When the first three of the enumerated four relays restore, the above- .traced marking path extending to the twentyfifth contact of the contact set 3H in the switch 3|0 is interrupted. When this path is interrupted, the holding circuit for R420 is broken, causing this relay to restore to interrupt, at RA42I, a point in the above-traced holding cir:- cuit for the busy relay R4I0. Thus, the three operated relays R4|0, R420 and R450 are caused to restore and the common apparatus 4|5 is conditioned for further use. In this regard, it will be noted that when the relay R450 restores, it interrupts, at RA454, the path for applying ground to the amplier start conductor C294, whereby the signal amplier including the tube 620 is rendered inactive. Upon restoring, the relay R450 also interrupts, at RA45I, a point .in the operating circuits for R430 and R440 and, at RA452 and RA453, the two paths described above for applying ground to the tone apparatus start conductor C401 and the ringing apparatus start conductor C408.

If the calling party desires he may disregard the busy signal and wait until the established connection is cleared out, in which case the apparatus 4|5 operates to extend the connection to the called line 322 immediately the calledl line becomes idle. In this regard it is pointed out that the above-traced holding cli/cuit for the busy relay R4|0 extends to grou `cl `as applied to the control conductor.C324. Hence, when the line 322 becomes idle and ground is removed from C324, the `holding circuit for R4H) is in- Upon . restoring, the busy relay R4|0 permits ringing current to be projected over the called line in the manner described immediately hereinafter.

In the event the called line 322 is idle at the 45 time it is selected by the switch 3|0, battery potential is present on the control conductor C324 thereof and, hence, the busy relay R4|0 is not energized. A short time interval after the switch 3|0 is operatedto the position corresponding to the line 322 or, more particularly, a short time interval after the relay R420 operates, the slow- 'to-release relay R430 restores to interrupt, at RA432 and RC433, a point in the previously traced testing circuit by way of which the busy relay R4|0 is energized. At RA432 and RC434, the relay R430, upon restoring, completes a circuit for energizing the control relay R400, this circuit extending from ground at RA45| by way of RA4||, the winding of R400, RC434, RA432,

60 RA42I, C334, the wiper 336 and its associated twenty-fifth contact to battery as impressed upon the control conductor C324 of the idle line 322. Shortly following its energization over the cir.- cuit just traced, the slow-to-operate relay R400 operates to complete, Iat RA40I, a circuit for energizing the ringing relay R560, this circuit extending from ground at RA45I by way of RA422, RC424, RA40|, C463 and the winding of R560 to battery. Upon operating, the relay R550 completes, at RA561, a holding circuit for itself, which circuit exten-ds from ground at RA45| by way of RA422, RC424, C462, RA561 and the Winding of R560 to battery. At RA568, the relay R560, upon operating, applies ground to the control conductor C324 of the called line 3.22, thereby to mark .this line as busy in the bank contacts of the connector switches to which it extends. Specifically, the above-mentioned marking path extends from ground at RA568 by way of C464, RA42I, C334, the wiper 3I6 and its associated twenty-fifth contact to the control conductor C324. At RA565 and its associated grounded contact RC566, the re1ayfR560 prepares la locking circuit, traced hereinafter, for thel slow-acting ring cut-olf relay R550. At RA56| and RA562, therelay R560 completes a path for projecting ringing current over the called line 322, thereby to energize the signal device at this substation. Specifically, the path traversed by the ringing current extends from one terminal of the ringing current source, not shown, by way of C405, C461, RA551, RA56I, C330, the wiper 3|3 and its associated twentyfifth contact, the line conductor C326, the signal' device at the ycalled substation, the line conductor C325, the wiper 3|1, C335, RA563, RA562, RA552, RC553 and the upper Winding of R550 to the other terminal of the ringing current source. The resulting operation of the signal device provided at the called substation indicates that this substation is being called. The relay R560 also completes a path for transmitting ring-back-tone energy to the input circuit of the signal amplifier tube 620, this path being substantially the same as that traced previously for -the busy-tone current, but extending from the ringing current source by way of C405, the condenser 403, C468, RA569, RA558 and RA542, rather than by way of C406, the condenser 404, RA4|3 and C465. 'I'he ring-backtone energy delivered to the input circuit of the amplier tube 620 is ampliiied by this tube and v delivered to the loud speaker |36 for reproduction, thereby to indicate to the calling party that the desired called substation is being rung.

'Ihe desired conversational connection is completed when the call is answered at the called substation `A. More particularly, when the receiver at the substation A is removed from its hook, a direct current bridge is placed across the line conductors C325 and C326 of the line 322 to complete a direct current circuit substantially similar to that, -traced previously and traversed by the alternating ringing current, for energizing the upper winding of the ring cutoff relay R550. When its upper winding is energized the relay R550 iirst closes the X contacts associated with RA556, thereby to complete the above-mentioned holding circuit for` itself, this circuit extending from ground at RC566 by way of RA565 and the two windings of R550 in series to battery. At RA552 and RA551 and their respective associated resting contacts, the relay R550 opens two points in the above-traced path for projecting ringing current over the selected called line 322 and, in so doing, opens two points vin its own operating circuit. At these same arpletes, at RA53I, a holding circuit for itself, thisy phone |35 is conducted to the microphone over the circuit Just traced. Upon` operating, the relay R540 prepares, at RA54I, a circuit, traced hereinafter, for energizing the relay R530. At RA542, the relay R540 opens a further point in a path for conducting ring-back-tone current by way of the anti-side-tone impedance network 630 to the input circuit of the amplifier tube 620.

Following the operation of R540, the desired conversational circuit is completely established, talking battery being supplied to the loop extending to the called substation A through the windings of the back bridge relay R5|0, and talking battery being supplied to the transmitter microphone |35 over the circuit, traced above, for energizing the relay R540. Signal currents incoming to the special substation and traversing the windings 503 and 504 of the repeating transformer 500 cause induced voltages to be developed in the winding 50| coupled to the two windings 503 and 506, whereby signal currents corresponding to the incoming sign-al currents are caused to iiowthrough the anti-side-tone impedance network 630. More particularly, the path followed by currents traversing the winding 50| may be traced as extending from the lower terminal of this winding by way of RC529, RA528, C516, RA616, the two windings 6|2 and 6| I, the inductance element 63|, the resistor 632, C512, RC522, RA52I, C514, the two windings 5|3 and 6H, RAB'H, C515, RA524, RC525,`

back to the upper terminal of the winding 50|. The resulting induced voltages developed in the winding SI5 are impressed through the coupling transformer 6|8 and the vol-tage dividing resistor 6|9 between the input electrodes of the amplier tube 620, thereby to cause corresponding amplified currents to flow in the output circuits of this tube. Such currents cause induced voltages to be produced in the winding 629 of the output coupling transformer 621, which voltages are impressed through the potentiometer |4| on the input terminals of the loud speaker |36 for reproduction.

Audio frequency currents developed by operation of the microphone |35 are caused to circulate in the closed low impedance transmission circuit comprising the primary winding |38 of the transformer |31 and the 'condenser |39. Such currents cause corresponding induced voltages to be developed in the secondary winding |40 of the transformer |31 which are impressed by means of the two conductors C|46 and C|4`| between opposite terminals of the anti-side-tone impedance network 630. More particularly, signal currents traversing the winding |40 divide at the terminals of the anti-side-tone impedance network and follow the two paths traced previously, whereby the well-known anti-side-tone effect is realized.

The release of the established communication connection is primarly under the control of the calling party. If the called party at the substation A restores his receiver to its hook before the calling key switch is returned to normal, the established called partly loop circuit extending to the repeating transformer 500 is interrupted, causing the back bridge relay R|0 to restore in the usual manner. Upon restoring, the relay R|50 interrupts, at RA5|| and RC5|3, the above-traced circuit for energizing R540 and, at RA5|| and RC5|2, it completes a circuit for energizing R530, the latter circuit extending by way of RA521, RA54| and the winding of R530 to battery. Upon operating, the relay R530 comcircuit extending from ground at RA555 by way of RA53| and the winding of R530 to battery. It is noted that the relay R530, although of the slowto-operate type, operates to complete its holding circuit before the slow-to-release relay R540 restores to open the operating circuit for R530. At RA532, the relay R530, upon operating, opens a further point in the above-traced operating circuit for R540 and, in so doing, opens a i'urther pnint in the circuit by way of which energizing current is delivered to the microphone |35. At RA534, the relay R530 applies ground by way of C410 to the two start conductors C401 and C408, thereby to reinitiate the operation of the tone apparatus and the ringing apparatus providing this apparatus is not already in operation. At 1`\.A542,l the relay R540. upon restoring, completes an alternative path including C469 for impressing busy-tone voltage upon the input circuit of the signal amplifier tube 620, thereby to cause a busy-tone signal to be reproduced by the loud speaker |36. The busy-tone signal produced by operation of the loud speaker |36 indicates to the calling party that the key switch v |00 has not been restored to normal.

The purpose of providing the relay R530, which operates in response to the release operation performed at the called substation, is that of positively preventing eaves dropping on the part of a subordinate, following the termination of a conversation between the subordinate and th calling executive. relay and its associated circuits, the called party at the substation A could, after hanging up, again remove his receiver from its hook to recomplete the talking connection, provided the operated key switch |00 was not restored to normal bythe calling party. In this regard, it will be appreciated that the back bridge relay R5|0 would respond to the recompletion'of the loop circuit to again complete the circuit for delivering energizing current to the transmitter |35. Obviously, with this connection reestablished, the subordinate could listen to any conversation being carried on in the oflice of the executive. With the arrangement illustrated, however, the relay R530, upon operating, is held operated independently of the back bridge relay R5|0 and, in its operated position, holds interrupted, at RA532, the above-traced path by way of which energizing current is delivered to the transmitter microphone |35. Thus, the channel for transmitting signal currents outgoing from the special substation to the called substation is rendered completely inactive and eavesdropping on the Vpart of a subordinate is positively prevented.

As indicated above, the common control apparatus 4|5 is released in response to the restoration of the operated key switch 00 to its normal position. When this switch is returned to normal the three operated coding relays R200, R2|0 and R220, and the start relay R450 are all caused to restore in the manner pointed out above. When the three enumerated coding relays restore, the above-traced holding circuit for R420 is interrupted, causing the last-named relay to restore and interrupt, at RA422 and RC424, the circuit over which R560 is being held energized. During the restoration of the relay R560, the contact springs RA563 and RC564 are moved into engagement before the loop `circuit extending to the called substation is interrupted, provicling the called party has not restored his receiver to its hook. When the enumerated springs Thus, in the absence of this.

' ductors C339 and C335.' -This condenser absorbs the voltage transient which occurs when the talking-loopi is interrupted at RAEBI :following the restoration oi.' R560 completely to normal and thus prevents the transient from being reproduced as an objectionable "click by the receiver at the called substation. When R560 falls back, it interrupts, at RA565 and RC566, a point in theholding circuit for the ring cutoi relay R550, whereby the last-mentioned relay is caused to restore. The relay R550,

upon restoring, opens at RA55| a point in the path connecting the condenser 543 between the link line conductors C335 and C339. Upon restoring, the relay R550 opens, at RA555, a point in the holding circuit for R530. If R530 is operated, lt restores to remove, at RA534, multiple ground from C410. Following the restoration of R530, the common control apparatus 4|5 is entirely released.

If the executive desires not to be disturbed by incoming calls routed over the line 323 while the above-described direct line call is in progress, he may preclude such incoming calls by operating the transfer switch H0, thereby to mark the line 323 as busy in the bank contacts of the connector switches having access thereto. More particularly, when the start relay R450 operates it prepares a path for impressing ground potential upon the private or control conductor C329 of the line 323. This path may be completed by operating the transfer switch H without operating the answer switch lil and extends from ground by way of the middle and left-springs of the. switch ill, the middle and left springs of the switch H0, CI52, RA213, 025| and RA455 to the private vconductor C329. When this path is completed,

the line 323 tests busy to all automatic switches attempting to gain access thereto and thus the executive substation is guarded against :incoming calls over this line. f

Following the completion of the direct line connectionin the manner just described, the substation apparatus 600 may, if desired, be used in lieu of the auxiliary transmitting and receiving the apparatus 600 is resting upon its supporting cradle or hook, the loop circuit extending from the substation apparatus 600 by way of the line 323 to the switchingapparatus 320 is open andthe relays R610, R660 and R690 are deenergized, whereby the line conductors C515 and C516 are connected by way of the armatures RA61I and RA616 and their respective associated resting contacts through to the lineterminals 643 and 644 of the network 630. Also, with the hand set resting on its supporting means such that the relay R610 is restored, the two line conductors C321 and C328 of the substation line 323 are connected by way of the armatures RA614 and RA615 and their respective associated resting` contacts to the line conductors of the cord extending to the substation apparatus 600. Solong as all of the calling switches and the answer key switch III occupy their respective normal positions, the amplifier start conductor C294 is disconnected from ground so that the operating circuit, traced hereinafter, for R610 is interrupted independently of the hook or cradle switch provided in the apparatus 600- may be utilized in the usual manner in the initiation and answering of calls outgoing from and incoming to .the executive substation.

If, after the above-described direct call connection is established, it is desired to use the hand set embodied in the apparatus 600 as .the transmitting and receiving means in communicating over the direct call connection, the hand set is removed from its supporting hook or cradle, thereby to connect a direct current bridge between the line conductors of the cord extending to the apparatus 600. As a result, a loop circuit is completed over which the relay R680 is energized, this circuit extending from one side of the exchange battery through one winding of the line relay associated with the line 323 and by Way of C321, RA614, the winding of R690, the bridge across the line conductors of the cord extending to the apparatus 600, RA615, C328, and through the other winding of the line relay associated with the line 323 to the other terminal of the exchange battery. When energized over this circuit, the relay R680 operates to complete, at

' RA68| an obvious circuit for energizing the relay R690. The slow-to-operate relay R690 now partially operates to close the "X contacts associated with RA693, thereby to complete a circuit for energizing the relay R610, this circuit extending-by way of the grounded start conductor C294,

1 RA693, RA692, RA613, and the lower winding of R610 to battery. Shortly thereafter, the relay R690 completes its operation to open, at RA692, a

point in the circuit just traced and to prepare, at

RA692, an alternative circuit, traced hereinafter, for energizing the lower winding of R610. At RA69I, the relay R690 interrupts the path normally short-circuiting its upper winding, thereby to render itself fast to release. The armature springs of the relay R610 are so adjusted that the two armatures RA614 and RA615 are moved from engagement with their respective associated resting contacts and into engagement with their respective associated working contacts before the armature RA692 of the relay R690 is moved out of engagement with its associated resting contact. Thus, upon the initial energization ofthe lower winding of R610, a holding circuit is completed for this relay and the relay R680 before the operating circuit, just traced, is interrupted. I'his holding circuit extends from the grounded conductor C294 by .way of the upper winding of R610, RA615 and its associated working contact, the closed bridge between the line conductors of the cord extending to the apparatus 600, the winding of R680, RA614 and its associated working contact, and the lower winding of R610 to battery. Energizing current is delivered to the transmitter embodied in the hand set of the apparatus 600 over this holding circuit. Immediately after the completion of the above-traced holding circuit, the armature RA692 of the relayR690 is moved from engagement with its associated resting contact to interrupt the operating circuit for R610.

At RA61I and RA616 and their respective associated resting contacts, the relay R610, upon fully operating, disconnects the line conductors C515 and C516 from the line terminals 643 and 644, respectively, of the anti-side-tone impedance network 630 and connects these line conductors by way of the condensers 618, 619 and 682 to the line conductors of the cord extending to the substation apparatus 600. Thus, the transmitter and receiver incorporated in the substation apparatus 600 are substituted for the auxiliary transmitting and receiving means in the established connection.

When the operating circuit for R610 is completed, a relatively heavy current iiows through the upper winding oi the relay, which current is materially reduced when the operating circuit is broken and the holding circuit for R610 is completed. Due to the collapse of the magnetic field occasioned by interruption of the operating circuit, a relatively large transient voltage tends to be developed across the windings oi.' R610, which, if not absorbed, would be impressed through the condensers 616 and 619 across the repeating transformer winding 50| or the winding 30|, depending upon the position of the relay R520. If allowed to assume its full value, such a transient would cause an objectionable click in the receiver being used at the distant end of the connection. To obviate this dimculty, the relay R610 is provided with a set of armature springs associated with RA611 which are included in an obvious path for short-circuiting the signal channel comprising the two line conductors C515 and C516 during the initial operation of R610. The springs of this set are so arranged that they are not moved to the open-circuit position to interrupt the short-circuiting path noted until after the transient is substantially damped out. By this arrangement the transient voltage is not4 impressed on the signal channel of which the line conductors C515 and C516 form a part, but is dissipated in a closed circuit comprising the condensers 610 and 619, the two windings of R610, the winding of R660 as shunted by the by-pass condenser 602, the exchange battery and RA611.

It will be noted that, when the relay R610 operates to connect the substation apparatus 600 to be used in communicating over the established direct line connection, the line terminals 643 and 40 644 of the anti-side-tone impedance network 630 are open-circuited and, thus, this network is unbalanced so that electrical coupling is incidentally provided between the signal current receiving and signal current transmission paths extending to the loud speaker |36 and from the microphone |35, respectively. Also, acoustical coupling exists between the microphone I 35 and the loud speaker |36, and the signal amplifier comprising the vacuum tube 620 is still energized and in condition for-operation. Due to the two couplings noted, a local oscillatory condition tends to be set up in the network comprising the signal amplier, the transmitter 35, the loud speaker |36, and the anti-side-tone impedance network. If this oscillatory condition is allowed to persist, howling or singing results. To obviate this difculty, the relay R610 is provided with a pair of' armature springs for short-circuiting the input circuit to the loud speaker |36 when this relay occupies its operated position. More particularly, when the relay R610 operates, it completes, at RA612, an obvious path for short-circuiting the winding 629 of the transformer 621. therevby effectively to interrupt the local oscillatory of the cord extending to this apparatus. When this occurs, the above-traced holding circuit for the relays R616 and R660 isinterrupted. Thus, the relays R610 and R660 are caused to restore. Upon restoring, the relay R660 opens the operating circuit for R690, causing the last-mentioned relay to restore. The relay R610, upon restoring, disconnects the substation apparatus 600 from the established communication connection; interrupts, at RA612, the short-circuiting path across the winding 629; and reconnects the line conductors C515 andC516 to the line terminals 643 and 644, respectively, of the anti-side-tone impedance network 630.

From the foregoing description, it will be apparent that, when once the relay R610 is operated, it is only deenergized in response to the replacement of the substation hand set upon its supporting hook or cradle. Hence, to release a direct call connection which has been established in the manner previously described and to which the apparatus 600 isv operatively connected, it is necessary to restore the operated one of the calling switches of Fig. 1 to normal and to replace the substation hand set upon its supporting hook or cradle. When the operated calling switch is returned to its normal position, the operated relays of Figs. 2, 4 and 5 are caused to restore in the manner described previously, it being noted that, when the start relay R450 falls back, ground potential is removed from the ampliiier start conductor C294 to open a point in the operating circuit for the relay R610. When the substation hand set is placed upon its supporting hook or cradle, the operating and holding circuits for the relays R610, R660 and R690 are interrupted in the manner just described.

As indicated above, the substation apparatus 600 is normally connected and arranged to be used in communicating over connectionsestablished between the executive substation and one of the other substations on calls incoming to the executive substation. The special substation apparatus illustrated in Figs. 1 to 6, inclusive, is, however, so arranged that the auxiliary transmitting and receiving means comprising the microphone |35 and the loud speaker |36 may, if desired, be used in lieu of the regular substation apparatus 600 toanswer incoming calls. For example, if a call is initiated at the substation A and routed by way of the automatic switching equipment 320 and the line 323 to the substation apparatus 600 at a time when the line 323 is idle, ringing current is projected over the line to actuate the signal device 64I, thereby to give an audible indication that the executive substation is being called. Obviously, the call may be answered in the usual manner by removing the receiver embodied in the substation apparatus 600 fromlits associated hook. In this case the resulting senu'ential operation of the two relays R660 and R690 is without effect since ground potential is not present upon either of the conductors C294 and C694 and. hence, the relay R610 is not energized. Alternatively, the auxiliary transmitting and receiving means comprising the loud speaker |36 and the transmitter microphone 35 may be used. In case the answering party elects to use the last-mentioned means, the answer switch is operated to its off-normal position to close the springs |25 and thereby complete a circuit for energizing the control relay R210. This circuit extends from ground by way o1' the switch springs |25, the signal lamp |26, CI21 and the winding of R210 to battery. Upon operating, the slow-to-release relay.R210 completes a lcircuit for energizing the relay R520. The lastmentioned circuit extends from ground by way cf the springs |25, the springs |24, CI52, RA213, and the Winding `of R520 to battery. When energized over the above-traced circuit, the signal lamp |26 is illuminated to indicate that the answer switch ||I occupies its off-normal position. 'The relay R210, upon operating. also completes, at RA212, a path for applying ground to the amplifier start conductor C294, whereby the signal amplifier comprising the vacuum tube 620 is conditioned for operation in the manner described previously. At `RA212 and RA214, the relay R210 prepares a circuit, traced hereinafter, for energizing the relay R610. At RA21I, the relay R210 completes a direct current bridging path between the line lconductors C321 and C328 of the called line 323,

which path includes the two windings 303 and 304 of the repeating transformer 300. More particularly, the bridging path noted extends from the line conductor C321 by way of the winding 304, the resistor 306, C299, RA21I, C298 and the winding 303 to the opposite line conductor C328. The operated connector switch by way of which the connection is routed to the line 323 responds to the completion of this path in the usual manner by interrupting the ringing operation.

As indicated above, the relay R520 is energized in response to the operation of the answer switch lll to its off-normal position and the resulting operation of the relay R210. When energized, this relay operates to complete, at RA528', the above-traced circuit for energizing R540 and by way of which energizing current is delivered to the transmitter microphone |35. At RA521, the relay R520 opens a point in the above-traced operating circuit for R530, thereby to prevent the last-mentioned relay from operating in response to the operation of R540. At RA523, the relay R520 opens a closed and obvious path normally short-circuiting the winding 30| of the transformer 300. At RA525, the relay R520 completes an obvious path for short-circuiting the winding 50| of the transformer 500. The purpose of this channel short-circuiting arrangement is to preclude the possibility of cross talk. To this end, the relay R520 is provided with the'two armatures RA523' and RA525', which function to shortcircuit the channel not in use in an obvious manner. associated resting contacts RC525 andi RC529', the relay R520 disconnects the line conductors C515 and C516 from the winding 50| of therepeating coil 500, and, at these same armatures and their respective associated working contacts RC526 and RC529, connects the line conductors C515 and C516 by wayof the link line conductors C296 and C201, respectively, to the opposite terminals of the repeating transformer Winding 30|.-

Thus, the repeating transformer 300 is incorporated in the newly established talking connection in lieu of the repeating transformer 500, which latter transformer is used on subordinate calls originated at the special substation. Since the impedance characteristics of the loop'circuits respectively including the repeating transformers 500 and 300 may be different, it is desirable to provide alternative line balancing circuits arranged alternatively to be included in the. anti/- slde-toneimpedance network 630. As described above, with the relay R520 deenergized, thel line balancing circuit comprising the inductance element 63| and the resistor 632 is operatively included in the anti-side-tone impedance network At RA524 and RA528 and their respective 630. When, however, the relay R520 operates, it substitutes the otherA balancing circuit, namely, that comprising the inductance element 63| and the resistor 632', for the first-mentioned balancing circuit in the anti-side-tone impedance network. This is accomplished through the operation of R520 to disconnect, at RA52I, the conductor C514, extending to the upper terminal of the winding 6|3, from the conductor C512 and to connect this conductor to the conductor` C513. It will be understood that the line balancing circuit comprising the inductance element 63|' and the resistor 632' is adjusted to have a frequencyimpedance characteristic substantially matching that of the equivalent impedance of the repeating transformer 300 and the line impedancesasso ciated therewith as determined at the line terminals 643 and 644 of the impedance network 630. Similarly, the line balancing circuit comprising the inductance element 63| and the resistor 632 is adjusted to have a frequency-impedance characteristic substantially the same as that of the equivalent impedance of the repeating transformer 500 and the line impedances associated therewith as determined at the line terminals 643 and 644 of the network 630.

Following the operation of R520, the desired conversational connection is fully completed.

After the connection is completed the answering party may, by removing the telephone receiver or hand set of the substation apparatus 600 from its supporting hook or cradle, release the auxiliary transmitting and receiving means comprising the loud speakerjl36 and the transmitter |35 and continue the conversation by using the regular substation transmitter and receiver. the receiver of the telephone instrument 600 is removed from its supporting element the three relays R610, R680 and R690 respond in the exact manner previously described to disconnect the telephone instrumentA of the apparatus 600 from the line 323 and to connect this instrument to the link line conductors C515 and C516. It will be noted that with the relays R210, R520 and R610 oper-ated, the substation apparatus 600 is operatively connected to the line 323 over an alternative channel, namely, that including thelink line C515, C516, the link line C296, C291, and the transformer 300. With the relays R210 and R520 operated it is obvious that, by restoring and removing the substation hand set from its supporting hook, the answering party may substitute one set of apparatus for the other as many times as desired during a single conversation. When, however, the answer switch is restored to normal, the above-traced operating circuit for the relay R520 is interrupted .to cause this relay and the relay R540 to restore in the When order named, and the series circuit for energizing the signal lamp |26 and the relay R210 is inter-` rupted causing the last-mentioned relay to restore. When R210 restores, it removes ground from the amplifier start conductor C294, thereby to render the signal amplifier inactive and to open a point in the operating circuit for R610, and opens, at RA21I, a point in the above-traced bridging'path between the conductors C321 and C328 of the line 323. Following the restoration of the three relays R520, R540 and R210, the auxiliary transmitting. and receiving means are ventirely disconnected from the line 323. If desired, the telephone receiver embodied in the substation apparatus 600 may be retained olf its supporting element when the answer switch III is restored to normal. In such case the substation apparatus is directly connected to the line 323 when the relay R010 restores in response to the removal ot ground potential from the start con- -ductor C294, and may be used in the usual mannel'.

As pointed out previously, to initiate a call vat the special substation intended for a substation connected to a line not directly accessible to the special switching means, such, for example, station B connected to the line 92|, it is necessary to employ the dial operated calling device conventionally included in the substation apparatus 600. Such a call is initiated in the usual manner by removing -the hand set embodied in the apparatus 600 from its supporting cradle to cause a bridging path to be placed across the line conductors C321 and C320 of the line 323. As a result of the completion of this path, one of the non-numerical switches included in the switching equipment 320 responds by extending the calling loop circuit through to one of the numerical impulse responsive switches of the exchange, whereby the seized switch is conditioned to respond to impulses transmitted thereto by way of the calling line 323. When the calling loop circuit is completed, the relays R600 and R690 sequentially operate, but without effect since the conductors C294 and C694 are not grounded and accordingly the relay R610 is not energized. The seized impulse responsive switch and one or more additional impulse responsive switches successively respond to the impulses of successive digits dialed at the calling substation by extending the connection to the called line 32| in the usual manner. If, following the dialing operation, the selected called line is found busy, busy tone is returned over the calling loop and is reproduced by the receiver included in the apparatus 600 to give an audible indication that the desired connection cannot be obtained. On the other hand, if the selected called line is idle, ringing current is projected thereover to actuate the signal device at the called substation and ringback tone is conducted back over the calling loop circuit to indicate to the calling party that the called substation is being rung. When the call is answered at the called substation, the desired conversational connection is completed in the usual manner.

Following the completion of a connection to the substation B in the manner just described, the auxiliary transmitting and receiving means comprising the transmitter microphone |35 and loud speaker |36 may be substituted for the transmitter and receiver embodied in the hand set for use in transmitting andvreproducing signal currents.` Such substitution is accomplished by operating the answer switch III to its off-normal position to cause the operation of the three relays R210, R520 and R540, and then replacing the hand set of the apparatus 600 upon its supporting element. The three relays noted operate in the exact sequence and perform the same functions as are described in detail in a preceding portion of the specification. In addition, the relay R210, upon operating, completes analternative circuit for energizing the relay R610, this circuit extending from ground at RA212 by way of RA214, C694, RA692RA613 and the lower winding of R610 to battery. is completed the relay R610 operates to complete the holding circuit for R600 and R610, and to perform the various switching functions previously explained. When the hand set of the substation apparatus 600 is replaced upon its sup- When this circuit porting element, the holding circuit for the two relays R610 and R000 is interrupted, whereby the three relays R000, R690 and R610 are caused sequentially to restore in the order named. The relay R610, upon restoring, connects the line conductors C515 and C516 to the line terminals 643 and 644 and opens, at RA612, the path short-circuiting the transformer winding 629. Thus, the auxiliary transmitting and receiving apparatus is operatively connected to the line 323 over a channel including the link line C515, C516, the link line C296, C291, and the transformer 300.

The purpose of first operating the answer switch Ill and then replacing the hand set of the apparatus 600 upon its supporting element is to prevent the established automatic switch train from being prematurely released. Thus, if the hand set is replaced upon its supporting element before the answer switch I is operated, the loop circuit extending to the operated switch train forming a part of the switching equipment 320 is interrupted. If the last numerical switch of the train, namely, the connector switch is of the calling party release type, the established switch train will release during the interval when the calling loop circuit is open and preceding operation of the answer switch When, however, the answer switch Ill is operated prior to replacement of the hand set upon its supporting element, the relay R210 completes, at RA21I, a multiple loop circuit which precludes the release of the operated switch train.

It will be appreciated that the regular substation transmitter and receiver may be substituted at will for the transmitting microphone |35 and the loud speaker |36 by removing the hand set or receiver from its supporting hook or replacing the same thereon, the three relays R610, R690 and R690 cooperating to eiect the transfer in the exact manner previously described.

As indicated in the introductory portion of the specification, a calling party at the executive substation may utilize the special substation apparatus to initiate a direct call to one of the subordinate lines during the course of a conversation over an established connection to another of the lines extending to the exchange housing the automatic switching equipment 320. If the ensuing conversation is being held over a connection utilizing the regular substation transmitter and receiver embodied in the substation apparatus 600, the second call to the subordinate line may be initiated in the exact` manner described above. For example, if the calling party is using the regular substation apparatus 600 to converse over an established connection extending to the substation B and desires to secure information from the subordinate having an office in which the substation A is located, the direct call to the substation A is initiated by operating the calling switch to the lower olf-normal position in the manner described above. The coding relays R200, R2|0 and R220, the common equipment 4|5 and the rotary switch 3| 0 respond to the operation of the switch |00 to its off-normal position by performing the previously described functions, whereby a talking connection including the loud speaker |36 and the transmitter microphone |35 is established between the executive substation and the substation A. As pointed out previously, one of the functions of the relay R450 is that of impressing ground potential upon the amplifier start conductor C294 thereby to condition the amplifier for operation and to prepare one of the operating circuits for the relay R610. Since, however, the relays R680 and R690 are fully operated when the relay R450 operates, this operating circuit for the relay R610 is not completed, `it being held open at the operated armature RA692. Thus, so long as the hand set is not replaced upon its supporting element the transmitter and receiver thereof are connected to the line 323 for communication thereover. Also, the auxiliary transmitting and receiving meansare operatively connected for use in communicating over the established direct line connection. After the two connections have been established if the hand set is replaced upon its supporting element and is then removed therefrom, the relay R610 is caused to operate to disconnect the auxiliary transmitting and receiving apparatus from the direct line connection, to disconnect the substation apparatus from the line 323 and to connect this apparatus for use in communicating over the established direct llinie connection. More particularly, when the hand set is replaced upon its supporting element, the calling loop circuit is interrupted causing the two relays R630 and R630 sequentially to restore. 'Ihese relays reoperate sequentially when the hand set is thereafter removed from its supporting element, the relay R600 rst closing the X contacts associated with RA633 to complete the previously traced operating circuit for the relay R610. Thus, the relay R610 is caused to reoperate to effect the above-described transfer operation.

If, with the two connections established it is desired lto connect the hand set to the direct line connection without releasing the first connection, namely, that extending by way of certain of the automatic switches of the switching equipment 320, the answer switch and the transfer switch ||0 are sequentially operated to their off-normal positions. When the answer switch I I i is operated the relays R210 and R520 sequentially operate and the relay R540 restores, all in the manner previously explained. The relay R210 completes, at RA21I, the above-traced multiple calling loop circuit, thereby to prevent the release of the operated switches in the equipment 320, and completes, at RA212 and RA214, the above-traced alternative circuit for energizing the lower winding of the relay R610. The relay R610 now operates to complete the previously traced holding circuit for itself and the relay R630. With the three relays R210, R520 and R610 operated the substation apparatus is operatively connected by wayv of the link line C515, C515, the link line C236, C291 and the transformer 300 to the line 323. When the transfer switch 0 is operated following operation of the answer switch the relay R520 restores to dis- -connect the substation apparatus from the line 323 and to connect this apparatus for use in ccmmunicatingover the established direct line connection. Obviously, by operating the transfer switch ||0 between its two positions the substation apparatus 600 may be transferred from one connection to the other at will. Also, by removing the hand set ofthe apparatus 600 from its supporting element and replacing the hand set upon its supporting 'element the hand set and the auxiliary transmitting and receiving means may be interchangeably connected to either of the two established connections. 'I'he switching apparatus is released in the manner set forth above when the operated calling switch |00, the answer switch and the transfer switch ||0 are restored to normal.

If the loud speaker |36 and the transmitter |35 are being used as the receiving and transmitting means in an established connection extending to the substation B, for example, and

it is desired to use the special substation appa.-l

ratus to initiate a call to one of the subordinate lines, as, for example, the line 322, directly accessible to the special substation equipment,` it is necessary to operate the calling switch |00 to the off-normal position corresponding to the line 322 and following this, to operate the transfer switch l0 to its oil-normal position. When the calling switch |00 is operated to the oir-normal position corresponding to the line 322, the apparatus illustrated in Figs. 2, `3, 4 and 5 operates to cause a connection to be extended to the line 322 in the exact manner previously described. In this regard it will be recalled that the three relays R210, R520 and R540 are operated. Hence, the amplifier comprising the vacuum tube 620 is conditioned for operation and the line conductors C515 and C516 incoming to the anti-side-tone impedance network 630 are connectedthrough to the repeating coil 300. The operated condition of the three relays R210, R520 and R540 in no way interferes with the operation of the other relays included inthe common control apparatus 4I5Ato cause the connection to the desired subordinate line 322 to be be established. When the transfer the relay R520 falls back, it disconnects the line conductors C515 and C516 from across the terminals of the transformer winding and connects these line conductors to the respective terminals of the transformer winding 50|, thereby operatively to include the auxiliary transmitting and receiving means comprising the loud speaker |36 and the transmitter microphone in the established connection extending to the called subordinate line 322. The relay R520, upon restoringfalso disconnects the conductor C514 from the conductor C513 and connects it to the conductor C512, thus substituting the line balancing circuit comprising the inductance element 63| and the resistor 632 for the balancing circuit comprising the inductance element 63| and the resistor 632' in the anti-side-tone impedance network 630. With the relay R520 in its restored position, the original circuit for supplying energizing current to the microphone |35 and for holding R540 operated, is interrupted at RA528'. At this time, however, the back bridge relay R5|0 is in its operated position so that a multiple circuit extending from ground at RA5| i' and b-y way 'of RC5|3 and RA532 is completed for maintaining R540 operated and for supplying energizing current to the microphone |35.

It will lbe noted that to effect the transfer operation just described the transfer switch ||0 is operated to its off-normal position without restoring the answer switch toits normal position. So long as the last-mentioned switch is' retained in its off-normal position, the abovetraced circuit by way of Vwhich vthe relay R210 is energized is held completed and. so long as R210 is energized and remains in its operated position, the loop circuit extending tothe substation B and including the line-322 is maintained completed at RA21I. After the two con- 75

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