US1521771A - Telephone-exchange system - Google Patents

Description

Jana I H. L. HOFFMANN TELEPHONE EXCHANGE SYSTEI Filed April 5, 1920 2 Shuts-Shoot 2 I /ZZ /Z5 //7 /20 l/d /z/ Patented Jan. 6, 1925.

UNITED STATES PATENT OFFICE.

HENRY L. HOFFMANN, OF NEW YORK, N.

Y., ASSEG-NOB TO "\VESTERN ELECTRIC COM- PANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEPHONE-EXCHANGE 23 Application filed April 3,

To all whom it may concern:

Be it known that I, HENRY L. I-IOFFMANN, a citizen of the United States, residing at New York, in the county of New York and State of New York, have invented certain new and useful Improvements in Telephone- Exchange Systems, of which the following is a full, clear, concise and exact description.

This invention relates to telephone exchange systems and particularly to systems comprising a toll exchange and an automatic telephone exchange.

An object of the invention is to provide improved and simplified circuit arrangements for use in the extension of connections from a toll exchange to subscribers of an automatic telephone exchange, involving a repeater, comprising means ior repeating mice currents and means for repeating impulses, to be used in connection with an incoming selector at the automatic exchange, said means for repeating voice currents being normally disconnected from the trunlr line and connected thereto when the last switch of the train of switches to be operated, is set.

In systems of this type it is necessary to maintain the voice current repeating means disconnected from the outgoing conductors eve which the impulses are transmitted to said switches, during the time such impulses re being transmitted.

The invention consists in the provision of a circuit arrangement wherein the relay, which switches in the repeater, is included in the test and release wire without interfering with the testing and releasing operations, the arrangement being such that, when the final switch is set on the called line, a circuit change is produced, which causes the i\\lt0liiii,9; relay to operate.

Other no and useful features will be set forth in the following description and the accompanying drawings in which Figs. 1 and 2 illustrate diagrammatically, the means which may be employed by an operator at a toll. exchange in calling a subscriber of an automatic exchange. The two exchanges are connected by trunk lines, one oi which is shown terminating at one end in the re peater C and the other end in a jack 6 at the toll exchange.

1920. Serial No. 371,049.

Referring to Fig. l of the drawings, that portion of the circuit to the left oi? the dotted portion of the trunk is lo ated at the toll exchange a; l that to the right thereof, at the auto-mule exchange. A portion only of the cord 0 nit, at ill, toll cpcratofls position has been shown. since such circuits are well known. It wl he understood that long distance calls for subscribers at the automatic exchange will arrive at the toll operators position and such calls may be extended through the meuiiun oi? cord circuit A and over the trunk inc to the automatic exchange. lhe repeater U comprises a repeating coil 17 for repeating voice currents over the trunk line to the automatic exchange, and a relay 8 for repeating impulses, transmitted by an impulse sender at the operators position, directly to the stepping magnets oi the switches.

The repeater O is located at the automatic exchan e and associated with an incomfirst selector such as D shown in Arg. 2. At F, a connector switch is shown. The station of a subscriber B is shown who switci F. At E, a seconu selector switch is schematically shown There may be a number oi switches such as wired and equipped the same itch l) shown on Fig. 2, interposed between the first selecto and the connector switches. The sets 01 impulse for setting the various switches of the train are successively conveyed directly to the operating magnets oi the switches,

trio impulse relay 8.

The invention will be more readily understood ilrom a detailed description of the operation.

Assuming that the open .tor at the toll eimhange desires to establish a connection with the subscriber B at the automatic exchange, the calling plug oil? cord circuit A. may be inserted into jack 6 of the trunk cirthe autowhereupon relay 8 opercuit extending to the repeater C matic exchange s lower normal contacts of dialing through relays 3 and 4, by

ringing key 7, rings of plug 0 and jack 6, conductor 41, inner normal contacts of relay 12, right-hand winding of relay 8, and resistance 31 to grounded battery. Relay 8 in attracting its armature completes circuits for the energization of slow-torelease relay 9 of the trunk circuit C and. slow-to-release relay 108 of the selector, said circuits extending from ground, through the armature and front contact of relay 8 and thence in parallel to battery, one path extending through the winding of relay 9, and the other through the upper normal contacts 0715 relay 25, conductor 101, inner left-hand normal contacts of relay 109 and winding of relay 108.

The operator may now actuate dialing key 10, thus replacing the loop for relay a loop through and proceed to the contacts 42 of sender i send impulses to select the desired line The sender schematically shown at S is, for the purpose of convenience, represented as the well known dial or rotary type although it is to be understood that it may he oi 11 well known type adapted to cause a llllllfli ity of series of interruptions in a line circuit.

With each interruption of the contacts 4-2 of the sender S, relay 8 will retract its armature thereby completing a circuit from grounded battery, through the winding otl' primary magnet 111, winding it relay 110, front contact and left-hand armature of relay 108, outer left-hand normal contacts of relay 109, conductor 102, lower normal contacts of relay 25, and haclr contact and armature of relay 8 to ground. Magnet 111 in response to these impulses operates to step the selector switch brushes 116, 117 and 118 from group to group of the trunk lines terminating at said switch.

The relay 110 is energized in series with magnet 111, and being of the slow-to-relcase type does not retract its armature during the sending of a series 01? impulses. In attracting its armature relay 110 completes a circuit for relay 113 which then locks over a circuit extending from grounded battery, through the winding of relay 113, front contact and armature of relay 113, vertical oil?- normal contacts 151, which are closed in the first vertical step of the brushes, righthand contacts of secondary magnet 112, and right-hand alternate contacts of relay 108 to ground. Relay 108bcing of the slowto-release type does not retract its armatures during the sending of a series of impulses. At this point it should he noted.

that the left-hand winding of relay 1 1,

which eventually operates over the release conductor 103, is shunted by the direct path to ground at the contacts of relay 108, tor

the battery at the winding of relay 113.

At the termination of the series oi impulses, the circuit of relay 8 1S maintained closed for an appreciable time so that relay 110 and magnet 111 deenergize. The switch stops the selected group of contacts and as relay 110 retracts its armature di 2 to the prolonged interruption of its circuit, a circuit for secondary magnet 112 is completed saiil circuit extending from grounded battery, through the left-hand cont oi magnet 112 normal contacts l l zlit-ilfi'illte contacts of relay 113, contacts 151, r1 t-hand normal contact oi 121a it E12,. and right-hand alternate contacts of relay 108 to ground. Upon energizing, magnet 112 operates to rotate the inches lie, 11? and, 118 one step into eng'einent will, the first set of contacts of Near the end of its M 112 opens its interrupter breaking the locking cir- 11.3. The interrupter contacts however until the test brush contact. Upon de- M re 1?; opens the energizing circuit 0 magnet 112. Magnet 112 retracts arii res to again close its in- ;ter contacts, The operation now deupon whether or not the first trunk the terminals of which are now encd by the selector brushes, is busy or not. l is airi line is busy, ground potential .7 upon the test terminal with 118, in engagement, and .i. serve to maintain relay energized a ad to again energize magnet lilagnet 112 in energizing moves the and 118 into engagement next set ol' cont-acts. It will be mlhil that as long as brush 118 engages grounded or busy contacts, relay 113 will remain one .zed and the stepping magnet t n; oi: relay are not opened achcd the advance the brushes oi the selector D Hay-step. l Vhen an idle or nongrounoed contact is reached, assumed to be the test contact 121, and magnet 112 retracts its armature, relay 113 will retract its armature, thereby completing a circuit tor the relay 109, said circuit extending ti it grounded batt e winding of relay 109, normal contacts of relay 113, off-normal contacts ii, right-hand contacts of magnet 112, r thand alternate contacts of relay 108 to groinid, It will be noted that relay 11 does not energize at this time due to the shunt to around at relay 108 for the hatt ry at the winding oi? relay 109.

Upon energization oi? relay 109, a circuit will be completed for, slow-to-release relay 128 of the connector switch F, said circuit extending from grounded battery, through the winding of relay 128, inner left-hand u nal contacts of relay 136, terminal 125, 1 122, terminal 119, brush 116, inner t-hand alternate contacts of relay 109, conductor 101, upper normal contacts of relay 27 and thence in parallel with slow-tocontacts 42 of the sender will cause the re-* traction of the armature of relay 8 as before. lVith the first retraction of the armature of relay 8, a circuit for slow relay 129 and stepping magnet 131 will be completed, said circuit extending from ground, through the armature and back contact of relay 8, lower normal contacts of relay 25, conductor 102, left-hand alternate contacts of relay 109, brush 117, terminal 120, brush 123, terminal 126, outer left-hand normal contacts or relay 136, left-hand contacts of relay 128, contacts 160, winding of slow-to-release relay 129 and winding of magnet 131 to battery. Magnet 131 will be energized and will move the connector brushes 143, 144, and 145 one step. At the first step of the switch off-normal contacts 160 will be opened and contacts 161 and 162 will be closed. Relay 129 is also energized and being of the slow-torelease type holds its armature attracted throughout the remaining impulses of the tens series which pass through contacts 162 and the alternate contacts of relay 129. In accordance with the remaining impulses of the tens series, magnet 131 is repeatedly energized and deenergized and steps the connector switch brushes 143, 144 and 145 from group to group of the subscribers line contacts terminating therein until the tens group, in which the wanted line terminates, is reached. The circuit for relay 8 will then be maintained closed thus holding open the circuit for relay 129 and magnet 131. Relay 129 then retracts its armature thus disconnecting slow-to-release relay 129 and magnet 131 from the impulse circuit and connecting slow-to-release relay 132 and secondary magnet 133 thereto.

The operator now sets the impulse sender to send the final or units series of impulses. \Vith the first retraction of the armature of relay 8, a circuit is closed through slow-torelease relay 132 and secondary magnet 133 similar to that over which relay 129 and magnet 131 were energized when the tens series of impulses were sent. Magnet 133 and relay 132 are both energized; the former moves the brushes one step and the latter completes a circuit at its front contact and inner armature for relay 134. Relay 134 grounded battery, through the Winding, front contact and right-hand armature of relay 134, and front contact and right-hand armature of relay 128 to ground. Relay 14 of trunk circuit C is not operated due to the shunt to ground at the contacts of relay 128 for the battery at the winding of relay 134.

In accordance with the remaining impulses sent by the sender S, magnet 133 is repeatedly energized and deenergized, moving the brushes from set to set of the line terminal contacts of the selected tens group.

WVhen the connector switch brushes are positioned on the terminals of the called line, the circuit of relay 8 will remain closed, causing it to hold up its armature thus holding open the circuit for relay 132 and magnet 133. Relay 32 being of the slowto-release type does not retract its armatures until the sending of the series of impulses ceases.

Assuming that the called line is idle, in which case battery will be present upon the test terminal 148 of the called line, a circuit will be completed for relay 135, said circuit extending from battery at terminal 148, brush 145, left-hand winding of relay 135, left-hand contacts of relay 134, back contact and outer armature of relay 132, front contact and right-hand armature of relay 134 and front contact and right-hand armature of relay 128 to ground. Relay 135 thereupon attracts its armatures and completes a locking circuit for itself, said circuit extending from grounded battery, through the right-hand winding and inner right-hand armature and front contact of relay 135, and front contact and right-hand armature of relay 128 to ground. A circuit is also completed for relay 136, said circuit extending from grounded battery, through the winding of relay 136, back contact and inner armature of relay 132, front contact and right-hand armature of relay 134, and front contact and right-hand armature of relay 128 to ground.

Relay 136 in attracting its armatures extends the calling station to the conductors of the called station, and opens the circuit for slow-torelease relay 128. Relay 128 in deenergizing, removes the shunt at its right-hand contacts for relay 14 and current through the windings of relay .136, 134 and 135 in parallel flows over the release conductor 103 through the left-hand winding of relay 14 to ground at the left-hand contacts of relay 9 of trunk circuit C. Relays 136, 134 and 135 are thus maintained energized and relay 14 is energized over this circuit. Relay 14, in operating, at its right-hand armature, completes a locking circuit for itself and a circuit for the energization of relay 25, and at its left hand alternate contacts short circuits its iii) left-hand winding, Relay 25 in energizing, connects battery and ground through repeating coil 17 to the trunk conductors 101 and 1.02 now extended through to the called substation B, and closes the primary side of the repeating coil.

The operator may now restore dialing key to normal and then actuate listening key 11 to ascertain it .the called line is busy. Sincethe called station is idle, no busy tone will beheard. The operator may then actuate ringing key 7, thereby completing a circuit for ringing torrent responsive relay 18, said circuit extending from ground, through the upper alternate contacts of key 7, tips of jack 5 and plug 6,.conductor 40, upper letbhand winding of repeating coil 17, winding of relay 18, condenser 19, lowermost alternate contacts of relay 25, lower left-hand winding of repeating coil 17 conductor 41,, rings of jack .6 and plug 5 and lower alternate contacts or key 17 to source of alternating current 1. Relay 18 thereupon energizes and completes a circuitfor relay 16, said circuit extending from ground, through the contacts of relay 18, winding of relay 16, and left-hand \i'inding of relay to grounded battery. .lela ys l6 and 12 are thereby energized, the former serving to project ringing current from the source 50 over the talking conductors 102 and 101 to the called substation, and the latter serv ing to .discom'iect battery and ground from the conductors 4.0 and 41 of the trunk circuit C, thereby maintaining relay 8 energi-zed through the alternate contacts of relay '12. Restoration of key 7 to normal releases relays E18, 16 and 12.

Upon the. response of the called subscriber, relay of trunk circuit C is energized over a circuit extending from grounded battery, winding of relay 15, resistance 21, lower right-hand winding of repeating coil 17, 'lmver normal contacts of relay 16, lower .al- :ternate contacts of relay 25,, and thence over conductor 102 through to the substation loop and back to conductor 101, through the upper alternate contactsot relay 25, upper normal contacts of relay 16, upper' righthand winding of repeating coil 17, and re ista-ucc to ground. Relay 15 in energizingcompletcs a circuit at its contacts for relay 12 which in energizing again i eniovcs battery and ground from conductors i0 and 41, thus decnergizing relay 3, and it in turn extinguishing lamp 2, thus indicating to the operator that .the called station ll has answered,

Station B is now connected for conversation, talking battery for said station being supp'liei'l through resistances 20 and 21, and the right-hand windings of repeating coil 11.7.

lluring the time that the called subscriber ilias the receiver removed vfrom the switchhook, the operator may change cords with out releasing the connection since relay 12 will be energized thus maintaining relay 8 operated.

Upon completion of conversation and restoration of the receiver to the switchhook at station B, relay 15 deenergizes and in turn. relay 12. The latter in deenergizing connects battery and ground through relay 8 to and the normal coutactsof relay 12 to relay .3 of thecord circuit A. Relay 3 thereupon energizes, thus lighting lamp 2 and indicating to the operator that the subscribcro't station B has restored the receiver to the switchliook.

The operator ma. then remove plug 5 from jack (3, there y opening the circuit for relay 8. Relay 8 then retracts its armature thus opening the circuit for slowetor elease relay 9. Relay 9, in deenergizing, releases relays 1.4 and and rei-noves ground from release conductor 1053 thereby releasing relay 109 of selector 1) and relays 136, 1.35 and 13 1 of connector F. Belay 109, in releasing,completes a circuit for release magnet l 15over a path extending fromgrounded battery, through the winding of magnet 11.5, nit-normal contacts [50, right-halal armature and'back contact of relay 10$), nnd front contact and armaturcoi relay 108 to ground. ,Upon actuation, magnet 1.15 restores the selector switch to v.i'lornial in the well ki iown manner. Helay 136 of connector I, in releasing, complctes a circuit for release magnet 130 over a path extending from grounded battery, through the winding of magnet 130, off-nor- -mal contacts 161, rigl-it-lijand armature and back contacts of relay 136, and back con tact and right hand armature of relay 128 to ground. Upon actuation, magnet 130 restores the rwnnector switch .F to normal.

Assuming that the wanted line busy, in which case the test-,contact 148 will have a ground potential thereon, relay 135 will not operate. Relay 1.36 will energize however as soon asrelay .132'releases, and relay 134 will bemaintainedenergized over paths heretofore described in the case of a successful call.

As soon as relay 1% energizes, v,current will flow over the releaseconductor through the winding of inlay it to energize relay 14 as before; relay 14 in energizing locks up and completes a circuit for relay ;25, .thereby rendering the trunk circuit continuous.

A busy tone will 111,181}lJGfillilQSllllttQtl.0X81 a path extending tromgrouod, through the secondary winding 14,1 of the busy signal device, condenser 1&0, outer left-hand normal contacts of relay 135, inner left-hand alternate contactsot relay 136, terminal 125, brush 122. terminal 119, brush 116, inner left-hand alternate contacts of relay 109, conductor .101, upper alternat contactsof Jun relay 25, upper normal contacts of relay 16, upper left-hand winding of repeating coil 17, and resistance 20 to ground. This tone in the secondary of repeating coil 17 is induced into the primary. The operator after having operated sender S may restore dialing key to normal and actuate key 11 to connect her set (not shown) to the cord circuit whereupon she will hear the busy tone indicating to her that the called line is busy.

The operator may then withdraw plug 5 from jack 6 whereupon all relays of trunk circuit C are released and relay 109 of selector switch C and relays 136 and 134: of connector switch F are released as heretofore described. The release magnets of selector switch D and connector switch F are thereupon actuated and restore their respective switches to normal.

hat is claimed is:

1. In a telephone exchange system, an operators position, sender, an automatic telephone exchange called subscribers line circuit, a connecting circuit including a trunk line, a repeater, and a train of automatic switches for establishing a connection between said positionand said line circuit, said repeater being provided with a repeating coil normally disconnected from said connecting circuit for repeating voice currents and also with means for repeating impulses from said sender to operate said switches, electromagnetic switching means adapted upon energization to connect said repeating coil to said connecting circuit, and means for operating said switching means as soon as connection is established with the called line circuit.

2. In a telephone exchange system, an operators position, an automatic telephone exchange called subscribers line circuit, senders therefor, a connecting circuit including a trunk line, a repeater, and a train of automatic switches for establishing a connection between said position and said line circuit, said repeater being provided with a repeating coil normally disconnected from said connecting circuit for repeating voice currents and also with means for repeating impulses from said sender at said operators position to operate said switches, relays adapted upon energization to connect said repeating coil to said connecting circuit and means for operating said relays as soon as connection is established with the called line circuit.

3. In a telephone exchange system, a calling station, a called station, senders therefor, a connecting circuit including auto matic switches, a repeating relay in said circuit operated by said sender at said calling station to set said switches, a repeating coil in said connecting circuit having a cc of talking battery connected thereto,

repeating coil being normally disconnected from the called end oi the connecting circuit, and means for connecting said re pcating coil in circuit with the called station as soon the last series of impulses has he ,1 transmitted.

In a telephone exchange system, a calling s ation, :1 called station, means including a repeater and a train of automatic switches for interconnecting said stations, a relay associated with the first switch of the train, means for preventing the operation of said relay including a circuit at each succeeding switch, and means for operating said relay as soon as the last switch of the train is set.

5. In a telephone exchange system, a calling station, a called station, senders therefor, a connecting circuit including a train of automatic switches, a repeating relay in said circuit operated by the sender of said calling station to set said switches, a repeating coil between said relay and said switches, having a source of talking battery connected thereto, said repeating coil being normally disconnected from the called and of the connecting circuit, relays associated with said connecting circuit adapted upon energization to connect said repeating coil to the called end of the connecting circuit; and means for energizing said relays as soon as the last series of impulses has been transmitted.

6. In a. telephone exchange system, a manual exchange, an automatic exchange, a two conductor trunk circuit extending between said exchanges, means including a repeater and a train of automatic switches for establishing a connection between an operators position at said manual exchange and a subscribers line terminating at said automatic exchange, a pair of talking c0nductors and a third conductor for each of said automatic switches, electromagnetic switching means associated with the first switch of said train, a circuit for said switching means successively extended over said third conductor through said train of switches, and means for operating said,

switching means over said circuit as soon as the last switch of the train is set.

7. In a telephone exchange system, an operators position, a called station, means tor connecting said operators position with said called station. including a train of automatic switches, a pair of talking conductors and a release conductor for each of said switches, a source of talking battery at the first switch of said train, a relay associated with the first switch of the train adapted upon energization to connect said source of talking current to the talking conductors of said switches, a second relay associated with the first switch of the train adapted upon energization to energize said first mentioned relay, means for preventing the operation of said second relay, including a circuit at each succe 'ug switch, duritches and means inc; the setting oi said i I and relay soon as for 1 train is set on the energizing saio the last switch oi the called line.

8. in a telephone exchange system, an operat n."s po. on, a called station. means in'ouding a ii iii: circuit and a train of automatic switches for establii ihingg a connection between said position and said called station, a relay associated with said trunl; circuit, mean assiiiciaied with each switch of h train tor prevcntin the operation of said relay during the establishment oi the connection, and means for oper" ating: said relay as soon as the terminals of the called line have been reached.

9.121 a telephone exchange c stom, a calli and called station. means including a train of switches for interconnecting said stations, a pair of talking coi'iductors and a third conductor for each of said switches, a relay associated with the first switch of said train, adapted to be operated over said third conductor, means at each of said switches for preventing; the operation of said relay over said third conductor until the last switch of said train has been set and means for operating said relay as soon as the last switch of? the train is set upon the terminals of the called station.

10. In a telephone system. a series of automatic switches inchidina' talking conductors and a test conduct 3 a reiieater at the first of said switches comprising means for re eatin im ulses to succccdin switcles and for repeating voice currents, said voice current repeating means being; normally disconnected troin the talking: conductors. means for operating said impulse repeating means to transmit impulses over said talking conductors beyond said repeater, to set said swltches and a relay included 111 said test conductor at said first switch and operating when the last set o! innnilses is tliLDSDfllttOCl to connect said voice current voice current repeater being normally disconnected from the talking conductors, means for operating said impulse repeat-- ing means to transmit impulses over said talking; conductor beyond. said repeater to set said switches, a relay included in said test conductor at said first switch shunted while the connection is progressing from switch to switch, and means for opening sald shunt when the last switch is set to 13. In a telephone exchange system, an

operatoi"s cord circuit, an automatic switch. an in'ipulse Sel'lClGI associated with said cord circuit for setting said switch, a repeating coil between said sender and said switch conductively separating said cord circuit and said switch, and means for automatically connecting said coil in circuit with said cord circuit and said switch as soon as said switch has been set.

'14. In a telephone exchange system, an operators cord circuit, an automatic switch. :1 called station, an impulse sender associated with said cord circuit for setting said switch on the terminals of said called station, a repeating coil between said cord circuit and said switch, means for connecting said coil in circuit with. said cord circuit and said called station as soon as the terminals of the called station are reached, means for informing the operator of the busy condition ort the line of the called station, and operator controlled means for ringing the called station it the line of the called station is idle.

15. In a telephone exchange system, a calling; station, a train of switches, a dial for setting said switches, sources of direct current associated with the first switch of the train, a repeatingcoil between said dial and said train of switches conductively separating: said sources of direct currenhand means for automatically connecting said coil in circuit with said switches as soon as the last switch of the train has been set.

16. In a telephone exchange system, a calling station, a called station, a trunk circuit, means includingia train of switches and said trunk circuit For interconnecting said stations, a repeating coil in said trunk circuit, a lay-pass around said repeating coil over which said switches are set by said calling: station, and means for connecting; said repeating coil in circuit with said calling and called stations as soon as the last switch of thetrain is set. I

17. In a telephone exchange system, a. first ofiice, a cord circuit therea-t, a second oflice, a trunk circuit extending between said oflices, a repeating coil, a train of switches and a called station at said second ofiice, an impulse sender for the cord circuit, a bypass around said repeating coil for oontrollllt) ling said switches, and means operative as soon as the last switch of the train is set to open said bypass and to connect said repeating coil in circuit with said cord circuit and said called station.

18. In a telephone exchange system, a first exchange, a second exchange, a trunk circuit extending between said exchanges, an operators position at the first exchange, a called station at the second exchange, a train of switches at the second exchange, means including an impulse sender at the first exchange for setting said switches over said trunk circuit, sources of talking and signaling current at the second exchange associated with said trunk circuit, a repeating coil in said trunk circuit conductively separating said sources of current and having its primary and secondary windings normally disconnected from the ends of the trunk circuit, and means operating as soon as the both windings of the coil in the trunk circuit.

19. In a telephone system, subscribers lines, an operators position, a supervisory signal thereat, a trunk circuit terminating at one end before said operators position, an automatic switch at the other end oi said trunk circuit, means at the operators position for connecting with said trunk circuit, means for sending impulses over said trunk circuit, means responsive thereto for operating said switch to extend said trunk to a called line, said trunk being open at both ends during the transmission of impulses, and means operative as soon as said switch has been set for closing both ends of said. trunk to place said supervisory signal under the control of the called subscriber.

In witness whereof, I hereunto subscribe my name this 31st day of March A. D., 1920.

HENRY L. HOFFMANN.

Images