US1231085A - Telephone-exchange system. - Google Patents

Description

ALB. SPERRY.. 'TELEPHONE EXCHANGE SYSTEM.

APPLICATION FILED SEPT. 22. i916.

Patented June 26, 1917.

, A. B. SPERRY.

TELEPHONE EXCHANGE SYSTEM.

A APPLICATION FILED SEPT. 22. 1916. 1,231,085, Patented Junez, 1917.

A. 3 SHEETS-SHEET 2.

205 AJC-20a 4 III-IIl A. B. SPERRY. `TEUEPHONE EXCHANGE SYSTEM.

APPLICATION FILED SEPT-22. |916.

Patented June 26, 1917.

3 SHEETS-SHEET 3'.

UNITED STATES PATENT OFFICE.

ARTHUR B. SPERRY, OF NEW YORK. N. Y., ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 0F NEW YORK.

TELEPHONE-EXCHANGE SYSTEM.

Specification of Letters atent.

Patented June 26, 1917.

` Application filed September 22, 1916. Serial No. 121,568.

To all u'lmmit 'may concern.'

lie il known that. l, Au'ruun B. Srnlun', a. citizen of the United States, residing at New York, in the count)y of New York and State of New York, have invented certain new and useful Improvements in Telephonellxchange Systems, of which the following is a full, clear, concise, vand exact description.

This invention relates to telephone eX- change systems employing machine switching, and particularly to that type of system wherein a connection leads through a number of oliices which are interconnected by two-wire trunks, and wherein a set of repeating relays is located vbefore the first selector to repeat impulses from the sender to the automatic switches.

Its object is to provide a. system of this character wherein the impulses are repeated over a local grounded circuit from the repeating relays to the switches in the originating oliice, and to provide for automa-tically changing the control circuit from a grounded circuit to a metallic circuit when a trunk is selected leading to a distant oice.

ln accordance with this invention, an electromagnetic switch is associated with the repeating-relay set to control the character of the impulse circuit, this switch being in turn controlled by a device connected to the test wire of one of the switches at the origina-ting office in such a way that, when such switch is set upon a trunk level leadingr to a distant oliice, the electromagnetic switch is operated by said device to change the impulse circuit from a grounded circuit to a metallic circuit. Preferably, the electromagnetic switch consists of adouble-wound differential relay which, as long as a connection is lacing set up over a local grounded circuit including switches at the originating oiiice, remains inert as current is iiowing through its windings in opposite directions. When, however, a trunk group is selected leading to a distant. oiice, the said device shunts one of the windings, and the relay thereupon energizes and includes a marginal relay at the originating office in circuit with a repeating relay at the distant oliice, the circuit being completed at an armature and contact of the stepping relay at the originating oliice. Thus, as the sender is intermittently operating the stepping relay at the originating oice to set the switches at such otce,

the impulses are transmitted over a grounded circuit includingr au armature and hack contact of the stepping rela-y at such ollice. lVhen, however, the outgoing trunk group is selected and the differential relay operates, the impulses are transmitted over a metallic circuit leading to the distant office and controlled at an armature and front contact of the stepping relay at the originating oiice.

Referring to the drawings, Figures 1, 2 and 3 taken together, illustrate one embodiment of the invention.

The invention will be best understood by describing in detail, the operations involved in setting up a connection. Upon the removal of the calling subscribers receiver, the line relay LR is energized over the line circuit, and in attracting its inner armature, grounds the multiple 100 of the calling line in the finder, and in attracting its outer armature, grounds commutator segment 101 of the group to which substation A belongs. The attraction of the outer armature of relay LR also energizes relay 102, common to the group of lines to which substation A belongs, over the following circuit: free pole of battery, relay 102, outer armature and front contact of relay LR, conductor 103, left armature and back contact of slow relay 104 to ground. Relay 102, in attracting its left armature, locks itself up and closes a circuit for relay LR, independently of the initial energizing circuit leading to the calling substation, whereby, even though the receiver is replaced upon the hook, the finder will complete its operation. The attraction of the right armature of relay 102 closes the following circuit for primary magnet PM to drive the linder in a primary direction: free pole of battery, magnet PM, armature and back contact of said magnet, side switch arm 105 and its first contact, armature and back contact of the escape magnet 106 of the finder, armature and-back contact of release magnet RM of the finder, armature and back contact of` slow relay 107, relay 104 to ground, at thc right armature and front contact of relay 102. Primary magnet PM steps around the brush shaft 108, until the commutator brush 109 thereon reaches the grounded segment 101, whereupon the following circuit is closed to energize the escape magnet to pass the side switch into position 2: free pole of battery, magnet PM, arm 110, escape magnet 106, conductor 111,

' 101 to ground arm 112 and its first contact, conductor 113, commutator brush 109, commutator segment at the left armature and front contact of relay 102. The primary magnet locks up in series with the escape magnet when this circuit is closed, thus positively positioning tbebrush shaft. As the side switch moves into position 2, the escape magnet denergizes, and in position 2 the following circuit is closed for the secondary magnet SM to step the brushes 114, 115, 116 over the multiples 117, 100 and 118 containing those of the calling line: free pole of battery, magnet SM and its armature and back contact, arm 105 and its second contact, armature and back contact of magnet 106, armature and back contact of magnet RM, left armature and back contact of relay 107, conductor 138, relay 104 to ground at the right armature and front contact of relay 102. During this movement the right winding of test relay TR is in parallel with the secondary magnet SM, and these magnets energize together. At the end of the stroke of the armature of magnet SM, the

test brush 116 of the line finder is in engagement with a multiple 118. If this multiple is grounded. as would be the line to which it belongs were engaged in conversation, the relay TR would remain energized by its left winding and side switch arm 112, and in maintaining its armature attracted, would prevent the escape magnet from energizing over a circuit including brush 115 and multiple 100. However, mul tiple 118 of the calling line is not grounded,

' and consequently when the brushes 114, 115,

116 engage the multiples 117, 100 and 118 thereof, the following circuit is closed to energize the escapement magnet 106 to pass the side switch into position 3: free pole of battery, magnet SM, arm 110 and its second contact, magnet 106, armature and back contact of relay TR, arm 119, brush 115, multiple 100, inner armature and front contact of relay LR to ground at the left armature and front contact of relay 102. Magnets SM and 106 are locked in this circuit until the side switch passes out of its second position. In position 3 arm 112 closes the circuit of the cut-off relay CO, and grounds multiple 118 of the calling line to cause the same to test busy to other finders and to other connectors. The relay CO, in attracting its armatures, denergizes the relay LR, whichv in retracting its armatures removes the ground from multiples 100 and commutator lsegment 101 to render the line and group non-calling. As the side switch moves into position 3 the following circuit is closed to energize a stepping relay 121: free pole of battery, left winding of relay 121, arm 119 and its third contact, brush 115, multiple 100, through the telephonie apparatus at substation A, multiple 117, brush 114, arm

the case if4 120, right winding of relay 121 to ground. Relay 121 in attracting its armature, energizcs a slow relay 122, which, in attracting its right armature, removes ground from the conductor 123 leading to the release magnet RM to prevent premature release. Relay 104, being slow to release its armatures, is maintained energized continuously during the setting of the inder switch and does not retract its armatures completely until after the relay 122 has become energized. Thus thc ground on the back contact of thc relay 122 is not transmitted to the release magnet RM. The attraction of the right armature of relay 122 closes the following circuit for release relay RR, (Fig. 2) of the irst selector: grounded right armature and front contact of relay 122, right armature and back contact of relay 107, conductor 140, right winding of relay DR, conductors 141, 132, 213, armature and back contact of magnet RM, relay R'R to free pole of battery. Simultaneously with the closure of this circuit a circuit is closed through both windings of relay DR as follows: free pole of battery, windings of relay DR, to grounded conductor 140. Relay DR is differentially wound and is not adapted to attract its armatures under the above condition. Relay RR is energized by the closure of this circuit however, and relay 122, in attracting its left armature, connects the back contact of the stepping relay 121 to conductor 124 leading to the primary magnet PM of the first selector FS 2).

The calling party now operates his sender to transmit the first digit of the wanted number. This sender interrupts the line circuit in the well-known manner, and each time the relay 121 denergizes, an impulse is sent over the following path to the primary magnet PM, of the irst selector; ground, armature and back contact of relay 121, left armature and front contact of slow relay 122, conductors 124, 125, right armature and lower contact of relay DR, conductors 126, 200, side switch arm 201 of the first selector, left armature and front contact of release relay RR, magnet PM, free pole of battery. Relay 127 in parallel in the circuit just traced, being slow acting, remains energized during the transmission of the impulses. This relay disconnects the condensers 128, 129 from the conductors 126, 130 to prevent the discharge thereof from interfering with the character of the impulses. The attraction ofthe lower armature of relay 127 energizes the escapement magnet 240 of the first selector over the following circuit: ground, lower armature and front contact of 'relay 127, con- y ductors 130, 202, arm 203 and its first contact, magnet 240, right armature and Vfront contact of relay RR, release magnet RM, free pole of battery. The escape magnet energizcs when this circuit is closed, but the release magnet RM is marginal and (loes not. Upon the restoration of the. sender S,^tlie relay 121 (Fig. 1) remains energized, and after a short interval the slow relay 127 retracts its armatures, the retraction of the lower armature breaking the circuit of the escape magnet 240 of the first'selector, which thereupon passes the side switch into the second position. In position 2 arm 204 completes the circuit of the secondary magnet SM, to drive the brushes 205, 206, 207 over the multiples 208, 209, 210 of the selected group of trunks.

When the first selector side switch reaches position 2, the escape magnet 240 is again energized over side switch arm 211 and secondary off -normal contact CON. This contact opens on the first secondary step and the escape magnet is dependent for its maintained energization upon the grounds of the busy test multiples 210, as will hereinafter appear. When the first ungrounded multiple 210 is reached, escape magnet 240 becomes denergized, releasing the side switch into position 3. Arm 204 in moving off its second contact, opens the stepping circuit, thus stopping the selector upon the seized trunk. When arm 211 reaches its' third contact, the relay R2R2 of the second selector is energized over the following circuit to interpose a break in the circuit of release magnet R2M2 and connect the primary magnet P2M2 to the back Contact of the stepping relay 121: free pole of battery, relay R2,R2, armature and back contact of magnet R2M2, conductor 212, multiple 210, brush 207, arm 211, conductor 213, conductor 132, right winding of relay DR to ground at the right armature and front contact of relay 122.

The calling party now operates the sender to transmit the second digit of the called number. Each time the relay 121 retracts its armature, an impulse is transmitted over the circuit previously traced up to the side switch arm 201 of the first selector, from whence it passes over the third contactof such side switch arm to brush 205, multiple 208, side switch arm 215 and its first contact of the side switch of the second selector, left armature and front contact of relay RZRZ, magnet PZMZ, free pole of battery. Magnet P2M2 thus steps the brush shaft of the second selector in a primary direction to position a set of brushes in operative relation to the desired outgoing-trunk group. As before, the slow relay 127 energizes upon the i'irst impulse and remains energized during the transmission of impulses. This relay again disconnects the condensers and grounds the lower talking conductor to energize theescape magnet 217 of the side switch of the second selector. This circuit is as follows: ground, lower armature and ungrounded, z'. e., idle,

front contact of relay 127, conductors 130, 202, arm 203 and its third contact, brush 206, multiple 209, arm 21S and its first contact, magnet 217, right armature and front contact of relay RZRZ, magnet 'R2M2, free pole of battery. When the sender restores and maintains 'the line circuit closed, the resultant continued energization of the relay 121 causes the slow-relay 127 to become denergized and remove ground from the lower talking conductor. This results in the denergization of the escape magnet 217 of the second selector which passes the side switch into the second position. The brushes 219, 220, 221 are now in operative relation to the desired group of trunks and assuming this group leads to a distant office, a commutator brush 216 secured to the brush shaft 226, and grounded over arm 229 and its second contact, is connected to a conductor 227 joined to the test wire 228. This ground shunts the right winding of differenti-al relay DR, resulting in the energization of this relay to change over the sending circuit from a local grounded sending circuit to a metallic interoflice sending circuit. Rela DR is Wound to hold up over both its win ings in series to ground at the right armature and front contact of relay 122 when once energized, independently of the ground on the test wire 228 applied by the commutator brush 216, this ground being removed upon the rst secondary step of the second selector brush shaft.` In position 2 secondary magnet S2M2 steps the brushes 219, 220, 221 over the selected group of trunks 222, 223, 224. The tion is again energized over secondary offnormal contact S2O2N2 and side switch arm 225. When the test brush 221 reaches an trunk multiple 224` the escape magnet becomes denergized and passes the side switch into position 3, in which position arm 225 grounds the test multiples 224 of the seized trunk.

When the arms 215218 of the second selector reach their third contacts, the following circuit is closed to energize the repeating relay 300 vat the distant office: free pole of battery, left winding of relay 133, (Fig. 1) lower armature and back contact of relay 127, conductor 130, conductor 202, arm 203 and its third contact, brush 206, multiple 209, arm 218, brush 220, multiple 223, conductor 301, lower left-hand winding of repeating coil RC, both windings of relay 300, upper left-hand winding of repeating coil RC, conductor 302, multiple 222, brush 219, arm 215, multiple 208, brush 205, arm 201, conductor 200, conductor 126, upper armature and back contact of relay 127, right winding of relay 133, left armature and front contact of relay DR, left armature and back contact of relay 134 to ground at the armature and front contact of relay escape magnet in this posi-y .121. 'llxe relay 133 is marginal and does not become energized when this circuit is closed. due to the presence of the high-resistance right-hand winding of relay 300. Relay 300, however, energizes and in attracting its armature, encrgizes a slow relay Ril. which connects a slow relay 303 to the back contact of relay 300, and interposes a break in the circuit of release magnet RRMB.

The calling party now operates his sender to transmit the next digit of the wanted number. Duc to the energized condition of the relay DR, the relay 121, in operating in response to the sender, interrupts the metallic circuit traced leading to the distant oliice and including the repeating relay 300.

Relay 300 denerglzes upon each retractionv -of the armature of relay 121, and transmits an impulse to the primary magnet IRM3 of the incoming" selector IS over the following circuit: ground, armature and back contact ot' relay slow' relay RiRi, conductors 305, 352, arms 314, 315 and their iirst contacts, magnet I3M3, free pole of battery. The magnet 1MM3 steps the brushes 307, 308, 309 in a primary direction in operative relation to the desired group of trunk terminals 310, 311, 312. At the first retraction of the armature of relay 300, slow relay 303 is energized, and in attracting its armature, energizes the escape magnet 313 of the incoming selector side switch. This circuit is as follows: ground, armature and front contact of relay 303, arm 306 and its iirst contact, arm 318 and its rst contact, conductor 316, escape magnet 313, free pole of battery. Upon the restoratlon of the sender and the consequent maintained energization iof relay 121, relay 300 remains energized, and after an interval. slow `relay 303 retracts its armature, denergizing the escape magnet 313 which passes the side switch into position 2. In this position arm 316' closes the circuit of the secondary magnet SM, which steps the brushes 307, 308 and 309 over the multiples 310, 311 and 312 of the selected group. In position 2. arm 317 energizes the escape magnet over the secondary otlnormal contact S3O3N3. When an ungrounded multiple 312 is reached, escape magnet 313 becomes denergized and passes the side switch into position 3, in which position arm 317 grounds the test multiples 312 to cause the seized trunk to test busy to other incoming selectors and energizes relay RR which interposes a break in the circuit of release magnet R-M4 of the connector C. The side switch of the incoming selector remains in position 3 during the setting of the connector until the response of the called party.

The calling subscriber now operates his sender t0 transmit the tens digit of the called number, and the relay 300 is again in- 300, armature and front contact of` terlnittcntly operated, as described. Each retraction of the arma-ture of this relay transmits an impulse to the primary magnet IHM1 of the connector over the following circuit: ground, armature and back contact of relay 300, armature and front contact oi. relay R3R3, conductor 305, arm 314 and its third contact, arm 315 and its thir'd contact, brush 307, multiple 310, conductor 319, arm 320 and its first contact, left armature and back contact of busy relay BR, arm 321 and its first contact, magnet I4M4, free pole of battery. In parallel with the circuit just traced a slow relay 322 is energized and remains energized during the transmission of the impulses. This relay, in operating, energizes the connector side switch escape magnet 323. When the sender restores and maintains relay 121 (Fig. 1) energized, relay 300 (Fig. 3) remains energized, and after an interval slow relay 322 retracts its armatures. This relay in retracting its armature opens the circuit of the escape magnet 323 and the escape magnet deenergizes,

passing the side switch into position 2. In position 2 the arm 321 substitutes secondary magnet SM4 for the primary magnet 1MM4 in the stepping circuit. Consequently, when the calling party operates his sender to transmit the units digit of the wanted number, and causes the intermittent operation of relay 300, an impulse is sent upon each retraction of the armature of such relay over the circuit previously traced to the side switch arm 321, from whence it passes to the secondary magnet S*M4. When the sender restores and maintains the relay 121 energized, the relay 300 remains energized, and after an interval, slow relay 322 retracts its armature, again denergizing the escape magnet 323 which passes the side switch into position 3. In position 3, assuming the line to be idle, ringing current is applied to the called line. In this position arm 339 applies ground to test multiples 338 of the called line to cause the same to test busy to other and operates the cutoff relay of connectors, such line to remove the control of the line called relay of the selected line from the party. In position 3 the slow relay 322 is again energized over arms 321 and 325. ,This relay, in attracting its armature, again energizes the escape magnet, which in attracting its armature, applies ringing current to the called line over the following circuit: free pole of ringing generator 324, ringing cutoi relay RGO, armature and front contact of magnet 323, arm 336, brush 327, multiple 328. bell 329, condenser 330, multiple 331, brush 332, 'arm 333 and its third contact, conductor 334, arm 325 to ground.

When the called party removes his receiver, the ringing cutoff relay RCD energizes, opening the circuit of escapement magnet 323, whichy in denergizing, passes the side switch into however, being slow, holds its armature attracted after the side switch arm 335 reaches its fourth contact which results-in an impulse being transmitted over the following circuit to operate the escape magnet 313 of the incoming selector: grounded spring 356 and armature of relay 322, arm l335 and its fourth contact, conductor 350, terminal 311, brush 308, arm 318 and its third contact, arm 306 and its third contact, conductor 351, armature and back contact of relay 303, magnet 313 to free pole of battery. This magnet is momentarily operated to pass the side switch to position 4. In position 4 the relay 337 is includedin circuit with the called line over the following circuit: free pole of battery, left winding of relay 337, lower `right winding of repeating coil RC, arm 306 and its fourth contact, arm 318 and its fourth contact, brush 308, multiple 311, arm 336 and its fourth contact, brush 327, multiple 328, through the telephonie apparatus at substation B, multiple 331, brush 332, arm` 333 and its fourth Contact, multiple 310, brush 307, arm 315 and its fourth contact, arm 314 and its fourth contact, upper winding of repeating coil RC, right winding of relay 337 to ground. Relay 337, in attracting its armature, short-circuits the highresistance right winding of the relay 300. The resultant increase in current causes the marginal relay 133 (Fig. 1) to energize. This relay, in attracting its armature, energizes the slow relay 134 which, in attracting its left armature, connects ground to the right winding of relay 133, so that this relay is maintained energized independently of relay 121 controlled by the calling party. Relay 134, in attracting its right armature, grounds the test conductor 132 from which the release relays RR are branched oil. The release of the switches except the finder is thus transferred to the called party as soon as he responds.A The attraction of this armature also provides a new holding circuit for the relay DR and energizes a slow relay 107, the function of which will be described.

Talking current is fed to the calling party over-the of relay 121. `The condensers 128, 129 inductively connect the calling end of the circuit with the middle section, and the repeating coil RC inductively connects the called end of the circuit with .the middle section. The subscribers may now converse.

The calling magnet pole of battery, normal contact replacement of the receiver by the arty closes the circuit of release M of the finder as follows: free magnet RM, primary oil- PON, right armature and back contact of relay 104, right armature and back contact of relay 122 to ground. The replacement of the receiver by the position 4. Relay 322, v

'RR of the connector.

vgization of the relay called party denergizes relay 337 which in retracting its armature, opens the shortcircuit about the high-resistance right winding of relay 300. The inclusion of this high-resistance winding in the circuit of marginal relay 133 causes this relay to deenergize. In retracting its armature this relay opens the circuit of relay 134, which, in retracting its right armature, removes ground from the test wire 132, from which release relays RR and RZR2 of the first and second selectors, respectively, are branched ofi'. These relays in retracting their armatures, close the circuits of release magnets RM, and R2M2, which thereupon Withdraw the holding pawls from the brush shafts, and when such shafts reach their normal positions, the opening of contacts PON and P2O2N2 causes the denergization of these magnets. These release magnets restore the side switch arms in the well-known manner.

The `release of these selectors denergizes the relay 300. Relay 300, in retracting its armature, opens the circuit of slow relay RBRS. This relay, after a slight interval, retracts its armature, closing the following circuit for the release magnet R3M3: free pole of battery, magnet R3M3, off-normal contact P3O3N3, armatures and back contacts of relays R3Ra and 300 to ground. Relay RSMS, in operating, restores the brush shaft of the incoming selector, and in attracting its right armature, locks itself up. When the brush shaft reaches its normal position and opens contact P3AO3N3, this locking circuit is opened to denergize the release magnet. The release magnet, in operating, restores the side switch in the well-known manner. Arm 317 of the incoming-selector position, opens the circuit of release relay This relay, in retracting its armature, closes the circuit of release magnet RM* of the connector, over off-normal contact PON which opens when the connector brush shaft reaches its normal position.

The function of the slow relay 107 (Fig. 1) will now be explained. Whilethe finder is operating, the group relay 102 interposes a break in the in-starter wire 135 to prevent interference, and when the finder makes connection with the calling line A and operates the cutoff relay, the lresultant dener- 102 restablishes the continuity of the in-starter wire. A call then comin in from another group would pass over tile following circuit to the next idle finder: in-starter wire 135, right armature and back contact of relay 102, slow relay 104, armature and back contact of relay 107. armature and back contact of magnet RM. armature and back contact of magnet 106, side switch arm 105 and its tary, ycontact of relay RGO, arm 335 and its secthird contact, out-starter wire 136 to the primary stepping magnet of the next idle finder. The slow relay 104, in attracting its left armature, removes the control of the group relay 102 from the line relays of the group to which substation A belongs. The relay 107, while included in this circuit, does not energize, due to the presence of resistance 137. When the called vparty at substation B responds and applies ground to relay 107, in parallel with the resistance 137 this relay responds and connects the instarter wire 135 to the out-starter wire 136, independently of the sideA switch arm 105. If the called party replaces his receiver when the in-starter wire 135 is grounded, by reason of an incoming call from another group, and the calling party then replaces his receiver, relay 107, although its original energizing circuit is opened by the retraction of the armature of relay 134, does not become denergized since the circuit characteristics are such that the relay 107, while not energizing in series with the resistance 137, will hold up through this resistance. Consequently, the replacement of the receiver by the calling party at substation A, although causing the release of the finder and the restoration of the side switch arm 105, does not interfere with the starting circuit to the inder beyond, since the relay 107 maintains its armature attracted to provide a circuit in substitution of that leading through the arm 105. When the ground 1s removed from the in-starter wire 135, due to the finder having found the line initiating the call, relay 107 becomes denergized and reconnects conductor 138 to the side switch arm 105.

If the selected line had been busy either as a called or as a calling line, the test multiples 338 thereof would have been grounded either over the side switch arm 112 of the finder (Fig. 1), or over the side switch arm 339 of the connector (Fig. 3). When the connector test brush 340 is set upon such grounded multiple 338, a circuit is closed for a busy relay BR, in series with the escape magnet 323, this circuit for the escape magnet being closed before the original energizing circuit for the same is opened as spring 356 of slow relay moves out of engagement with the armature of such relay, and is as follows: free pole of batescape magnet 323, armature and back ond contact, make-before-break contact 341` relay BR. contact 342 thereof, arm 330 and its second contact, brush 340 to grounded multiple 338. The rela-y BR, in attracting its right armature. locks its circuit to ground over arm 325. independently of the ground on the multiple 338, and in attracting its left armature, applies a busy signal to the calling linev from a busy tone apparatus 343.

When the calling party replaces his receiver, the relay 121 becomes denergized, and after an interval relay 122 denergizes. The retraction of the right armature of relay 122 closes the circuit of release magnet RM of the inder, and removes ground from the third wire 132. The removal of ground from this" conductor results in the release of the remaining switches, as already described in connection with a successful call.

When the second selector SS is set by the calling subscriber in operative relation to a trunk group terminating either in third selectors or connectors in the originating office, the commutator brush 216 of the second selector rests on an open segment and consequently the relay DR is not operated. The succeeding switches are then set over a grounded circuit controlled at the armature and back contact of the stepping relay 121. When the called party responds, ringing cutoff relay RCO at the connector operates, as described, which results in the side switch being advanced to position 4. Side switch. arm 335 then transmits an impulse over the lower talking conductor to energize relay 133. This relay, in operating, energizes relay 134 which shunts the right winding of differential relay DR. Relay DR, in attracting its right armature, extends the grounded left armature of relay l134 to the right winding of .relay 133. This relay is then maintained energized over the called line circuit. In the case of a local call, ground on the third wire 132 is transmitted over side switch arm 225 (Fig. 2) of the second selector'to the test conductor 380 of the connector. Release relay R4, R4 of the connector is thus held energized in parallel with the release relays of the selectors, and when the called party replaces his receiver at the conclusion of the conversation. the deenergization of the relay 134 removes ground fro-m the test conductor, and the relays RR of the selectors nand connector become denergized, and at their armatures and back contacts close thecircuits of the release magnets of the switches.l

What is claimed is:

1. ln a telephone exchange system, the combination with telephone lines, senders associated therewith, trunk lines including automatic switches adapted to interconnect said lines, said trunks including test conductorsdriving magnets for said switches. a local grounded control circuit for said magnets, a set of repeating relays normally controlling said circuit and operated by said senders to set said switches, a group of two-wire trunks connected to one of said switches. other automatic switches in which said trunks terminate. driving magnets therefor, control relays for said magnets at said switches, a metallic control circ-uit including one of said trunks, the associated control relay, and contacts of said repeating relays, means associated with sald re- 'peatingrelays adapted to render inoperative said grounded control circuit and to render operative said metallic control circuit, and a device associated with the test conductor of the switch containing said trunk group and adapted automatically to operate said means when said group is selected.

2. In a telephone exchange system, the

combination with telephone lines, senders associated therewith, trunk lines including automatic switches adapted to interconnect said lines, said trunks including test conductors, automatic switches adapted to intei-connect said lines, stepping magnets for said switches, a local grounded stepping circuit for said magnets, a set4 of repeating relays normally controlling said circuit and operated by said sender to set said switches, a group of two-wire trunks connected to one of said switches, an olice wherein said trunks terminate in' automatic switches, stepping magnets for said switches, stepping relays for said switches, a metallic stepping circuit including one of said twowire trunks, the associated relay and contacts of said repeating relays, an electroresponsive device associated with said repeating relays adapted to render said grounded stepping circuit inoperative and to render said metallic stepping circuit op erative, said device being included in the test conductor, and a switchassociated with the test conductor ofthe switch containing the two-wire trunk group and adapted when said group is selected automatically to operate said device. n -1 3. In a telephone exchange system, the combination with telephone lines, senders associated therewith, trunk lines including automatic switches adapted to interconnect said lines, said trunks including a pair of talking conductors and a test conductor, driving magnets for said switches, a local grounded control circuit for said magnets, a set of repeating relays normally controlling said circuit and operated by said senders to set said switches, a group of two wire trunks connected to one of said switches, other automatic switches -in which said trunks terminate, driving magnets therefor, control relays for said magnets at said switches, a metallic control circuit including one of said two wire trunks, the associated control relay and contacts of said repeating relays, an electroresponsive device associated with the repeating relays and included in the test conductor but unresponsive to the initial electrical condition thereof, said device being adapted when operated to render inoperative said grounded control circuit and to render operative said metallic control circuit, and a device associated with the test conductor of the switch containing the two wire trunk group and adapted automatically to operatesaid device when said trunk group is selected.

4. In a telephone exchange system, ,the combination with telephone lines, senders associated therewith, automatic switches adapted to interconnect said lines, stepping magnets therefor, a local grounded stepping circuit for said magnets, a set of repeating relays normally controlling said circuit and operated by said sender to set said switches, a group of two-wire trunks connected to one of said switches, an oflice wherein said trunks terminate in automatic switches, stepping relays for said switches, a metallic stepping circuit including one 'of said twowire trunks, the associated relay and contacts of said repeating relays, a doublewound differential relay associated with said repeating relays adapted to render said grounded stepping circuit inoperative and to render said metallic stepping circuit operative, a circuit for said rrelay including both windings differentially during the setting of said switches at the originating office over the grounded circuit, and means associated with the switch containing the twowire trunk group adapted, when said group is selected, to shunt one of the windings of said relay whereupon the same operates.

5. In a telephone exchange system, the combination with telephone lines, senders associated therewith, automatic;` switches adapted to interconnect said lines, stepping magnets therefor, a local grounded stepping circuit for said magnets, a set of repeating relays normally controlling said circuit and operated by said sender to set said switch, a group of two-Wire trunks connected to one of stepping magnets for said switches,

said switches, an olice'wherein said trunks terminate in automatic switches, stepping magnets for said switches, stepping relays at said Switches, a metallic stepping circuit including one of said two-wire trunks, the associated relay at the distant olice and contacts of said repeating relays, a double- Wound differential relay associated with said repeating relays adapted to render said grounded stepping circuit inoperative and to render said metallic stepping circuit operative, a circuit for said relay including both windings. differentially during the setting of the switches at the originating oilice over the grounded circuit, and an auxiliary step-by-step switch on the brush shaft of the switch containing the two-wire trunk group adapted when said roup is selected, to shunt one of the win ings of said dili'erential relay whereupon the same operates.

6. In a telephone exchange system, the combination with telephone lines, senders associated therewith. automatic switches interconnecting said lines, stepping magnets interconnecting said lines,

VAbe inclu therefor, a local grounded stepping circuit for said magnets, a set of repeating relays includin a fast and a slow relay adapted to ed in circuit with the calling line, the said fast and slow relays controlling at their back and front contacts, respectively, said local grounded stepping circuit, a group of two-wire trunks connected to one of said switches, an otlice wherein said trunks terminate in automatic switches, stepping magnets for said switches, stepping relays at said switches, a marginal relay associated with said repeating relays, a metallic stepping circuit including said marginal relay, one of said two-wire trunks, a stepping relay at the distant ofiice and a front contact of said fast relay, 'a double wound differential relay associated'with said repeating relays and adapted, when energized, to close said metallic circuit and to interpose a breakin said grounded circuit, said differential relay including both windings in opposition during vthe setting of said switches at the originating oiiice over the grounded circuit, and means associated with the switch containing the two-wire trunk group adapted, when said group is selected, to shunt one of the windings of said relay whereupon the same operates and closes said metallic stepping circuit, whereupon the continued operation of the line relay by the sender intermittently interrupts the Icircuit and operates the stepping relay at the distant oiice to set the switches thereat.

7. In a telephone exchange system, the combination with telephone lines, senders associated therewith, automatic switches for stepping magnets therefor, a local grounded stepping circuit for said magnets, a set of repeating relays including a fast and slow relay adapted tobe included 1n circuit with the calling line, the

said fast and slow relays controlling at their lmy name this 21st day of September,

back andfront contacts, respectively., said4 local grounded-stepping circuit, a group of two-Wire trunks connected to one^of said switches, an oiiice wherein said trunks terminate in automatic switches, stepping mag- `nets for said switches, stepping relays at said switches, a marginal with said repeating relays, a metallic stepping circuit including said marginal relay, one of said two-wire trunks, a stepping relay at the distant oiice and a front contact of said fast relay, a double wound diiferential relay associated with said repeating relays and adapted, when energized, to close said metallic circ 't and to interpose a break in said grounded circuit, a circuit for said differential relay including both windings in opposition during the setting of said switches at the originating office over the grounded circuit, means associated with the switch containing the two-wire trunk group adapted, when said group is selected, to shunt one of the windings of said relay whereupon the same operates and closes said metallic stepping circuit, whereupon the continued operation of the line relay by the sender intermittently yinterrupts the circuit and operates the stepping relay at the distant olice to set the switches thereat, a high resistance included in circuit with said stepping relay at the distant oiiice, a relay energlzed by the response of the called party, adapted to close a shunt about this highresistance to operate the marginal relay at the originating oilice, and means actuated upon the energization of said marginal relay for placing the release of the switches under the control of the called party.

In witness whereof, I hereunto subsrillo;

relay associated ARTHUR B. SPERRY.

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