US2210739A - Through supervision system - Google Patents

Through supervision system Download PDF

Info

Publication number
US2210739A
US2210739A US242151A US24215138A US2210739A US 2210739 A US2210739 A US 2210739A US 242151 A US242151 A US 242151A US 24215138 A US24215138 A US 24215138A US 2210739 A US2210739 A US 2210739A
Authority
US
United States
Prior art keywords
relay
trunk
contact
circuit
cord
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
US242151A
Inventor
Horace W Ulrich
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
AT&T Corp
Original Assignee
Bell Telephone Laboratories Inc
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Bell Telephone Laboratories Inc filed Critical Bell Telephone Laboratories Inc
Priority to US242151A priority Critical patent/US2210739A/en
Application granted granted Critical
Publication of US2210739A publication Critical patent/US2210739A/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

Links

Images

Classifications

    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04MTELEPHONIC COMMUNICATION
    • H04M5/00Manual exchanges
    • H04M5/08Manual exchanges using connecting means other than cords

Description

6, 1940- H. w. ULRICH 2,210,739
THROUGH SUPERVISION SYSTEM Filed Nov. 25, 1938 2 Sheets-Sheet 1 IIHI- NIGHT AL ARM RELEASE INVENTOR HWULR/CH BY A Woe/DEV Aug. 6, 1940. H. w. ULRICH 2,210,739
THROUGH SUPERVISION SYSTEM Filed Nov. 25, 1958 ZSheets-Sheet 2 MEL FIG 2 Ti v A TTORNE V Patented Aug. 6, 1940 UNITED STATES 2,210,739 THRQUGH SUPERVISION SYSTEM Horace W. Ulrich, Newark, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application November 25, 1938, Serial No. 242,151
8 Claims.
This invention relates to telephone systems and more particularly to two-way trunk circuits between two exchanges, as between a toll office and a tributary central office serving both local 5 battery (magneto) and common battery subscribers.
An object of the invention is to provide through supervision over the trunk to the toll operator between a calling common battery subscriber l served by the tributary office and the toll office,
regardless of the direction of the call.
A feature of the invention whereby the foregoing object is attained resides in terminating the trunk at the tributary exchange in a jack and so arranging the circuits of the trunk and the tributary ofiice cord circuit that when both ends of the trunk are connected, a tributary common battery subscriber will, when connected to the trunk, control the supervision to the toll operator.
Other features of the invention will appear and be understood from the following description and accompanying drawings of which Fig. 1 illustrates a common battery subscriber A adapted to be connected by a cord circuit SC, of a so-called toll tributary exchange, to a twoway trunk line L extending to a so-called toll center; and
Fig. 2 shows the toll center end of the trunk line L and a toll cord circuit TC adapted to extend the trunk line L to a subscriber B.
A description of the operation of the circuit of the invention will now be given first assuming that the subscriber A (Fig. 1) has placed a call for a subscriber such as B (Fig. 2) which subscriber is reached over the trunk line L.
When subscriber A lifts his receiver to initiate a call, his line lamp (not shown) is lighted in the usual manner. The operator inserts plug 2 of cord circuit SC in jack I of the subscribers line, whereupon answering sleeve relay 3 of the cord operates from ground on the sleeve of jack I in series with the supervisory lamp 4. Relay 3 closes its right-hand front contact thereby connecting ground and battery through the upper and lower windings of supervisory relay 5, to the subscribers line which is now closed due to receiver having been removed from the switchhook, whereupon relay operates. The operation of relay 3 also opens its outer right back contact thereby disconnecting the middle winding of relay 5, which winding is normally in series with the upper and lower windings across the cord. Operation of relay 5 connects ground at its inner lowermost front contact to short-circuit supervisory lamp 4 and prevents it from lighting at this time. Relay 5 also closes its inner lower front contact thereby connecting ground, through high resistance 6, to the sleeve of the front or calling plug 1. The value of resistance 6 is such, however, that the front or calling supervisory relay 48 is not operated at this time nor is lamp 49 lighted.
When the operator inserts plug 1 into jack 8 of the trunk line L, the auxiliary jack spring contacts 9 are immediately closed thereby completing an obvious circuit to operate the trunk sleeve relay II) which operates and closes its contacts II, I2, I3 and I4. The closure of contact I I lights the busy lamp I5, and multiples thereof, and closure of contact I2 connects relay I6 to the sleeve of jack 8 which is now connected to ground in the cord by Way of resistance 6 and the lower front contact of relay 5. Relay I6 which is ultrasensitive operates in series with the high resistance.
When the tip and ring contacts of the plug 1 are in engagement with the corresponding springs of the jack 8, relay 50 operates in a circuit including battery, contact SI of relay 47, upper left winding of repeating coil 22, tip contact of the jack and plug, all three windings of relay 50 which are connected in series over contact 52 of relay 48, ring contact of the plug and jack, lower winding of repeating coil 22 and resistance 53 to ground at contact 54 of relay 41. Relay 5!! in operating closes its lowermost contact and connects ground, over the right-hand back contact of relay 48 to light lamp 55 in the cord circuit.
Closure of contact I3 of relay I6 connects ground to relay I'l over a path including contacts I8 of relay I9. While relay I1 operates immediately following the closure of contact I 3, I
in the path just traced, which includes contact iii of relay I9, the operation of relay I6 and the consequent closure of its left front contact, completesa short circuit around contact i8 which includes contact 20 of relay 86 so that a subsequent operation of relay I9 will not release relay I'I. When relay Il operates due to operation of relay Ill, it short-circuits the high resistance (lower) winding of the polarized relay 2| which is bridged across the condenser in the right-hand side of transformer 22. This reduction in resistance in bridge of the line causes marginal relay 23 (Fig. 2) at the toll ofiic'e to operate which in turn closes the following circuit: battery, wind ing of relay 24, front contact of relay 23, contact 25 of relay 24, contact 26 of relay 21, and both windings of relay 28 to ground through high resistance 29. The left-hand winding of relay 23 is of low resistance but its right-hand winding is sufficiently high so that relay 24 can not operate in series therewith. Relay 28, however, is sufiiciently sensitive to operate in this circuit, whereupon it releases the normally operated relay 30 which closes its right-hand back contact to operate the night alarm relay 3I which looks up the relay 32 under the control of key 80. Relay 3| operating, closes a circuit to energize the night alarm buzzer 33. Release of relay 30 also tact 36 of relay 2 closes its left-hand contact which short-circuits the right-hand high resistance winding of relay 28 and the high resistance 29 thereby lowering the resistance of the previously traced circuit for relay 24 to a point Where it can operate. Operation of relay 24 closes its contacts 34, 35, 35 and El. The closure of contacts 34 connects battery, over contact 38 of relay 39 to the sleeve of jack it! and multiples thereof to hold the line busy to calls in the opposite direction. Closure of contact 35 and the subsequent opening of contact 25 looks relay 24, operated under the control of relay 23, and opens the previously traced circuit for operating relay 28 which thereupon releases thereby permitting relay 30 to reoperate. Closure of contact '36 lights the line lamp 4i and multiples thereof over the left back contact of relay 2'! and closure of contact 3'! lights the busy lamp 42 and its multiple.
When the toll operator answers by inserting plug 33 jack as, relay 3%! is operated due to the closure of auxiliary contact 44, the circuit including normally closed back contact 45 of relay 5. Relay 39 in operating operates relay 27, looks up over its left-hand contact and con- Relay 35; opens its contact 38 thereby disconnecting the busy potential applied over contact as of relay 24 as the sleeve of plug A3 now supplies such a busy potential to the multiples of jack it Further, relay 39 in operating reverses at its left-hand contact the connection of relay 23, and its battery supply, to the line whereupon polarized relay 2% at the tributary oflice (Fig. l) which up to this time has been released due to the direction of the current flowing in its winding, now operates, and in turn operates relay ll. Relay ll in operating opens its contacts 5! and 54 thereby disconnecting battery and ground from the transformer 22 and thence to the line which causes relay '50 of the cord to release thereby extinguishing lamp 55. Relay ll also closes its contact 56 thereby closing a circuit to operate relay l9 which can be traced from battery, right-hand winding of relay I9, contact 5% of relay ll, contact 57 of relay l9, and contact i of'relay H] to ground. Relay l9 operates and locks itself at contacts 58 under control of contact 58 of relay ll which in turn is controlled from the distant 'toll office. Relay l9 also closes its left contact 59 thereby completing a second locking circuit for' itself including its left-hand winding which can be traced from bottery, left winding, contact 59 of relay l9 and contact S3 of relay in to ground. Relay l0 being under control of the plug '5 of the tributory ofiice, it will be noticed that relay I9 is now held operated until both ends of the trunk line become idle.
When relay l9 operated it opened its contact is over which relay l! was first operated. Relay 21', however, does not release as it is still held operated over a path in parallel with contact I8 which is due to the left-hand front contact of relay :5 and contact as of relay 86. It will now be noted that control of relay I"! which in turn controls supervision over the lines to the distant toll oflice is now under control of both the plug 7, i. e. its controlled relay Ill, but also under control of the calling supervisory relay 5, which, while operated maintains ground on the sleeve of the plug '5 and jack 3 of the trunk line to hold relay it operated. It will therefore be obvious that during the existence of the connection to the trunk line, any movement of the calling subscribers switchhook will release relay 5, in turn releasing relay is which in its turn will release relay H. The release of relay ll will reconnect the lower high resistance winding of relay 2| in the bridge across the line thereby causing relay 23 at the toll ofnce to release. The release of relay 23 releases relay 2t which closes its contact Ell thereby grounding the sleeve of jack 4i] in a circuit including contacts 6! of relay 39.
ing coil it, thereby preventing ringing current 2 from. passing through the repeating coils to the lines. At contacts 39, relay 36 connects ground to hold relay 2'? operated when relay 39 releases, to be later referred to, and at contacts 58 it connects ground to the sleeve of jack Ml, to keep supervisory lamp 52 lighted as it is assumed that the calling subscriber A has disconnected and consequently relay 24 is released as previously mentioned.
Operation of relay as also opens its contact and immediately closes contact H and also closes contact 72 thereby placing relay 39 under control of interrupter I which alternately disconnects and connects ground thereto thereby causing that relay to alternately release and reoperate. The release of relay 39 reverses the current flowing back over the trunk and releases relay 2! which in turn releases relay 4;! which reconnects battery and ground at the mid point .of the left hand side of repeating coil 22 which causes supervisory relay 56 in the cord circuit to operate to light supervisory lamp 55. As the interrupter I alternately releases and operates relay 3? the lamp in the cord circuit is flashed as long as the toll operator continues to ring.
In case it is desired to retain supervision at the tributarycffice, i. e. not transmit through supervision to the toll center, the tributary operator will make connection to the trunk line by way of the so-callcd plug supervision, jack 53,'whereupon relay t l instead of relay IE operates. Under this condition the ground on the sleeve of plug 1 is ineffective as the sleeve of jack 53 is not connected to relay [8. Relay 84 closes its right-hand contact, thereby lighting the busy lamp l5 and its multiples and operates relay H which short-circuits the high resistance winding of relay 2| thereby operating relay 23 at the toll ofiice as before to light the line lamp 4|.
When the toll operator answers, relay 39 operates as before described to reverse the line battery and causes relay 2! at the tributary office to operate which operates relay cl. Relay 4'! disconnects at contacts 5i and 54, battery and ground at the mid-point of the left-hand winding of repeating coil 22 thereby releasing relay 5!) which extinguishes lamp 55 in the cord circuit. The closure of contact 55 of relay 41 operates relay It in a circuit from ground, contact '13 of relay 64, outer right back contact of relay IE), not now operated, contact 57 of relay i9, contact 58 of relay 47 and right winding of relay l9. Relay I9 closes its contact '58 thereby locking it self operated under control of relay 57, i. e.
under contro1 of the distant toll ofiice, but does eration at this time.
not find a locking ground for its left-handwinding under control of the tributary cord SC as relay [0, which supplied this ground when jack 8 was in use, is not now operated.
As the sleeve or jack 63 is not connected to relay [6, as is the sleeve of jack l8, movement of the calling subscribers switchhook does not affect relay I7 and hence supervision is restricted to the tributary cord circuit SC and is received on supervisory lamp 4 in the usual'manner only. The supervisory lamp 62 in the toll ofiice remaining dark until the tributary operator disconnects from the jack 63 whereupon relays 13 and I! release in turn releasing relays 23 and 24 which now light lamp 62.
Non-through supervision jack 63 is also used for connecting calling magneto (local battery) stations to this trunk line. In this case due to the fact that magneto station line circuits do not have the jack sleeve grounded, the answering cord sleeve relay 3 does not operate at any time and the answering cord supervisory relay 5 only in response to ringing current occasioned by the subscriber turning the crank of the telephone set generator. Under this condition the tributary operator receives supervisory signals from both ends of the cord on the lamp 55.
Now let us assume a call in the opposite direcitno, i. e. a call from the toll center (Fig. 2) to a common'battery subscriber A which is reached through the tributary office (Fig, 1)
When the toll operator inserts plug 43 in jack 4!], relay 39 operates from grounded contact 44 of the jack which reverses the battery supplied tothe line and causes polarized relay 2| of the tributary end of the trunk to operate.
Relay 39 also closes its inner right contact to operate relay 2'! and by closing contact 6! connects ground, supplied over contact 60 of relay 24, to the sleeve of the jack 4!! and plug 43 which causes cord supervisory lamp 62 to light and sleeve relay (4 to operate. Operation of relay 2! closes its inner right contact to light the busy lamp 42 and multiples thereof. Operation of relay 2! at the tributary end of the trunk operates relay 4'! which causes the delayed operation of relay l9 as follows: closure of contact 55 of relay 41, operates relays in the following circuit; battery, right winding of relay 19, contacts 56 of relay 41, contacts 51 of relay l9, outer right back contact of relay Ill, back contact of relay 64, both windings of relay 15, and high resistance 15 to ground. Relay 15 operates in this circuit but due to the high resistance of the right-hand winding of relay 15 and resistance 16, relay l9 does not receive suflicient current to cause its op- Operation of relay "(5 opens its contacts and permits normally operated slow release relay 1'! to deenergize. Vvhen relay TI releases it closes its right-hand contact and aperates relay 18 which closes a circuit to operate .the night alarm buzzer 19 which locks up under control of a release key. The closure of the left hand contacts of relay H short-circuits the righthand winding of relay 15 thereby decreasing the resistance of the circuit including relay l9, which relay now operates and locks up the ground at its contact'58 under control of contact 56 of relay 41. The opening of contacts 51 of relay l9 breaks the circuit through relay 15 which releases and permits relay 1'! to reoperate. Operation of relay 4'! also closes its contacts ill and 82 which light the line lamp 83 and busy lamp l5 including multiples thereof. All calls from the toll center are answered in the through supervision jack 8 and, consequently when the operator at the tributary oflice notes the lighted line lamp 83 she inserts plug 1 in jack 8. Due to the fact that relay 41 is operated at this time, battery and ground are disconnected from the trunk and contacts 5| and 54 and therefore supervisory relay does not operate and the supervisory lamp remains dark.
When the plug is inserted in jack 8, auxiliary contacts 9 are closed thereby operating relay In which at its contact maintains the busy lamp l5 lighted independent of the circuit previously traced which was controlled by contact 82 of relay 41. Closure of contact l2 of relay I 0 connects relay Hi to the sleeve of jack 8 in readiness for operation, if and when a called common battery subscriber is connected to the other end of the cord and the subscriber answers. ,When this occurs relay l6 operates in series with high resistance 6 thereby closing its front contact and closing a circuit to operate relay I! which can be traced from ground, contact [3 of relay In, front contact of relay It, contact 20 of relay 86, and windings of relay I! to battery. Relay l1 operates and short-circuits the high resistance winding of relay 2! and causes relay 23 of the toll center to operate which in turn operates relay 24 thereby locking itself under control of relay 23. Relay 2'! is also held operated at contact. 36 of relay 24 under control of the tributary oflice and independent of the holding effect exercised by the relay 39. Operation of relay 24 also opens its contact thereby removing the ground from the sleeve of the jack 40 and extinguishing the supervisory lamp 62 as an indication that the called subscriber has answered. Any further movement of the called subscribers switchhook will release relay 24 to reapply grounded'contact 60 to the sleeve of the jack to relight the supervisory lamp 62.
In case the call from the toll center was for a magneto subscriber at the tributary ofiice, which would be answered in the same manner as before by the tributary operator, the response of the called subscriber will not operate relay [6 due to the fact that supervisory relay 5 is not affected by a magneto subscribers switchhook and consequently it is incumbent on the tributary operator, in such cases, to manually relay the subscriber's answer to the toll operator by op erating the key 84. When this key is operated, it completes a circuit to cause operation of relay I! which can be traced from ground, contacts l3 of relay IIJ, normal back contact of relay I6, contacts of key 84 and winding of relay 86 to battery.
Relay 85 closes its left front contacts thereby connecting ground at contact I? of relay [0 to relay I! which operates and extinguishes the toll cord supervisory lamp 62 as previously described. Re-
lay BB locks up at its front contact under control of contact 56 of relay 41. This locking circuit can be traced from battery, winding of relay 86, contacts 85 of relay 59, contacts 56 of relay 4?, and contacts 58 of relay lfito ground. This locked condition of relay 86 maintains relay ll operated and the toll cord supervisory lamp 62 extinguished under control of the plug 1 of jack 8. When the conversation is completed, the tributary operator removes plug 7 from jack 8 thereby releasing relay Ill which releases relay [1 and relights the toll cord supervisory lamp 62. Release of the trunk by the toll operator restores the circuit to normal thereby releasing relay 39 which reverses the line battery to release relay 21 which in turn releases relays 41, IS and 86.
What is claimed is:
1. In a telephone system, a first operators position, a subscribers line terminating thereat, a trunk line also terminating at said position in a jack having a sleeve terminal, a cord circuit at said first position for interconnecting said line and trunk and having a sleeve conductor cooperating with said jack sleeve when said cord and trunk are connected, a second operators position terminating the other end of the trunk, a supervisory relay in said first position cord circuit controlled over said subscribers line, a link circuit at said second position adapted to be con nected to the trunk, supervisory means in said link circuit, relay means in said trunk at the second position end thereof controlled over the trunk for controlling said supervisory means when said trunk and link are connected, means in the first position end of the trunk for transmitting signals over said trunk to operate said relay means at the other end, means responsive to connection of said first position cord circuit to said trunk jack to operate said signal transmitting means, locking relay means in the trunk at the first position operated when the distant link circuit is connected to the trunk, and thereafter held operated until both ends of the trunk are disconnected, for transferring control of said signaling means from said means responsive to connection of the cord to the trunk jack to said supervisory relay over said connected sleeve conductors of the trunk and cord.
2. In a telephone system, a first operators position, a subscribers station line terminating thereat, a trunk line also terminating at said position, a cord circuit at said first position for interconnecting saidline and trunk, a supervisory relay in said first position cord circuit controlled over said line, a second operators position terminating the other end of said trunk, a cord circuit tliereat adapted to be connected to the trunk, means in said trunk responsive to connection of one end of said first position cord circuit to the trunk to transmit a signal thereover independent of the cord supervisory relay, locking means in the trunk operated when the second position cord circuit is connected to the trunk following the connection of the first position cord circuit thereto, and thereafter held operated untilboth cords are disconnected from the trunk, for transferring control of said first means to the supervisory relay in the first position cord circuit, said control being exercised over the connected sleeve circuit of the cord and trunk.
3. In a telephone system, a first operators position, a subscribers station line terminating thereat, a second operators position, a trunk line extending between said positions, a cord circuit at the first position for connecting the subscribers line to the trunk and including supervisory means controlled over the subscribers line, a cord circuit at the second position adapted to be connected to the trunk and including a supervisory signal device, means controlled over said trunk for controlling said device, and other means in said trunk at the first position controlled over the sleeve circuit of the cord by said supervisory means for controlling said means to control said device.
4. In a telephone system, a first operators position, a subscribers station line and a trunk line terminating thereat, a cord circuit at said position for interconnecting said trunk and subscribers line, a second operators position termimating the other end of said trunk line, a cord. circuit at said second position including a supervisory signal device, means in said trunk for controlling said signal device, a supervisory relay in said first position cord circuit controlled by the subscribers station switchhook, and means, including the interconnected sleeve conductors of said first position cord and trunk, controlled by said supervisory relay, for controlling the means in said trunk.
5. In a telephone system, a first switchboard, a subscribers station line terminating at said switchboard, a toll switchboard, an automatic signaling trunk extending between said switchboards, a cord circuit at the first switchboard adapted to connect said subscribers line to the trunk line, a cord circuit, including a supervisory signal device, at the toll switchboard, adapted to be connected to the trunk line, means in said trunk for controlling said supervisory signal when the associated cord circuit is connected to the trunk, supervisory means associated with the cord circuit at the first switchboard responsive to removal of the subscribers station receiver while the cord circuit is connected to the line, and means including the sleeve conductors of the cord at the first switchboard, effective when said supervisory means responds, to control said trunk means.
6. In a telephone system, a first operators position, a subscribers line terminating thereat, a trunk line also terminating thereat, a cord circuit thereat for interconnecting said line and trunk, a supervisory relay for said cord circuit controlled over the subscribers line, sleeve conductors for said cord and trunk arranged to be interconnected when said cord and trunk are connected, a second position terminating the other end of the trunk, a link circuit at the second position for connecting to the trunk, a supervisory signal device associated with said link circuit, and means in said trunk controlled by said supervisory relay over the connected sleeve conductors of the cord and trunk for controlling the signal device in the link circuit.
'7. In a telephone system, a first operators position, a subscribers line terminating thereat, a
trunk line also terminating thereat, a cord circuit thereat for interconnecting said line and trunk, a supervisory relay for said cord circuit controlled over the subscribers line, sleeve conductors for said cord and trunk arranged to be interconnected when said cord and trunk are connected, a second position terminating the other end of the trunk, a link circuit at the second position for connecting to the trunk, a line relay associated with the trunk at the second position, means responsive to connection of the first position 'cord to the trunk for operating said line relay independent of the condition oi the other end of the cord and means responsive to the connection of said link circuit to the trunk to trans-
US242151A 1938-11-25 1938-11-25 Through supervision system Expired - Lifetime US2210739A (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
US242151A US2210739A (en) 1938-11-25 1938-11-25 Through supervision system

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
US242151A US2210739A (en) 1938-11-25 1938-11-25 Through supervision system

Publications (1)

Publication Number Publication Date
US2210739A true US2210739A (en) 1940-08-06

Family

ID=22913649

Family Applications (1)

Application Number Title Priority Date Filing Date
US242151A Expired - Lifetime US2210739A (en) 1938-11-25 1938-11-25 Through supervision system

Country Status (1)

Country Link
US (1) US2210739A (en)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2703342A (en) * 1952-10-15 1955-03-01 Stromberg Carlson Co Transmission of control signals in a telecommunication system
US3037086A (en) * 1960-08-01 1962-05-29 Bell Telephone Labor Inc Private branch exchange telephone system

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2703342A (en) * 1952-10-15 1955-03-01 Stromberg Carlson Co Transmission of control signals in a telecommunication system
US3037086A (en) * 1960-08-01 1962-05-29 Bell Telephone Labor Inc Private branch exchange telephone system

Similar Documents

Publication Publication Date Title
US2210739A (en) Through supervision system
US2055647A (en) Telephone line system
US2700069A (en) Measured service telephone system
US2302587A (en) Telephone trunking system
US2312385A (en) Telephone system
US2310481A (en) Two-way trunk circuit
US2572104A (en) Private branch exchange telephone system
US1919317A (en) Telephone system
US2314518A (en) Telephone system
US1747833A (en) Telephone-switchboard system
US2186219A (en) Telephone system
US2130482A (en) Telephone system
US2031922A (en) Telephone system
US2276515A (en) Telephone switchboard
US2305939A (en) Universal cord circuit
US2173924A (en) Telephone system
US2568138A (en) Combined line and tripping relay circuit for telephone lines
US2073003A (en) Switchboard cord circuit
US1832263A (en) Telephone system
US2342586A (en) Repeater system
US1881669A (en) Telephone system
US1314804A (en) John e
US2702832A (en) Telephone system
US2132857A (en) Telephone system
US1786480A (en) Toll-service trunking system