US1315445A - Planodrat tl co - Google Patents

Description

T. G. MARTIN.

AUTOMATIC TELEPHONE SYSTEM.

APPUCATION min MAY 31.1w.

1,315,445. PatentedSept. 9,1919.

P k g i J g t: I {I H "'gu UNITED STATES PATENT OFFICE.

TALBOT G. MARTIN, OF CHICAGO, ILLINOIS, ASSIGNOR TO AUTOMATIC ELECTRIC COMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS.

AUTOMATIC TELEPHONE SYSTEM.

Specification of Letters Patent.

Patented Sept. 9, 1919.

Application filed May 81, 1917. Serial No. 171,815.

To all whom it may concern:

Be it known that I, TALBOT G. MARTIN, a citizen of the United States of America, and a resident of Chicago, county of Cook, and State of Illinois, have invented certain new and useful Improvements in Automatic Telephone Systems, of which the following is a specification.-

My invention relates in general to automatic telephone systems, but more particularly to such systems as comprise a, plurality of exchanges interconnected by trunk lines; and the object of the invention is to provide an improved repeater for use in the interoflice trunk circuits, whereby the quality of telephonic transmission for a certain class of calls, hitherto below the standard, is improved greatly and is made substantially equal to the average quality of transmission in the system.

The above indicates in general terms the purpose and scope of the invention. The manner in which the same is to be used, its mode of operation, and the advantages derived from its incorporation in a telephone trunking system of the character described, will all be fully explained hereinafter, reference being had to the accompanying drawings.

In the drawings Figure 1 is a schematic diagram illustrating the general layout of a portion of a system which is well adapted for the use of my improved repeater, while Fig. 2 is a complete circuit diagram of two selector switches and a repeater, wired in accordance with the invention, which are shown only symbolically in Fig. 1.

Referring to the layout Fig. 1, I have shown at the right of the dotted line certain apparatus in one of the main exchanges of the system, while to the left is shown a branch exchange subsidiary to the main exchange. The main exchange may be a five digit system, and local connections will then be established through the medium of individual line switches, first, second, and third selector switches, and connector switches. For example, the subscriber at substation A may establish connection with the subscriber at substation A by means of the individual line switch C, the first selector E, the second selector F, the third selector H, and the connector I. This is the usual and well known percentage trunking system and needs no detailed explanation.

The branch exchange may be a three digit system comprising individual line switches, first selectors and connectors. A calling subscriber at substation A may extend a connection to the substation A by means of line switch C, the first selector E and the connector I. Subscribers in the branch exchange of course can also call subscribers in the main exchange, and for this urpose a certain bank level (assuming that t e Strowger type switches are used) of the first selectors, such as the selector E is reserved for trunk lines extending to the main exchange. Thus the selector E not only has access to a plurality of groups of connectors, such as the group which includes the connector I, but also has access to a group of trunk lines extendin to the main exchange and terminating t ere in first selector switches, termed incoming selectors. 'One of these trunk lines is the trunk line 2 which extends by way of the repeater R to the main exchange and terminates in the incoming selector E. The incoming selectors have access to second selectors of the local trunking system in the main exchange and their banks therefore are multipled with the banks of certain local first selectors such as the selector E. For the purpose of giving main exchange subscribers access to lines in the branch exchange,acertain bank level of a group of third selectors may be assigned to trunk lines extending from the main to the branch exchange. One of these trunk lines is shown extending from the bank of third selector H by way of repeater R to the branch exchange where it terminates in the selector E.

The other main exchanges of the system, of which there may be one or more, are preferably each accessible to subscribers 1n the main exchange shown from 'a partlcular first selector level. Thus a certain level of the selectors E and E may be assigned to a group of trunk lines extending to a second main exchange and terminatmg there n second. selectors. One of these trunk lines 1s the trunk line 3 which extends by way of the repeater R to a second selector (not shown) in the second main exchange. It will: be seen now that the main exchange subscriber at substation A can extend a connection to the second main exchange by way of line switch C first selector E, repeater and trunk line 3; while the branch subscriber at substation A can extend a connection to the same point by way of line switch Q, first selector 111 repeater It tru-nk line 2pm.coming selector E, repeater R, and trunk line 3.

From the foregoing 'bri e'tfi description o l a typical trunking system the purpose and utility of my invention may easr y be understood. When the subscriber at substation A extends a connection over the trunk line 3 his transmitter is supplied with tallking current from the front bridge or controlling relay of the repeater R'. which must be oi sulliciently low resistance to permit the flow of the requisite amount of talking current. But when the branch subscriber at substation A extends a connection over trunk line 3 transmitter current is supplied from the the front bridge of repeater R", and thus it will be seen that in this latter case it is extremely desirable that the front bridge in repeater R- be of high resistance, or there will be excessive transmission loss. By my invention the repeater R is so designed that when it is seized by a local first selector such as the selector E the usual front bridge will remain across the talking circuit during conversation for the purpose of supplying transmitter current to the calling line in the customary manner; but when it is seized by an incoming selector such as selector E, when the called subscriber answers an additional coil of high resistance and impedance is inserted in the bridge, thereby greatly re ducing the transmission loss.

The manner in which this is accomplished can best he explained by describing briefly the operation of the incoming selector E and the repeater R during the establishment of a connection, and sucli doscri 'tion wi l] now be proceeded with, reference "ein had to Fig. 2. First, however, certs-in pu Bications will be cited as illnstraflin well known features of the equipment have red, in order to avoid the necessity of too elaborate an explanation. The mechanical construction of the switches is shown in U. S. Letters=Patent Nos. 5%1-5321 and 815,1"7-6', both granted March 13, 1306, to Keith, Ericltsong and Erickson. Cir'cuitsof a modern two wire automatic system are shown in U. 8. Patent No. 13,901', reissued April 1'3, 1915tto' Nowforth. A repeater halving the general functions of the ones heroin shown is disclosed U. 8. Patent to Martin, No 1,136,912, granted A- (11 20 12915:.

To proceed with t! e operation, when the subscriber at mibetatioir Al" removes Ilia m cover to initiate the call, the line switch C is operated in the usual manner to extend the line of substation A to an idle first selector switch, which we may assume to be the selector E "he callin r device at substation A" is now operate in accordance with the first digit of the desired number, resulting in the operation of selector E whereby an idle trunk line extending to the main exchange is selected and connection established therewith. Assn-min that the selected trunk line is the trunk inc 2, when it is seized by the selector E the front bridge or controlling relay of the repeater It is energized over the calling subscrihefis loop; and as a result of the energization of this control'lin relay a bridge is closed across the coiu-iliictors of the trunk line extendin r between the repeater R and the se lector ill at the main exchange. The line relay [(1 ol' the selector 1*) is accordingly energized and upon attracting its armature (loses a circuit for the slouacting release relay 11. Upon energizing in turn, relay 11 prepares circuits for the Vertical and rotary magnets ll and 15 in the well known manner.

The ceiling subscriber may now operate his calling device in accordance withthe second digit of the desired number, thereby producing a; series of interruptions or inipulses in the circuit of the controlling relay oi the repeater R. These interruptions are repeated in the usual way into the circuit of the line relay 10 0f the selector E in the main exchange. It follows that the line relay 10' is deenergized a corresponding number of times. and at each deiinergizati-on' an impulse ilrom ground G is transmitted through the slow acting relay 12 and the vertical magnet 14 in series. The vertical magnet in response toithese impulses operates to step up the switch shaft until the wipers 40; 41 and 4-2 arrive at the horizontal level of bank contacts which constitute terminals of the trunk lines extending to the second main exchange.

The relay 1 2 is energized in series with the vertical magnet, and being slow acting; retains its armature during the series of inn pulses. Upon the first upward movement of the shaft the off normal springs 1-8 are closed, thereby eomplbting' an initial energizing circuit tor the stepping relay 13 which may be traced as follows: ground at (2- working contact of armature" 2'11 and the said armature, armature 22 and its working c intact, ofi' normal s rings 18, andr thewinding of relay 13 to' attlery. Upon energizing, relay 13 looks itsel f to ground G at its upper armature 23 and at its lower armature 9L prepares a circuit for the rotary magnet 15 which is held openghowever. during the ertical movement oi the switch by the slow acting relay 12. At this point it; should h s mentioned that by the closure of 011 normal springs 18 the line switching relay 16 and the step ing relay 13 were connected in series. ut since the junction of the two relays has already been connected to ground as described, relay 16 is short circui-ted and remains inoperative for the resent.

When the slow acting re ay 12 denergizes at the end of the series of impulses it completes at the back contact of its armature 22 the circuit of the rotar magnet 15 extending as follows: groun at G, working contact of armature 21 and the said armature, armature 22 and its resting contact, armature 24 and its working contact, and the winding of the rotary magnet 15 to battery. Upon energizing, the rotary magnet operates to rotate the wipers 40, 41 and 42 into engagement with the first set of bank contacts in the level opposite which they were raised. Near the end of its stroke the rotary magnet opens its interrupter contact, thereby breaking the looking circuit of the steppin relay 13. Upon deenergizing, relay 13 breaks the energizing circuit of the rotary magnet which accordingly retracts its armature to again close its interrupter contact. The 0 eration now depends upon whether or not t e first trunk line, the term inals of which are now engaged by the selector wipers, is busy or not. If this trunk line is busy there will be a round potential upon the test contact with which the private wiper 41 is in engagement. and this ground potential will serve to maintain the switching relay 16 short circuited and to again energize the stepping relay 13. The rotary magnet circuit is therefore again completed and the foregoing operations are repeated, whereby the selector wipers are advanced into enga ement with the next set of contacts. It wil be seen then that the stepping relay 13 will operate to advance the switch wipers step by step through the medium of the rotary magnet as long as the rivate wiper 41 continues to engage grounded or busy test contacts. When an idle or non grounded test contact is reached, assumed to be the test contact 44, and the rotary magnet retracts its armature, the

switching relay 16 will no longer be short circuited but will instantly energize in series with the relay 13. Owing to the high resistance of relay 16 relay 13 remains inoperative. Upon energizing, relay 16 opens at its armature 25 a contact in the circuit of the release magnet 27 to guard against the release of the switch upon the deenergization of relays 10 and 11; shifts the private wiper 41 from connection with its upper terminal into connection with its lower terminal at armature 29; and at armatures 28 and 26 disconnects the conductors of the trunk line 2 from the windings of the line relay 10 and extends them by way ofwipers 40 and. 42,

bank contacts 43 and 45, conductors 46 and 48, and normally closed contacts of the reversing relay 106 to the windings of the double wound line relay 100 of the repeater R.

Upon energizing, the line relay 100 closes the usual circuit for the slow acting release relay 101. The relay 101, upon energizing, completes a holding circuit for the selector E as follows: ground at G, Working contact of armature 111 and the said armature, winding of relay 105, conductor 47, bank contact 44, private wiper 41, armature 29 and its working contact, winding of relay 16, interrupter contact of the rotary magnet 15 oil normal springs 18, and the winding oi the stepping relay 13 to battery. It is to be understood that this circuit is established before the release relay 11 has had time to deenergize. The relay 105 of the repeater which is included in the holding cir" cuit is of low resistance so that the test contact 44 and its multiples in the banks of the other incoming selectors are brought suliiciently near to ground potential to make the trunk line busy. This ground potential also extends over conductor 50 to multiple test contacts in the banks of the local selectors such as the selector E where the trunk line 3 is made busy also. It will be observed that a branch of the above described holding circuit extends through the coil 17 at the selector E to battery. This multiple path permits sufficient current to flow through the winding of relay 105 at the repeater to energize it, with results which will be described later.

Proceeding now with the operations at the repeater R resulting from the energizetion of the relay 101, at its armature 112 this relay prepares a circuit for the slow acting relay 102 and at its armature 111 closes a circuit through the polarizing winding of relay 103 as follows: ground at G, working contacts of armature 111, and the lower winding of the said relay 103. The lower winding of relay 103 is of high resistance and few turns, being composed in part of German silver wire, and relay 103 is therefore not sufficiently energized to attract its armatures. A still further result of the energization of relay 101 is the closure of a bridge across the conductor of the trunk line 3 extending to the second main exchange which may be traced as follows: upper conductors of trunk line 3, armature 110 and its working contact, armature 113 and its resting contact, upper windin of relay 103, spring 119, contact 120, right liand winding of impedance coil 107, armature 121 of the line relay 100 and its working contact, to the lower conductor of trunk line 3. By the closure of this bridge the line relay of the second selector at the distant main exchange is energized with'results similar to those f-Q. 1,ana,m

which were described in the case of the line relay 10 of the selector E. Although the upper wind in of relay 103, which is included in the bridge as traced, is connected in series with the line relay of the distant second selector,- the relay 103- is not operativcly energitacd for its windings are so connectcd that their energizing effects at this time are in' opposition.

The calling subscriber ma now 0 crate his calling device in accor ance wit the third digit of the desired number, thereby producing another series of interruptions III the circuit of the controlling relay of repeater R, which interruptions are now repeated into the circuit of the relay 100 of the repeater R. The line relay 100 is accordingly dei-inergized a number of tii'nes corresponding to the number of interrup tions produced in its circuit and at each decnergization transmits an impulse to the slow acting relay 102 over the following path: ground at G armature 122 and its resting contact, armature 112 and its working contact, and the winding of slow acting relay 102 to battery. The slow acting relay 102 remains energized continuously during the intermittent deenergization of line relay 100 and completes the bridge across the conductors of the trunk line 3 over a new path. exclusive of inductive windings, which may be traced as follows: upper conductor of trunk line 3, armature 110 and its working contact, armature 113 and its working contact, and the armature 121 of the line relay 100 and its working contact, to the lower conductor of trunk line 3. While this new bridge excludes the upper winding of relay 103 and the ri ht hand winding of the impedance coil 10 it will be seen that it still includes the armature 121 of the line relay 100, and it follows therefore that at each denergization of the line relay an interruption will be produced in the circuit of the line relay of the second selector at the distant main exchange. This second selector is accordingly operated in the usual manner to establish connection with a trunk line extending to an idle' third selector. At the end of the series of interrnptions the slow acting rela 102 de'encrgizes and the bridge across t e trunk line 103 is reestablished over the path first traced.

The calling subscriber may now operate his calling device for the remaining digits in the called number with the result that the selected third selector and a connector switch are operated to complete the connection to the desired called line. After the connection is completed the called subscriber may be signaled in the customary manner by means of automatic equipment associated with the connector switch. When' the called subscriber answers the signaling operation is stepped nnd a relay in the connector,

which may be the usual answerin bridge, is energized over the called subscri ers line to reverse the direction of current flow over the conductors of the trunk line 3. As a result ct this operation the direction of ourrent flown in the upper winding of relay 103 is reversed and since now the two windings of this relay assist each other it will at once energize: By the attraction of armaturcsi 11-4 and 115 the heavy talking con'duc- 7b tors oi the repeater are extended through tothe conductors of the trunk line 3 independent of contacts of relays 100 and 102; and by the attraction of armature 116 a circuit is completed in an obvious manner for the reversing relay 106. Relay 106 is accordingly energized and at its armature 126 connects the left hand winding of impedance coil 107 in series in the bridge across the trunk line 3. The left hand winding of impedance coil 107 is of high resistance and impedance tor the purpose of reducing as far as possible transmission loss due to this bridge. As a further result of its energiza'tion relay 1 06 reverses the conductors 46 and 8 as regards their connections to the windings of line relay 100, and since the relay 105 is now energized these con-ductors after the encrgization of relay 106 will extend to the windings of the line rela 100 by way of the windings of the inipe ance coil 104. These circuits may be briefly traced as follows: conductor 16, working contact of armature 127 and the said armaturc. lower winding of impedance coil 10-1 and the lower winding of relay 100 to ground. Conductor 418, working contact of armature 128 and the said armature, upper winding of impedance coil 104, and the upper winding of line relay 100 to battery. By the inclusion of the impedance coil 104- in the front bridge of the repeater R the impedance of the bridge is greatly increased and a considerable transmission loss which would otherwise occur is avoided. The transmission is further benefited by the connection of condensers 130 and 131 in multiple, respectively, with condensers 132 and 133. This operation is also brought about by the energization of relay 106. It will be seen then that when a connection is extended throu -h the incoming selector E and the repeater that the impedance and resistance of both the front and back bridges of the repeater are greatly increased with the results already mentioned. Whenthe calling subscriber replaces his receiver the a paratus is restored to normal in a manner w hich is well understood and which need not be described herein.

When the subscriber at substation A extends a connection to the second main exchange by way of the trunk line 3 the operations are as follows: Upon the removal of the receiver the line switch G operates to extend the callin line to a first selector switch, which we wi I assume to be the selector E. The line relay 150 of the selector E is accordingly energized over the conductors 152 and 163 to which the calling line has been extended by the operation of the line switch and closes the usual energizing circuit for the slow acting release relay 151. The relay 151, upon energizing, connects ground to the conductor 144 in order to maintain the line switch in operated position. By the operation of the calling subscribers calling device the selector E is now operated similar to the manner in which the selector E was operated to select an idle trunk line extending to the second main exchange, and we will assume in this case also that the trunk line 3 is the one selected. Upon the seizure of the trunk line 3 the line relay 100 and release relay 101 are energized, the latter relay serving to complete a holding circuit for the selector E which as in the previous case includes the winding of relay 105. It will be observed, however, that the selector E has no coil cor responding to the coil 17 of the selector E, and it follows therefore that when the holding circuit for selector E is completed the current flow through the winding of relay 105 will be insufficient to enable it to energize. In other words, relay 105 is a marginal relay which will energize only when the repeater R is seized by a selector switch which is provided with a coil 17. Needless to say all selectors similar to the selector E are provided with such resistance coils while the selectors such as the selector E are not so provided.

The apparatus in the distant main exchange is now controlled through the medium of the repeater R in the manner previously described until the connection is finally established with the line of the called subscriber. When the receiver is removed at the called substation the relays 103 and 106 of the repeater are energized as before and with the same results with one exception, however. Since the relay 105 in this case is deencrgized the operation of relay 106 serves merely to reverse the conductors 49 and 51 as regards their connections to the windings of line relay 100 and does not operate to include the wmdings of the impedance coil 104 in these connections. This is true because when the relay 105 is deenergized its armatures 117 and 118 short circuit the windings of relay 104 at their resting contacts. It will be seen then that the line relay 100 of the repeater alone is left in bridge of the conductors 49 and 51 during conversation, thereby afi'ording an ample supply of transmitter current to the calling substation. The reversal of current flow in the callin line produced by the energization of re ay 106 may be used to cause the actuation of a meter or coin collecting device in any well known or desired manner.

Having described my invention what I consider to be new and desire to have protected by Letters Patent will be pointed out in the appended claims.

What I claim as my invention is:

1. In a telephone system, two subscribers lines, means including a repeater for setting up a circuit connection from either of said lines to a called line for talking purposes, a controlling relay bridge across said circuit in said repeater, and means in said repeater effective to increase the impedance of said bridge during conversation only when a particular one of said first lines is included in the connection.

2. In a telephone system, two subscribers lines, means including a repeater for setting up a circuit connection from either of said lines to a called line for talking purposes, a controlling relay bridge across said circuit in said repeater, and means comprising a marginal relay in said repeater effective to increase the impedance of said bridge durin conversation only when a particular one 0 said first lines is included in the connection.

3. In a telephone system, two subscribers lines, means including a repeater for setting up a circuit connection from either of said lines to a called line for talking purposes, a controlling relay bridge across said circuit in said repeater, and means in said repeater effective to increase the resistance of said bridge during conversation only when a particular one of said first lines is included in the connection.

4. In a telephone system, a trunk line including a repeater, means for setting up a circuit connection over said trunk line from two difl'erent oints for talking purposes, a controlling re ay bridge across said circuit in said repeater, an impedance coil for insertion in the bridge, and discriminating means in said repeater efl'ective to cause the inclusion of said coil in the said relay bridge only when a connection is extended over said trunk line from a particular point.

5. In a telephone system, a trunk line including a repeater, means for setting up a circuit connection over said trunk line from two difierent oints for talking purposes, a controlling re ay bridge across said circuit in said repeater, an impedance coil, and diseliminating means comprising a marginal relay in said repeater effective to cause the inclusion of said coil in the said relay bridge only when a connection is extended over said trunk line from a particular point.

6. In a telephone system, two subscribers lines, means includin an electromagnetic switching mechanism or setting u a circuit connection from either of said ines to a to said switches,

called line for talking urposes, a battery supply bridge across sai circuit included in said mechanism for supplying talking current to a calli line, means or supplymg one of said first ines when calling with talking current from a different oint, and automatic means for increasing t e resistance of said bridge when the connection is extended from the calling line last mentioned.

7. In a telephone system, first and second subscribers lines, means including a switching mechanism controllable over either of said lines when calling for extending a talking connection to a on led line, a battery supply bridge in said mechanism, and discrlnunating means for increasing the impedance of said bridge if a particular one of said first lines is included in the connection.

8. In a telc ihone system, first and second subscribers lines, .means including an electromagnetic switching mechanism controlla'ble over either of said lines when calling for extending a talking connection to a called line, a lntttery supply bridge in said mechanism, means for sup lying talkii current by way of said brl go to the saic first line after the connection is completed, and means whereby it the said second line is the calling line the resistance and impedance of mid bfidge is automatically increased.

9. In a telephone system, first and second subscribers lines, means including an electromagnetic switching mechanism controllable over either of said lines when calling for extend a talking connection to a called line, a attery supply bridge in said mechanism, and discriminating means for changing the resistance of said bridge if a particular one of said first lines is included in the connection.

10. In a telephone system, selector switches of two difl'erent kind-s, a repeater accessible to said switches, means including either one of said switches said re. ater for connecting calling and called lines, a battery supply bridge in said re, eater, and discrimihating means whereby if an established conincludesa selector switch of particular kind the impedance of said bridge is increased.

11. In a telephone ystem, selector switc es of two difl'erent kinds, a repeater accessible to said switches, means including either one of said switches and said repeater for connecting calling and called lines, a battery sirpply bridge in said repeater, and discriminoting means whereby if an established connection includes a selector switch of pfll'lilclller kind the resistance of said bridge is altered. V

12 In a telephone system, selector switches of two different kinds, a repeater accessible means including either one of said switches and said re eater for connecting calling and called llnes, a battery supply bridge in said repeater, discriminating means whereby if an established connec tion includes a selector switch of particular kind the impedance of said bridge is increased, the said means con'lprising a marginal relay in said repeater, a conductor including a winding of said relay adapted for seizure by said switches, means in the switches of one kind for causing a certain current flow in said conductor, and means in the switches of the other kind for causing a different cur-rent How in said conductor.

13. In a telephone system, selector switches of two different kinds, a repeater accessible to said switches, means including either one of said switches and said repeater for connecting calling and called lines, a'battcry supply bridge in said repeater, discriminating means whereby if an established connection includes a selector switch of particular kind the resistance of said bridge is altered, the said means comprising a mar ginal relay in said repeater, a comluclor including a winding of said relay adapted for seizure by said switches, means in the switches of one kind for causing a certain current flow in said conductor, and means in the switches of the other kind for causing a different current flow in said conductor.

14. In a telephone system, a trunk line consisting of talking conductors accessible from two different points, means for extending a connection from either point over said trunk line to a called telephone line, a battery supply bridge connected across the talking conductors of said trunk line, and automatic means efl'ective only when a connection is extended over said trunk line from a particular point for increasing the impedance of said bridge.

15. In a telephone system, a trunk line consisting of talking conductors accessible from two difi'erent points, means for extending connection from either point over said trunk line to a called telephone line, a battery supply bridge connected across the talking conductors of said trunk line, and automatic means efl'ective only when a connection is extended over said trunk line from a particular point for altering the resistance of said bridge.

16. In a telephone system, two selector switches, a repeater accessible to said. switches, means including either one of said switches and said repeater for connecting calling and called lines in a talking circuit, a controlling relay bridge in said repeater, whereby if an established connection includes one of said selector switches talking current is supplied to the talking circuit via said last mentioned selector, and means whereb if an established connection includes he other of said selector switches the .im edance of said bridge is increased.

7. In a telephone system, a trunk line ill] including a repeater, means for setting up a circuit connection over said trunk line from two diiierent points for talking purposes, a controlling relay bridge across said circuit in said repeater, an impedance coil for insertion in the bridge, and a marginal relay in said repeater efl'ective to cause the inclusion of said coil in the said relay bridge only when a connection is extended over said trunk line from a particular point.

18. In a telephone system, first and second subscribers lines, means including a switchin mechanism controllable over either of said lines when calling for extending a talking connection to a called line, a battery supply bridge in said mechanism, and a marginal relay for increasing the impedance of said bridge if a particular one of said first lines is Included in the connection.

19. In a telephone system, first and second subscribers lines, means including an electromagnetic switching mechanism controllable over either of said lines when calling for extending a talking connection to a called line, a battery supply bridge in said mechanism, means for supplying talking current by Way of said bridge to the said first line after the connection is completed, and a marginal relay for automatically increasin the resistance and impedance of said bridge if the said second line is the calling line.

20. In a telephone system, first and second subscribers lines, means including an electromagnetic switchin mechanism controllable over either of said lines when calling for extending a talkin connection to 3. called line, a battery supp y bridge in said mechanism, and a marginal relay for changing the resistance of said bridge if a articular one of said first lines is included 111 the connection.

21. In atelephone system, selector switches of two different kinds, a repeater accessible to said switches, means including either one of said switches and said repeater for connecting calling and called lines, a battery supply bridge in said repeater, and a marginal relay for increasing the impedance of said bridge if the established connection includes a selector switch of a particular kind.

Signed b me at Chica 0, Cook count State of Illlnois, this 24th a of May, 19l

TALBOT MARTIN.

copies of this patent may be obtained for are cents each, by addressing the "Commissioner of Patents, Washington, D. 0."

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