US1550801A

US1550801A - Automatic telephone system

Info

Publication number: US1550801A
Application number: US359327A
Authority: US
Inventors: Charles L Goodrum
Original assignee: Western Electric Co Inc
Current assignee: AT&T Corp
Priority date: 1920-02-17
Filing date: 1920-02-17
Publication date: 1925-08-25
Anticipated expiration: 1942-08-25

Description

Aug. 25',v 1.925.

1,550,801 c. L. GoonRuM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 1920 'I Shouts-Sheet 3 /ni/fw for: Ww/fs L, bad/um by Affy.

A ug. 25, 1925. 1,550,801

C. L. GOODRUM AUTOMATIC TELEPHONE SYSTEM Filed Fab. 1'?, 1920 vsheets-sheat 4 /H van for.' Vzw/as l.' Gow/mm l Amig. 25, 1925.

C. L. GOODRUM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 1920 7 Sheets-Sheet 5- Aug. 25, 1925.

1,550,801 C. L. GGODRUM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 192C '7 Sheets-Sheet 6 I 4/rf/ vak/fak [7Min/f5 Wd/fam Aug. 25, 1925. l 1,550,801

C. L. GOODRUM AUTOMATIC TELEPHONE SYSTEM Filed Feb. 17, 1920 l. '7 Sheets-Sheetl 7 IlT- /h van/0n y (har/@s L .Cdad/w71 by lfm vAbw.-

Patented Aug. 25, 1925.

UNITED STATES PATENT OFFICE.

CHARLES L. GOODRUM, 0F NEW YORK, N. Y., ASSIGNOR T0 WESTERN ELECTRIC COM: PANY INCORPORATED, OF NEW YORK, N. Y., A CORPORATION 0F NEW YORK.

AUTOMATIC TELEPHONE SYSTEM.

Application filed February 17, 1920. Serial No. 359,327.

T0 all whom, t may concern.'

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

The present application is a continuation in part of my application Serial Number 329,922, filed OctOber 1l, 1919.

This invention relates to telephone systems wherein connections between lines may be established under the control of the subscribers and without the intervention of operators.I

I-Ieretofore such so-called automatic telephone systems employed either machineswitching means or combinations of great numbers of relays.

The object of this invention is to produce a system employing a small number of relays per line in which all of the functions performed in modern systems using machineswitching means will be efliciently performed.

Through the use of novel circuit arrangements and a novel method of selection, the above object has been accomplished.

The present invention contemplates a system in which the method of selection of a path to a called substation simulates the method used in modern telephone practice, that is to say, a large group of lin-es is served by a small group of trunks, the proportion of one to the other being determined by the traffic condition. The calling line is extended over a seized trunk to a particular large group of lines, whereupon another trunk is seized and over this the line is further eX- tended to a particular subgroup ofthe selected large group and, thereafter, a particular line is selected. So far the similarity is complete.

By using mechanism individual to each line it has heretofore been possible to obviate trunk-hunting by preselecting a trunk and holding it ready for the first incoming call and by using a common mechanism it has heretofore been possible to reduce the number of pieces of mechanism. The present invention contemplates an arrangement which combines the advantages of both sysi tems; a system in which a common mechanism is employed and in which an idle trunk is preselected and held ready for the first incoming call. In other words, instant connection to an idle trunk is accomplished through the use of a common mechanism and this applies not only to the selection of a trunk upon the initiation of a call, but to the selection of a trunk after the first digit selection has been made and so on.

In accordance with this invention, each line is provided with the usual line and cutoff relay, but the line relay performs a new function. I-Ieretofore this piece of apparatus merely functioned to initiate the movement of other apparatus and the calling line was identified through the method of changed potential on the third or test conductor in a system employing a common mechanism to respond to the calling lines, such for instance, as the well-known line finder switch system. The line relay in this system energizes connecting mechanism to instantly connect the calling line to the preselected trunk, thus being employed not only to startithe common mechanism in motion, but also to identify the calling line.

When the call has been extended over a trunk and the irst digit selection has been made, a pair of contacts are closed which perform functions similar to those of the line relay to actuate the next stage common mechanism and to identify the calling trunk.

In a ten per cent four-digit system there would be available for the use of a group of 100 lines, 10 trunks. According to the present invention each of these trunks leads to ten selecting mechanisms each serving a different group of 1000 lines. Entering each of these selecting mechanisms will be 100 trunks in groups of l0, incoming from l() groups of 100 lines each. Leaving each of these selecting mechanisms will be 10 trunks each leading to ten similar selecting mechanisms which, in turn, each serve a different group of 100 lines. In a similar manner each of these mechanisms have 100 incoming and 10 outgoing trunks. These last mentioned ten trunks each go directly to a selecting mechanism leading to a particular 100 lines.

In a ten per cent three-digit system the arrangement will be the same with the exception that one of the selecting mechanisms will be absent. Therefore, each of the trunks leading from the mechanism wherein the calling lines are identified will lead to a selecting mechanism in turn leading to a different group of 100 lines. Entering this Selecting mechanism will be 100 trunks in groups of ten incoming from l() groups of 100 lines each. Leaving each of these mechanisms will be l() trunks each leading to a selecting mechanism having direct control over a group of 100 lines.

While it has been found in commercial practice that the ten per cent. system gives the best results, this invention is not limited to such a system.

A calling line is identified through the method of electrically distinguishing two circuits one each in the principal direction of a two coordinate system, and through these circuits connecting mechanisms are instantly energized to connect the line thus identified to an idle preselected trunk. rlhereupon the subscriber operates a calling device to actuate the mechanism included in the trunk whereby one of the lO branches of this outgoing trunk is identified in a. particular one of the Al0 selecting mechanisms to which it leads. The identification of the trunk is accomplished in a similar manner to the identification of the calling line. rlhis action is then repeated in the second selecting mechanism after the second stage selection has been made by the calling subscriber. The mechanism in the seized trunk between the second stage selecting mechanism and the selecting mechanism leading to the particular 100 lines out of which one is to be selected7 is actuated by the calling subscriber. Here the identification of the called line is made through the method of electrically distinguishing two circuits each in the principal direction of a two coordinate system, the only difference being that in the previous identifications made the two circuits were electrically distinguished simultaneously while in this last stage the two circuits are electrically distinguished one after the other and in response to directive operations on the part of the subscriber. rather than automatic operations on the part of the calling line or the calling trunk.

The means for identifying the calling line comprises contacts on the line relay. The means for identifying the calling trunk comprises contacts controlled over the trunk and having` a similar function to the contacts on the line relay. Such contacts associated with the calling trunk may be actuated either by a small step-by-step switch or by a combination of relays known in the art as count-- ing relays. The question of preference between the two methode of actuating contacts is merely a question of commercial feasibility and hence both methods are illustrated and described in the following specification.

The method of preselecting an idle trunk entails no limiting action whatsoever and occupies only that amount of time which it tal-:es an electrical relay to become energized. The selection of idle trunks isj therefore, to all intents and purposes instantaneous and as soon as one trunk is seized another is preselected and ready for instant use. Therefore, a plurality of calling lines may all be served practically with no loss of time. Upon the seizure of the first idle trunk a relay is actuated to preselect the next idle trunk and if a second calling line immediately initiates a call it will seize the second preselected trunk and in turn a third trunk will be preselected through the mere energizing of a relay associated with the second trunk. Since it is almost impossible for two lines o initiate a call at exactly the same instant there will be no probability of two lines seizing the same trunk.

The release of the connection is under the control cf the calling subscriber. lVhen he replaces his receiver on its hook, release of all selected mechanism involved in the connection will be instantaneous since it inif'olves only the return to normal of connecting mechanisms having no movement eX- cept that from a normal to an actuated posi-- tion or vice versa.

ln the drawings, Fig. l is a diagrammatic view of the trunking arrangement employed. F ig. 2 diagrammatically represents the selecting mechanism by means of which a calling line is connected to an idle trunk. Fig. 3 diagrammatically represents the distribution of the trunks to the selecting mechanisms leading to different groups of lines. Fig. diagranmiatically represents selecting` mechanisms by means of which the beforenientioned trunks are connected with trunks leading to selecting mechanisms having functions similar to those of final selectors or connectors. Fig. 5 diagrammatically represents the selecting mechanisms which are equivalent to the final selectors or connectors, and through which connection of a trunk to a called subscribers line is made. Fig. (l diagrammatically represents a detail of F 8 showing the identifying means by which a trunk identifies itself in the next stage selecting mechanism. Fig. 7 diagrammatically represents a detail of Fig. 3 and is to be used in connection with Fig, 8. Fig. S is a detail view which diagrammatically represents a portion of Fig. 5. lt is an organization of circuits by means of which the calling subscriber controls the selecting mechanisms which are equivalent to the final selector or connector to make the final connection to the called subscribers line, actuate his call bell and perform other functions vi'ttt13991131'.

ordinarily performed in such a mechanism. Fig. 9 is a schematic representation of the release circuit of an established connection.

The general trunking arrangement is shown in Fig. 1. The square numbered 102 represents the connecting mechanism into which lines numbered 0 to 99 enter and out of which trunks, which, for convenience, we will number 0 to 9, lead. n a similar inanner square 103 represents the connecting mechanism into which lines to 199 enter and from which trunks 10 and 19 leave. In aI fully equipped system there will be ten of these groups oit mechanisms serving lines 0 to 999 inclusive, and out of which trunks 0 to 99 lead. Trunks 0 to- 99 then go into connecting mechanisms in the same manner. For instance, square 104 represents a connecting mechanism into which trunks 0 to 99 enter and out of which trunks which, for convenience, we will number 1000 to 1009, lead. These trunks 0 to 99 also enter nine other such connecting mechanisms, of which 105 and 106 are shown. A calling subscriber in the group, say 103, would be connected to a trunk in the group 10 to 19. If this calling subscriber desired another subscriber in the rst thousands group he would manipulate his calling dial to identiy this trunk in that one of the ten mechanisms into which his trunk enters which leads to the large group of lines numbered 10004999. 7e will assume that 104 is the desired one and that by sending one impulse, he has identified the trunk to which he had been connected in the group 104 and thereby selects a trunk in the group 1000 to There are ten such mechanisms as 104 intowhich trunks 0 to 999 enter and out of which trunks 1000 to 1099 lead. The trunks 1000 to 1099 enter ten mechanisms similar to 104, one of which is here shown numbered 107. The subscriber, having picked out a line in the first thousands group, now wishes to select a line in the lirst hundreds group of the first thousands group by selecting the first of the mecha.- nisms that is numbered 107. By sending one impulse he is connected to one of a group of trunks represented by 108. This trunk takes him into a connecting mechanism 114 serving lines 1100 to 1199.

With this short description and with the aid of the numbering on Fig. 1, the entire layout will be plain.

A similar view of a three-digit system may be obtained by calling the trunks 0 to 99 entering 104, lines 0 to 99 and the trunks 100 to 199 entering 109, lines 100 to 199 and the trunks 200 to 299 entering 113, lines 200 to 299, etc., and disregarding the remainder of the figure to the right. Trunks 1000 to 1009 would then become trunk 0 to 9 and the group of trunks 1000 to 1099 would become a group of trunks 0 to 99. Mechanism 114 would then serve lines 100 to 199, there being no thousands digit.

A similar diagram for a live-digit system could be built up by calling the lines 0 to 99 entering 102 trunks 0 to 99 and providing a similar layout.

The following specification is based on a three-digit system rather than a four-digit system, as this smaller system will illustrate suciently every ifea-ture of this invention.

In order to facilitate the description, only a small number of incoming lines and outgoing trunks are shown. It will be understood, however, that the incoming lines and the number of outgoing trunks are unlimited. Let it be assumed that this system is designed on the decimal basis and that the trunking arrangement is what is known as the 10 per cent system. Let it be further assumed that the subscribers lines shown in Fig. 1 belong to that group of lines numbered 200 to 299 inclusive. Six of these lines have been shown, 210 to 212 inclusive, and 220 to 222 inclusive. The invention will be illustrated by describing the establishment of a connection between line 220 and line 112.

When line 220, as numbered on the drawing Fig. 2, initiates a call, line relay 1 is energized. Through its armature it places ground potential on two circuits, each in the principal direction of a two coordinate system. The lirst group of circuits 2 is associated with the tens designation of the calling lines and the group 3 is associated with the units designation of the calling lines. Thus all the line rela-ys associated with lines 220 to 229 inclusive, upon actuating their armatures, will ground circuit 5 in group 2. Thus, also, line 220 will ground circuit 8 in group 3; line 221 will ground circuit 9 in group 3; line 222 will ground circuit 10 in group 3 and so on. Returning to the function of line relay 1, it will be seen that circuits 5 and 8 are grounded. Following these circuits as they lead to the right and upwards it will be seen that relay 12 is energized.

The group of relays 12, 22 and 23 individual to the trunk 27 is one of a number of similar groups employed for the purpose of preselecting a trunk. As will appear from the following description, the fast relay 12 and slow releasing relay 23 of a preselected trunk are actuated, relay 22 of a busy trunk is actuated and all relays of idle trunks are deenergized. Thus the trunk represented in the group of rela-ys below the one being considered is represented as being a busy trunk and its relay 26 (corresponding to relay 22) is energized. The energization of relay 26 causes the energization of relays 12 and 23 of the next idlel trunk and this puts the trunk 27 in the preselected condition, hence, upon the energization of the line iii- relay 1, the. circuits Yi'or the energization of' the groups and sub-group relays are cempieted through the armatures and contacts of relay Through Vthe. corresponfclinglett-hand armature ot relay 12, circuit will be grounded, which will, in turn, establish a circuit for relays 13 and 22. This circuit may be traced from ground at the armature and innermost contact ol` relay 1, circuit 5, corresponding lett-hand armature and contact or relay 12, circuit 15, winding ot relay 13, winding oit relay 22, to battery and ground. Tiel-ay 13 locks up in a circuit extending lfrom ground, the arn'iature and contact of relay 23, conductor 24, right-hand armature and Contact of relay 13, and thence over the circuit previously described to battery and ground. rt the same time through t-he outer contact ot relay 1 a circuit is established which may be traced from ground, the armature and outer contact of relay 1, circuit 3, corresponding` right-hand armature and contact ot relay 12, circuit 18, winding of relay 25, winding oi relay 22, to battery and ground. Relay 25, in a similar' manner to relay 13, locks up to the ground on conductor 2li. Relay 22 being energized with

relays

13 and 25 moves its armature from its back to its front contact. lt should be stated at this point that previous to the energization ot relay 22, a` circuit was normally established 'from ground, the armature and iront contact of relay 2G associated with a busy trunlr, the armature and back contact ot relay 22, and thence in parallel through the windings of relays 12 and 23. Upon the energization oi' relay 22, relay 12 becomes deenergizefcl, but relay 23 being slow to release, does not immediatelyv remove the ground from conductor 2a. The purpose oi this is to allow a sufficient interval tor trunk 2?, associated with relay to return ground from another point to hold relays and 13 energized. The energization of

relays

13 and 25 will connect line 220 to trunk 27, through the outer set of three armatures and contacts ot relay 13 and the right-hand set of contacts oit relay 25. Upon the establishment of this circuit, the ground on conductor 2li will be extended to conductor 23 through inner right-hand armature and iront contact oif relay 25 and thence to the third wire of line 220 through the inner armature of the outer set of three armatures of relay 13 where it will energize cutoff relay 29.

Connection has now been established between line 220 and trunk 27 and ground is placed on the conductor 24 by the liront contact and armature of relay 23 to hold

relays

13, 25 and 29 energized, until ground is returned on conductor 211 from trunk 27. Relay 22 having become energized, relay 12 becomes deenergized as described, and in turn relays 3() and 31, associated with the rascher next trunlr 32, become energized through the transicr of the ground at the armature of relay 26 to the windings of

relays

30 and 3 Associated with each trunk, as shown in Fig. 3, is a mechanism indicated by squares 33, 34 and Square 34 is associated with trunk 27 and the mechanism represented ther-cat is shown in Fig. G. llt will be seen that when the talking conductors of trunk 2T are connected to the calling substation, relay 33 will be energized. This relay, in moving its a mature trom its back to its 'liront contact, energizes a slow-releasing electroinagnet 3-1 which, besides controlling the holding pawl ot a small step-by-step switch, also controls two armatures. At the outer lett-hand one ot' these armatures, a ground is placed on the lead 24 to replace the ground hold on this lead at the armatin'e ot slow-releasing relay The connection is now in a stable` condition and operations arc suspended until the subscriber on line 22() manipulates his calling device.

it was assumed that line 220 was to establish a call to line 112. The subscriber, theretore, manipulates his calling device to send a single impulse over the connection. This impulse is in the form of an interruption of a line circuit in accordance with modern automatic telphone practice. This single impulse deenergizes relay 33 during a sufficient interval to establish a circuit from the armature and back contact of relay 33, the inner left-hand armature and Contact of relay 34, the winding of stepping magnet 35 to battery and ground, whereupon the brushes 36 and 37 are stepped on to their first contacts.

Before going further with the description ot the operation, it will be necessary to explain F 3 more in detail. ln this ligure, trunk 27 is shown with three sets of taps, 3S, 39 and 490, these being representative of the ten taps which would be used in a ten per cent system as assumed. At the righthand portion of Fig. 3, two squares, l1 and l2, are shown. These squares represent lineconnecting mechanism similar to that shown in Fig. 2. The square l1 representing the connecting mechanism associated with the lines whose members are 0 to 99 inclusive, and the square 112 representing the mechanism associated with the group of lines whose members are 100 to 199 inclusive. The set of taps 33 ot the trunk 27 leads to a connecting mechanism which in turn leads to a group of lines numbered 100 to 199 inclusive. The set of taps 39 leads to a mechanism which in turn leads to a group of lines numbered 200 to 299 inclusive, and in a similar manner seti of taps 40 leads to a mechanism which in turn leads to a group of lines numbered 300 to 399, inclusive.

In further explanation of this system,

and referring to Figures 2, 3, 4 and 6, it

' may be stated that each trunk leading from a non-numerical switching mechanism such as is shown in Figure 2, is provided with a connecting mechanism actuating means indicated in Figure 3 and shown in detail in Figure 6. This means comprises 'two brushes, 36 and 37, arranged to travel over and make contact with sets of terminals connected to two sets of conductors arranged in the two principal directions of a two coordinate system. The brushes 36 and 37 move together so that they may both be placed on their first terminals connected to 45 and 51, on their second terminals connected to 59 and 60 or on their third terminals connected to 61 and 62. Thus, by one, two or three steps of the brushes 36 and 37 the trunk 27 may be identified in the non-numerical switching mechanism shown in Figure 4, non-numerical switching mechanism II in Figure 3 or non-numerical switching` mechanism III in Figure 3.

It should be noted that no two of the connecting mechanism actuating means' shown in Figure 3 are connected in the same manner. The set of brushes shown directly above 36 and 37, Figure 3, connect through their first terminals, not with 45 and 51 as at 36 and 37, but with 45 and the conductor to the left of 51. The set of brushes shown directly below 36 and 37 connect through their first terminals with 45 and the conductor to the right of 51. The set of brushes to the right of 36 and 37 connect through their first terminals with the conductor to the right of 45 and with 51. lVe thus have a connecting mechanism actuating means individual to each trunk, so associated with the sets of contacts leading to the different non-numerical switching means to which the trunks lead, that each trunk may be identied in each non-numerical switching means through the method of electrically distinguishing two contacts, one each in the principal direction of a two coordinate system.

The system may be readily explained in terms of a three coordinate system. Thus a set of trunks as 0 to 99, each having a single point of ingress as at 102, 103 and so forth, and each having a plurality of points of egress as at 104, 105, 106 and so forth, are arranged to be identified at their points of egress by conductors such as 45, 51, 59, 60, 61, 62 and so forth arranged in a three coordinate system. Each trunk might be said to be on a different line in one of the three principal directions of such a system, and as the pointers (the brushes 36 and 37) are moved one, two or more steps in that direction, they contact with two contacts one each in the remaining two principal directions of the three coordinate system.

Since the subscriber on line 220 has manipulated his sender to transmit a single impulse, brushes 36 and 37 will come to rest on their first terminals. Stepping magnet 35 is so constructed that its armature which controls the ratchet wheel 43 is quick to act but the armature shown at the left-hand end being of much lighter construction, and under a much smaller spring pressure, is slow to release. After the single interruption of the circuit, relay 33 attracts its armature and holds it in this position thereafter. Magnet 35 becoming deenergized after a short interval, the left-hand armature retracts and closes a circuit from ground, the outer left-hand armature and contact of magnet 34', conductor 24, the outer left armature and contact of relay 44, the lefthand armature and two contacts of magnet 35 and thence in parallel to the brushes 36 and 37. In this manner two conductors each in the principal direction of a twocoordinate system are momentarily grounded. The conductor grounded by brush 36 and numbered on Fig. 3 as conductor 45, leads through a corresponding left-hand armature and contact of relay 46 to conductor 47, relay 48, conductor 49, winding of relay 50 to battery and ground. The conductor grounded by brush 37 and numbered on the drawing 51, leads through a corresponding right-hand armature of relay 46, to conductor 52, winding of relay 53, conductor 49, winding of relay 50, to battery and ground. Thus through a movement of one step by brushes 36 and 37, relays 48 and 53 are energized and the trunk 27 is eX- tended through a middle set of three armatures and contacts of relay 48 and through the set of three right-hand armatures and contacts of relay 53 to a trunk 54. In a manner similar to that heretofore described, conductor 55 is temporarily grounded by the left-hand armature and contact of relay 56 until a ground has been returned from conductor 55 of trunk 54. Also in a similar manner to that heretofore described, relay 50 becomes energized and transfers the ground at its armature to the windings of relays 57 and 58 of the neXt idle trunk.

If the relay 33 had been deenergized twice to cause the stepping magnet 35 to move the ratchet 43 two steps, then brushes 36 and 37 would have each been stepped to their second contacts. In this case conductors 59 and 60 would have been grounded and consequently relays in the mechanism entered by tap 39 of trunk 27 would have been energized. Also, in a similar manner, if brushes 36 and 37 had been given three steps,

conductors

61 and 62 would have been grounded and relays in the mechanism entered by tap 40 of trunk 27 would have been energized.

Upon the seizure of trunk 54, ground on conductor 55 will be extended through the DIlGI fight-hand armature of relay 53, third Wire conductor 621 corresponding armature and contact of relay 48, third wire conductor of trunk 27, winding of relay 44 (Fig. 6) to battery and ground. Relay 44 in energizing extends the ground from conductor 55 through its inner lett-hand armature and contact to conductor 24 and at its right-hand armature opens the circuit of relay 33. Relay 33 becomes deenergized and, after an appropriate interval, magnet 34 in turn becomes deenergized. Upon its deenergization, magnet 34 retracts the holding pawl 63 and the brushes 36 and 37 are returned to normal. Thus brushes 36 and 37 are similar in function to the Contacts on the line relay 1, and the relay 44 is similar in function to cut-ott relay 29.

Trunk 54 leads through a combination of relays represented by square. 64. This is shown more in detail in Fig. 8.

In Fig. 8 it will be seen that when trunk 54 is seized by the line 220, line relay 65 will become energized. This relay, through its left-hand armature and front contact, energizes sl-ow-to-release relay 66, which, through its right-hand armature, grounds conductor 55, thus replacing the ground at the armature and contact of relay 56. At its outer left-hand armature relay 66 places a ground on conductor 155. rlhe connection is nowl again in a stable condition and awaiting the further operations of the calling subscribers sending device.

It was assumed that subscriber on line 220 would establish a call to line 112, and it has been seen how through the sending of a. single impulse, that subscriber has selected a trunk leading to a group of lines all ot whose 100s digits are one.

The calling subscriber now manipulates his sending device to interrupt the circuit of line relay a single time. Upon the retraction of the armature of relay 65 a circuit is established from ground, left-hand armature and back Contact ot relay 65, inner lefthand armature and contact of relay 66, inner left-hand armature and contact of relay 68, conductor 84, winding of relay 67, conductor 83, outer left-hand armature and contact of relay 71, outer right-hand armature and contact of relay 72 to battery and ground. Relay 67 becomes energized and through its right-hand armature and front contact places a ground on one terminal of relay 69 whose other terminal is connected through conductor 83 to battery and ground, whereby relay 69 is energized. The ground placed on conductor 84 while the armature of relay 65 is resting on its back contact is also extended through the letthand armature and back contact of relay 75, the winding of relay 76 to battery and ground. Relay 76 becomes energized and by means of its left-h and armature and contact extends a circuit from ground, the outer letthand armature and contact of relay 66, the left-hand armature and contact of relay 67 conductor 81, the said armature and contact of relay '7 6 to one terminal of relay 75. Relay 75, however, is shunted at this time by the ground on conductor 84. Upon the attraction of the armature of relay 65 immediately after the interruption of the circuit of relay 65, the shunt around the winding of relay is removed and it becomes energized in series with relay 76. Since there is only a single interruption of the line circuit for transmitting the digit 1, only the pair of counting relays 75 and 76 will become in this manner energized. The conductor 77 which is one of the conductors in the two groups of conductors each in the principal direction of a two-coordinate system, is now connected through the right-hand contact and armature of relay 76 and the back contacts and righthand ari'nat-ures ot the other lower coimting relays to conductor 80. A short interval after relay 65 has become energized relay 67 retracts its armatures. Thereupon a circuit is closed from ground, the right-hand armature and contact of relay 67, the inner right-hand armature and front contact oi? relay 69, the outer right-hand armature and back Contact of relay 7 9, conductor 80 and thence to conductor 77. Conductor 77 is connected to relay 78 and the grounding ot this conductor causes the energization of relay 78 which through its right-hand armature and contact locks up to the ground on conductor 155 supplied by the Yfront contact and outer left-hand armature of relay 66.

After relay 67 has become deenergized, and before relay 69 becomes deenergized, ground is extended to relay 70 the other terminal ot' which is connected to battery. Relay 70 becomes energized and through its armature and contact extends ground through the normal lett-hand contact and armature ot relay 79. The other terminal ot relay 79 is, however, connected to conductor 155 and as this constituted a` shunt circuit relay 79 does not at this time become energized.

Shortly after relay 67 becomes deenergized, relay 69 retracts its armatures. At its outer right-hand armature it removes the ground 'from conductor 81 whereby the counting relays are released. At its inner right-hand armature it removes ground from the winding of relay 70 but the latter holds up momentarily in series with relay 7 9. The latter energizes and by so doing' at .first shunts relay 7 0 through the lefthand continuity contacts of relay 79 and then opens the circuit of relay 70, whereby it becomes deenergized. Relay '79 is now held energized over the following circuit: ground, outer left-hand armature and contact or' relay 66, conductor 155, winding of relay 79, left-hand armature and alternate contact of relay 7 9, resistance 98, conductor 83, outer left-hand armature and back contact of relay 71,` outer right-hand armature and contact of relay 72 to battery and. ground.

The apparatus is again in a stable condition awaiting the receipt of the last series of impulses.

In accordance with our previous assumption, the subscriber will now send a train of two impulses. Having picked out a group of lines whose hundreds digit was 1 and Whose tens digit was 1, the calling subscriber wishes to pick out a particular one of these lines Whose units digit is 2 in order to connect with the line numbered 112. Two impulses will be sent and relay will be deenergized twice in quick succession. Upon the first retraction of its left-hand armature, the relay 67 will become energized, as previously described. Upon the first interruption, the circuit for relay 76 will again be established and relay 76 will become energized. At the end of this interruption, this circuit will be opened but relay 76 in the meantime having attracted its leftehand armature, it will remain energized in series with the winding of relay 75, as heretofore described. Relay 75 becomes energized and transfers the circuit of conductor 84 through its left-hand armature and front contact, the left-hand armature and front contact of relay 86 to the winding of relay 87. Upon the beginning of the second impulse, relay 65 closes the circuit heretofore described which now includes the winding of relay 87 whereby relay 87 becomes energized. Shortly thereafter the circuit including conductor 84 is opened at the left-hand armature and contact of relay 65 and the shunt being removed from around relay 86, it becomes energized in series with the winding of relay 87. Conductor 89 is now included in a circuit including the front contact and right-hand armature of relay 86, the back contacts and rightehand armatures of the rest of the upper row of counting relays to the conductor 82. During the sending of these two impulses relay 67 has become energized through a circuit from ground, the armature and back contact of relay 65, the inner left-hand armature and front contact of relay 66, the inner left-hand armature and back contact of relay 68, conductor 84, winding of relay 67 condactor 88, back contact and outer left-hand armature of relay 71, back contact and outer right-hand armature of relay 72 to battery and ground. As before, relay 67 causes the energization of relay 69. Shortly after the sending of this last :series of impulses relay 67 retracts its armatures. A circuit is now established from ground, the right-hand armature and back contact of relay 67, the inner right-hand armature and contact of relay 69, outer right-hand armature and front contact of relay 79, conductor 82, and thence to conductor 89. Conductor 89 is one of the conductors in the second group of conductors each in the principal direction of a two coordinate system, and is connected to the winding of relay 90. Relay 90 is energized and in attracting its armatures locks itself through its left-hand armature and contact to the ground on conductor 155.

The ground from the armature and back contact of relay 67 is also extended to the winding of relay which becomes energized. Relay 70 in attracting its armature entends this ground through the inner righthand armature and contact of relay 79, the left-hand armature and normal contact of relay 68 to the Winding' of relay 68, but since the other terminal of relay 68 is connected to the conductor 155 it is shunted and does not energize at this time. A short interval after the deenergization of relay 67, relay 69 retracts its armatures. The first effect of this is to remove the ground from conductor 81 whereby the counting relays are restored tol normal. Next the ground is removed from the terminal of relay 70 but it remains energized in series with relay 68 which now that the shunt is removed becomes energized. It is held energized through a circuit from ground, the outer left-hand armature and contact of relay 66, conductor 155, winding of relay 68, right* hand armature and alternate contact of relay 68, resistance 99, conductor 83 and thence to battery and ground over the path previously described. The energization of relay 68 causes the deenergization of relay 70.

Let it be assumed that line 112 is, at this time, involved in a connection and is therefore in a busy condition. This busy condition will be denoted by a ground on the third conductor which will be extended through the innermost left-hand armature and contact of relay 78, the innermost righthand armature and Contact of relay 90, conductor 91, the left-hand armature and front contact of relay 69 (before relay 69 becomes deenergized), the inner rightnhand armature and normal Contact of relay 71, winding of relay 71 to battery and ground. Relay 7l becomes energized and locks itself through its inner right-hand armature and alternate contact to conductor 155. By attracting its outer left-hand armature, relay 71 removes battery and ground from conductor 83 whereby relays 68 and 79 are deenergized, or more strictly speaking, relay 79 is deenergized and relay 68 is prevented from being energized as above described. Through its inner left-hand contact relay 71 connects the source of busy tone 92 to one winding of the called subscribers battery feed coil whence by induction this tone is transmitted to the calling subscriber.

Attention is now directed to Fig. 9. 1t will there be seen that relay G5 through its right-hand armature and 'liront contact controls the slow-releasing relay (5G which, in turn, at its contacts and armatures, places a ground on com luctor and conductor 155 to hold the relays controlling the connection in an energized condition. 1t will be seen that ground on conductor 155 holds relay 71 in an energized condition. 1t also holds relays 78 and 90 oitl F ig. 5. Ground on conductor 55, in turn, holds relays 4S and 53 in parallel and relay 5() in series with these two energized, in Fig. 4. 1t holds relay 44 et Fig. 6 energized and through the innermost lett-hand armature and front Contact of relay 44, this ground is extended to Conductor 24 where it holds relays 13 and 25 in parallel with relay 22 in series energized and also holds the cutoi'l' relay E29 energized. 1f the calling subscriber hea-rs the busy tone connected to the line through the agency of relay 71, he will replace his receiver on its hook and therethrough relay will become deenergized. After a very short interval, relay 66 will become deenergized and by removing the ground from

conductors

55 and 155, will cause the instant deeuergization of all the relays just described whereupon the connection will be disestablished and all connecting mechanisms returned to their normal. condition.

le will assume now that the line 112 is idle in which case no ground will be returned over conductor 91 to energize relay 71, as described. Therefore, atter relay 69 has become deenergized, relay 68 becomes energized in the manner heretofore described. Thereupon a` circuit is completed from ground, the outer right-hand armature and bacl contact ot relay 71, the left-hand armature and back contact of relay 69, conductor 91, the innermost right-hand contact and armature ot relay 90, the innermost left-hand contact and armature of relay 78, the winding' of cutoi'l relay 101 to battery and ground, whereby line 112 is marked as busy. At the same time, a circuit is established from ground, the outer right-hand armature and contact of relay 69, the outer left-hand contact and armature of relay G8, the winding ot relay 7 4, the outer left-hand armature and contact of relay 71, the outer right-hand armature and contact of relay 7 2 to battery and ground. Relay 74 attracts its armatures and connects a source of ringing current 97 through the winding of a rnging cutoli relay 94 to the line of the called subscriber. The current in this circuit is insuiiieient to cause the energization et' relay 94 until the subscriber in answer to the summons of his` bell removes his receiver 'from its hook. Thereupon the flow of ringing current is increased and relay 94 becomes energized.

1t will be noticed that prior to the energization ot' relay 94 that ground from conductor 155 was extended to one terminal ot relay 72, the other terminal of which is connected through resistance 190 to battery and ground. However, ground from conductor 155 also extends through the contact and armature of relay 94 to the other terminal et relay 72, whereby it is shunted until relay 94 becomes energized. lllhen this occurs, relay 72 becomes energized, and by means of its inner right-hand armature and contact opens the shunt circuit so that it cannot again become effective. Through its lefthand armatures and front contacts, relay 72 connects the calling and called subscribers in a conversational circuit` Relay 72 at its outer right-hand armature removes battery and ground from conductor 83, whereby relays 68 and 79 are deenergized.

Attention is again called to Fig. 9. The release circuit is new in the condition shown in Fig. 9 with the exception that instead ot relay 71 being locked to conductor 155, the ground will extend to the winding of relay 72 and relay 71 will hold relay 101 energized by means of the outer right-hand armature and contact ot relay 71. At the end of the conversational period, the calling subscriber, by placing his receiver on its hook, will cause the instant disconnection otl the relays involved in the circuit shown in Fig. 9 in a manner similar to that heretotore described.

The called line is guarded against intrusion by ground potential from ground at the outer right-hand armature and back contact oit relay 71, lett-hand armature and back contact of relay G9, conductor 91 through the corresponding contacts and armatures of relays 90 and 78 to the third or test conductor or line 112.

Attention is called to Fig. 7 wherein a substitute arrangement for the counting relays o-t Fig. 8 is shown. This arrangement is very similar to that shown in Fig. 6 comprising a stepping magnet 98 connected to the conductor S4 and a holding magent 99 connected to the conductor 81. The impulse ot current supplied over the conductor 84 causes the stepping magnet to rotate the wipers ot this small step-by-step switch into connection with the contacts, whereby conductor 77 may be grounded over the conductor 89 by one step and the conductor 89 grounded over conductor 82 by two steps. The operation ot this is obvious when taken in connection with the description of the operation ot the counting relays.

What is claimed is:

l. In a telephone system employing relays for interconnecting subscribers telephone lines, a holding and release trunk comprising a continuous path extending through each stage for maintaining energized the interconnecting relays thereat.

2. In a telephone system employing switching devices having permanently paired active and passive contacts for interconnecting subscribers telephone lines, a holding and release trunk comprising a continuous path extending through each switching stage for maintaining energized the switching devices thereat.

3. In a telephone system employing switching devices having permanently paired active and passive contacts for interconnect-- ing subscribers telephone lines, a holding and release trunk comprising a continuous non-inductive path extending through each switching stage for maintaining energized the switching devices thereat.

4. In a telephone system employing switching devices having a simple movement of slight extent for interconnecting subscribers telephone lines, a holding and release trunk comprising a continuous path extending through each switching stage for maintaining energized the switching devices thereat.

5. In a telephone system employing switching devices having simple movements of slight extent for interconnecting telephone lines, a release trunk comprising a continuous non-inductive path extending through each switching stage.

In witness whereof, I hereunto subscribe my name this 11th day of February A. D. 1920.

CHARLES L. GO ODRUM.