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Telephone switching system.

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US1110027A
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US
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line
exchange
extension
branch
switch
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William Aitken
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William Aitken
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    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04MTELEPHONIC COMMUNICATION
    • H04M7/00Interconnection arrangements between switching centres
    • H04M7/0096Trunk circuits

Description

W. AITKEN.

TELEPHONE SWITCHING SYSTEM.

APPLICATION FILED 00124, 1910.

3 SHEETS-SHEETI W. AITKEN. TELEPHONESWITGHING SYSTEM. APPLICATION TILED 00T.24,1910.

Patented Sept. 8, 1914.

3 SHEETSSHBET 2.

fizz/672207? ZflzZ/z'am 56 2/2272 21/2272 asses a? W W APPLICATION FILED OCT. 24, 1910.

Patented Sept. 8, 1914.

3 SHEETS SHEET 3,

Wb Wm NM NM 0 o O o o o 9 O ww M.

%T\ Q ZR.

WILLIAM AITKEN, OF LIVERPOOL, ENGLAND.

TELEPHONE SWITCHING SYSTEM.

Specification of Letters Patent.

Patented Sept. 8, 1914.

Application filed October 24, 1910. Serial No. 588,645.

To all whom it may concern:

Be it known that I, VILLIAM AITKEN, a

subject of the King of England, anda resiforded on connections between the central exchange and extensions of a branch exchange.

At the present time branch exchange switchboards are so arranged that clearing or supervisory signals are displayed at the branch exchange switchboard on all connections made through the switchboard. This clearing signal is given when the subscriber replaces h1s receiver on the hook at the termination ,of a conversation, and is the same whether theconnection be of local or extension lines to each other, or of an extension line to an exchange trunk line. The clearing signal, in case of a connection be tween an extension line and exchange trunk line, is given to the central exchange open ator when the private branch exchange operator disconnects the connecting cords or circuits from the exchange trunk line. For example, if a connection has been made at a private branch exchange between a central exchange and an extension lino terminating in the private branch exchange, and the receiver at the extension station is replaced upon the hook at the termination of the conversation, the clearing signal will show onlyat the branch exchange, the branch exchange connecting or transfer circuit being so arranged as to prevent the display of the clearing signal at the central exchange.

Should there be no regular operator at the branch exchange, or should the operator be negligent in removing the, connecting circuit from the exchange trunk line, the central' exchange operator underithese conditions is usually instructed to disconnect from the trunk line upon receiving the supervisory. signal from the central exchange subscriber. This leaves the trunk line, which the branch exchange subscriber was previously using and with which he is still connected, apparently available at the central exchange for other connections. The exchange operator may, therefore, use this trunk in attempting to complete a connection to this private branch exchange and will probably fail to get the branch exchange operator, but will obtain connection through the branch exchange with the extens on line previously connected, thereby causing confusion and a delay in securing the connection desired. Again, if an extension line subscriber, after the terminat on of a central exchange connection, desires a connection to an extension line terminating on the branch exchange board, and if the branch exchange attendant is attending to other duties or has been negligent in removing the connecting circuit or clearing the previous connection, the subscriber will, when he attempts to signal the branch exchange operator and give his order for the extension line connection desired, experience considerable delay in securing the connection.

In my invention I employ novel and improved means whereby, on a connection through the branch exchange between any extension line and the central exchange, there is obtained, when the extension subscriber replaces his receiver on the hook at the termination of a conversation, an automatic disconnect at the branch exchange, which restores all apparatus and lines employed in the connection to normal and at the same time gives the central exchange a clearing signal, thereby insuring a positive operation between the branch exchange and the central exchange and preventing delays occurring at the central exchange, due to slow operating on the part of the attendant at the branch exchange, and thus increasing the efiiciency of operating.

In the drawings which accompany and form a part of this s ecification, I have illustrated, two forms or embodiments of my invention, one applicable to use with common battery systems and one arranged to use with magneto systems.

Figure 1 shows a sectional elevation of my improved switching mechanism taken on the line AA of Fig. 2; Fig. 2 illustrates a section of the same taken on the line B-B of and will su s'equently describe,

Fig. 1; Fig. 3 shows a detail of the ratchet -mechanism taken on the line C--C of Fig.

1; Fig. 4 showsin diagram complete apparatus and circuit arrangements of my m-' vention as applied to a branch exchange of the common battery type; Fig. 5 shows in diagram complete apparatus and circuit a'rrangements of my invention as a plied to magneto type branch exchange; Fig. 6 represents in diagram an tromagnetic switch, by which, in connection with certain circuit arrangements, I am enabled to maintain connections electromagnetically during conversation, and to attain automatic disconnection of such connections on the cessation of conversation. In the figure an electromagnetic coil 12 is carried on a core 12, and 1s provided with a magnetic cup 15, which serves to bring the poles in such relation that the magnetic circuit may be closed by means of the armature 7. Cup 15 is supported by a series of brackets 9, 10, 11, which are attached to the cup 15 and serve to support the switching-mechanism upon the inner surfaceof the switchboard 25 in any convenient position. A shallow metallic cup 14: is secured to the magnetic cup 15 and is supported thereby, supporting in turn the metallic ring 13, whichcarries upon its outer surface the groups of springs 16, 17, 18, etc. The number of these groups of springs is limited only by the diameter of the ring 13 and the spacing of the groups thereon, the number employed depending upon the number of extensions to be served from a particular branch exchange. The armature 7 is carried upon the spindle 1, which further carries an arm 8 rigidly attached thereto. This arm 8 terminates in a knob 8 so positioned upon the arm 8 and so shaped as to register with and o erate, on depression, the switches, as 16, 1?, 18. A bushing 6, secured in an aperture provided therefor in the switchboard 25, admits and supports in a slidable manner the spindle 1. A hollow ratchet l is attached to the bushing 6 in a rigid manner and is provided with serrations corresponding with the number and spacing of the groups of switches employed. A exible switch lever 2 is rigidly attached to the spindle 1 near its outer extremity and moves therewith. One end of the switch lever is bent to make contact simultaneously with the two conducting rings 20 and 21 when the spindle is alternative method depressed. These rings are placed one within the other upon the face of the case 25, and are insulated from each other. The other end of the switch lever 2 is bent at substantially a right angle, and is provided with a tooth 2, which is formed to fit the serrations in the ratchet 4 and acts as a detent pawl. with a spring 5, wh1ch abuts at'one end upon the bushing 6, and upon the other end upon a ring 1 rigidly attached to the spin dle 1. This spring 5 is contained within the hollow portion of the ratchet member 4 and isadapted to maintain, through its compressive tension, the armature 7 normally out of contact with the poles of magnet 12. A knob 3 is securely fastened to the extended portion of the spindle 1 and serves as a means whereby the mechanism may be operated. It will be noted that as the groups of springs 16, 17, 18, etc., are radially spaced upon the ring 13, and as the ratchet is provided with serrations which coincide with the spacing of the groups of springs, that the arm 8, bearing the button 8, may be rotated to position the button 8' over any group of springs that may be selected. The switch lever 2 further acts as an indicator, and the positions of the groups of switch springs may be designated upon the outer surface of the case 25. Thus any particular group of springs may be selected by turning the knob 3 until the lever registers with the designating mark of the group desired. Thebutton 8 is held over the group-of springs in the position selected I by means of the ratchet mechanism 4 and 5. The switches of a group are operated by means of the button 8, when the spindle 1 is depressed by means of the knob 3, to break one set of contacts and make another. It will be readily understood by those skilled in the art that, if the energization of the magnet 12 is accomplished by the operation just described, the spindle 1, carf rying the arm 8, button 8' and lever 2, will ;be retained in its depressed position by gmeans of the attraction of the magnet 12 for the armature 7 until such time as the i magnet 12 will be denergized and allow the spindle, carrying the armature 7, to be returned to normal by means of the spring 5,

and thereby restoring the apparatus to nor- I mal. The contact rings 20 and 21, together with that portion of the lever 2 bearing Luse with a branch exchange operated on energizing circuit is broken, whereupon the The spindle 1 is provided operation of @a branch briefly explained, as the circuit -;tl ie reonnnon battery principle,-I have shown' "diagrammaticallythe electromagnetic multiple' zcontactswitch, previously described, at 16, 17, 18, 19, 12, 7 and 8, and the circuit eonneetions neeessa-ry to-adapt it to use with. a.- branch exchangehaving four extension lines and one trunk to the central exchange. .The balance of the circuit necessary to the exchan e is also shown; however, this part wil be only} arrangementis common and will be readily understood by Ithose skilled in the art. Conductor47 of the exchange line is connected to anouter contact of each of the groups of switches 19,18, 17 and 16. The conductor 48 of the exchange line, after passing through the non-inductive relay 50, is connected to the remaining outer contact of each of the groups of .switches, 16, 17, 18 and 19. One contact of the relay is connected through conductor 29 with one terminal of the magnet 12, the other terminal of which is connected to the local battery 9, which in turn is connected throu h its other terminal and conductor 30 to t e remaining contact of relay 50. Four exten-' sion lines radiate from the branch excl i afiige' ese and are shown at 31, 32, 33 and 34. lines are connected to the movable springs of each of the switches 16, 17, 18 and 19, respectively. The movable springs of the extension line jacks 41, 42, 43 and 44 are connected to the inner contacts of the switches 16, 17 18 and 19, respectively. The exchange trunk line 47-48 is provided with a drop 52, which is bridged in series with a condenser 53 across the line. key 54 is connected directly across the trunk line and is provided with a holding coil 55, wich is connected to one set ofcontacts onth'e key. The operators telephone set,

' shown at 56, is connected to the other set of contacts on the key 54 and is also connected to two contacts on key 27 of the operators cord circuit 26. The drop 52 is conceived to be automatically restored when the key 54 is operated to place the operator in speakingconnection with the exchange. The drop signals associated with the ans'wering jacks-41, 42, 43 and 44 are restored by the introduction therein of the plug of the operators cord circuit 26. The mechanical restoration of line signals is well understood in the art, and it is not deemed necessary to enter more fully into the description.

The operation of the circuits and mechanism is as follows: When a calling signal is received from the central exchange the operator at the branch exchange operates key 54 to make contact between the movable and upper contacts, as viewed in Fi 4, thereby connecting the operators telep one set 56 with the trunk line and at the same the ringing position, ingcurrent upon the extension line and tension station just si The trunk time automatically restoring the shutter of drop 52. Afterlearning the extension line desired by the central. ,exchange, the attendantinserts the calling plug of the cord circuit 26 in the jack associated with the ex tension line wanted and throws key 27 to thereby placing ring ringing the bell at the extension subscribers station. After receiving the response from the subscriber run the operator moves the multi-contact switch to the group of switches, as 18, associated with the ex aled and depresses the knob 3, thereby reaking contact between the movable contact springs and the inner contact springs on the switch 18, and making contact between the movable and outer contact springs on the same switch, thus connecting the-conductors of the extension line, as 33, to the conductors 47 and 48 of the trunk line, and therefore placing the extension station in direct communication with the exchange. I The subscriber upon the extension line, having answered r. the signal, will therefore have his receiver removed from the hook, thereby completing a circuit for the passage of current from the common battery at the central exchange through the non-inductive relay 50, which is actuated-thereby to attract its armature and close the local circuit of the electromagnetic multiple contact switch through the coil 12 and local battery 9, thereby energizing magnet 12 and maintaining the.100 armature 7, carrying the arm 8 and the button 8, in a depressed condition, thus maintaining the exchange connection to the extension line. The non-inductive relay 5Ov is used in order that the talking efliciency of the circuit may not be prejudicially affected. The key 54 is provided with a holding coil 55 in order that the operator, while signaling and receiving the answer of the extension subscriber desired, may no hold the exchange trunk busy by throwing the key 54 to a position to place coil 55 across the trunk. At the termination of. the conversation the subscriber replaces the receiver upon the hook, thereby opening the 1115 circuit of the trunk line and giving the operator at the central exchange the disconnect signal,-and also at the same time opening the circuit through relay 50, deenergizing the relay and thereby opening the local circuit of the magnet 12, which results in denergization of magnet 12, andthereby allowing armature 7, arm 8 and button 8 to be returned to normal by means of spring 5, Fig. 1. This restores switch 18'to normal and serves to disconnect extension 33 from exchange conductors 47 and 48. When a call is received from an extension line for an exchange line, the operator at the branch exchange calls'the exchange by placing the extension line upon the exchange trunk by means of the electromagnetic multiple contact switch, as described. This results in signaling the exchange operator, and the branch exchange operator or subscriber may request the called number from the exchange operator. Connections between extension lines are accomplished by means of the regular cords and plugs, as shown at 26, the battery for the operation of the subscribers set on the extension line being shown at 28.

Referring to Fig. 5, in which I have illustrated an adaptation of my invention to use with branch exchanges of the magneto type, I have shown such a branch exchange, together with one trunk line and four extension lines. I have illustrated diagrammatically the electroma etic 'multiple contact switch, previously escribed, at 16, 17, 18, 19, 12, 7 and 8. The exchange trunk line is represented by conductors 47 and 48, and the terminals of the four extension lines are represented at 31, 32, 33 and 34. Conductors 47 and 48' are multiplied to the outside contacts of switches 16, 17, 18 and 19. The

extension lines 31, 32, 33 and 34 are connected to the movable springs of the switches 16, 17, 18 and 19, respectively, while the inner contacts of said switches are connected to the movable springs of the answering jacks 41, 42, 43 and 44 associated with the extension lines 31, 32, 33 and 34, respectively. The jacks are associated with drop signals which may be of any desired type and need no explanation, as their action is well understood in the art. The exchange trunk 47 and 48 is provided with a ringdown drop signal 52, which is normally bridged across the line through the normally closed contacts on the operators or trunk key The operators telephone set 56 is bridged across the remaining contacts of key 55 and may be connected across the trunk thereby. A local battery 9 is connected to a terminal of the coil of the operating magnet 12 of the electromagnetic multiple contact switch, the remaining terminal of the coil 12 being connected to one contact of the switch '20 and 21, while the other contact of the switch is connected to one contact of the switch 54 through conductor 29, and the other contact of switch 54 is connected through conductor 30 to battery 9, thus placing the coil 12, switch 20, 21, and switch 54 in series in the circuit of the local battery 9. Switch 54 is arranged to open the local battery circuit through the magnet coil when ring-down drop 52 is energized to release its shutter. The ring-down drop 52, which is of the ring-down type usual in magneto type systems and requires generator current for its operation, is conceived to be restored whenever the key 55 is operated to connect the operators telephone set to the exchange trunk line, this restoration being accom lished in an one of a number of ways wel known in t e art. Switch 20 and 21 is a diagrammatic representation of the rings 20 and 21 and a portion of the lever 2 of Fig. 1, and is adapted to o erate to close whenever the knob 3 of the e ectroma etic switch is depressed.

- he operation of my invention, when adapted to use with branch exchanges of the magneto type, is as follows: When the call is received from the exchange the operator at the branch exchan moves key 55 to a. talking position, there y brid 'ng the operators telephone set 56 across t e trunk and disconnecting the drop 52 from the line.- This movement of key'55 also, by mechanical means, restores drop 52 to normal and allows switch 54 to close. After learning the requirements of the exchange the branch exchange attendant places the calling plug of the cord circuit 26 in the 'ac-k, as 43, associated with the extension line desired and moves key 27 to a ringing position, thereby placing ringing current u on the extension line-and ringing the bel at the extension subscribers station. After receiving a response from the subscriber rung, the operator moves the indicator of the electromagnetic multiple contact switch to the group of switches, as 18, associated with the extension just si naled, and depresses the knob 3, thereby making contact between the movable contact springs and the inner contact springs on the switch 18 and making contact between the movable and outer contact springs on the same switch, thus placing the conductors of the extension line, as 33, in connection with the trunk line conductors 47 and 48, and therefore placin the extension in direct communication wit the exchange. Upon the depression of the knob 3 of the electromagnetic multiple contact switch in the operation of connecting the extension line to the exchange trunk line, switch 20 and 21 will be closed, and, as switch 54 has previously been closed by the restoration of drop 52 to normal by the operation of the key 55, there will be completed a local circuit 'as follows: key 54, conductor '30, battery 9, coil of ma et 12, switch 20-21 and conductor 29. fiagnet 12 will therefore be energized to attract its armature 7 and will thus retain the electromagnetic multiple controlled switch in a depressed condition until the local circuit is broken. The connection between the extension line called and the exchange is thereth'gneratoricurieht received fi'oin tha exf-i tension statiiin thdreleases the'shutter 53. y The shatter-{when released, opens switchiyl ,1

and hen'ce opens the local circuitf-throf the electroma netic, deenergizin this in'a l'iet and cau'si'n the far'mature afa'yi'g arm 8;buttm 8 and-jvera, to'be rest-m5 to normaitthrough the-tension ofispring 5;, Fig. 1, 'thirsdisconnct'ing the extension ine' from the exchange line and connecting the iinie to its associated ac'lfijthereby restoring the apparatus at the branch change to. name:

a call-is received rpm extension 1ine. f o =a1r-' exchan e line, the: operator atifthe; branch f exchangh notes the number of the extension signaling m'aij moves the indicator. of the electromagnet'ic multiple contact switch to the group, oflsprings associated with the calling extension. 'lhe' operator then depresses the electromagnetic switch, having previ usly operated switch 55, to remove the rin down dropy52"fr om the extension line an place the 'operators telephonese't 56 across the -ex-' change trunk line. The operator thenjreqii'ests the subscriber at" the extension 'in-' strument to ring again, thus signaling" the cent'ral'exchange, the connection being main-f tained during conversation, and being auto-' matically disconnected on the cessation of conversation, as previously described. Local calls, or calls between'extension lines on nected" to the same'branch exchange, are handled in 'the usual ,way by means of the" jacks 41, 42, 43 and 44" andthe connecting cord'circuit 26. H I

It will be noted in the application of my invention disclosed in Fig. 4 that the necessity for a holding coil associated with switch 54 could be elimlnated in systems infwhijch ringing current is'supplied from the central exchange, by providing a ringing key for each extension line, in which case'the 'attendant at the'branch-excha'nge, 'upo'n receiving a call for an. extension, would depress the key associated with'the line called and ring until the line indicator jdenoted a reply, and then move the multiple contact switch to the group of springs associated with the line calling, thus obviating the necessity for a holding coil being placed across the exchange trunk while the operator is callin an extension.

It-is 0 vious that, when my device is utilized in connection with common battery systems in which theexchange trunk is of comparatively short length and consequently of low resistance, the use of the non-inductive relay 50 is not necessary and I may eliminate the use of local battery 9 and cause the energization of magnet 12 by means of current from the central station battery supplied over the exchange trunk 47 and 48, as illustrated in Fig. 6, 1n 6 which I have shown in diagram the magnet i frequently retained long.

r'nent of'the service.

The h ttn es f the "fa ilities'iaitorded ant at onceceases, thus eliminating 0' a c establish. the at can; h n

per "011' le at on. at the Inge. extentithe' nd r i' 'n t reby lending to; gr atly increase t e speed of'telep the efficiency of a eratingi Farther, in th as 1 n ht. h' 'ugh 'ia h as change, the 'nightyor; alarmfbell i'sij providing an audible signal. "But frequently l' is not connected with the supervisory signals, so that i'i'ofaudible'signal given pon the CQSSBElQiOh O f c'onversa-j tion; therefo e a connection between an ex;

tension line and the the night bel after conversation tends to the detrihas been. completedan 4 It is obvious from the foregoing that branch exchanges having more than one trunk line may be equipped with an'electr'o'ma" netic multi le contact switch for each trunk' 'i'n'e, the ension line and jack connections being multiplied through each, switch, thus enabling the operator at the branch exchange'to connect any extensionv to any trunk and vice versa.

j While I have. illustrated and described ertain embodiments'of my invention, it is understood that numerous changes may be made therein without departin from the spirit 'or' narrowing the scope'o my inventlon'.

hone service and central, exchange" is :H Ying thus describ a' my invent'itoi-lgwhat I- claim vas new and desire to secure, by

.United States Letters Patent is:

1. 'Iri'a telephone switching system, ,an exchange; a branch exchange; a trunk'line extending the rfebetween;v a plurality of extension l nes radiating from said branch ex? changefia-plurality of line jacks, one asso ciated with each of said extension lines;

a'" switching mechanism at said branch exchange and associated with said trunk line,

said extension-lines being normally in c'ontabt'with their respective line jacks through said switching mechanism, said switching mechanism being adapted to selectively remove one of said extension lines from connection with its respective line jack and place said extension line in connection with .said trunk line; electromagnetic means associated with said switching mechanism whereby said extension line is maintained in connection with said trunk line during traffic over such connection, and whereby said extension line may be automatically releases 2. In atelephone switching system, an cx change; a branch exchange; a trunk line extending therebetween; a pluralit of extension lines radiating from said ranch exchange; a line jack associated 'with each 01' said extension lines; an electroma etic switch mechanism at said branch exc ange and associated with said trunk line; said line jacks being n'ormall connected with their respective extension ines'throu b said switch mechanism, said switch mec anism being adapted'to selectively remove one of said extension lines from connection with its respectiveline jack and place said extension line in connection vith saiditrunk line' said electroma etic switch mechanism being further a apted to maintain said extension line in connection with said trunk line during traffic over such connection and. to automaticall disconnect said extension line from said trunk line, and restore said extension line toconnection with its" respec tive'line jack at the instant of termination of said trafiic.

3. In a telephone switching system, anexchange; a branch exchange; a plurality of trunk'lines extending therebetween; a plurality of extension lines radiating from said branch exchange; a a plurality of switching mechanisms at said branch exchange, one associated with each of said trunk lines; a pluralit of line jacks, one associated with eac of said extension lines, said line jacks being normally connected through each of said switching mechanism in multiple with their respective extension lines, each of said switching mechanisms being adapted to selectively remove one of said extension lines from connection with its respective line jack and place said I extension line in connection with said trunk line; electroma etic means associated with each of said said extension line is maintained in connection with said trunk line during trafiic over such connection, and whereby said extension line may be automatically released from connection with said trunk line and restored switching-mechanisms whereby .to connection with its respective line jack I at the termination of said trafiie.

4. a telephone switching system, a pill,- & ralit of exchanges; a branch exchange, trunij lines extending between said exchanges and said branch exchange; a plurality of extension linesradiating from said i branch exchange a plurality o electromag E netic switch mec 'amsms at said branch 'exchange, each of said switch mechanisms being associated with one of said,trunk lines respectively a line jack associated withone i of each of said extension lines, said line jacks being normally coniiected' throplgh each of said switch mechanisms in m ti lev with lmagnetic switch mechanism being adapted to selectivel remove any one of said exten- :sion lines rom connection with its associated line jack and place said extension line selectivel in connection with any one of said tru lines; said electromagnetic switch 'mechanism being further adaptedto maintain said extension line in connection with said trunk line during traiiic over such connection, and further to automaticall release said extension line from said trun line I and restore said extension line to connection with its respective line jack at the termina- :tion of said traflic. I

5. In a telephone stem, a switching device consisting of a s aft capable of rotary and longitudinal movements; a contact actuating member carried by said shaft and partaking of its movements; a group of sprin contacts lying adjacent to the circular path of said contact actuating member; means for manually rotating said shaft to select one of said group; means for moving said shaft longitudinally to cause said contact actuating member to engage the s rings of said group; an electromagnet a apted to hold said shaft in its engaging position, and means controlled over a connected telephone line for denergizing said magnet to release 'said shaft, substantially as described.

Signed by me at Liverpool, in England, in the presence of two witnesses.

v WILLIAM AIIKEN. Witnesses:

F. C. BERTIE,

F. 'STRIPLING.

Itheir respective extension lines;,sai electro- I

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2445820A (en) * 1945-03-31 1948-07-27 Rca Corp Motor control switching system and device therefor
US2454702A (en) * 1946-02-23 1948-11-23 Union Switch & Signal Co Circuit controller
US2640895A (en) * 1947-10-24 1953-06-02 Hans P Boswau Impulse-responsive stepping device actuated by magnetic torque
US2786904A (en) * 1954-05-13 1957-03-26 Gen Railway Signal Co Control switch for railway traffic controlling systems
US2936341A (en) * 1957-03-04 1960-05-10 North Electric Co Multiline telephone substation set
US2942076A (en) * 1957-08-07 1960-06-21 Marshall F Thompson Selective electrical control apparatus for parliamentary voting systems
US2991337A (en) * 1958-09-02 1961-07-04 Gen Electric Master selector switch

Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2445820A (en) * 1945-03-31 1948-07-27 Rca Corp Motor control switching system and device therefor
US2454702A (en) * 1946-02-23 1948-11-23 Union Switch & Signal Co Circuit controller
US2640895A (en) * 1947-10-24 1953-06-02 Hans P Boswau Impulse-responsive stepping device actuated by magnetic torque
US2786904A (en) * 1954-05-13 1957-03-26 Gen Railway Signal Co Control switch for railway traffic controlling systems
US2936341A (en) * 1957-03-04 1960-05-10 North Electric Co Multiline telephone substation set
US2942076A (en) * 1957-08-07 1960-06-21 Marshall F Thompson Selective electrical control apparatus for parliamentary voting systems
US2991337A (en) * 1958-09-02 1961-07-04 Gen Electric Master selector switch

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