US2154547A - Teletypewriter system - Google Patents

Description

A ril 18, 1939. c. R. WALKER TELETYPEWRITER SYSTEM Filed Dec. 11, 1935 2 she e'ts-sheet 1 INVEA ITOR C. R. "44L KER H ATTORNEJK April 18, 1939. c. R; WALKER 2,154,547

' TELETYPEWRITER SYSTEM 7 Filed De c. 11, 1935 2 Sheets-Sheet 2 AT TORNEV Patented Apr. 18, 1939 UNITED STATES TELETYPEWRITER SYSTEM Charles R. Walker, Jersey City, N. J., assignor to American Telephoneand Telegraph Company, a corporation of New York Application December 11, 1935, Serial No. 53,950

' 18 Claims. (01. 173-2) This invention relates to teletypewriter systems and, more particularly, to systems which include a teletypewriter having contacts for performing various functions.

It is an obfect of this invention to mount contacts, for performing various functions, in a teletypewriter to be operated by the teletypewriter mechanism.

It is also an object of this invention to open l and close, by means of a central teletypewriter,

telegraph lines leading from a teletypewriter oilice to otherteletypewriter stations.

It is a furthergobject of this invention to provide a central teletypewriter oflice with an iml proved switching device for switching various circuits.

It is an additional object of this invention to enable a subscriberat a teletypewriter station to connect his teletypewriter through a teletype- 0 writer central oflice to another teletypewriter station without the assistance of an operator at the central ofiice thereby effecting a saving in operating time.

It is a further object of the invention to enable subscribers having their lines connected together at a central oifice to disconnect the lines at the central omce without the assistance of an operator at the central office thereby effecting a further saving in operating time. 3 These and other objects of the invention are attained by means of a plurality of contacts mounted in a teletypewriter at. various predetermined positions. The contacts are adapted to be operated by the'teletypewriter mechanism in 35" response to certain preassigned teletypewriter signals which may, if desired, be sent from other teletypewriter stations. ment of the invention, the operation of the contacts serves to operate a relay which operates and 40i'1ocksup to perform any desired function, such as switching a telegraph'bircuit. Also in the preferred embodiment of the invention, the operation of'another contact unlocks the relay and causes it to deenergize thereby restoring the sys- 45' tern to its normal condition, If desired, a plurality of these relays may be provided for performing a variety of functions, such as starting the teletypewriter'motor of an outlying teletypewriter station. I 50" These and other features of the invention will now be described in detail with reference to the drawings inwhich:

Fig. 1 represents a teletypewriter central oflice and shows a schematic diagram in perspective 55 of a teletypewriter provided with special contacts In the preferred embodiand circuit connections leading to other teletypewriter stations; and

Fig. 2 shows the other teletypewriter stations to which the teletypewriter central oflice shown in Fig. 1 is connected. 6

The teletypewriter T, shown in Fig. 1, is similar to the teletypewriter disclosed in Patent 1,745,633 granted February 4, 1930 to S. Morton et al. The disclosure of this Morton et al. patent is incorporated herein by reference as a part of this specifllo cation. However, the invention is not limited to this type of teletypewriter nor is it limited to the specific structure shown in the drawings which is a preferred embodiment of the invention and is shown and described in order to illustrate-and explain the features and principles of operation of the invention.

For purposes of illustration, teletypewriter T is shown to be located at a teletypewriter central office although it may be located at an ordinary 2o teletypewriter station if desired. Connected to this central oflice by telegraph lines L1, L2, L3,

and L4, respectively, are the subscribers stations S1, S2, S3, and S4 each of which is provided with a teletypewriter T1, T2, T3, and T4, respectively, and a teletypewriter motor M1, M2, M3, and M4, respectively. Each of the motors M1, M2, M3, and M4 is operated by a power supply source B1, B2, B3, and B4, respectively. At stations S1 and $2 the energizing circuits leading from the power supply sources to the motors are opened and closed by manually operable switches A1 and A2, respectively, controlled by the local attendant at each station.

The energizing circuit of the motor M3 is normally open at the armature and contact A3 of relay 43 and is adapted to be opened and closed by any control method that may be desired, such as that shown in Patent 1,913,431 granted June 13, 1933' to W. Daum. The disclosure of this 40 Damn patent isincorporated herein by reference as a part of this specification. The energizing "circuit of motor M4 is normally open at contact A4 .of relay Ra located at the central ofiice. It is to be understood that these different motor con-' trol circuits may be varied as desired and that they are shown for' the purpose of illustrating the wide extent to which the invention may be applied. It should also be understood that the invention is capable of use with any other type of line circuits, such as duplex circuits.

The teletypewriter T at'the central office has a motor M which is operated by a power supply source B over an energizing circuit closed by a manually operable switch lever A. Motor M operates teletypewriter T by means of gears G1 and G2 which alternately raise and lower the printing bail I to operate the pull bars selected by the selecting mechanism 2 as is described in the Morton et al. patent. Mounted in the teletypewriter T are four contacts 01, C2, C3, and C4 all of which are normally open except contact C3 which is normally closed. These contacts are adapted to be operated by the teletypewriter mechanism as will be explained subsequently.

In the operation of the system, let it first be assumed that the subscriber at station S1 wishes to be connected to the subscriber at station S2.

The subscriber at station S1 normally has his switch A1 closed during business hours so that his motor M1 is energized by his local power supply source B1. Accordingly, the subscriber transmits, by means of his teletypewriter T1, a combination of significant current impulses, corresponding to a preassigned upper case permutation code signal, out over line L1 and conductor IT to the central ofiice. At the central office, the preassigned combination of significant current variations is received by the receiving magnet R which operates the selecting mechanism 2 of the teletypewriter T for selecting the pull bar 3. Pull bar 3 is then elevated by the reciprocating printing bail I. This elevates the left end of lever 4 and causes its right end to drop down thereby allowing the left end of lever 5 to be pulled down by spring 6. Consequently, the right end of lever 5 is raised to allow the spring contact C4 to close.

A circuit is now closed from grounded battery I, through relay R4, over line conductor 8, through contact C4, along conductor 9, then conductor I0, through the normally closed contact C2, conductor II, conductor l6, and then to ground I2. As a result, relay R4 becomes energized and operates both its armatures. This switches the line conductor II to contact I3, joining conductor 26, contact I4, line conductor I8, through station S4, conductor 21, and then out over line L2 to the subscriber's station S2. At the same time, relay R4 locks up from grounded battery I through contact I5, over conductor 9, conductor I0, through the normally closed contact C2, conductor II, conductor I6,'and then to ground l2.

Since the subscriber at station S2 normally has his switch A2 closed during business hours, his motor M2 will be energized at this time by his local power supply source B2. Accordingly, the system is now in condition for communication to take place between the subscriber at station S1 and subscriber at station S2. Significant current variations transmitted by the teletypewriter T1 will travel over line L1, conductor I I, contacts 49 and 50 of. relay R conductor 42, relay R, operated armature and contact I3 of relay R4, conductor 26, operated armature and contact I4 of relay R4, conductor I8, through station S4, conductor 21, and then out over line L2 to the teletypewriter T2 at station S12. Due toboth motors M2 and M4 being disconnected from their respective power supply sources B2 and B4 at this time, neither teletypewriter T3 nor teletypewriter T4 will record the signals sent from teletypewriter T1 to teletypewriter T2.

Thus, the subscriber at station S1, which may be located at a remote point, has connected himself through the central oflice to the distant subscribers station S2 without any assistance from an operator at the central oflice. In other words, the subscriber at station S1 has switched both his and the other subscribers lines I1 and I8 from their terminations at batteries I9 and 20, respectively, to thejoining conductor. 26 thereby switching the line conductors II and I8 together. Consequently, a considerable saving in operating time is effected since the subscriber at station S1 simply sends the code combination assigned to select pull bar 3 which closes contact C4 to operate relay R4 which immediately switches the two lines I! and I8 together without any act by an operator at the central omce.

This saving in operating time is of importance.

in certain types of business where time is of the essence and promptnessin putting calls through is essential.

When the subscribers at stations S1 and S2 have finished communicating with each other, either one of them may eiiect a disconnection of their lines at the central office by transmitting a preassigned permutation code signal from either station S1 or station S2. This signal is received by receiving relay R at the central office which operates the selecting mechanism 2 to select pull bar 23. Bail I then elevates pull bar 23 thereby momentarily opening the normallyclosed contact C2. The opening of contact C2 opens the locking-up circuit of relay R4. Consequently, relay R4 releases its armatures and switches the line conductors I1 and I8 from the joining conductor 26 to batteries I5 and 20, respectively, thereby disconnecting the two stations S1 andSz from each other and restoring the system to its normal condition. Since this disconnection was accomplished by either subscriber without the assistance of an operator at the central ofiice, a further saving in operating time is effected.

The nature of the business of. the subscriber at station S1 might be such that it would be desirable for him to send a message to both the subscriber at station S2 and the subscriber at station S4 (which may, for example, be an outlying station near the central office) and have the message recorded practically simultaneously by the teletypewriters T2 and T4. To accomplish this conference connection, it is first necessary to start the teletypewriter motor M4 at station I S4 by energizing relay R3. The subscriber at station S1 energizes the relay R: by transmitting a certain preassigned permutation code combination of signal impulses out over line L1, conductor I'I, contacts 49 and 50 of relay R1, conductor 42, relay R, and then to battery I9. This code combination will cause the selecting mechanism 2 of teletypewriter T to select pull bar 2| which will then be operated by the reciprocating printing bail I.

Bail I will elevate pull bar 2| and cause it to momentarily close contact C3. A path for energizing relay R2 is now closed from grounded battery 22, through the winding of relay R3, contact C2, conductor I6, and then to ground I2. As a result, relay R3 operates its armatures and locks-up through a path from battery 22, winding of relay R3, left armature and contact of relay R2, conductor, 9, conductor ill, normally closed contact C2, conductor I I, conductor I6, and then to ground I2. At the same time, the right armature of relay R3 is operated and closes a path from power supply source B4 at station S4, over line L4, line conductor 24, operated right armature and contact A4 of relay R2, line conductor 25, line L4, motor M4, and then back to power supply source B4 thereby energizing motor M4 and starting it into operation.

The subscriber at station S1 then transmits the v permutation code signal for selecting pull bar 3 in the teletypewriter T at the central omce. When bail i elevates pull bar 3, contact C4 is momentarily closed and relay R4 operates its armatures and locks-up as was described above. The messages sent now by the subscriber at station S1 will travel out over line L1, conductor I1, conductor 42, through relay R, left armature and contact I3 of relay R4, joining conductor 26, right armature and contact I4 of relay R4, conductor I8, through the teletypewriter T4 at station S4, conductor 21, line L2, and then to the teletypewriter T2 at the station S2. Thus, messages sent fromstatio-n S1 will be recorded practically simultaneously on both the teletypewriters T2 and T4. The subscriber at station S2 can now transmit messages to both stations S1 and S4 which will be practically simultaneously recorded by both teletypewriters T1 and T4. Likewise, the subscriber at station S4 can send messages which will be recorded on both teletypewriters T1 and T2.

At the close of communication, any of the subscribers at stations S1, S2, or S4 may disconnect this conference connection by transmitting the preassigned code signal assigned to select pull bar 23 at the central otlice which will momentarily open the normally closed contact C2 as was described above. The opening of contact C2 will open the locking-up circuits of both relays R3 and R4 and will cause them to release their armatures. This switches the communication circuit from conductor 26 to batteries I9 and 20 as was described above. At the same time, the energizing circuit for motor M4 is opened at the contact A4 of relay R3. Consequently, motor M4 stops running. The system is thus restored t its normal condition.

In the event that the subscriber at station S1 desires to communicatewith the subscriber at station S3. (which may, for example, be an outlying station near the central ofice), he will transmit a preassigned upper case code signal which, upon beingreceived by relay R, will efiect the selection of pull bar 21 by the selecting mechanism 2 and, at the same time, shift the platen 38 to its upper case position, shown in the drawings in the manner described in the Morton et al. patent. The elevation of pull bar 21 by bail I will, by means of the ratchet 28 and gear 29, drive the top of bar 30 downward in an arcuate path. Bar 30 will then hit lever 3I thereby tilting downward the right end of lever 32. This drives the top end of lever 32 to the right and allows spring 4|] to pull down the right end of lever 33 into latching engagement with the top end of lever 32. Consequently, the left end of lever 33, which is normally located immediately beneath the printing bail I as shown in the drawings, is driven upward to close contact C1.

The closure of contact C1 closes a path from grounded battery 34, through the winding of relay R1, along conductor 35, through contact C1, conductor 36, conductor I6, and then to ground I2, Consequently, relay Rioperates both its armatures and locks-up through its right armature over a path from grounded battery 34, winding of relay R1, operated right armature and contact of relay R1, conductor 31, conductor I0, normally closed contact C2, conductor I I, conductor l6, and then to ground I2.

It might be mentioned here that, if it were not for this locking-up circuit, relay R1 would otherwise be deenergized when the reciprocating printis due-to the fact that the left end of lever 33 has been tilted up into the position normally occupied by the printing ball I. Thus, when bail I moves back to its normal position, it will strike the left end of lever 33 and force it downward into the position shown in the drawings thereby allowing the spring contact C1 to open. This will drive the right end of lever 33 upward thereby allowing spring 39 to pull back the top end of lever 32 into the lower notch in the right end of lever 33. This elevates the right end of lever 32 into the position shown in the drawings and restores this system of levers to its normal condition.

When relay R1 is energized, it operates its armatures and the two left armatures operate and open the make-before-break contacts 49 and 50 thereby closing a path from battery I3, left armature of relay R4, through relay R, over conductor 42, contact 50, outer left operated armature of relay R1, conductor 48, line La, teletypewriter T3, over line L3, conductor 4|, operated inner left armature and contact 49 of relay R1, conductor I1. line L1, and then to the teletypewriter T1 at station S1. The subscriber at station S1 then transmits motor start signals, such as those as described in the Daum patent mentioned above, which effect the energization of a relay 43 similar to the motor control relay disclosed in the Daum patent, Upon being energized, motor control relay 43 operates its armature to close the contact A: thereby closing the energizing circuit of motor M3 and starting motor Ma into operation.

Communication may now take place between the two stations S1 and S3, the signals from station S1 passing over line L1, conductor I'I,operated inner left armature and contact 43 of relay R1, conductor 4I, line L3, through the teletypewriter T3 at station S3, back over line L3, conductor 48, outer left operated armature and contact 50 of relay R1, conductor 42, through relay R, and then over the left armature of relay R4 to grounded battery I9. It will thus be understood that relay R will follow all signals sent over the communication circuit. It might be mentioned here that this is the case with all connections between the four subscriber stations S1, S2, S3, and S4; that is, relay R follows all the signals sent between these stations to provide a complete record at the central oflice of all messages. It is to be understood that this arrangement may be changed if desired and that the circuit connections may be designed to cut relay R out of some of the connections to the subscribers stations.

At the close of communication, either the subscribers station S1 or S3 will send the permuta-' tion code signal for selecting pull bar 23 which momentarily opens contact C2, as described above, with the consequent opening of the locking-up circuit of relay R1. Relay R1 now releases its armatures thereby opening the circuit leading to station S3. The motor at station S3 may be stopped by either the subscriber at station S1 or by the subscriber at station S2 in 'the manner described in the Daum patent.

vIf the subscriber at station S1 wishes to send a message to be recorded by the teletypewriters ing-up circuits of both relays R1 and R4 thereby restoring the system to its normal condition.

Also, if the subscriber at station S1 wishes to send a message to be recorded by the teletypewriters at each of stations S2, S3, and S4, he will follow the same procedure as that outlined in the preceding paragraph except that he will, in addition, send the permutation code signal to select pull bar 2|. The operation of pull bar 2| will close contact Ca and operate relay R3 which will close the energizing circuit of the motor M4 at station S4 thereby conditioning station S4 for reception and/or transmission of messages. Any of the subscribers may disconnect this conference connection in the manner described above.

It is to be understood that this specific application of the invention has been shown and de- 1 scribed for purposes of explaining the principles and features of operation of the invention. Many changes may be made in the construction shown without exceeding the scope of the invention. More switching contacts to be operated by the pull bars of the teletypewriter at the central oflfilce may be added whenever desired to perform any function that may be required. Likewise, the circuit connections of the various subscribers stations may be varied as desired. Whereas for the sake of simplification only one dominant subscribers station (station 31) has been shown, it is to be understood that other dominant subscribers stations may readily be added with their necessary contacts to meet the individual requirements of any particular system. In short, the scope of the invention is to be limited only by the claims appended hereto.

What is claimed is:

l. A teletypewriter system having in combination a first teletypewriter station, a second teletypewriter station, a transmission line for connecting the two stations, said first station having a teletypewriter motor having a normally open energizing circuit, said second station having a teletypewriter including a plurality of pull bars, a reciprocating bail for momentarily elevating the pull bars individually, a switching relay also located at the second station for closing the energizing circuit of the teletypewriter motor at the first station, an energizing circuit for energizing said switching relay, a control contact located in the teletypewriter and connected in series with said energizing circuit of said switching relay for controlling the energization of the switching relay, and means for operating said control contact in response to the elevation of a preassigned pull bar by the bail.

2. A teletypewriter system including in combination a first teletypewriter connected by a communication circuit to a second teletypewriter having a plurality of pull bars, a reciprocating bail .for momentarily elevating the pull bars individually, a plurality of switching relays, a plurality of energizing circuits, each energizing circuit being individual to one of the switching relays, a plurality of control contacts located in the second teletypewriter, each of said control contacts being connected in series with an individual energizing circuit for controlling the energization of the switching relay individual to each particular energizing circuit, each of said control contacts being operable by a predetermined pull bar individual to each control contact.

3. A teletypewriter station having in combination a teletypewriter including a plurality of pull bars, a reciprocating bail for momentarily elevating the pull bars individually, a normally unoperated switching relay, an energizing circuit for said switching relay, a normally open contact located in the teletypewriter and connected in series with said energizing circuit for controlling the energization of the switching relay, said normally open control contact being momentarily closed by a preassigned pull bar when elevated by the bail, a locking-up circuit for said switching relay, a normally closed control contact located in the teletypewriter and connected in series with said locking-up circuit, said normally closed control contact being momentarily operated by a second preassigned pull bar whereby the locking-up circuit is momentarily opened.

4. A teletypewriter system having in combination a teletypewriter central ofiice having a teletypewriter with a plurality of pull bars, a reciprocating bail for momentarily elevating the pull bars individually, a plurality of telegraph lines, a plurality of teletypewriter stations connectable to the central ofiice by the telegraph lines and normally disconnected from each other, and instrumentalities for enabling an operator at one of the teletypewriter stations to establish a conference connection by connecting his station through the central ofilce to a plurality of the other teletypewriter stations without the assistance of an operator at the central ofiice, said instrumentalities including a plurality of contacts operable by the pull bars when elevated'by the bail.

5. A teletypewriter system including in combination a teletypewriter central oflice having a teletypewriter with a plurality of pull bars, a plurality of telegraph lines, a plurality of teletypewriter stations connectable to the central office by the telegraph lines and normally disconnected from each other, and instrumentalities for enabling an operator at one of the teletypewriter stations to connect his station through the central office to one of the other teletypewriter stations, said instrumentalities including a plurality of switching contacts mounted in the teletypewriter at the central oifice in operative relationship with said pull bars.

6. A teletypewriter system including in combination a teletypewriter central ofilce having a teletypewriter with a plurality of pull' bars, a reciprocating bail for momentarily elevating the pull bars individually, a plurality of telegraph lines, a plurality of teletypewriter stations connectable to the central oflice by the telegraph lines and normally disconnected from each other, a plurality of normally deenergized relays for connecting the teletypewriter stations together in various combinations, each of said relays having an energizing circuit and a locking-up circuit, each energizing circuit having a normally open contact connected in series therein for controlling the energization of the relay individual to each particular energizing circuit, each of said control contacts being operable by a predetermined pull bar individual to each control contact, and an instrumentality operable by a pull bar for unlocking any of the relays which happen to be energized and locked-up at a particular time whereby any of the teletypewriter stations which were connected together are now disconnected.

'7. A teletypewriter system including in combination a teletypewriter central ofiice having a teletypewriter with a plurality of pull bars, a reciprocating bail for momentarily elevating the pull bars individually, a first teletypewriter station, a first telegraph line for connecting said station to the central oflice, a second teletype- .writer station, a second telegraph line connecting said second station to the central oilice, a

conductor adapted to join said telegraph lines nism together with recording mechanism, a.

transmission line connecting the first station and the second station, a transmission line extending between the second station and the third station, contacts at the second station controllable by the permutation code selector mechanism thereat for cutting the third station into and out of circuit relation with the first and second station, means for operating said contacts from the first station to connect the third station into operative circuit relation, instrumentalities for cutting said third station out of operative circuit relation, and means for operating said instrumentalities from either the first or the third stations.

9. A printing telegraph system comprising in combination a first teletypewriter station, a second teletypewriter station, a telegraph line extending between said stations, a second telegraph line extending away from said second station, at least one additional teletypewriter station remotely connected to said second telegraph line, each of said stations being provided with permutation code selector mechanism and recording mechanism, switching contacts in said second station for switching said second telegraph line into and out of circuit relation with said first telegraph line, means for operating said switching contacts under the control of said permutation code selector mechanism, means for connecting said first and second stations into operative circuit relation independently of said switching contacts, means for operating said switching contacts from said first station for effecting the switching of said second telegraph line into operative circuit relation with said first telegraph line, means for disconnecting said second telegraph line from operative Gil".- cuit relation with said first telegraph line, and operating means at said additionalstation for efiecting the operation of said last-mentioned means.

10. A teletypewriter system including in combination a teletypewriter central office having a teletypewriter comprising a two-position platen assembly with means for shifting the platen assembly from one position to the other, a plurality of telegraph lines, a plurality of teletypewriter stations each provided with a teletypewriter and normally disconnected from each other and connectable to the central ofice by the telegraph lines for the transmission and reception of permutation code signals, switching apparatus for enabling an operator at one of the teletypewriter statipns to connect his teletypewriter through the central office to one of the other teletypewriter stations, actuating means for actuating said switching apparatus, and control means for operating said actuating means in response to the reception by the teletypewriter at the central oflice of a preassigned permutation code signal that has been preceded by a difierent preassigned permutation code signal, said control means including alever with operating means. therefor and an instrumentality attached to the platen assembly for permitting said lever to be operated only when the platen assembly is in a preassigned position.

11. A teletypewriter system including in combination a teletypewriter central office having a teletypewriter, a plurality of telegraph lines, a plurality of teletypewriter stations normally disconnected from each other and connectable to the central ofiice by the telegraph lines for the transmission and reception of permutation code signals, switching apparatus at the central office for enabling an operator at one of the teletypewriter stations to connect his teletypewriter through the central oflice to a second teletypewriter station, means in the teletypewriter at the central oflice for enabling the operator at the first teletypewriter station to effect the operation of said switching apparatus at the central ofiice for effecting said connection, means at the central office for disconnecting said first and second stations, and operating means in the teletypewriter at the central oifice for enabling the operator at said second teletypewriter station to efiect the operation of said last-mentioned means.

12. A communication system including in combination a firstteletypewriter station, a second teletypewriter station having a teletypewriter, a communication channel connecting the two stations, a plurality of other teletypewriter stations each having a communication channel leading to the second station, said communication channels being normally disconnected from the teletypewriter at the second station, switching apparatus for automatically connecting a plurality of the communication channels to the teletypewriter at the second station in any combination that may be desired for simultaneously receiving a message, a plurality of control contacts in the teletypewriter at the second station for controlling-the operation of said switching apparatus, a plurality of pull bars in the teletype- Writer at the second station for operating said control contacts, and selecting mechanism in the teletypewriter at the second station responsive to significant current variations transmitted from the first teletypewriter station to the second teletypewriter station for operating said pull bars.

13. A communication system comprising in combination a communication line, a first station having a teletypewriter for transmitting permutation code signals over the communication line, a second station having a teletypewriter for re-' ceiving permutation code signals transmitted over the communication line by the teletypewriter at the first station, a plurality of other teletypewriter stations each having a communication path extending to the second station, switching means at the second station for selectively connecting any one of said communication paths to the teletypewriter at the second station, a p1urality of pull bars in the teletypewriter at the second station for effecting the operation of said switching means, control means for operating the pull bars in accordance with permutation code signals transmitted by the teletypewriter at the first station and received by the teletypewriter at the second station. 14. A communication system including in combination, a first teletypewriter station, a second teletypewriter station having a teletypewriter, a communication channel connecting the two stations, a plurality of other teletypewriter stations each having a communication channel leading to the second station, said communication channels being normally disconnected from the teletypewriter'at the second station, switching apparatus for automatically connecting a. plurality of the communication channels to the teletypewriter at the second station, in any combination that may be desired for simultaneously receiving a message, a plurality of control contacts in the teletypewriter at the second station for controlling the operation of said switching apparatus, a plurality of pull bars in the teletypewriter at the second station for operating said control contacts, and selecting mechanism in the teletypewriter at the second station responsive to significant current variations transmitted from the first teletypewriter station to the second teletypewriter station for operating said pull bars, instrumentalities at the second station for disconnecting the communication channels from the teletypewriter at the second station, and means for operating said instrumentalities from any of the teletypewriter stations that become connected to the teletypewriter at the second station.

' 15. A teletypewriter system including in combination, a teletypewriter central oflice having a teletypewriter, a plurality of telegraph lines,

a plurality of teletypewriter stations normally disconnected from each other and connectable to the central oflice by the telegraph lines for the transmission and reception of permutation code signals, switching apparatus for enabling an operator at one of the teletypewriter stations to connect his station through the central omce to one of the other teletypewriter stations, means for operating said switching apparatus in response to the reception by the teletypewriter at the cen-- tral office of a preassigned permutation code signal that has been preceded by a different preassigned permutation code signal, instrumentalities for disconnecting this connection, and means for operating said instrumentalities from either of the teletypewriter stations that become connected through the central oflice.

16. A teletypewriter system including in combination a teletypewriter central oflice having a teletypewriter with a platen assembly provided with an upper case position and a lower case position, shift mechanism for shifting the'platen from one of said positions to the other, a plurality of telegraph lines, a plurality of teletypewriter stations connectable to the central ofllce by the telegraph lines and normally disconnected from each other, normally ineffective instrumentalities for enabling an operator at one of the teletypewriter stations to connect himself through the central ofiice to one of the other teletypewriter stations, and control means attached to the platen assembly for movement therewith for conditioning said instrumentalities for effective operation.

17. A teletypewriter system including in combination a teletypewriter central oflice having a teletypewriter with a platen assembly provided with an upper case position and a lower case position, shift mechanism for shifting the platen from one of said positions to the other, a plu rality of teletypewriter stations connected to the central office by a plurality of telegraph lines, normally unoperated switching apparatus at the central office for switching said telegraph lines together, and control means for operating the switching apparatus in response to the reception by the teletypewriter at the central oflice of permutation code signals of a preassigned case .transmitted over one of the lines by one of the stations.

18. A teletypewriter system including in combination a central ofiice having a teletypewriter with a plurality of pull bars and a platen assembly provided with an upper case and a lower case position, shift mechanism for shifting the platen from one of said positions to the other, a plurality of subscribers teletypewriter stations, a plurality of communication lines extending from the subscribers stations to the central office but normally disconnected from each other, switching means for selectively connecting the communication lines together at the central office, said switching means including contacts mounted in the teletypewriter at the central office, and control means for enabling certain of the pull bars to eflect the operation of said contacts only when the platen assembly is in a preassigned case position.

CHARLES R. WALKER.

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