US2245756A - Communicating system - Google Patents

Description

J1me 1941- L. c, CAVANAUGH EI'AL 4 I COMMUNICATING SYSTEM I Filed April 28, 1939 2 Sheets-Sheet 1 INVENTORS C (avafiauy/z flan R0 2; f y m J ATTORNEYS J1me 1941- L. c, CAVANAUGH EI'AL 2,245,755

COMMUNICATING SYSTEM Filed April 28, 1939 2 Sheets-Sheet 2 wmw 1N VENTORS Zu/re C 601 4/704/ fi 3% f7. Var/200 ATTORNEYS Patented June 17, 1941 TED STATES.

COMIVIUNICATING SYSTEM Luke C. Cavanaugh and Henry Vernon Roohk, Cleveland, Ohio Application April 28, 1939, Serial No. 270,602

17 Claims.

Our invention relates to acoustic devices comprising a plurality of stations capable of sending and receiving sound, but more particularly to intercommunicating telephonic or radio systems in which each of a plurality of stations can'send to, or receive from, another station the voice of the person without the possibility of the other stations interrupting the conversation.

Heretofore intercommunicating systems have been made in which a plurality of stations were-1o wired to permit sending and receiving at each of the stations but there was a possibility that during a two-way conversation any of the other stations in the system could cut in thereby causing a disagreeable interruption.

One of the objects of our invention is to eliminate the above mentioned disadvantage by providing an intercommunicating system comprised of a plurality of stations each of which can send to and receive from any other station in the system but which, when two stations are mean versation, automatically prevents any other station from interrupting.

Another object of our invention is to provide an intercommunicating system which has no bells or buzzers.

A still further object of our invention isto provide a wiring system which will enable the expensive and bulky parts of an intercommunicating system to be combined and placed in a central" 3 out-of-the-way unit thereby reducing the size of the desk units and reducing the cost of the system.

Another object of our invention is to provide combined sending and receiving apparatus in a unit and to provide a minimum of simple controls to control the unit.

Another object of our invention is to provide a wiring system for an intercommunicating system whereby each sending and receiving unit 40 may, upon initiating a call, become a central or" master station. r

A further object of our invention is to reduce the number of manual operations necessary to carry on a conversation over an intercommunicating system.

Another object of our invention is to provide an intercommunicating system which has selective relays common to all sending and receiving units.

In an electrical intercommunicating system some space must be provided for the necessary parts and it is an object of our invention to provide a wiring system which will enable'many of those parts to be placed in a convenient central out-oi-the-way location thereby reducing the size of the sending and receiving units which usually are in a prominent place, as upon an ofilce desk.

Another object of our invention is to provide an intercommunicating system having a plurality of voice transmitting lines all of which are short circuited out when the system is at rest.

Another object of our invention is to provide for breaking the short circuit on the line of the calling party and on the line of the called party but on no other line when a party initiates a call.

Another object of our invention is to furnish an intercommunicating system having a plurality of normally disconnected transmitting and receiving stations and to provide for selectively connecting any pair of stations in the system and, upon the connection of a pair of stations, to prevent any other station in the system from interrupting the established connection.

Another object of our invention is to furnish a plurality of normally disconnected transmitting stations and to provide for connecting any station to any other station and, by establishing that connection, preventing any other station from initiating a call while that connection exists.

Another object of our invention is to provide an intercommunicating system having a plurality of stations capable of initiating a call and a oneway-amplifier and to cause the current to flow through the one-way-amplifier in the correct direction by the selecting ability of a relay.

Another object of our invention is to provide an intercommunicating system having a plurality of stations each capable of sending and receiving signals in which the originator of a call need not operate a switch to control his apparatus to determine whether he is to send or to receive.

Another object of our invention is to provide an intercommunicating system having a plurality of stations each capable of sending and receiving vocal or other signals in which the receiver of the call operates a switch thereby controlling a relay which causes his vocal or other signal to enter a one-way-amplifier in the proper direction to be transmitted to the party originating the call.

Still another object of our invention is to provide an intercommunicating system having a plurality of signal sending and receiving stations and to allow any one station to initiate a call but once that initiating station is connected to the called station, to prevent any other station from initiating a call until the first connection is broken and, upon blocking out all other stations,

. station devices.

Figure 2 is a circuit diagram: which when combined with Figure 1 constitutes another embodi ment of my invention using a pole changer relay;

and

Figure 3 is a circuit diagram which when combined with Figure 1 constituted another embodiment of my invention showing a two-way ainplifier.

With reference to Figure 1 of the drawings,

, one embodiment of our invention may comprise a plurality of station devices H) and H, a plurality of selective relays l2, 13, l and 65, an interlocking relay [5, and station control push-button switches 26 and 21.

While we have illustrated but two station'devices Ill and l I, it is to be clearly understood that our invention may comprise a larger number of arranged to be of the same construction and may be of the type which serves both as the transmitting and receiving unit, thus eliminating the necessity of utilizing one device for transmitting the sound and another device for receiving the sound. In the embodiment in Figure l, the station devices are constructed of such parts that they serve as a self-contained unit for sending and receiving sound without aid of auxiliary amplifiers in the sound transmitting circuits.

The illustrated relays may be of any suitable type and may comprise a plurality of movable contact-s operated by a single armature. The system is arranged to be energized by means of bat- Each of the station devices are.

teries 34 and but it is to be understood that any other suitable source of electrical energy may be provided. ,The plurality of relays may be assembled in a suitable box or other container and placed in some out-of-the-way place in a closet or .store room of an ofiice .or inany other suitable location. The station'devices l9 and H may be convenientlypositioned near the person using the system. Tlius, for example, in an ofiice system the stations IB and II may be positioned upon office desks in separate rooms or upon near-by locations convenient for the use of the persons.

- The push buttons 26 and 2?, while shown sepaboxesor containers which house the sending and.

receiving devices 10 and H. The lamps 1-0 and 3-! may be arranged to be incorporated in the container or housing for the station devices Ill and I l, respectively. The conductors which lead from ton switches and lamps may be connected to a plurality of terminals mounted upon a suitable 7 input terminal block 32, which may be positioned near or embodied in the relay assembly unit which isjpositioned in some out-of-the-way place. The

relay assembly unit may also comprise an out- I put terminal bloclr 33 having a plurality of terminals to which the conductors may be connected.

In describing the operation of our system, let us first assume that the system is at rest, which is the position shown in Figure 1 of the drawings. In this position, the station devices It) and H are connected in a shorted circuit. The shorted circuit for the station device l0 may be traced as follows: Beginning with the upper terminal of the station device 10 the shorted circuit extends through conductor 52, terminal 53, conductors 5 and 55, movable contact 58 of the relay 13, conductor 5?, movable contact 58 of the relay l5,

conductor 59, terminal 60, and conductor 6| to the lower terminal of the station device Ii]. The shorted circuit for the station device I! may be traced as follows: Beginning with the lower terminal of the station device II the shorted circuit extends through conductor 40, terminal 4i, conductors 42 and 43, movable contact 44 of the relay IZ, conductor 45, movable contact 46 of the relay i4, conductor '41, terminal 48, and conductor 49 to the upper contact of the station device l l. 7

Now let it be assumed that the person at the station device. desires to communicate with the person at the station device Iii. Under this assumption, it is only necessary for the person at station H to depress the push button switch 27 and close contacts 12,13 and 81. The depressing of the push button switch 21 establishes control circuits for energizing the selective relays I3 and 14, which in turn establish electrical contacts for establishing a transmitting communicating circuit between the station device I l and the station device [0. The control circuit for energizing the selective relay 13 maybe traced as follows: Beginning with the battery 34 curw rent flows through conductors 63 and '64, mov- 40 push button switch 21, conductors l4 and I5,

terminal 16, and conductors l1 and 18 to the opposite sides of the battery 34. fhe current ionenergizing the'sele'ctive relay 14 may be traced I to the opposite side of the battery-34 as previously traced for the circuit which energized the relay, Hi. The energization of the selective relays i3 and I4 from the battery 34 by depressing the switch'push button 21 causes the armatures 22 tacts.

and 23 to be actuated which shifts the movable transmittingcommunicating circuit between the the station-devices and the associated push butst'ationdevice I1 and the station device It may 'be traced as follows: Beginning with the lower device Ill. The circuit thus traced maybe connected to a ground '95 through conductor 92, terminal 93 and a switch 94. The other side of the communicating circuit between the station device II and the station device I8 may be traced as follows: Beginning with the upper terminal of the station device II communicating current flows through the conductor 49, terminal 48, conductor 41, movable contact 46, conductors 98 and 99, terminal I30, switch- IilI, conductor I82, terminal I53, conductors I84 and I85, movable contact 56, conductor 51, movable contact 58 of the relay I5, conductor 59, terminal 60and conductor 6i to the lower terminal of the station device Ill. The establishment of'the transmitting communicating circuit between the station device II and the station device I8 permits the persons to communicate back-and-forth with each other. I A 1 As previously explained, oursystem is not limited to two station devices but may include any number of such devices. When other station devices are connected in our system, additional selective relays are employed with circuit connections passing through them in the same manner as the circuit connections pass through the illustrated relays for the station devices I8 and II. In other words, for each additional station device added to our system, two additional selective relays of the same type as illustrated need be added. A

We provide in the embodiment of our invention an interlocking relay I6 which prevents a subsequently operated station device from interfering or interrupting the selected transmitting communicating circuit established :by a prior operated station device such, for example, the station device I9, nor any newly added station device, can not take the control away from the station device II, so long as the push button switch 21 is depressed. The manner in which we provide for excluding other station devices from interfering with the selected transmitting communicating circuit established by the selective relays, is by energizing the interlocking relay I6 upon the closing of the contacts of the operated selective relays which establish the transmitting communicating circuit between two of-thestations. Thus,- for example, when the selective relays I3 and I4 are energized, a circuit is established for The circuit for energizing the interlocking relay l6 may be traced as follows: Beginning with the battery 34 current flows through conductors 63 and I88, the relay winding of the relay I6, conductor I09 to a connection I81 at which point the current divides part going through the contacts of the relay I3 and part going through the contacts of the relay I4. The current which flows through the contacts of the relay I3 may be traced from the connection I91 through conductors H8 and III, movable contact 68, the

winding of the relay I3, conductor 69, terminal 18, conductor H, the contacts 12 and 13 bridged by the depressed push button switch 21, conductors 14 and 15, terminal 16, and conductors 11 and 18 to the other side of the battery 34. The path of current flowing from the connection I81 through the contacts of the relay I4 may be described as follows: conductors H2 and H3, movable contact 82 of the relay I4, the :winding of the relay l4, conductor 83, terminal 84, conductor 85, lamp 3 I, conductor 86 to the contacts 81 and 13 and from there the current flows back to the opposite side of the battery 34 as previously miovable contacts 6| and 65 to be shifted to the opposite side. The shifting of the movable contact 65 to the opposite stationary contact prevents the'battery 34 from being connected in circuit relation with the selective relays I2 and I5, even though the push button switch 26 may be depressed by the person at the station device I0. In'other words, when the person at the station device II depresses the push button switch 21 and selectively energizes the relays I3 and I4, after which the interlocking relay I6 is immediately energized, then the person at the station device Ill can not, by depressing the pushbutton switch 26, take the control away from the station device II by interrupting the selected communicating transmitting circuit. In this manner, the interlocking relay I6 excludes a subsequently operated station from interfering with the selected and established circuits efiected by the operation of a prior station device. The interlocking relay I6 also prevents any additional station devices to our system from interfering with the selected and established transmitting communicating circuit when once established by a person at another stationdevice.

The purpose of the lamps 38 and 3| is to in-. dicate to the operator at each of the station devices, as to whether or not another station has been established a transmitting communicating circuit. Thus, for example, should the operator at the station device II have control of the system and should the operator at the station device IO depress the push button switch 26, the lamp 30 would fail to light since the connection is broken from the battery 34 at the movable contact 65. Therefore, when a person depresses a push button switch at a station and the lamp energizing the coil of the interlocking relay I6.

does not light then this is an indication that some other station has control of the system.

When the operator at station device I I finishes his conversation with the operator at the station device I8 he merely releases the push button switch 21. Upon the releasing of the push button switch 21 the system is again at rest. Now, should the person at the station device III, at some later time, desire to communicate with the person at the station device -II then the person at the station device I6 merely depresses the push button switch 26 bridging the contacts H1, I I8 and I I9 establishing circuits from the battery 34 for energizing the selective relays I2 and I5, which establishes a transmitting communicating circuit from the station device II] to the station device". The control circuit for energizing the selectiverelay I? may be traced as follows: Beginning with the battery 34 current flows through the conductors 63 and 64, movable contact 65, conductors 86, 89 and 19, movable contact 88, the winding of the selective relay I2, conductor I2I, terminal I22, conductors I23 and I24, the contacts In and H8 bridged by the depressed push button switch 26, conductors I25 and 15, terminal 16, and conductors 11 and 18 to the other side of the battery 34. The circuit for energizing the selective relay I5 may be described as follows: Beginning at the battery 34 to energized conductor 66, as previously exwinding of the relay I5, conductor l33, terminal I28, conductor I29, lamp 35, and conductor I36 to'the contact H9 to the common contact H8 and from there the current flows back through thecommon circuit as previously described to the opposite side of the battery 34. The energization of the selective relays i2 and I establish a communicating transmitting circuit from the station device. I0 to the station device II. The .circuit from the upper terminal of. the :station .device It to the lower terminalof the. station device I I is the same as previouslydescribed. The circuit between the'lower, terminahof the station device Iii and the upper terminal of the station .device .I 1 maybe traced as .follows: Beginning with the lower terminal. .of the station device I0, the circuit extends through conductor .BI, terminal 60, conductor 5.9, movable contact 58, the conductors I34, conductor 9! terminal 100, switch IilI, conductor 02, terminal. I03, con- -ductor I04, conductor Hid-movable .contact 44., conductor-4.5, movable contact 46, conductor 41, terminal 48, conductor 49 to. the upper terminal .of the station device .I I. Upon the establishment of the transmitting communicating circuit through the relays I2 and. I5, .as just described, .the person at the station, device and the person at the station device 5.! may communicate with each other. Upon the .energizationof theselective relays i2 and I5 and upon the actuation of the armatures 2! and .24, acircuitis established for energizing the interlockinglrelay -I.6 which prevents the person at station .I I, by

depressing the .push button switch 21,..as .well as a person at any additionally added stations from interferring with the circuit establishedby the personat the station Iii. The circuit .for energizing the interlocking relay It by the closing of the contacts of the selective relays I2 and .may be described as follows: Beginning with the battery 34 current flows through the conductors .63 and I03, the winding of the relay I5, and conductor E89 to the connection IE1, at which point the current divides, one path extending through conductors H3 and H4, movablecontact I21 of the relay I5, the winding of therelay I5, conductor I33, terminal I28, conductor I29, lamp 30, conductor I3il, the contacts H8 and NB bridged by the depressed push button switch 26,

conductors I25 and #5, terminal 15 and conductors TI and I3 to the opposite side of the battery 34. Beginning with the connection I01, the other path extends through conductors -II2 -and'l32, movable contact 38, the winding of the relay --I2, conductor I21, terminal 622, conductors I23 and I 24,, contacts i ll and i I3 bridged by-the depressed push button switches at any additionally added stations would prevent them from interferring with the established communicating transmitting circuit between the station I0 and the station I I, so long as the operator at the station I0 retained the push button 26 depressed. 7

Since the selective relays for the subsequently operated stations can not be energized when -a station has control and since the de-energized selective relays short circuit the station devices not in use, then it is impossible for a subsequently operated station device to cause interference-with the prior selected station devices. The selective relays I2, I3, I4 and I5 andthe interlocking relay the battery 34 'is 'pre- I6 may be surrounded by a shield I38 which is-'= connected to a ground I31 to eliminate static interferences in the communicating system.

As explained before, in the embodiment shown in Figure 1, the station devices I0 and II produce .suificient communicating transmitting energy to enable communication to be carried ,on :therebetween without the employment of a centralized amplifying device. In Figure 2, we have illustrated an embodiment which, when combined with the embodiment in Figure 1, comprises a complete communicating system having .a centralized amplifier I40. The output terminal block 33 in Figure 2 is a duplication of the terminal block in Figure 1, but the switches IOI and :94-are in their open position. Thus, for example, .the communicating current from the. station device I I, instead of flowing through the switch IOI .as explained in Figure 1, is caused to flow, in Figure 2, from terminal I00 through conductors -I4I and I42, movable contact I43, and conductor I44 to the input terminal I83 of the amplifier 14,0, and from the amplifier the amplified current flows from the output terminal I04 through conductor I45, movable contact I45, and conductors I41 and I48 to terminal I03 and from there to the station device I0 as previously explained. The opening of the switch 94, in FigureZ, causes the terminal 93 to be grounded through conductor I58, terminal I59 of the amplifier and con ductor I to a ground ISI which is also connectedto a shield I62 which eliminates static interferences in the equipment shown in Figure 2. The amplifier I40 is, a one-way amplifier, in that the communicating current must enter the amplifier, through the input terminal I63 and leave. the amplifier through the terminal I64. In order that communication may be carried on between the station device It the station device I] in. both directions with a one-way amplifier I40, we have employed a pole changer relay I38 which provides for directing the incoming communicating current from either of the :two station devices I0 or II into the input terminal I63 of the amplifier I40. The pole changer .relay. I38 is arranged to be energized by circuits established either by depressing the push button switch 28 at the station device. I0 or by depressing the push button switch. 29, at ,the station device I I. Thus, for example, should station device II have control of the communicatingsystem and desire to carry on a conversation with the .person at the station device I0, then under this condition, the push button switch 21 is depressed and communicating current ,from the station II upon reaching the terminal I00, as previously described, flows through conductors I4:I and I42, movable contact, I43, conductor I44 to the :input'terminal I63 and from the amplifier M0, the amplified current flows from the output terminal I.64.throug h conductor I45, movable con- 'tact..I46, and conductors I41 and I48 to the terminal I03 which is connected to the station device I0, as previously described. In order that the person at the station device I0 may com- .municateEback to the person at the station device :II, it :isnecessary to depress the .push button switch 28 which energizes the pole changer relay I38 .and causes the armature I30 to move the :movable contacts I43 .and I46 to the opposite set of stationary contacts. Under the energized position of the pole changer relay I38, communicating, current -from the station device I0 upon reaching zthe terminal I03, as previously described, fiows through conductors I48-and I50, ..movable contact I43, conductor I44 to the input communicating current from station device I6 terminal I63 of the one-way amplifier I30 and from the amplifier the amplified current flows from the output terminal I63 through conductor I45, movable contact I86, and conductors I39 and IQI to the terminal I and from there to the station device II, as previously. explained. When the person at the station device Ill has finished his message he releases the push button switch 28, after which the person at the station device II may communicate with the person at the station III. The person at station device I I, for example, cannot take the control away from the person at station device I0 by depressing the push button switch 29 and thus the person at station H cannot communicate with the person at station device I0,'until the person at station I0 releases the push button switch 28. The same conditions prevail with any newly added station devices. In the event that the person at the station II] should have control of the system by the prior depressing of the push button switch 26, then it is necessary for the person at the station device II to depress the push button switch 29 to energize the pole changer relay I38 to establish circuit for causing the communicating current to fiow in the input terminal I63 of the amplifier M0. The circuit for energizing the pole changer relay I38 upon the depressing of the push button switch 28 may be traced as follows: Beginning with the battery 34 current flows throughthe conductors 63 and 64, movable contact 65, conductor I15, terminal I14, conductor I13, the coil of the relay I38, conductor I12, terminal I1I, conductor I10, terminal I69, conductor I68, contacts I61 and I66 bridged by the push button switch 28, conductor I30, contacts 12 and 13 bridged by the push button switch 21, conductors 14 and 15, terminal 16,"and conductors 11 and 18 tothe battery 34. The circuit for energizing the pole changer relay I38 upon the closing of the push button switch 29 may be traced as follows: Beginning with the energized terminal I14, current flows through conductor I13, the coil of the pole changer relay I38, conductor I12, terminal I1I, conductor I10, the terminal I69, conductors I68 and I19, contacts I18 and I11 bridged by the depressed push button switch 29, conductor I16 and I24, contact H1 and H8 bridged by the push button switch 23, conductors I25 and 15, terminal 16, and the conductor 11 and 13 to the battery 39.

The energization of the interlocking relay I6 also controls the filament to the amplifier I30. Thus, when the interlocking relay I6 is energized current from the filament battery 35 fiows through conductor II, movable contact 6I, conductor I52, terminal I53, and conductor I54 to the filament terminal I65 of the amplifier I All and from the amplifier the filament current flows from the terminal I59 through conductor I55, terminal I56, and conductors I51 and18 to the opposite side of the filament battery 35. In this manner the amplifier I40 is de-energized until the interlocking relay I6 is energized by the closing of the contacts of the selective relays of our system.

In Figure 3, we show an arrangement of a twoway amplifier I83 and I84 which obviates the necessity of the pole changer relay I38. Thus, for example, communicating current from station device II which fiows from the terminal I09 through the conductor I35 enters the input terminal I95 of the amplifier I84 and the amplified current leaves the output terminal I96 and fiows through conductor I86 to the terminal I03. The

upon reaching the terminal I03 flows through conductor I33 and to the input terminal I94 of the amplifying device I33 and the amplified current fiows from the output terminal I93 through the conductor I85 and to the terminal I00 which is connected to the station device II. Therefore, communication may be carried on between the station devices I0 and II without the operation of the push button switches 28 and 29. The filament terminals I91 and I98 of the amplifiers I83 and I84 are inter-connected together by conductor I31 and both are connected to the filament battery 35 through a conductor I82 leading to the energized terminal I53. The ground terminals I39 and 200 of the amplifiers I83 and I84 are connected together by conductor I88 and both are connected through the conductor I89 to the other energized terminal I56 from the fi1a-' ment battery 35. Both of the ground terminals I99 and 200 are connected to a ground I9! through a conductor I which is also connected to a shield I92 to the ground. The object of the shield is to eliminate static interferences in the equipment. In the embodiment of Figure 3, as just described, the amplifier I84 has the property of amplifying the communicating current from the station II to the station I0 and the amplifier I33 has the property of amplifying the communicating current from the station III, to the station II and each have the property of selecting its own directional current.

In our system each station device I5 and II, and the additional station devices which may be added thereto, upon initiating a call become a central or master station and prevent any other station in the system from interrupting the established communicating connection. In addition, our system is completely de-energized when at rest and thususes a minimum of electrical energy.

While we have described our invention with a certain degree of particularity, it is understood that the present disclosure has been made only by way of example and that numerous changes in the details of construction and the combination and arrange-ment of parts may be resorted to without departing from the spirit and scope of the invention as hereinafter claimed.

We claim as our invention: 1 I

1. An electrical transmission system comprising, in combination, a plurality of station devices, each capable of sending and receiving sound, transmission circuit connections interconnecting the station devices with each other, selective relay means for selecting and establishing a transmission circuit connection between at least two of said station devices, control circuit connections for governing the selective relay means, a switch associated with each station device for selecting and establishing control circuit connections to govern the selection of the selective relay means and the establishment of a transmission circuit connection between at least two of said station devices, and an interlocking relay means controlled by the selective relay means, said interlocking relay means governing the control circuit connections to prevent the subsequent actuation of a switch at one of said station devices from interrupting a selected transmission circuit connection established by the prior actuation of a switch at another station device, and amplifying means for amplifying the selected transmission circuit in each of two directions and means for governing the operation of the amplifying means in accord with the interlocking relay.

2. An electrical transmission system comprising, in combination, a plurality of station devices, each capable of sending and receiving sound, transmission circuit connections interconnecting the station devices with each other, selective relay means for selecting and establishing a transmission circuit connection between at least two of said station devices, control circuit connections for governing the selective relay means, a switch associated with each station device for selecting and establishing control circuit connec-. tions to govern the selection of the selective relay means and the establishment of a transmission circuit connection between at least two of said station devices, an interlocking relay means controlled by the selective relay means, said interlocking relay means governing the control circuit connections to prevent the subsequent actuation of a switch at one of said station devices from interrupting a selected transmission circuit connection established-by the prior actuation of a switch at another station device, a two-way amplifying, means included in the selected transmission circuit. connection, and means for causing the interlocking relay to govern the operation of the amplifying means.

3. An electrical transmission system comprising, in combination, a plurality of station devices, each capable of sending and receiving sound, transmission circuit connections interconnecting the station devices witheach other, selective relay means for. selecting and establishing a transmission circuit connection between at least two of said station devices, control circuit connections for governing the selective relay means, a switch associated with each station device for selecting and establishing control circuit connections to govern the selection of the selective relay means and the establishment of a transmission circuit connection, between at least two of said station devices, an interlocking relay means controlled by the selective relay means, said interlocking relay means governing the control circuit connec tions to prevent the subsequent actuation of a switch at one of said station devices from interrupting a selected transmission circuit connection established by the prior actuation of a switch at another station device, amplifying means included in the selected transmission circuit con-. nection, and means for causing the interlocking relay to govern the operation of the amplifying means.

4. An electrical transmission system comprising, in combination, a plurality of station devices, each capable of sending and receiving sound, transmission circuit connections interconnecting the station devices with each other, selective relay means for selecting and establishing a transmission circuit connection between at least two of said section devices, control circuit connections for governing the selective relay means, a switch associated with each station device for selecting and establishing control circuit connections to governthe selection of the selective relay means and the establishment of a transmission circuit connection between at least two of said station devices, an interlocking relay means controlled by the selective relay means, said interlocking relay means governing the control circuit connections to prevent the subsequent actuation of a switch at one of said station devices from interrupting a selected transmission circuit connection established by the prior actuation of a switch at another station device, one-way amplifying means having input and output terminalmeans included in the selected transmission circuit connection, pole changing relay means for establishing the selected transmission circuit connection through the amplifying means in a direction to cause the current flowing in the selected transmission circuit connection to enter the amplifying means through the inpunt terminal means and leave through the output terminal means, and means located at each station to govern the pole changing relay means.

5. An electrical transmission system comprising, in combination, a plurality of station devices, each capable of sending and receiving sound, transmission circuit connections interconnecting the station devices with eachother, selective relay means for selecting and establishing a transmission circuit connection between at least two of said station devices, control circuit connections for governing the selective relay means, a switch associated with each station device forselecting and establishing control circuit connections to govern the selection of the selective relay means and the establishment of a transmission circuit connection between at least two of said station devices, an interlocking relay means controlled by the selective relay means, said interlocking relay means governing the control circuit connections to prevent the subsequent actuation of a switch at one of said station devices from interrupting a selected transmission circuit connection established by the prior actuation of a switch at another station device, one-Way amplifying means having input and output terminal means included in the selected transmission circuit connection, pole changing relay means for establishing the selected transmission circuit connection through the amplifying means in a direction to cause the current flowing in the selected transmission circuit connection to enter the mplifying means through the input terminal means and leave through the output terminal means, means located at each station to govern the pole changing relay means, and means for causing the interlocking relay means to govern the operation of the amplifying means.

6. In an apparatus of the character described, the combination of a central exchange comprising selective relay means and interlocking relay normally in a shorted circuit, and means at said central station adapted to select and break the short circuit upon at least two of said station devices and establish a transmitting circuit between the said selected station devices, said means at said central station upon establishing the selected transmitting circuit preventing a second transmitting circuit from being made while said selected transmitting circuit is established.

7. In an electrical system, the combination of a plurality of station devices, each capable of sending sound to and receiving sound from any other station device in the system, each of said station devices being normally in a shorted circuit, meanslocated at each station device for establishing a circuit connection between a station device and another station device and breaking the shorted circuits of said connected station devices, anda central exchange for causing the circuitconnection established to exclude all other station devices from establishing subsequent circuit connections in the system until the prior circuit is interrupted. I

8. An electrical system comprising, in combination a central exchange, a plurality of station devices capable of sending and receiving sound, each of said'station devices being normally in a shorted circuit, a sound transmitting circuit connected to each of said station devices including a two-way amplifying means through said central exchange, a selecting control circuit connected to each of said station devices, means located at each of said station devices for selecting the control circuit to connect said transmitting circuit to any other station device through said central exchange and locking out all other selecting control circuits at the central exchange to prevent all the other station devices in the circuit from being connected to the selected stations, and means for breaking the shorted circuits of said selected and connected station devices.

9. In an electrical system of the character described, the combination of a plurality of stations including a first and a second station normally located in shorted circuits and adapted to send and receive signals, each of said stations being normally in a shorted circuit, circuit connections including switch means associated with said first station for connecting said first station to said second station by breaking each of the shorted circuits and by establishing a transmitting circuit between them to permit said first station to send signals to said second station, a one-way amplifier adapted to normally receive current from the first station and amplify same for the second station, and means associated with said second station for directing current from the second station through said one-way amplifier in the proper direction and amplify same for said first station.

10. In an apparatus of the character described, the combination of a plurality of station devices capable of sending and receiving sound, each of said station devices being normally in a shorted circuit, a central exchange, relays at said central exchange for breaking said shorted circuits and establishing a signal transmitting circuit between said station devices, and a one-way-repeater at said central exchange, said relays also acting to direct the fiow of current from said station devices through said one-way-repeater in one direction, and to prevent the establishment of any other signal transmitting circuit while one circuit is made.

11. An electrical system comprising, in combination, a central exchange, a plurality of station devices capable of sending and receiving sound, each of said station devices being normally in a shorted circuit, a sound transmitting circuit including means for amplifying in two directions connected through said central exchange to each of said station devices, a selecting control circuit connected through said central exchange to each of said station devices for breaking said shorted circuit and establishing two selected stations in said sound transmitting circuit and for locking out all other selecting control circuits at the central exchange to prevent all the other station devices in the circuit from being connected to the selected stations.

12. In an electrical system of the character described, the combination of a plurality of stations including a first and a second station adapted to send and receive signals, each of said stations being normally in a shorted circuit, circuit connections including switch means associated with said first station for connecting said first station to said-second station by breaking each of the shorted circuits and by establishing a transmitting circuit between them topermit said'first station to-send signals to said'second station, a one-way amplifier adapted normally to receive current from the first station and amplify same for the second station, and means associated with said second station for directing current from the second station through said one-way amplifier in the proper direction and amplify same for said first station, said switch means also operating means to prevent the subsequent actuation of a switch at one of the other of said plurality of stations from interrupting the circuit between the said first and second stations.

13. In an electrical system of the character described, the combination of a plurality of stations including a first and a second station adapted to send and receive signals, each of said stations being normally in a shorted circuit, circuit connections including switch means associated with said first station for connecting said first station to said second station by breaking each of the shorted circuits and by establishing a transmitting circuit between them to permit said first station to send signals to said second station, amplifying means included in the transmitting circuit and means for causing said switch means to govern the operation of the amplifying means.

14. In an electrical system of the character described, the combination of a plurality of stations including a first and a second station adapted to send and receive signals, each of said stations being normally in a shorted circuit, circuit connections including switch means associated with said first station for connecting said first station to said second station by breaking each of the shorted circuits and by establishing a transmitting circuit between them to permit said first station to send signals to said second station, a two way amplifying means included in said transmission circuit and means for causing said switch to govern the operation of the amplifying means.

15. An electrical system comprising, in combination, a plurality of station devices normally in shorted circuits, a plurality of relay means located at a central exchange spaced from said station devices, a sound transmitting circuit including means for amplifying in two directions connected through said central exchange to each of said station devices, a selecting control circuit connected to said relay means and to said station devices for actuating said relays to break said shorted circuit and establish two selected stations in said sound transmitting circuit, the actuation of said relay means locking out all unselected control circuits at the central exchange to prevent the unselected station devices from interrupting the selected station devices.

16. An electrical transmission system comprising, in combination, a plurality of station devices normally in shorted circuits, transmission circuit connections interconnecting the plurality of station devices with each other, first relay means for establishing a transmission circuit between two selected station devices and breaking the shorted circuits of said selected and connected station devices, second relay means for preventing the interruption of the established transmission circuit, amplifying means in said transmission circuit, and means for causing said second relay means to govern the operation of the amplifying means.

17. In an electrical system, the combination of a. plurality of stationdevices, each capable of sending sound to and receiving sound from: anyother station device in the system, each of said station devices being normally in a shorted circuit, means located at each station device for governing the establishment of a circuit connection between a station device and another station device and the breaking of the shorted circuit of said connected station devices, a central exchange having means for causing the circuit connection established to exclude all other station devices from establishing subsequent circuit connections in the system until the prior circuit is interrupted and grounded shielding means around the said central station to reduce the static interference inthe said system.

- LUKE C. CAVANAUGH.

H. VERNON. ROOHK.

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