US2440249A - Key controlled numerical digit register equipped with discharge tube means - Google Patents

Description

Aprll 27, 1948. G. DEAKIN 2,440,249

KEY. CONTROLLED NUMERICAL DIGIT REGISTER EQUIPPED WITH DISCHARGE TUBE MEANS Filed April 21. 1944 4 Sheets-Sheet 1 April 27, 1948. G. Dr-:AKIN

KEY CONTROLLED NUMERICAL DIGIT REGISTER EQUIPPED WITH DISCHARGE TUBE MEANS Filed April 21, 1944 4 Sheets-Sheet 2 AGENT April 27', I1941s.

G. DEAKIN v .KEY CONTROLLED NUMERIGAIJ DIGIT REGISTER EQUIPPED wITH .DISCHARGE TUBE MEANS Filed April 21,

4 Sheets-Sheet 5 AGENT G. DEAKIN KEY CONTROLLED NUMERICAL DIGIT REGISTER April. 27, 194s. 2,440,249

EQUIPPED WITH DISCHARGE TUBE MEANS y 4 Sheets-Sheet 4 Filed April 2l, 1944 AGENT Sk ES E MSR atented` pr. 27, M

KEY CONTROLLED NUMERICAL DIGIT REG- ISTER EQUIPPED WITH DISCHARGE TUBE :MEANS Gerald Deakin, New York, N. Y., assignor to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application April 21, 1944, Serial No. 532,142

25 Claims. l

This invention relates to new and usefulimprovements in switching systems of the type useful in controlling telecommunication.

The object of the invention is to provide `a simple key-set controller selectively switching in circuit a relatively small number of resistances for controlling as many different switching operations as there are keys, For instance, a ten-button key set is provided with ve resistances for controlling the performance of a desired one of ten switching operations.

In accordance with the present invention, a responsive or registering device is interposed between the key set and the switching device. It comprises electric discharge devices arranged in aY number of groups less than the number of keys.

In the preferred embodiment here disclosed six identical vacuum tube circuits have their grids connected in multiple to one wire of a two-wire line connected with the key set. When the register is idle, the common lead is connected to ground through, e. g. a resistor of, e. g. 5,000 ohms. When the register is picked up, which in an automatic telephone system means that the line is closed to ground at the central office through the subscribers set, the free end of a resistor of,e. g. 1,000 ohms also attached to this common lead is connected to a common battery of 48 volts. It will be assumed, for the purpose of determining the specific resistances and potentials used, that the maximum total loop resistance of a subscribers line will not exceed 500 ohms and that the leakage resistance from the one line wire to ground or from the one line wire to the other will not be less than 25,000 ohms. Other resistance values could be used, but an insulation resistance of not less than 25,000 ohms is easily realized in any modern cable plant.

In operation, the potential of the common lead depends upon the value of the combined series line and key controlled resistance to ground in multiple with any leakage. An ample series of potentials may be obtained to operate any one of six tube circuits in multiple with the aforementioned leakage. The operation of one, two or three of these tube circuits makes it possible to select any one of ten digits, to change over from one selection to the other, or to eiect a premature release should the calling subscriber hang up before the register is disconnected in the normal way after completing selection of the called line. The use of tube circuits in this manner eliminates timed circuits, timed relays or step-by-step means 2 now so generally used in dial-controlled register circuits. K

In the drawings:

Figs. 1 is a diagram of a telephone substation circuit equipped with a key set;

Fig. 2 is explanatory of the order in which the key contacts are operated;

Figs. 3-8 show the mechanical arrangement of the key set, Fig. 3 being a section along 3-3 of Fig. 5, Fig. 4 a section along 4--4 of Fig. 6, Fig. 5 a section along 5 5 of Fig. 3, Fig. 6 a section along 6 6 of Fig. 5, Fig. 7 an end view of the key set with part of the wall broken away, and Fig. 8 is explanatory of the manner in which the contacts are operated; c

Fig. 9 illustrates a telephone set equipped with a key controller;

Fig. 10 is a diagram oi' the register circuit;

Fig., 11 is explanatory of the functioning of the control circuit; and

Fig. l12 is a tabulation of the resistance values used and potentials applied to the register.

subscribers set circuit (Figs. 1, 2 and 9) When the subscriber removes the handset from the swltchhook, the ringer is disconnected and the a and b wires of the line circuit are connected through the usual `induction coil, transmitter and switch-hook contacts. The ringer is disconnected so that the condenser shall not receive an appreciable charge between the keying of digits and thus produce unwanted potentials.

The mechanical details of the subscribers keys will be disclosed below in connection with Figs. 3-8 after their electrical functions have been explained in connection with Figs. 1 and 2.

When any key l-O is depressed, contact l of the home contact nest closes and short-circuits the induction coil and transmitter. Immediately after that, contact 2 opens and removes a shortcircuit around resistor RX and the iive keying contacts CI to C5. Resistor RX is adjusted individually at each station so as to make the combined series resistance of RX plus the line wire LR in the loo`p to the central oilice 500 ohms i-5%. To facilitate adjustment at the sub-station, it is proposed to use a plug-in radio type resistor for RX,

The above-mentioned operations take place immediately upon the depression of any key. When any one of keys l to 4 is about M1 depressed, one of the contacts CI to C4 is momentarily opened by the key in passing, thereby momentarily introducing a resistor RI R2, R3 or R4, as the case may be, into the line circuit. Each resistor Rl to when it is released, it restores 'instantly'to its n'lor-Y mal position under the tension'of al spring with'- out switching in any of the resistances RI to R5. The keys 'may be depressed very rapidly or Svery slowly; the speed of their operation is without effect on the responsive means.

When any one of the keys to 0 is depressed, three contacts are momentarilyopened.4 instead of two, the i-lrst when the key is 1A depressed,.the second when the key is 1A; way down, and the third when the key is.% depressed.

'Ihe contacts which' each key momentarily opens when it is depressed are indicated by'the letters CI, C5, etc. placed immediately below the key button on the key contactfdiagram (Fig,l 2). The contacts are listed topV downin th .order in which they are opened.V It `will befseen that contact C5 is alwaysfthellas'tto open. It introduces resistor R5 into the line lcircuit which operates the change-over tube circuit in the register. A

Should the subscriber hang .up before .com pleting keying, the line circuitwillbe openedland the shunt will be removed from the "5,000 ohm resistor R1 in the register circuit. (Fig. `10) .theremeans, such as shown in Figs. 5 and 6, is a sliding plate I3 which engages the 4home contact nest Il and is normally kept in the position shown by the tension of the spring nest. When amr key is depressed, the slanting part I4 of the plunger I (Fig. 5) forces the sliding plate I3 to the left, thereby operating the home contact nest.

The arrangement of the pins in sliding bars BI to B5 (see Fig. 8) agrees with the diagram (Fig, .10.

2). YFor example, rwhen key 6 is depressed and released, theiollowing operations take place in the" order Vlisted t a. Sliding plate I3 operates home contact nest b...When the key is about 1A Way down, wiper 6 engages -pin-PI 'and forces bar BI to the right, thereby opening vcontact CI. Almost immediately thereafter, part 6 releases the pin and bar 4BI is forced back sufficiently byV contact CI to by permitting the potential ofthe common lead to increase toa maximum and operate .therelease tube circuitNo. 6. v

Key set assembly (Figs, 358) One form of a ten-button key set is shown in these figures. It comprises ten keyY plungers I of which a rectangular portion pierces a housing 2 and is free to slide in said housing. The plunger is slipped through .the housing from the bottom, after which button :i is attachedto it in any suitable manner. The lower' part 'fof .the plunger I is round and over this partis slipped retractile spring 4, which rests against thejbottfom plate 5 of the housing and againstthe rectangular part of the plungerat' the'. top.v V',Ihuslthe spring normally keeps the plunger'in the position shown. A slanting wiper 6 projectsirom Y.each plunger.

When any key is depressed, it longitudinally displaces Atwoor three of five horizontallbars, BI, B2, B3, B4, and B5 in accordance' .with the setting of pins, PI, P2, P3, P4, ete., whichproject -from the barsin patterns that determine the sequence of contacts CI-C5 Vthat will be operated by the key. Each of the iive bars engages atone end a contact CI toC5 (see particularly Figs, 6 and 7). Part 'I is welded tobars BIv and`B5 and operates the twoouter contacts CI `and C5 when either bar is moved by a key. Part 8 projects from'BZ and B4 and operates contacts C2 and CII. Barl.B3 operates contact C3 direct. The other ends of the ve bars rests against an insulated piece 9 attached to band spring!) (see Figl'); Spring 9 keeps all the five bars in engagementwith their corresponding C contacts. Movement of the spring` 9 is limited by back stop I2. The ve bars are heldin position by slotted insulator ,supports IIJ (Fig. 5) which are attached to the bottoni of thehousingt.

' Supported from the housing by any suitable allow its closure.

c.,When .the key is about le waydown, bar..BS is similarly/'forced 'to the, right.throughJP,Y and momentarilyopens contact C3.`

d. When the key is about.% way down, bar.B5 is kforced, lto theright by P5 andcontact C5 .is

momentarily opened.

e. When the key is all the way down, bar B5 is released and contactC5 re-closes.

',f. When pressure is removed. from. the `key button, the plunger is quickly lrestored tofnormal by retractile spring 4. Wiper 6, however, .engages the three pins in question fromtheunder side, thereby forcing them to .the ,left against common back stop 9. Contacts CI, C3 V and C5 are thus not opened on the returnstroke. As .Wiper-.,wpasses eac-h pin, spring!!,restoresv thebar to normal.

When the `plunger is fully restoredtoits normal position, sliding plate I 3 Vresumes its 'the second. This is particularlyl truen the ease of key o which operates baises, B2i and 135.

Slubscrzfbers set assembly (Fig.9)

One form of table set l25 equipped with handset 20 and key set 2| is shown in'IEig '9. fI get the keys out of the way when removing and replacing thehandset, and to obtainthe required depth for the key set without disturbing thenorial arrangement of ringer, induction coilv and.v ,con-

.denser attached to the base, the keys are backof the handset. -The keys are guarded against accidental operation or damage by 'sidea'rid ` Vrear walls 22 and 23. v

Thedepression 24 in -frontof .the .key set..2l and the outward slope of the Wel/1123 atjithe rear vof lthe keys afford a meansof pickingupthe table s et without disturbing thehandset.'

Operation of the register'vial) For the details of the register circuit, selectors,

lline finders and associated devices attention is directed tomy Vcopending application Serial No. 521,160, `iiled February 5, '1944'.'1-Ier'e only 'so muchY isrshown `as is necessary for illustrating automatic means lfor variably lresponding to the actuation ofthe key set, for registering the response vand transmitting it to selectors or the like used. e. g. in the establishment oraconnection Vbetween calling and called subscribers -When the subscriber removes the handset 20,

an idle register (Fig. ,10) vand a iirst linender Whena Y digit keyed by the subscriber has been recorded, the a Wirecof the selector-circuitis grounded are connected with the callingline.

and apotential is placed o n the bf wireso that theselection of thealledline caribe-made. AIn

.the present casegXR relays are substituted for the digit switches oirny application SerialNo. 521,-

160, filed February 5, 1944, ,in the same VIrl-'inner as in my copending application SeriallSIo.` 527,921,

led March 24, 1944, now Patent No. 2,433,345, dated Dec. 30. 19,47-

When relay BAR releases, -it opens thecircuit of slowreleasingrelay LDLR, thereby-momentarily grounding both sides 0f the .calling line forth@` purpose of discharging Ythe Yenergy stored in the line. AWhen-LBR makes its back contact, the V2 20 grids of thestlx triade circuits are connected to the b wire. ;R13R,operat es HTR. lI-ITR oonnectsresistor R1, also attached to the b` Wire,

to theexchangel v olt battery. 'I'he `secondary winding ofthe dialingtone-transformer TT is 2 5 bridged across the calling line and the primary circuit is closed over a front contact of RBR. If required, filter F may be placed inthe common valve leadtoprevent false operation of the valve circuits due to possible undesirable potentials 30 generated by theqdialing tone circuit. ap-

plies high tension battery to the valve circuits.

, RBRalso vOperates RIR which Controls release and the vdischarge tube meansmarked Tube Ct.

#6 in Fig. 10 contrOlsthe release as will be here- -inafter explained in vgreater detail. ating .potential for which leach pairof triodes is set is shown in Figs. 11 Vand 12. When the potentials applied-to the grids of a pair of trlodes bear a predettir11 linedV relationship Ato one another, i. ve. when they are Vthe same, then the associated gas valveGVv strikesy andoperates its GVR relay.

Whenthe connectionis lestablished with the register, the subscribers set is bridged acrossthe line and the subscriber hears the --dialing tone.

vThe resistance of theloop circuit, as previously `stated, is A500 ohms orflessand the potential on a common lead (point E-I,"Figs. 10 and 11) is Well below the Operatinglimit of any ofthe Valve circuits. -l

When the subscriberY depresses a key for the .iirst.digit, his induction coil `and transmitter are Ashort-circuited and RX (Fig. 1) is cut into -the line circuit. 'The potential of point-E-l is now 12.7 volts (Fig. 12). This is the non-release con- 7 dition during keying. If key 1 is depressed, then the next stepis the insertion of resistor Rl, which :increases the loop. resistance to 3500 ohms and thepotential of the common tube lead to approximately `32..volts, .whereupon the No. lpair of .17

The 45 ,dischargetube means marked Tube Ct. #5 controls the change-over from one, digit to the next,

The Voper- 50 .triodesfy the. grid offone of which is permanently connectedto this and only this potential, operates and restheassociated gas valve, all inthe well-known manner. GVRI operates. When zikey 1 is nearly fully depressed, it momentarily cuts in R5 (Fig. 1) vand applies potential of approximately 18 volts, whereupon the No. 5 pair Aof triodesoperates and energizes GVRS (Fig. 10).

GVR5 grounds the armature of relay GVRI, thereby causing XRI to operate. GVRS also operates DTR which looks up over a front contact of RI R and disconnects the dialing tone. Shortly after GVR5 makes its iront contact, itv opens its back contact and releases HTR. HTR falls oii, de-ionizes GV and releases GVR of tube circuitsNos. 1 and 5. When ground is removed from the winding of XR, the latter locks up over a front contact in series with the Winding of TRI andaront contact of RCR. TRI operates, and transfers the tube circuits to the second set -of XR relays over back contacts of TR2.

The second and following digits are, keyed in lthe; same manner as the iirst, tube circuits Nos. 1 to 4 or combinationsof them being operated according to the key depressed as indicated in the -following table. The tube circuit No. 5 is always the last tobe operated. It locks up the proper XR relay or relays and prepares immediately for thenextselection.

GVR and PN-Poten- Key Depressed XR Relay tial Con- Operated nected l PN-l 2 2 3 3 4 4 l+2 A 5 1+3 6 1+i 7 2-1-3 8 2-1-,4 9 Sel-4 10 I l.As explained in my copending applications, in multi-potential controlsystems like the one here disclosed, the selectors are startedby the register Aand hunt for a potential corresponding to the one connected in the' register. When the selector nds the desired potential its driving circuit is --opened by the firing of the gas valve in the register.

Ordinarily the registeris released upon the completion of selection. However, should Vthe subscriber hang up before selection is complete, .the line circuit is opened and the potential of point E-l is increased to the'maximum as indicated in Fig. 12, applying 40 volts to the grids and operating tube circuit No. 6. GVRS opens the circuit ofthe release relay RIR which restores the circuit to normal.

What is claimed is:

1.111 a telecommunication system, a plurality of lines, each having a multi-digit number, means including a plurality of stages .of numerical selector switches for establishing a, connection between a calling and a called line, a plurality of keys for each line, a normally inactive register comprising digit switching means for Variably operating the selectors in accordance with the different digits of the called number, said register comprising a plurality of electric discharge tube means less innumber than the keys associated with a line, means for establishing a conductive connection from the keys to the tube` means, a source of direct current potential, means con- 5 .,trolledlby. the keys over the connection for selectively operating the tube means from said source, and means controlled by thev tubes for variably controlling the digit switching means.

2. The system according to claim l', and in which the register comprises a digit Vswitchv .for each digit, means controlled by .the tube means for operating all the digit switches, and means controlled by one of the tube means for successively connecting the other tube means from one digit switch to the next.

3. 'Ihe system according to claim l, and in which one of the tube means operates upon each actuation of any key, and means controlled by the last-mentioned tube means for preparing the register for the next digit.

4. The system according to claim 1, and in which each line is provided with ten keys and the register four discharge tube means, means controlled by four keys for selectively operating the four tube means, and means controlled by each of the other six keys for operatingtwo of sai-d tube means in six diierent combinations.

5. The system according to claim l .,-and'fin which each line is provided with ten keyseiand the register with four discharge tubemeans,means controlled by four keys for selectivelyV operating the four tube means, means controlled by each of the other six keys for operating two of said tube means in six different combinations, and a fifth tube means in the register operable upon the actuation of any key, and means controlled by the iifth tube means for associating the four tube means with the digit switching means.

6. The system according to claim l, and release means for the register controlled by one of the tube means. Y Y

7. The system according to claim 1, a plurality of resistances for each line, contacts controlled by the keys for selectively connecting said resistances in the connection for selectively operating said tube means.

8. The system according to claim 1, and a control grid for each tube means connected in multiple with said connection.

9. The system according to claim 1, and in which each tube means comprises a pair of tubes.

10. The system according to claim 9, and in which each tube is `a triode, a multiple connection from the grids of one triode of each pair to said connection, and connections for applying different potentials to the'grids of the other trio'des of each pair.

11. In a switching system, a source of D. C. potential, a switch variably operable depending on the potential applied thereto, a plurality of two-wire lines, means for connecting a line with the source and the switch, keys associated with each line, home and keying contacts controlled by l said keys, at least four different resistances connected in series with one another between the line wires over home contacts, a shunt normally closed V.around each resistance by. a different keying contact, means for actuating the home contacts when any key is operated, and means controlled by each key for successively momentarily opening diierent keying contacts.

l2. In a switching system, a source of D. C. potential, a switch variably operable depending on the potential applied thereto, a plurality of two-wire lines', means for connecting a line with the source and the switch, keys associated with each line, home and keying contacts controlled by said keys,'ve different resistances connected in series with one another between one line wire and a normally open home contact, the later connected with the'other wire,'a shunt normally closed around each resistance by a dlierent keyingcontact, a sixth resistance for each line in series with the five resistances of a value approximately equal to the diierence between the loop resistance of the line and a predetermined value, a shunt around the six resistances including a normally closed home Contact, means for actuating the home contacts when any key is operated, and means controlled by thekeys for successivelyv momentarily opening different keying contacts.

13. In a switching system, a source of D. C. potential, a switch variably operable depending on the potential applied thereto, a plurality of two-Wire control circuits, means for connecting one of the circuits with the source and the switch, ten keys associated with each line, a pair of normally closed keying contacts for each key, normally open and normally closed home contacts, ve different resistances for each line connected in series with one another between one line wire and a normally open home contact, the latter connected with the other wire, a shunt normally closed around each resistance by a diiierent keying-contact, a sixth Vresistance for each line in sei'es with the iive resistances and having a value approximately equal to the difference between the loop resistance of the line and a predetermined value, a shunt around the sixth resistance and the keying contacts including a normally closed home contact, means for actuating the home contacts when any key is operated, a longitudinally movable plunger for each key, and means controlled by each plunger for successively momentarily opening predetermined keying contactsv during its movement in one direction.

14. In a telecommunication system, a plurality oi lines each having a multi-digit number, means including a plurality of stages of numerical switches for establishing a connection between a calling and a called line, a register for successively controlling the operation of said switches, a controller comprising ten keys associated with each line, each key adapted to variably operate the register in accordance with the different digits of the called number, a plurality of but less than ten discharge devices in the register ccnnected to a common lead, key resistances, contacts Vcontrolled by the keys for connecting the key resistances in series with the line, the potential of the common lead depending upon the value of the combined line and key resistances in series to which said devices are responsive, a relay associated with each device and controlled by them, and a selector control circuit controlled by said relays in accordance with successive digits of the called number.

15.4The system according to claim 14, and in which one of said relays controls the change-over of the register control from one selector to the next.

16. The system according to claim 14, and in which one of said relays controls the release of the register.

17. In lan automatic telephone exchange system, a plurality of subscribers two-wire lines each having a multi-digit number, means including a plurality of stages of numerical selector switches for establishing a connection between a calling and a called line, a plurality of registers for successively controlling the oper-ation of said switches, ten keys associated with each line for Vvariably operating a register in accordance with the different digits of the called number, means for connecting a calling line with an idle register, six pairs of triodes in each register, one triode in each pair having its grid connected in multiple to a common lead to one of the wires of the calling line, a first connection to ground from the common lead over a resistance closed while the register is idle, a second connection from said common lead to battery over a resistance, key resistances, contacts controlled by the keys for connecting the key resistances in series with the line, the potential of the common lead depending upon the value of the combined line and key resistances in series to ground and in multiple with leakage resistance, a different D. C. potential connected to the grid of the second triode of each pair, a pair of triodes being adjusted to operate when the potentials applied to the grids of the triodesbear a predetermined relationship to one another, a relay associated with each pair of triodes and controlled 4by them, and a selector control circuit controlled by said relays.

18. In an automatic telephone exchange system, a plurality of subscribers two-wire lines each having a multi-digit number, the loop and leakage resistances of a line having predetermined values, means including a plurality of stages of numerical selector switches for establishing a connection between a calling and a, called line, a plurality of registers for successively controlling the operation of said switches, ten keys associated with cach line for variably operating a register in accordance with the different digits of the called number, means for connecting a calling line with an idle register, six pairs of triodes in each register, one triode in each pair having its grid connected in multiple to a common lead to one of the wires of the calling line, a first connection to ground from the common lead over a relatively high resistance closed while the register is idle, a second connection from said common lea-d to battery over a relatively low resistance, key resistances, contacts controlled by the keys for connecting the key resistances in series with the line, the potential of the common lead depending upon the value of the combined line and key resistances in series to ground and in multiple with leakage resistance, a .different D. C.

potential connected to the grid of the second triode of each pair, a pair of triodes being adjusted to operate when the samepotentials are applied to the grids of both triodes, a relay associated with each pair of triodes and controlled by them, and a selector control circuit controlled by said relays.

19. In an automatic telephone exchange system, a plurality of subscribers two-wire lines each having a multi-digit number, the loop resistance of a line being less than about 500 ohms and the leakage resistance less than 25,000 ohms, means including a plurality of stages of numerical selector switches for establishing a, connection between a' calling and a called line, a plurality of registers for successively controlling the operation of said switches, ten keys associated with each line for variably operating a register in accordance with the different digits of the called number, means for connecting a calling line with an idle register, six pairs of triodes in is idle, a second connection from said common lead to a 48 volt battery over a 1,000 ohm resistance, key resistances, contacts controlled by the keys for connecting the key resistances in series with the line, the potential of the common lead depending upon the value of the combined line and key resistances in series to ground and in multiple with the leakage resistance, a different D. C. potential connected to the grid of thc second triode of each pair, a pair of triodes being adjusted to operate when the same potentials are applied to the grids of both triodes, a relay associated with each pair of triodes and controlled by them, and a selector control circuit controlled by said relays.

20. In an automatic telephone exchange system,` a plurality of subscribers two-wire lines each having a multi-digit number, means including a plurality of stages of numerical selector switches for establishing a connection between a calling and a called line, a plurality of registers for successively controlling the operation of said switches, ten keys associated with each line for variably operating a register in accordance with the different digits of the called number, means for connecting a calling line with an idle register, six pairs of triodes in each register, one triode in each pair having its grid connected in multiple to a common lead to one of the wires of the calling line, a iirst connection to ground from the common lead over a resistance closed while the register is idle, a second connection from said common lead to battery over a resistance, key resistances, contacts controlled by the keys for connecting the key resistances in series with the line, the potential of the common lead depending upon the value of the combined line and key resistances in series to ground and in Vmultiple with leakage resistance, a different D. C.

potential connected to the grid of the second triode of each pair, a pair of triodes being adjusted to operate when the same potentials are applied to the grids of both triodes, a relay associated with each pair of triodes and controlled by them, a selector control circuit controlled by four of said relays.

21. The system according to claim 20, and in which one of said relays controls the change over to the next.

22'. The system according to claim 20, and in which one of said relays controls the release of the register.

23. In an automatic telephone exchange system, a plurality of telephone substations, switch- Vidual contacts controlled by each of the keys each register, one triode in each pair having its and a common contact controlled by all the keys, a device selectively operable` under the control of the key contacts over said circuit, a ringer and a condenser in series in the circuit at each station, and means oper-able while any key is operated for disconnecting the ringer and condenser from the circuit.

24. In an automatic telephone exchange system, a plurality of telephone substations, switching devices for connecting a calling with a called station, a circuit leading from each station towards the switching devices, a source of D. C. Potential in said circuit, a plurality of resistances at each station connected in the circuit, a'set of keys, contacts short-circuiting the resistances controlled by the keys, a ringer, a condenser and circuit at eachstation, 4said' home Vcontact short-v circuiting allthe keys, andmeansl responsive to the actuation of-vany' key, of opening the home contact and maintaining it 'open'while thekey is operated;

Y 25.`In. an automatic telephonenexchangesys'' tem, a source ofl D. C.,v potential, a plurality of telephone substations, switching'devicesforfcontrolling V,the connectingof a calling with a called station, a, two-wire circuit-leading :from each station-towardsthe switchingdevices, means for connecting the Vsource with 'the' callingiline, five resistances at each station connected 1n series in the circuit, ten keys at each station; contacts. controlled `Joy; saidA keys. Yshort.;clrcuitimg the .resistapnces,` means controlled; by 'the .keysrfor sedenser-"fand a Y. normallyV Vclosed f'home icontact in y seriesi'in lthe'cireuit at= each station, said home contactfshortfcircuitn'g allthe keys'and resistances, and v4:means` responsive' to the f actuation of rany key, fory Vopening the --home contact and maintaining it f open while'the'key is operated'. GERALD DEAKIN.

REFERENCES CITED Th'efollowing vreferences are of recordinthe' l'e ofv thispatent:

UNI'I'ED STATES'PATENTS Number: Name-.r Date.v

1,337,715 M'cQuarrie" 4A131210; .1920 1,854,106@V Car.penter1 V- Apr. 12, 1932 1,982,290 Gardne-rf Nov. 27, 1934 2,269,022 Hech't ,Jan. 6,'.1942 2,307,523?` Parkinson-1 Jan: 5; 1943

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