US2285972A - Telephone system - Google Patents

Description

June 1942- w. H. T. HOLDEN ETAL 2,285,972

TELEPHONE SYSTEM Filed June 22, 1940. 4 Sheets-Sheet 1 LOWEST LINE GROUPS mu. 1: HOLDEN //v VEN TORS: r: .4. HUBEARD m H. MARTIN v A T TORNE Y June 9, 1942.- w. D N ET AL 2,285,972

TELEPHONE SYSTEM I Filed June 22, 1940 4 Sheets-Sheet 2 ATTORNEY June 9, 1942. w. H. 'r. HOLDEN ETAL TELEPHONE SYSTEM Filed June 22, 1940 4 Sheets-Sheet 3 nut //v v/v TORS-f'. A. HUBBARD By hf H. MART/M v ATTORNEY June 9, 19 w. H. T. HOLDEN arm. 2,235,972

TELEPHONE SYSTEM Filed June 22, 1940 4 Sheets-Sheet 4 MMZHOLDEN INVENTORS: F. A. HUBBARD w. H. MART/N ATTORNEY Patented June 9, 1942 TELEPHONE SYSTEM William H. T. Holden, Woodside, N. Y., Francis A. Hubbard, Maplewood, N. J., and William H. Martin, Chappaqua, N. Y., assignors to Bell Telephone Laboratories,

Incorporated, New

York, N. Y., a corporation of New York Application June 22, 1940,Serial No. 341,816

8 Claims.

This invention relates to telephone systems and ha for its object the improvement of such systems by the use of novel equipment.

More specifically, the invention contemplates a novel line circuit for a subscribers line which does not involve the usual line and cut-off relay as parts thereof and, in this respect, one novel feature of the invention is the disposition of telephone lines upon the cross-points of a cross-bar switch, each hold magnet of which forms a part of the line circuit of the individual line disposed upon the cross-points controlled by said magnet.

Another feature of the invention is a novel arrangement of electron distributors which are adapted to operate in response to the initiation of a call to identify the cross-point of the line making the call and to control the circuits which operate the appropriate select and hold magnets that cause the extension of said line to an idle link channel available in common to the lines of the group of which the calling line forms a part. This is accomplished through the application of a discriminating potential upon an electrode individual to the line in a distributor allocated to a group of lines, and through circuits responsive to an impulse generated by the electrode when the scanning beam of the distributor strikes it.

Another feature of the invention is the ap plication of an auxiliary cross-bar switch through whose operated cross-points the respective hold magnets of the various individual line circuit are made accessible.

A further feature of the invention contemplates the use of other scanning distributors by which the magnets of said auxiliary cross bar switch are controlled.

These and other features of the invention will be more apparent from a consideration of the ensuing description in connection with the drawings, in which:

Fig. 1 shows a cross-bar line switch assembly accommodating six hundred lines divided into vertical subgroups of one hundred lines each;

Fig. 2, which should be placed directly below Fig. 1, shows the auxiliary cross-bar switch unit through whose cross-points the hold magnets of the line switch in Fig, 1 are accessible;

Fig. 3, which should be placed to the right of Fig. 2, shows two of six electron distributors and amplifiers controlled thereby as well as the common release circuit; and

Fig. 4, which shouldbc placed to the right of Fig. 3, shows two other electron distributors and amplifiers controlled thereby.

Reference is here made to the application of W. W. Carpenter, Serial No. 214,356, filed June 17, 1938, now U. S. Patent 2,235,803, granted March 18, 1941, for a detailed description of a telephone system of the type in which the present invention may be employed.

In such a system, a calling subscriber, upon initiating a call, is connected to an idle link channel and thereover to a district junctor circuit and to a sender Via sender selecting switches. The subscriber then dials the wanted number which is registered in the sender. When the sender has received the digits identifying the ofiice in which the wanted line is located, it associates itself with an idle marker through a marker connector and transmits these digits to the marker. The marker translates said digits and operates a route relay in consequence thereof which identifies the trunks leading to the wanted office. The present invention has to do with a line circuit for such a system having neither line nor cut-off relays and which operates in conjunction with electron scanning devices that operate to extend the calling line to an idle link channel.

According to the present invention, the line circuit comprises a hold magnet of a cross-bar switch, a loop extending between the line station and the contacts of said magnet, a resistance and a source of current connected to said contacts whereby upon the line initiating a call, current will flow over the loop and through said resistance. A conductor is provided for each line which extends from one side of the resistance to an electrode within an electron distributor which is adapted to hold one hundred such electrodes, each connected to a different line. The electrodes, or anodes, are circularly disposed within the distributor envelope and can be struck by the electron beam in succession if the beam is caused to rotate around the anode ring by suitable sweep controls at the envelope deflecting plates. The potential of any anode is altered when the associated line initiates a call, and when the electron beam strikes the anode a secondary electron emission is induced which, after suitable amplification, operates a thyratron device having a relay in its space current path that operates to close a partial circuit to the hold magnet of the calling line circuit, and is further impressed upon the grids of two other electron distributors .whose electron beams rotate synchronously with common electrode.

that of the line distributor; The first of these two other distributors has its one hundred circularly disposed anodes multiplied in ten groups of ten each and the other has its one hundred similarly disposed electrodes multiplied in ten groups of ten, each group being made up of corresponding anodes in each of the groups of ten. Since all three beams rotate synchronously, the beam of the line distributor strikes the anode whose potential has been altered by the calling condition of its associated line at the same time that the beams of the two other distributors strike the correspondingly disposed anodes which considering the manner in whichzthey areseparately multiplied, will indicate the tens and units desig- 1 nation of the calling line. These anodes, when struck by the electron beams, induce secondary electron emissions which, through suitable amplifiers, operate two other thyratron devices that complete circuits to the appropriate select and hold magnets of an auxiliarycross-bar switch that, in operating, close the. cross-point through which the linehold magnetcircuit is completed by the relay operated inresponse to theline dis-- tributor beam'striking the anode of the calling line. The operated select magnet of the auxiliary cross-bar switch locally controls the-select=rnagnet of the line switch and the operation of said magnetas well, as the line hold magnet, serves 1 extendthe calling line to the link.

The operation of the invention will now be described in detail with respect to the circuits and apparatus shown in the above-mentioned drawings.

In Fig. l is shown a subscribers line group of six hundred lines disposed in vertical subgroups of one hundredlines at the cross-points of the cross-barswitch assembly in the numerical order indicated in each of thecircles designating a subscribers line. The cross-bar switch shown in Fig. 1, as well as that shown inFig. 2, is dis closed in Patent 2,021,329, granted November 19, 1935, to J. N. Reynolds and reference is made thereto for a complete description of its construction, and operation.

Each line circuit, as, for instance, line circuit A, which is line 99 in the first verticalsubgroup of lines, has a conductor 39 extending to an anode 99 in the electron discharge distributor Bill. Similarly, all other lines in this group have their respective identifying conductors extended to correspondingly numbered anodes in distributor 39 l. The remaining five groups of one hundred lines each have their respective identifying conductors connected to correspondingly numbered anodes in the other distributors, namely, distributors 392 to 3%. This distributor, of which six are used as line distributors for a six hundred line group, namely, 30! to 356, of which the first and last only are shown in Fig. 2, and two more, namely, M30 and 46d shown in Fig. 4 which are used as identifying distributors, are of, the kind described in Patent 2,217,774 granted to A. M. Skellett on Oct. 5, 1940. The distributor comprises a cathode, one hundred anodes mounted in a cylindrical boundary coaxial with and encompassing the cathode beam, means for concentrating the electrons emanating from the cathode into a single radial rotating beam focussed and impinging upon the anodes in sequence, and a collector ring which acts as a The concentrating means comprises a two-phase current supply Elli for producing a magnetic field normal to the oathode and an electrostatic field in step with the :however, that the invention is not so limited,

it being evident that auxiliary switching facilitiescan be augmented and the line group increased correspondingly, in which event the line distributors would also have to be increased in accordance with the number of lines which can .beaccommodated by a single distributor.

In the operation of the invention, a subscriber,

saysubscriberA,.initiates a call and in so doing completes a circuit which is traced from battery, resistance 3!, inner contacts of, bold mag- ,net I62, subscribers loop, outer contacts of hold magnet I82 to ground. The current flowing through this circuit establishes a negative potential to the left of resistance it! which is applied, over conductor 99 to anode. 955 of theelec tronic distributor 35!. th distributor, caused by the 24 20-cycle supply 375 connected to the two sets of coils outside of the envelope and deflecting the beam electromagnetically as explained in the abovementioned patent to Skellett, sweeps a surface in the form of a circle that coincides with the ring. formed by the one hundred anodes to which the conductors of the first one hundred line subgroup are connected. Consequently when the beam strikes the anode connected to f conductor 99 of the calling line (this anode nowbeing more negative than normal), a current impulse is pro-' duced by the electron emission induced through the circuit formed by the line anode, in the collector electrode 39'! of the distributor, conductor 308, resistance 3H, positiVe battery 326i. The collector electrode 337 is a circular electrode adiacent to the anodes and is connected to the outside of the distributor by a collector conductor 398 which extends, in turn, to the grids of vacuum tubes 322 and 323 and to the resistancecapacity coupling consisting of resistance elements 3|! and H8 and capacity element M9.

The current to the collector electrode 367 is due to the secondary electrons emitted from the target anodes when the electronic beam 399 is incident thereon. When an anode is more negative than its normal value, due to the associated line initiating a call, the voltage available to draw the secondary electrons to the collector 301 is increased, which cause the collector'current in conductor 3&8 to increase. The point in the cycle of rotation of the primary beam at which this current increase will occur is characteristic of the identity of a particular anode. For example, as there are one hundred anodes distributed around 360 degrees of the circle, and as the 24 field is a bipolar one, the 2 I current or voltage causing the beam to be deflected will go through 360 electrical degrees for one complete sweep around the circle. Thus if the twenty-fifth anode is the one that corresponds to the calling line, the phase angle between the impulse in the collector circle and the so supply will be 90 degrees, while if it were the twentyfourth electrode the phase angle would be 87.4 degrees. In this way, the collector current, when it is produced, identifies which one of the one hundred lines is calling.

When the collector current impulse appears The revolving beam of on conductor 338, it is transmitted to the grids on vacuum tubes 322 and 323 of amplifier 3| l. The voltage impulse of the collector is applied through the resistance-capacity coupling above described. Battery 320 furnishes the usual positive bias for the collector electrode. Battery 32| furnishes grid bias to the amplifier vacuum tubes 322 and. 323, the grids of which are multipled. The output of the tube 322 is applied to the grid of the gas-filled tube 324 which has anode battery supplied to it from positive battery source 329 through resistance 34! contacts of relay 3 32, conductor 325, and causes it to become conducting when the impulse is received from the collector by the amplifier tube 322. The operation of gas-filled tube 324 causes the operation of relay 33l which operates to ground start conductor 348 to initiate the operation of a control of the kind disclosed in the above-mentioned Carpenter application, performs functions noted hereinafter and lowers the voltage on the anode of each of the other five gas-filled tubes which are connected together by conductor 325 to resistanoe 35!. It also lowers the anode voltage on the vacuum tubes 322 and 323 of amplifier 3! I as well as on the other vacuum tubes of the other five amplifiers EH2 to 315, so that they are ineffective after gas-filled tube 324 becomes conducting. This reduction in anode voltage results from the drop in resistance 34! due to the arc from tube 322 reaches the grid of gas-filled tube 324, the same impulse, amplified, is also transmitted through tube 323, transformer 326 to conductor 32?, which connects with the control grids 4-0! and hil of the electronic distributors 409 and M39 respectively, shown in Fig. 4. 45

These distributors are also of the cathode ray type but difier from distributors of the 3!!! type in that they have a control grid 48!. This grid is connected to conductor 32? and through the secondary winding of transformer 325 and conductor 343 to a source of negative battery 408. The grid bias potential is of such a value as to suppress the electron beam except when an impulse of the prop-er polarity is received from the transformer 3'28 at which time the change of grid bias due to the impulse releases the beam for impingement upon whatever anode is in line with the direction indicated by the magnetic field outside of the distributor envelope. The arrangement of the anodes in each of the di tributors and 5% is as follows:

In each of the distributors Add and iii! there are one hundred anodes which correspond in position to the one hundred anodes in each of the line distributors Ziill to 3&5. The leads, however, are not brought out from each of the anodes. They are instead, multipled in the following order: In distributor 4th, the anodes 0 to G3 are multipled to a single conductor which is brought out. Likewise, anodes iii to ES, 23 to 29, etc. are groups each connected to its own group conductor. In distributor 46%, on the other hand, anodes iii H1, 23, 33, etc. up to and including anode 93 are multipled together and one conductor is connected thereto and brought out 7 therefrom. Simfiarly, anodes 0|, ll, 2|, 3|, etc. are multipled and a single conductor connected to them and brought out. There is, therefore, a total of ten conductors each of which is connected to every tenth anode around the circle.

Each anode conductor from distributor 49!] extends to the grid of a gas-filled tube of which one tube 4H] of thyratron circuit IXil is shown connected to conductor 492 which extends to the first group of ten anodes of distributor 4%; another conductor 403 which extends to the tenth group of anodes in distributor 4-06 is connected to the grid of gas-filled tube MB of thyratron circuit TX9, while the intermediate conductors for the second to ninth groups of anodes are connected to the grids of the intermediate gasfilled tubes MI to H8 of circuits TX! to TXS which are not shown but indicated. Similarly, each conductor from distributor 490 extends to the grid of a gas-filled tube of which one tube 423 of thyratron circuit TYii is shown connected to conductor 4M extending to the first group of ten anodes connected to said conductor; another conductor 495 which extends to the tenth group of anodes is connected to the grid of gas-filled tube 429 of thyratron circuit TY9, while the intermediate conductors for the second to ninth groups of anodes are connected to the grids of the intermediate gas-filled tubes 42! to 423 of circuits TY? to TY3 which are not shown but indicated. The anodes of the gas-filled tubes of the circuits T'X i to TXQ are connected to positive battery through the normally made contacts of relay 4% while the anodes of the gasfilled tubes of the circuits TYii to 'IYii are connected to positive battery through the normally made contacts of relay iiil. The filament of each of the tubes is heated by a local battery source which is not shown.

The revolving fields which cause the electron beams to traverse the circles of one hundred anodes in distributor tubes ii to and the virtual beam of distributor tubes 538 and Add to traverse the circles of the anodes in these tubes, are all in synchronisin since th y derive their motive power from the same 22o driving supply 375. In other words, when the beam in any one of the line tubes 31:?! to strikes an anode having a negative potential thereon, due to a call having been started on the associated line, the impulse transmitted along conductor 32! will release the virtual beam in both distributors 429 and 4353' and strike anodes in their respective envelopes which correspond to the position of the anode struck by the beam of the line distributor. However, since the anodes of distributor see are multipled. consecutively in groups of ten, it follows that the group containing the anode struck by the beam thereof identifies the tens digit of the calling line. Similarly, since corresponding anodes in each group of ten anodes of distributor it fi are multipled together, it also follows that the anode struck by the beam in this distributor identifies the units designation of the calling line. It has been assumed, however, that the main group of six hundred lines is divided into subgroups of one hundred lines each. the present invention, the tens and units designations of the calling line are used to identify the cross-point in auxiliary switch 2% through whic accessibility may be had to the hold magnet the calling line circuit.

When the beams of distributors and 4 39' strike respective targets in the same relative positions as the target associated with the calling line in any one of the distributors 36! to 303 when struck by the beam of said distributor, the grid of one of the tubes in each of the two groups of circuits TXE! to TXEI and TYiB to TY3 is made sufficiently positive to override the negative grid bias, in consequence of which these tubes, one in each group, become conducting. If it is assumed, for example, that the calling line is located in the first group and is line 99 in that group, then the beam of line distributor 3E3! will strike anode 99 therein and cause the operation of relay 33! as already explained. Simultaneously, the beam of distributor 4% will be directed to the ninth group of anodes and that of distributor 4% will likewise be directed to its own ninth group of electrodes. Upon the arrival of the impulse to lower the bias of grids 4m and 4M, therefore, both beams are activated thereby causing tubes M8 and 429 to break down and become conducting.

The cathode of each tube in circuits TXt to TXQ is connected to a corersponding select magnet of the auxiliary cross-bar switch 203 while the cathode of each tube in circuit TYfi to TY9 is connected to a corresponding hold magnet in said switch. Thus the cathode of tube 4H] of circuit TXll is connected to select magnet 2m and that of tube 459 of circuit TX9 is connected to magnet 259 while the cathodes of intermediate thyratron circuits are connected in corresponding numerical order to the intermediate select magnets 2| i-28 (not shown). Also, the cathode of tube 42$! of thyratron TYS is connected to the hold magnet 22%] through reactance 249 and that of tube 429 of circuit TYU is connected to hold magnet 229 through reactance 249. The cathodes of the intermediate tube circuits TY! to TYS are connected to the intermediate hold magnets 22! to 223 through reactances 24! to 248 all of which are not shown.

Hence when tube 4| 9 breaks down and becomes conducting for the reasons above set forth, a circuit path is completed which extends from positive battery, contacts of relay 4%, space current path through the tube 449 and cathode thereof, conductor 439, winding of select magnet M9 to ground, causing said magnet to operate. Upon its operation, the magnet causes the fingers of the ninth horizontal channel of the crossbar switch 238 to be set for engagement upon the operation of any of the hold magnets 22E! to 229 and further closes an obvious circuit for relay H3 which operates and connects the sleeve conductors of the ten links ii to 9 having access to the calling line through the link control circuit. According to the teachings of the abovementioned Carpenter application, the control circuit tests the district junctor groups (not shown) to which the above links have access, selects the lowest numbered junctor group to which the idle links are available from the calling line and applies ground to the select magnet of a selected link, thus causing said magnet (as, for instance magnet Hi9 and the other select magnets multipled thereto) to operate.

As already stated, the operation of the control circuit was started by ground applied to conductor 348 at the time relay 33! operates. the operations above described leading to the selection of an idle district junctor and its connection to an idle link have been completed, substantially as described in the above-mentioned Carpenter application, conductor 349 is grounded by the control circuit.

When

Similarly, when tube 429 breaks down and becomes conducting a circuit path is completed which extends from positive battery through the contacts of relay 401, space current path through the tube 429 and cathode thereof, conductor 449, reactance 249, winding of hold magnet 229 to ground. After an interval determined by the inductance of reactance 249 which should be made of a value sufiicient to allow magnet 2l9 and relay H9 to operate in advance of magnet 229, magnet 229 operates. In conjunction with the operated select magnet 2i 9, hold magnet 229 causes the cross-point 2M to close its six contact pairs.

The hold magnet 102 of the calling line is 0perated over a circuit completed by relay 33| which is one of a group of six relays 33] to 336, there being one such relay in the cathode circuit of each gas-filled tube associated with each one of the six amplifiers 3 to 3E6. The front contact of each of these relays is connected to one of the six channel conductors extending across the auxiliary switch while the six vertical wires of the switch 230 are multipled to corresponding vertical conductors in all ten vertical units of the switch. The back contact of relay 33] is wired in a chain through the armatures and back contacts of the remaining five relays 332 to 336. Consequently when relay 33l operates in the amplifier 3-H of the calling group tube, it completes a circuit extending from ground on conductor 34-9 (applied thereto in the control circuit), through the winding of relay 344, right front contact of relay 33!, conductor 352, contact set No. 1 of cross-point 20!, conductor 23I, hold magnet N32 to battery. Hold magnet I02 operates and since a link select magnet is already operated, a cross-point is operated by which the line A is extended to the link controlled by said operated select magnet. The link is connected to the district junctor which, in turn, is connected to a sender for the further extension of the connection in the well-known manner.

The operation of hold magnet I02 disconnects potential from the anode in the line distributor which is connected to conductor 99. This, however, will have no effect on the thyratron 324 which remains conducting.

Relay 344 also operates in the above circuit and closes an obvious circuit for slow-release relay 345 which operates and extends a path from conductor 34'! to the back contact of relay 344. Now when the district junctor becomes connected to the link and is ready to lock in, it applies a holding ground over the sleeve conductor I03, the efi'ect of which is to hold operated hold magnet I02 and apply ground to conductor 352, thereby causing relay 344 to be short-circuited and release. Relay 345, being slow to release, will remain operated and the ground from conductor 349 will be extended over the back contact of relay 344 to the front contact of the still operated relay 345 arid thence over conductor 341 to the winding of rela 202. This relay, in operating, grounds conductor 238 which closes obvious circuits for relays 342, 406 and 431. These relays operate and disconnect positive battery from the anodes of the operated gas-filled tubes 324, M9 and 429 causing them to extinguish, the former opening the circuit of relay 33! causing it to release, and the latter opening the circuits of select and hold magnets H9 and 229, respectively. These magnets, upon releasing, cause cross-point 20| to open, while select magnet 2I9 further causes the release of select magnet I09 of the line switch assembly. Hold magnet 102, however, is held operated over the sleeve conductor in the usual manner and thereby holds line A connected to the link. The release of relay 33! opens the circuit of relay 344 which, being slow. to release, releases after an interval and opens the circuit of relay 345 which thereby releases also, thus restoring all parts of the circuit to normal.

In the event of an abandoned call, no ground will be applied to the sleeve conductor I03, and the circuit will be released by timing means in the control circuit which applies ground to conductor 349 if release is not received in the normal manner.

While the present embodiment of the invention has been described with reference to a cross-bar automatic telephone system, it is evident that it may be readily applied by anyone skilled in the art to other types of systems.

What is claimed is:

1. In a telephone system, a station line, link circuits, switching means for extending a con- .nection from said station line to any one of said link circuits, an operating magnet for said switching means, said station line extending over normal contacts of said magnet and including a source of current and a resistance, and means responsive to the potential drop through said re- .sistance upon the initiation of a call on said line .for controlling said switching means to connect .said line to a link circuit.

2. In a telephone system, station lines, link .circuits, switching means for extending connections from said lines to said link circuits having -.an operating magnet for each of said lines, each :of said lines extending over normal contacts of :its associated magnet and including a source of current and a resistance, and electronic devices reach comprising an evacuated envelope and a source of electrons for producing a focussed electron beam operative in response to the potential drop through said resistance upon the initiation of a call on one of said lines for controlling said switching means to connect said line to a link circuit.

.3. In a telephone system, station line, link circuits, a cross-bar switch having hold magnets .allocated to said station lines, said lines and link circuits terminating in the cross-point contact sets of said switch, each of said lines extending :over normal contacts of the hold magnet allocatred thereto and including a source of current and :a. resistance, and means responsive to the potential drop through said resistance upon the initiation of a call on one of said lines to operate the hold magnet allocated to said calling line whereby a set of cross-point contacts of said switch is operated to connect said line with one of said link circuits.

4. In a telephone system, station lines, link circuits, a cross-bar switch having hold magnets allocated to said station lines, said lines and link circuits terminating in the cross-point contact sets of said switch, a start circuit for said lines comprising electron devices each having an evacuated envelope and a source of electrons adapted to produce a focussed electron beam, said devices being responsive to any one of said lines when initiating a call, and means controlled by said devices for operating the hold magnet allocated to a calling line whereby a cross-point contact set of said switch is operated to connect said line with one of said link circuits.

5. A starting circuit for the lines of a telephone system, said lines being disposed in groups upon the cross-point contact sets of a cross-bar switch, comprising an electron distributor for each line group, two identifying electron distribubtors common to all groups, means responsive to a line initiating a call for operating the line group distributor and the two identifying distributors for identifying the location of said calling line upon the cross-bar switch, and means responsive to the operation of said line distributor and said identifying distributors for operating the hold magnet on said-cross-bar switch which is individual to said calling line.

6. In a telephone system, the combination with a, group of lines disposed upon the cross-points of a cross-bar switch of an electron distrubtor, comprising as many anodes as there are lines in said group and having a revolving electronic beam adapted to impinge upon each of said anodes in succession, a conductor extending between each of said lines and a corresponding anode in said distributor, means responsive to one of said lines initiating a call for altering the potential of its corresponding anode over its said associated conductor whereby upon the impingement of said electronic beam upon said anode a current impulse is produced, and means operative in response to said current impulse for closing a current path to the hold magnet controlling the closure of that cross-point on said switch which is individual to said calling line.

7. In a telephone system in combination, a group of lines disposed upon the cross-points of a cross-bar switch, link circuits available to said lines through said cross-points and means for connecting one of said lines when calling to an idle one of said link circuits, comprising an electron distributor common to said lines and having an anode for each line, two identifying electron distributors having respectively an anode for each line that marks a digital disposition of the correlated line on the cross-points of said cross-bar switch, means for synchronously impinging the electron beams of each of said distributors upon corresponding anodes, means responsive to the electron beam of said electron distributor impinging upon the anode of a calling line for producing a current impulse which is effective to produce two other impulses when the electron beams of said identifying distributors impinge upon corresponding anodes, switching means operative in response to said last two current impulses for closing a path to the hold magnet of said calling line and for providing accessibility to the select magnets of said link circuits available to said calling line, and means responsive to said first current impulse for completing the circuit of said hold magnet.

8. In a telephone system in combination, a group of lines disposed upon the cross-points of a line cross-bar switch, link circuits available to said lines through said cross-points and means for connecting one of said lines when calling to an idle one of said link circuits, comprising an electron distributor common to said lines and having an anode for each line, two identifying electron distributors having respectively an anode for each line that marks a digital disposition of the correlated line on the cross-points of said cross-bar switch, means for synchronously impinging the electron beam of each of said distributors upon corresponding anodes, means responsive to the electron beam of said electron distributor impinging upon the anode of a calling line for producing a current impulse which is effective to produce two other impulses when the electron beams of said identifying distributors impinge upon corresponding anodes, an auxiliary cross-bar switch providing 7 access to the hold magnets of said line crossbar switch through the cross-points thereof, means operative in response to said last two current impulses for operating the select and hold magnets of said auxiliary cross-bar switch to close the cross-point giving access to the hold 10

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