US2235153A - Chain start circuit - Google Patents

Description

N E D L Q H T. H w

CHAIN START CIRCUIT Filed July 1l, 1939 n q.-- Ein. -1M .I :ul

' /NVENTOR l WHT/HOLDEN BV Summum] un NS .In

A T TORNE V Patentes Mar. 1s, 1941 PATENT OFFICE CHAIN START CIRCUIT William H. T. Holden, Long Island City, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application july 11, 1939, Serial No. 283,735

i 12 Claims.

This4 invention relates to chain start circuits and more particularly to a start circuit that employs electronic devices which are rendered operative in a desired order to initiate the operation or selection of whatever appliances are controlled thereby. By an electronic device I mean an evacuated vessel containing a cathode, an anode and one or more control electrodes and filled to a predetermined pressure with some inert gas like neon, argon, helium and the like which,

' when the potential on any one of its control electrodes becomes sufficiently positive with respect to the potential of the cathode or 'current carrying electrode, breaks' down and becomes conducting to allow a ow of space current between the cathode and anode until the anode potential is rendered sufficiently negative with respect to the cathode potential. Current through the device or tube can then be restarted by making the potential of the control electrode again suciently positive with respect .to the potential of the cathode.

In the usual chain start circuit it is necessary to employ a relay that functions in response to a suitable signal to start the operation of an' available one of a group of devices and thereafter to advance the start wire to a succeeding relay which, in turn, performs a similar function. Of necessity, the utility and efiiciency of this kind of a circuit depends upon the speed of the relays themselves and the reliability of the contacts thereon, the failure of any one of which, in extending the start wire serially over the operated relays, will effectively block its further use. In a start circuit using electronic devices, starting operations are exceptionally fast because of the speed of tube breakdown and series contact chains, therefore, are dispensed with, thus avoiding failure through a break in the chain itself and thus resulting in a start circuit which is fast and reliable.

By way of illustration, the principle and operation of the invention will be described with reference to its application to the line-finder switches of a telephone system the lines of which terminate on terminal banks accessible to said switches, although it will be evident to all skilled in the electrical arts that the invention is adaptable to any electrical system involving the selection, preselection, or allotment of electromechanical devices adapted to a wide variety of uses.

In certain types of telephone systems in which selective switching devices are employed, particularly those adapted for the extension of connections in the full automatic manner, the subscribers' lines terminate at the central office in the Amultiple banks of line-finder switches, and to secure the advantages incident thereto these lines are usually divided into relatively large groups the number of finder switches reserved for preferred use by any one line group being dependent, of course, upon the size of the group and its calling rate. The groups of lines may be subdivided into smaller groups and the finder switches serving such groups are provided with l0 as many brush sets as there are subdivisions in the group. Thus, according to one well-known practice which is herein used to illustrate the operation of the invention, the lines are divided into two hundred line groups, each being subl5 divided into one hundred line subgroups that appear on separate terminal banks accessible to nder switches equipped with two sets of brushes, that is, an upper set for engaging the terminals of the one hundred lines appearing in the upper 20 bank and a lower set for engaging the terminals of the one hundred lines appearing in the lower bank. These subgroups of one hundred lines are again subdivided into smaller groups of ten lines each located on a different terminal level, and 25 the lines in corresponding levels in the two terminal banks are grouped together as a unit with respect to a group of line-finder switches to which these lines have a preferred access when initiating calls. Accordingly, the start wire cir- 30 cuit which constitutes the present invention is used, in a system of 'this kind, to start available line-finder switches in a preferred group and in other groups if none are available in the preferred, for connection to the terminals of calling 35 lines'inresponse to calls initiated therefrom.

Referring to the drawing, there is shown in schematic manner-two lines X and Y belonging, respectively, to the upper and lower groups of one hundred lines terminating on the multipled 40 terminal banks of what are known as two huny dred pointf switches, while directly below the circuit thereof -is shown the chain start circuit which is the subject of the present invention.

The invention will be more readily understood 45 from a detailed description of the operation of the circuits shown in the drawing.

Upon the removal of the receiver at substation X, the line relay |6| operates over the following circuit: battery, right winding of relay |6|, right 50 inner contacts of cut-off relay |62, line con-. ductor |36, through the substation loop at X, returning to ground over the line conductor |31 and the right outer contacts of relay |62. The line relay |6| operates and, in attracting its 55 right front contacts, operates the group relay |00 over the following path: grounded right front contacts of relay |6I, conductor 0|, right contacts of relay |02, winding of relay |00, battery. Relay |00, in attracting its upper contacts, locks itself to ground on conductor |0| and also applies ground to the segment |03 of the vertical commutator |10 which is common to the major groupof lines to which substations X and Y belong and which is traversed by the brush ||2 when the brush shaft (not shown) is stepped vertically by a circuit shortly to vbe described. In attracting its lower armature, relay |00 connects positive battery |04 over'the lower front contact of relay |00 to the charge path comprising an essential part of the start circuit.

The start circuit comprises an electronic tube for each line-finder switch, a relay connected to the anode thereof such as, for example, tube |52 and relay l 02, respectively, two series resistances ||3 and |5 for each tube the control electrode of which is connected to the mid-point thereof, and a grounded condenser |5| intermediate the second resistance ||5 of one tube and the first resistance I3 of the next tube,`the last condenser in the chain completing a loop to the bottom back contact of the first group relay 00 over conductor |51. The cathode of each tube is conected to a source of negative potential on a normal set of contacts of relay |53 of the linefinder circuit operatively associated with said tube. Since the total number of line-finder switches available to all the lines are divided into groups normally preferred by level group divisions, the tubes themselves partake of the same number of divisions as the line finders themselves in that the connection of positive battery |04 to the charging circuit takes place through the contacts of the group relay common to a level group of lines as, for instance, relay |00, or relay |56 or relay |58 when any or all of these relays operate in response to calls initiated from calling lines in the correlated groups as already described, fox` example, for line X in the level group controlling the operation of group relay |00. The object of applying positive battery |04 through the individual group relays is to insure the choice of a line-finder switch forsonnection available in the preferred group the start circuit is arranged to cause the breakdown of the tube belonging to the rst line-finder switch available in the next choice group. 'Ihese selective operations continue until an available line nder is chosen.

As can be seen from the drawing and as already explained, the start circuit is essentially a closed loop of paired series resistances (I |3| I5) with paths to ground through condensers |5| between each pair of resistances, and an impedance through a tube and a relay (|02) between each two resistances of a pair to a source of positive potential |04. If the source of positive potential connected to each of the tube relays should be applied to any point of the loop, a charging circuit would be established for all the condensers through the connecting resistance network and the time taken for each of them to reach a particular potential would depend, of course, upon its remoteness from the charging source. Whence lt follows that the condenser nearest the charging source would be charged to that particular potential before all others. Now since the control electrode of each tube is connected to the loop ahead of each condenser and is separated from it .by one of the paired resistances ||5, it further follows that the electrode of the tubes immediately preceding the condenser nearest the charging source will reach a positive potential value higher than that of the condenser itself by the value of the potential drop in the intervening resistance ||5 earlier than the condenser. As the potential charge is propagated throughout the loop, the electrodes of all tubes connected thereto will be charged, in time, to the same potential, but the time at which they become lso charged will be different for each tube, that of the tube most remote, electrically, from the charging source being the last to reach this value. Now if the charge to which the tube control electrodes are subjected will reach breakdown potential at different times it is obvious that the tube nearest the charging source will reach this potential value ahead of all others, and when it breaks down and thereby establishes a discharge path therethrough, the electrodes of all other tubes will be raised to no higher potential than that reached by them when the first tube breaks down, because of the direct current path for the anode battery now established through the conducting tube. Since the control electrodes of the other tubes now cannot be charged to the requisite breakdown potential, said tubes remain inoperative.

Referring, now, to the drawing for an understanding of the manner in which a charge of the loop start circuit permits the selection of an available line finder, when positive battery |04 is applied to the bottom front contacts of relay |00, the charging potential is propagated along the loop causing all the condensers |5| to begin to charge with condenser |5| ,to the right of tube |52 beginning to charge first and condenser |5| to the right of the last tube |65 beginning to charge last.. When the first condenser is partially charged, the control electrode of tube |52 is made positive and this tube breaks down between its cathode, which is connected to ground on the back contacts of relay |53 over conductor and its anode which is connected to positive battery |04 via the winding of rel-ay |02. Space current flows through the tube |52 and relay |02 operates to perform the function of starting the line-finder switch controlled thereby to engage the terminals of the calling line, as explained hereinafter. For the purpose of further explaining at 'this time the operation of the chain start circuit, it will be asumed that when the linender switch has engaged the termin-als of the calling line X, relay |53 will operate, relay |00 will release and, due to the disruption of the cathodeanode circuit at the back contacts of relay |53, tube |52 will be reset. Under these circumstances, relayl |02 will release, thereby reestablishing the normal unoperated condition of the chain start circuit except for the fact that the cathode conductor |1| of tube |52, lbeing disconnected from ground by the operation of relay |53, renders said device inoperative and thereby protects the line-finder switch taken into use for service with line X from reseizure by any other calling line in the group of line X or any the same level group as line X, sayline Y, initiates a call, resulting in the operation of the line relay |63 and the common group start relay |00 over the common conductor |0'| and the back contacts of relay I 02, then the connection of positive battery |04 over the bottom front contacts of-relay |00 will not be effective again to operate tube |52 because cathode potential has been removed therefrom by the operation of re-` lay |53, as before mentioned. Hencelwhile the connection of positive battery |04 .to the bottom front contact of relay |00 will again establish a charging circuit .to all condensers |5|| of the start circuit through the paired series resistances I=|3fl=|5 ||3-|l|5, it follows that lthe -tube which this time will reach breakdown potential rst is the one next to tube |5`I since, inthis case, this tube is nearest to the charging source |04. -For completeness of disclosure Ithe drawing has been made so that the first and last tube of Aeach group of switches is shown. fIh-at is, tube |52 would be the i'lrst and tube |54 Iwould be the last of the group off switches normally serving as rst preference to the level group oflines indicated by lines X and Y. However, if tube |54 'were the one immediately succeeding tube I 52,

then it would be the one that would be rendered conducting upon the initiation of a call by line Y following the resetting of device |52, whereupon its operation would be followed by the operation of the line-finder swit-ch controlled` thereby to engage the terminals of line Y. In this manner it is seen'that, as the lines in a level group exhaust the line-finder switches normally available to them, they will, thereafter, have access -to the succeeding group 0f preferred line finders due to the fact that the tubes of the busy line nders of their own group will have been rendered non-conducting by the operation of the relay A|'53 in .the respective line nders. This process of selection will continue down through all groups until the last line-finder switch is taken into use.

|Illf simultaneous calls originate among lines in diierent level groups, line-finder switches in each of the preferred available groups will be started simultaneously. .'Thus iiV three calls originate simultaneously, one in the level group designated by group relay |00, another in the level group designated by group relay |56, and another in the level group designated by group relay |58, then these three relays will operate in the man- 'ner already described for the operation of relay |00. Positive battery |04 will then establish three charging circuits: one through'the bottom front contact of relay |58, the resistance combinations |i|3||5 to parallel grounds through condensers |5| on either side of the tubes 0f the grou-p including Ithose designated |59 at one end and |65 on the other and resulting in the breakdown of the tube first charged to breakdown potential which, from what has already been said, may be assumed to be tube |59 if not rendered non-conducting by the busy condition of its associated line nder; another charge path -will be completed through the bottom front contact of relay |56, resistance combinations I'|3-||5 .to parallel grounds through thecondensers I5| on either side of the tubes in the group that inwhich will be tube |55 if not rendered non-conducting by a busy condition of its associated line nnder; while athird charging path will be completed through the bottom front contact of relay |00, the series resistance combinations ||3| I5 to parallel grounds on either si-de of the tubes in the group including tube |52 at one end and tube |54 ,at the other. Tube |52 will, of course, break down first if it is not rendered inoperative by the busy condition of its associated line-finder switch. 'The Ithree tubes thus rendered conducting will operate their corresponding anode relays |02 and cause the operation of their respective associated line-finder switches in the manner now to be described for the switch disclosed in the drawing assumed to be associated with tube |52 which, from the previous description, was rendered conducting by the initiation of a call by the line X.'

The operation of the relay |02, due to the conducting state of tube |52 and by the opening of its right contacts, temporarily removes relay |00 from control of other subscribers to prevent interference, while'bythe closure of its left contacts it initiates the vertical movement of the associated line finder, The circuit for initiating this Vertical movement is as follows: ground, top outer contacts of relay |00, lef-t'contacts of relay |02, -conductor |34, armature |05 of relay |53 and its back contact, conductor |06, bottom inner back contact of relay |16, conductors |01 and |06, interrupter contacts of thevertic-al magnet |09, interrupter contacts of rotary magnet ||0, winding of relay |61 to battery. Rel-ay |61 operates, and in attracting its top set of contacts, connects this ground 'to the winding of slowrelease relay |168 and vertical magnet |09 in series, by VJway of the top normally made contacts of relay |16, whereupon the vertical magnet op-4 erates and steps the finder. brush shaft (not shown) one step in its vertical movement. Re-A lay |61, upon energizing, funther locked itself over the following circuit: battery through the winding of said relay, interrupter contacts of magnets Iii 0 and |09, respectively, conductors |08 and II'I, bottom inner contacts of relay |61 to ground. -The vertical magnet |09, at the end of its stroke, Opens the locking circuit of relay |61 as Well as t'ne original energizing circuit of said relay, whereupon Ithe relay |61 releases and opens the circuit of the vertical magnet |09. The vertical magnet thereupon releases, and in closing its initerru'pter contacts, again energizes relay |61 over its original energizing circuit, whereupon said relay locks up over its locking circuit, previously described, and recloses the circuit of vertical magnet |09 causing it t-o operate and advance the brush shaft another step. This interaction between relay I 61 and .the vertical magnet |09 continues until the grou-p brush H2 of the finder reaches the grounded segment |03 of :the calling group, whereupon relay |16 operates over the following circuit: battery through winding of relay |161, interrupter contacts of rotary magnet H0, conductor |15, upper winding of relay |16, conductor 4, brush ||2 and segment |03..to ground on the top inner contacts of relay |00. Relay |16 operates and at its top normally closed contact opens the vertical stepping circuit. At the moment that the grou-p brush H2 engages grounded segmentl03, relay |61 is s'till locked up .to its own bottom inner cont-acts since this locking circuitis opened only at the extreme limit of :the stroke of the vertical magnet |09. When this occurs, however, relay |61 does not release since it has another circuit including the upper winding of relay |16 to ground on segment |03. When slow relay |68 releases, which occurs shortly after relay |16 operates, the following circuit is closed to operate t-he rotary magnet ||0 to rotate the brushes over the calling line group: battery through the Winding of magnet ||0, back contacts .of relay |68, top front contacts of relay |16, conductor ||6, top contacts of relay 61, interru'pt contacts of magnets ||0 and |09, respectively, conducto-rs |08 and to ground at the bottom inner front contacts of relay |61. Magnet I l0 operates, rotates the brushes one step and -a-t the end of its stroke opens the locking circuit of the relay |61 and also moves the group brush ||21 off the grounded group contact |03. Now if the first two sets of terminals encountered by the brushes are termin-als of non-calling lines, the brush shifting relay |69, which was operated when relay |16 operated at the end of the vertical movement over a cir-cuit including the bottom outer contacts of relays |61 and |16, remains operated over the following circuit: battery through the winding of relay |69, its armature ||1 and front contact, conductor ||8, normally made contacts ||9 of relay |53, conduct-ors |20 and |2|, brush |22, test multiple |23, conductor |24, left back contacts of a, cut-off relay corresponding to cut-off relay |64 of line Y to ground at the right back contacts of a line relay correspon-ding to line relay |63 of line Y. Therefore, the following circuit is completed to operate relay |61 to transmit another impulse to the rotary magnet ||0 to ad- Vance the nder from t-he terminals of these two lines which are non-calling, to the next two lines: battery through the winding of relay |61, interrupter contacts of magnets ||0 and |09, respectively, conductors |08, and |3|, armature |26 of relay |53, armature |25 and front contact of relay |69, test brush |30, test terminal |28, conductor |29, back contacts of a cut-off relay corresponding to cut-off relay |62 of line X, right back contacts of a line relay corresponding to line relay |6| off line X .to ground. Relay |61, in operating, again locks up over the cir-cuit heretofore traced, including its bottom inner armature and front contact, and again operates the rotary magnet ||0 over the following circuit: ground on the bottom inner contacts of relay |61, conductors and |08, interrupter contacts of the magnets |09, ||0, respectively, top contacts of relay |61, conductor ||'6, .top front contacts of relay |16, back contacts of relay |68, winding of magnet 0 ,to battery. The rotary magnet operates to move the brushes one step and at the end of its stroke opens the locking 'circuit trlaced. for the relay |61 which, in opening itstop contacts. opens the circuit of the rotary" magnet. This interaction between relay |61 and rotary magnet ||0 continues until one of the sets of brushes encounters a calling line, which is indicated by the absence of groundat its test terminal. It having been assumed that the calling line is substation X, no ground is present on its terminal |28 but ground is present on test terminal |23 of the non-calling substation Y. Consequently, relay |69 is held up from the ground on test multiple |23. There being no ground, however, at test terminal |28, relay 61 cannot 8 again energize to cause the reoperation of the rotary magnet. The switch, therefore, comes to rest on the calling line multiple. During the movement of the finder over the non-calling line terminals, relay |53 is not operated since its .2,235,153 winding is short-circuited over the following path: ground on conductor |3| usal-ready traced,- to one lside of its Winding and ground from the lower set o'f contacts on off-normal contact springs |33, now closed, conductor |32 to the other side of its winding. When, however, brush |30 reaches test terminal |28 of the substation X which is calling and therefore is not grounded, relay |53 energiaes over the following circuit: battery through the winding of relay |61, interrupter contacts of magnets ||0 and |09, respectively, con-ductors |08 and winding of relay |53, conductor |32 to ground on the lower contact of off-normal switch |33. Relay |53 operates and in attracting its lowermostarm-ature, maintains relay |69 energized from the ground on the off-normal switch |33. Rel-ay |69, therefore, will remain energized until release takes place. In attracting its armature |12, relay |53 removes ground from the cathode'of tube |52, causing thereby a disruption of the space current therethrough and the release of relay |02. In attracting its armature |26, relay |53 applies a direct ground from the olf-normal switch |33 to the cut-'off relay |62 of the calling line X to operate the same. This circuit is as follows: battery through the winding of relay |62, left winding of relay |6| and its left contacts, conductor |29, terminal |28, brush |30, armature |25 and front contact of relay |69, armature, |26 and front contact of relay |53, conductor |32 to ground at the off-normal switch |33. FIhe cutoff relay |62, in operating, deenergizes the -line relay |'6|. Ground is applied tothe test multiple |29 of substation X in the other nders in which it appears, to render the same non-calling, this ground being supplied over the following oath: oli-normal switch |33, conductor |32, armature |26 of relay |53 and its front contact, armature |25 and front contact of relay |69, brush |30 to multi-ples |28. Relay |6|, upon releasing, a-ls-o releases the group relay |00 which, in turn, removes positive battery |04 from the start charging circuit .to restore the starting circuit to the control of the other lines. The cut;

off relay |62 is now locked over its' left front contacts to ground on the'o-n'ormal switch |33 over a previously described path.

With the operation of relay |53 the line conductors aire extended to the rst selector S, the tip conductor extending from one side of the station X, conductor |31, Iterminal |68 and brush |49, tcp front contacts of relay |69, amature and front contact of relay 53, top outer contacts of release relay 11 to conductor |13 of selector S. The ring conductor extends from the other side of station X, conductor |36, termin-a1 |41 and brush4 |46, bottom innermost front contacts of relay |69, armature |38 and front con- |11 to conductor |13 of selector S. As is well known, these conductors extend to aline relay in the selector S and to a source of current thereat which will cause the relay to operate over the line loop in readiness to receive dial impulses. In response to these'rpulses, the selector S is then positioned on the termina-ls of anv appropriate connector VC by one series of dial impulses, the connector itself being then positioned on the terminals of the called station Z by the next two series of dial impulses transmitted from the dial at station X, al1 in accordance with well-known automatic telephone practice. Since this part of the operation as well as the release of the con- Itact of relay |53, inner contacts of release relay nection forms no part of this invention, further description thereof is omitted.

Assume, now, that the call has been initiated by substation Y and that substation Xis noncalling, In this event, ground would be present on the multiple |28 of substation X and the multiple |23 of substation Y would be ungrounded due to the attraction of the right armature of the line relay |63 of substation Y. The operations would be the same as just described up to the time the test brushes |30 and |22 reach the terminals |26 and |23, respectively. No ground being present on the test terminal |23 of line Y, relay |69 would release when relay |61 released due to the opening of its locking circuit by the rotary magnet H0. Relay |69, in releasing, connects the trunk talking conductors to the brushes |39, |40 and consequently to the talking conductors |4|, |42 of substation Y, disconnecting such trunk from lthe brushes |46 and |49. Although in the case assumed ground is present on the test multiple |28 and the armature |25 of the relay |69 is attracted when the brush |30 reaches such contact, the relay |69 releases its armature |25 before this ground is applied long enough to permit the relay |61 to again attract its armature and for the rotary magnet |||l to energize suiciently to actuate its pawl to give thender another step. The relay |53 operates in this case exactly as it did in the other; that is to say,

due to the removal of the shunt about its winding, caused by the brush |22 engaging the open terminal |23. In attracting its armature |26, relay |53 operates the cut-olf relay |64 of substation Y.l

While I have described my invention and the.

method and means utilized for practicing the same in connection with its specific application to a line-lnder start circuit, it is to be understood that various other applications and embodiments thereof may be made by those skilled in the art without departing from the spirit of the invention as dened within the scope of the appended claims.

What is claimed is:

1. The method of operating an electronic device out of a plurality of such devices all adapted to operate upon the application of a definite potential thereto, which consists in propagating said definite potential along a circuit network joining all said devices, whereby the operation of one device precludes the further propagation of said deiinite potential.

2. A circuit for operating an electronic device out of a plurality of such devices, all adapted to operate upon the application of a definite potential thereto comprising a circuit network joining all of said devices, a source of said denite potential, and means for applying said source to any point of said circuit network for thepropagation thereover of said definite potential whereby the operation of one of said devices establishes a conducting path through said device and through said network which precludes the further'propagation of said denite potential over said circuit network.

3. A starting circuit for the selection of one of a plurality of work appliances comprising a device for each appliance adapted to become electrically conducting upon the application of a denite potential thereto, means responsive to the conductivity of a device for operating its associated work appliance, and means for applying said definite potential to a preferred one of said devices.

4. A starting circuit for the selection' of one of a plurality of work appliances comprising a device for each appliance adapted to become electrically conducting upon the application of a definite potential thereto, means responsive to 5 the conductivity of a device for operating its associated work appliance, and means for applying said denite potential progressively to all of said devices. whereby the conductivity of one device arrests all other devices from attaining said definite potential.

5. A starting circuit for the selection of one of a plurality of work appliances comprising an electronic device for each appliance adapted to become electrically conducting when a definite potential is applied thereto, means responsive to the conductivity of one of said electronic devices for operating its associated work appliance, and means for applying said definite potential progressively to all of said electronic devices whereby the conductivity of one electronic device arrests all other` devices from attaining said denite potential.

6. A start wire circuit for the selection of work appliances in each of a plurality of groups of such appliances comprising an electronic device for ea-ch appliance adapted to become'electrically conducting upon the application of a definite potential thereto, a source of potential common to all of said electronic devices, means responsive to the conductivity of one of said electronicdevices for operating its associated work appliance, and means individual to each group of electronic devices for connecting said source of potential progressively to all electronic devices whereby the electronic device nearest to said means reaches said definite potential first and whereby the resulting conductivity of said electronic device arrests all other devices from attaining said deni-te potential.

7. A start wire circuit for the selection of work -appliances in each group of a plurality of groups of such appliances comprising an electronic device for each appliance adapted to become electrically conducting upon the application of a denite potential thereto, a source of potential common to all of said devices, means responsive lit to the conductivity of one of said devices for operating its associated work appliance, means individual -to each -group of electronic devices for connecting said source of potential progressively to all of saidelectronic devices, said connection being effective through said means to apply said potential first to the electronic device nearest -to said Imeansl whereby said device reaches said denite potential before lall other devices.

8. A start wire circuit for the selection of one of a plurality of work appliances comprising a device for each appliance adapted to become electrically conducting when a denite potential is applied thereto, means responsive to .the conductivity of one of said devices for operating its Iassociated work appliance, an impedance network connecting 'all of said devices comprising a resistance elemen-t on 'each side of each of said devices, a parallel condenser connected to ground on one side of each of said resistances, a source of potential, land means for applying said source of potential to any point of said impedance network whereby the device nearest the point of -application of said potential' is .charged to said denite potential first and whereby the conduc` tivity of said device prevents all other devices from being charged :to said denite potential,

9. In a rtelephone system, a line, a line-linder 75 ilnder switch' for initiating the operation thereof, and means responsive to a cali originated from said line for 'operating said electronic means.

' of lsaid automatic switches, means responsive to said operated electronic devicel for causing its as-- sociated automatic switch to hunt over said .terminals in search of the terminals of Said calling line, and means responsive to the connection of said switch with the terminals of said line for rendering said electronic means inoperative.V

11. In a telephone system, subscribers lines arranged in a plurality of groups, automatic switches arranged in a plurality of groups for serving all of said lines and arranged to be taken for use in a given order, each group of switches arranged to serve as rst choice a diiierent one of said groups of lines, a st-arting circuit for all 4oi' said switches comprising a gaseous conductor 2,235,153 switch, electronic means individual .to said linedevice for each switch, means responsive to a callin any group of lines for operating the device of a free switch in the group normally serving such a group of lines, and means responsive to the operation of said device for initiating the operation of its associated switch.

12. In a telephone system, subscribers lines ar ranged in a plurality of groups, automatic switches arranged in a plurality of groups for serving all o1' said lines and arranged .to Iloe taken for use in a given order, each group oi' switches arranged to serve as iirst choice a different one of said groups of lines, a starting circuit for ally of said switches comprising a gaseous conductor device for each switch, means responsive to a call in any group of lines for operating the device of a free switch in the group normally serving such group of lines and .to operate the device of a free switch in the next preferred group should all switches in the first group be unavaillable, and means responsive to the operation of said device for initiating the operation of its as-r sociated switch.

WILLIAM H. T. HOLDEN.

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