US1922237A - Automatic telephone system - Google Patents

Description

Aug. 15, 1933. A TT- 1,922,237

AUTOMATI C TELEPHONE SYSTEM ori ihal Filed July 13, 1931 2 Sheets-Sheet 1 IH'U'ETIEF Hlexandat Jul-m EEEEH Aug. 15, 1933. A, J, ESCOTT AUTOMATIC TELEPHONE SYSTEM Original Filed July 13, 1931 2 Sheets-Sheet 2 IHUEIL Dr Hlexande'r Jul-m BEE 5:11

Patented Aug.15. 1933 7 I 1,922,237

UNITED STATES PATENT OFFICE Alexander John Escott, London, England, assigner to Siemens B1'otl1ers & Company Limited, London, England Application July [13. 1931', Serial Nb. 550,323, 7 e and in Great Britain August 7, 1930.. Renewed October 17, 1932 i 20 Claims. (cine-1a The present invention relates to automatic This may be done as followsjBoth Switches step telephone systems, and'more particularly to the on receipt of the first impulse of the switch setselecting switches for use in such systems. ting train, one being stepped to the commence- The selecting switches employed in automatic ment of the first group of outlets, the other to telephone systems are usually arranged to step the'commencement of the second group of out- 60 in response to a train of impulses consisting of ts. The succeeding impulses drive the switches from one to ten impulses, to an initial position alternately, that resting at the commencement at one end of a. group, of outlets, and then to of the first group being the first to drive, the

hunt automatically fora freeoutlet in the group. switch last driven preparing a circuit for the Where the selecting switch is of thetype which driving of the other switch. At the end of the 65 has motion in one plane only, usually a rotary switch setting impulse train the switch which motion, and in which all the wipers on the rests at the commencement of the earlier group switch shaft partake in the hunting motion, it of contacts hunts over that group for a free out is sometimes arranged that the impulses take let. V

l5 ei fect on a marking switch which marks in the In order to reduce the hunting time and yet 70 banks of the selecting switch the group of outto have-a large number of. outlets we may conlets to be searched, the selecting switch being nect the outlets of a group to a few consecutive stepped automatically to the commencement of contacts only and provide the switches with'more the marked group for a free outlet. The markthan one set of wipers andfcontact banks and ing; switch is thus used temporarily until a selectarrange for simultaneous. search over. all the 7 ing switch has found a freeoutlet and may then wiper sets of a switch.

be released, and arrangementshave been pro- Reference will now behadto the accompanyvided whereby a marking switch may be availing drawings. Fig. I ShOWSB/ PaiIflOf swit'ches able in common to a number of selecting switches. embodying the inventionand suitable for use in In the present invention two. single-motion a register controller system. Fig. 2 shows answitches are combined toiorm a singleselector, other embodiment whichmay beused in a direct outlets being provided in the banks of each impulsing system.

switch, and one switch may. be used as a mark- The arrangements shown in Fig. .1 will first ing switch for the other. i be considered;

30 It may be arranged that outlets of a group are The selector comprises two 'slng le-niotion provided in both switchesand a set of outlets switches each having seven wipersfian'd correof a group in one switch is marked by the other spending contact banks of 51 contacts each. switch in response to. an impulse train. and if The switch wipers are designated Xl-Q-X'lfor one these outlets are all busy the switch which apswitch which will be referred to as switch xjand plied the first marking has. a marking applied Yl Y'l for theother switchwhichwill be re 90 to it and it thereupon steps to the marked posiferred' to as switch Y. Outlets to a further tion and commences hunting for a free outlet switching stage orother circuits are divided into in a further set of outlets in the same group. 7 groups/some oi the outlets of agroup being con- This arrangement, whilst suitable for use with nected to switch X and others to switch Y. In

4.0 a system in which the selectors are set under the example taken it is assumed that there are .95

the control of a register controller in which the ten groups of eight outlets each','the first four pause between successive impulse trains can be outlets of a group being connected to switch Y controlled so as to permitof the complete searchand the last four outlets of the group being coning of a group, may not be convenient in a direct nected to switch X" Further, for the purpose impulsing system except for small groups of of equalizing the searching time for all the 190 outlets such as lines to a small private branch groups. the groups are connected to switch X in exchange, owing to the time requiredto step the the reverse order to their connection to switch Y. switches to the'marked positions and to hunt Theselector is of the type which tests free by over a complete group of outlets. reason of battery potential on the test wire, .the In a modification suitable foruse with a direct free condition of an outlet also being battery .l05

impulsing system, it maybe arranged that the .potential on the test wire. The impulse relay two switches comprising a selector act in turn as is designated A, the release relay B, and the dialmarkers' at each impulse of a train, oneswitch ling relay C. HX is the test relay for switch X being marked by an odd number of impulses and and JHY for switch Y. R. is a relay which conthe other switch by an even numberof impulses. nects up test relay HY when the wipers of switch .111

Y have reached a group marked by impulses in which search is to be made and relay P is operated when switch Y has failed to find a free outlet to mark the remainder of the group to be searched for by switch X. The driving magnet of switch X is designated XDM and that of switch Y, YDM.

When not in use the selector rests in a normal position with its wipers on the first contacts in their banks as shown in the drawings. For convenience in drawing only parts of the banks have been shown, the remainder of the banks being connected in a manner similar to the parts shown.

The selector tests free by reason of a battery connection over winding (I) of relay C, wipers Y5 and X5 to wire c and when seized from a previous switching stage relay C operates and at contact 02 opens a point in the circuit of magnet YDM to prevent switch Y being stepped by a false impulse. Contact 04 disconnects battery from wire (1. Relay A is now operated over wire I) from the register controller and at contact a2 connects relay B to the test wire 0, and at contact a1 opens a point in the impulsing circuit for magnet XDM. Relay B operates and locks to the test wire over its contact b1. Contact b2 pre pares a circuit for magnet YDM, contact 123 prepares a circuit for magnet XDM and contact b4. prepares a circuit for marking switch Y, this circuit being open at this time at contact 03.

Relay A responds to impulses and repeats them over contacts a1 and 01, low resistance winding (II) of relay C, and contact 123 to magnet XDM. Magnet XDM steps switch X in accordance with the impulses received and'on leaving its normal position wiper X5 opens the circuit of winding (I) of relay C. At the end of the impulse train relay C releases and connects earth over contacts b4 and c3, and wiper X2 and the contact to which it has been stepped to mark a group of contacts in the bank of wiper Yd. The groups of contacts correspond to the outlets from switch Y, and, as regards the first group, they are connected to contacts 2'5. The second group is connected to contacts 7l0, each succeeding group being connected to four contacts, the first contacts of each group being five contacts apart. Contact 02 closing, acircuit is closed for magnet YDM and this magnet steps switch Y in search of the marked group, the magnet interrupting its own circuit at contact ydm. When .the

marked group isreached relay R operates over wiper Y4, the group marking contacts in its bank and the bank of wiper X2, contacts 03, D4 to earth. Contactrl connects test relay HY to test wiper Y3 and contact 1-2 prepares a holding circuit for relay R should no free outlet be found and switch Y is stepped, to contacts 6, ll, 16succeeding groups "of outlets. Switch Y continues to step over the contacts of the marked group in search of a free outlet connected to its wipers Y6, Y7, Y3. If a free outlet is found relay HY operates over wiper Y3 to battery on the test bank contact and opens the magnet circuit at contact hy i, stopping .further driving. Contact hyl short circuits the high resistance winding of relay HY to mark the outlet engaged and contacts hyZ and 71.113 connect the speaking wires a and 1) through over Wipers Y6 and Y'I. Relay A releases and battery is connected from wire a at contact hy2.

Should all the outlets of the group connected to switch Y be engaged the switch is stepped on to one of the contacts 6, 11which in the bank of wiper Y3 are commoned and connected to relay P. Relay P operates in series with relay HY.

The operation of relay HY opens the direct connection of battery over contact 04 to the a wire and of relay A to the 1) wire but alternative circuits are closed over one of the contacts 6, ll, 16in the banks of Y6 and Y7 for these battery connections. The battery connection to wire a transmits a signal to the register controller which holds up further impulse transmission until the battery connection is removed.

When relay P operates contact p2 connects earth over wiper Y2, one of the bank contacts 6, l1,--to a set of group marking contacts in the bank of wiper X4. Contact pl closes a circuit from earth over self-interrupting contact :cdm of magnet XDM, contacts had and 123 for magnet XDM. Switch X is thereupon stepped until the marked group is reached after which it continues to step over the outlets of the group in search of a free outlet. When such an outlet is encountered relay HX operates in a circuit from earth over contact p2, wiper Y2, one of the contacts 6, 11, 16marked group contacts in the bank of wiper x4, relay HX, test wiper X3 to battery on the test bank contact. Contact have opens, stopping further driving of the switch and contact hat]. short circuits the high resistance winding of relay HX to mark the outlet engaged. The speaking wires are switched through at contacts 71x2 and M23 to wipers X6 and X7 and relay A is released and battery is disconnected from wire a, signalling to the register controller that impulsing may be resumed. Relays P and R remain operated.

If all the outlets of the group connected to switch X are busy the switch is brought to rest on contacts next after those connected to the last outlet of the group by the operation of relay X to a battery connection to contacts 6, ll, 16 in the bank of wiper X3. The corresponding contacts in the banks of wipers X6 and X? are not connected and the circuits over wires a and b are opened, thereby effecting release of the register controller and switches subsequent to the first selector which remains held over the calling loop and from which a busy signal is given.

The release ofthe preceding switches, whether due to the above or to release of the first selector by the calling subscriber clearing disconnects earth from wire 0 and releases relay B. Contact b3 falling back closes a homing circuit over wiper X1 for switch X and the switch is returned to its normal position. Switch Y performs only a hunting function, and, therefore, need not be returned to a normal position, but it may be so restored if required by connecting magnet YDM over its self-interrupting contact and a back contact 01' 192 to a homing wiper Y1 and earthed segment as shown in the drawings. If, however, switch Y has no normal position, winding (I) of relay C will be connected direct to wiper X5 and no advantage is gained by transposing the connection between the banks of wipers Y2 and X4.

Fig. 2 shows an arrangement suitable for use in a direct impulsing system, and, for the sake of example, a pair of single-motion switches having banks of twenty-five contacts each are used as a selector. Each switch has eight wipers and corresponding contact banks and outlets to a further switching stage or other circuits are grouped, a group of outlets being connected to the banks of one switch, part of the group being in one bank and part in another, simultaneous search over the outlets in the two banks being provided for. As regards the groups themselves alternate groups are connected; different switches, thus, the odd groups are connected to switch Xandthe even groups to switch Y, diii'erentiation between the groups being pro- 'vided by a pair of relays P and Q. successive impulsesof a train being repeated to alternate switches.

The selector tests free by reason of battery potential on the test Wire and: whenseizedconnects earth to. the test, wire to mark it engaged and to hold preceding selectors during the setting operation, the holding earth being returned from a succeeding switch on its seizure When the selector is picked up relay C operates by means of its winding. (I) over the test wire in series with the test relay of the preceding selector which switches through and operates relay A. Contact cl prepares the impulsing circuit for the switch, magnets, contact 02 opens a homing circuit for the switches and contacts c3 and 04 open the; circuits of the test relays HA and HB during-group selection. Relay A operating, contact a1 closes the circuit of relay B, which opcrates, and at contact b1 connects earth to the incoming test wire to hold preceding switches and to mark the selector engaged. Contact b2 opens apointin the homing circuit of switch X, contact b3 further prepares the impulsing circuit for the switch magnets, contact b4 prepares the circuit of relay P, and contact b5 that of relay Q, contacts b6 and b7. close points in the circuits of the test relays. Impulses for the setting of the selector are now transmitted to relay A which responds. The first impulse is repeated at contact al to relay P which operates by means of its. winding (I) and locks up over its contact 122 independently of relay Q. Contact all) p1 connects winding (1) of relay Q to the impulse repeating circuit but the Winding is short tact ydm, magnet YDM to battery; a parallel circuit is closed for magnet XDM, the magnets XDM and YDM operate, and step the wipers of both switches to the second bank contacts. The

, off-normal contacts :cnl, $122, 11111, and 11112 are operated, contacts rnl and ynl opening the circuit of winding (I) of relay C to leave that relay dependent for continued energization on impulsing. At the. end of the impulse, relay A releases and the short circuit is removed from winding (I) of relayQ. Relay Q operates over this winding in series with winding (I) of relay P and at contact ql opens a point in the energizing circuit of relay P. Contact q3 opens a further point in this circuit and in conjunction with contact a2 prepares a circuit ,for winding (II) of relay Q. Thus, atthe end of the first impulse, relays P and Q are operated and switch X is connected up with itswipers restingon its second contacts which are the contacts associated with the firstoutlet of the first group; switch Y is resting with its wipers on its second contacts which are associated with the first outlet of the second group, but the. wipers are not connected. In the example taken it will be assumed that the outlets of the first group are connected to contacts 2 6, those of the third group to contacts 8-12, those of the fifth group to contacts 1418, and those of the seventh group to contacts 20 -23, all in the banks of switch X. The outlets of the even groups are connected to similarly numbered contacts in the bank of switch Y; I I I leases due to the commencement of a second impulse a circuit is closed from earth over contact b3, winding (II) of relay C, contacts 01, p3,

wiper Y2 and second. bank contacts, commoned contacts 2-7 in the bank of wiper X1, contacts xdm, magnet XDM to battery. The magnet operates and by means of its contact acdm, steps the wipers of switch X to contacts 8. Switch Y is now connected up with its wipers resting on contacts 2 which are associated with the first outlet of thesecond group and switch X is resting with its contacts resting on contacts 8 associated with the first outlet of the third group but its speaking and test wire wipers are disconnected at contacts p4, p5, p6, p7, g5, and qfi, which contacts connect the speaking and test wires wipers Y3--Y8. I

If a third impulse is transmitted relay P is operated and a circuit is closed over the front of contacts 113, wiper X2 and its eighth bank contact, commoned contact 2-47 in the bank of wiper Y1 for magnet YDM. The switch Y is stepped to, its eighth contacts and its wipers Y2'Y8 are disconnected at contacts of relays P and Q which connect in circuit wipers X2--X3 which are now resting on the first outlets of the 7 third group. Further impulses effect the alternate stepping of switches X and Y in a similar manner.

At the end of the impulse train relay (3 releases and search over the outlets of the required group takes place. For the sake of example it will be assumed that an odd number of im-- pulses have been received and in consequence relays P and Q are operated and locked up over their windings (I) connecting switch X in circuit. On the release of relay C the test relays HA and HB are connected to earth over contact bland the impulsing circuit for the switch magnets is opened at contact cl. A stepping circuit is closed, however, at contact 02 for the magnet of theswitch which is to search, in the example switch X. Magnet XDM, therefore, energizes in a circuit over contacts :cdm, 3on2, g4, 02, hafi, 11b5, toearth and the wipers are stepped over con tacts connected to outlets of an odd group,

test for a free outlet being made simultaneously by relays HA and HB. Wipers X3, X4, and X5 give access to one set of outlets of a group and wipers X6, X7, and X8 to another set, and when a free outlet in one of the sets is encountered, the

corresponding test relay operates. Assuming for a moment that a free outlet is encountered in the first-mentioned set, relay HA operates in a circuit from earth over contacts 121, ha2, b6, 03, winding (I) of relay HA, contact 106, wiper X5,

and bank contact'to battery connected to the extend the speaking wires and disconnect relay A, and contact ha5 opens the magnet circuit to prevent further stepping, contact ha6 opens to prevent release of relay Q when relay A releases and contact ha? closes a holding circuit for relays P and Q against the release of relay B which takes place shortly after the release of relay A. If the first free outlet is found in the bank of wipers X6-.-X8, relay HB operates and similar operations ensue. If, however, a free outlet is found simultaneously in both sets both test relays operate but relay HA on operation closes a hold ing circuit for itself at contact hal and opens the locking circuit of relay HB so thatthe latter is not held but is released by the subsequent opening of contact 127.

Should all the outlets in the group be engage the wipers are brought to rest on busy contacts immediately succeeding those connected to outlets. These contacts are numbered 7, 13, 19, and 2%. Relay HA operates over these contacts and relay S, which is connected to them in the test banks and relay HA, locks up. Relay A is held in a circuit from battery over its right-hand winding, busy contact in the bank of wiper X4, contacts :05, hal, the calling loop, contacts .7013, p4, wiper X3, and busy contacts, left-hand winding of the relay, front contact 81, contacts controlled by a busy signal cam device to earth and the subscriber receives a busy signal.

Precisely similar operations take place if switch Y is used instead of switch X.

As only twenty-five contacts are provided in the banks of each switch the number of groups would usually not exceed eight, except in cases where some of the groups are very small. It will be assumed then that eight groups only are provided. At the commencement of the eighth impulse relay P is released, switch X is stepped from its twentieth contact to the twenty-fifth, while switch Y remains on contacts 20. If a ninth impulse is dialled relay P is reoperated and a circuit is closed over the twenty-fifth bank contact and wiper twentieth bank contact and wiper Y1 for magnet YDM and switch Y is stepped to its twenty-fifth contact. The test relay HA operates immediately over a battery connection shown and a circuit is closed over the secondary of the transformer N U T and twenty-fiith contact in the banks of wiper X3 to the a wire and the subscriber receives a distinctive tone informing him that he has dialled a non-existent number.

Similarly, if is dialled, relay P will release at the commencement of the last impulse but there will be no circuit for magnet XDM and the subscriber will receive the tone as described above.

The selector is released by disconnection of earth from the test wire in a succeeding'circuit. The test relay releases and opens the speaking and test wire circuits, releases relays P and Q if 0 erated, and prepares the operating circuit of relay P. A homing circuit for the switch magnets is closed as followsz-earth over contacts hb5, ha; 02, then over contacts 122, amZ, to magnet XDM and over contacts q l yn2 to YDM and switch.

2. A switching device comprising two automatic switches, groups of outlets accessible to each switch, means for causing either switch to hunt for and connect with an idle outlet in a marked group of outlets, and means in each of said switches for marking outlet groups in the other of said switches.

3. A switching device comprising two singlemotion switches, groups of outlets accessible to said switches, means for operating either switch to designate a particular group of outlets accessible to the other switch, and means for operating the said other switch to hunt for and connect with an idle outlet in the designated group.

4. In a telephone system, a trunk line, a switching device comprising two automatic switches terminating said trunk lines, groups of outlets accessible to said svitches, means for operating either switch to designate a particular group of outlets accessible to the other switch, and means for causing said other switch to connect said trunk line with an idle outlet in the designated group.

5. In a telephone system, two selective switches, groups of outlets accessible to said switches, means in each switch for marking groups of outlet groups accessible to the other switch, and means in each switch for selecting an idle outlet in a group marked by the other switch.

6. In a telephone system, two selective switches, groups of outlets divided into sub-groups accessible to the respective switches, means for seizing said switches simultaneously, means for causing the first switch to hunt for an idle outlet in one of said sub-groups, and automatic means for causing the other switch to hunt for an idle outlet in the corresponding sub-group when all outlets in said one sub-group are busy.

7. In a telephone system, a selector comprising two single-motion switches, a group of outlets part of which are accessible to one of said switches and the remaining of which are accessible to the other of said switches, means for operating said one switch to hunt over the accessible outlets of the group for an idle outlet, and means responsive to the failure of said one switch to find an idle outlet for operating the other of said switches to hunt over the remaining outlets of the group for an idle outlet.

8. In a telephone system, two selective switches, groups of outlets accessible to said switches, means for directively operating the first switch to mark a group of outlets accessible to the second switch, means for causing the second switch to hunt for an idle outlet in the marked group, means in the second switch for marking a corresponding group of outlets accessible to said first switch when all outlets of the first marked group are engaged, and means for causing said first switch to hunt for an idle outlet in the second marked group.

9. A switching device comprising two automatic switches, groups of outlets divided into subgroups accessible to respective ones of said switches, means including one switch for marking a sub-group of outlets to cause the other switch to hunt for an idle outlet therein, and means including the other switch effective in case no idle outlets are available in the marked sub-group for marking the corresponding sub-group accessible to said one switch to cause the same to hunt for an idle outlet therein.

10. In'combination, a switching device comprising two single-motion switches, groups of outlets divided into sub-groups accessible to the respecto hunt for anidle outlet in the marked subgroup, means for causing said other switch to mark the corresponding sub-group accessible to said one switch responsive to all trunks of the first sub-group being found busy, and means for an idle outlet 11. In a telephone system, we single-inotion switches, wipers for said switches, groups of con tact sets accessible to the respective switches via their wipers, means for directively operating the first switch to position its wipers on a setoi contacts to marl: a particular group of contact sets accessible to the second switch, means for moving the wipers of the second switch over the marked contact group in search of an idle contact set and to a position beyond the last contact set in the group in case no idle contact sets are ava11-=' able, and means including a wiper of said second switch thereafter effective to mark a particular group of contact sets accessible to said first switch. i

12. In a telephone system, two automatic switches, each of which is adapted to mark groups of outlets accessible to the other switch and each of which is adapted to'hunt for. an idle outlet in a marked group, means for directively operating one switch to initiate the marking and hunting operations, and means for restoring both switches to normal position.

13. In ,a telephone system, a trunk line, two. automatic switches each of which is adapted to mark groups of outlets accessible tothe other switch and eachof' which is adapted to hunt for an idle outlet in a marked group, and meansczontrolled over the trunk line fordirectively operating one switch to initiate the marking and hunting operations whereby said line is connected with an idle outlet accessible to one or the other of said switches.

14. In a telephone system, a switching device comprising two automatic switches each of which is adapted to control the hunting operation of the other by designating a particular group of outlets to be hunted over, and groups of outlets divided into sub-groups accessible to therespec ,,tive switches so arranged that the order of access to corresponding subgroups is'reversed for the switches. a

:15. A switching device comprising two automatic switches, means in the device responsive .to a series of impulses, and means including said last means for operating said switches simultaneously responsive to the first impulse of the series and for operating said switches alternately responsive to succeeding impulses of the series.

16. A switching device comprising two automatic switches, groups of-outlets accessible to said switches, means for alternately causing each switch to mark a particular group of outlets accessibleto the other switch, and means for causing the said other switch. to pass over the marked outlet group. i H

17. A switchingdevice comprising two automatic switches, two sets of wipers for each switch, groups of outlets accessible to the respective switches via said wipers, means in the device for determining the particular switch to be used in a connection, and means in'the device for determiningthe particular set of wipers of said particular switch which are to connect withone of said outlets. V

18. In a telephone system, a switching device comprising two step-by-step switches, means for 19. In a telephone system, a trunk line termimating in a switching device comprising two autoc s t itcheswipers for the switches, means itting a series of impulses over the device energized and dcon ve to alternate impulses-of the series, and r ns'controlled by said relays for connecting thetrunlrlineto' the wipers of either of said switches.

as. In a telephone system,a trunk line terminating in a switching device comprising two automatic switches, groups of outlets accessible to the switches, means fortransm-itting digit impulses over the trunk line, means inthe device for responding to said digit impulses, means for causing eitherof the switches to hunt for an idle outlet in an accessible group responsive to the receipt of a digit designating that particular operate said switches alternately, a

group, and means for applying a distinctive tone to the trunk line responsive to the receipt of a digit greater than the number of outlet groups accessible to said switches. 1

21. In an automatic switch, means for receiving a series of impulses, means for operating the switch one step responsive to the first odd numbered impulse of the series, and means for operating the switch a plurality of steps responsive to each subsequent even numbered impulse of the V series.

22. In an automatic'switchhaving access to groups of contacts, a control relay, means for operating and releasing said relay, means responsive to the operation of said relay for advancing the switch one step to the first contact of one of said groups, and means responsive to the release of said relay for advancing the switch a plurality of steps to the first contact of a succeeding group.

23. Inan automatic switch, means for receiving a series of impulses, means for operating the switch one step responsive to the first odd num bered impulse of the series/and means for opcrating the switch a plurality of steps responsive to each subsequent odd numbered impulse of the tic.

sponsive to the receipt of a series of impulses for setting each switch in selective relation with a group of outlets, and means dependent on the V 7 value of the impulse series for determining which one of the switches is to huntfor an idle outlet in a selected group.

26. In combination, a switching device comprising two separate switches, groups of outlets accessible to the respective switches, means for transmitting a series of impulses to said device to set each of said switches in selective relation with a group of trunks accessible thereto, means operative in case the impulse series consists of an even number of impulses for causing one switch to hunt for an idle outlet in its selected group, and means operative in case the impulse series consists of an oddlnumber of impulses for causing the other switch to hunt for an idle outlet in its selected group.

2'7. In combination, a switching device com prising two separate switches, groups of outlets accessible to the respective switches, a control relay, means for energizing and deenergizing said relay responsive respectively to alternate impulses of a received impulse series, means controlled by said relay for operating-said switches alternately to set them in selective relation with groups of outlets, and means controlled by said relay for determining which of said switches is to hunt for an idle outlet in a selected group.

28. In a switching device, four separate sets of wipers, groups of contacts accessible to said wipers, means responsive to the receipt of a series of impulses for setting said wipers in selective relationwith groups of outlets, means controlled by the value of the impulse series for selecting a pair of said wiper sets, and means for automatically selecting a particular wiper set of the selected pair.

29. A switching device as claimed in claim 27 in which the selection of a particular Wiper set of a selected pair is controlled by the busy or idle condition of outlets accessible to the wiper sets.

30. In combination, a switching device comprising two switches, a control relay in said device, means for repeatedly energizing and deenergizing said relay responsive to a series of impulses received by thedevice, means responsive to the initial energization of the relay for operating both switches one step, means responsive to each subsequent energization of the relay for operating one switch a plurality of steps, and means responsive to each deenergization of the relay for operating the other switch a plurality of steps.

ALEXANDER JOHN ESCOTT;

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