US2245748A - Automatic switch for use in telephones or like systems - Google Patents

Description

June 17, 1941. c, ETAL 2,245,748

AUTOMATIC SWITCH FOR USE IN TELEPHONES 0R LIKE SYSTEMS Filed March 2,' 1938 4 Sheets-Sheet I INVENTORS CHARLES EDMUND BEALE [DR/5 TREVOR RICHARDS man u mum/m m,

June 17, 1941. c. E. BEALE ETAL' 2,245,748

AUTOMATIC SWITCH FOR USE IN TELEPHONES OR LIKE SYSTEMS Filed March 2, 1938 4 Sheets-Sheet 2 Jay/z INVENTORS CHARLES EDMUND BEALE lDR/S TREVOR RICHA:

Patented June 17, 194i TED STTES PATENT OFFIC AUTOMATIC SWITCH FOR USE IN TELE- PHONES R LIKE SYSTEMS Application March 2, 1938, Serial No. 193,415 In Great Britain March 11, 1937 9 Claims.

The present invention relates to automatic switches for use in telephone or like systems and is more particularly concerned with switches suitable for the selection of trunk lines in automatic trunk exchanges. The invention is moreover particularly concerned with group selector switches of the so-called two-digit type, that is, requiring the transmission of two digits to complete their operation and preferably these switches operate on the longitudinal and rotary principle. The chief object of the invention is to adapt such switches so that they will more readily meet the requirements of automatic trunk exchange working.

According to one feature of the invention in a group selector switch for use in telephone systems a timing device permanently associated with the switch is eifective in the event of all the lines of a selected group being busy to prevent the busy signal being returned to the calling party until after the lapse of a predetermined time during which the switch is enabled to conneot with any line in the group which becomes free.

According to another feature of the invention in a group selector switch for use in telephone systems after unsuccessful hunting has taken place over the lines of a selected group a timing device permanently associated with the switch comes into operation to control a circuit for causing the switch to perform a further hunting operation.

A further feature of the invention is that in a group selector switch for use in telephone systems after unsuccessful hunting has taken place over the lines of a selected group, the wipers are restored to the beginning of the group and caused to perform a further hunting operation thereover after the lapse of a predetermined time under the control of a timing device permanently associated with the switch.

Still another feature of the invention is that in a group selector switch for use in telephone systems in the event of all the lines of a selected group being busy the switch wipers come to rest at the beginning of the group and are set in operation to hunt for a line as soon as one becomes free, the transmission of the busy signal being delayed for a predetermined time under the control of a timing device permanently associated with the switch.

The invention will be better understood from the following description of two methods of carrying it into effect, reference being had to the accompanying drawings comprising Figs. 1-4.

Figs. 1 and 2 when arranged together side by side form the circuit diagram of a mil-outlet twodigit group selector of the vertical and rotary type suitable for use in an automatic trunk exchange and adapted to deal with a trunk group contained within a single level while Figs. 3 and 4 when similarly arranged show the circuits of a switch having a group extending over a plurality of levels.

The mechanical construction of the switches considered is such that they have so-called rectangular movement, that is to say release takes place by further operation of the rotary magnet to advance the wipers clear of the bank after which they fall to the normal level and rotate back home beneath the bank.

The selectors for the purpose of the description are assumed to be employed as third trunk selectors, that is to say they are taken into use after two trains of impulses have been transmitted from the calling point.

Referring now to Figs. 1 and 2, it will be assumed that an originating operator at an automatic trunk board provided with key-operated impulse sending equipment desires access to a group of ten trunks situated on level 3 of the switch, this main group including a sub-group of five high grade trunks which are located on outlets 6 to I] of the level.

When the switch is seized from the preceding trunk selector, a loop is extended over the incoming negative and positive conductors from an impulse sender which is temporarily associated with the operator's keyset during the setting up of the connection and this operates relay A in series with relay DF which however is non-operiative under this condition due to the differential arrangement of its windings. A battery potential is also extended from the operators position circuit over the S lead and this operates relay CS by way of its right-hand winding, the left-hand winding being short-circuited by earth over armature co t. Relay CS at armature csl operates relay B which thereupon at armature b2 operates relay C by Way of its right-hand winding in series with resistance Y0 and at armature b5 applies a guard earth to the incoming P lead. Relay C at armature cl prepares an impul-sing circuit to the vertical magnet VM by way of its low resistance lefthand winding.

When the first train of three impulses is received as a result of the keying up by the operator of the third digit 3 in the number of the desired trunk line group, relay A responds and on each release of this relay a circuit is completed to energise the vertical magnet VM from earth over armatures bi, csl, cl, are and al, lefthand winding of relay C, armature e3, rotary offnormal contacts NR5, magnet VM to battery and accordingly the switch wipers are raised to level 3.

On the first vertical step the off-normal springs N are mechanically operated and at contacts NI earth is extended from armature bl over armature ht, contacts NR3 and armatures e2 andb? to short-circuit the right-hand winding of relay C so that this relay is made slow to release and will remain held during the impulse train by way of its left-hand winding but will release after a short interval on completion of the train.

Assuming that the operator wishes to gain access to any one of the group of ten outgoing lines on this level she will have keyed up 1 as a fourth digit. As a result of this operation, immediately after the end of the third digit the sender transmits a signal which momentarily unbal ances the incoming loop, for example, by the connection of earth potential thereto so that the differential relay DF now operates while relay A remains held on its upper winding. Relay C at this time still remains held operated due to its slow release characteristic. Relay at armature dfl operates relay TA over its middle winding and this relay thereupon locks temporarily over its left-hand winding by way of armature ms, off-normal contacts N3, rotary magnet interrupter contacts rm and armature tad to earth. The short-circuit is new removed from the left-hand winding of relay AR at armature tel, while the right-hand winding of relay TA is connected at armature ta-2 to the test Wiper P in preparation for the testing for a free outlet when the switch wipers are subsequently rotated. Relay AR upon operating, at armature ari looks over its right-hand winding in series with resistance YB and provides an additional shortcircuit to the right-hand winding of relay C at armature m3, while it disconnects the impulsing circuit to the vertical magnet VM at armature ar5.

On the release of relay C, a self-interrupted driving circuit is completed for the rotary magnet RM from earth, armature ta l, interrupter contacts Tm, off-normal contacts N3, armatures 03, 115, sol and hl, magnet RM to battery. On the first energisation of the magnet RM the Wipers are advanced into engagement with the first set of contacts in the level selected, whereupon the rotary oil-normal springs NR are mechanically operated and at contacts NR3 prepare for the removal of the short-circuit from the right-hand winding of relay C while the rotary magnet interrupter contacts rm are also operated.

If the test Wiper P encounters an engaged outlet, that is, one marked by the presence of earth potential, relay TA will be maintained operated over its right-hand winding and the disconnection by the contacts rm of the holding circuit for this relay over its left-hand winding is therefore Without effect. However the rotary magnet RM releases and when the interrupter contacts again close the magnet is again energised to advance the Wipers to the next set of contacts. This stepping operation continues automatically until a free outlet in the selected group is found or alternatively the wipers are rotated to the 11th.

position where they remain until a predetermined interval elapses after which they are automatically released to the rotary normal position and are caused to research the level in a manner to be described later.

Assuming first that there is a free outlet in the level, when this is encountered there will be no earth potential extended over the P wiper to hold relay TA operated when the interrupter contacts rm open and this relay therefore quickly restores and at armature m4 opens the magnet driving circuit, the outlet seized being immediately guarded from earth over armatures b5 M2 and m2. A circuit is now completed from earth, armature b6, windings of relays H and BK in series, armatures sr2, m5 and M4, magnet RM to battery and accordingly relays H and HX are operated, the magnet RM being non-operative in this condition. The short circuit is also removed from the right-hand winding of relay C at armature ms and this relay re-operates. Relay HX at its armatures hurl-729:5 switches all the incoming leads except the P lead through to the wipers and over the bank contacts on which they are resting to a relay set terminating an outgoing trunk line. At the same time relays A and OS are released and the latter opens the circuit of relay B which commences to release slowly due to its slug. On the re-operation of relay C, the incoming P lead is also extended through, a guard earth being maintained on this lead at armature b5 and b8 until an earth is returned from the trunk relay set seized. On the release of relay B relay AR is released, but relays H and HX remain locked to the P conductor in series with armature hi and the magnet RM and consequently maintain a holding circuit for relay C at armature hZ independently of armature b2.

At the end of the conversation the operator, on receiving the clearing signals on the calling and answering sides of the connecting circuit utilised, throws her release key and in response to this operation the trunk relay set is caused to remove the hold earth from the P lead and this brings about the release of relays H and HX. The release of relay HX opens the through connection of the incoming leads to the wipers while relay H opens the circuit of relay C at armature 722. A self-interrupted driving circuit is now completed for the rotary magnet PM from earth via delayed alarm equipment which is arranged to give an alarm in case the mag-netshould be held operated for longer than a predetermined period, lead I l fault release jack FRJ, armature b3, contacts SSN I, armatures c4 and tad, contacts rm and N3, armatures 03, T15, sol and hi, magnet RM to battery. The wipers are accordingly advanced to the 12th position whereupon they restore vertically under gravity and then rotate under spring control under the banks back to their full normal position where the magnet driving circuit is "opened by the off-normal contacts N3.

A momentary unguard interval comprising the sum of the release times of relays H and C is provided on the incoming P lead to allow preceding switches to release with a minimum of delay. On the release of relay C the guard earth is replaced on this lead over contacts N2 and armature c2 and it is then maintained until the switch reaches normal, when it is removed on the restoration or" the elf-normal springs.

In case all the outlets in the level selected prove busy, the wipers are rotated to the 11th position where the cam springs S are mechanically operated and since in this position the P contacts are left unwired, relay TA releases quickly and opens the' magnet driving circuit. The operation of the cam springs -S at contacts 'S'l reconnects both windings of relay RL and opens moves the short-circuit from the right-hand winding of relay C so that it re-operates while at the same time a circuit is completed for relay SR. This relay thereupon at armature srl extends earth over the PS wiper and bank and common lead It to a group busy equipment, one of which is provided for each group of outgoing trunk lines. By this means a lamp on the delay supervisors position can be lit to indicate that this particular trunk group is busy and that at least one switch is searching over it. Relay SR also at armature 8T2 disconnects theoperating circuit for relays H and HX which would ordinarily be completed on release of relay TA so that these two relays fail to operate and consequently relays CS, B and AR remain operated. Relay A restores to normal when the impulse sender drops out of the connection after sending its full complement of digits since no direct current loop is then maintained from the operators position. A new circuit for relay TA is also prepared at armature sr3 while an operating circuit is completed for the slow-to-operate relay S at armature sr5. On the operation of relay C relay TA is energised over its middle winding in the following circuit: earth, armature bl, off-normal contacts N l, rotary off-normal contacts NR2, armatures c5 and $13, windings of relays AR and TA in series to battery, the latter relay locking temporarily over its left-hand winding to earth via armature M4.

The relay SO preferably comprises a solenoid type relay with its plunger associated With a delayed action device of the dashpot or escapement type, so that a delay in operation of the order of ten seconds is obtainable, the delay period being adjustable.

After the lapse of this period, relay SO operates and at armature s02 completes a circuit for relay RL over its right-hand winding. Armature rll now opens the circuit of relay SR which releases and also applies earth to again short-circuit relay C which commences to release. At armature r12 a locking circuit is completed for relay TA over its middle winding, at armature r13 the right-hand winding of relay TA is disconnected from the P wiper, at armature 1'14 the magnet SSM of an auxiliary step-by-step switch is energised, at armature 1'Z 5 an energising circuit for the vertical magnet VM is prepared, and at armature TZ'B relay S0 is locked independently of armature sr5. On the release of relay 0 after its slow period, magnet VM is energised in series with the left-hand low resistance winding of relay RL from an earth supplied over armature tad, interrupter contacts rm, off-normal contacts N3, armatures c3 and 115, and contacts NR1 whereupon the rotary release of the wipers along the path over which they have been set is initiated. The vertical magnet pawl when operated is arranged to engage with an extension arm on the rotary detent and move it out of engagement with the rotary ratchet member so as to permit the release of the wipers in a rotary direction, for instance, in the manner described in specification No 2,028,689, dated January 21, 1936.

' When the wipers reach the rotary normal position, the vertical ratchet member re-engages with the vertical holding detent thus preventing further release of the wipers to their fully normal position. In this position the rotary offnormal springs NR are, restored while at the commencement of the release movement the 11th step cam springs S were also restored.

" The restoration of contacts NRI and NR2 disthe energising circuit for the magnet VM. Relay RL' upon releasing opens the energising circuit of the magnet SSM at armature r14 and since the switch SS is assumed to be of the direct drive type the energisation of the magnet SSM will have advanced Wiper SSL to position 2 and on the disconnection of the magnet circuit the wiper will remain in this position. Relay RL also at armature rl6 opens the circuit of relay SO which is arranged to release quickly, and at armature 1Z5 prepares an energising circuit for the rotary magnet RM. On the release of relay S0 at self-interrupted driving circuit is again completed for the magnet RM so that re-search of the group takes place in an identical manner to the initial search.

If all outlets in the group are again found to be busy, the wipers are again advanced to the 1 1th position where on the operation of relay SO after its slow period relay BL is again operated to bring about a further rotary release and subsequent re-search. At the same time the switch SS advances its Wiper SSI to the next position. If no free outlets are encountered during a predetermined period which is measured by counting the number of re-searches at intervals of the order of ten seconds each, earth is extended over wiper SSI and cross connection between the bank contact on which it is resting to terminal l3 and thence by way of armature M4 to battery through the right-hand winding of relay G0 which operates. For example if contact I in the SSI bank is cross connected to terminal I3 relay GO will not be operated until a period of the order of sixty second-s has elapsed since the commencement of the initial search.

Relay 00 upon operating locks over its armature col and at the same armature disconnects the energising circuit for relay SO so that rotary release of the wipers cannot be again brought about. At armature 003 an earthed busy tone transformer is connected up to the lower winding of relay A over common lead l0 so that busy tone is extended over the speaking conductors to the operator in a balanced circuit while a flicker earth is extended over common lead l2, armature 004, the low resistance right-hand winding of relay CS and armature has to the incoming S lead to flash the operators calling supervisory lamp in well-known manner.

If a free outlet is encountered on one or other of the re-searches, it will be noted that on the rel-ease of relays CS and B in turn in consequence of the operation of relays H and HX, a homing circuit is completed for the switch SS from an earth supplied to common lead II by way of delayed alarm equipment, fault release jack FRJ, armature b3, the SS switch elf-normal contacts SSN I, armature 00-2, magnet SSM to battery. The magnet SSM upon energising completes a circuit for relay CO over its left-hand winding from earth, armature b6 and the magnet interrupter contacts ssm and relay CO thereupon opens the energi-sing circuit for the magnet SSM at its armature 002. The magnet in turn releases and releases relay CO which again completes the circuit for the magnet and the interaction continues until the switch reaches the home position when the elf-normal contacts SSNI restore and open the magnet driving circuit. This immediate homing arrangement reduces the time involved in freeing the selector for further use at the conclusion of the conversation. j

' If a busy signal is returned to the operator due to the failure of the specified number of researching operations, release of the switch is brought about by the operator removing battery from the S lead. Thereupon relays CS and B release followed by relays C, AR and CO. The switch SS then homes as just described and when it reaches normal position contacts SSNl complete the usual restoring circuit for the main switch.

Considering now the case in which the operator requires access only to the sub-group of high grade trunks Within the main group of trunks on the level, 6 is keyed by the operator as a fourth digit in order that the switch wipers will be advanced to the first outlet of the sub-group, from which position automatic rotary hunting over the sub-group only may take place.

The circuit operation proceeds as previously described to the end of the vertical train of pulses When after a slight interval relay C releases, an ordinary interdigital pause being provided in this case, since a train of pulses will follow as a fourth digit as distinct from the previous condition in which in response to the fourth digit, an unbalance signal was transmitted immediately after the sending out of the third digit. Relay C upon releasing completes a circuit for relay CR from earth, armature bl, contacts Ni, armatures hi, ar3, 12.95 and 07, lower winding of relay CR, contacts NR1, magnet to battery, the magnet being non-operative in this condition. .At arma ture art a circuit is now completed for relay E which at armature el locks up independently of armature crZ. Relay E also at armature 02 removes, the short-circuit from relay which reoperates while at armature e3 an impulsing circuit is prepared for the rotary magnet RM. Relay C at armature c1 disconnects the energising circuit for the relay CR over its lower winding but this relay now remains held on its upper winding by way of armatures or], all, m5, cl, cc! and hi to earth.

On reception of the next train of six impulses transmitted from the sender in response to the keying of the fourth digit 6, relay A again responds and each time it releases earth is extended via the left-hand winding of relay C and armature c3 and h! to energise the rotary magnet RM so that the wipers are rotated step by step to th desired position. Due to its slug relay GR remains operated over its upper winding during the train of impulses while relay C also holds operated since its right-hand winding is shortcircuited on the first rotary step atcontacts NR3 and armature e2. I

At the end of the train of pulses relay C restores after a short interval and at armature cl opens the holding circuit of relay CR which commences to release slowly. At armatures 0T3 and c6 relay HS is now connected to the Pl wiper and since th wipers have been positioned to the first outlet of the sub-group, relay HS will operate on its upper winding in series with a resistance battery potential connected to the first outlet of the group on the Pl bank. Relays HS looks over its lower winding by way of armatures gi, 71.35, art and 723 contacts Ni and armature bl and at armature hs3 prepares a further energising circuit for the rotary magnet RM. Relay C on releasing also connects an earth to the winding of relay TD by way of armatures c and c4.

In, the, first trunk in the sub-group is found to be free, relay ID will operate in series with resistance YB since there will be no busying earth extended back over the P wiper and metal rectifier MBA in a conductive direction to prevent its operation. Relay TD thereupon at armature tdl applies a guard earth on to the outlet seized and at armature tdZ completes an operating circuit for relays H and HX from the earthened P lead. These relays in operating extend the incoming leads through in the normal manner and reoperate relay C by removing the short-circuit across its right-hand winding at armature as, the energising circuit for relay E being opened at the same armature so that this relay releases after a short interval due to its slug. Relays A, CS and B are released in the normal manner and at armature 05 the energising circuit for relay TD is opened so that this relay releases while the P lead is extended through over. armatures c2 and hi. Both windings of relay HS are now disconnected so that this relay releases after a slow period due to its slug while relay CR restores to normal at the end of its slow release period. The circuit conditions during conversation are therefore as before described.

If the first trunk in the sub-group is engaged, an earth is extended over the P wiper and rectifier MRA to shunt relay TD and prevent its operation. On the release of relay CR after its delay period, the energising circuit for relay E is disconnected at armature 072 since relay HS is operated and on'the release of relay E the shortcircuit is removed from relay C at armature e2. Relay C now re-operates and at. armature 05 extends an earth over armatures g2, hs3 and hi to energise the rotary'magnet RM whereupon the switch wipers are advanced to the next outlet. Earth is new extended over armatures c4 and um, operated rotary magnet interrupter contacts rm and armatures hs2, hsl and e5 to operate relay G by way of its upper winding. Thereupon at armature gl earth is extended by way of the lower winding of relay G to the winding of relay TD while the lower winding of relay HS isdisconnected but this relay remains held for a further short interval due to its slug. Moreover at armature g2 the rotary magnet energising circuit is opened and the magnet restores and opens the energising circuit of relay G at contacts rm. If this second outlet in the sub-group is also busy, relay G will release since the earth extended over the P wiper prevents it holding on its lower winding in series with-the winding of relay TD. On the release of relay G, a circuit is again completed for relay HS over its lower winding by way of armature gl, this relay having been held operated during this time due to its slug, while at armature 92 the magnet RM is again energised. Interaction between relay G and th magnet RM now continues until a free outlet in the subgroup is reached, or alternatively the wipers are rotated to the last outlet in the sub-group.

If a free outlet is reached, relay G will hold in series with relay TD which operates and energises relays H and HX as before. On the operation of relay H, relays G, HS and TD are released at armature 71.3 and the circuit conditions during conversation are thus as usual.

If all outlets in the sub-group prove to be busy, when the wipers reach the last of these outlets relay G will hold operated over its upper winding, armatures c5 and hsl high resistance YD, armature c5, Pl wiper and bank to an earth which marks the last line of the sub-group. On the release of relay HS after its slow period, relay G then remains held over its upper winding, armatures e5, hsl and tail, contacts NR3, armature h3, contacts NI, and armature bl to earth and accordingly busy tone is returned to the operator by theconnection of the busy tone transformer to the lower winding of relay A over lead l and armatures g3 and hs4.

Facilities are also provided whereby the operator may select any particular trunk line of a group except the first of a main group or the first of a sub-group, but if this trunk is found to be busy, no hunting takes place but the switch camps on it until it becomes free. Generally the above facility is likely to be used when it is important to employ the highest grade trunk line available for the xtension of a call.

In this case, on the completion of the train of rotary setting pulses, relay C restores and opens the holding circuit of relay CR in a normal manner. Relay HS is connected as before to the Pl wiper but since the wipers have not been positioned on to the first line of a sub-group, this relay cannot operate. Earth is also connected at armature 05 to relay TD which operates in series with resistance YB if the selected trunk is free and the operation proceeds in a normal manner. If however the selected trunk is busy, the earth potential encountered by the P wiper will prevent relay TD from operating. Relay CR releases after a short interval but relay E now remains held over armatures el, hs5, m3 and Ms, contacts NI and armature bl to earth. An earth is therefore maintained on the left hand terminal of relay TD over contacts NR2 and armatures c5 and e4 and the switch wipers will remain on the selected outlet until it becomes free, when relay TD will operate and operate relays H and EX to switch the connection through. When this occurs the operator is signailed by the darkening of her supervisory lamp due to the changeover from a low resistance earth normally applied to the S lead by way of the right-hand low resistance winding of relay CS to a high resistance earth connected to this lead by way of a high resistance relay in the trunk relay set associated with the selected trunk line.

Leads NA and N13 and resistance YA are utilised in connection with the inclusion of thermionic repeaters, resistance YA serving as a line balancing arrangement during the setting of the switch.

Rectifier MBA is provided toprevent an earth by way of the winding of relay TD from being extended on to the P wiper during the camp-on period, which would influence other switches connected to the same outlet.

Considering now the circuit operation of the selector shown in Figs. 3 and 4, it will be assumed for the purpose of description that the operator desires access to a trunk group of thirty outlets located on levels 3, 4 and 5.

When the switch is seized, relays A and OS are operated whereupon relays B and C are op- 'erated with results as described in connection with Figs. 1 and 2.

Assuming that the operator desires to obtain access to any trunk in the group, she keys digits 3 and I as third and fourth digits respectively and in response to the first train of impulses received the switch wipers are raised to level 3. On the first vertical step the off-normal springs are operated and at contacts NI apply an earth to short-circuit the right-hand winding of relay C as in the previous case.

After the sending out of the third train of impulses the sender immediately transmits an unbalance signal as a result of the keying digit l as a fourth digit and this signal brings about the operation of relay DF. This relay at armature dfl energises relay TA over its middle winding and this relay locks temporarily over its left-hand winding and'at armature ta5 removes the short-circuits from the right-hand winding of relay AR. Relay'AR now operates in series with the winding of relay TA and at armature arl locks over its left-hand winding in series with resistance YB, at armature ar4 provides an additional short-circuit to the right-hand winding of relay C at armature ar3 connects relay RC to the right-hand vertical wiper VW and at armature 0W5 operates relay AX. This relay thereupon at armature c932 connects'up the common lead It to magnet SSM, disconnects any impulsing circuit to the vertical magnet VM at armature (m4 and connects relay CF to the lefthand vertical wiper at armature (1x5.

The contacts of the vertical bank are arranged to receive marking'potentials so as to indicate to the switch Whether the whole of the desired trunk group is engaged and also whether there is a free trunk available on any particular level. The former indication is eilected by extending a 'common lead from all the left-hand vertical bank contacts corresponding to the levels on which the group is situated to a so-called group busy equipment. This is in association with the trunk relay sets terminating each of the outgoing trunks in the group and when all the trunks in a group are engaged, the group busy equipment removes the battery marking potential from the commonlead. As soon as a trunk in the group becomes free, the battery marking potential isreplaced. The indication as regards the individual levels is eifected by extending a lead common to all the ten trunk relay sets associated with the level in question to the corresponding contact on the right-hand vertical bank, so that in the event of all the trunks being engaged no earth will be applied to the corresponding contact of the right-hand vertical bank. The contact on the right-hand vertical bank which corresponds .to the last level of a group is earthed permanently for a reason which will shortly be apparent.

Returning again to the circuit operation; while relay C still holds up relays GF and RC test the condition of the whole group and of the level to which the Wipers have been raised.

If there is a free outlet in the group, the lefthand vertical wiper encounters a resistance battery potential whereupon GF operates over its two windings in series and thereupon at armature gfl short-circuits its lower high resistance winding so as to apply a low resistance earth potential to the vertical wiper by way of its upper winding. This arrangement prevents a GP relay in another switch from switching to the same battery and thereby ensures that one switch only may hunt over a particular group at a time so that two switches can never simultaneously set out in search of one free outlet. If the free outlet is in level 3 the right-hand vertical wiper encounters an earth potential supplied from the resting contacts of a relay in the disengaged trunk relay set and this operates relay RC which thereupon at armature rcl prepares av self-interrupted driving circuit for the rotary magnet RM and disconnects the hunting circuit for the vertical magnet VM. On the release of relay 0 after its slow period, the circuit for the rotary magnet RM is completed from earth, armature taB, rotary magnet interrupter contacts rm, contacts N2, armatures 03, M2, gf2, rcl and hl, magnet RM to battery and the switch thereupon rotates in search of the free outlet in the level. When this is found, relay TA releases and operates relays H and HK in the usual manner which switch the lines through and operate relay C. Relays A and CS are now released and open the holding circuit of relay B while on release of the latter, relays AR, AX, RC, and GF are released.

At the end of the conversation the release of the switch takes place in the manner described for the switch of Figs. 1 and 2.

If all the trunks on level 3 are engaged and if there is a free trunk on level 4 or level 5, relay GF will operate as before during the release time of relay C, but relay RC will not operate due to the absence of earth on the third right-hand vertical bank contact. Hence on the release of relay C, a self-interrupted driving circuit for the vertical magnet VM will be completed from earth over armature rcl, vertical magnet interrupter contacts om, magnet VM to battery and the switch thereupon raises it wipers vertically until they reach a level serving the group in which there is a free outlet. Relay RC now operates to cut the drive at armature rel and at the same armature completes a self-interrupted driving circuit for the rotary magnet whereupon the operation proceeds as before described.

If during the vertical search however all outlets in the group become busy due to the arrival of incoming calls over the trunk lines which are worked on a bothway basis, the switch will step vertically until it reaches the last level serving the group where the right-hand vertical wiper encounters a permanent earth marking. Relay RC then operates to cut the drive as before. Since no outlets are free on this level the switch thereupon rotates to the 11th position where relay TA releases in the normal manner to stop the rotation. The cam springs S are mechanically oper ated in this position and at contacts SI relay CO is energised and at armature e02 connects a busy tone transformer over common lead H) to the lower winding of relay A whereupon busy tone is returned to the operator and a flashing signal is given by the operation of armature 003.

If all trunks in the group are found to be engaged, the vertical wipers on contact 3 of the vertical bank encounter no potentials to operate either relay BC or relay GF and therefore on release of relay C no circuit will be completed for either the vertical or rotary magnets. The switch now remains on this lowest level of the group for a predetermined period and if no trunk in the. group becomes free during this time a busy indication is returned to the operator. If a trunk in the group should become free during this period relay GF and subsequently relay RC will operate and the switch will hunt for the trunk in the manner described above.

Considering now the method of timing, on the operation of relay AX the magnet SSMv is connected up to common lead l4 over which earth pulses are extended at intervals of the order of eight seconds. Thus if no trunk in the group becomes free the magnet SSM responds to these pulses and advances its wiper SSI step by step round the bank. After a predetermined period has elapsed, as determined'by the contact on the SS! bank which is strapped to terminal 13 connecting with the left-hand Winding of relay CO. earth is extended over the SSI wiper and bank to operate relay CO and this relay upon operating opens the pulsing circuit to the magnet SSM at armature col, connects up a busy tone to the incoming speaking conductors at armature 002 and extends a flicker earth from common lead 12 on to the incoming S lead at armature 003 to flash the operators supervisory lamp and at armature 004 releases relay AR. This relay on releasing releases relay AX and disconnects relay RC from the right-hand vertical wiper while relay AX disconnects relay GF from the left-hand vertical wiper.

The operator may also obtain access to individual high grade trunks in the group by keying the appropriate third and fourth digits and if the selected outlet is found to be busy no hunting takes place but the switch camps on until it becomes free in the manner described in connection with the switch of Figs. 1 and 2.

We claim:

1. In a selecting system, a selector switch, lines accessible to said switch divided into groups, means for operating said switch to elect a group of said lines and to cause the same to then automatically pass over the lines of said group to hunt for an idle line in the elected group, a timing device associated with said switch, means for starting the operation of said device in the event all of the lines in said group are found busy during said second operation, and means controlled by said device for causing a second hunting operation of the switch to select an idle line after a predetermined interval.

2. A selector switch as claimed in claim 1 in which there is means for returning a busy signal to the calling subscriber controlled by said timing device after the lapse of a predetermined time.

3. A selector switch as claimed in claim 1 having means for transmitting a busy signal to the calling line, the operation of said last means being delayed for a predetermined time under control of the timing device.

4. A selector switch as claimed in claim 1 in which the timing device comprises a step by step switch and in which there is means for transmitting impulses to the step by step switch at predetermined intervals.

5. A selector switch as claimed in claim 1 in which the timing device comprises a step by step switch and in which there is an impulse generating means consisting of a slow relay for sending impulses at predetermined times to the timing device.

6. A selector switch as claimed in claim 1 in which the timing device has means for causing the switch to repeat the testing of the lines a predetermined number of times if necessary and for then transmitting a busy signal to the calling line in case the tested lines are still busy.

7. Ina telephone system, a selector switch having access to lines divided into groups and subgroups, means for operating the switch to elect a group of said lines, and to then automatically hunt for a line in the elected group, means for operating the switch to a certain point to elect a sub-group of lines and for then automatically selecting an idle line only in the elected sub-group, and a timing device effective. to. transmit a busy signal to the calling line only after a predetermined time.

8. A selector switch for use in .a telephone system, having access to trunk lines divided into groups, said switch having means controlled from a calling line for causing it to elect a group of lines and automatically and repeatedly test the lines of the selected group in search for an idle trunk line, means for transmitting a busy signal to the calling subscriber, a timing device permanently associated with said switch for controlling the repeated testing of said lines and for preventing the transmission of said busy signal until after a predetermined time.

9. In a telephone system, a selector switch with wipers having access to lines divided into groups, means for setting the wipers at the beginning of a particular group and causing them to advance over the lines of a group in search of an idle line, means for resetting the wipers to the beginning of the group in case all the lines thereof are busy and for again operating them to search for an idle line in the group in case one has become idle, and means for applying a busy signal to the calling line only after a predetermined time in case an idle line has not been formed within that time.

CHARLES EDMUND BEALE. IDRIS TREVOR RICHARDS.

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