US1543893A - Telephone system - Google Patents

Description

June 30, 1925.

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UNITED STATES PATENT OFFICE.

SAMUEL B. WILLIAMS, OF BROOKLYN, NEV YORK, ASSIGNOR TO WESTERN ELECTRIC COIVIPANY, INCORPORATED. OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEPHONE SYSTEM.

Application filed March 31, 1923.

To a?! whom it may concern:

Be it known that I. SAMUEL B. \ViLLiAMs, a citizen ot the United States of Ameri -a, iesiding at Brooklyn, in the county of Kings and State oi New York, have invent ed certain new and useful Improvements in 'i'elephonc Systems, of which the following is a full. clear. concise, and exact description.

This intention relates to telephone systems and more particularly to systems employing automatic switching apparatus for the establishment of connections.

The object of the invention is to secure a more cti'icient and flexible switching ar raagcment by an improved method of distributiug the selector switches which receive incoming calls and by assigning certain common controlling equipment to said switches in accordance with the distribution thereof.

A feature of the invention is the arrange ment of a plurality of selective switches to which incoming calls are distributei'l, in a nmaoer of groups. together with a registerand controlling equipment for each n onp for selectively operating any switch in the corresponding group.

,i further feature relates to the provision of a nmnber of registering; devices and a common controlling device for each group otswitches. said controlling device being associablc with any registering device and with any switch of the group to which it teloings for operating such switch.

Another feature. relates to an arrangement. in a system designed to receive a numtcr oi' classes ot calls. whereby calls of a numter oi ditl'erent classes may be routed to each ot the several groups of switches, each group having its individual controlling equipment l'or serving all switches therein.

(Ether tcat'ures and advantages contemplated by the invention will be taken up in detail hereinafter and pointed out in the appended claims.

(onsidering the drawing. Figs. 1 to 12. inclusive. taken in the order illustrated in Fig. 1 3 disclose the details of a telephone system incorporating the features of the present invention. Fig. 1 illustrates a line ."witch of the coordinate type in which the subscribers lines terminate and also two of a numtcr ot' trunks outgoing to a district clector switch,

Serial No. 29,019.

Fig. 2 shows certain control elements associated with an outgoing trunk.

Fig. 3 discloses a district selector switch coordinate type. l i 4 illustrates a group selector switch coordinate type in which terminate trunks incoming from the district selectors.

Fig. 5 shows a coordinate connector switch for completing connections to called lines.

Fig. 6 represents a sender selector switch also of the coordinate type.

Figs. 7 and 8 ilh istrate one of a number common register senders. Fig. it shows. at the left of the sheet. a sender cmuiector device 'i'or connecting any of the senders with the district marlier or selection controlling device.

The remainder of l ig. 9 together with Fig. 1t! disclose the common marker for controlling the district switches.

11 shows a sender connector device for connecting the line marker with any of the several register senders. This tigure also show testing relays for testing the condition of trunks interconnecting the group selector and connector switches together with certain control elements ot the line marker.

Fig. l2 discloses the remainder of the line marker.

Figs. 14 and 15 comprise a scl'iematic showing of a telephone system empli'iying switches and controlling equipment arranged in accordance with the principles of this nvention.

Referring tirst to Figs. 14 and 15. the subscribers lines entering the exchange ap' pear as calling lines in a number of coordinate line switches. It the lines are of diilcrcnt classes they may be conveniently grouped to appear in the line switches according to the class. For example flat rate lines may appear in a group of line switches Hut) of which two switches 140:1 and Hot are shown. Message rate lines appear in the group l-tttl of which one line switch HUS is illustrated. Also coin box lines may appear in the third group 1492 represented by the single line switch H06.

The first group selector or district switches are arranged in a number of selector gronp particularly suitable to the method in which the controlling apparatus exercises its con trol Three of these over these swit hes. groups l-l T. this and H09 are illustrated.

Group 1407 comprises a plurality of dis trict selector switches 1411, 1412, 141? and 1414. The second group 1408 comprises a series of district selectors 1.415, 1410, 1417 and 1418 while the third group 1409 includes a number of district switches 1419, 1,420, 1421 and 1422. The trunks 1410 and 1430 outgoing from the line switches of the group 1400 are shown distributed in any arbitrary manner to the district switches of the several selector groups. Some of these trunks lead to switch 1411, others to the switch 1415, others to the switch 1419, and still others lead to other switches in each of the different selector groups. This distribution of the trunks is purely arbitrary and may be varied to suit the tratiic requirements.

In a similar fashion the trunks 1431 and 1432 outgoing from the line switches of the other two groups 1401 and 1402 are distributed among the district switches of the several selector groups.

The trunks outgoing from the dillcrent selector groups may lead through as many succeeding switches as are neccssacv. in the system illustrated, however, these trunks lead to group selector switches, which in turn, have access to connector switches for making connection with called subscribers" lines. The group selectors and connectors constitute terminal line groups of which there may be any suitable number as, for example. five. Two of these groups 1440 and 1441 are illustrated. The group 1440 comprises a plurality of group selectors 1442 1444, etc, and a plurality of connector switches 1442i and 1145. The second group 1441 consists of group selectors 1440 and 1448 and connector switches 144T and 1449. In like manner other terminal line groups are employed each comprising as many group selectors and connectors as are heresary.

Trunks 1423 which lead from the group 1407 of district switches are multipled to appear in all district selectors of that group and lead to the group selectors ot the several terminal line groups. ll'hilc only a few trunks are illustrated, it is understood that; these are symbolical 01' all groups and that each selector group is provided with at least one group ot trunks leading to each of the terminal line groups and appearing in the group selector switches 1442, 1441-, 1446, 1448, etc. Likewise, the trunks outgoing from the second and third selector groups 1408 and 1409 are multipled before all the district switches of the respective groups and lead to the group selector switches 01' the sei'eral terminal line groups. Several group selectors in each terminal line group have acces thrr'iugh intcrconnecting trunks to the dillerent connector switches. which in turn, have access to the called lines.

In accordance with the division of the district selectors into groups, so is the controlling equipment arranged on a similar plan. In Fig. 15 the elements 1500 to the left of the left hand vertical broken line represent controlling equipment which is individual to the first group 1407 of district switches. The elements 1501 represent the controlling equipment individual to the second group 1408 and the elements 1502 represent the controlling equipment individual to the third group 1409. The controlling equipment 1500 consists of a plurality of register senders 1503, 1504, 1505, 1506, a common dis trict marker 150T and a number of sender selector switches 1508 and 1509. The tum-- tion of the sender selector switches 1508 and 1509 is to connect to an idle sender 1503, 1504, etc., any one of the trunks leading from the line switches to the switches of group 1407. As soon as the sender has received the record it becomes associated with the common district marker 1507, which in turn, becomes associated with the particular district selector of the group 1407 to which the extended calling line leads.

ln a like manner the sender selectors ot the second and third selector groups serve to connect trunks incoming to the district switches of these groups through to the register senders 1510, 1511, 1516 and 1517, which are in turn associable with the corre sponding district markers 1513 and 1519. The district marker 1513 is associable with any one of the district switches of the group 1408, while the district marker 1519 is associable with any one of the district switches of the group 1409.

For controlling the terminal lino group switches there are provided a number of line markers 1523, 1524, one for ea h of the terminal line groups. The marker 1523, for example, is individual to the group 1441 while the marker 1524 may be individual to the group 1440. The marker 1523 serves to control the group selector and connector switches and to this end it may be associat ed with any group selector and with any connector in the group it controls. The marker 1523 is associalole with any one of the senders relating to any channel in the exchange. By this means any sender that has been taken for use may be connected to any line marker to control a group selector and connector switch in any terminal line group.

Gcnclal description of system and apparaitus.

The line switch 100, selector switches 300 and 400, connector switch 500 and the sender selector switch (300 employed in the system are of the coordinate type such as shown and described in British Patent No. 183.438. issued to Vestern Electric Company, Limited, accepted September 6, 1923, and in the TAD patent of S. B. Williams, No, 1,517,331, granted Dec. 2, 1924. Reference may be had to the latter patent for a more complete disclosure of features which are not specifically detailed in the following descriptions. The subscribers lines enter the exchange and appear in the coordinate line switches too in vertical rows and 111. each line having access to a group oi horizontal links such as the groups represented by links 106 and to? and links 108 and 109. Line 101, for instance, has access to the group of links con taining the links 106 and 107. Each of the outgoing trunks 111 and occupies a full vertical row 01 contacts 112, 113 and leads to the district switches.

Trunk 114. which is shown in tall, appears in the district switch 300 in the vertical row of contacts 308. Another trunk 115 is shown occupying the vertical row 0t contacts 309. District selector switch 300 also has a plurality of groups of outgoing trunks appearing in rows 311, 312. 313, etc. Each outgoing trunk is shown having an appearance before a single horizontal link such as the links 303, 301. 305. 306 and 307. 'lhese trunks are multipled as explained to appear before corresponding horizontal links in all of the district switches of the same group. Since the subscribers lines as called lines are divided into a number of groups as called lines known as terminal line groups the trunks outgoing from the district switches are arranged as explained hcrcinbetore to have access to any of the terminal line groups of switching apparatus. The group ot trunks appearing in the upper section 302 of the vertical row 311 in switch Still lead to the group selector switches of a particular terminal line grottp. ()ne of these trunks 350 is shown leading to the group selector switch 400. The group of trunks occupying the lower section 301 of the same vertical row may lead to the group selectors ot the same or of a different terminal line. group as desired. Other groups outgoing from the district switches are distributed to other terminal line grou 'is. In addition to these groups of trunks. the district switches may have access to trunks extending in other directions.

The groups of trunks outgoing from the group selector 100 and from other group selectors in the same terminal group are distributed to a suitable mnnber of connector switches. ()ne of these connectors 500 is shown and a single trunk r22 is illustrated. The called subscribers lines appear in the connector switches in groups. For instance. the vertical rows 52% and each represents a group of subscribers lines. Each line is accessible over a group 0t horizontal links. The line 516 appears in the group occupying the vertical row 52? and is act-ssible by a group of links. two of which 501 and 502 are shown. A second line 526 in the same group is accessible to another group of horizontal links 5041, 505.

The district marker shown in Figs. 9 and lll includes two of testing relavs ltlllt'l and hill-i. The set llltlt'l serves to test the llJl' ".-1)lll':ll links of the section 302 in any district selector switch of this group and also to test the outgoing trunks which have access to these horizontal links. conncctiug d vice tone which comprises a. number ot ntultiwoniart relays 1015. 1016. etc. serves to ccuncct the. set of relavs lllllll with the proper group ol' outgoing trunks. the multicoutact relays each being individual to a tliti'ert-ut i'roup. iiihe he. the other set of the test relays too?) serves with re spect to the sct ot horizontal l nks 301 in all sw tches and the outgoing trunks to wh ch thee links have access. Also a con nccting 1 i it will consistin ol inulti-cou -ct relays ilk 5;. ltri l. lllltl. i020, 1025 and serve to t th nut 1(81' testing with the llll dual switches of the group. l?cla vs EH2 for intaierc. are in=livit ta to the particular dis trict swit h :ioa Elli line inarltcr s l wn in l 'igs. 11 and 12 ill) from the controllii sender. it also inhides the lllfltl t' act relay translating device 1201 whi h is operated according to the settings ol the registers to select the magnets 514, 515 individual to the di'l'- it "cut verti al rows of contacts in the connector switches in which subscribers lines terminate. Moreover. the line marker includes a inulti-contact relay translatn'ig device 12th; which serves to associate the re tcr 1201 with a connecting device 510 mar vidual to the proper connector switch 50!). The dev icc 51d. consisting of relays 511. 512. etc. serves to associate certain test relays with the horizontal links of the group having access to the called lines of the desired units dcs ignation. In Fig. 11 the set of testing relays oi the uiarljcr are associable through the nutlti-contact relay connecting devices ti l and hi2 with the groups ol' trunks outgoing llOlil the selector switch llld and other switches and with thc horizontal links ol' these group selectors.

onsider that the subscriber of line lot desires to hold conversation with the sub scriber o'l' line 516 appearing in the same central ollicc. It may also be assumed that the directory number of the line 510 is t 2 I? l. \Vhcn the calling subscriber 1ell1()\ eshis receiver from the switchhook. a circu t is closed from hatter) through the winding of line relay 121. inner contact 01 cut oil too Ill)

120, conductor 104. through the substation loop, conductor 1113, to ground lirough the outer contact 01 relay 120. Re-

ay 121 operates and a circuit is closed from iat'tery through the winding of relay 129, right baclt contact of relay 128, leiit contact oi relay 121. conductor 156 to ground.

Relays 125, 125.) and 131) enclosed within the broken rectangle pertain to the vertical group oi lines in the row ill) of the switch 1110. These relays are brought into action whenever one oi the lines oi the corresponding group initiates a call.

ltelay 121) completes a circuit 't'roni bat tery through the winding of relay 131. inner front contact of relay 129 to the grounded conductor lilo. llelay 1211 also closes a circuit l i1; oi relay 131), outer trout contact of relay 129, lei't: barlt contact of relay 135, outer right contact of relay 137, conductors ion and 15a to ground. llelay lili'l operates and closes a circuit for relay 12o. Itelay 128 locks through its left contact and the contacts 1 relays Ml and 132 to ground. Relay tilt), in operating. completes a circuit r in battery through its right hand winding and inner right contact. winding oi re lay .135, outer right contact 01" relay 1:17, con- ;luctor 158 to ground. llelay 1235 operates and closes a circuit. irom battery through the winding of slow-towelease relay ll-ltl, inner right. contact 01' relay 135, inner right back rontart of relay 137, coinluetor 158 to ground. llelay 1211i closes a substitute holding circuit for relays 1 1i and 1215.

"ith relay 135 operated a circuit is closed from battery through the winding of the yertical operating magnet, 1113. outer right trout contact of relay 1111), left front contact otrelav 13.). contact of relay 13113, conductor 13a to ground. The magnet lit} operates and prepares the contacts in the vertical row 1111. Magnet 1113 also closes a circuit from ground through its contact, conductor 155 inner eontact of relay 13-1, outer le'tt contact o1 relay litl. right contact ot relay 121, lett hand winding of the horizontal group relay 122 to battery. .telay 122 is a group relay indiyidual to the group of horizontal links including the links ltlti and 1 7. Similarly. relay 123 is a group relay individual to the other group of horizontal links illustrated.

Relay 122 operates and closes a circuit from battery through its right hand winding and right contact, right back contact of relay 123, winding of relay 131, conductor 15%) to ground. Relay 13-1 operates in this circuit. Another circuit is closed from battery through the winding of the vertical operating magnet 118. brush 114 of allotter switch 1153. contact of slow release relay 135), left contact oi relay 137. outer right contact of relay 135, conductor 138 to ground. It is assumed that the first one of the outgoing trunks 114 is idle and that the allottcr switch 1413 is standing in a position to allot this trunk as shown in the drawing. The allotter switch 143 has a position for each of the several trunks which lead out of the line switch 100 to the succeeding distriet selectors. Consequently, the vertical mag net 118 relative to the idle trunk 114 up crates and prepares the contacts in the ver tical row 112. Magnet 118, in operating, completes a circuit from battery through its winding and inner contact, conductor ltitl, winding oi relay 137, outer right contact ol' relay 135, conductor 158, to ground. Relay 137, however, being shunted by the circuit above traced does not operate this time. Magnet 118, furthermore, closes a circuit from ground through its outer contact, conductor 151, winding oi trunk relay 146 to battery. lvith relay 146 operated a circuit is completed t'roi'n battery through the winding of relay 139. winding and contact of stepping magnet 13b. brush 1 15, contact of relay 116 to ground over conductor 151. The magnet 13H operates, interrupts its own circuit and advances the brushes 14-1 and 14.3 to the next set of terminals. It the following trunk is idle then the allotter switch comes to rest, but if it is busy, the switch continues until an idle trunk is found. When the switch leaves the position shown it opens the shunt around relay 137 and this relay operates in series with nraguet 118. ltelay 137 opens the circuit of relay 1:11; which commences to deenergize. Relay 136 being slow, holds its armatures for an interval after the release of relay 137 which is suflicient for certain operations to take place.

During the interval mentioned a circuit is closed from ground over conductor 15kt, inner right front contact of relay 137, outer contact of relay 134, left back contact of relay 123, left front contact of relay 122. back contact of relay 127, assun'iing the tirst link 107 is idle, winding of magnet 125 to battery. Magnet 125 operates and etl'ects a connection between the link 107 and the incoming line and also between the link and the outgoing trunk 114. Immediately that. this connection is established, a circuit is closed from battery through the winding of magnet 125, winding of relay 127, contacts of the switch 100, over the sleeve conductor to ground at the outer contact o1 magnet 118. Relay 127 remains normal in this circuit: to prevent a false operation 01' a succeeding magnet as 121. Another circuit is closed from battery through the winding of cut-oil relay 120 over the sleeve eonductor to ground at the contact of magnet; 118. Relay 120 opens the circuit of relay 121, which in turn, opens the original energizing circuit of relay 129.

Should another line in the same vertical group attempt simultaneously to make a call while a subscriber in a succeeding vertical group is calling his attempt to make such a call will be unsuccessful. When the line relay 121 releases following the opening o 1' its circuit by relay 120, it opens the holding circuit of relay 131, but since the second subscriber in the same group is calling relay 131 is held energized over a parallel circuit through the left contact of the line relay similar to relay 121. Relay 131 in remaining energized preserves the locking circuit of relay 128 which prevents the rcenergization of relay 125). A second subscriber in the tirst group, or any group therefore, cannot succeed in making a call until all simultaneously waiting calls in subsequent groups have been handled. In this latter event a series circuit is closed for relay 132 through the back contacts of all relays similar to relay 120 and relay 132 operates to unlock relay 128.

At the instant relay 146 energizes the ground potential on conductor 151 is extended through the contact of this relay over conductor 150, through the back contact of relay 204, conductor 206, thence through the winding of relay 613. Relay 613 operates and locks through its winding and inner left contact, conductor 207, outer lett back contact of relay 201 to ground. Relay 613 completes a circuit from ground through its outer right contact, conductor 200, winding of slow-to-release relay 204 to battery. Relay 204 operates and applies holding ground to the conductor 150, whereby magnet 125 and relays 127, 120 and 146 are held energized when magnet 118 becomes decnergiied. Relay 61.3 operates similarly to the relay 120 of the line switch and brings the group relays 614, 615 and 616 and the control relays 619, 620, 621, 617, 622 and 623 into play to cause the extension of a connection through the sender selector switch 600 to the allotted idle register sender.

With relays 616 and 617 operated a circuit is closed from battery through the winding ol' vertical. magnet 629, contacts of relays 616 and 617, outer right contact of relay 623 to ground. h'liagnet 629 operates and brings about the energization of relay 612, which in turn, locks in a circuit including the relay 621. Relay 617 also closes a circuit trom battery through the winding of vert cal magnet 631. brush 627 of allotter 612 contact of slow relay 625, lett contact of relay 623. contact of relay 617 to ground. The magnet 631. operates and closes a looking circuit through its winding and contact and the winding of relay 623, the latter relay. however, remaining (leenergized due to the shunt around its winding. Magnet 631 also closes a circuit from ground through its outer contact, conductor 638,

through the brush 628, contact and winding of magnet 624, winding of relay 625 to battery. Magnet 624 steps the switch 626 forward to the set of terminals representing the nevt idle sender and in so doing removes the shunt from the winding of relay 623. Relay 623 opens the circuit of slow-to-release relay 622 which commences to deenergiye. During the interval that relay 622 is deenergzxing a circuit is closed from ground through the inner right front contact of relay 623, right contact of relay 621, left front contact of relay 612, front contact of relay 611, assuming the link 608 to be busy, back contact of relay 609, winding of the hori- Zontal magnet 610 to battery. Magnet 610 operates the switch and connects the incoming trunk cmiductors through the link 607 to the group of conductors 632 leading to the selected idle sender.

lnunediatcly that the connection is established a circuit is closed from battery through the winding of relay 201, conductor 210, contacts of switch 600, conductor 638 to ground at the contact of magnet 631. Relay 201 operates and opens the circuit of relay 613 which causes the deenergization of relays 612 and 619 provided no other sender is being sought through the same vertical group in the switch 600. Relay 201 also closes a holding circuit for the relay 204 which maintains a holding ground potential on the conductor 150. Furthermore, relay 201 connects the tip and ring conductors of the trunk through the sender selector switch to the impulse relay 700 of the sender. This results in the closure of a circuit from battery through the resistance 718, winding of relay 700, outer lett contact of relay 703, conductor 640, through contacts of the switch 600. conductor 641, outer right contact of relay 201, conductor 149, thence through the line switch 100 and the subscribers substation loop and returning over the other side of the line to conductor 148 to the inner right contact of relay 201, conductor 64-2, through the switch 600, conductor 643. inner left contact of relay 703. right hand winding of the balancing coil 701 to ground. Relay 700 operates and closes a circuit For slow-to-release relay 702. Relay 702 places ground on the holding con dnctor 720. This ground potential is trans mitted. by way of conductor 638 and results in a holding circuit for the magnet 610 and relays 600 and 201 in substitution of the circuit ten'iporarily maintained by the vertical operating magnet 631.

Following the series of operations just related slow relays 136 and 622 become deenergized. Relay 136 opens the circuits of relays 130 and 135. Relay 130 opens the circuit of magnet 116 and relay 135 opens the holding circuit of magnet 118 and relay 137. Magnet 116 causes the release of relays 134 and The release of relay 131 opens the shunt around the winding of relay 127 which operates in series with magnet 125. Vith the vertical magnets deenergized the connection is now held through the line switch 100 due to the maintained energization of relay 125 which, as above mentioned, is held in a circuit closed to ground over the sleeve conductor 150. Relay ($22, in releasing, similarly brings about a release of the corresponding elements in the sender selector switch 600 and the connection through this switch is now held by the magnet 610, the circuit of which is controlled by the slow relay 702 in the sender. The apparatus is now in condition for receiving the first series of impulses. The walling subscriber, consequently, manipulates his impulse transmitter 16 to send a single impulse repr senting the first digit of the wanted number.

At the time relay 702 operates it closes a circuit from battery through the Winding of relay 706, left back contact of relay 704, left contact of relay 705, conductor 720, to ground at the left contact of relay 702. Relay 702 also closes a circuit from battery through the resistance 73.), left hand winding of relay 707 to ground at the right contact of relay 702. relay 707, however, does not operate since its winding is shunted by the following path: battery, through the resistance 730, contact of relay 700 to ground at the right contact of relay 702.

In response to the first interruption of the line circuit by the calling subscriber, relay 700 releases and opens the shunt around the winding of relay 707 and this latter relay operates. Relay 707 closes a cir uit from battery through the winding of relay outer let't contact of relay 707 to the grounded conductor 72 Relay 70 t operates and locks in a circuit through its let". front contact, left contact of relay 70.) to ground on conductor 720. lelay 70l opens the original circuit of relay 706. but this latter relay remains energized in a circuit traceable through the outer right front contact of relay 707 to the grounded conductor 720. Relay 707 closes a circuit through the left hand winding of relay 708 and relay 708 operates and closes a circuit from battery through its right hand winding and inner right contact to the grounded conductor 720.

Relay 708, in operating completes a circuit from battery through its left hand winding and left contact, conductor 724', outer right back contacts of counting relays 715. 71-1, 713 and 712 in series, winding of 'elay 712, outer left back contact of relay 713, conductors 725 and 726, outer right con tact of relay 705, left front contact of relay 704, left con-tact of relay 705 to the grounded conductor 720. So long as rclay 707 re iains operated relay 712 is shunted and do s not receive sullicient current in the circuit traced to energize. At the. end of the impulse rclay 700 operates and shunts relay 707 and this relay releases, opening the original circuit through the left hand winding of relay 708. Relay 712 now operates in series with the left hand winding of relay 705%. Relay 712 locks in a circuit from battery over conductor 727, inner right contact and winding of relay 712, outer left back contact of relay 713 to ground over conductor 725.

It should be noted at this point that the armatures of the counting relay 712, which is also true of the remaining counting relays, so arranged that the inner right contact is closed slightly in advance of the opening of the outer right back contacts. By this arrangement the locking of the relay is certain to occur before the energizing circuit is ol'iened.

Immediately that the relay 712 opens its back contact the circuit through the left hand winding of relay 708 is opened. It is to be noted that the left hand winding of relay 708, when energized in series with relay 712, is sufiicient to overcome the difi'erential effect of the right hand winding and maintain relay 708 actuated. However, since relay 712 is so adjusted that its right inner front contact closes before the right outer back contact is opened, a shunt etlcct is produced on the left hand winding of relay 705. This shunt circuit may be traced from battery, conductor 727. right inner armature and contact of relay 712, right outer armature and back contacts of relays 712. 713. 711, and 715 in series, conductor 7:2-t and thence through the left winding of rclay 708 to battery. Since, lmwevcr, there would be an appreciable interval between the openii'ig of the ircuit through the lctl, winding of relay 708 and the complete do energization of said relay because of the self inductan e of the left hand ninding thereo f, the effect of the right win ling is to produce an immediate decncrgiaation and quick release of this relay when the llux produced by the right winding is dill'erentiated by the declining flux of the le ft l and winding.

After the usual interval has elapsed l'ollowing this ilnpulsc, slow-to-rclease relay 700 becomes deenergized and a circuit is completed from ground o cr conductor 720. contact of relay 700, right contact of relay 70 inner right contact of relay 705, cond uctor right back contact of relay 00H, winding of relay 807 to battery. Relay so? operates and closes a cir uit 'l'rom batter through its winding and inner upper con tact, thence over conductor 830, winding of relay 705, through contacts of relays 70l and 705 to ground. Relay 705, however, being shunted by the above circuit does not operate at this time. Relay 807 serves to con-

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