US2257894A - Telephone system - Google Patents

Description

Oct. 7, 1941. P. wlNsoR, JR

TELEPHONE SYS TEM 4 sheet'S-sheet 1 Filed Nov.A 2, 1958 4 Sheets-Sheet 2 P. WINSOR, JR TELEPHONE lSYSTEM Filed Nov. 2, 1938 pdfwlll" n mm. Wm.

Oct. 7, 1941.

/N VE N TOR P. W/NSOR JR. BV

A TTORNE V N @Px Oct. 7, .1941. l P. wlNsoR, JR

TELEPHONE SYS TEM Filed Nov. 2, 1938 4 Sheets-Sheet .3

, W QN n .nl

/NVENTOR RW/NSOR' JR.

Arron/vn oct. 7, 1941. P WINSO'R, JR 2,257,894

TELEPHONE SYSTEM File'd Nov. 2, 1938 4 Sheets-Sheet 4 III-Ill j f WH"- l L .m M1 K l d" e :l

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/NVENTOR P. W/NSOR JR.

Afro/wr Patented Oct. 7, 1941 f.

Paul Winsor, Jr.,'Lincoln Park, N. J., assignor to Bell TelephoneLaboraItories, Incorporated, :New York, N. Y., a corporation of New York Application November 2, 1938,' serial No. 238,368

c claims. rc1.- 179-18) l Thisinvention relates to vtelephone systems and particularlyV to systems comprising automatic switches.

It is -the object of the invention to increase the reliability f and increase the life of automatic switchesin telephone or other'communication and signaling systems, to reducey the cost of maintenance, and to prevent faultyv operation and the disablement of theswitching mecha- 1n-ism of such systems.

According to this invention each of the trunk, selector or connector circuits connected to the banks of an automatic switch comprising a set of brushes and a bank of terminals is arranged soy asr to eliminate or' minimize the normally eX'- is-ting difference in potential between adjacent terminals in the bank. ByV eliminating or reducingthis difference in potential, leakage currents are minimizedand possible adverse effects on .the insulation are prevented.

As applied to outgoing repeater circuits such, for instance, as are used in systemsl comprising switches of the well-knownstep-bystep type, the invention is a circuit arrangement whereby .battery is normally connected through one wind'- ing of the line relay to one of the two terminals of'a set tol which the talking conductors of the circuit are connected, Land the other talking terminal of this.' set is connected through another winding of the line relay to the sleeve or test i,

terminal. oi the set, there being neither battery nor ground potential connected to the last-mentioned winding until the circuit is seized. Upon seizure of the circuit, the usual temporary connecti'on of ground potential to the test terminal by the seizing switch to prevent seizureby an*- other switch, is also effective to` complete the ircuit'ior operating the line relay of the seized circuit.

winding of the linelrel'ay tothe other of the two i talking terminals.A According to this `feature all oi the terminals, ora set to which a selector. connector orrepeater circuit is connected, are normally at the samev battery potential.`

According to a iurther'feature of the1 invenalf@ i, minals.

` tion, thev electrostatic vshields 'separating vadj acent levels of the bank,comprising` the talking terminals areoconnected through a high resistanceto battery potential so -as to minimize or eliminate any difference in potential between the talking terminals in adjacent levels.

For a furtherand ,completedescription Vof the invention reference will be made to the drawings which show schematically a.y portion of an `automatic system in whichthe various Afeatures of the invention are embodied. The inyention is not limited to the particular system or type of switches shown in the drawings. but is in general applicable to all automatic switching .sys-

tems and to all switches comprising a,v set vof wipers and a terminal bank. Referring yto the drawings: 5 f

Fig. `1 shows acalling subscribers station A and line L|,a line inder LF and anassociated first selector SI, .and a second` selector S2; Fig. 2 shows anoutgoing repeater circuit OR anoltrunkT;v f lFig. 3 showsan incomingor third selector S3 land a fourth selector S4; .and

Fig-i shows a connector C and acalled subscribers line'LZ and station B. v

Each of the subscribers stations'A and Bis 0i the usual .typeY employed inautomatic .telephone systemsand is equippedv with a. dial or other impulse sender for/.usein controlling Vthe operation of eachof the selector and-connector switches involved in establishing connection .with acalledsubscribers line. A line circuit, compri-sing vvtheusual lineV and cut-oi relay, is asi sociated with each of lines LI and. L2, but'isjnot shown in the drawings.- Y

lThe Vline-finder switch yelector switches vSMS2, S3 and S4 and connector switch C areof Ythe well-known two-motion step-by-step type.

. -Adescription of the structure. of such switches -is liound in Automatic ITelephony by Smith and Campbell, Zndfedi-tion. -Each of the line-finder, sel-ector'and connector switches is represented by a set of brushesand one orfmore sets of ter- All of the relays and other circuit apparatus associated with the line iinder LF have been omitted; but a description of two-motion line-iinder switches isto be'found in the' patent to R. Lr'Stokelyn No. 1,799,654 granted April '7, 1.9731. The line, release' and-cut-through-relays of the'rst selectorv Si are shown but thev other circuit apparatusv associated with switch S-I has been omitted. Reference -may be `hadto .A-utof matic Telephony,aforementioned.for a complete description ofthe operation ofthe selector SI.

'Ihe bank of the selector SI differs from the usual bank assembly in that the electrostatic shields 20 separating the tip and ring terminals in each level from the ring and tip terminals in adjacent levels are connected through a high resistance to battery instead of being connected to ground.

The selector, repeater and connector circuits S2, OR. S4 and C, which are shown in detail in the drawings, differ from similar circuits heretofore known in that they are arranged in accordance with the various features of the invention to prevent deterioration of the insulation in the banks of switches having access to these circuits. The selector S2 is similar to the usual selector circuit in that battery is normally connected through one of the windings of its line relay 3| to the ring terminal 22 in the bank of the preceding selector SI; but, instead of ground being normally connected through the other winding of the line relay to the tip terminal 2| in the bank of the selector SI and instead of the associated sleeve terminal 23 of selector SI being normally connected neither to battery nor ground potential, the sleeve terminal 23 is connected through the upper winding of relay 3| to the tip terminal 2|, and the sleeve terminal 23 is further connected through a high resistance 30 to battery. Thus, normally, the tip, ring and sleeve terminals 2|, 22 and 23 are all normally connected to battery potential and, therefore, there is no leakage current through the insulation separating-the tip and ring terminals. Since the sleeve terminals are in a separate bank unit from the tip and ring terminals, the high resistance connection of the sleeve terminal to battery may be omitted; and in this case, with the sleeve terminal 23 connected through the upper winding of relay 3| to the tip terminal 2|, the terminals 2I and 22 are normally free of both ground and battery potential. However, the most complete protection is the normal connection of battery potential to the tip, ring and sleeve terminals together with the connection of battery potential to the electrostatic shields between the tip and ring terminals of adjacent levels.

The outgoing repeater OR differs from the usual circuit arrangement in that the upper winding of the line relay 5I is connected to the test conductor 59 instead of being connected to ground potential, so that normally the tip terminal of the sets of terminals to which the repeater is connected is normally connected neither to battery nor to ground potential. In this case the electrostatic shields 40 between the tip and ring terminals of adjacent levels in the bank of selector S2 may be connected to ground potential in usual manner as shown in Fig. 1. A high resistance may be added, however, for normally connecting battery potential to both the tip and test terminals, in which case the electrostatic shields may also be connected through a high resistance to battery potential.

The selector S4 differs from the usual selector arrangement in that the additional relay 16 is provided for normally disconnecting the windings of line relay 1| from the tip and ring terminals in the bank of selector S3. And'the connector C differs from theusual connector arrangement in that the sleeve or test terminals in the banks of selectors having access thereto are connected through one of the windings of the battery being connected through the other winding of relay 9| to the ring terminals inthe banks of these selectors. While not so shown in the drawing, a high resistance may be provided for connecting battery to the sleeve conductor; so that all three terminals of each set to which the connector C is connected are normally at the same potential.

To-facilitate further vexplanation of the invention, the operation of these circuits will be described in establishing a connection between the calling station A and called station B.

When the call is originated at station A, va line nder LF hunts for and connects the line LI to the associated rst selector SI. In responseV to the dialing of the rst digit of the called subscribers number, the selector SI is operated to select the level corresponding to the digit dialed and to hunt for and seize the rst idle second selector S2 in the selected group, the "cut-through relay I3 of selector SI being operated to extend the connection from line LI through brushes I1 and I8 in usual and wellknown manner. The operation of relay I3 disconnects the windings of the line relay II of selector S| from the calling line, and relays II and I2- are thereby successively released. But since relay I2 is slow in releasing, ground potential is temporarily connected through front contacts of relays I2 and I3 to the sleeve brush I9 and test terminal 23 to guard the seized selector S2 against seizure by another first selector before the guarding ground is supplied by the selector S2 as hereinafter described.

The connection of ground to brush |9 and terminal 23 by the operation of relay I3 of selector SI completes a circuit for operating line relay 3| of selector S2. This circuit is traced from battery through the lower winding of relay 3|, a back contact of relay 34, terminal 22, brush I8, a front contact of relay I3, middle brush of line iinder LF, one of the conductors of line Ll, through the calling station A, back through the other conductor of line Ll, upper brush of line nder LF, another front contact of relay I3, brush I1, terminal 2l, a back contact of relay 34, upper winding of relay 3|, normally closed conltact of the 11th rotary step springs, thence through test terminal 23, brush I9 and the front contact of relay I3 to ground at the front contact of relay I2. The operation of relay 3| closes a circuit for operating the slow-to-release relay 32. Relay 32 connects ground through sleeve terminal 23 and brush I9 to hold the selector SI and line-finder LF in operated position after relay I2 of selector SI releases. When the calling subscriber dials the second digit of a called sub- 'scribers number, relay 3| is alternately released and reoperated in response to each dial impulse. The first release of relay 3| closes a circuit for operating the vertical stepping magnet 31 and relay 33 in series. The operation of magnet 31 steps the brushes up to the first level of the bank. When the shaft and brushes move out of normal position the vertical off-normal springs 36 are actuated thereby closing a circuit including a. front contact of relay 33 for operating relay 35. Relay 35 locks through the back contact of the rotary stepping magnet 38. When relay 3| reoperates, magnet 31 releases. Each succeeding release and reoperation of relay 3| causes the reoperation of magnet 31 to step the brushes to the next level. Since relay 32 is slow in releasing the release of relay 3| in response to dial impulses ls ineffective to cause the release of relay 32. Relay 33 is also slow in releasing and remains operated until relayv3 l has received alljof the-impulses inV the` train createdby the dialing of thefirst digit. When allof theimpulsesin the-train have been` received'and the ybrushes advanced to--select the desired level, the'- release of relay 33 Vclosesa circuit for operating the rotary stepping magnet 38 thereby advancing the brushes into engage ment with the first set of terminalsr inthe selected level. The operation of magnet 38 causes the release of relay 35 and the release o f relayl 35 causes the release of magnet 38. If this rst'set of terminals connects to a busythird selector or to a= busy outgoing repeater, the sleeve? terminal isA marked by ground potential land as soon., as magnet 3.8releases the winding of relay 35is connected to brush 43 to cause thev reoperationvof relay 35 and magnet38 and thus advancethe brushes to the next set of terminals. Thllsrelay 35 and magnet 38 `control each other to conti-nue the advance of the brushes until a setof vter-- minals connecting to an idle selector orrepeater is reachedand since the terminal then engaged byY brush 43v is not connected toground, relay 34 is operated in a circuit which includes the wind.- ing of relay 35. Relay 35 is marginal and does not reoperate in series with the winding of relay 34. Relay 34 disconnects the windings of relay 3-l from the calling line and connects the calling line to the selected set of terminals, whichwe will assume to be terminals'45, 4B and 41 connected to the outgoing repeater OR. The release of relay 3 l causes the release of relay 32 but relay 32 is slow in releasing so4 that it` does not disconnect the holding ground potential from terminal 23 until a holding ground potential has been connected to terminal 41 bythe seized repeater OR as hereinafter described.4 Before relay 32 releases, the ground connected to terminal 23 is further connected through aront contact of relay 34 to brush-43 to guard the repeater OR against seizure by another 'selector inthe group having access to this repeater.

Upon seizure of repeater OR and temporary connection ofground throughA brush- 43a toterminal 41, relay is operatively energized bythe current in a circuit traced froml battery through the lower winding `of relay 5l, a back Contact of relay 54, terminal 46, brush 42, a lower front centact of relay 34 thence through selectorSl, line inder LF and the calling line LI as, above described, then through an upper front contact of relay 34, brush 4I, terminal 45:, a back contact of relay 54, upper winding of relay: 5|, test conductor 59, test terminal 41:, brush 43, fron-t contact of relay 34v to groundat affront Contact of relayv 32. Relay 5i! closes a: circuit for operating the' slow-to-release relay 52. Relay Fa-connects ground to conductor 59 and terminal e41-tohold rela-y 5l operated in series withl the calling` line, te hold relay 34 of selector S2 and to `hold selec,- tor Sl and line finderLE1v inusual manner after relay 32 of selector SI releases The operation of relays 5l; and 52 closesva'bridge comprisingthe lower winding of relay 55 andupper winding of relay teliv across the `conductors of trunk T to cause the operation ofthe line relay 5,1 Vof the incoming third selector S3 in the called cflice. Relay 52 also closes a circuit for energizing the lower, biasing winding of polarized relay 55; and the current through the upper winding of relay 56 is at this time in the direction which maintains the armature in its normal position. Relay 55 may or may not be operatedv by the energization of its lower winding alone. When lthe' calling subscriber dials each of the remaining digits 'of the called; subscribers nmnber,;.re1ay 5I: alter'- nately; released 'andUreoperated` in respmisetc eachimpulse. The release of relay-v 5l in'. respense: tothe rst impulse of each train causes the'roperation *ofv relay 53 and being slcvv4 iny releasing relay: 53- remains operated untill all` of; the impulses-in the train have been received-. While operated, relay 53 short-circuits the wind-ingsrof relays 55 and 56 to insure proper response of the line relays of the succeeding switchesl to which the impulses arebeing repeated. Each release and reoperation of relay 5I opens vand closes the bridge across the trunk T thereby repeating the impulses ofeach train in succession to the. line relay of the next switch through which the d esired connection is being established.

Assuming that the trunk T is kconnected to the i incoming third selector S3 in the called oice.,

closure-of the bridge across trunk T upon seizure of repeater OR causes the operation of the line relay El of selector S3. Relay 6L closes a circuit for operating the slow-to-release relay-62,. The rst train of dial impulses repeated by relayy 5.1 'of repeater OR is received by relay 6l and effects the advance of'.y the brushes of selector S3.tose1ect a desired level. The-brushes are thereafter ad,- vanced toselect and seize an idle set of terminals in the selected level and the cut-throughv relay 63: is thereupon operated to extend the connec- "tion tov a fourth selector S4 which isv connected `to the seized terminals; The operation of; the cut-through relay 63gcauses the successive`y release-of yrelays t1 and 62; and, until relay,A 62, re-v leases, ground is connected throughja front-contact'of relay53 and the test brush of selector S3 to the test terminal 39to temporarily guard selector S4 against seizure by another vthird selector. VThe temporary connection oi-ground potenti-al to terminal 39 causes the operationof relay-16 of "selector S4. Relay 1liy connects the windings` of line relay 1i in serieswith windings of krelaysj55 and 56 of repeater OR; and relayr1! operates, closing acircuit for operating the slow-to-release vrelay 1:2. -Relay 12 connects holding lground potential to terminal to Vhold selector S3 after relay 62 releases. The vertical stepping magnet 11 of selector S4 is thereupon operatedfby .the responseA of relay 1! to the neX-t train of dialimr pulses received and( repeated byrelay 5I of repeater OR to select a desired group. The opera;- tion of relay 13 in series with magnet 1-1 and actuation of the vertical ofi-normal contacts 89 closes a circuitfor operating relay- 15. When relay 13 releases after all of the impulses inthe trai-n have been received by relay 1l the .rotary stepping magnet 18 is operatedto advance the brushes into engagement with the rst set oi terminals in the selected level. If the rst set of terminals is-busy. the magnet 18 is thereupon controlled by stepping relay 1-5 to advance'the brushes to hunt for and seize the first idle set rof terminals inthe selected level. Upon finding an idle set ofterminals, relay 14 is operated to extend the connection to the connector C which is connected to the seized set of terminals.- The operationv of relay 14 temporarily connects ground from the front contactV of relay y:1 2 through brush 83 to the test terminal to` prevent seizure of connector C'by another selector. The y'operation of relayv 14 Valso causes the release 0I relays 1l and 12. Before relay 12 releases,` holding; ground potential is connected to conductor -89 and sleeve brush 83, as hereinafter described, l to hold selectors S3 and S4.

When ground isml@.stealA throughA freir-.tA con?.

tacts of relays 12 and 14 and brush 83 to sleeve conductor 89, relay 9| operates in series with the windings of relays 55 and 56 of repeater OR. Relay 9| closes a circuit for operating slow-torelease relay 92. Relay 92 connects ground to conductor 89 to hold relay 9| and to hold the preceding switches S3 and S4 in usual manner after relay 12 of selector S4 releases. Each of the'impulses created by the dialing of the next to the last digit of the called subscribers number is repeated by relay 5| of repeater OR to cause the alternate release and reoperation of relay 9|. The rst release of relay 9| closes a circuit for operating relay 93 and the vertical stepping magnet in series. The operation of magnet |0| steps the brushes up to the rst level of the bank. When relayl9| reoperates magnet |0| releases but relay 93 is slow in releasing and remains operated until all of the impulses in the train have been received by relay 9|. The vertical OIT-normal springs |99 are actuated as soon as the shaft and brushes move out of normal position, the circuit for reoperating magnet |9| being thereafter traced through the front contact of relay 93 and a front contact of springs |00 instead of through a back contact of springs |00.Y Being slow in releasing, relay 92 does not release during the response of relay 9| to dial impulses. Each succeeding release and reoperation of relay 9| due to impulses corresponding to the next tothe last digit dialed, causes the brushes to be raised one step thereby selecting the level corresponding to the tens digit of the called number. When relay 93 releases at the end of the impulses of the tends digit, and relay 9| responds to the rst impulse of the units digit ol the called number,rtherotary stepping magnet v|02 is operated in parallel with relay 95. Relay 95 connects its winding and the winding of the rotary stepping magnet to the impulse contact of relay 9| independent of relay 91 and connects the winding of busy test relay 91 to test brush |09. The operation of magnet '|92 advances the brushes one step into engagement with the rst set of terminals in the selected level. When relay 9| reoperates at the end of the first impulse of the units digit, magnet |92 releases but relay 95 is slow in releasing and remains operated until all of the impulses have been received by relay 9|.

Each succeeding release and reoperation of relay 9| causes the operation and release of magnet |02 to advance the brushes one step. The brushes are thereby advanced into engagement with the terminals of the called line. If this line is busy, the sleeveterminal engaged by brush |09 will be connected to ground in usual manner and relay 91 operates before relay 95 releases. In this case relay 91 is locked operate-d, after relay 95 releases, to ground at a front contact of relay 92 and the busy tone source BT is connected through a talking condenser 90 to the ring conductor 88 to transmit a busy tone to the calling station. If, however, the called line is idle, relay 91 does not operate and when relay 95 releases after the last dial impulse has been received, relay v98 is operated by the energization of its upper winding in series with brush |09 and the cut-off relay (not shown) of the called line. Relay 98 locks through its lower winding under control or relay 92, connects ground to brush |09 to hold the cut-off relay of the called line, and closes a ringing circuit from source through the upper winding and lower back contact of relay 96, inner lower front contact of relay 99, brush |08, called subscribers line and through the ringer (not shown) at station B, brush |01, inner` upper front contact of relay 98, to ground at a back contact of relay 96. When the called subscriber answers the call, the current through the upper winding of relay 96 is increased suiiciently to cause the operation of this relay. Relay 96 locks under control of relay 92, opens the ringing circuit and closes the talking circuit, the windings of relay 94 being connected in series with the talking battery to line L2. Relay 94 operates interchanging the connections between conductors 81 and 88 and the windingsof relay 9| so that the current through the windings of relays 55 and 56 of repeater OR is 'reversed and polarized relayV 56 operates. Relay 56 closes a circuit for operating relay 54. Relay 54 includes both windings of relay 55 in the loop across trunk T and interchanges the connections between the calling line and the windings of relay 5|' to reverse the current over the calling line as an answering supervisory signal. The switches LF, Si and S2 in the originating oflice are held by ground from the front contact of relay 52 of repeater OR, the switches S3 and S4 are held under control of relay 92, and the connector switch C is held under the control of relay 92 and also under the control of relay 94.

When the called subscribed replaces the receiver, relay 94 releases causing a reversal of current over trunk T to release relays 55 and 54 of repeater OR.' When the calling subscriber replaces the receiver, relay 5| releases thereby causing the release of relay 9| and the release of relays 55 and 56 if operated. The release of relay 5| causes the release of relay 52; and the release of relay 52 disconnects ground from sleeve terminal 41 thereby causing the release of relays 34 and |3 and the return of the line finder LF to normal in usual manner.

The release of relay |3 causes the return of 'selector Sl to normal in usual manner. The release of relay 34 closes a circuit for operating release magnet 39`and selector S2 is thereupon restored to normal; The release of relay 9| causes the release of relay 92, disconnecting ground from conductor 89 to cause the release of relays 16 and 14 and the return of selector S3 to normal in usual manner. The release of relay 14 closes the circuit for operating release magnet 19 and selector S4 is restored to normal. With both of relays 92 and 94 released, relays 96 and 98 release and release magnet |03 operates restoring connector C to normal.

What is claimed is:`

1. In a telephone system, a switch comprising a set of wipers and two banks of terminals, two of the wipers being adapted to engage the terminals in one bank and a test wiper being adapted to engage the terminals in the other bank, each `of said banks comprising a plurality of levels, means for operating said switch to select an idle set 0f terminals and for connecting ground potential to the test terminal of the selected set, a relay individual to the selected set of terminals, a grounded battery, the nongrounded terminal of vsaid battery being normally connected through one winding of said relay to one of the terminals of said selected set, another terminal of said selected set being normally connected through the other winding of said relay to the test terminal of said set, electrostatic shields between the adjacent layers of insulation of adjacent levels of said one bank, and a high resistance connecting the nongrounded terminal of the battery to each of said shields.

2. In a telephone system, a switch comprising a set of wipers and two banks of terminals, two oi the wipers being adapted to engage the terminals in one bank and a test wiper being adapted to engage the terminals in the other bank, each of said banks comprising a plurality of levels, means for operating said switch to select an idle set of terminals and for connecting ground potential to the test terminal of the selected set, a relay individual to the selected set of terminals, a grounded battery, the nongrounded terminal of said battery being normally connected through one winding of said relay to one of the terminals of said selected set, another terminal of said selected set being normally connected through lthe other winding of said relay to the test terminal of said set, a high resistance normally connecting the non-grounded terminal of the battery to said test terminal, said nongrounded terminal of the battery also being connected through said high resistance and said other winding of said relay in series to said other terminal of the selected set, electrostatic shields between the adjacent layers of insulation of adjacent levels of said one bank, and a high resistance connecting the non-grounded terminal of the battery to each of said shields.

3. In combination in a switching system, a battery, a switch comprising a set of wipers and a bank of terminals, means for moving said wipers into engagement with any set of terminals in said bank, repeaters, each of said repeaters being connected to a different one of said sets of terminals, means associated with said switch and including the test wiper of said set of wipers for testing an engaged set of terminals and for seizing an idle set of terminals, aline relay for each of said repeaters, one terminal of each of said sets of terminals being normally connected through one winding of the line relay of the associated repeater to one pole of said battery, another terminal of each of said sets being normally connected to one end of another winding of the line relay of the associated repeater, and the test terminal of each of said sets being normally connected to the other end of the said other winding of the 'line relay of the associated repeater, and a circuit closed upon seizure of any of said sets of terminals by said switch for operating the line relay of the repeater connected to the seized terminals, said circuit including said battery, both windings of said relay, the terminal to which said one winding of the line relay is normally connected and including the test terminal to which the other winding of the line relay is normally connected.

4. In a telephone system, a switch comprising a set of wipers and a bank of terminals having a plurality of levels, means for operating said switch to select a desired level and for operating said switch to select an idle set of terminals in the selected level, means effective upon seizure of a set of terminals to connect ground potential to the test terminal, a relay individual to the selected set of terminals, a grounded battery, the

non-grounded terminal of said battery being normally connected through one winding of said relay to one of the terminals of said seized set, another terminal of said seized set being normallyi connected through the other winding of said relay to said test terminal, electrostatic shields between adjacent layers of insulation of adjacent levels of said bank, and a high resistance connecting the non-grounded terminal of the battery to each of said shields.

5. In combination, a switch having a bank of terminals, means for operating said switch to select a set of terminals in said bank, means eiective upon selection of a set of terminals for connecting ground potential to th'e test terminal of the selected set, a relay individual to said selected set of terminals, a grounded battery, one terminal of said selected set being normally connected through one winding of said relay to the non-grounded pole of said battery, another terminal of said selected set being normally connected through another winding of said relay to the test terminal of said set, and means comprising a high resistance element normally connecting the non-grounded pole of said battery to said test terminal and also to and through said other winding of said relay to said other terminal of the selected set.

6. In combination in a switching system, a battery, a switch comprising a set of Wipers and a bank of terminals, means for moving said wipers into engagement with any set of terminals in said bank, selectors, each of said selectors being connected to a diierent one of said sets of terminals, means associated with said switch and including the test wiper of said set of wipers for testing an engaged set of terminals and for seizing an idle set of terminals, a line relay for each of said selectors, one terminal of each of said sets of terminals being normally connected through one winding of the line relay of the associated selector to one pole of said battery, another terminal of each of said sets being normally connected to one end of another winding of the line relay of the associated selector, and the test terminal of each of said sets being normally connected to the other end of the said other winding of the line relay of the associated selector, a. circuit closed upon seizure of any of said sets of terminals by said switch for operating the line relay of the selector connected to the seized terminals, said circuit including said battery, both windings of said relay, the terminal to which said one winding of the line relay is normally connected and including said other terminal and the test terminal to which the other winding of the line relay is normally connected, a high resistance element for each of said selectors normally connecting the non-grounded pole of said battery to the test terminal of the set to which the selector is connected and also normally connecting said pole of the battery to and through said other winding of the line relay of said selector to said other terminal of the set to which said selector is connected.

PAUL WINSOR, JR.

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