US1898063A

US1898063A - Telephone system

Info

Publication number: US1898063A
Application number: US464156A
Authority: US
Inventors: Rodney G Richardson
Original assignee: Associated Electric Laboratories Inc
Current assignee: Associated Electric Laboratories Inc
Priority date: 1930-06-27
Filing date: 1930-06-27
Publication date: 1933-02-21
Anticipated expiration: 1950-02-21
Also published as: FR717671A

Description

Feb. 21, 1933. R. e. RICHARDSON TELEPHONE SYSTEM 3 Sheets$heet 1 Original Filed June 27, 1930 Inuen or Rnclne E Richardsun QQP r H! 5 mm: im BIA. 55 SQ W8 T|.l||.

IA.-- IIIIJ 7 May Feb. 21,' 1933. R. G. RICHARDSON TELEPHONE SYSTEM 3 Sheets-Sheet 2 Original Filed June 27, 1930 xi m2: BF

lllllu vll i Imreniczr Hudn Er Richardson VIII I Feb. 21, 1933- R. G. RICHARDSON TELEPHONE SYS TEM Original Filed June 27, 1930 3 Sheets-Sheet NED-h [I I. l nd SEWWNF J MR h W m m 1 v! luvliluvliilfi Tn ill fl Q, n 3n QM M kl. m SE38 \\h H 0 NR a 965 5 fin an w n i4 I n U n u an v VII Q "F lllflVuI III m y ukk 'llll W ma v lla v in vlilu H d In. NW} NWNR "I! 1 nllll Nu m MP Nhw I V 3 3* -I -w I QR NQ MN v I N w ENQSE 383% mw fl Patented Feb. 21, 1933 UNITED STATES PATENT OFFICE RODNEY G. RICHARDSON, OF CHICAGO, ILLINOIS, ASSIGNOR- BY MESNE ASSIGNMENTS,

T ASSOCIATED ELECTRIC LABORATORIES, INC., 0F CHICA 0, ILLINOIS, A CORPO- BATION OE DELAWARE TELEPHONE SYSTEM Application filed June 27, 1930, Serial No. 464,156. Renewed August 18, 1932.

impulses transmitted over a calling telephone line in an automatic telephone system are responded to by a line relay connected in series with the line. Owing to the widely differing conditions (including differences in re- 3 5 sistance and in insulation amongst the various lines connected to an automatic telephone exchange) it is diilicult to maintain the adjustment of the line relay of an automatic switch so that it will follow faithfully, and without J undue distortion, the impulses received from all lines. if the line relay is rendered too sensitive in orderto enable it to respond satisfactorily to the impulses received over long lines, the relay tends to remain operated for too high a percentage of the time when impulses are being received from short lines. Accordingly, it is proposed to interpose a vacuum tube between the line and the automatic switch in place of the line relay hereto 3 fore employed, as it is observed that substantially the same plate-current variation may be secured in a vacuum tube under a wide range of changes of the grid potential. Accordingly, in the system disclosed herein, the grid element of a vacuum tubeis connected to an incoming line conductor of an automatic switch to receive the incoming impulses, while the operating magnets of the switch are controlled in accordance with the plate current 3 of the vacuum tube, which is caused to start and stop underthe control of the incoming impulses received by the grid element of the vacuum tube.

According to one method of carrying out the invention, an impulse-receiving vacuum tube is placed at each automatic switch and the operating magnet-s of the switch are connected directly in the plate circuit of the vacuum tube, while according to another method of carrying out the invention, vacuum-tube repeaters are interposed between the calling telephone lines and the automatic switches to intercept all of the incoming impulses and to repeat them to the various automatic switches by means of the usual line relays in the automatic switches. The line relays used in the automatic switches are thus enabled to respond satisfactorily as they are always operating under substantially the same conditions, and are not subjected to the widely varying line conditions as is the case when they are connected directly to the calling lines.

t is a further object of the invention to provide a vacuum-tube type of impulse repeater which may be introduced in a standard automatic telephone system as an outgoing repeater in place of the existing outgoing re peaters employing line relays. This repeater is useful in multi-oifice systems wherein the line conditions are not too severe, so that the line relays respond satisfactorily when connected directly to the line, but do not respond so satisfactorily when operated by the electromagnetic impulse repeaters on interoflice calls, often involving long trunk lines.

Other objects and features of the invention have to do with the successful solution of the problems arising in connection with the application of the vacuum tubes to the specific circuits. These include the light ing and extinguishing of the filament circuits of the vacuum tubes; the securing of the correct normal grid potentials for the vacuum tubes; and the provision of suitable circuits for enabling standard supervision to be carried out.

Referring now to the drawings comprising Figs. 1 to 4,they show by means of the usual circuit diagrams a sufiicient amount of apparatus in a telephone system embodying the features of the invention to enable the invention to be understood.

Figs. 1 and 2 taken together show the switching apparatus and controlling appara tus involved in setting up a connection from a calling substation A1 in one office to a called substation B1 in another office. Of this apparatus, the l1nesw1tch LS1 may be a regular rotary l1nesw1tcl1 such as is commonly used in automatic telephone systems; the

office selector 0S1 is assumed to be a vertical and rotary selector having bank contacts arranged in the usual horizontal rows or levels; the thousands and hundreds selectors ThSl and HSl may be mechanically and electrically similar to the oiiice selector CS1, and the connector C1 may be of the same type of vertical and rotary mechanism. In order to lower the resistance of the circuit over which talking current is supplied to the calling line on the interotfice call, and to avoid the use of a third conductor in the interoifice connection, and to afford reliable interoflice impulsing, the repeater R1 is interposed between the office selector CS1 and the outgoing trunk line comprising conductors 141 and 142 leading to the distant ollice and terminating in the incoming thousands selector ThSl.

Fig. 3 shows apparatus in a multi-oiiice system similar to the system of Figs. 1 and 2 except that the vacuum-tube impulse repeater R2 is inserted in the trunk line between the lineswitch LS2 and the otiice selector 0S2, all of the switching apparatus being of standard construction and provided with the usual line relays such as the line relay 331 of the ollic-e selector CS2. The selectors 0S2, T7LS2, and HSQ may be similar to the selectors shown and desc'ibed in the patent to Stehlilr, 1,729,803, granted October 1, 1929; the connector C2 may be similar to the connector C in the Stehlik patent, and the lineswitohes LS1 and LS2 shown herein may be similar to the lineswitch LS of the Stehlik patent.

The vacuum-tube impulse repeater shown in Fig. 4 repeats the impulses directly from the incoming line to the outgoing line without the interposition of a repeating relay, and it may be used preceding a first selector as in Fig. 3, or it may be used as an outgoing repeater as in the case of the repeater R1, Fig. 1. Like the repeater B2 in 3, the repeater R3 in Fig. 4 is arranged to repeat impulses to standard switches employing line relays.

Detailed description; F 7198. J and 2 Referring now particularly to Figs. 1 and 2, adetailed description of the operation of the apparatus shown therein will now be given. For this purpose it will be assumed that the subscriber at substation A1 in the first office desires to converse with the subscriber at substation B1 in the second ofiice. To do this, the subscri oer at substation A will remove his receiver from the switchhook and dial the digits in th number of the subscriber substation B1.

When the receiver is removed at substation Al, the lineswitch LS1 operates over

line conductors

2 and 3 to extend a connection to an idle first, or oifice, selector. Assuming that the oltice selector CS1 is the first one found to be idle, a connection is made thereto by wipers 101-103 of the lineswitch LS1 by way of conductors 10410G. When this occurs, release relay 107, whose lower winding is connected between ground and conductor 106, in series with resistance 116, operates over the calling line and over conductor 104 which is supplied with a negative potential from the exchange battery through resistance 123. Upon operating, release relay 107 places a ground potential on release trunk conductor 105 at its upper armature so as to maintain the selector guarded against other lineswitches, and to close the usual holding circuit (not shown) for the lineswitch LS1. At its lower armature, release relay 107 closes a lighting circuit for the filament of the tube 121 through resistance 122 and through the inner-lower armature of relay 110 from the regular exchange battery. At the normally-closed contacts controlled by the lower armature of relay 107, a point in the circuit of release magnet 115 is opened to prevent premature operation of the release magnet. Responsive to the filament on the tube 121 becoming lighted when acircuit is closed for it by relay 107 through resistance 122, current tends to flow from the filament of the tube to the plate element thereof, and through relay 108 and vertical magnet 113, owing to the positive potential imposed on the plate element of the tube by the special plate battery associated with vertical magnet 113. This current flow will beheld to a minimum value 3 or prevented altogether (depending upon the resistance of the line of substation A) owing to the negative potential from the exchange battery which is impressed on the grid element of the tube 121 through the calling line. The relatively high resistance 116 is inserted in the circuit between the lower winding of release relay 107 and ground so as to maintain the grid element of the tube at a relatively high negative potential.

It will be noted that, when the lighting circuit is closed for the filament of the tube 121, a parallel circuit is closed through contacts of vertical magnet 118 for the upper winding of relay 107, strongly energizing the relay so as to insure that it will remain operated during the transmission of a series of impulses. By this arrangement, the line current needs to be only suiiicient to operate the relay far enough to close the filament circuit, whereupon the circuit through the upper winding becomes effective to operate the relay the remaining distance. The upper winding of relay 107 is shunted by the resistance 117 so as to enable alocal circulating current to flow after the circuit of the upper winding has been opened, rendering the relay slow to release.

At this time it may be pointed out that the exchange battery usually has a potential i lay 108.

is momentarily opened.

of from e6 to 50 volts, while the vacuum tube 121 may require potential of from three to seven volts, depending upon the tube used. Accordingly, the average potential of the filament or" the tube 121 r. n a few volts of ground poten l Vhen the calling device 51.1.1 suhstaeio operated in accordance with the i in the desired number, the line is opened a corresponding number of times. Each time the line circuit is opened, and upon the removal of the negative potentialapplied over the line from conductor 10% above mentioned, the grid elen'ient of tube 121 becomes positive in potential with respect o the filament, owfng to the connection through the lower relay 10? and resistance 116. 1 the full operating current to flow special plate battery through magnet 113 and relay 108 each time the line circuit is momentarily opened. Each time the line is a closed at the calling device sub ion 111 following a momentary interruption, current again flows over the line. and the potential of the grid element of tube 121 is changed from positive to negative, stopp' tnc current flow in the plate circuit througn magnet 113 and re V11 11 this occurs. reitical magnet in closes the circuit 1" relay 10?. vertical magnet, the

d step v-step unti 1J3 the desired love 1 1 s i e 1 4-1 1 Ella T816383 c116 (30121160 011 ill C3216 L118 1111i;

circui" is not closed. During the tim the circuit of the upper winding of relay 10;

, the relay is maintained operated by the circulating current thi igh the upper wind" in series with resistance 117.

The series relay 108 is p ovided with a conducting slee e. as indicated by the inner par-rule verti lines, and the v is maintained operated between successive impulses to the vertical ma ct by the circulating current flowing in this conductii 9; sleeve.

When relay 10S first operates it prepares a circuit for stepping closed at off-normal the shaft of the n its normal position by E S0011 11S car 1. agnet 11o.

1 Relay 109 op rates and closes a locking cir- J! '1. 1. 4- "L. H 1, Y7, 1. cuit ior itseii a. lab per a nature, and. 4L, W. 1. L 11s ion er armature it prepares an operating the initial circuit of the now-locked stepping relay 109 and completes the circuit of rotary magnet 11 1 from the grounded release-trunk conductor 105. Magnet 114i operates to advance the wipers 118-120 into engagement with the first set of contacts in the level opposite which they have been raised. Near the end of its stroke, magnet 11 1 opens the circuit of stepping relay 109 at the contacts shown to the right of the magnet, whereupon relay 109 falls back and opens a further point in its locking circuit at its upper armature, the same time opening the circuit of tie rotary magnet at its lower armature. The

rotary magnet accordingly falls back and again connects up relay 109.

The further operation depends upon w eth r the trunk line connected to the first bank contact is busy or idle. If this trunk line is idle, it is seized by the operation of relay 110, but if it is busy relay 110 is shunted by the ground potential on the release trunk conductor thereof and encountered by test wiper 119, and relay 109 operates again through test wiper 119 and through oil-normal contacts 112, and again closes a locking ci cuit for itself at its upper armature, and cloes the circuit of the rotary magnet at its lower armature. In this way, the rotary magnet is again operated to advance the wipcrs to the rent set of bank contacts. 7

This alternate operation of stepping relay 109 at magnet 11 1 continues until an idle set of hank contacts is reached. lVhen an idle 28430, for example, switching relay 110 is no longer short circuited by the apphcation of ground potential to its lower terminal, whereupon it operatesin a circuit from the grounded release-trunk conductor 105 through the oil-normal contacts 112, in series with relay 109. Relay 109 is not operated at this time on account of the relatively high resistance 01 relay 110. Upon operating, relay 10 opens a further point in the test circuit and prepares the holding circuit for the selector at its inner upper armature; opens the ilament-lighting circuit and the holding circuit for the upper winding of reiay 10': at its inner lower armature; and at its upper and lower arniatures it disconnects the incoming conductors 10 1 and 106 from resistance 123 and from the lower winding of relay 107 and extends them by way of

ank contacts

128 and 130 to the repeater R1.

the repeater R1, line relay 131 enere calling line in series with re- 139 an closes a circuit for release relav 132. Release relay 132 operates and applies a. ground potent al at its inner upper armature to the assoc f d. release trunk conductor extending to bank contact 129, thereby completing the holding circuit for the selector G81 and the lineswitch LS1 through the inner upper armature of switching relay 110.

This holding circuit is completed, it will be understood, before the slow-acting release relay 107 of the olfice selector 0S1 has had time to fall back responsive to the opening of the circuit of its lower winding at the lower armature of relay 110 and responsive to the opening of the circuit of its upper winding at the inner lower armature of relay 110. In practice, the inner lower armature of relay 110 is adjusted so as to break first in order to extinguish the filament of the vacuum tube 121 and prevent a flow of plate current responsive to the disconnection of the lower winding of relay 107 at the lower armature of relay 110, and the consequent shifting of the grid potential of tube 121 from positive to negative potential.

At the time the filament is extinguished and the circuit of the upper winding of relay 107 is opened, the upper winding of relay 107 is shunted by the resistance 117 as before explained and it is also shunted at this time by the low-resistance circuit including resistance 122 and the filament of lamp 121. The self-induced current from the upper winding of relay 107 flows through these two shunt circuits, rendering the relay sutficiently slowto-release to maintain conductor 105 grounded until release relay 132 of the repeater R1 operates as above pointed out.

hen relay 107 .falls back, it removes ground potential at its upper armature from conductor 105, leaving this conductor grounded through wiper 119 and the inner upper armature of release relay 132 of repeater R1. At its lower armature, relay 107 opens a further point in the filament circuit and in the circuit of its upper winding, at the same time preparing a circuit for release magnet 115 through off-normal contacts 111, which circuit is maintained open for the time being at the inner lower armature of switching relay 110.

In the repeater R1, release relay 132, in addition to grounding the release trunk conductor at its inner upper armature, as above pointed out, closes a bridge across the outgoing trunk conductors 141 and 142 at its upper armature through contacts of relays 133 and 134 and through the marginal supervisory relay 135. Current now flows in this bridge circuit and over conductors 141 and 142 in series with the line circuit of the thousands selector ThSl, Fig. 2, which thousands selector is the same as the ofiice selector 0S1, Fig. 1, except for its position in the switch train. The current flow is limited by the resistance in the selector TkSl similar to resistance 116 in the selector 0S1. Ac cordingly, the marginal supervisory relay 135 is not operated at this time. he thousands selector ThSl is prepared for operation in the manner described in connection with the selector 0S1.

In the repeater R1, release relay 132 also J. the

closes a circuit at its lower armature through resistance 137 for lighting the filament of the tube 136. The flow of plate current through relay 133 from the special battery associated therewith (which may be the same battery associated with vertical magnet 113 of the switch 0S1) is prevented by the application of ne ative potential to the grid of tube 136 from the regular exchange battery through the upper winding of line relay 131 and over the calling line to the grid of the vacuum tube. The high resistance 139 is included in circuit with the lower winding of the line relay for the same purpose as described in connection with resistance 116 of the selector 0551, and the upper winding of line relay 131 is shunted by resistance 143 in order to reduce the inductance of the circuit through which negative potential is supplied to the grid element of tube 121.

When the next digit is dialled, the line circuit is opened a corresponding number of times. Each time the line circuit is opened, the grid potential of tube 136 is changed from negative to positive, whereupon an im pulse of current flows through repeating relay 133 connected in the plate circuit of the vacuum tube. Each time relay 133 operates, it opens the bridge across conductors 141 and 142 at its lower armature, repeating the impulse to the thousands selector ThSl, Fig. 2. As a result, the thousands selector ThSl, Fig. 2, responds to the second digit dialled by raising its wipers opposite the desired level of bank contacts and rotating them in search of an idle trunk line. It may be assumed that the trunk line extending to the hundreds selector HS1 is the one taken for use.

As a further result of each operation of relay 133, a circuit is closed for relay 134. Relay 134 is slow in releasing on account of the resistance 120 shunted around its winding. Accordingly, relay 134 operates responsive to the first impulse of the series and remains operated throughout the series of impulses. When it operates, relay 134 disconnects conductor 141 from the upper talking conductor of the repeater and the upper portion of the bridge through relay 135 and connects it to the lower terminal of relay 135 and lower talking conductor. By this arrangement, the inductive relay 135 is cut out of the impulsing circuit in order to give a quicker building up of current following each interruption in the line circuit.

When the calling subscriber at substation A1 dials the hundreds digit in the desired number, the repeater R1 operates in the manner before described to repeat the impulses of the digit to the selector H81. The selector HSl responds in the manner described in connection with the selectors OS1 and ThSl to select an idle trunk line, which trunk line will be assumed is the one comprising conductors 231-233 and extending to the connector C1. When this trunk line is taken for use, line relay 201 therein operates and closes a circuit for release relay 208. Release relay 203 places ground potential on release trunk conductor at itsupper armature; opens a point in the circuit release magnet 217 and places grou d potential on conductor 234 at its inner u p t its lower digit in the desired number, this digit is re- 4 peated by the repeater R1 of Fig. 1 to the connector C1 by way of the connection set up through the selectors TltSl and H51. hen the line circuit first interrupted by the repeater R1 and the grid potential at tube 211 is consequently rendered positive. for the first time, the vertical magnet 218 operates from the special battery thereat through series relay d, off-normal springs 213 and 2155, and through th plate circuit of the vacuum tube, to ground by way of the lower armature of relay 203. Slow-acting relay 20s operates in series with the vertical magnet 218 and maintains the vertical magnet circuit intact after the oil-normal springs have shifted, as they 2; do upon the first vertical step of the switch.

h vertical impulse following the first is delivered to the vertical magnet through springs 215 and 214v and through the lower armature of the operated relay 204.

By the operation of the vertical magnet 218, the wipers 220-222 are raised step-bystep until they stand opposite the desired level of bank contacts.

At the end of the vertical movement, relay 204 falls back and transfers the plate element of the vacuum tube from the vertical magnet to the rotary magnet.

W hen the units digit of the number is dialled, the impulses of current are delivered through the plate of the vacuum tube 211 to the rotary magnet 219 and slow-acting relay 207 in parallel. By the operation of the rotary magnet 219, the wipers 220222 are rotated step-by-step until they come into engagement with the bank contacts otthe line of substation B1. Relay 207 operates in parallel with rotary magnet 219 upon the first impulse and, being slow acting, remains operated throughout the digit. At its upper armature, relay 207 disconnects test wiper 221 from the test winding of switching relay 208 and connects it to busy relay 206, and at its lower armature it shunts the lower contacts of busy relay 206 so as to prevent the filament or". tube 211 from becoming extinguished in case busy relay 206 should operate during the rotary movement due to wiper 221 encountering a busy test contact.

The operation ensuing at the end of the rotary movement depends upon whether the called line is ousy or idle. It the called line is busy, there is a ground potential on the testcontact thereof which is encountered by wiper 221, and busy relay 206 operates when the wiper lands on the contact. lVith busy relay operated, a locking circuit is closed for relay 206 when relay 207 falls back. This locking circuit is from ground on the grounded conductor 23% through contacts ot' relays 206 and 207. With relay 206 operated and relay 20'? restored, the filament of tube 211 is extinguished, thereby preventing further operation or" the rotary magnet 219. At its lower armature, relay 206 connects up the busy-tone lead 251 to the lower talking conductor through the associated condenser, thereby causing a busy tone to, be transmitted back over the established connection to the calling line. Upon hearing this busy tone, the calling subscriber is expected to replace his receiver and cause the established connection to be released.

Assuming now that the line at substation B1 is idle when called, busy relay 206 is'not operated when the wipers 220221 come to rest on the contacts of the called line, and

when relay 207 falls back it completes at itsv upper armature a circuit for energizing switching relay 208 through its test windin This circuit is from ground on the grounded conductor 23a through the upper armature of busy relay 206 and through the upper winding of relay 208 through wiper 221by way of contacts of relay 207. The circuit of relay 208 is completed to battery by way of the cut-ofi relay the lineswitch (not shown) of the called line. Upon operating, relay 208 closes a locking circuit for its lower winding from release-trunk conductor 232 at its inner lower contact-s; applies ground potential directly to test wiper 221 at its inner upper armature; extinguishes the filament of the vacuum tube 211 at its middle lower armature; and at its upper and lower armatures connects up the wipers 220 and 222. When this occurs, ringing current from ringing lead 250 is sent out over the called line and returns by way o1 wiper 220 and the upper talking conductor to battery by way of the upper winding of ring-cut-ofi' relay 205. c

When the called subscriber responds to the ringing of his bell by removing the receiver and substituting a direct-current bridge for the ringer bridge, which includes a condenser, ring-cut-ofi" relay 205 operates and closes a locking circuit for its lower winding at its inner lower armature, and at its upper and lower armatures opens the ringing circuit and connects the calling and called lines together telephonically. When this occurs, back-bridge relay 202 of the connector operates over the called line; disconnects the shunting resistance 223 at its upper armature; and at its lower armature shunts the resistance 209, so as to increase the flow of talking current in the calling line in case the calling line is in the same olfice as the called line, in which case no repeater is involved. In the present case, shunting resistance 209 increases the flow of current over the connection set up between the repeater R1 and the connector C1 sufficiently to bring about the operation of the marginal supervisory relay 135. Relay 135 operates; disconnects the shunting resistance 143; and shunts the resistance 139 associated with line relay 131, thereby increasing the flow of current in the calling line so as to supply the transmitter at the calling telephone with the sufiicient amount of talking current.

The calling and called subscribers may now converse with each other as desired, the talking circuit being outlined by the heavy conductors.

When the conversation has terminated, the two subscribers replace their receivers. When the receiver is replaced at substation B1, back-bridge relay 202 in the connector C1 falls back and again inserts resistance 209 into the line circuit, whereupon the marginal relay 135 in the repeater R1 again falls back and inserts the resistance 139 in the circuit of the calling line.

It may be pointed out that the increase of line current brought about by relays 202 and 135 may be used to operate a meter to assess a charge against the calling subscriber it so desired, and that the increase when the called man replies and the decrease when he again hangs up may be used to give supervision in case the call is coming, for example, from a private branch exchange employing an operator.

When the receiver is replaced at substation A1, repeating relay 133 of the repeater R1 is operated to open the line circuit extending to the connector C1, Fig. 2. Also, line relay 131 falls back and opens the circuit of the slow-acting release relay 132. A moment later, the slow-acting release relay 132 falls back and opens a further point in the bridge across conductors 14:1 and 142 at its upper armature; eXtinguishes the filament of tube 136 at its lower armature, permitting relays 133 and 134 to fall back; and at its inner upper armature it removes ground from the associated release trunk conductor, thereby permitting the lineswitch LS1 to release in the usual manner, and opening the circuit of switching relay 110 of the ofiice selector 0S1. hen switching relay 110 falls back, it closes at its inner lower armature a circuit through contacts of release relay 107, and off-normal contacts 111,

for release magnet 115. The release magnet operates and restores the selector 051 to normal, whereupon the release-magnet circuit is opened at off-normal contacts 111.

in the connector C1, Fig. 2, line relay 201 falls back when its circuit is opened at the repeater R1, and it opens the circuit of release relay 203. Release relay 203 removes ground potential from release trunk conductor 232 at its upper armature, thereby permitting the selectors ThSl and H51 to release in the manner described in connection with the oilice selector CS1. The removal of ground potential from releasetrunk conductor 232 also permits relays 205 and 208 to fall back. At its inner upper armature, release relay 203 closes a circuit through oil-normal contacts 216 for release magnet 21?. Release magnet 217 operates and restores the connector ()1 to its normal position, whereupon the circuit of the release magnet is opened at contacts 216.

The entire connection is now released and the apparatus involved therein may be used in setting up other connections.

The system of Fig. 3

Referring now particularly to Fig. 3, the system shown therein Will be described. As pointed out hereinbefore, instead of employing a vacuum tube in each switch, the system of Fig. 3 employs a vacuum-tube repeater in the line ahead of each first selector, while all of the switches which respond to impulses employ standard circuit arrangements. In addition, the repeater R2 of Fig. 3 differs from the repeater R1 of Fig. 1 in that the grid potential of the vacuum tube 311 is normally positive in the repeater R2 after the repeater has been taken for use, and the grid potential is rendered negative each time the calling line is interrupted. Furthermore, the repeater R2 has been arranged to respond to standard reversed-battery supervision in stead of to the high-resistance supervision of Figs. 1 and 2.

In order to explain the operation of the apparatus of Fig. 3 in detail, it Will be as sumed that the subscriber at substation A2 in one office desires to converse with the subscriber at substation B2 in a distant office. hen the receiver is removed at substation A2, the lineswitch LS2 operates in the usual manner to seize an idle trunk line. It will be assumed that this trunk line is the one extending to the repeater and comprising conductors 313315. When this trunl: line is seized, line relay 301 operates over the calling line in series with resistance 306 and over

conductors

313 and 315. Upon operating, line relay 301 closes a circuit through resistance 309 for the filament of the tube 311. lhe tube 311 accordingly becomes lighter. it will be noted that the resistance 310 is also in series with the filament of the tube 311. As indicated by the relative sizes of the

resistance symbols

309 and 310, resist ance 310 is a fairly low resist -ce. while the resistance 300 is a relatively-high resistance. is a result, the filament of the tube 311 is maintained at an average potential very nearly as negative as the negative terminal of the exchange battery. On the other hand, the grid element of the tube 311 is now maintained at a potential very nearly as positive as the positive terminal (the grounded terminal) of the exchange battery owing to the ground connection to the grid from the grounded conductor 315, over the line of substation A2, the relatively high resistance 306 eing inserted in circuit between the grid element of tube 311 and the negative terminal of the battery, in series with the upper winding of relay 301. lVith the grid element thus maintained at a relatively positive potential with respect to the filament, condi tions are favorable for flow of current in the plate circuit. it wbe noted that no separate plate battery is used and that the plate relay 303 has its upper terminal connected directly to ground. This arra g ment is employed because the character of the vacuum tube 311 are such that the voltage of the exchange battery is suthcicnt to drive an ample operating current through relay 303. Accordinsrlv. relay 303 now operates and closes a circuit for relay its lower armature, at the same time opening a point in the circuit of the slow-ac upper armature. Relay pares a circuit for relay 30 1 at its inner noper armature; places ground pote .al on release-trunk conductor 31 1 at its upper armature; and at lower armature closes an additional filament lighting circuit so as to maintain the filament oi": the tubes 311 lighted during impulsing in case relay 301 falls back. As further result of the energizat on of repeating relay 303. a circuit is closed at the lower armati. thereof for the line relay 331 of the office selector @552, thro gh the upper winding of the electro-polarized relay 305 anl through the impedance coil 312. Accordinfly. line relay 331 operates to prepare the oiiice selector CS2 for operation.

When the calling subscriber dials the first digit in the desired number, the line is interrupted at the calling device a corresponding number of times. Each time the line is interrupted, the potential of the grid element of tube 311 is changed from positive to negative, whereupon the current flow through the plate circuit and through relay 303 ceases. Relay 303, accordingly, falls back each time the line circuit is opened, and each time it does so it opens the circuit of the line relay 305 of the otlice selector 0S2. Relay 303 also opens the circuit of the slow-acting release relay 302 each time it falls back, but relay 302 remains operated owing to the shunt around its winding through resistance 307. Each time lb falls back relay 303 also closes a circuit for the slow-acting relay 304. Relay 30% operates the first time its circuit is closed and remains operated throughout the series of impulses on account of being shunted by the resistance 308. Relay 30-1 shifts the circuit of the line relay of the oflice selector 032 so that it excludes the impedance 312 and the upper winding of 305 so as to improvethe impulsing circuit.

At the end of the digit, relay 303 remains ener ized and relay 30 ifalls back.

Responsive to impulses of the first digit,

the ofiice selector 0S2 raises its wipers 333- 335 until they stand opposite the desired level of bank contacts, whereupon the wipers are rotated in the usual manner in search of an idle trunk line. When an idle trunk line is found, the one extending to the thousands selector T7182, for example, the switching relay 332 operates in the usual manner to seize the trunk line and complete the connection thereto. The thousands selector ThS2 is an inco-mingselector in the distant exchange, but no outgoing repeater is required at the outgoing end of the trunk line because the repeater R2 is already in the connection to repeat the impulses. llt is unnecessary to extend a releasetrunk conductor between the two exchanges because the office selector CS2 and the lineswitch selector LS2 are held operated over conductor 314 by the ground potential supplied by release relay 302 of the repeater R2.

Responsive to the dialing of the remaining digits in the desired number, the thousands selector ThS2, the hundreds selector HS2, and the connector C2 are operated in the usual manner to make connection with the line of substation B2.

When the subscriber at substation B2 responds, he back-bridge relay (not shown) in the connector C2 is operated to reverse the connect-ions between the incoming line conductors and the linerelay of the connector, thereby reversing the current flow in the incoming conductors, causing the current flow to be reversed in the back bridge of the repeater R2, including the upper winding of the electro-polarized relay 305. hen this occurs, the current flow in the two windings of the electro-polarized relay is in agreement and the relay operates. At its upper armature, relay 305 short circuits the resistance 306 so as to increase the'flow of talking current, and at its lower armature it removes the shunt from around the lower winding of line relay 301 so as to balance the talking circuit.

The subscribers may now converse as desired, and when the conversation is completed they replace their receivers.

When the receiver is replaced at substation B2, the current flow in the line conductors between the repeater R2 and the connector C2 is reversed back to normal, whereupon the electro-polarized relay 305 restores.

When the receiver is replaced at substation A2, repeating relay 303 falls back and opens the outgoing line circuit at its lower armature, incidentally operating relay 304 at its upper armautre. Line relay 301 also falls back. Release relay 302 falls back after a slight interval and opens the circuit of relay 304 at its inner upper armature; extinguishes the filament of the vacuum tube at its lower armature; and at its upper armature removes ground potential from the release trunk conductor 314 so as to permit the lineswitch LS2 and the ofiice selector 0S2 to release.

Responsive to the opening of the line circuit at the lower armature of relay 303, the switches ThS2, HS2, and C2 in the distant oflice restore.

The connection is now completely released.

The repeater R2, it will be noted, is so arranged that it will cause the connection to be released and cause itself to be freed from a line which may be so poorly insulated as to hold line relay 301 operated, because repeating relay 303, which responds to fluctuations of the grid potential at vacuum tube, falls back and opens the circuit of release relay 302, when the receiver is replaced at the calling substation, whether the line rel y 301 falls back or not. When the release relay 302 falls back, the lineswitch LS2 releases and frees the repeater, permitting the line relay 301 to fall back and extinguish the filament of the tube in case it was prevented from doing so by the leak current flowing over the calling line.

The repeater of Fig. 4

Referring now particularly to Fig. 4, the repeater shown therein will be described. When this repeater is seized, either by a lineswitch or by a selector, depending upon the location of the repeater in the trunking layout, line relay 405 operates over

conductors

401 and 403 and closes a circuit for the slowacting release relay 406. Relay 406 operates and at its upper armature places ground po tential on the release trunk conductor 403. At the lower armature of relay 406 a circuit is closed for the filament of the vacuum tube 411 from the battery 412. It may be pointed out that the battery 412 is individual to the repeater. VVhen the filament becomes lighted, current flows through the plate circuit of the vacuum tube from the exchange battery through the lower winding of line relay 423 of the automatic switch to which the repeater R3 is connected, over the negative trunk conductor 422, filament-plate circuit of the vacuum tube 411, relay 407, positive trunk conductor 421, and the upper winding of line relay 423 to ground. At the same time, there is a branch circuit from the upper terminal of relay 407 through the contacts thereof and through the upper winding of the electropolarized relay 408 to ground through the contacts of relay 410, in parallel with the upper winding of relay 423. Relay 408 does not operate in this branch circuit because relay 407 quickly energizes in the main circuit and opens the branch circuit. Line relay 423 operates in series with relay 407 and over conductors 421 and 422 and prepares the associated switch for operation in the usual manner.

When a digit is dialled following the seizure of the repeater the incoming line circuit is broken a corresponding number of times. Each time the line circuit is broken, the grid potential at the tube 411 shifts from the positive potential formerly applied over the line to the grid of the tube to a negative potential applied through the upper winding of line relay 405 and resistance 349 to the grid of the tube. lVhen this occurs, the current flow in the plate circuit of the tube 411 ceases, whereupon relay 407 of the repeater and line relay 423 in the automatic switch fall back, relay 407 again connects up the branch circuit including the upper winding of relay 408.

Each time the line circuit is closed, following an interruption, relays 407 and 423 operate again, relay 407 again disconnecting the branch circuit before relay 408 has time to operate.

At the end of the digit, the positive potential remains applied to the grid of the vacuum tube 411, and relays 407 and 423 remain operated.

After the connection has been extended through the automatic switches to the called line and the called subscriber has responded, the connection between the line relay now supplying current to the back bridge of the repeater and the line conductors incoming to such line relay are reversed in the usual manner, whereupon the current flow tends to reverse in the back bridge of the repeater. The current flow cannot be reversed through the vacuum tube 411, however, and relay 407 falls back and does not again operate. In this case, ne ative potential is supplied by way of the upper conductor 421 and through the contacts of relay 407 to the upper winding of the electro-polarized relay 408, the circuit being completed to ground by way of the contacts of relay 410. The electropolarized relay 409 now operates and closes a circuit for

relays

404 and 410 in parallel. Relay 404 reverses the connections between line relay 405 and the

incoming conductors

401 and 403, at the same time cutting out the resistance 349 and removing the shunt from around the lower winding of relay 405. The incoming line circuit is thus balanced to give a quiet talking circuit; the current flow is increased to give a suflicient supply at the transmitter of the calling line; and the current flow is reversed in the calling line. It will thus be seen that the repeater R3 affords both high-resistance supervision and reversed-current supervision. Either, or both, may be utilized as desired.

When relay 410 operates, it closes a looking circuit for itself at its inner lower armature to the grounded release-trunk conductor 402, at the same time opening its initial circuit; extinguishes the filament of the vacuum tube 411 at its middle lower armature; and at its lower armature it completes a new bridge across the conductors 421 and 422 through the impedance coil 413 and the lower contacts of release relay 406.

When the receiver is replaced at the called substation following the conversation, the current flow in conductors 421 and 422 is reversed back to normal, whereupon the current fiow reverses through impedance 413 and the upper winding of electro-polarized relay 408. hen this occurs, relay 409 restores, but relay 410 remains locked up. Belay 404 restores and reverses the current flow in

conductors

401 and 403 back to its normal direction, restoring the incoming line to conditions existing before the call was answered.

It is to be noted that the vacuum tube 411 plays no part in the operation of the repeater following the answering of the call because relay 410 extinguishes the filament thereof and maintains it extinguished until the connection is released.

When the receiver is replaced at the calling substation and the current flow ceases in

conductors

401 and 403, line relay 405 falls back and opens the circuit of the slow-acting release relay 406. Relay 406 falls back and opens the bridge across conductors 421 and 422 at its lower armature to permit the connection set up through the succeeding automatic switches to be released, and at its upper armature it removes ground potential from release trunk conductor 402 so as to permit the repeater to be freed by the automatic switch that seized it.

Additional explanation A few points in connection with the operation of the several circuits shown in the drawings will now be explained more fully.

Referring first to Fig. 1, it will be noted that the lower winding of relay 107 is included in the line circuit between the positive pole of the exchange battery and the grid element of the vacuum tube 121. When the break occurs at the calling substation, the lower windin of relay 10? tends to continue the current flow by means of the well known self induction, momentarily rendering the grid element of the tube 121 more positive than ground potential. This is effective in quickly building up the current flow in the filament plate circuit of the vacuum tube. On

I the other hand, when the line circuit is closed following an interruption, due to the inductiveness of the lower winding of relay 107, the current flow is momentarily prevented through the lower winding of the relay and the potential of grid 121 is momenwhich is connected in shunt of the upper;

winding of relay 131, making the line circuit practicallynon-inductive except for the lower winding of the line relay 131, which is connected between the grid element of the tube and ground. A similar arrangement is employed in the connector C1.

In the repeater R2 of Fig. 3, the lower winding of relay 301 is normally shunted so as to remove the inductive lower winding from the impulsing circuit, and the inductive upper winding of relay 301 is connected between the negative pole of the exchange battery and the grid of the vacuum tube 311 so as to add the self-inductive action of the upper winding to the fluctuations of current caused by the line interruptions to bring about more violent fluctuations of the grid potential of the vacuum tube. By this arrangement, the grid element of the tube 311 becomes more negative when the line isbroken than it is after the line has been broken for an instant, and the grid element is rendered more positive when the line circuit is closed after being broken than it is an instant after the line circuit is closed, thus bringing about the desired abrupt changes in the flow of plate current. The same arrangement is employed in the repeater R3 of Fig. 4. I i

It will be noted that in all cases the line circuit is balanced and the talking current is increased when the called subscriber answers, as there is no longer need for the dialling condition to be maintained.

What is claimed is:

1. In an automatic telephone system, means including a train of automatic switches controlled from a calling subscribers line for setting up a connection from the calling line to a called line responsive to series of impulses, a repeater interposed between the calling line and the train of switches, and a vacuum tube in said repeater arranged to respond to the impulses received over the calling subscribers line and repeat themto the automatic switches.

2. In combination, an automatic progressively-movable switch, a line incoming to said switch, a vacuum tube having its grid circuit multiply connected to said line, and

a motor magnet for said switch connected in the plate circuit of said tube.

3. In combination, an automatic progressively-movable switch, a line incoming to said switch, a relay for controlling the release of said switch bridged across said line, a vacuum tube, a grid circuit for said tube shunted around said relay and including the two conductors of said line in series, and a plate circuit for said tube including the motor magnet of said switch.

4. In combination, an automatic switch, a line incoming to said switch, an operating magnet for said switch arranged to be controlled over said line to position said automatic switch, a vacuum tube in said switch having its grid element multiply connected to the incoming line, and a circuit for the operating magnet of said switch including the plate element of the vacuum tube.

5. In combination, a selector switch, a line incoming thereto, a vacuum tube, a relay arranged to be energized in series with said line to close a filament-lighting circuit for said vacuum tube, the grid element of said vacuum tube being connected to a conductor of said incoming line, an operating magnet for said selector switch, and a circuit for said operating magnet including acurrent source and the filament and plate elements of the vacuum tube.

6. In an automatic telephone system, an automatic switch including a vacuum tube and an operating magnet, a line incoming to said switch, means responsive to current flow over said incoming line for closing a filamentlighting circuit for said vacuum tube, said vacuum tube being arranged to respond to interruptions in the current flow to transmit impulses to said operating magnet, a switching relay and means for operating it after said automatic switch has been positioned by said operating magnet, and contacts on said switching relay for opening the filamentlighting circuit of the vacuum tube.

7. In combination, a repeater, a line incoming thereto, a line relay bridged across said line in series with the battery, a vacuum tube having its grid connected to the negative line conductor, means for closing the line to thereby establish a positive potential onsaid grid, a plate circuit for said tube including a relay, and a line outgoing from said repeater closed by said relay.

8. In a repeater, an incoming line, an outgoing line, a vacuum tube, a grid circuit for said tube including the two conductors of the incoming line in series, a plate circuit or said tube including a relay, and contacts on said relay for repeating impulses over said outgoing line.

9. In combination, a repeater, a line incoming thereto, a line relay bridged across said line in series with the battery, a vacuum tube having its grid connected to the positive line conductor, means for closing the line to energize said line relay and to establish a negative potential on said grid, an outgoing line closed responsive to the operation of said line relay, a plate circuit for said tube, and a relay included in said plate circuit for interrupting the said outgoing line.

10. In a telephone system, an automatic switch arranged to be controlled over a calling line to set up a telephone connection to a called line, a vacuum tube interposed between said line and said automatic switch and arranged to control the setting operation of said automatic switch responsive to the voltage variations resulting from current variations in the calling line, a resistance element inserted in said line in order to increase the voltage variations at said vacuum tube, and means automatically efiective after said automatic switch has been positioned to remove said resistance element from the circuit in order to increase the current flow over the calling line.

11. In combination, a telephone line entering a telephone exchange. means for applying a current source to said line at the said exchange and for setting up a current flow thereover, means at the distant end of the line for interrupting the current flow, automatic switching apparatus in the exchange, a trunk line leading to said switching apparatus, and a vacuum-tube repeater interposed between said telephone line and said trunk line and arranged so as to be responsive to the interruptions of said current flow over said telephone line to transmit impulses over said trunk line to said automatic switching apparatus.

12. In an impulse repeater for use in repeating impulses to automatic switching apparatus, an impulse conductor incoming to said repeater, a return conductor, a vacuum tube in said repeater arranged to repeat impulses responsive to voltage variations on the incoming line resulting from impulses of current transmitted thereover, and an inductive winding included in circuit between said incoming line conductor and the return conductor to accentuate the voltage variations on the grid of the vacuum tube.

18. In a telephone system, arepeater, a

.trunk line outgoing from said repeater, a

vacuum tube in said repeater, and a plate circuit for said tube including the two conductors of said trunk line in series and a source of current at the distant end thereof.

14. In a telephone system, a repeater, atrunk line outgoing from said repeater, a vacuum tube having its filament-plate circuit connected in bridge of said trunk line, an automatic switch, a line relay for said switch energized over said trunk line and being responsive to impulses transmitted thereover by interruptions in said bridge, means in said switch for reversing the battery connections to said trunk line, and means in said repeater responsive thereto for connecting another bridge across said trunk line.

15. In a telephone system, a repeater and an automatic switch located in difierent offices, a vacuum tube included in said repeater, and a line relay in said switch connected in series with the plate of said tube.

16. In a telephone system, a repeater, a trunk line outgoing from said repeater, a vacuum tube having its filament-plate circuit connected in bridge of said trunk line, means for seizing said repeater, and means responsive to said seizure for rendering said filament-plate circuit conductive.

17. in an automatic telephone system, means including a train of automatic switches for setting up a telephone connection responsi e'to impulses series transmitted thereto from a calling line, an impulse repeater included in said train and having a vacuum tube therein arranged to repeat the incoming impulses to the succeeding switches in the train, and means controlled by impulses sent out from the calling line for operating the first portion of said switch train to extend a connection to said repeater.

18. in a telephone system, an automatic switch arranged to be controlled over a calling line to set up a telephone connection to a calld line, a. vacuum tube interposed between said line and said automatic switch and arranged to control the setting operation of said automatic switch responsive to the voltage variations resulting from current variations in the calling line, a re sistance element inserted in said line in order to increase the voltage variations at said vacuum tube, and means automatically responsive to the removal of the receiver on the called line for cutting out said resistance element in order to increase the current flow over the calling line.

19. In a telephone system, an automatic switch arranged to be operated responsive to series oi interruptions in the current flow over a calling line to set up a connection to a called line, a resistance element in circuit with said line during the time said switch is being controlled, and means responsive to the removal of he receiver at the called station for removing said resistance element so as to increase the current flow over the calling line.

20. in a telephone system, a calling line EX- tending to an impulse repeater, circuit connections at said impulse repeater for supplying current to said calling line, a resistance element connected in circuit with said incoming line to limit the current flow thereover, an automatic switch, line (onnecting said repeater and said automatic switch, circuit connections automatic switch for sup- 7 plying current to said line, a resistance element inserted in the line circuit at said automatic switch to reduce the current flow in the line between the repeater and the automatic switch, means on the calling line for producing series of interruptions in the current flow thereover, means at the repeater responsive to each interruption for interrupting the current flow in the line extending between the repeater at the automatic switch, said automatic switch being responsive to the interruptions in the line incoming thereto to set up a connection to a called line, means responsive to the removal of the receiver on the called line for cutting out the resistance element at the automatic switch to increase the current flow over the line extending back to said repeater, and means in the repeater responsive to the increase of current flow for cutting out the resistance element thereat to increase the current flow over the calling line. 21. In a switching system, an automatic switch, an impulse sending device, a line interconnecting said device and said automatic switch, circuit connections to said line at said automatic switch for setting up a currentflow thereover, a vacuum tube at said automatic switch having a grid element, a separate connection from the grid element of said vacuum tube to said line, a plate circuit for said vacuum tube, said impulse sender being effective to intermittently interrupt the current-flow over said line to impress voltage variations on the grid element of said vacuum tube, and means operating under the control of the plate circuit of said vacuum tube responsive to voltage variations on the grid element to position said automatic switch.

22. In a switching system, a sender, a vacuum tube, a line extending between said sender and said vacuum tube, circuit connections at the vacuum tube for setting up a current-flow over said line through said sender, a separate connection between the grid element of said vacuum tube and said line, said vacuum tube having a plate circuit and circuit connections for sending a current-flow therethrough under th control of the potential on the grid element, said sender being effective to intermittently interrupt the current-flow over said line to produce voltage variations on the grid element of said vacuum tube, and an automatic switch operative responsive to the current variations in the plate circuit of the vacuum tube responsive to the grid-voltage variations to extend said line.

23. In a switching system, a line, a current source at one end of said line, means for setting up a current-flow over said line from said current source, means at the opposite end of said line for intermittently interrupting the current flow, a vacuum tube on the same end of said line with said current source, the grid element of said vacuum tube being connected to said line to enable the vacuum tube torespond to the voltage fluctuations occasioned by the intermittent interruption of the current-flow at the distant end of the line, and

an automatic switch controlled by the plate circuit of said vacuum tube to extend said line responsive to the voltage variations on the grid element of the vacuum tube.

24. In a telephone system, a trunk line divided into two sections, a telephone line, automatic switching apparatus controlled over said line to extend a connection from said telephone line to the first section of said trunk line, means on said telephone line for sending series of impulses to the first section of said trunk line, means including a vacuum tube having its grid element connected to the first section of said trunk line for repeating said impulses from the first section of said trunk line to the second section thereof, another telephone line, and means including a train of automatic switches controlled over the second section of said trunk line responsive to the repeated impulses to complete a connection from the second section of said trunk line to said other telephone line.

25. In a switching system, an impulse sender, a vacuum tube, a line extending from said impulse sender to said vacuum tube, said line being normally open, means for closing said line, a relay operative responsive to the closure or" said line to close a filament-lighting circuit for said vacuum tube, said im pulse sender being effective to intermittently interrupt the current-flow in said line to produce voltage variations on said line, an automatic switch operative under the control of said vacuum tube respons've to said voltage variations to set up a connection, said relay being slow releasing so as to prevent it from falling back responsive to the current interruptions, and an additional holding circuit for said relay controlled by said vacuum tube, whereby the tendency of said relay to hold up during momentary interruptions is increased, while the relay is permitted to fall back responsive to a prolonged interruption of current-flow.

26. In a switching system, a switching station, a line incoming to said station and connected to a source of current therein, a gridcontrolled thermionic valve having its grid connected in multiple to said incoming line so that potential changes occurring on the line responsive to current interruptions produced at the distant end thereof cause current changes in the plate circuit of the valve, and automatic switching apparatus responsive to the current variations in said plate circuit to link a corresponding extension to said line. 7

27. In a telephone system, an impulse repeating device having a line relay, aline 1ncoming to the device and connected to a current source by way of a winding of said line relay, a three-element vacuum tube having its grid element connected to the line in. multiple relationship with the line relay, whereby interruptions in the line at the distant end thereoi cause voltage changes at the grid element which in turn cause current changes in the plate circuit of the tube, and an automatic switch operable responsive to the current changes in said plate circuit.

28. In a signalling system, a'vacuum tube, an electro-responsive device connected in circuit with the plate element of said tube and arranged to be controlled thereby, a line in coming to the grid element of'said vacuum tube, an impulse transmitter, means for operating said impulse transmitter to transmit impulses over said line to the grid element of said tube in a circuit relatively free from inductance to cause said tube to operate said device, and an inductive coil connected in circuit with said line at the vacuum tube to accentuate the potential swing of said impulses at the grid element, whereby a more faithful response of said device is obtained.

29. In a telephone system, a trunk line, a selector switch, means for operating said switch to seize said trunk line, a vacuum tube having its grid-filament circuit bridged across said trunk line and rendered operative responsive to said seizure, and means controlled by said tube for maintaining said selector operated.

30. In a telephone system, a trunk circuit including a vacuum tube, an automatic switch, means for operating said switch to connect with said trunk circuit, and means controlled by said tube responsive to said connection for supplying holding potential to said switch.

31. In a telephone system, a trunk, means for establishing a connection to said trunk, a vacuum tube controlled over one conductor of said trunk, a plate relay for said tube, and a slow-acting relay controlled by said plate relay for holding up said connection.

32. In combination, an automatic switch, a line extending to said switch, a vacuum tube having its grid connected to and controlled over said line, a plate circuit for said tube including a relay, and a motor magnet for said switch controlled by said relay.

33. In a telephone system, an automatic switch, a line, a vacuum tube, a circuit including the two sides of said line in series over which potential is supplied to the grid of said tube from the exchange, a local circuit for altering the potential on said grid when the line circuit is interrupted, and means included in the plate circuit of said tube for controlling said switch responsive to such alterations of grid potential.

34. In combination, an automatic switch, a line, means including a vacuum tube controlled over said line for operating said switch, means in the plate circuit of said tube for controlling the release of said switch, a grid circuit including said line and effective RODNEY G. RICHARDSON.