US2205551A - Telephone system - Google Patents

Description

J1me 1940- s B. WILLIAMS TELEPHONE SYSTEM 2 Shes tS-SIIGGC l n I i O 9 I Q N UK Filed July 8, 193

A Hg? IN [/5 N TOR By 5.8. W/LL/AMS MW m A T TOR/VEV June 25, 1940. s B. w LL AMs 2,205,551

' TELEPHONE SYSTEM Filed July 8, 1939 2 Sheets-Sheet 2 CONS TAN 7" CURRENT SOURCE /N l EN TOR -S. B. WILLIAMS A TTORNE V Patented June 25, 1940 UNITED STATES PATENT OFFICE TELEPHONEv SYSTEM Samuel B. Williams, Brooklyn, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 8, 1939, Serial No. 283,352

6 Claims.

This invention relates to telephone systems and particularly to automatic systems in which the subscribers are charged for the service rendered by means of message registers.

The objects of the invention are to prevent the operation of the message registers from interfering with the functions of the switching equipment; to insure against false operation of the message registers in the event that unstandard conditions occur in the circuits associated therewith; to test the message register operating circuits to determine that they are in proper condition before applying operating currents thereto; and to secure other improvements in systems of this character.

A feature of the invention is a system in which a single control conductor is utilized for controlling the hunting movement of the line finder that seizes the subscribers line and for applying operating current to the subscribers message register, in which the test relay of the line finder, which energizes to stop the movement of the line finder when it seizes the terminals of the calling line, is permitted to operate in case the finder encounters the terminals of a line on which message registration is taking place, and in which the test relay so operating prepares a circuit, depending upon the busy condition of the line encountered, to maintain the continued movement of the finder over the busy line thus encountered. This feature has special utility in connection with party line systems where two message registers are connected through polarizing devices, such as rectifiers, to the combined message register and hunting lead of the line finder switch. The presence of these rectifying devices in this lead increases the resistance thereof and raises the potential on the multiple terminals of the lead in the line finder switch. If, therefore, a finder encounters the multiple terminals of this lead at the instant message registration is taking place, the potential thereon is sufficient to operate the testrelay of the finder. This operation would, except for 4 the provisions above explained, result in the trical connection with each other. If these operating conductors are crossed the current supplied over the common operating conductor from the constant current source is insufficient to operate both registers.

Another feature of the invention is a testing arrangement in which a test is made automatically to determine whether the message register lead of any particular line is in proper condition before operating current is applied thereto. If the operating lead is in engagement accidentally with the corresponding lead of some other line, a test relay operates and serves to withhold the application of the register operating current and to give an alarm.

These and other features and advantages of the present invention will be described more fully in the following detailed specification.

In the drawings accompanying the specification Figs. 1 and 2 illustrate an automatic telephone system partly in diagrammatic manner and partly in detail.

Fig. 1 shows a two-party subscribers line appearing in a line finder switch together with a portion of one of the links comprising a line finder and a district selector switch; and

Fig. 2 shows the remainder of the finder-district link circuit together with the succeeding switches used in establishing a connection to a called line.

The subscribers line I, having two substations thereon, appears in the terminals of line finder switches, such as switches F and F-l. Each of the finder switches is paired with an individual district selector switch comprising a link circuit. For example, the finder F is paired withthe district selector S, and the relays and other equipment associated with these switches constitute the link circuit. In addition to the various relays of the link. circuit a sequence switch is provided having numerous circuit-controlling contact springs which advance through successive' positions for performing the necessary switching operations.

The district selector switches have access to finder switch seizes the calling line. The fourth brush of the finder switch has two functions. One of these is to extend the test or hunting circuit whereby the finder is able to determine which line is calling. The second function is to furnish a circuit for the operation of the subscribers message registers. Since it is necessary to discriminate between two message registers, where the subscribers line has two parties thereon, the operation of one or the other of these registers must be performed over a single circuit, namely the one extending through the fourth brush of the finder switch. More specifically, the brushes 2 and 3 of finder F are the ones that extend the talking circuit. Brush 4 is the sleeve brush, which completes the operating circuit for the cutoff relay 5. The fourth brush 6 is the one that controls the hunting operation and also the message register operating circuit.

The discrimination between the message registers i and 8 of the party line I is obtained by means of rectifying devices 9 and ii). The registers T and 8 are connected through a resistance l i and a contact of the line relay l2 to the single hunting and message register control conductor [3. By poling the rectifiers 9 and It oppositely it is possible to select the register 7 by applying current of one polarity to the conductor l3 and to select the other register by applying current of the opposite polarity.

Discrimination between the two substations i4 and I is made by means of a party test in the link circuit. This test is made at the proper stage in the progress of the call to determine whether a ground connection exists on the line. If the party at substation It is calling, a ground connection is applied through the switchhook contacts and the ringer It to the tip conductor is of the line I, whereas if the party at substation E5 is. calling no ground connection is present.

While the invention is not necessarily limited to any particular kind of system, it has been illustrated in connection with an automatic system employing switches of the panel type. In view of their well-known character the structure of these switches, such as the line finders F and Fl and the selectors S, 8-! and S--2,

has been largely omitted from the disclosure. Also the controlling circuits associated with the line finder and district selector switches F and S have been abbreviated as far as possible in order to facilitate an understanding of the present invention. However, reference may be had to the following patents for an understanding of the detailed circuits and operation of systems of the type to which this invention is applicable: Taggart et al. 1,513,351 of October 28, 1924, Irvine 1,541,356 of June 9, 1925, and Johnson et al. 1,859,924. of May 24, 1932. In particular, the patent to Taggart et a1. and the patent to Irvine disclose a system of this character in which identification tests are made on two-party lines for the purpose of message registration.

The rectifying devices 9 and I0 may be of any suitable type, such as the well-known copperoxide rectifier.

The operation of the system will now be described, and to this end it may be assumed that the subscriber at station M wishes to converse with the subscriber of line H. When the calling party removes his receiver from the switchhook, a circuit is closed from battery through the line relay I 2, left contact of cutofi relay 5, over the line conductor I8, through the closed loop at substation l4 and returning over the line conductor I9 to ground through the right back contact of relay 5. Relay l2 at its right front contact applies battery potential to the hunting lead I3. At its left contact relay l2 closes a circuit from ground over the start conductor through closed contacts of sequence switch cam 2|, conductor 22 through the winding of the line finder relay 23 to battery and ground.

The cam 2! is one of a plurality of contactmaking elements controlled by the sequence switch magnet 24, which advances the cams through a succession of positions to cause the opening and closing of the various contacts for circuit controlling purposes. It may be assumed that the sequence switch 24 is standing in position 2 at this time awaiting the seizure of the link, to which it is individual, comprising the finder F and district selector 8.. In position I the sequence switch serves in the well-known manner to control the allotment of the link. In position I of the sequence switch the link comprising the line finder F and selector S awaits the preselection by a sender link circuit, not

shown. As soon as the preselection takes place the sequence switch 24 advances into position 2 where, as stated, it awaits a call. The purpose of the sender selector link, with which we are not particularly concerned in connection with this invention, is to associate the subscribers line, after it has been seized by the line finder F, with an idle central office register sender.

Relay 23 in operating locks in a circuit from battery through its winding and contact, contacts of sequence switch cam 25, normal contacts of relay 35 to ground through the normal commutator segment 3| of the finder switch. The segment 3| has ground potential connected thereto in all positions of the finder F except the extreme uppermost position of the brushes. Relay 23 in operating also closes the driving circuit for the line finder F. This circuit may be traced from battery through the winding of the up-drive magnet 26, inner lower contact of relay 23, contacts of sequence switch cam 21,

middle lower front contact of relay 23, left contact of sequence switch cam 28 to ground at sequence switch cam 29. The magnet 26 causes the brushes 2, 3, 4 and 6 of the line finder F to move upwardly over the terminal bank in search of the terminals of the calling line I. As the finder moves upward the hunting brush 6 passes over the hunting terminals of the subscribers lines, and the test relay 38 tests these terminals for the presence of battery potential, indicating the calling condition of a line. As soon as the brushes 2, 3, 4 and 5 encounter the terminals of the line I the test relay 30 operates in a circuit from ground through the contactsof sequence switch cam 32, winding of relay 30, conductor 33, brush 6, terminal 34, lead l3, front contact of relay l2 to battery. Relay 30 opens the holding circuit of relay 23. Relay 23 remains energized until the brush engages an insulating segment of the centering commutator 36. Thereupon relay 23 releases to open the circuit of the power magnet 26 and bring the line finder F to rest centered on the terminals of the calling line I. While the sequence switch is in positions 2 and 3 the line is held busy by means of a circuit from ground over cam 29, cam I M, back contacts of relay 23 to conductor 40.

Relay 23 on releasing closes a circuit for driving the sequence switch 24 out of position 2. This circuit may be traced from battery through the winding of the sequence switch magnet 24, contact cam 31, upper contacts of cam 38, through the contacts of relay 39, which relay is in an operated condition at this time, middle lower back contacts of relay 23, left contacts of cam 28 to ground at cam 29. In position3 of the sequence switch, and from there on to position 17 A a circuit is maintained over the sleeve conductor for operating and holding the cutofi relay energized and for placing a busy condition on the multiple sleeve terminals accessible to other find-er switches. The busy sleeve circuit may be traced from ground through the left contacts of cam 29, conductor 4|], resistance 4!, brush 4, terminal 42, conductor 43, winding of the cutoff relay 5 to battery. This ground potential also appears on the multiple sleeve terminal 44 in the line finder switch FI and similarly on the terminals of other line finder switches having access to this line. Relay 5 operates and releases the line relay 12 to remove ground from the start conductor 20 and also to remove calling battery potential from the hunting lead 13.

In the manner well known in the art and disclosed in detail in the Taggart et al., Irvine, and Johnson et al. patents, above mentioned, the line finder-district link progresses to the point where the calling subscribers line is extended by Way of sequence switch cams 45 and 46 and conductors 47 and 48 to an idle register sender, not shown; the subscriber at the calling station manipulates his dial 49 to set the registers of the sender in accordance with the designation of the called line H; the fundamental circuits are prepared, and the selectors S, Sl and 8-2 are operated under the control of the register sender to extend the connection to the called line. During these proceedings the sequence switch 24 associated with the link is advanced from position to position, passing into position ll following the extenson of the connection to the called line.

As the sequence switch 24 reaches position II a test is made of the calling line I to determine which one of the two stations has initiated the call. In preparation for this test a circuit is closed from battery through the winding of relay 50, conductor 5|, contacts of sequence switch cam 52 to ground. Relay 50 operates and extends the grounded conductor 5| through its front contact, over conductor 53 through the winding of relay 54 to battery. Relay 54 operates in this circuit and connects the test relay 55 to both talking conductors 56 and 51 of the link. Relay 55, therefore, is connected by way of finder brushes 2 and 3 to the talking conductors l8 and IQ of the line I. Since the party at station l4 has his receiver off the hook, the grounded ringer I6 is connected through the upper switchhook contacts to both line conductors l8 and IQ. Test relay 55 operates in this circuit and closes a circuit from ground over its front contacts, conductor 59, outer left contact of relay 60, conductor 6| through the Winding of relay 52 to battery. Relay 62 operates and locks through its upper front contact to ground through sequence switch cam 63. Relay 52 remains locked through position 18 of sequence switch 24 to record the fact that the test revealed the presence of a ground potential on the calling line, indicating that the call was initiated by the party at station 14. As soon following the arrival of the sequence switch in position H as the constantly-driven interrupter 64 operates its right contacts a circuit is completed from battery through the left winding of relay 65, right contacts of interrupter 64, contacts of cam 52 to ground. Relay 65 operates and looks through its inner left contact to ground on conductor 5|. As soon thereafter as interrupter 54 closes its left contacts a circuit is completed from the grounded conductor 5|, left contacts of said interrupter, outer left front contact of relay 65, conductor 65, middle lower contact of relay 54', winding of relay 23 to battery. Relay 23 operates and closes a circuit for driving the sequence switch out of position II. This circuit may be traced from battery through the magnet 24 of the sequence switch cam 31, left contact of cam 38, lower front contact of relay 23, through the contacts of cams 28 and 29 to ground. As the sequence switch passes out of position H 4 it opens the left contacts of cam 52, and ground is removed from conductor 5| to permit the release of relays 50, 54, 55 and 65. The release of relay 54 opens the operating circuit of relay 23.

In subsequent positions of the sequence switch, such as positions l3 and I4, the talking circuit is completed, and for conversation between the calling and called subscribers. At the time the called subscriber answers the supervisory relay 61 operates, closing a circuit from ground through the front contact of said relay, conductor 68,,contacts of sequence switch cam '69 to conductor 70. As soon thereafter as the powerdriven interrupter H closes its left contacts the conductor is extended through the right winding of relay 65 to battery and ground. Relay 65 operates and locks through its inner right contact to the grounded conductor 10. When thereafter interrupter H next closes its right contacts the grounded conductor 10 is extended by way of said right contacts, outer right front contacts of relay 65, through the winding of relay 60 to battery and ground. Relay 60 operates and locks through its inner left contact, upper left contact of sequence switch cam 12 to ground over conductor 13, Thus the relay 60 operates to record the fact that the called subscriber has answered.

After conversation has ceased and the calling subscriber has replaced his receiver on the switchhook, the sequence switch 24 is driven out of the talking position and into position l5. As the sequence switch enters position I5, the relay 55 is reoperated and, in its operated condition, is applied to the calling line to make a second test to determine whether the line is free from ground potentials. The operating circuit of the relay may be traced from battery through the winding thereof, conductor 14, middle left contact of relay 60, lower left contacts of sequence switch cam 12 to the grounded conductor '33.

Relay 55 operates relay 50 in a circuit from ground through the contact of relay 55, conductor 59, outer left front contact of relay 6!), conductor 75, conductor 5i, thence through the winding of relay 5!] to battery. Relay 5!] operates and extends the grounded conductor 5| through its closed contact to conductor 53, operating relay 54. Relay 54 connects the winding of the operated test relay 55 to the tip and ring conductors of the subscribers line I to determine whether relay 55 will hold after its energizing circuit has been opened; or, in other Words, to determine whether there is a ground connection on the subscribers line after he has replaced his receiver to remove the grounded connection at the ringer l6. Relay 54 in operating also closes a circuit from battery through the winding of relay 23, lower contact of relay 54, conductor 16, upper, left contacts of cam 12 to ground over conductcr I3. Relay 23 operates and completes the circuit above traced for driving the sequence switch 24 out of position I5 and into position I6. As the sequence switch leaves position I5 the lower left contact of cam I2 opens the energizing circuit of the test relay 55, leaving this relay dependent for its maintained energization upon he circuit closed through the contacts of relay 54 to the line conductors. If the line has ground potential thereon, relay 55 is maintained energized and in turn holds relays Ell and 54 to prevent the sequence switch from advancing further. Relay 54 energizes the time measuring and alarm device Eli, and, after an interval has expired, this device gives an alarm to indicate the presence of an abnormal condition. If the subscribers line is in its normal condition and has no ground connection thereon, relay 55 immediately releases. The release of relay 55 permits the release of relays 59 and 54, provided the test relay II, which will be described presently, has not operated at this time to indicate an unstandard condition on the message register operating conductor l3.

In positions I5 and I5 of the sequence switch a second test is made, simultaneously with the testing of the subscriber's line, to determine whether the hunting and message register operating lead 5 3 is in accidental connection with the corresponding lead of some individual subscribers line. If two of these leads are crossed, the danger is that the application of metering potential to one of them will cause the false operation of the register on the other line.

The test for the crossed condition of the operating leads is made by the test relay TI. This relay is differential, and its windings and the associated resistance elements constitute a bridge circuit in which the operating winding of the relay is insuhicient to attract the relays armature if the lead IE to which the test relay is applied is in standard condition. However, the test relay will operate in case the resistance of the lead I3 is abnormally low due to a parallel path to ground through the message register of an individual subscribers line. When relay 50 operates following the movement of the sequence switch into position I5, the conductor i8, connected to the test relay IT, is extended through contacts of cam I9, conductor 8!], closed contact of relay 55, conductor 33, brush 6, terminal 34, conductor l3, back contact of relay I2, resistance H, to the conductor BI common to the registers i and ii. Also the operation of relay 50 connects the conductor 82 to ground. With these circuits closed an electrical bridge arrangement is established in which three of the arms are formed respectively by the resistances 83, 84 and 85, in which the fourth arm is formed by the resistance II and the register 8, in which the left winding of relay I! is connected across the balance points of the bridge, and in which the battery and the right winding of relay Ill and the resistance 88 are connected in series with the bridge. The left winding is differential to the right winding and receives suflicient current in the opposing direction to prevent the relay from operating when only the conductor I3 of one line is included in the fourth. arm of the bridge.

Should the conductor l3 of the party line be accidentally crossed with the corresponding conductor of an individual line, the combined resistance would be sufficiently low to cause the operation of the difierential relay T. to prevent the subsequent application of metering potential to the conductor 93. It follows, of course, that the parallel paths present when the conductors [3 of two lines are crossed result in a lowering of the resistance connected in series with conductor 33. And in the case assumed, namely, when a two-party line is crossed with an individual line, the combined resistance is so lowered that relay 71 operates. This is true by reason of the fact that the conductor I3 of an individual line is substantially lower in resistance since it does not include a rectifier nor the series resistance II. Should the conductors I3 of two different party lines become crossed, the combined resistance would be somewhat higher than the ease assumed, and it might be that the test relay would not be adjusted to detect this condition. In such a case, however, false operation of the registers on the party lines is prevented by limiting the current supply. This feature Will be explained later.

Assuming, therefore, that conductor #3 of line I is crossed with the corresponding conductor of an individual line, relay 'I'I operates and locks in a circuit through its windings in series and its inner lower contact to ground over conductor 89. In operating, relay 'I'I closes a grounded circuit by way of its outermost contact, conductor 99 and thence through the windings of relays 54 and Eli to battery, holding these relays energized after the sequence switch has passed out of position Ita to release relay 55 in the event the tip and ring conductors of the subscribers line are in normal condition. The maintained energization of relay 54 prevents the sequence switch from advancing out of position i6, and a circuit is closed from ground through the uppermost contact of relay 54 to the timing and alarm circuit 9i. After a measured interval has expired the circuit 9i gives the necessary alarm to call attention to the fact that the equipment has encountered trouble.

If, however, the subscribers line I is free from ground potential, and if the message register operating lead I3 is not in contact with the corresponding lead of an individual subscriber's line, when the sequence switch moves into position l5 and into position I6 as above described, the test relay 55 releases, and the test relay 1'! fails to operate. Consequently, relays 50 and 54 release as the sequence switch 'moves out of position I5 4 and into position I5. leases the relay 23, and a circuit is closed from battery through the winding of sequence svitch magnet 24, contacts of cam 31, contacts of cam 92, back contacts of relay 23 and thence to ground through the contacts of cams 28 and 29. The

sequence switch moves out of position I6, through position II and into position I8.

In position I! metering current is applied to the conductor I3 to operate the register individual to the calling party on the line I. The metering current is supplied from the current supply device 93 and the circuit therefor may be traced from the positive pole of said device over conductor 94, through the lower front contact of operated relay 62, conductor 95, right front contact of relay 6|], conductor 96, contacts of cam 91, thence over conductor 33, brush 6, and terminal 34 to conductor l3 through the back contact of relay I2, resistance I I, conductor 8i, through the rectifier 9 and the winding of register I to ground and returning through the inner lower front contact of relay 62 and conductor 93 to the negative pole of the supply circuit 93. Since the current flowing in this circuit passes only through the rectifier 9, only the mes Relay 56 resage register I will operate, charging thefc all to the subscriber of station I 4.

As the sequence switch moves into position I8 the necessary release circuits are closed for restoring the line finder switch F .and the other switches to their normal position. The sequence switch then'advances into position I; where it awaits allotment. i

When the other party on the line I, namely, the party at station I5, makes a call, the operation of the system is substantially the same, except that the test of the line by the relay 55 fails to find a ground potential thereon. There being no ground potential on the line when the party at station l5 calls, relay 55 does not operate in position II of the sequence switch, and

relay 62 consequently remains deenergiz'ed. When, later, the sequence switch is in position I I, the metering circuit is closed, and current of the opposite polarity flows thereover to operate the register 8 individual to the station I5. This circuit may be traced from the negative pole of the constant current supply device 93, conductor 98, lowermost back contact of relay 62, thence over conductor 95 as above tracedtothe common conductor 8| through the rectifier I0, winding of register 8 to ground and from ground through the inner lower back contact of relay 62, conductor 94 to the positive pole of the current supply device 93.

It has already been explained how the system is guarded against possible false operation of a message register when the conductor I3 of a party line is in accidental connection with the corresponding conductor of an individual subscribers line. This safeguard is afiorded by the test relay 1'! which does not operate in circuit with the normal resistance of the lead I3 of. a party line but does operate in circuit with the relatively low combined resistance of lead I3 of a party line and the corresponding lead (crossed therewith) of an individual subscribers line. It is also desirable, of course, to prevent false operation in the event that the leads I3 of twopartylines should become accidentally crossed. Since both of these leads contain the resistance element II and the rectifier element, the combined resistance is substantially greater than that encountered when a party line and an individual lineare crossed. In such a case it might be .difficult for the relay TI to distinguish between the resistance of a single party line lead and the combined re,- sistances ofv two party line leads. It ispossib-le, however, to prevent the false operation of .the registers by limiting the amount of current flowing over the message register operating circuit to a value which is insuflicient to operate either of the registers of the two party lines whose leads have become crossed. This is accomplished by means of the constant supply device 93, which converts alternating current into direct current and supplies it to the output conductors 94 and 98. For this purpose alternating current is supplied over conductors 99 and IE0 to the constant current rectifier 93.

Thus if the conductors I 3 of two party lines are crossed at the moment metering current is applied over conductor 95 to the conductor I3 of the line against which it is desired to assess a call, the lowered resistance, which would otherwise tend to draw the greater current required for operating both registers is without efiect. The constant current device 93 maintains its output current at a constant value regardless of. the increased load, and this value is insufficient to current rectifying device suitable for use in this system is shown in the patent to Bodde, No. 1,298,443 of March 25, 1919.

It will be recalled from the foregoing .descrip' tion that the single lead I3 is used for the hunt ing operation of the line finder and later for, the message register operation. Since each line is multipled before a number of final switches, it is possible for a moving finder switch to test the lead I 3 of a line in the hunting operation at the same instant that another finder, whichlhasalready seized said line for a conversational con 'nection, is applying message register operating current to the same lead. Under these conditions the resistance included in the lead I3 may be suificient to raise the potential of the hunting terminal to a point where a test relay in the hunting finder operates. Inasmuch as the oper ation of the test relay normally indicates that the finder has found the calling line it is seeking, it is necessary to provide meansfor ignoring the operation of the test relay under the conditions above mentioned. Assume, for. example, thatthe subscribers line, I has made a call, that the con,- nection was established by Way of the line finder switch FI, that conversation is over, and that message register operating current is being'applied by way of conductor I01, brush I08, terminal I09 to the lead, I3 to the line I. Assume also that. the finder switch F, which is seeking some other calling line, encounters the terminals of line I at this particular instant. It will berecalled thatduring the hunting operation of the finder switch F, sequence switch 24 is in position 2 and the'rela'y 23 is operated and locked through its inner upper contact, contacts of cam.25 to ground through the commutator, 3|. When the brush 6 of the finder F encounters the terminal 34, having the message register operating potential thereon, current flows by way of the brush 6 and conductor 33 through the winding of relay 30,:contact cam 32 to ground. This-operates the relay 30,,but relay 23, which normally would .release at this time to terminate the hunting movement of the finder, is held in another circuit to insure the continued movement of the finder F past the line I and in search for theother calling line. The holding circuit for relay 23 may 'be traced from battery through the winding and inner upper contact thereof, contacts of; cam 25,,

make-before-break contacts of relay. 3EI', conduc- 1 releases and recloses the normal holding circuit of relay 23 through the commutator 3|.

What is claimed is:

1'. In a telephone system, line finder switches, subscribers lines appearing in the multiple terminals of said finders, each line having a message register, a combined test and register operating conductor for each line appearing in the test terminals of said finders, means associated with a finder that has seized a line to render the seized line busy and to apply a source of register-operating potential to said test terminals to operate the register or". said seized line, means for applying a calling potential to the test terminals of a calling line and for initiating the hunting movement of one of said finders, test means operative in response to said calling potential to stop the hunting finder on the terminals of the calling line and also operative in response to said register-operating potential in case the hunting finder engages said seized line at the moment registeroperating potential is present on the test terminals thereof, and means for preventing the hunting finder from stopping on said seized line when the said finder test means is operated by said register-operating potential.

2. In a telephone system, line finder switches, subscribers lines appearing in the multiple terminal sets in said switches, each terminal set including a sleeve terminal and a hunting terminal, message registers for said lines, means responsive to a call on one of said lines for applying a calling potential to the hunting terminal thereof and for initiating the hunting movement of a line finder switch, a test relay in the line finder responsive to the calling potential on said hunting terminal for causing the finder to seize the terminals of the calling line, means controlled by the operated finder switch for placing a busy potential on the sleeve terminal of the seized line, means controlled by the operated finder switch for applying a register-operating potential to the hunting terminal of the seized line to operate the associated message register, means responsive to a call on a second line for initiating the hunting movement of a second finder switch, a test relay in the second finder switch responsive to the calling potential on the hunting terminal of a calling line for seizing the same and also responsive to the register-operating potential on the hunting terminal of said firstmentioned line in case said second finder encounters the hunting terminal of said first line at the moment the said register-operating potential is present thereon, and means associated with said second finder switch for rendering its test relay ineffective to stop said second finder on the terminals of said first line and for causing said second finder to continue its movement in search of said second-mentioned calling line.

3. In a telephone system, subscribers lines having message registers, a control conductor for each of said lines for operating the message register thereof, a common current supply conductor connectable to the control conductor of any calling line, and a source of constant intensity current for operating a desired register over a circuit including the common supply conductor and the control conductor associated with the desired register, the current supplied from said source being sufficient to operate the register of any one of said lines but insuificient to operate the registers of two lines in case the control conductors of said lines are accidentally in contact with each other.

4. In a telephone system, subscribers lines having individual message registers, control circuits individual to said lines for operating the respective registers, a line finder switch for seizing a calling line, a common supply conductor connectable by said finder switch to the control conductor of the seized line, and a constant current supply source associated with said finder switch for supplying operating current to the register of the seized line over a circuit including the common supply conductor and the individual control conductor, the current from said source being independent of the resistance of said circuit and sufiicient to operate a single register but insuflicient to operate two registers in case the control conductors of two lines are in contact with each other.

5. In a telephone system, subscribers lines having message registers, message register conductors for operating said registers, a selective switch for seizing a calling line, a common operating conductor connectable through said switch to the message register conductor of the calling line, resistances in said message register conductors, a test relay connectable to said common conductor for testing the resistance in series therewith and operative if the message register conductor of the calling line is in contact with that of a second line, a source of current applicable to said common conductor to operate the calling line message register, and means controlled by said test relay to withhold the application of said source.

6. In a telephone system, party lines, individual lines, message registers for said lines, control conductors individual to said lines for operating the associated message registers, resistance elements in the control conductors of said party lines, a finder switch for seizing a calling line, a common supply circuit connectable through said switch to the control conductor of the calling line, a test relay connectable to said common conductor for testing the resistance in series therewith and operative if the control conductor of the calling line is in contact with that of a second line, a source of current applicable to said common supply circuit to operate the calling line message register, and means controlled by said test relay to withhold the application of said source.

SAMUEL B. WILLIAMS.

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