US2102769A - Service observing system - Google Patents

Description

SERVICE OBSERVING SYSTEM Filed July 29, 1936 4 Sheets-Sheet l lNl EN TOR h. W ULRICH A TTORNE Y Dec. 21, 1937. H. w. ULRICH SERVICE OBSERVING SYSTEM Filed July 29, 1936 4 Sheets-Sheet 2 Sing.

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INVENTOR By H. W ULRICH A TTORNEV Dec. w ULRlCH SERVICE OBSERVING SYSTEM Filed July 29, 1956 4 Sheets-Sheet 3 1/ WUL A TTOR/VE V Dec.'21, 1937. H ULRlCH 2,102,769

SERVICE OBSERVING SYSTEM Filed July 29, 1936 4 Sheets-Sheet 4 lNl/ENTOR H. W. ULRICH BY Patented Dec. 21, 1937 STATES one starts PATET SERVICE OBSERVING- SYSTEM Horace W. Ulrich, Newark, N. J., assignor to Bell Telephone Laboratories, Incorporated,

4 Claims.

This invention relates to service observing systems and more particularly to such systems in which the service upon any one of a plurality of telephone lines may be observed from a central point.

Systems of this character are well known and an arrangement on which the present invention is based is disclosed in U. S. Patent 1,19%,721 dated August 15, 1916 which arrangement has been used extensively for the past twenty years,

with the exception that the regular operators cord circuit shown in Fig. 1 of the patent, eliminates, in practice, the message register key 35 shown, the control of the contact 46 being trans-- ferred to the coin collect key 33; in other words, key 33 is used both as a message register key and as a coin collect key, that is to say, when the cord circuit is connected to a coin box line, such as line No. 1 shown, the operation of key 33 connects the high voltage negative battery 34 to the tip conductor of the line to dispose of coin 29 in the coin box at the subscribers station. At the same time 36-volt battery 46 is connected to the sleeve circuit of the line but as no message register is associated with this line it performs no useiul function at this time. Conversely, if the cord circuit is connected to the message rate line 26 which is provided with a message register 32 connected to its sleeve circuit but has no coin box, operation of key 33 will connect the high voltage negative battery 34 to the line as before which, however, performs no function as there is no coin box present on the line but battery, 44 connected to the sleeve will cause the message register to operate.

The'foregoing describes the standard arrangement of coin box and message rate lines and an operators cord circuit adapted to be used with either type of line,

While in many exchanges it is not customary to terminate both such types of lines at the same operators position it is occasionally desirable to do so and by the foregoing arrangement of the cord circuit either coin disposal or message registration is effected by the simple operation of a single key. I

In order to avoid annoying subscribers with a click when either a coin is collected or when the message register is operated it has heretofore been the practice to collect coins and register calls at the end of the conversation, i. c.,.after the calling subscriber has hung up. It is at exchanges in which this operating practice is followed that the service observing circuit arrangement in accordance with the foregoing Patent 1,194,721 is adapted for use.

Conditions under which collection of coins at the start of conversation is desirable are described in a c'opending application filed August 8, 1935 of Anderson, Bascom et a1. Serial No. 35,225, and in the copending application filed May 19, 1936 for H. W. Ulrich Serial No. 80,522 (Case 24) a similar but somewhat improved circuit arrangement is disclosed which is also arranged for coin collection at the start of conversation, the cord circuit of which is shown having a single key which when operated connects both high voltage coin battery to the tip of the line and message register battery to the sleeve of the line.

Now considering the operation of the service observing system as described in the before-mentioned Patent 1,194,721 it Will be noted, see page 3, lines 67 to 107, that when the operator answers the call with her cord circuit, relay 522 operates which completes a Wheatstone bridge circuit which is at that time sufiiciently unbalanced to energize coil l3! of the voltmeter relay 525 in a direction to close contact I32 thereby operating relay I35 which, in turn, reenergizes relay 2t! to hold lamp 201 lighted as long as the subscriber has his receiver off the hook.

When the receiver is restored the resistance of the third arm of the bridge is changed sufiiciently to balance the bridge thereby deenergizing coil |3| of the voltmeter relay which opens contact I32 thereby releasing relays E35 and 29! in turn and extinguishing lamp 201 as an indication to the observer that the subscriber has hung up. The fundamentals of such a VVheatstone bridge arrangement for. controlling a lamp signal in service observing systems are disclosed and claimed in the U. S. patent to Bascom 1,232,133 dated July 3, 1917.

On page 7, lines 40 to 45 the specification states the voltmeter relay operates in the same manner as before described when the operator answers a call on line 26 which is equipped with a message register in its sleeve circuit.

As pointed out in the' specification after the operator has answered a call on line 28 the voltmeter relay is operated to hold lamp till or 235 (depending on whether key 266 or 29s is operated) lighted. On page 6, lines 117 et seq. it is stated that the operator registers the call on line 26 byconnecting the 36-volt battery 44 to the sleeve of the line which operates the meter magnet 32. Magnet 32 in operating short circuits its right-hand winding thereby lowering its efiective resistance.

Although not specifically pointed out in the specification it will be observed that when relays I43 and H8 operate resistance I29 is not connected to the sleeve wire as was the case when relays I I3 and I M operated. The reason for this is that the meter magnet 32, which has the same resistance, when both windings are connected in series, as resistance I29 is substituted therefor in the fourth arm of the Wheatstone bridge which now comprises a parallel arrangement of the sleeve cut-off relay 43, meter magnet 32 and relays I I1 and H8. It will be obvious, therefore, that if the operator should register a call on line 26 while this line is under observation and the calling subscribers receiver is off the hook as indicated by the lighted lamp 281 (or 235) when the meter magnet operates its right-hand winding is short-circuited thereby lowering the resistance of the fourth arm of the bridge which unbalances the bridge in the opposite direction and causes the voltmeter relay to open its contact I32 thereby extinguishing lamp 207 or 235 and giving a false indication to the observer. Due to the fact that magnet 32 is locked in an operated condition during the remainder of the connection the bridge remains in this unbalanced state and cannot be relighted.

1e lamp 281 (or 235) Hence, further observation on the line is interfered with.

As hereinbefc-re mentioned, however, this difficulty has not arisen in the past as ithas been the standard operating practice to collect coins and register calls at the end of conversation and not at the start of conversation and consequently lamp 2d! (or 235) has already been extinguished by the replacing of the calling subscriber's receiver. However, in case it is desired to use this service observing arrangement in exchanges where the operating practice is to collect coins and register calls at the start of conversation, as described in the application referred to, the foregoing defect is serious and some steps appear necessary to prevent falsely extinguishing lamp 28! (or 235) before the calling subscriber hangs up.

An object of this invention, therefore, is to prevent falsely extinguishing the receiver off the hook lamp in service observing systems of the type disclosed in Patent 1,194,721.

A feature of the invention whereby the foregoing object is attained resides in means responsive, when message register potential is connected to the sleeve of a message register line connected to the service observing equipment and subsequently disconnected, to hold the voltmeter relay operated under control of the subscribers switchhook, thereby maintaining the receiver ofi the hook lamp lighted.

More specifically the foregoing feature resides in connecting a predetermined resistance element in parallel with the resistance in the second arm of the Wheatstone bridge circuit when the message register in the fourth arm of the bridge is operated and the message register potential is disconnected from the sleeve, thereby compensating the bridge, for the change in resistance of the fourth arm by the operation of the register, sufiiciently to maintain a difference in potential between the terminals of the voltmeter relay winding to hold this relay operated.

Another feature related to the foregoing resides in additional means comp-rising a second voltmeter relay which is connected in series with the first relay, when message register potential is applied to the sleeve of either a message register or coin box line and subsequently removed, and operated if the connected line is not equipped with a message register, to disconnect the parallel resistance from the second arm of the bridge, which was connected when the message register potential was removed from the sleeve, and. to restore all of said means andsaid bridge to the condition prior to application of message register current to the line sleeve.

The invention will be understood by referring to the accompanying drawings, Figs. 1, 2, 3 and 4 of which when placed end to end as shown in Fig. 5 illustrate the essential parts of the observing system as originally described in Patent 1,19%,721, together with the modifications thereof (shown on Figs. 2 and 3) in accordance with the present invention. Referring specifically to the various features of the drawings, Fig. l is identical, in all essentials, with Fig. l of Patent 1,194,721, including reference characters, with the exception that key 35 of the patent has been eliminated, the function of this key, which is to connect battery 44 to the sleeve of the plug, having been combined with key 33 by the addition of an extra pair of contact springs on key 33.

Fig. 2 is identical with Fig. 2 of the patent, including reference characters, except for the following:

a. Addition of resistance IZS -A and its associated key which connects to the tip of jack I42 in the same manner as resistance I29 is connected to the tip of jack I40.

b. Addition of a second grounded armature (I553) to relay H8.

0. Addition of two conductors I61 and I68.

d. Elimination of grounded resistance I30, which was shown in error in the patent.

In this figure (Fig. 2) also, certain current sources, relays and resistances such as relays II5, M5, and I46 and resistance I65 have been shown by dotted lines which were merely indicated by the table of Fig. 4 of the patent but not shown in original Fig. 2. For example, by referring to relay I 53 of the patent, it will be noted "iat one terminal of its winding appears as a lead A and by referring to the table shown in Fig. 4 we note that lead A of relay I I3 when used with line I, should be connected to terminal I of the group of relays, resistances and current sources shown at the bottom of the figure which, in the present case, is direct battery.

Fig. 3 is identical with Fig. 3 of the patent, including reference characters, except for the addition of a circuit arrangement whereby the present invention is achieved. which is shown within the dotted rectangle Y at the lower left of the figure and by the further addition of leads with which this arrangement is connected to the rest of the system. In this connection it will be noted that conductors I61 and I68, added to Fig. 2. are extended to and terminate in this new circuit arrangement, that the conductor originally extending from Fig. 2 to the moving coil l3! of voltmeter relay 126 of Fig. 3 of the patent has been openedat points I'iO and I'll and serially extended through the new circuit arrangement and that. an additional lead I'I3 has been extended from the new circuit arrangement to ground through resistance I28. Further, it will be noted that the table identified as Fig. i of the patent has been omitted.

The present Fig. 4 is an abbreviation of Fig. 5 of the patent which shows enough of the circuit to illustrate the operation of the invention. No changes, however, have been made, certain apparatus and omitted.

The invention will now be described in detail.

In Patent 1,194,721 beginning with line 58 on page 2, there will be found a description of the operation of the observing circuit when the subscriber on coin box line I removes his receiver from the switchhook and deposits 'a coin in the coin box associated with his station. "Briefly, this circuit operation may be'summarizedas follows:

Removal of the receiver and deposit of a coin operates line relay I5 which lights line lamp I0 and operates relay I 0|.

Relay IOI operates relay I02 which in turn operates relay 2M. Relay 20I lights lamps 204 and 205 at the service observing desk which indicates to the observers that a call is being made on one of the lines which are connected to the observing equipment. Operation of relay I02 also operates relays I09 and H0. Relay I09 operates relays II3 and H4. Relay II3 extends the calling line toward the service observing desk and relay H4 operates relay II2 which releases relays I 09 and H0. Relay II4 operating, locks itself and relay I I 3 operated over a circuit which includes relay I05 which relay operates and causes relay I02 to release. As long as relay I05 is operated all other lines are prevented from being connected to the observing trunk because relay I02 can not operate due to the removal of ground at the. back contact of relay I05. Release of relay I02 does not extinguish lamps 204 and 205 as relay I I2, now operated, furnishes ground connections having merely been to hold relay 23I operated.

' lights lamp 201' and connects the observers telephone set to the observing trunk D.

It is further assumed that key 206 is operated before the regular operator answers the calling line by inserting the answering plug of the cord circuit E into the jack of line I.

Now, when the call is answered by the regular operator by inserting the answering plug in the jack of line I, cut-oil? relay I? and relay II'I operate from the sleeve battery of the cord. Relay He is in series with relay I I! but does not operate due to its resistance. .Operation of out oif relay I1 releases line relay I5 which in. turn extinguishes the line lamp I6 and releases relay II2. Operation of relay III operates relays ME, I 20 and 622. Relay I20 extends the trunk through to the observing desk and relay I22 operates relay 208 which in turn lights lamp 280 to indicate to the observer that the operator has answered the call. The operation of relay I22 also connects, at its lower contact, battery to lead I12 which extends to the voltmeter I26 of Fig. 3 thereby completing a Wheatstone bridge circuit in which resistance I2'l forms the first arm, resistance I20 the second arm, resistance 2i and lamp I8 in parallel and in series with resistance I0 forms the third arm, and three parallel paths form the fourth arm which consists of the following: First path, winding of cut-01f relay I I; second path, winding of relays II! and i I8 in series; third path, resistance I28. Ground is connected at the junction of the second and fourth arms and the exchange battery is connected to the junction of the first and third arms.

In this connection it should be pointed out that when the coin subscribers line i is connected to the observing circuit by connecting plugs 24 and 25 with jacks I39 and I40, respectively, resistance I29 is connected to the tip of jack I40 by operating the associated key, this being done to make the combined resistance of the fourth arm of the bridge, the same as it would be if the message rate line 26 were connected to the observing circuit which line (26) includes the message register magnet 32 in parallel with the cut-01f relay 43 to ground. In other words, resistance I29 compensates the bridge for the absence of meter magnet 32 when a coinline is connected.

It will, of course, be understood that when the message rate line 26 is connected to the observing circuit, resistance I29-A, for example, associated with jack I42 will not be required and hence the associated key will be opened.

The moving coil I3I of the voltmeter relay I23 is connected across the foregoing described bridge the condition described the bridge is unbalanced.

sufficiently and in such a direction that sufiicient current flows through the coil I3I from the junction of the third and fourth arms to the junction I of the first and second arms to cause the voltmeter contacts I32 to close.

Closure of contacts: I32 short-circuits condenser I33 and resistance I34 and operates relay I35 which in turn supplies ground to hold relay 2! operated which otherwise would release due to the release of relay H2. 207 lighted at the observing desk to indicate to the observer that the subscriber has his receiver ofi the hook. Relay 20! will release for an in stant due to the release of relay H2, but it will immediately reoperate as soon as relay I35 operates. Thisxmay cause lamp 201 to flicker momentarily.

Restoration of the subscriber's receiver will release cord supervisory relay 22 which removes resistance 2| from parallel relation to lamp I8 thereby changing the resistance of the third arm of the bridge to such an extent that the bridge becomes balanced and coil I 3| of the voltmeter Functions 1, 2, and 3 described in line 108 on page3 through line '74 on page 4 of Patent 1,194,721 will not be redescribed herein as they are not directly related to the invention of the present application.

In case a subscriber on message rate line 26 calls, line relay 42 operates which operates relays IN and I02 as before, which latter relay operates relays 20I, I09 and H0. Relay 20I lights lamps 204 and 205 as usual and relay H0 operates relays I43 and H5. Relay H6 operates relays II2 and I05. Relay H2 in operating removes ground from relays I09 and H0 which release and relay I 05 in operating opens the circuit of relay I02 which releases and which also breaks the operating circuit of relays I00 and 0.

It will be assumed the same observer (position F) observes the lighted lamp 204 and operates relay 206 as before thereby extinguishing lamps 204 and 205, and lighting lamp 201 in their stead. 7

When the regular operator answers the call by inserting the answering plug of cord circuit E in the jack of line 26, line cut-off relay 43'and relay II'I operate. Cut-off relay 43 releases line This maintains lamp relay 42 which in turn extinguishes the line lamp and releases relay II2 which momentarily releases relay 20I. Lamp 261, however, is only momentarily extinguished as relay MI is immediately reoperated by the operation of relay I35 when the voltmeter relay I26 operates and closes its contacts I32 which follows the operation of relay I22, which relays together with relays H9 and I20 operate in response to operation of relay II1. Operation of relay I22 also operates relay 208 as before which lights lamp 2") as an indication to the observer that the operator has answered the call.

In accordance with the new operating practice before referred to, the operator 'will inquire the wanted number and establish the desired connection, but either simultaneously with plugging up the called line or prior thereto, will register the call on the calling subscribers meter 32 by operating key 33 of the cord circuit which opens the talking conductors of the cord, connects negative 110-Volt battery, 34 through relay 36 to the tip of the cord and line and 39-volt battery 44 through the magnet of the position register 45 to the sleeve circuit of the cord and line.

There being no coin box associated with line 26, connection of battery 34 to the tip of the line has no effect, but as this line is equipped with a message register 32 connected to the sleeve, operation of key .33 causes the message register to operate which short-circuits its high resistance left-hand winding thereby materially reducing the jack sleeve resistance to ground. The call is now registered on line meter 32 and also recorded on position register 45, which also operates.

Marginal relay H8, in series with relay II1, now operates due to the higher voltage of battery 44 connected to the jack sleeve and at its inner contacts connects ground to lead I61 .to operate relay I14, the circuit for which can be traced from ground, inner contact of relay I I8, lead I61, contacts I of relay I16, winding of relay I14, lead I68 and contacts of relay II1 to battery.

Relay I I8 also closes its upper contact to operate relay 230 which relay in operating lights lamp 236 and operates buzzer 23I, as an indi-' cation to the observer that the operator has registered the call on the line under observation.

When key 33 is released the 39-volt battery 44 is disconnected from the sleeve of the cord whereupon relay II8 releases as it is so designed as to require more current to hold it operated than is supplied by the normal 24-volt exchange battery. Magnet 32, however, holds up on the exchange battery during the remainder of the connection.

When relay II8 releases it disconnects ground from lead I61 whereupon relay I16 operates in,

series with relay I14 in the following circuit: Ground, contacts of relay I11, contacts of relay I14 (now operated), windings of relays I16 and I14 and lead I68 to battery at the operatedcontacts of relay 1.

Relay I16 in operating opens its contacts I18 thereby removing the short circuit that has heretofore existed across coil I19 of a-second voltmeter relay similar to I26.

Relay I16 also closes its upper contact thereby connecting a high resistance I80 in parallel with resistance I28 thereby maintaining a surficient unbalance in the bridge circuit, and in the proper direction, to hold coil I3I energized and contacts I32 closed which, otherwise, would be open due to the unbalance caused by shortcircuiting the high resistance winding of magnet 32 of the message register.

The coil I19 of'the second voltmeter relay is now in series with coil I 3I across the bridge, but being of slightly lower resistance the current therein is not suflicient to cause its contacts I8I to close.

As relay I35 is held operated by voltmeter relay I26 lamp 201 remains lighted as an indication that the subscribers receiver is still off the hook.

Returning now to the case where the coin line number I was under observation, it will be recalled that when the operator answered the call a bridge circuit was established the fourth arm of which was made up of a parallel arrangement of cut-oflf relay I1, resistance I29 and relays H1 and H8 in series, resistance I29 being substituted at this time to compensate for the resistance of message register 32 which is present in the circuit when a message register line is observed.

In accordance withthe operating practice previously referred to, the operator upon receiving the wanted subscribers number, before plugging up and ringing on the called line, operates the combined coin collect and message register key 33 in the cord circuit which, as before described, connects negative 1l0-volt coin battery 34 to the tip of the line and 39-volt message register battery 44 to the sleeve circuit.

Connection of 110-volt battery to the tip of the line causes coin magnet 30 to deposit the coin 29 in the proper receptacle as is well known.

Connection of the 39-volt battery 44 to the sleeve causes marginal relay H8 to operate, as before described in connection with message register line 26, which in turn operates relay I14.

When key 33 is released, following the disposal of the coin, relay I16 operates in series with relay I14, as previously described. Relay I16 in operating connects the high resistance I80 in parallel with resistance I28 in the second arm of the bridge, as before described, and removes the short circuit from coil I19 thereby connecting it in series withcoil I3I of the voltmeter relay I26. Under this condition, however, as no change occurs in the resistance of the line sleeve due to the absence of the message register and the substitution therefor of the resistance I29, the bridge is! unbalanced to such an extent that a larger current flows from the junction of the third and fourth arms of the bridge through coils I19 and I3I in series to the junction of the first and second arms which current is sufiicient to cause coil I19 to close its contact I8I which completes a circuit to operate relay I82 which can be traced from ground, resistance I83, winding of relay I82, contact I8I, contact I84 of relays I16 and I68, and contact of relay II1 to battery. Relay I82 operates and closes an obvious circuit to energize slow-operate relay I11 which soon opens its contact and releases relays I14 and I16. Relay I16 in releasing, disconnects the high resistance I80 from the second arm of the bridge and again short-circuits voltmeter coil I19 which opens its contact WI. The original bridge unbalance again exists which is sufiicient to hold contact I32 closed, relays I35 and I operated and lamp 201 lighted.

It will be obvious that it was necessary to remove the high resistance I80 from the second arm of the bridge as soon as it was connected, on a coin line, as otherwise the unbalance of the bridge would have been such that the voltmeter relay coil I3l would have received such an increase in current that disconnection of resistance 2| from the third arm of the bridge when the Calling subscriber hung up would not have reduced the current through coil I3l sufficiently to allow contact I32 to open. Therefore, lamp 201 would have remained lighted as a false indication to the observer. 1

The remainder of the system functions in the same manner as described in Patent 1,194,721

and therefore a repetition of its description appears unnecessary. I

What is claimed is:

1. In a service observing system, a subscribers line having a message register, adapted to reduce its effective resistance when operated, associated with the line, an operators cord circuit adapted to be connected thereto having means therein for operating the message register, a service observing circuit connected to said line, a signal lamp associated with said observing circuit, means comprising a voltmeter relay in a Wheatstone bridge arrangement established when the cord is connected to the line, said voltmeter' relay being adapted to operate to light said lamp when the subscribers receiver is off the hook, one arm of said Wheatstone bridge arrangement including said message register, and means eifective when said message register is operated to compensate said bridge for the change in resistance of its message register arm to maintain said voltmeter relay operated.

2. In a service observing system, a subscribers line having a cut-off" relay in its sleeve circuit and a double-wound message register magnet in shunt thereto, an operators cord circuit adapted to be connected to said line having a source of current, a supervisory signal and a resistance serially connected in its sleeve conductor, a second resistance, a supervisory relay in said cord adapted when operated to connect said second resistance in shunt to said signal, a second source of current of a higher voltage than the cord sleeve source for operating said message register, means in said cord for connecting said second source to the sleeve thereof, said register being adapted when operated to lower its resistance in shunt to the cut-off relay, a service observing desk, a trunk line, having a sleeve conductor, for extending said subscribers line to said desk, means responsive to a call on said subscribers line for connecting said trunk sleeve conductor to the subscribers line sleeve, a pair of relays in series in the trunk line sleeve conductor and in shunt t the line cut-off relay when the cord is connected to the line, one of said relays being marginal and adapted to operate only on the higher potential of said second source, a voltmeter relay, a pair of high resistance elements, means responsive to the connection of said cord circuit to said subscribers line to establish a Wheatstone balance circuit, the first and second arms including respective ones of said high resistance elements, the third arm said supervisory signal and its series resistance and said second resistance in shuntto the supervisory signal when the cord supervisory relay is operated, the fourth arm including a parallel arrangement of said line cut-off relay, message register magnet and pair of trunk line sleeve relays, and the bridge arm of the balance including the winding of said voltmeter relay, said resistances being so proportioned that when the cord circuit is connected to the subscribers line and the cord supervisory relay is operated a difference in potential will exist between the ends of the bridge arm sufficient to cause the voltmeter relay therein to operate in one direction to close one of its contacts and, when the supervisory relay is released, the potential difference between the ends of the bridge arm will be insufiicient to hold the voltmeter relay operated, a lamp signal at said desk adapted to be lighted, when the voltmeter relay contact is closed, as an indication that the subscribers receiver is off the hook, and means jointly controlled by said trunk sleeve relays so arranged that the change in resistance of saidmessage register magnet caused by its operation, thereby causing a change in resistance of the fourth arm of the balance, will simultaneously effect a change in the opposing arm of the balance to maintain a suflicient difference of potential between the ends of the bridge arm to hold the voltmeter relay operated as long as the message register is operated and the subscribers receiver is off the hook.

3. In a service observing system, a subscribers line having a cut-off relay in its sleeve circuit and a double-wound message register magnet in shunt thereto, a second subscribers line having a cut-off relay in its sleeve circuit, a resistance, equivalent to that of said first line message register, connected in shunt thereto, an operator's cord circuit adapted to be connected to either of said lines and having a source of current, a su pervisory signal and another resistance serially connected in its sleeve conductor, a third resistance, a supervisory relay in said cord adapted when operated to connect said third resistance in shunt to said supervisory signal, a second source of current of a higher voltage than the cord sleeve source for operating the message register in the first line, means in said cord for connecting said second source to the sleeve thereof, said register on the. first line being adapted when operated to lower its resistance in shunt to its associated cutoff relay, a service observing desk, a trunk line, having a sleeve conductor for extending either of said subscribers lines to said desk, means responsive to a call on either of said subscribers lines for connecting said trunk sleeve conductor to the calling line sleeve, a pair'of relays in series in the trunk line sleeve conductor and in shunt to the cut-off relay of the connected line, one of said relays being marginal and adapted to opcrate only While said second source is connected to the sleeve, a voltmeter relay, a pair of high resistance elements, means responsive to the connection of said cord circuit to either of said subscribers lines to establish a Wheatstone balance circuit, the first and second arms of which include respective ones of said resistance elements, the third arm including said supervisory signal and its series resistance and said third resistance in shunt to the signal when the cord supervisory relay is operated, the fourth arm including a parallel arrangement of the line cut-off relay, the pair of trunk line sleeve relays in series and the message register or its equivalent resistance,

depending on which line is calling and connected to by said cord, the bridge arm of the balance including the winding of said voltmeter relay, and said first cord sleeve current source connected across the remaining point of the balance, said plurality of resistance being so proportioned that when the cord circuit is connected to the subscribers line and the supervisory relay is operated a difference in potential will exist between the ends of the bridge arm suiilcient to cause the voltmeter relay therein to operate in one direction to close one of its contacts and, when the supervisory relay is released, the potential difference hold it operated, a lamp signal at said desk adapted to be lighted when the voltmeter contact is closed as an indication that the subscribers receiver is 01? the hook, means jointly controlled by said trunk sleeve relays when the current source is connected to the sleeve of the message register line and subsequently disconnected to maintain a suiiicient difierence in potential across said voltmeter relay to hold it' operated. as long as the message register is operated, and the receiver is off the hook, and further means including a second voltmeter relay connected in series with the first voltmeter relay in response to the joint operation of said trunk sleeve relay and the subsequent release of one of them and operated, if the non-message register line is connected, to immediately restore the Wheatstone balance to the condition existing before the second source was connected to the subscribers line sleeve.

4. In a service observing system, a subscribers line having a cut-off relay in its sleeve circuit and double-wound message register magnet in shunt thereto, an operators cord circuit adapted to be connected to said line having a source of current, a supervisory signal and a resistance serially connected in its sleeve conductor, a second resistance, a supervisory relay in said cord adapted when operated to connect said second resistance in shunt to said signal, a second source of current of a higher voltage than the cord sleeve source for operating said message register, means in said cord for connecting said second source to the sleeve thereof, said register being adapted when operated to lower its resistance in shunt to the cutofi relay, a service observing desk, a trunkline, having a sleeve conductor, for extending said subscribers line to said desk, means responsive to a call on said subscribers line for connecting said across the voltmeter relay will be insufiicient to first and second arms including respective ones of said high resistance elements, the third arm said supervisory signal and its series resistance and said second resistance in shunt to the supervisory signal when the cord supervisory relay is operated, the fourth arm including a parallel arrangement of said line cut-off relay, message register magnet and pair of trunk line sleeve relays, and the bridge arm of the balance including the winding of said voltmeter relay, said resistance being so proportioned that when the cord circuit 'is connected to the subscribers line and the cord supervisory relay is operated a difference in potential will exist between the ends of the bridge arm suflicient to cause the voltmeter relay therein to operate in one direction to close one of its contacts and, when the supervisory relay is released, the potential difference between the ends of the bridge arm will be insufficient to hold the voltmeter relay operated, a lamp signal at said desk adapted to be lighted when the voltmeter relay contact is closed as an indication that the subscribers receiver is off the hook, and means jointly controlled by said trunk sleeve relays to maintain a sufficient difference of potential across said voltmeter relay to hold it operated when the message register magnet is operated before the receiver is replaced on the hook.

HORACE W. ULRICH.

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