US2335512A - Telephone ringing apparatus - Google Patents

Description

Nov. 30, 1943. R. H. HERRICK 2,335,512

TELEPHONE RINGING APPARATUS Filed July 3, 1941 3 Sheets-Sheet 1 llb ' LINE SWITCH SELECTOR TO comEc'roFzs INVENTOR. ROSWELL H. HERRICK ATTORNEYS NOV. 30, 1943. R HERRlCK 2,335,512

TELEPHONE RINGI'NG APPARATUS Filed July 3 1941 3 Sheets-Sheet 2 MARGINAL CONVERTING UNIT l7 RELAYS l9 iilBRATOR NO. 2

I VIBRATOR No.3 T?

RA OR N0 INVENTOR. ROSWELL H. HERRIOK M02547, M M

ATTORNEYS Nov. 30, 1943. R. H. HERRICK TELEPHONE RING-ING' APPARATUS Filed July}, 1941 5 Sheets-Sheet 3 lllll INVENTOR. ROSWELL H. HERRICK un thy-2 358 I'll Ill-Ill ATTORNEYS Patented Nov. 30, 1943 TELEPHONE RENGING APPARATUS Roswell H. Herrick, Oak Park, 111., assignor to Automatic Electric Laboratories, Inc., a corporation of Delaware Application July 3, 1941, gerial No. 426M335 24 Claims.

The present invention relates to telephone ringing apparatus and more particularly to improvements in ringing apparatus of the character used in small exchanges where facilities for selective ringing on party lines are required.

In installations of this character, ringing currents of the required frequency or frequencies are derived from the exchange battery through converter channels which usually include converters of the pole changer type. This type of converter conventionally includes a pole changer relay which functions alternately to energize the two halves of the primary winding of an associated transformer from the exchange battery, so that an alternating voltage of the desired frequency is produced across the secondary winding of the transformer. Under conditions of heavy load, when the contacts of the pole changer relay are required to make and break the highly reactive circuits over which the transformer primary windings are alternately energized, serious and objectionable arcing and pitting of the contacts may occur.

It is an object of the present invention, therefore, to provide improved ringing apparatus of the character described wherein no moving parts are used in converting the direct voltage of the available direct current source into alternating current of the desired frequency.

It is another object of the invention to provide improved ringing apparatus of the pole changer type wherein provisions are made for limiting the currents traversing the contacts of a pole changer relay to exceedingly minute values regardless of the load demands on the apparatus.

. According to another and more specific object of the invention, the variations in the currents traversing the primary windings of the conversion transformer are effected through appropriate control of thermionic space current paths which are included in circuit with the primary windings.

In accordance with another object of the invention provisions are made for automatically establishing an alternative conversion channel, from which the space current paths of the thermionic means are excluded, when certain abnormal conditions, such, for example, as failure of the thermionic means, are present in the apparatus.

According to another object of the invention improved and exceedingly simple arrangement is provided for indicating the condition of the apparatus and for producing a distinctive signal indication when an abnormal condition is present therein.

It is another object of the invention to provide improved ringing apparatus of the character described which operates to provide a plurality of output frequencies in a positive, reliable and improved manner and includes a minimum of circuit equipment.

According to another and more specific object of the invention a single transformer is utilized in the apparatus, successively to deliver output currents of the required difierent frequencies.

The novel features believed to be characteristic of the invention are set forth with particularity in the appended claims. The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in connection with the accompanying drawings, in which Figs. 1 and 2, when laid one above the other in the order named, illustrate an automatic telephone system having incorporated therein improved ringing apparatus constructed and arranged in accordance with the present invention; and Fig. 3 illustrates a modified arrangement of the ringing apparatus which may be used in conjunction with the automatic switching equipment shown in Fig. 1 in lieu of the ringing apparatus shown in Fig. 2.

Referring now more particularly to Figs. 1 and 2 of the drawings, the system there illustrated may be provided in a small unattended exchange which serves one thousand lines or less. The system includes a plurality of single party lines,

- one of which is indicated at H] as extending to the substation A, and a plurality of multiparty lines one of which is indicated at H as serving the substations B and C. These lines terminate in the exchange in line switches 20 and 2|, respectively. The automatic switching equipment of the exchange further includes a plurality of selectors, one of which is indicated at 25, which are accessible to the various line switches and in turn have access to a plurality of groups of connectors, one of which includes the connector 30. Preferably the line switches 20, 2|, etc., are of the well-known rotary type of which several commercial embodiments are available. Each of the selector and connector switches of the system is preferably of the well-known Strowger type having embodied therein a wiper carriage structure which is adapted to be translated vertically to bring the wipers thereof to a position opposite a particular level of bank contacts and then rotated to bring the wipers into engagement with a particular set of contacts in the selected level. Such movement of the Strowger switch wipers is conventionally obtained by providing vertical and rotary magnets and suitable relay equipment for controlling these magnets and for effecting the required signaling and line switching operations. If desired, the selector 25 may be of the form illustrated and described at page 69 at seq. of the book entitled Telephone Theory and PracticeAutomatic Switching and Auxiliary Equipment, Written by Kempster B. Miller and published in 1933. The connector 30 may be of the character disclosed at pages 82, 83 and 84 of this book.

In order more clearly to explain the operation of the circuits to which the invention pertains, a

portion of the control equipment embodied in the connector 30 has been illustrated in detail. More specifically this equipment comprises a slow-tooperate ringing control relay RIIIO, a slow-acting ring cut-off relay RI I and a ringing frequency selecting switch I20 of the well-known minor type. In brief the switch I20 comprises a single set of bank contacts I2I, a wiper I22, an operating magnet I23 for driving the wiper I22 over the contacts of the contact set I 2 I, a release magnet I24 which when operated permits the wiper I22 to be restored to normal, and a set of oil normal springs I25. As indicated above the line switches individual to the various subscriber lines are of the well-known rotary type. The switch 2| is illustrated as including a line relay RIM) which includes windings normally bridged across the conductors of the line II. If desired, these switches may be of the form disclosed at pages 48, 49, 50 and 51 of the above-cited book written by Miller.

The substation equipment provided at each of the substations A, B, C, etc., of the system may be of any desired commercial form. In brief, each of these substations includes a transmitter, a receiver, a bell or other form of signal device, an anti-side-tone induction coil and a hook or cradle switch which is arranged to be controlled by the associated receiver to complete a substation circuit of the well-known anti-sidetone type. In this regard it is pointed out that the ringers provided at the substations served by the multiparty lines of the system are of the well-known tuned reed type, such that each thereof Will respond only to ringing current of a particular frequency.

The ringing apparatus which. is provided in the exchange is arranged to be controlled by the automatic switching equipment just described and functions to provide the difierent ringing current frequencies through which selective signaling over the multiparty lines is eflected. More particularly, the operation of the equipment is controlled by a manually operable switch 236 and a start relay R235 which is arranged to be energized over the common start lead 295. Briefly considered, this equipment comprises a pair of converting units 51 and I3 which may selectively be used in converting the direct cur-- rent of the exchange batter I5 into alternating current of the desired frequency. More specifically, a transfer relay R289 is provided for selectively associating the two converting units I1 and I8 with the pulse generators SI, 32, 33, etc., having different pulse periodicities. For the purpose of successively connecting the different pulse generators, one at a time, to the one of two converting units selected by the transfer relay R280, a set of cyclically operating sequence relays I9 is provided. This set of relays includes a timing relay RZ IB, transfer relays R250, R250, R2Ic, etc., individual to th several pulse generators 3I, 32, 33, etc., and a reset relay R290. Each of the several pulse generators 3H, 32, 33, etc., is of the pole changer type and includes a relay provided with a weighted armature having a specific period of vibration. For example, the pulse generator SI comprise a relay 225 which is equipped with a Weighted armature 226 having a natural period of vibration of 16 cycles per second. The relay comprising the generator 32 may be equipped with a weighted armature designed to vibrate at 33 cycles per second and the relay comprising the generator 33 may be designed to vibrate at a rate of 50 cycles per second.

More specifically considered, the converting unit I'I comprises a transformer 29!! having a pair of primary windings or winding section 2022 and 283, and a secondary winding 25H which is arranged successively to be connected to the ringing conductors 292, 293, 294, etc., during each cycle of operation of the sequence relays I9. The secondary winding 2M is shunted by a condenser Edla having the function of providing a low impedance path for the radio frequency components of the alternating current developed during operation of the unit. The two primary windings 202 and 203 are arranged to be energized in opposite directions from a source of di rect current 35 over circuits which respectively include the space current paths of two electron discharge tubes 284 and 285. Each of these tubes is of the well-known pentode type utilizing an indirectly heated cathode. For example, the tube 264 comprises an anode 204a and a cathode 2940 separated to define a space current path, and a control electrode 2841) which may be subjected to a fixed or varying potential to determine the current flow over the space current path. The tube further comprises a suppressor grid which is directly connected to the cathode of the tube, a screen electrode 204d, and a cathode heater 204e, For the purpose of normally biasing the control electrodes 2342i and 2051) of the two tubes 2M and 2215 to substantially the same negative potentials so that currents of substantially equal magnitude normally traverse the two primary windings 2e: and 263, a biasing network i provided which includes a voltage dividing resistor 20'! bridged across the exchange battery I5. It will be noted that the voltage across the resistor section 28111 is negatively applied to the cathodes 2840 and 2050 of the two tubes and that the larger voltage appearing across the two series connected sections 251a and Zil'lb i negatively applied to the control electrodes 2341) and 2552) through the resistors 208 and 2% respectively. In order to vary the potential of these control electrodes in accordance with the operation of any one of the pulse generators to which the converting unit I 1 may be connected, circuits are provided whereby these electrodes may be controlled through the resistors 2H) and ZII respectively, the contacts of the sequence relays I9, and the contacts of the pulse generators 3!, 32, 33, etc.

In general, the arrangement of the converting unit I8 is similar to that of the converting unit IT as described above. Thus the unit I3 comprises a transformer 2I3 which is provided with a secondary winding 2M having a resistor 2I8 and a condenser 2 I9 connected in shunt therewith and arranged to be successively connected to the ringing current leads 2,92, 293, 294, etc., by the sequence relays l9 when the transfer relay R289 is restored. The transformer 213 .further comprises a pair of primary windings H and H5 which are bridged by the condenser 2H and are arranged to be alternately energized in opposite directions from the exchange battery i5 under the control of the pulse generators 3i, etc. The energizing circuits for these two windings commonly include. a low-resistance, high impedance, iron-core choke 22] which is provided for the purpose of suppressing noise currents that may be produced during operation of the unit. Further to this end, a condenser 223 is provided which shunts each of the two windings 2G5 and 216 during those periods when the windings) are respectively energized.

As indicated by the above explanation, when the converting unit I8 is connected for use, the primary windings 2E5 and M6 included therein are arranged for .direct energization under the control of the pulse generators 3!, 32, 3-3, etc, so that conditions are established whereby the pole changing or circuit controlling contacts of the pole changing relays may be subjected to arcing and sparking when heavy load demands are imposed on the system. In order to minimize these effects the transformer 22.3 is designed to possess a high leakage reactance and a condenser 2!? having a compromise capacitance value, is used. In this regard it may be pointed out that if a condenser 2!? of large capacitance is used, sparking at the pole changing contacts during the circuit break periods may be substantially entirely eliminated. However, the energy storage in a condenser of this size results in an enhanced pitting of the contacts during the make periods thereof. On the other hand if a small condenser is used the pitting may be substantially entirely eliminated but the spark suppression obtained is totally inadequate. By effecting a compromise between these two values, however, reasonably good spark suppression, accompanied by a not objectionable amount of pitting, may be obtained regardless of the load demands on the system.

As indicated above, the transfer relay R5238 is provided for selectively associating the two converting units l! and ill with the remaining portions of the ringing equipment. It is more specifically pointed out that the converting unit H is normally utilized in the production of the alternati. currents of different frequencies. To this end, a marginal relay R295 is provided which is included in the common portion of, the paths over which the positive terminal of the current source 35 is connected to the two screen electrodes 284i) and Z liib. This relay is margihed to operate and hold its operated positio when traversed by current of a value approximatel one and one-half times the screen electrode current of each tube. As thus margined the relay R285 normally holds its operated position, but will r store if the screen current flow to either of the two tube .is interrupted. In order to exclude the winding of the relay R295 and the current source 35 from the paths traversed by the periodically changing output currents of the two tubes 2M and 285, a pair of by-p-ass condensers 23'? and 20.6 are provided which respectively shunt the relay R235 and the source 35.

Neglecting for the present the mode of operation of the ringing apparatus illustrated in 2 of the drawings and the manner in which this apparatus is controlled, the system as described above is substantially conventional in form and its operation is well understood in the art. In brief when a subscriber such, for example, as the subscriber at the substation A lifts the receiver provided at this substation from its supporting hook or cradle to initiate a call intended for the substation B, for example, a loop circuit is completed for initiating the operation of the line'switch 25] to select a trunk leading to an idle selector. If the selector 25 is seized by the line switch 251, the calling loop circuit is extended to this selector and the selector is conditioned to respond to the first series of impulses dialed at the calling substation A. With the selector 25 in this condition, the usual dial tone signal is returned over the established loop circuit to the calling subscriber to indicate that the dialing operation may be started. When the first digit is dialed at the calling substation, the dial tone signal is arrested and the wipers of the selector 25 are elevated to a position opposite the level of bank contacts terminating the trunks leading to the connectors having access to the desired line i 5. During the interdigit pause between the first and second digits, the wipers of the selector 25 are automatically stepped over the contacts of the selected level until a trunk leading to an idle connector of the selected group is found. Assuming that the connector 353 is the first available idle connector of the group, the calling loop circuit is extended to this connector when the wipers of the selector 25 are stepped into engagement with the contacts terminating the trunk line extending to this connector. When the connector M is thus seized, certain of the control relays embodied therein and more particularly the line and release relays, operate to complete a path for impressing ground potential upon the start conductor 2295 thereby to initiate the operation of the rin ing apparatus in the manner explained below. In response to the dialing of the second digit the wipers 0f the connector 31! are stepped vertically to a position opposite the level of bank contacts in which are terminated the lines of the ten line subgroup including the called line it. Following this digit and during the interdigit pause between the second and third digits, the connector is conditioned to rotate its wipers step by step in accordance with the impulses of the third digit of the directory number designating the line i 5. When the third digit is dialed, the wipers of the connector Si! are positioned to engage the particular contacts terminating this line and at the end of the digit the control circuits of the connector are conditioned to repeat the impulses of the fourth digit to the operating magnet I23 of the frequency selecting switch 526. This magnet responds to the impulses of the fourth digit by operating the wiper I22 into engagement with the particular contact of the contact set l2! having impressed thereon ringing voltage of the particular frequency required to operate the harmonic ringer provided at the desired substation B. If it be assumed, for example, that the ringer provided at this substation is constructed to respond only to ringing current of 33 cycles the fourth digit dialed at the calling substation will comprise two impulses so that at the conclusion thereof the wiper I22 is left standing in engagement with the contact terminating the ringing conductor 2%. During the operation of the switch i253 and when the wiper l2? is stepped oil normal, the oil-normal springs 1525 are closed to prepare the operating circuit forthe release magnet I24, this circuit being held open through break contacts controlled by the release relay of the connector until the connection is released.

In accordance with conventional practice suitable provisions are made in the connector 30 for testing th selected line II to determine the idle or busy condition thereof, for returning the usual busy tone signal over the established loop circuit to the calling subscriber in the event the called line is busy, and for projecting ringing current of the selected frequency over the called line in the event this line is idle. Assuming that the called line is idle, a circuit is completed through operation of certain of the relays embodied in the connector 39 for energizing the lower winding of the slow-to-operate ringing control relay RI Oil in series with the winding of the cut-off relay conventionally embodied in the line switch 2!. When this circuit is closed the cut-off relay of the line switch 2I partially operates to disconnect the windings of the line relay RI3II from the conductors of the called line II, thereby to clear this line of the shunt impedance represented by these windings. The relay RIIlIl also operates when energized in this series circuit. In operating, this relay first locks to ground over a prepared path which includes its preliminary make contacts I82. At its oontacts'llil and I03, the relay RIDU completes a circuit for projecting ringing current of the selected frequency over the conductors of the line I I. In the present case the circuit traversed by the ringing current may be traced as extending from the upper terminal of one of the two transformer windings 2IlI or 2H, the contacts 28! or 282, the contacts 2', MI, etc., the ringing conductor 293, the wiper I22 and its engaged second contact, the contacts H5 and I03; the conductor I ID, the windings of the ringer provided at the substation B, the conductor No, the contacts IIII and III, the upper winding of RI I I] and the exchange battery I5 to the opposite or grounded side of the transformer winding 20I or the transformer winding 2I4. From a consideration of this circuit it will be noted that the illustrated X wiring is used throughout in the ringing apparatus illustrated in Fig. 2 of the drawings. With this wiring arrangement, grounded ringing is utilized in signaling over all the lines of the system. If, however, the requirements of the system are such that superimposed battery ringing must be used, the requirements are met by utilizing the illustrated Y wiring arrangement in lieu of the illustrated X wiring arrangement.

Portions of the current transmitted over the above-traced ringing circuit are obviously shunted through the harmonic ringers respectively provided at the other substation served by the line II. The ringer provided at the substation B is, however, the only ringer associated with this line which responds to current of the particular frequency of 33 cycles. Operation of this ringer serves to signal the called subscriber that he is being called. A portion of the ringing current is also shunted over the calling end of the connection, whereby the receiver provided at the calling substation is energized to indicate to the calling subscriber that the called substation is being signaled.

The desired talking circuit between the calling and called substations is fully completed when the call is answered at the called substation B. More particularly, when the receiver provided at this substation is removed from its supporting hook or cradle a direct current conductive bridge is connected across the conductors I la and I w of the line II to complete a circuit for energizing;

the upper winding of th slow-acting ring cutoff relay RI I 0. This circuit extends from ground by way of the transformer winding 20I or 2I4 in use, the contacts 282 or 28I, the contacts 21 I, 26I, etc., the ringing conductor 293, the wiper I22, the contacts H5 and I03, the conductor II b, the bridge across the conductors IIb and Ila at the substation B, the conductor I Ia, the contacts IN and III, and the upper winding of RI III to the negative terminal of the exchange battery I5. When its upper winding is energized in this circuit the relay RI III first locks to ground over a prepared path which includes the preliminary make contacts II3. Thereafter the relay RIIO opens its contacts III and H5 to interrupt its operating circuit as traced above and to open two points in the circuit for transmitting ringing current over the line II. At the contacts I I2 and I it, the relay RI I0 completes the desired talking circuit between the calling and called substations. When this relay operates it also functions to open the path, not shown, over which a portion of the ringing current is transmitted over the calling end of the connection, whereby the ringback signal is terminated in the usual manner.

Preferably the control relays of the connector 3! are so arranged that the release of the operated switch train is entirely under the control of the calling subscriber at the substation A. With such an arrangement, the connection is cleared out at the calling substation when the loop circuit extending through to the connector 30 is opened to cause the release of the line and hold relays embodied in this switch. When these relays restore, the line and selector switches 20 and 25 are released and the control apparatus embodied in the connector 30 is restored to normal, all in a manner well understood in the art. During the release of the control apparatus in the connector 30 the previously traced holding circuit for the two relays RIIlfl and RIIO are interrupted, causing these relays to restore. Also, the start conductor 295' is disconnected from ground in the connector 30 and ground potential is removed from the private conductor of the line II to cause the deenergization and release of the cut-01f relay embodied in the line switch 2|. When the line and hold relay of the connector 30 restore a circuit is also completed for energizing the release magnet of the Strowger switching mechanism embodied in the connector 30, whereby the wiper carriage structure of the mechanism is restored to rotary and vertical normal. larly a circuit is completed by way of the offnormal springs I25 for energizing the release magnet I24 of the frequency selecting switch I28, whereby the wiper I22 of this switch is restored to normal. Incident to the release of this switch, the off-normal springs I25 are opened to deenergize the release magnet I24.

Referring more particularly to the operation of the ringing apparatus illustrated in Fig. 2 of the drawings, it is pointed out that to condition this apparatus for operation the manually operable switch 236 is operated to complete a circuit for energizing in multiple the cathode heaters 2046 and 2056 of the two tubes 204 and 2%, respectively. This circuit extends from the negative terminal of the exchange battery by way of the conductor I6, the contacts of the switch 236, the current limiting resistor 2I2, and the parallel-connected heaters 2Me and 2Il5e to the grounded positive terminal of the battery I5.

With these two heaters energized, the space current paths through the two tubes are rendered conductive to the extent permitted by the negative biasing potentials impressed upon the control electrodes 204D and 2051) from the voltage dividing resistor 201. Accordingly, the primary windings 202 and 203 of the transformer 200 are energized in parallel circuits which commonly include the current source 35 and respectively include the space current paths of the tubes 204 and 205. The relay R295 is also energized by the current, supplied by the source 35, which traverses the screen electrodes 204d and 205d. With this relay operated a circuit is completed at the contacts 296 for holding the transfer relay R280 energized. With the relay R280 in its operated position, the contacts 283 and 285 are disengaged so that the converting unit !8 is disassociated from the sequence relays l9 and the pulse generators 3!, 32, 33, etc. At the contacts 284 and 286 of the relay R280 paths are prepared over which control pulses of ground potential may alternately be impressed upon the two control electrodes 20417 and 20512 through operation of one of the pulse generators 3!, 32, 33, etc. At the contacts 28! the circuit is held open over which the ringing conductors 252, 293, 294, etc., may be connected to the high potential terminal of the secondary winding 2M. At the contacts 282 a circuit is prepared over which the ringing conductors may successively be connected to the secondary winding 20! under the control of the sequence relays !9.

With the apparatus in the condition described above, if ground potential is impressed upon the start conductor 295 through operation of one of the associated connectors in the manner previously described, an obvious circuit is completed for energizing the start relay R255. This relay, in operating, closes its contacts 25'! to complete a circuit for energizing in parallel the windings of the pole changer relays respectively comprising the pulse generators 3!, 32, 33, etc. One branch of this circuit extends from ground by wayof the series-connected right windings of the relay 225, the resistor 235, the contacts 2 3'! and the conductor it to the negative terminal of the battery l5. When this branch circuit is completed the weighted armature 226 is attracted toward the pole faces of the relay 225 until the contacts 22'! and 228 are respectively engaged. When the contacts 22! are moved into engagement a circuit is completed for energizing the differentially related left windings of the relay 225. This circuit extends from ground by way of the armature 226, the contacts 22?, the inductance coil 23!, the series-connected left windings of the relay 225, the contacts 237, and the conductor 6 to the negative terminal of the battery !5. When the relay 225 is thus difierentially energized the attractive force exerted on the armature 226 is reduced substantially to zero with the result that this armature is released and swings through its normal position to a position wherein the contacts 229 are engaged. Incident to this movement of the armature 226 the contacts 22'! are opened to deenergize the left windings of the relay 225, and the contacts 228 are disengaged. The deenergization of the left windings of the relay 225 is retarded somewhat due to the presence of the condenser 232 in order to permit the armature 226 to swing to the extreme position required for engagement of the contacts 229. When the current traversing these windings is reduced substantially to zero an-attra'ct'ive force is again exerted on the armature 226 so that this armature is again actuated to its attracted position wherein the contacts 22? and 228 are respectively engaged. During the last-mentioned movement of the armature 225 the contacts 229 are obviously opened. It will he understood from the above explanation that the vibration of the weighted armature 226' alternately to close the contacts 228 and 2253 continues so long as the start relay R255 is maintained in its operated position; It will also be understood that the condenser 232 eiiectively shunts the contacts 22'! during the break p'eriods'of these contacts for the purpose of minimizing sparking thereat. Further, the action of the inductance coil 23! in the circuit contrclied by the contacts 22? serves to prevent the generation of transient currents of radio frequency during the break periods of these contacts. y

In operating, the relay R235 also closes its contacts 233 to complete a circuit for initiating the operation of the sequence relays I91 More specifically, when the contacts 235 are closed, a circuit including the contacts 243' is completed for charging the condenser 246-fro1n the exchange battery i5. This condenser initially provides a low impedance path in'shunt with the winding of the relay R250 for by-passing current around this relay winding, and thus functions to delay the operation of the relay R240. As a charge builds up across the electrodes of this condenser the shunting effect of the condenser decreases, so that after a short interval the currenttraversing the winding of the relayRZMl and the resistor 245 in series is sufiicient to cause the operation of this relay. In operating the relay R240 opens its contacts 243' to interrupt its own operating circuit and to interrupt the charging circuit for the condenser 246. After these circuits are opened the condenser 2 30 discharges through the resistor 245 and the winding of the relay R240 in series, thereby to maintain this relay in its operated position for a given time interval. At its contacts 254 the relay R240 opens a point in the operating circuit for the reset relay R290; At its contacts 25! the relay R240 opens the path normally short-circuiting thecondenser 249. At its contacts 242 the relay R240 completes a circuit for charging the condenser 249 in series with the winding of the sequence relay R250, this circuit extending from the grounded terminal of the exchange battery I5'by way of the contacts 242, the condenser 249', the contacts 255, the winding of R250, and the conductor It to the negative terminal of the battery !5; The initial rush of current over this circuit is sufficient to cause the operation of the relay R2561 In operating this relay first locks to ground over a path including its preliminary make contacts 256' and the contacts 29!. After this locking circuit is completed the relay R255 closes its contacts'25'4 to prepare a circuit for charging the condenser 249 in series with the winding of the second frequency selecting relay R265 and opens its contacts 255 to interrupt the cir'cuit for charging the condenser 249 through its own winding. By the time the relay R250 has operated to perform the functions just described" the condenser 249 is fully charged so that current flow through the winding of the relay R250isnrevehted.

After a given time intervalthe current which is passed throughvth'e' winding of the relay R240 from the condenser 24 6is' sufiicieritly" reduced to permit the release of thisrela'y. In restoring the relay R240 opens its contacts 242 to interrupt the common portion of the available circuits over which the condenser 240 may be charged, and closes its contacts 24! to discharge this condenser through the resistor 248. At its contacts 244 the relay R240 reprepares the operating circuit for the reset relay R200. At its contacts 243 the relay R240 completes the above-traced parallel circuits for charging the condenser 240 and for energizing its own winding. It will be understood from the above explanation that the relay R240 continues to operate and restore at intervals which are determined by the capacitance value of the condenser 245 and the resistance values of the two resistors 245 and 241. More specifically, the resistance value of the resistor 245 determines the release period of the relay R240, while the resistance value of the resistor 241 determines the period required to produce operation of this relay following its release. As the relay R240 continues to operate and restore the sequence relays R200, R210, etc., which individually correspond to the pulse generators 32, 33, etc. are successively energized. Thus incident to the second operation of the relay R240, the condenser 249 is charged in series with the winding of the relay R200 over a circuit which extends from ground by way of the contacts 242, the condenser 240, the contacts 254 and 251, the winding of R260 and the conductor !5 to the negative terminal of the battery Hi. When its winding is thus energized the relay R200 first locks to ground over a path including its preliminary make contacts 208 and the contacts 20!. After this locking circuit is completed the relay R260 closes its contacts 260 to extend the condenser charging circuit through the winding of the next sequence relay and opens its contacts 261 to interrupt the circuit over which the condenser 240 is energized in series with its own Winding. The manner in which the other sequence relays, indicated by dash lines in the drawings as being connected between the relay R260 and the relay R210, are successively energized and locked up in response to the continued intermittent operation of the timing relay R240 will be clearly apparent from the preceding explanation. After all of these intervening relays are operated the charging circuit for the condenser 249 is extended to include the winding of the last frequency selected relay R210. Accordingly when the timing relay R240 next operates, the condenser 249 is charged in a circuit which includes the conductor 16, the winding of R210,

the contacts 216, the chain-connected contacts of the intervening frequency selecting relays, the contacts 265 and 254, the con-denser 240 and the contacts 242. When its winding is energized in this circuit the relay R210 locks to ground over a path including the contacts 211 and 205. After this path is completed the relay R210 opens its contacts 210 to interrupt the charging circuit for the condenser 249. Shortly following the operation of the relay R210 and when the timing relay R240 next restores, the prepared operating circuit for the reset relay R2 90 is completed, this circuit extending from ground by way of the contacts 244 and 218, the resistor 203, the winding of R200, the resistor 204, and the conductor E0 to the negative terminal of the battery l5. It will be noted that the Winding of the relay R200 is paralleled by the condenser 202. Accordingly when the circuit just traced is completed the major portion of the current is shunted through this condenser away from the winding of the relay R290 to delay the operation of this relay for a short time interval. At the end of this interval the relay R290 operates and opens its contacts 29l to interrupt the established locking circuitsfor each and all of the sequence relays R250, R250, R210, etc. Incident to the release of the relay R210 the contacts 210 are opened to interrupt the circuit for energizing the relay R200. After this circuit is opened, the condenser 202 discharges through the resistor 294 and the winding of the relay R290 to delay the release of this relay for a short time interval. At the end of this interval the relay R290 releases and closes its contacts 29! to reprepare the locking circuits for the various sequence relays. Following the release of the sequence relays and the reset relay R200, the network I9 is conditioned for a second cycle of operation. In this regard it will be understood that the relays of this network continue to operate in a cyclic manner so long as the start relay R235 is maintained in its operated position.

Each time one of the sequence relays R250, R200, R210, etc., is operated, the corresponding one of the pulse generators 3|, 32, 33, etc., is connected to control the potentials of the control electrodes 2041) and 205?). Thus during each interval when the relay R250 is operated the contacts 252 and 253 are closed to prepare paths over which ground potential may alternately be impressed upon these control electrodes by the relay 225 of the generator 3|. One of these paths extends from ground by way of the armature 226, the contacts 229, 252, 262, 212 and 204 and the resistor 2m to the control electrode 20422. The other of the two paths extends from ground by way of the armature 226, the contacts 228, 253, 264, 214 and 286, and the resistor 21! to the control electrode 2052). Each time the control electrode 2042) is thus connected to ground the potential thereof is made positive with respect to the potential of the cathode 2040, so that the current traversing the space current path of the tube 204 and the primary winding 202 in series is sharply increased. A correspondingly sharp decrease in the current traversing this circuit also occurs when the contacts 229 are open. Each time the contacts 228 ar closed to complete the path for impressing ground potential upon the control electrode 2051) of the tube 205, this electrode is similarly made positive with respect to its associated cathode 2050 so that the current traversing the series connected winding 203 and space current path of the tube 205 is sharply increased. A corresponding sharp decrease in this current occurs each time the contacts 228 are opened to restore the normal negative bias to the control electrode 20527. It will be understood that since the two paths described above are alternately completed under the control of the relay 225 the current traversing the two primary windings 202 and 203 of the transformer 200 is alternately increased and decreased, whereby an alternatin induced voltage is developed in the secondary winding 20L The frequency of this voltage is obviously determined by the rate at which the two paths for impressing ground potential upon the control electrodes 204?) and 20517 are alternately completed and hence by the rate of vibration of the weighted armature 226. This generated alternating voltage has a more or less square wave form and accordingly it is exceptionally well suited to the requirements of harmonic ringer control. Moreover the wave form of the generated alternating voltage is completely lacking in the high voltage peaks which normally occur in the output voltage of conventional pole changer converters at the beginning and end of each half cycle. Since the generated wave form is of the square top variety the harmonic content thereof is comparatively large but is not so excessive as to cause cross ringing between the various lines of the system.

After a predetermined time interval as measured by the operation of the timing relay R240, the second sequence relay R260 is caused to operate in the manner explained above. In operating, this relay opens its contacts 262 and 264 to disconnect the control electrodes 20412 and 205?) from the contacts of the pole changer relay 225. At its contacts 263 and 265 the relay R280 connects the control electrodes 2941) and 20% to the contacts of the pole changer relay comprising the pulse generator 32. The manner, in which the contacts of the last mentioned relay are alternately closed to transmit ground pulses to the two control electrode 204b and 2051) is exactly the same as described above with reference to the operation of the relay 225. In the present case, however, the frequency at which these ground pulses are impressed upon the two control electrodes is double the frequency at which the two electrodes are connected to ground by the relay 225. Accordingly, the output frequency of the apparatus is increased two-fold. In operating, the relay R260 also opens its contacts25l to disconnect the ringing conductor 232, which is individual to the generator 3I, the selecting relay R250 and the first ringing frequency, from the high potential terminal of the secondary winding 20!. At its contacts 26I' the relay R260 connectsthe second ringing conductor 293, which is individual to the pulse generator 32, to the high potential terminal of the secondary winding 20I. Thus during the period which separates the operation of the relay R260 from the operation of the next succeeding sequence relay, ringing voltage of the second ringing frequency is impressed upon the ringing conductor 293. As the sequence relays continue to operate, the control electrodes of the two tubes 204 and 205 are successively connected to be controlled by different ones of the pulse generators, and the corresponding ringing conductors are successively connected to the secondary winding 20 I. Thus when the last sequence relay R210 operates, it opens its contacts 212 and 214 and closes its contacts 213 and 215, whereby the potentials on the control electrodes 2041) and 205?) are controlled in accordance with the operation of the pulse generator 33. In operating the relay R210 also opens its contacts 2' and closes its contacts 21!, whereby the high potential terminal of the secondary winding 20I is connected to the ringing conductor 294 and is disconnected from the remaining ringing conductors.

The above described operation of the ringing apparatus continues until the start relay R235 is deenergized and restores. In releasing this relay opens its contacts 231 to interrupt the operating circuits for the pole changer relays respectively comprising the various pulse generators 3|, 32, 33, etc. At its contacts 238 the relay R230 opens the above-traced operating circuit for the timing, relay R240 whereby the cyclic operation of the sequence relays I9 is discontinued.

As indicated by the aboveexplanation the two relays R295 and R280 are held energized so long as the converting unit I1 is operatingin its normal and intended manner. If, however, one of the two tubes 204 and 205 should become defective or the current source 35 should fail to render the unit I1 inoperative, the current traversing the winding of the relay R295 is decreased to a value insuflicient to maintain this relay in its operated position. In releasing, the relay R295 opens its contacts 296 to deenergize the relay R280. The latter relay, upon restoring, opens its contacts 284 and 286 to interrupt the common portion of the paths for transmitting ground pulses to the control electrodes of the tubes 204 and 205. At its contacts 283 and 235 the relay R280 connects the primary windings 2I5 and 2I6 of the transformer 2I3 to be alternately energized in accordance with the operation of the pulse generators 3|, 32, 33, etc., during different periods of each operating cycle of the sequence relays I9. At its contacts 282 the relay R280 opens a point in the common portion of the paths over which the various ringing conductors 292, 293, 294, etc., may be connected to the high potential terminal of the secondary winding 20I. At its contacts 28I the relay R230 prepares a common path over which the ringing conductors may successively be connected to the high potential terminal of the secondary winding 2I4 during each cycle of operation of the network I9. Thus following the release of the transfer relay R280 the converting unit I8 is included in the conversion channel in lieu of the converting unit I1. In this regard it will be noted that with the relay R280 in its restored position and th sequence relay R250 operated, the two primary windings 2I5 and 2I6 are alternately energized under the control of the pole changer contacts of the relay 225. The circuit for energizing the primary winding 2I5 extends from ground by way of the armature 226, the contacts 229, 252, 262, 212 and 283, the winding 2I5, the choke coil 22I and the conductor I6 to the negative terminal of the battery I5. This circuit is obviously completed at the contacts 229 only during the back stroke of the armature 226. The circuit for energizing the primary winding 2 I 6 extends from ground by way of the armature 226, the contacts 228, 253, 264, 214 and 285, the winding 2I6, the chokecoil 22I and the conductor I6 to the negative terminal of the battery I5. This circuit is completed at the contacts 228 only during the forward stroke of the armature 226. The alternate energization of the primary windings 2I5 and 2 I 6 over the circuits just traced causes an induced alternating voltage to be developed in the secondary winding 2 I4 having a frequency which corresponds to the frequency of vibration of the relay 225. This voltage is impressed upon the ringing conductor 292 over a path which includes the contacts 28I, 2'II, 26I and EM. The circuits over which the primary windings 2 l5 and 2 I6 are energized under the control of the other pulse generators 32, 33, etc., during different periods of each operating cycle of the sequence relays I9 are in all respects substantially similar to the circuits described above and will be fully apparent from the preceding explanation.

With the converting unit I8 included in the conversion channel, the condenser 2I'I functions to minimize sparking and pitting of the pole changer contacts in the manner previously described. Also, rapid fluctuations in the voltage across the exchange bus conductors over which current is supplied from the battery I5 to the talking circuits set up through the automatic switching equipment of the exchange, are prevented by the action of the high impedance choke coil 22!. Such voltage fluctuations tend to occur during these periods when the battery I is not fully charged and the internal resistance thereof is low, and are occasioned by the heavy current surges which occur incident to closing and opening of the circuits for energizing the primary windings H5 and 216. The choke coil 22l tends to retard the current build-up during each surge period with a resulting smoothing action on the voltage of the battery l5. The retarding effect of the coil 22! is in part compensated for by the action of the condenser 220 which is charged during each period when the respective circuits for energizing thev windings 2l5 and 2N are both open, and alternately dis charges through one of these windings and then the other as the energizing circuits therefor are alternately completed. The radio frequency components of the voltage developed across the secondary winding of the transformer 213 are suppressed through the action of the condenser 2l9. Further, the resistor 2H3 functions to damp the secondary winding 2M so that the peaks which occur in the voltage produced across this winding at the beginning and end of each voltage cycle are prevented from assuming abnormal values when only a light load is imposed on the apparatus.

As indicated above, the relay R295 is controlled in accordance with the current traversing the screen electrodes 234d and 235d in parallel. The purpose of providing this particular circuit arrangement is to permit the converter unit I! to be arranged for either class A, class B or class C v operation, depending upon the particular output voltage requirements. If an undistorted flat top output voltage wave is required the unit may be operated class A in which case the two tubes 204 and 255 are biased to operate on the straight portions of their grid-voltage, plate current curves. On the other hand, if distortion is permissible and a large output is required, the tubes 204 and 295 may be biased substantially to cut off or beyond for class B or class C operation. In the latter case, only a small current, if any, traverses the primary windings 202 and 263 during periods when the pulse generators are inactive. Current of appreciable magnitude does, however, traverse the screen electrodes 23M and 205d in the latter case. Accordingly, it is more desirable from the standpoint of flexibility of operation, to control the relay R295 in accord ance with the current traversing the screen electrodes of the tubes rather than the current traversing the anodes of the tubes.

Referring now more particularly to the modified arrangement of the ringing apparatus as shown in Fig. 3 of the drawings, this apparatus comprises a transformer 300 having a secondary winding 30! which is arranged to be connected successively to the ringing conductors 392, 333, 394, etc., under the control of a sequence switch 32!. The transformer 300 is also provided with a pair of primary windings 302 and 303 which are arranged to be energized in opposite directions from a direct current source 35a over circuits which respectively include the space current paths of the two thermionic tubes 304 and 365. These tubes are of the well-known pentode type and respectively include anodes 305a and 395a, cathodes 335C and 3050, control electrodes 304b and 335b, cathode heaters 30% and 395e, and screen electrodes 304d and 385d. The cathodes of these two tubes are biased positively with respect to their associated control electrodes through the provision of a biasing resistor 306 which is connected to be traversed by the combined space currents of the two tubes. An additional bias voltage derived from the voltage dividing resistor 309 is impressed negatively upon the control electrodes 304D and 3052) through the resistors 301 and 308 respectively,

For the purpose of varying the potentials of the control electrodes 30% and 30% alternately to in crease and decrease the current traversing the primary windings 302 and 303 a plurality of electronic oscillators 3la, 32a, 33a, etc., having different output frequencies are provided. These oscillators, which are of the well-known tuned-plate, tickler feed-back type, are identical in wiring arrangement but have different circuit constants in order to produce the required voltage of different frequencies. More specifically, the oscillator 33a comprises a thermionic tube 3M of the wellknown pentode type. This tube is provided with a tuned output circuit which includes the winding 3!! of the transformer 3|5 and the portion 345a of a voltage dividing resistor 345. This circuit is tuned to the desired operating frequency by a condenser 320 of suitable capacitance which is connected in shunt with the transformer winding 3". The tube 3|4 is also provided with an input circuit which is coupled between the control grid 3l4b and the cathode 3I4c and includes the transformer winding 3l8, a resistor 3|9, and the portion 3 I 0a of a voltage dividing resistor 3| 0. It will be understood that the voltage drop across the resistor section 3l0a represents the biasing potential which is negatively impressed upon the cathode 3M0. The action of the resistor 3!!) in the oscillator input circuit serves to limit the grid current which may flow between the control grid 3l4b and the cathode 3| 4c of the tube. The transformer 3l5 also includes a tertiary winding 316 having a tapped center point which is connected to the junction between the two resistors 30'! and 308. The upper and lower terminals of the Winding 3|6 are connected to corresponding contacts of the two contacts 323 and 324 in the sequence switch 32!, so that when this switch occupies a particular setting the oscillator 33a is connected to control the potentials of the control electrodes 30% and 3051). The center tap of the winding 3"? is also multipled to the center taps of the transformer secondary windings in each of the other oscillators 3la, 32a, etc. These other oscillators also derive cathode biasing potentials from the voltage dividing resistor 3 I i] and anode potentials from the voltage dividing resistor 345 as indicated by the multiple connections extending thereto.

In order to control the sequence switch 321 so that the oscillators 31a, 32a, 3311, etc., are successively connected to control the potentials'of the grids 30% and 305?), a timing relay R350 is provided which is shunted by a path which includes the condenser 329 and the resistor 330 connected in series. The apparatus further comprises a start relay R360,a manually operable start switch 332, and a signal control relay R310. For the purpose of controlling the latter relay a signal control network is provided which includes the three electrode thermionic tube 343 having its output electrodes 340a-and 3400 connected in circuit with the relay R310 and its input electrodes 340b and 3400 coupled to the secondary winding 3M of the transformer 300. The coupling network between the transformer 300 and the tube 340 comprises ,a pair of series connected resistors 34! and 342 of like resistance values which are bridged across the secondary winding 30'! a direct current blocking condenser 343 and a biasing path which includes the resistor 344 and the section 309a of the voltage dividing resistor 309'. The relay R310 is arranged directly to control a signal device 348 which may be conveniently located in the exchang wherein the illustrated ringing equipment is provided.

In considering the operation of the ringing ap-' paratus illustrated in Fig. 3 of the drawings, it will be understood that this apparatus may be directly substituted for the apparatus shown in Fig. 2 of the drawings, to supply ringing voltages of the desired frequencies to the automatic switching equipment illustrated in Fig. l of the drawings. In order to condition the ringing apparatus for operation, the switch 332 is operated to complete a circuit for energizing in parallel the cathode heaters of the two tubes 304 and 305, the tubes respectively provided in the various oscillators, and the signal control tube 340'. This circuit extends from the negative terminal of the exchange battery Ia by way of the current limiting resistor 33!, th contacts of the switch 332 and the parallel connected cathode heaters to the grounded terminal of the battery l5a. When the cathode heaters of the respective oscillator tubes are energized to produce electron emission in these tubes, operation of the oscillators is initiated. In this regard it will be understood that the oscillator 33a, for example, is initially shock excited to develop an oscillatory condition in the output circuit thereof. Due to the coupled relationship between the two windings 3!! and 3l8 an oscillating voltage is impressed across the in put electrodes 3 I 4?) and 3 140 of the tube 3 l 4 which is of the correct phase and amplitude to sustain the oscillatory condition of the tuned output circuit of the tube. It will also be undersood that the tuning of this output circuit determines the frequency of oscillation of the current traversing the winding 3 I 7. Due to the coupled relationship between the two windings 3!! and 3l6 the oscil'- lating current in the first of these two windings causes an induced voltage to be developed in the winding sue. During succeeding half cycles of this voltage the upper terminal of the winding 316 alternates between a value which is positive with respect to the potential of the tapped center point of the winding and a value which is negative with respect to this point. Each time the upper ter-' minal of the winding 3H5 becomes negative with respect to the tapped center point, the lower terminal of the winding becomes positive with respect to this point. Conversely, each time the upper terminal of the winding 3 It becomes posi tive with respect to the tapped center point of this Winding, the lower terminal of the windingbecornes negative with respect to this point. Thus alternating control potentials are developed which, when impressed upon the control elec--' trodes 35419 and 335b, cause the currents traversing the primary windings 302 and 303 to be varied in opposite senses. In this regard it may be assumed that at the time the switch 332 is operated to initiate the operation of the various oscilla-' tors, the wipers 325, 326 and 32! of the sequence switch 32! occupy a setting wherein they engage their respective associated third contacts. With the two wipers 320 and 32'! in this position the upper and lower terminals of the tertiary wind ing Sit are connected to the control electrodes 33% and 305b, respectively. The resulting variations in potential impressed upon these control electrodes are such that the currents traversing the primary windings 302 and 303 are varied in opposite senses to produce an induced alternating voltage in the secondary windings 30!. In this regard it will be understood from the above explanation that When the control electrode 304!) is made more positive with respect to its associated cathode 3040 the control electrode 3051) is made correspondingly more negative with respect to its associated cathode 3050. Accordingly an increase in the current traversing the primary winding 302 is accompanied by a decrease in the current traversing the Winding 303. The re+ verse action occurs when the potentials of the control electrodes 304i; and 3051; are varied in the opposite sense. The rate at which these current changes occur corresponds tothe oscillating frequency of the oscillator 33a, so that an induced voltage of corresponding frequency is developed in the winding 30!.

Normally the control electrode 3401; of the tube 330 isnegatively biased with respect to the oathode 340:: to apoint approaching anode current cut-01f by the voltage drop across the section 309a of the resistor 309. In operation; this tube acts as a plate detector. Thus, when an alternating voltage is developed across the secondary winding 30i of the transformer 300, the major portion of the voltage wave is positively applied to the control electrode 340!) through the condenser 343, whereby the negative bias on this grid is substantially reduced. The decrease in the average negative potentialon the control electrode 3401) is accompanied by a corresponding increase in the average current traversing the space current path of the tube 340. As a result the winding of the relay R310 is sufficiently energized in a circuit including this path and the section 345a of the resistor 345 to operate. In operating, this relay opens its contacts 31! further to interrupt the incomplete circuit for energizing the signal device 348.

With the ringing apparatus in the condition described in the immediately preceding para graphs, an obvious circuit is completed for energiz ing the start relay R360 from the exchange battery 65a when ground potential is impressed upon the start conductors 335. In operating, this relay closes its contacts 362 to prepare the operating circuit for the signal device 348. At its contacts 363, the relay R360 prepares a path over which the secondary winding 300 may succes'sively be connected to the ringing conducf tors 332, 393, 394, etc., through the wipers" 325 of the sequence switch 32!. At its contacts 36!, the relay R363 completes the operating circuit for the relay R350, this circuit extending from ground by way of the contacts 352, the winding of R350, and the contacts 36! to the negative terminal of the battery l5a. Since the winding of the relay R350 is shunted by. theseries connected condenser 329 and resistor 330 it will be understood that the operation of this relay is delayed until the condenser 329 is substantially fully charged. At the end of this delay interval the relay R353 opens its contacts 352 to interrupt its operating circuit and the circuit over which charging current is delivered to the condenser 323. With these contacts open, the condenser 329 is permitted to discharge through the resistor 33!] and the winding of the relay R350. At the end of the interval required to discharge the condenser 329, the relay R350 releases and re'closes its contacts 352 to recomplete its oper-' ating circuit and the circuit for charging the condenser 329. It will be understood from the above explanation that the relay 123% continues to operate and restore at a relatively slow rate so long as the start relay R360 is maintained in its operated position. The rate at which this relay alternately operates and restores is determined by the capacitance value of the condenser 323, theresistance value of the resistor 33D and the impedance of the winding ofthe relay R3553. Each time the relay R350 operates it also closes its contacts dbl to complete a circuit including the contacts 36i for energizing the magnet 323 from the exchange battery iiia; The circuit is obviously opened at the contacts 35! each time the relay R35i3 restores. Each time the magnet 328 is thus energized it functions to advance the wipers 325, 326 and 321 one step, whereby the different oscillators 3|a, 32a, 33a, etc., are successively connected to control the space current paths of the 'twotubes 384 and 305. Thus it will be noted that when these wipers engage their respective associated first contacts the oscillator am is connected to control the two tubes 3% and 305 and hence the output frequency of the voltage developed across the secondary winding 30|.' Similarly when these wipers are operated to engage their respective associated second contacts the oscillator 32a is connected to control the two tubes 3&4 and 3t5, whereby an output vpltage of different frequency is developed across the'secondary winding am. The sequence switch 32I also functions successively to associate the different ringing conductors 392, 393, 394, etc.,

with the secondary winding 36!. Thus when the wiper 325' is operated to engage its associated first contact, the conductor 39 i is connected through the' contacts 363 to the high potential terminal ofthe winding 30!. Similarly when the wiper 32'5.i's operated to engage its associated second contact, the conductor 393 is connected to the high potential terminal of the secondary winding 3M. It will be understood, therefore, that as the different oscillators are connected to control the frequency of the voltage developed across the winding 30!, corresponding ones of the ringing conductors are connected to the high potential terminal of this winding.

As indicated above the relay R3?!) is normally maintained in its operated position so long as the frequency determin ng oscillators are functioning to cause the production of an alternating voltage across the secondary winding 39!. If, however, one of these oscillators should fail or one of the two tubes 30d and 365 should become defective, the output voltage across the secondary winding Sill will fail. When this condition arises, the ne ative bias is restored to the control electrode of the tube 340?) with a resulting decrease in the space current through this tube and the winding of the relay R370. This relay accordingly releases and closes its contacts 31! to complete a circuit including the contacts 362 for'energizing the signal device 3 :8 from the exchange battery l5a. The operation of the signal device 348 may be utilized to inform the exchange attendant that the ringing apparatus is defective. Alternatively the signaling circuit just described may be utilized to control automatic transfer means for cutting in a second set of ringing apparatus when the illustrated apparatus becomes defective to cause the release of the relay R316. It will be noted that this signaling circuit is only completed through the contacts 362 when the start relay R350 is operated. The

start relay in releasing also opens its contacts 36! to interrupt the respective operating circuits for the magnet 328 and the relay R'Jeil and opens its contacts 353 to interrupt the path over which the secondary winding 384 may successively be connected to the ringing conductors esz, 333, 394, etc.

While there have been described what are at present considered to be the preferred embodiments of the invention it will be understood that various modifications may be made therein and it is contemplated to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a telephone system which includes lines and switching means for setting up connectionsbetween said lines and for signaling over said lines, ringing apparatus controlled by said switching means and comprising, in combination, a transformer including a pair of primary windings andia secondary winding adapted to be connected by said switching means to impress the voltage developed thereacross upon any one of said lines, thermionic means including a pair of space current paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, and means for controlling said paths to produce alternate and periodic changes in the current traversing said circuits, whereby analternating voltage is developed across said secondary winding.

2. In a telephone system which includes lines and switching means for setting up connections between said lines and for selectively signaling over any one of said lines, ringing apparatus controlled by said switching means and comprising, in-combination, a transformer including a pair of primary windings and a secondary winding,-thermionic means including a pair of space cu-rrentpaths; a source of direct current, circuits commonly including said current source and respectivelydncluding different ones of said paths and different ones of said primary windings,

means for controlling said paths to produce alternate and periodic changes in the current traversing said circuits, whereby an alternating voltage is developed-across said secondary winding, cyclically operating means for periodically varying the periodicity ofthe changes in the current traversing said circuits, whereby alternating voltages of different frequencies are developed across saidsecondary winding during different periods of'each operating cycle of said last-named means, and circuits selectively controlled by said switching means for impressing the voltage developed across said secondary winding on a selected one ofsaid lines only during one of said periods of each operating cycle of said cyclically operating means.

3. In 'a telephone system which includes lines and switching means for setting up connections between said lines and for selectiveiy signaling over any one of said lines, ringing apparatus controlled by-said switching means and comprising, in combination, a transformer including a pair of primary windings and a secondary winding, thermionicmeans including a pair of space current paths and control electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including different ones'of said paths and different ones of said primary windings, means normally biasing said electrodes to substantially the same potential, a plurality of generators having different output periodicities and each operative to vary the potentials of said electrodes to produce an alternating voltage across said secondary winding, a plurality of ringing current conductors individually corresponding to said generators, means for successively connecting said generators to control the potentials of said electrodes and for simultaneously and successively connecting said secondary winding to corresponding ones of said conductors, and means included in said switching means for selectively connecting said conductors to a selected one of said lines.

4. In a telephone system which includes lines, switching means for setting up connections be tween said lines, and frequency selecting switches .for selectively connecting any one of a plurality of ringing current conductors to a selected line. ringing apparatus controlled by said switching means and comprising, in combination, a transformer including a pair of primary windings and a secondary winding, thermionic means including a pair of space current paths and control electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, a plurality of generators individually corresponding to said ringing current conductors and having different output periodicities, each of said generators being operative to vary the potentials of said electrodes to produce an alternating voltage across said secondary winding, and means for successively connecting said generators to control the potentials of said electrodes and for simultaneously and successively connecting corresponding ones of said conductors to said secondary winding.

5. Ringing apparatus comprising a transformer which includes a secondary winding and a primary winding having two sections, thermionic means having a pair of space current paths, means for energizing the two sections of said primary Winding over different ones of said paths, and means for controlling said paths alternately to vary the energization of said two winding sections in the same sense, whereby an alternating voltage is developed across said secondary Winding.

6. A converter comprising, in combination, a transformer including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, means for controlling said paths to produce alternate and periodic changes in the current traversing said circuits, whereby an alternating voltage is developed across said'secondary winding, and means for varying the periodicity of the changes in the current traversing said circuits, thereby to change the frequency of the voltage developed across said secondary winding. I

7. A converter comprising, in combination, a transformer including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths and including control electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, means for alternately and periodicall varying the potentials on said control electrodes to produce an alternating voltage across said secondary winding, and means for intermittently varying the periodicity at which the potentials impressed on said control electrodes are varied.

8. A converter comprising, in combination, a transformer including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths and including control electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, means normally biasing said control electrodes to substantially the same potential, a plurality of pulse generators operative to produce pulses of different periodicities and each operative alternately to vary the potentials of said control electrodes to produce an alternating voltage across said secondary Winding, and means for successively connecting said generators to vary the potentials of said control electrodes.

9. A converter comprising, in combination, a transformer including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths and including contro1 electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, means normally biasing said control electrodes to substantially the same potential, a plurality of thermionic oscillators operative to produce alternating voltages of different frequencies, circuits over which each of said oscillators may be connected to vary the potentials on said control electrodes so that an alternating voltage is developed across said secondary Winding, and means for controlling said last-named circuits so that said oscillators are successively rendered operative to control the potentials of said control electrodes.

10. A converter comprising in combination, a transformer including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths and including control electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, means normally biasing said control electrodes to substantially the same potential, a plurality of generators having different output periodicities and each operative to vary the potentials of said electrodes to produce an alternating voltage across said secondary Winding, a plurality of output conductors .individually corresponding to said generators, and means for successively connecting said generators to control the potentials of said electrodes and for simultaneously and successively connecting said secondary winding to corresponding ones of said conductors.

11. A converter comprising in combination, a transformer including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths and including control electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including different ones of said paths and different ones of said primary windings, means normally biasing said control electrodes to substantially the same potential, a plurality of thermionic oscil lators operative to produce alternatingvoltages of difierent frequencies and each operative to vary the potentials of said control electrodes to produce an alternating voltage across said sec ondary winding, a plurality of output conductors individually corresponding to said oscillators, and means for successively connecting said oscillators to control the potentials of said electrodes and for simultaneously and successively connecting said secondary Winding to corresponding ones of said conductors.

'12. A converter comprising, in combination,'a transformer including a secondary winding and a pair of primary windings, thermionic means including a pair of space current paths and control electrodes individual to said paths, a source of direct current, circuits commonly including said source and respectively including diiierent ones of said paths and different ones of said primary windings, a plurality of generators operative to produce voltages of different periodicities and each operative alternately to change the potentials of said control electrodes in the same sense, whereby an alternating voltage is developed across said secondary Winding, and means for successively connecting said electrodes to be controlled in accordance with the voltages of different ones of said generators.

13. Ringing apparatus comprising a ringing current conductor, a source of direct current, thermionic means having a pair of space current paths, a first conversion channel including said paths and operative to convert the voltage of said source into alternating voltage and to impress the alternating voltage on said conductor, a second conversion channel for developing an alternating voltage independently of said thermionic means and for impressing the alternating voltage on said conductor, and means for selectively rendering said channels active.

14. Ringing apparatus comprising a ringing current conductor, a source of direct current, thermionic means having a pair of space current paths, a first conversion channel including said paths and operative to convert the voltage of said source into alternating voltage and to impress the alternating voltage on said conductor, a second normally inactive conversion channel for developing alternating voltage independently of said space current paths and for impressing the alternating voltage on said conductor, and means controlled in accordance with the operative or inoperative condition of said first channel for rendering said second channel active.

15. Ringing apparatus comprising a ringing current conductor, a source of direct current, thermionic means having a pair of space current paths and screen electrodes individual to said paths, a first conversion channel including said paths and operative to convert the voltage of said source into alternating voltage and to impress the alternating voltage on said conductor, a second normally inactive conversion channel for developing alternating voltage independently of said space current paths and for impressing the alternating voltage on said conductor, a device controlled in accordance with the current traversing said paths by way of said screen electrodes, and means controlled by said device for rendering said second channel active when said first channel is rendered inoperative.

l6. Ringing apparatus comprising a ringing current conductor, a source of direct current,

thermionic means having a pair of space current paths, a first conversion channel including said paths and operative to convert the voltage of said source into alternating voltage and to impress the alternating voltage on said conductor, a second conversion channel for developing alternating voltage independently of said space current paths and for impressing the alternating voltage on said conductor, means for selectively rendering said channels active, a plurality of generators common to said channels and having different output periodicities, and means for successively connecting different ones oi said generators to control the frequency of the alternating voltage impressed on said conductor through the one of said channels which is active.

17. A converter compri ng, in combination, a pair of transformers each including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths, means including said space current paths for energizing the primary windings of one of said transformers, circuits for energizing the primary windings of the other of said transformers, a pulse generator, operative to control said paths so that the current traversing said path i alternately increased and to control said circuits so that the primary windings of said other transformer are alternately energized, and means for selectively rendering said generator operative to control said paths or said circuits.

18. A converter comprising, in combination, a pair of transformers each including a secondary winding and a pair of primary windings, thermionic means having a pair of space current paths and screen electrodes individual to said paths, means including said space current paths for energizing the primary windings of one of said transformers, circuits for energizing the primary windings of the other of said transformers, a pulse generator normally operative to control said paths o that the currents traversing the primary windings of said one transformer are alternately increased, said generator also being adapted to control said circuits so that the primary Windin-gs of said other transformer are alternately energized, and means controlled in accordance with the current traversing said paths by way of said screen electrodes for rendering said generator inoperative to control said paths and for rendering said generator operative to control said circuits.

19. In combination With a ringing converter which comprises an output transformer having a secondary winding, an electron discharge device comprising a space current path and a control electrode for controlling the current traversing said path, an output circuit including said space current path, means normally biasing said control electrode to a predetermined potential, circuit means for altering the potential of said electrode in response to the energization of said secondary winding, a signaling circuit, and means controlled in accordance with the current traversing said output circuit for controlling said signaling circuit.

20. In combination with a ringing converter which comprises a transformer having a secondary winding, an electron discharge device having input and output electrodes, an output circuit coupled to said output electrodes, means normally biasing said inputelectrodes to a predetermined difference potential, an input circuit coupling said input electrodes to said Winding to change the difference potential of said input electrodes when said winding is energized, thereby to change the current traversing said output circuit, a signaling circuit, and means controlled in accordance with the current traversing said output circuit for controlling said signaling circuit.

21. In a telephone system which includes lines and switching means for setting up connections between said lines and for signaling over said lines, ringing apparatus comprising, in combination, a ringing circuit over which current may be transmitted to a selected one of said lines, a direct current source operative to deliver energy to said circuit, an electron discharge tube provided with a space current path, means connecting said space current path in circuit with said source, and means for varying the impedance of said space current path so that a periodically varyin undulating current is transmitted to said circuit.

22. In a telephone system which includes lines and switching means for setting up connections between said lines and for signaling over said lines, ringing apparatus comprising, in combination, a ringing circuit over which current may be transmitted to a selected one of said lines, a direct current source operative to deliver energy to said circuit, an electron discharge tube provided with a space current path, means connecting said space current path in circuit with said source, and frequency determining means for varying the impedance of said space current path at a predetermined frequency so that a periodically varying undulating current of said predetermined frequency is transmitted to said circuit.

23. In a telephone system which includes lines and switching means for setting up connections between said lines and for signaling over said lines, ringing apparatus comprising, in combination, a ringing circuit over which current may be transmitted to a selected one of said lines, a source of direct current, thermionic means operative to convert the voltage of said source into alternating voltage and to impress the alternating voltage on said circuit, a plurality of frequency determining devices each operative to control the frequency of the alternating voltage impressed on said circuit by said thermionic means and respectively operative to establish different frequencies of said alternating voltage, and means for connecting different ones of said devices to control said thermionic means.

24. In a telephone system which includes lines and switching means for setting up connections between said lines and for signaling over said lines, ringing apparatus comprising, in combination, a ringing circuit over which current may be transmitted to a selected one of said lines, a source of direct current, thermionic means operative to convert the voltage of said source into alternating voltage and to impress the alternating voltage on said circuit, a plurality of frequency determining devices each operative to control the frequency of the alternating voltage impressed on said circuit by said thermionic means and respectively operative to establish different frequencies of said alternating voltage, and cyclically operating means for sequentially connecting said devices to control said thermionic means.

ROSWELL H. HERRICK.

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