US2214259A - Telephone substation apparatus - Google Patents

Description

Sept. 10, 1940. H. c. PYE 2,214,259

TELEPHONE SUBSTATION APPARATUS Filed July 22, 1938 H Id E an: E BY I a. J

ATTORNEYS Patented Sept. 10, 1940 UNITED STATES PATENT OFFICE 2,214,259 TELEPHONE SUBSTATION APPARATUS Application July 22, 1938, Serial No. 220,757

14 Claims.

The present invention relates to telephone substation apparatus and has for one of its objects the provision of an improved and exceedingly simple substation circuit of the anti-side-tone u booster type which is adapted for use in telephone systems of the automatic type.

Itis a further object of the invention to provide in a substation circuit an improved arrangement for substantially preventing electrical transients of oscillatory character from being developed during operation of the substation hook or cradle switch or during operation of the calling device embodied in the circuit.

It is another object of the invention to provide a substation circuit arrangement of the character described wherein a single condenser is utilized in the circuit for a plurality of difierent purposes, thus minimizing the amount of circuit apparatus required at the substation.

It is a further object of the invention to provide a substation circuit adapted for use in telephone systems of the automatic type and having embodied therein an improved and exceedingly simple arrangement for eliminating clicks in the substation receiver during operation of the calling device.

It is a still further object of the invention to provide a telephone substation circuit arranged in an improved manner such that the condenser included in the transmitter branch of the circuit is not exposed to high voltage surges which might puncture the insulation of the condenser.

It is an additional object of the invention to provide an improved substation circuit of the character described which is so arranged that grounded or metallic ringing may be utilized in signalling over the line extending to the circuit without modification oi the circuit.

The invention is illustrated in its embodiment 40 in a substation which comprises a telephone transmitter, a telephone receiver, an induction coil, a hook or cradle switch, and a calling or impulsing device interconnected to form a' substation circuit. For the puspose of signalling the substation attendant or subscriberjon calls incoming to the substation, there is also-provided a signal device arranged to be energized by ringing current projected by way of the line extending to the substation. More particularly, this signal device isprovided with an operating circuit which includes at least one of the conductors of the line extending to the substation and a condenser. When arranged for metallic ringing, the signal device is normally bridged between the 56 two conductors of the substation line in series with the condenser. On the other hand, when grounded ringing is to be used, the signal device is bridged between one of the line conductors and ground in a circuit which includes the condenser. In accordance with one feature of the present invention, the elements of the substation circuit, as enumerated above, are so connected and arranged that the signal device may be arranged for grounded or metallic ringing over the line extending to the substation without any change in the substation circuit.

In accordance with a further feature of the invention, there is provided a non-oscillatory circuit for substantially preventing electrical transients of oscillatory character from being developed during operation of the impulsing or calling device. Similarly, there is .provided a non-oscillatory circuit for substantially preventing electrical transients of oscillatory character from being developed during operation of the hook switch to disconnect the substation circuit from its associated line. More particularly, the first-mentioned circuit comprises the condenser, mentioned above, by way of which ringing current is conducted to the substation ringer, which condenser is connected in series with a resistor, to form a circuit arranged to shunt the impulsing springs of the calling device. This circuit, which is completed by operation of a set of springs embodied in the hook switch, is also utilized substantially to prevent transients of oscillatory character from being developed during the opening of the hook switch contacts which are utilized to connect the substation circuit to the line extending thereto. The arrangement contemplates the provision of means whereby the contacts of the hook switch which are utilized to connect the substation circuit to its associated line, are opened, before the circuit just described is interrupted, during operation of the hook or cradle switch to disconnect the substation circuit from its associated line. By virtue of this arrangement substantially no parasitic radio frequency voltages or currents, which might produce radio interference, are developed during the switching operations incident to the completion and releasing of a communication connection.

In accordance with a still further feature of I for thereafter interrupting the branch of the substation circuit by way of which signal current energy is delivered to the substation receiver. By virtue of this arrangement, the possibility of transients, developed by operation of the calling device impulsing springs, being reproduced as Iclicks by the substation rereiver is positively precluded.

Furtherfeatures of the invention pertain to the particular arrangement of the circuit elements whereby the above and additional operating features are attained.

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 drawing, in which Figure 1 illustrates a substation circuit, having embodied therein the features of the invention as briefly outlined above; and Figure 2 illustrates a modification of the circuit shown in Figure 1.

Referring now more particularly to Figure 1 of the drawing, the substation circuit there illustrated comprises a telephone transmitter It, a receiver ll, an induction coil l2, a calling device 13 and a hook switch 14, interconnected to form a substation circuit of the well-known anti-sidetone type. This circuit includes line terminals 6 and 9 terminating the conductors l8 and IQ of a line 20 extending to an exchange, not shown, of the automatic type. The induction coil l2 comprises three inductively coupled windings I6, 16 and I1, serially included in a branch circuit connected between the line conductors I8 and I8 of the line 20. This branch circuit also includes a coupling condenser 2| connected between the two windings l1 and 15, which condenser is effective to prevent the receiver from being energized by the direct current flowing over the line 20 when the substation is connected for use. The portion of the branch circuit including the two windings l6 and I1 and the condenser 2| is shunted by the transmitter l0, and the receiver II is connected between the line terminal 9 and the junction point between the two windings I6 and IT to shunt the winding l6 of the induction coil l2. Since the condenser 2| is connected between the two windings l1 and IE it is protected against high voltages appearing upon the line 20 which voltage surges might result in puncturing the insulation of the condenser.

For the purpose of giving an audible indication of calls incoming to the substation, there is provided a signal device in the form of a ringer 22 which may be arranged for grounded or metallic ringing, as desired. If metallic ringing is to be used, the X wiring shown in the drawing is used, whereas if the line 20 is arranged for grounded ringing, the Y wiring illustrated in the drawing is utilized. In either case the signal device 22 is normally connected to be energized over an operating circuit including the line conductor l8, a condenser 23 and normally closed contact springs 24 and 25 embodied in the hook switch H. For the purpose of operatively connecting the substation circuit for communication over the line 20, the hook switch I4 is also .provided with a pair of springs 26 which normally occupy the open circuit position.

The calling device I3 is of the conventional dial operated type and comprises a pair of contact springs 28 which are utilized to transmit short-circuitin'g the receiver II and the springs 60 and 6| are included in a path for short-cir-v cuiting the entire substation circuit during operation of the calling device I3.

In accordance with a further feature of the present invention, there is provided a normally incomplete, non-oscillatory energy absorbing circuit for substantially preventing electrical transients of oscillatory character from being developed during operation of the calling device 12 and, more particularly, during the initial portion of each period when the impulsing springs 26 are opened. This circuit comprises the condenser 23 and a resistor 63 which are arranged to be connected in series when the hook switch is operated to its oil-normal position to close the contact springs 24 and 21. Following the completion of this circuit and closure of the springs 26, the circuit is connected in shunt with the impulsing springs 28. Also, with the impulsing springs 28 in engagement, the circuit shunts the hook switch springs 26. It is pointed out here that the contact springs 26 are so adjusted that when the hook switch I4 is operated to its normal position from the oil-normal position shown in the drawing, the springs 26 are opened before the spring 24 is moved from engagement with the spring 21 to interrupt the energy absorbing circuit comprising the resistor 32 and the condenser 23.

Normally, the springs 26 are o'pen-circuited, the springs 24 and 21 are disengaged and the springs 24 and 25 are in engagement to maintain the operating circuit of the signal device 22 in a condition such that the device 22 will be energized when ringing current is projected over the line 20. With the enumerated springs of the hook switch l4 in the positions just described, if the line '20 is seized by one of the numerical switches provided in the exchange terminating the line and tests idle, ringing current is projected over the line to energize the signal device 22. If the line 20 and the signal device 22 are arranged for grounded ringing, the path traversed by the ringing current may be partially traced as extending by way of the conductor I 8, the condenser 23, the springs 24 and 25 and the windings of the device 22 to ground. On the other hand, if the line 20 and the signal device 22 are arranged for metallic ringing, the current-return path to the distant exchange is by way of the line conductor II. The call is answered in the usual manner by operating the hook switch l4 to its off-normal position, at which time the springs 24 and 26 are opened to interrupt the circuit for energizing the signal device 22; the springs 26 are closed operatively to connect the substation circuit to the line 20 and the springs 24 and 21 are moved into engagement to complete the above-described energy absorbing circuit. When the springs 26 are closed, a direct current bridging path is connected between the line conductors l6 and IQ of the line 20. This path may be traced as extending from the line conductor l6 by way of the hook switch springs 26, the impulsing springs 26, u

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the induction coil winding l and the transmitter in to the opposite line conductor IS. The completion of this path may be utilized in the usual manner to cause the operation of a backbridge relayembodied in the connector switch by way of which the connection is routed to the line 2|]. Since the transmitter I0 is included in this path, it will be understood that the transmitter is energized by the direct current flowing from the exchange battery through the windings of .the back-bridge relay embodied in the operated connector switch and over the line 20. Following the operation of the hook switch I4 to its off-normal position in the manner just described, signal currents incoming to the substation traverse the above-traced bridging path between the line conductors l8 and I9. Since these currents flow through the winding I5 of the induction coil l2, corresponding induced voltages are developed in the winding l6 which cause corresponding signal currents to traverse the windings of the receiver II. The signal currents flowing through the windings of the receiver II are reproduced as sound in the usual manner. Signal currents developed by operation of the transmitter follow three closed circuits. One of these circuits may be traced as extending from the line terminal 9 by way of the transmitter III, the winding IS, the impulsing springs 28, the hook switch springs 26 and the loop circuit comprising the conductors l8 and I9 back to the terminal 9. The second closed circuit comprises the condenser 2|, the two induction coil windings l1 and I6 and the transmitter Hi. In this regard, it is pointed out that the winding I1 is so poled with respect to the winding l5 that the induced voltages developed in the winding l5, as a result of the currents flowing through the winding I'I, aid the currents flowing directly through the winding I5 and, thus, the well-known booster effect is achieved. The signal currents traversing the winding I6 over the closed circuit just traced, are substantially equal and opposite to the currents induced in the winding l6 as a result of the signal currents flowing over the loop circuit extending to the line terminating exchange. Thus, the resultant voltage drop across the winding I6 for signal currents within the voice frequency range is substantially zero and the wellknown anti-side-tone effect is realized.

The substation circuit as described above is balanced to achieve the maximum possible suppression of the side tone during operation of the transmitter ID by properly proportioning the impedance of the induction coil winding l6 so that it effectively balances the impedance of the line 2|] and the other apparatus associated therewith, measured at the line terminals 8 and 9, when a connection extending by way of the line is completed between the substation illustrated and a second substation. For perfect balance, it is necessary that the voltage across the winding l6 resulting from current flowing in the closed 'loop circuit comprising the condenser 2| and the two windings l1 and I6 be equal and opposite to the voltage induced in the winding i6 and resulting from the coupling between the windings l6 and IT, at all frequencies within the operating range. Since the voltages just mentioned are in part determined by the constants or effective impedances of lines having different characteristics, the impedance of the winding I6 is proportioned to equalize the voltage components for lines having average impedance characteristics, whereby reasonably good side-tone suppression is achieved regardless of the character of the lines utilized inan established conversational connection.

In order to release the connection established in the manner just described after the conversation is ended, the receiver II is returned to its supporting hook to cause the hook switch l4 to operate to its normal position. During the return of this switch to normal, the springs 26 are first opened to interrupt the direct current circuit by way of which energizing current is conducted to the transmitter Hi. When the springs 26 are first opened, a transient voltage tends to be developed which may be of oscillatory character and may have high frequency components within the radio frequency range. Such transient voltages are objectionable since they interfere with radio reception. Since, however, the circuit, comprising the condenser 23 and the resistor 33, is not interrupted at the time the springs 26 are opened, the transient energy developed by the opening of the springs 26 is dissipated in the resistor 33 and only a minute portion of the transient energy is radiated. It'will be noted that the energy absorbing circuit is substantially free of inductance and hence, is non-oscillatory in character. Thus, the possibility of the absorbing circuit oscillating to cause a substantial amount of radio frequency energy to be radiated, is obviated. After the transient energy is dissipated in the manner just described, the springs 24 and 21 are moved out of engagement to interrupt the absorbing circuit and the springs 24 and 25 are moved into engagementv to reprepare the operating circuit for the signal device 22.

In order to initiate a call at the substation illustrated in Fig. 1, the receiver II is removed from its supporting hook in the usual manner to cause the operation of the hook switch to its off-normal position. In response to the operation of the hook switch, the above-traced bridging' path between the line conductors I8 and IQ of the line 20 is completed, thereby to cause the connection to be extended to one of the numerical switches provided in the line terminating exchange in the usual manner. Also, when the hook switch I4 is operated to its off-normal position, the above described energy absorbing circuit is completed through closure of the springs 2-4 and 21. Following the receipt of dial tone, the calling party may operate the impulsing device l3 to cause switch control impulses to be transmitted by way of the line 20 to the automatic switching equipment provided in the distant exchange.

When the dial of the impulsing device I3 is moved off normal during the forward stroke thereof, the actuating element 32 is operated to close the springs 29, and 3 I. When the springs 29 and 30 are closed, the receiver I I is short-circuited over an obvious path, thereby to prevent electrical transients occasioned by operation of the impulsing springs 28 from being reproduced as clicks in the receiver. At the springs 3|! and 3 I, the entire substation circuit is short-circuited, thereby to lower the impedance of the circuit traversed by the current impulses resulting from the opening and closing of the impulsing springs 26. During each return movement of the dial, the springs 28 are opened and closed a number of times depending upon the digit dialed. When these springs are open, the loop circuit extending to the automatic switching equipment in period of the springs 28, occurring during each impulse, voltage and current transients, similar to those described above and occasioned by opening the hook switch springs 23, are developed. Since, however, the energy-absorbing circuit comprising the resistor 33 and the condenser 23, is now connected in shunt with the springs 28, the transient energy is dissipated in the circuit without the production of high frequency oscillatory currents. Thus, the circuit comprising the resistor 33 and the condenser 23 also functions to prevent high frequency radio interference energy from being developed during operation ofthe impulsing device. After the desired connection to the distant substation is completed and the conversation is terminated, the connection may be cleared out at the substation illustrated in Fig. 1' in the manner explained previously.

Referring now more particularly to Fig. 2 of the drawing, the circuit there illustrated is in all essential respects similar to the circuit shown in Fig. 1 and, accordingly, like reference characters have been used to designate corresponding elements in the two circuits. The circuit shown in Fig. 2 difiers from that of Fig. 1 by the addition of a resistor 34 connected in series with the winding l6 of the induction coil I2, and also by the addition of a shunt spring 35 in the impulsing device l3. The resistor 34 is provided for the purpose of supplementing the resistance of the winding IS in balancing the substation circuit to achieve maximum suppression of the side-tone in the manner explained previously. The added shunt spring 35 is arranged to cooperate with the spring 3| for. the purpose of opening the branch of the substation circuit by way of which signal current energy is delivered to the receiver ll during operation of the impulsing device l3. More particularly, when the dial of the impulsing device I3 is moved off normal, the receiver II is first short-circuited at the springs 29 and 30, following which the entire substation circuit is short-circuited at the springs 30 and 3|. Thereafter, the springs 3| and 35 are opened to interrupt the branch circuit including the transmitter I and the induction coil winding l5. When the dial is moved back to normal, the springs 3| and 35 are first closed to complete the branch circuit, following which the springs 30 and 3| are opened to interrupt the path short-circuiting the substation circuit. Finally, the springs 29 and 30 are opened to interrupt the path shortcircuiting the receiver ll. As indicated above, the purpose of providing the arrangement for interi'upting the branch circuit comprising the induction coil winding l5 and the transmitter I0 is to obviate any possibility of electrical translents developed by opening the impulsing springs 28 from being reproduced as clicks in the receiver Thus, if due to unsatisfactory contact conditions between the springs 29, 30 and 3|, the entire substation circuit is not short-circuited or the receiver l l is not efiectively short-circuited, transient currents may traverse the winding l5 and cause induced voltages to be developed in the winding l6 which are reproduced as clicks by the receiver When the springs 3| and 35 are opened, however, it is impossible for transient energy, developed by opening of the impulsing springs 28, to be delivered to the receiver irrespective of whether or not the short-cirthe distant exchange is interrupted, thus arrestcuiting paths described above are effectively completed.

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

What is claimed is:

1. In a telephone substation circuit adapted to be connected to a two-conductor telephone line, a signal device, a condenser, said circuit being so connected and arranged that said signal device can be connected in series'with said condenser across the conductors of said line for metallic ringing or in series with said condenser and between one of said line conductors and ground for grounded ringing, an impulsing device including contacts operative to transmit control impulses by way of said line, a non-oscillatory circuit including said condenser, and means including said last-named circuit for substantially preventing electrical transients of oscillatory character from being developed during operation of said impulsing device. I

2. A telephone substation circuit adapted to beconnected to a two-conductor line and comprising, in combination, a transmitter, a receiver, an induction coil and an impulsing device interconnected to form a substation circuit of the anti-side tone type, a signal device, a condenser, said substation circuit being so connected and arranged that said signal device can be connected in series with said condenser across the conductors of said line for metallic ringing or in series with said condenser and between one of the conductors of said line and ground for grounded ringing, said impulsing device including contacts operative to transmit control impulses by way 01' said line, a non-oscillatory circuit including said condenser, and means including said last named circuit for substantially preventing electrical transients of oscillatory character from being developed during operation of said impulsing device.

3. In a telephone substation circuit adapted to be connected to a telephone line, a signal device, a condenser, an operating circuit for said signal device, said operating circuit including said condenser and at least one conductor of said line, an impulsing device including contacts operative to transmit controlimpulses by way of said line, a normally incomplete second circuit including said condenser for substantially preventing electrical transients of oscillatory character from being developed during operation of said impulsing device, a switch including a first pair of contacts for operatively connecting said substation circuit to said line and a second pair of contacts operative to complete said second circuit, means for operating said contacts in unison, said second circuit being so connected and arranged that, when completed, operation of said first pair of contacts to the open circuit position is substantially ineffective to produce electrical transients of oscillatory character, and means whereby, during operation of said switch, said first pair of contacts are moved to the open circuit position beforesaid second pair of contacts are opened to interrupt said second circuit. I

4. In a telephone substation circuit adapted to be connected to a telephone line, a signal device, a condenser, an operating circuit for said signal device, said operatingcircuit including said con- 7 denser and at least one conductor of said line, an,

impulsing device including contacts operative to transmit control impulses by way of said line, a normally incomplete non-oscillatory circuit including said condenser Iorsubstantially preventtion circuit to said line and a second pair of con-- tacts operative to complete said non-oscillatory circuit, means for operating said contacts in unison, said non-oscillatory circuit being so connected and arranged that, when completed, operation of said first pair of contacts to the opencircuit position is substantially ineffective to produce electrical transients of oscillatory character, and means whereby, during operation of said switch, said first pair of contacts are opened before said second pair of contacts are opened to interrupt said non-oscillatory circuit.

5. In a telephone substation circuit adapted to be connected to a telephone line, a signal device, a condenser, an operating circuit for said signal device, said operating circuit including said condenser and at least one conductor of said line, an impulsing device including contacts operative to transmit control impulses by way of said line, a normally incomplete non-oscillatory circuit including said condenser and a resistor and arranged to shunt said contacts, said non-oscillatory circuit being operative substantially to prevent electrical transients of oscillatory character from being developed by operation of said impulsing device, a switch including a normally open first pair of contacts operative into engagement to connect said substation circuit to said line and a second set of contacts including a contact operative to interrupt said operating circuit and to complete said non-oscillatory circuit, said non-oscillatory circuit being so connected and arranged that, when completed, operation of said first pair of contacts to the open-circuit position is substantially ineifective to produce electrical transients of oscillatory character, and. means whereby, during operation of said switch,

said first pair of contacts are opened before said non-oscillatory circuit is interrupted.

6. In combination with a telephone substation circuit adapted to be connected to a telephone line, a signal device, a condenser, an operating circuit for said signal device, said operating circuit including said condenser and at least one conductor of said line, a switch including a first pair of contacts adaptedto be moved into en-' gagement, thereby operatively to connect said substation circuit to said line, a normally incomplete circuit including said condenser and operative, when completed, substantially to prevent electrical transients of oscillatory character from being developed when said contacts are opened, a second set of contacts controlled by said switch and operative to open said operating circuit and to close said second-named circuit when said first contacts are moved into engagement, and means whereby said second-named circuit is interrupted only after said first contacts are opened during operation of said switch to disconnect said substation circuit from said line.

'7. A telephone substation adapted tobe connected to a two-conductor telephone line and comprising, in combination, a transmitter, a receiver and an induction coil interconnected to form a substation circuit, said circuit comprising one branch adapted to be connected between the conductors of said line and arranged to transmit signal current energy to said receiver, a normally incomplete path for short-circuiting said receiver, a normally incomplete path for shortcircuitlng said substation circuit, an impulsing device including contacts operative to transmit control impulses by way of said line, means responsive to operation of said impulsing device for sequentially completing said first and second mentioned paths, and means also responsive to operation of said impulsing device for interrupting said branch or said circuit after said paths are completed.

8. A telephone substation adapted to be connected to a two-conductor telephone line and comprising, in combination, a transmitter, a receiver and an induction coil interconnected to form a substation circuit, said circuit comprising one branch adapted to be connected between the conductors of said line and arranged to transmit signal current energy developed therein to said receiver, a normally incomplete path for shortcircuiting said receiver, a normally incomplete path for short-circuiting said substation circuit, an impulsing device including contacts operative to transmit control impulses by way of said line, means comprising contacts controlled by said impulsing device for sequentially completing said first and second mentioned paths, and means also responsive to operation of said impulsing device for interrupting said branch of said circuit after said paths are completed and before operation of said first-mentioned contacts is started.

9. A telephone substation circuit adapted to be connected to a telephone line and comprising, in combination, an induction coil including a plurality of coupled windings, a branch circuit adapted to be connected across the conductors of said line and having said windings serially included therein, a condenser included in said branch circuit and connected between two of said windings, a receiver connected across one of said windings between one line terminal of said branch circuit and the junction point between a pair of said windings, and atransmitter connected in shunt with the portion of said branch circuit which comprises two of said windings and said condenser.

10. A telephone substation circuit adapted to be connected to a telephone line and comprising, in combination, an induction coil including a plurality of coupled windings, a branch circuit adapted to be connected across the conductors of said line and having said windings serially included therein, a condenser included in said branch circuit and connected between two of said windings, a receiver connected across one of said windings between one line terminal of said branch circuit and the junction point between a pair of said windings, and a transmitter connected between said one line terminal and the junction point between said condenser and a second of said windings to shunt a portion of said branch circuit including said condenser.

11. A telephone substation circuit adapted to be connected to a telephone line and comprising, in combination, an induction coil including a plurality of coupled windings, a branch circuit adapted to be connected across the conductors of said line and having said windings serially included therein, a condenser included in said branch circuit and connected between two ofsaid windings, a receiver connected across one of said windings between one line terminal of said branch circuit and the junction point between a pair of said windings, and a transmitter connected in shunt with the portion of said branch circuit which comprises two of said windings and said condenser, said one winding being so poled with respect to the other windings that signal current energy developed by operation of said transmitter is' substantially prevented from being delivered to said receiver.

12. In a telephone substation circuit adapted to be connected to a telephone line, an impulsing device including contacts operative to transmit control pulses by way of said line, a normally incomplete second circuit including a condenser for substantially preventing electrical transients from being developed during operation 01 said device, a switch including a first pair of contacts for operatively connecting said substation circuit to said line and a second pair of. contacts operative to complete said second circuit, means for operating said contacts in unison, said second circuit being so connected and arranged that, when completed, operation of said first pair'oi contacts to the open circuit position is substantially inefiective to produce electrical transients, and means whereby, during operation of said switch, said first pair of contacts are moved to the open circuit position before said second pair 01 contacts are opened to interrupt said second circuit.

13. In combination, a telephone line, a substation circuit including an lmpulsing device comprising contacts operative to transmit control pulses by way oi! said line, a switch including a first pair of contacts operative to connect said substation circuit to said line and a secondpair of contacts, a normally incomplete path shunting said first-named contacts and said first pair or contacts in series, said path serially including a capacitor and said second pair of contacts, whereby it is completed in response to operation of said switch to move said first pair of contacts into engagement, and means whereby, during operation of said switch, said first pair of contacts are disengaged before said second pair of contacts are disengaged to interrupt said path.

14. In combination with a telephone substation circuit adapted to be connected to a telephone line, a signal device, a condenser, an operating circuit for said signal device, said operating circuit including said condenser and at least one conductor of said line, a switch including a first pair of contacts adapted to be moved into engagement, thereby operatively to connect said substation circuit to said line, a normally incomplete circuit including said condenser and operative, when completed, substantially to prevent electrical transients from being developed when said contacts are opened, a second set of contacts controlled by said switch and operative to close said second-named circuit when said first contacts are moved into engagement, and means whereby said second-named circuit is interrupted only after said first contacts are opened during operation oi! said switch todisconnect said substation circuit from said line.

HAROLD C. PYE.

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