US2084115A - Control circuits - Google Patents

Description

June 15, 1937. D. M. TERRY CONTROL CIRCUITS 2 Sheets-Sheefl Filed April' 23, 1936 nwe/vron D. M. TERR V 7/1 /42M% ATTORNEY June 15-, 1937. b. TERRY 2,084,115.

- CONTROL CIRCUITS Filed April 23, 1936 2 Sheets-Sheet 2 FIG? I0 os r By E A 7' TORNE V Patented June 15, 1&3?

UNITED STATES casein CONTROL CIRCUITS Donald M. Terry, Brooklyn, N; 3L, assignor to Bell Telephone Laboratories, Incorporated, New York, N. 53., a corporation of New York Application April 23,

14 Claims.

This invention relates to gain control circuits for carrier signal systems and particularly to protective circuits for gain control circuits.

One object of the invention is to provide a gain control circuit for a signal transmission line with protective circuits that shall prevent op- .eration of the gain control circuit in case the voltage of the signals on the line varies beyond predetermined limits.

Another object of the invention is to provide gain control circuits for a signal transmission line with protective circuits for preventing operation of the gain control circuits in case the signals on the transmission line vary beyond 15 predetermined limits and for operating an alarm in case the signals on the transmission line vary beyond predetermined limits for an extended period of time.

A further object of the invention is to provide gain control circuits for a carrier current transmission system with protective circuits controlled by a pilot current for preventing operation of the gain control circuits in case the pilot current on the transmission system varies beyond predetermined limits and for operating an alarm in case the pilot current on the transmission system varies beyond predetermined limits for an extended period of time.

In a carrier current signal system transmitting signals in one direction by means of carrier current within one frequency band and for transmitting signals in an opposite direction by carrier currents located within another frequency band, the'energy level of the signal waves ineach .115. frequency band is controlled by a pilot current having a frequency located within the frequency band. The tran mission line particularly if an open wire transmission line is subjected to various conditions which cause variations in the 5-0 line attenuation. In wet weather the attenuation of an open wire line is greatly increased. In order to maintain the quality of the transmission it is necessary to vary the gain of the signals on the line according to the variation in 4: attenuation. A transmission line may under trouble conditions be subjected to abnormally large changes in attenuation for which no ordinary gain correction would be in order and it may also be subjected to so-called hits which so are momentary changes in attenuation.

According to the present invention a carrier telephone transmission system having gain control circuits of the type disclosed in the application of J. W. Beyer and D. M. Terry, Serial No.

5.3 76,020, filed April 23, 1936, is provided with pro 1936, Serial No. 76,021

tective circuits to prevent operation of the gain control circuits in case the strength of the signalsvaries beyond predetermined limits. If the strength of the signals varies beyond predetermined limits for an extended period of time an alarm is operated.

In one form of the invention an amplifier in a signal'transmission line has the input circuit thereof connected to attenuation networks in the line by means of a variable condenser. The variable condenser is operated to raise or lower the gain by means of two synchronous motors which are selectively operated by a very sensitive relay. The sensitive meter relay is controlled by a control circuit connected to the transmission line at a point beyond the amplifier therein. The feedback circuit comprises means for selecting a pilot current and means for effecting full voltage rectification of the pilot current.

The sensitive relay is provided with the two small permanent magnets for holding the contact arm of the relay in either of its operated positions after operation. Releasing means controlled by' the synchronous motors is provided for returning the contact arm of the sensitive meter relay to initial or neutral position.

The feedback circuit which controls the sensitive relay governing the gain on the transmis sion line alsocontrols a relay of limited sensitivity. The second relay will only be operated in case the variationin the strength of the sig-' nals and the pilot current on the transmission line is abnormal or beyond that which can be corrected under control of the first relay. Upon operation of the second relay in case of a large change in strength of the pilot current on the transmission line, means is provided for breaking the-circuit of the two synchronous motors so that no control of the gain amplifier can be effected by the highly sensitive meter relay. The relay of limited sensitivity also controls means for operating an alarm in case thevariaticn in strength of the pilot current continues for an extended period of time.

In the accompanying drawings,

Fig. 1 is a diagrammatic view of control circuits constructed in accordance with the invention;

Fig. 2 is a diagrammatic view of a modification of the control circuits shown in Fig. 1; and

Figs. 3 vand 4 are diagrammatic views of the sensitive relays shown in Figs. 1 and 2 of the drawings.

Referring to Fig. 1 of the drawings a transmission line comprising input conductors I and 2 and output conductors 3 and 4 is provided with a gain control amplifier 5 which is connected to a second amplifier '6 by means of a transformer I. 'The transmission line shown in Fig. 1 of the drawings may represent one channel of a repeater station as shown in the application of R. W. Chesnut, Serial No. 5,696, filed February 9, 1935 now, Patent No. 2,049,195, issued July 28, 1936. Two attenuation networks 8 and 9 which may be of the type disclosed in the patent to R. W. Chesnut et a1. 1,880,800, dated October 4, 1932 are provided in the transmission line for controlling the operation of the gain control amplifier 5 in accordance with the operation of an adjustable condenser Ill. The condenser I0 comprises a rotatable arm I I mounted on a shaft I2. The shaft I2 carries a plate I3. The condenser plate I3 is movable adjacent to condenser plates I4, I5 and I6. The condenser plate I4 is connected to the line conductor I on one side of the network 6. The condenser plate I5 is connected to the conductor between the networks 8 and 9. The condenser plate I6 is connected to the line conductor I beyond the network 9. A condenser arrangement of this type is disclosed and claimed in the application of C. R. Eckberg, Serial No. 5,717, filed February 9, 1935.

The variable condenser I 0 is operated by a synchronous motor I! to raise the gain of the amplifier 5 and is operated by a synchronous motor I8 to lower the gain of the amplifier 5. The synchronous motor I! comprises a stator winding I9 and a rotor 20. The synchronous motor I8 comprises the stator winding 2| and rotor 22. The rotors 26 and 22 of the motors I1 and I8 are mounted on a shaft 23 carrying a pinion 24. The pinion 24 meshes with a gear wheel 25 mounted on a shaft 26. The shaft 26 not only carries the gear wheel 25, but a cam member 21 and a pinion 28. The pinion 28 meshes with a gear wheel 29 on the shaft I2 carrying the contact arm II of the variable condenser I0.

The two synchronous motors I! and I8 which rotate in opposite directions are selectively operated by two relays 3|] and 3| under control of a sensitive relay 32. The sensitive relay 32, which is shown in detail in Figs. 3 and 4, comprises an operating coil 33, a contact arm 34 which is adapted to be moved into engagement with contact members 35 and 36 and a restoring coil 31. The contact members 35 and 36 are in the form of small permanent magnets which engage and hold a small soft iron armature 38 on the contact arm 34 whenever the contact arm is moved to either of its operated positions. In order to restore the contact arm 34 to initial or neutral position the restoring coil 3! is energized to attract a core armature 39. The core armature 39 efiects operation of two bell crank levers 49 and 4|. One or the other of the bell crank levers 4B and 4t engages the contact arm 34 and moves it to neutral position. An adjustable stop r 42 is provided for limiting the movement of thecore armature 39.

The relay 30 is provided with switch members 43 and 44 and relay 3| is provided with switch members 45 and 46. The sensitive relay 32 is operated by means of a feedback circuit connected to the conductors 3 and 4 beyond the amplifier 6. The feedback circuit comprises two transformers 4! and 48 which are tuned by means of condensers 49 and 50 to select the pilot or control current on the transmission line and a full Voltage rectifier bridge 5|. The rectifier bridge 5| comprises the rectifiers 52 and 53 forming two arms of the bridge circuit and condensers 54 and 55 forming the other two arms of the bridge circuit. The output vertices of the bridge 5| are connected to the operating coil 33 of the sensitive relay 32 and the input vertices of the bridge are connected by the transformers 41 and 48 to the transmission line. The two rectifiers 52 and 53, which are preferably of the copper-oxide type, are oppositely connected so as to efiect rectification of each half of the pilot current voltage. The sensitive relay 32 may be adjusted to effect operation of the contact arm 34 by a half a decibel change in the strength of the pilot current.

The feedback circuit also controls the operation of a relay 56 of lower sensitivity. The relay 56 which comprises an energizing coil 51, a restoring coil 58 and a contact arm 56 adapted to engage contact members 60 and 6| is similar in construction to the sensitive relay 32. However, the sensitivity of the relay 56 is much lower when compared with the relay 32. If the relay 32 is set to be operated by a plus or minus one half a decibel change in the pilot current on the line, the relay 56 will be set to operate, for example, at minus five decibels or plus three decibel change of the pilot current.

The relay 56 of lower sensitivity directly controls the operation of a relay 62. The relay 62 by means of a switch arm 63 prevents energization of either synchronous motors I! and I8 from a source of alternating current 640. under control of the sensitive relay 32. The relay 56 also controls two heating cells 64 and 65 which, in turn, control the heating of two silver sulphide elements 66 and 6?. The silver sulphide element 66 is in the energizing circuit of a relay 68 and the silver sulphide element 61 is in the energizing circuit of a relay 69. The relay 68 is provided with switch arms 69a and Hi and the relay 69 is provided with a switch arm II. The relay 69 when operated controls an alarm 12. The alarm I2 is released by means of a relay I3 having a contact arm I4 under control of a key I5.

If the strength of the pilot current on the conductors 3 and 4 is lowered a half decibel, the strength of the rectified current supplied to the winding 33 of the sensitive relay 32 is reduced to effect engagement between the contact member 35 and the contact arm 34. A circuit is then completed from a battery I6 for operating the relay 39. The switch arm 43 when operated by the relay 3!! completes a circuit from the source of alternating current 64a for energizing the stator winding I9 of the synchronous motor IT. The circuit completed by the switch arm 43 may be traced from one terminal of the secondary winding of a transformer II through the switch arm 63 of a relay 62, switch arm 43 of relay 30 and winding I9 to the other terminal of the secondary winding. The synchronous motor I1 is operated to effect movement of the arm I I carrying the condenser plate I3 towards the left as viewed in Fig. 1 of the drawings. This movement of the condenser plate I3 serves to raise the gain of the amplifier 5. Immediately thereafter the cam member 21 is operated to place a ground on contact members 36 and I9. contact member 13 by operation of the cam member 21 serves to complete a holding circuit for the relay 30 from the battery I6 through the switch arm 44. Grounding of the contact member I9 serves to complete a circuit for energizing the Grounding of the restoring coil 31 of the relay- 3-2 from a battery'8'0. The synchronous motor II will continue in operation under control of the relay 38 until the cam member 2'! has completed one rotation to open: the holding circuit for the relay 30.

If the strength of the pilot current on the transmission line has not been raised to normal by the above operation the sensitive meter relay 32 is operated again and the above cycle of operations is repeated. If the strength of the pilot current goes more than a half decibel above normal value the sensitive relay 32 is operated by the feedback circuit to effect engagement between the contact arm 34 and the contact member 36. This operation of the sensitive relay 32 completes a circuit from battery 8| for operating the relay 3i The switch arm 48 of the relay 3i completes a circuit from the alternating current source 54a for energizing the winding 2I of the synchronous motor I8. The circuit for energizing the winding 2i of the motor includes the switch arm '63 of the relay 82. The synchronous motor I8 when operated efiects movement of the condenser arm II in a direction towards the right as viewed in Fig. 1 of the drawings to lower the gain effect by the amplifier 5. Immediately thereafter the cam member 21 places a ground on the contact members 18 and I9. Placing a ground upon the contact members 18 completes a holding circuit for the relay 3! from the battery BI through the switch arm 45. The placing of ground upon the contact member 79 completes a circuit from the battery 88 for operating the restoring coil 31 of the relay 32. The restoring coil 37 returns the contact arm to neutral position in the manner above described.

In case of a minus decibel change from normal in the strength of the pilot current on a transmission line, the sensitive relay 32 will not only be operated, but the relay 56 of limited sensitivity will be operated. Upon operation of the relay 55 the contact arm 59 engages contact member 68 for completing a circuit from a battery 82 for operating the relay 82. The switch arm 63 operated by the relay 62 prevents any operation of the synchronous motors I1 and [8 by the sensitive relay 32. The engagement of the switch arm 58 with the contact member 68 also completes a circuit from a battery 83 for energizing the heating coil 54 which, in turn, controls the silver sulphide element 66. The silver sulphide element 88 when heated for a short period of time, for example, five seconds, completes an operating circuit from the source of alternating current 88a for operating the relay G8. The relay 38 operates the switch arm 89a for energizing the restoring coil 58 of the relay 58 from battery 84. The restoring coil 58 operates in the manner above described to return the contact arm 59 to initial or neutral position.

The contact arm 59 when in engagement with the contact member 58 also completes a circuit for energizing the heating coil. 65 from a battery 85. The heating coil 65 controls the silver sulphide element 6'! in the circuit of the relay 58; The silver sulphide element '6'! does not complete the energizing circuit for the relay 89 until the element has been heated for a longer period of time than the element 68, for example, 25 seconds. The switch arm It also completes a circuit from the battery 85 for energizing the heating coil 8-5 when the relay 88 is operated. Thus, the heating coil 65 is energized either upon operation of the relay 55 or upon operation of the relay 68. If the disturbance on the transmission line called hit and does not continue for an extended period of time, then the relay 62 is released upon release of the relay 56 and control of the gain on the transmission line is returned to the highly sensitive relay 32. However, if the large change in strength of the pilot current in the transmission line continues for an extended period of time, then relay 55 is repeatedly operated to finally heat the silver sulphide element 81 suiiiciently to effect operation of the relay 89. Upon operation of the relay 69 a circuit is completed from a battery 88 for operating the alarm 72. The circuit for operating the alarm I2 extends from grounded battery S'S-through the alarm 12, contact arm 14 of the relay 13' and contact arm II of the relay 69 to the battery 86. The alarm I2 is stopped when the key 15 is closed to operate the relay T3. The relay 13 upon operation breaks the alarm circuit by means of contact arm M.

If the strength of the pilot current on the transmission line is raised at least three decibels above normal, the relay 56 of lower sensitivity is operated and the contact arm 59 engages the contact member 85. The operation of the relay 56 and the protective circuits controlled thereby is the same as that given heretofore when the contact arm 59 engaged the contact member 89 upon reduced level of the pilot current of the transmission line. In the above operation it will be noted that when a large change in the strength of the pilot line takes place in the transmission line, the protective circuits prevent any operation of the gain control circuits and that if this change in the pilot current continues for an extended period of time an alarm is sounded.

Referring to Fig. 2 of the drawings, a modification of the circuits shown in Fig. 1 is shown wherein the protective circuits have been changed somewhat. The gain control circuits shown in Fig. 2 of the drawings are exactly the same as the gain control circuits governed by the sensitive relay 32 shown in Fig. l of the drawings. Accordingly, similar parts have been indicated by like reference characters. Moreover, a detailed description of the gain control circuits shown in Fig. 2 will not be repeated.

The relay 55 of limited sensitivity shown in Fig. 2 of the drawings controls the operation of a synchronous motor 87 which is similar in construction to the synchronous motors II and I8. The synchronous motor 81 comprises a stator Winding 88 and a rotor 89; The rotor 89 is mounted on a shaft 98 which carries a pinion 9!. The pinion 8i meshes with a gear wheel 32 on a shaft 83. The shaft 93 also carries a pinion 84 which meshes with a gear wheel 95 on a shaft 86. The shaft 96 also carries cam members 97, 98 and 99. The cam member 9'! controls two contact members I88 and If for closing a holding circuit for the synchronous motor 81. The cam member 98 approximately 20 seconds after the start of the operation of synchronous motor 81 places a ground upon contact members I02 and I83. The cam member 99 closes contact members IM' and I85 about 5 seconds after the grounding of the contact members I82 and I83 by the cam member 98.

In case a large variation in the strength of pilot current takes place on the transmission line, the relay 55 of limited sensitivity will be operated to cause the contact arm 58' to engage either the contact member 69- or the contact member 8t. Upon engagement of the conwhich effected operation'of the relay 56 is a sotact arm with one of the contact members 69 and iii a circuit is completed from battery 86 for operating a relay I01. A switch arm L93 operated by the relay l! prevents any operation of the synchronous motors I! and [8 which control the variation in gain effected by the amplifier 5. Switch arm I09 operated by the re lay it? completes a circuit from the alternating current source of power 64 for energizing the Winding 88 of the synchronous motor 81.' Switch arms H9 and HI operated by the relay I01 form no useful function at this time. The synchronous motor 81 when operated effects rotation of the shaft 96 and the cam members 91, 93 and 99. The cam member 91 when operated closes the contact members I60 and llil to complete a holding circuit for energizing the winding 88 of the motor 81 irrespective of the position of the relay WI. The holding circuit for the winding 88 of the motor 81 is held closed for one complete rotation of the shaft 98. Approximately 20 seconds after the starting of the synchronous motor 81 the cam member 98 places a ground on the contact member 803 for completing a circuit from the battery 88 to energize the restoring coil 31 of the relay 3i! and for completing a circuit from the battery 34 through the restoring coil 58 of the relay 59. A ground is placed on the contact member IE2 at this time to insure operation of the relay Ill! from the battery N36. The cam member 93 approximately seconds after the grounding of the contact members i332 and H13 closes the contact members i134 and N35 for completing a circuit to op erate a relay H2 from battery H3 if the relay it! is in operative position at this time. The circuit for operating the relay H2 may be traced from ground through switch arm ill of the relay l'a', switch arm H4 of relay (3, contact members 165 and HM, relay H2 and battery IE3. A switch arm I I5 operated by the relay I I2 completes a holding circuit from the battery 85 for operating the alarm 12. A holding circuit for the relay H2 is completed by a switch arm H6. The holding circuit for the relay H2 may be traced from grounded battery H3 through relay !!2, switch arm H6, switch arm H4, switch arm iii and ground return to the battery H3. The relay H2 may be released and the alarm stopped by closing the key 15 which energizes the relay '53 for operating the switch arm H4 to break the energizing circuit of a relay H2.

In the protective circuits above described no operation of the synchronous motors l1 and it under control of the sensitive relay 32 can take place when the energy level of the pilot current on the transmission line varies sufiiciently to effect operation of the relay 56. The relay 58 or the cam member 98 insures operation of the relay it? to prevent any operation of the relays and iii. If the large energy change in the pilot current continues for an extended length of time, the cam member 99 insures operation of the alarm '52.

Modifications in the circuits and in the arrangement and location of parts may be made within the spirit and scope of the invention and such modifications are intended to be covered by the appended claims.

What is claimed is:

1. In combination, a signal transmission line, an amplifier on said line, two relays of different sensitivity, means for operating said relays according to the strength of a pilot current on the line, means controlled by the relay of higher sensitivity for controlling the gain of said amplifier to maintain the volume of the signals constant, and means controlled by the relay of lower sensitivity for preventing control of the amplifier by the relay of higher sensitivity when the strength of the pilot current varies beyond predetermined limits.

2. In combination, a signal transmission line, an amplifier space discharge device on said line, two relays of different sensitivity, means for operating said relays according to the energy level of a pilot current on the line beyond said amplifier, means controlled by the relay of higher sensitivity for governing the gain of said amplifier to maintain the volume of the signals constant, and means controlled by the relay of lower sensitivity for preventing control of the amplifier by the relay of higher sensitivity when the energy level of the pilot current varies beyond predetermined limits and for operating an alarm in case of large pilot current variations for an extended period of time.

3. In combination, a signal transmission line, an amplifier on said line, a, condenser potentiometer for controlling the gain of said amplifier, two relays of different sensitivity, means for operating said relays according to the energy level of a pilot current on the line beyond said amplifier, means controlled by the relay of higher sensitivity for operating said potentiometer to control said amplifier, and means controlled by the relay of lower sensitivity for preventing operation of said potentiometer when the energy level of the pilot current varies beyond predetermined limits.

4. In combination, a signal transmission line, an amplifier on said line, a condenser potentiometer for controlling the gain of said amplifier, a motor for operating said potentiometer, two relays of different sensitivity, means for operating said relays according to the energy level of a pilot current on the line beyond said amplifier, means controlled by the relay of higher sensitivity for operating said motor to control said potentiometer, and means controlled by the relay of lower sensitivity for preventing operation of said motor when the energy level of the pilot current varies beyond predetermined limits.

5. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on said line, a relay of limited sensitivity, means for operating said relay in case of large variations in energy level of the pilot current and means controlled by said relay for preventing control of said amplifier according to the pilot current level changes and for operating an alarm in case of large variations in energy level of the pilot current for extended periods of time.

6. In combination, a signal transmission line, control means governed according to the energy level of a pilot current on the line for maintaining the volume or" the signals on the line substantially constant, a relay of limited sensitivity, means for operating said relay in case of large variations in energy level of the pilot current, and means controlled by said relay in case of large variations in energy of said pilot current for preventing operation of said control means and for operating an alarm when the variation in energy of the pilot current continues for prolonged periods.

7. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on the line, a relay of limited sensitivity, and means operated by said relay in case of large variations in energy level of the pilot current for preventing control of said amplifier according to the pilot level changes.

8. In combination, a transmission line having a condenser potentiometer for controlling the gain of an amplifier, a motor for operating said potentiometer, two relays of difierent sensitivity, a control circuit connecting said relays to said line beyond the amplifier device and comprising a filter for selecting a pilot current and full voltage rectifier means for supplying rectified current to the relays, means controlled by the relay of higher sensitivity for operating said motor to control the energy level of the signal on the line, and means controlled by the relay of lower sensitivity for preventing operating of said motor and for operating an alarm in case the voltage of the pilot current rises above or falls below predetermined limits for extended periods of time.

9. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on the line, a relay of limited sensitivity having a contact arm held in operative position and restoring means, and means operated by said relay in case of large variations in energy level of the pilot current for preventing operation of said amplifier control means, for operating said relay restoring means to return the contact arm to initial position and for operating an alarm when the large variations of pilot current level extend for prolonged periods of time.-

10. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on the line, a relay of limited sensitivity having a contact arm held in operative position when operated and restoring means, means operated by said relay in case of large variations in energy level of the pilot current for preventing operation of said amplifier control means, means controlled by and operated a predetermined interval after operation of the relay for operating said restoring means to return the contact arm to initial position, and means controlled by said relay for operating an alarm in case of large change in level by the pilot current for a prolonged period.

11. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on the line, a relay of limited sensitivity having a contact arm held in operative position when operated and restoring means, a motor operated by said relay in case of large Variations in level of the pilot current, means operated by said relay for preventing operation of said amplifier control means, means operated by said motor after a predetermined delay for operating said relay restoring means, and means operated by said motor in case of a prolonged large change in level of the pilot current for operating an alarm.

12. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on the line, a relay of limited sensitivity having a contact arm held in operative position when operated and restoring means, a motor, means operated by said relay in case of large variations in the energy level of said pilot current for preventing operation of said amplifier control means and for operating said motor, means comprising a cam operated by said motor for insuring rotation of the motor for a predetermined period, means comprising a second cam operated by said motor for operating said restoring means and for preventing operation of said amplifier control means, and means comprising a third cam operated by said motor for operating an alarm in case of large changes in energy level of the pilot current for an extended period.

13. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on the line, a relay of limited sensitivity having a contact arm held in operative position when operated and restoring means, a motor, means comprising a control relay operated by said relay of limited sensitivity in case of large energy variations of said pilot current for operating said motor and for preventing operation of said amplifier control means, means comprising a cam member operated by said motor for holding the motor circuit closed for a predetermined period, means comprising a second cam member operated by said motor to become eifective after a time interval for operating the restoring means of said relay of limited sensitivity and for again operating said control relay, and means comprising a third cam member operated by said motor to become effective after said second cam member for operating an alarm in case of a large variation in energy level of the pilot current for a prolonged period of time.

14. In combination, a signal transmission line, an amplifier on said line, means for controlling the gain of said amplifier according to the energy level of a pilot current on the line, a relay of limited sensitivity having a contact arm held in operative position when operated and restoring means, a control relay for operating said restoring means, a thermal element in the energizing circuit of said control relay for effecting operation of said control relay when heated for a predetermined period, means operated by said relay of limited sensitivity in case of a large change in voltage of said pilot current for preventing operation of said amplifier control means and for heating said thermal element to operate said control relay, and means comprising a second thermal element heated under control of said control relay in case of large voltage change of the pilot current for a prolonged period for operating an alarm.

DONALD M. TERRY.

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