US2260977A

US2260977A - Control apparatus

Info

Publication number: US2260977A
Application number: US352643A
Authority: US
Inventors: Harry S Jones
Original assignee: Brown Instr Co
Current assignee: Brown Instruments Co
Priority date: 1940-08-14
Filing date: 1940-08-14
Publication date: 1941-10-28
Anticipated expiration: 1958-10-28

Description

ct. 28, 1941. H, s JONES 2,260,977

CONTROL APPARATUS Filed Aug. 14, 1940 2 Sheets-Sheet l HARRY s. JONES T BY iM NN A RNEY.

Oct. 28, 1941. H s, JONES `2,260,977

CONTROL APPARATUS Filed Aug. 14, 1940' 2 sheets-sheefz IL 42 LIL*9"43 }39BY @7W r TORNEY.

Patented Oct. 28, 1941 CONTROL APPARATUS Harry S. Jones,

Philadelphia, Pa., assigner to The Brown Instrument Company, Philadelphia, Pa., a corporation of Pennsylvania Application August 14, 194D, Serial No. 352,643

The present invention relates to safety control systems for fuel burners.

An object of the invention is to provide an improved safety control system for a fuel burner -operation when the resistance of the burner flame is within a predetermined range of values, and operates to shut down the burner when the burner flame resistance is without said range.

A further object of the invention is to providea safety control system of the flame responsive type which utilizes a pair of electronic discharge devices having different characteristics, both responsive to the resistance of the burner flame, the difference in their characteristics serving to fix the permissible range of flame resistance.

In existing burner control systems, various means have been employed for determining whether combustion is taking place, one such means comprising an electrode in engagement with the burner flame and connected to the system so as to provide a conductive path of relatively low resistance to ground through the flame. The change in conductivity of this path between the presence and absence of flame is sometimes employed to change the bias on the control grid .of an electronic valve for controlling a thermal safety switch. Since a low resistance path may be set up between the electrode and ground by carbonization, accidental contact, or other abnormal conditions simulating combustion, means have been provided in some safety systems of the prior art for establishing a minimum permissible value of flame resistance. When the resistance between electrode and ground falls below the established minimum, the system responds in the same manner as it does to absence of flame, that is, it shuts down the burner. Systems including these features have been quite complicated. The present invention avoids such complexity by having a relatively small number of parts, and being simple in operation.

The various features of novelty which characterize my invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, however, its advantages and specific objects attained with its use,

reference should be had to the accompanying drawings and descriptive matter in which I have illustrated and described a preferred embodiment of the invention.

In the drawings:

Fig. 1 is a wiring diagram of a preferred embodiment of my invention;

Fig. 2 is a graph of certain current characteristics of circuit elements in Fig. 1; and

Figs. 3, 4 and 5 are wiring diagrams of different flame responsive circuits which may be used to operate the safety control system of Fig. l,

Figure 1 shows a gas burner I supplied with gas through conduit 2 and the flow of gas to the burner is controlled by an electrically operated or other suitable valve 3. A pilot burner 4 is provided which is controlled by an electrically operated or other suitable valve 5, and means are provided for igniting the pilot flame including a pair of electrodes 6 which are connected to the terminals of a secondary winding 'I of an ignition transformer 8 having a primary winding 9 which is adapted to be energized from the alternating current supply lines L1 and L2.

The fuel valve operating circuit of my control system is controlled by means of a thermostat I0 which may be located in a room or space to be heated. The thermostat I0 may be of any suitable construction and includes a bimetallic element II connected by means of conductor I2 to line L1, and a contact blade I3 adapted to engage a stationary contact I4 which is connected to line L2 through a thermal safety switch I5 and winding I6 of a transformer II. A

The thermal safety switch I5 is preferably of the form disclosed in the Patent 1,958,081 issued to F. S. Dennison May 8, 1934. As shown more or less diagrammatically in the drawing, this switch comprises a stationary arm I8 and a movable arm I9 biased for movement away from arm I8 but normally held in engagement with the latter by means of a bimetallic element 20. Element 20 is rigidly secured at one end to a block 2| and is arranged to be heated by a coil 22 when the latter is energized through a circuit which will later be described. Upon energization of coil 22 for a predetermined period of time, elementv 20 will be warped suiiiciently in the counterclockwise direction to permit arm I9 acting under spring or other bias to separate from switch arm Il thereby interrupting the' circuit including thermostat Il). VThe switch I5 will remain locked in this position until manually returned to its normal closed position.

The transformer I1 which supplies power for the control system comprises the primary winding I6 and three

secondary windings

23, 24 and 25. The winding 23 supplies power to a circuit which may be traced from the lower terminal of winding 23, as it appears in the drawing, through a conductor 26, a parallel connection consisting of a resistor 21 and a condenser 28, and a rectifier 29, to the upper terminal of winding 23. The common terminal of resistor 21 and condenser 28 nearest rectifier 29 is grounded, as at 30. The rectifier 29 may be of any convenient half-'wave type. The purpose of this circuit is to establish a charge on condenser 28 having the polarity indicated on the drawing, so as to make conductor 26 positive with respect to ground, for reasons to be set forth later.

The secondary winding 24 supplies power for the output circuit of an electric discharge device 3|, which may be of any suitable type, such as the type known commercially as a Sylvania GAEGG, and comprises triodes A and B having different grid bias-output current characteristics. The triodes A and B have a common cathode 32. provided with a heater lament 33, and a common control electrode 34. 'I'he triode A also has an anode 35, and triode B has an anode 36. The heater filament 33 may be energized from any convenient source, which may, for example, be another secondary winding (not shown) of the transformer |1. The output circuit of triode'A maybe traced from the lower terminal of winding 24 throughV a conductor 31, a winding 38 of a relay 39, a conductor 40, anode 35, cathode 32 and conductor 26 to the upper terminal of winding 24. 'Ihe output circuit of triode B may be traced from the lower through conductor 31, a winding 4| of relay 39, anode 36, cathode 32, and conductor 26 to the upper terminal of winding 24.

The windings 38 and 4| of relay 39 have the same number of turns and are connected in opposition so that the magnetic flux produced by one winding is in the opposite direction to that produced by the other. Therefore, when the currents in the two windings are equal, the net flux acting on the relay armature is zero. Each of the windings 38 and 4| is provided with a parallel condenser 38A and 4|A to smooth out the fluctuations in the pulsating current passed by the triodes A and B, and thereby prevent chattering of the relay contacts. The energization of winding 38 is controlled by the triode A, while that of winding The difference in triodes is shown 4| is controlled by triode B. the characteristics of these two in Fig. 2. The curves CA and -CB show the change in output current of the respective triodes as the grid-cathode potential (grid bias) changes from a large negative value to zero. The ordinates (I) represent output current, While the abscissae (Eg) represent grid bias. Since the effects of these two currents on the net flux produced in the relay are opposite, the net flux-producing current tending to raise the switch arms is equal to the output of triode B minus the output of triode A. The variation of this net current as the grid bias of the triodes is varied is lshown by the curve CB-CA in Fig. 2. If the net current required to overcome the inertia of the relay and raise the switch arms is represented by the value L on the current scale, then the switch arms will be lifted whenever the net current CB-CA is greater than L. It may be seen by an inspection of Fig. 2 that. this will occur whenever the grid bias lies between the values indicated by the dotted lines M and N.

The grid bias of triodes A and B is controlled,

terminal of winding 24 than before.

when the burner is in operation, by an input cirlcuit which may be traced from the positive terminal of condenser 28 through

resistors

42 and 43, a conductor 44, a switch arm 45, a front contact 46, a conductor 41, an electrode 48 in the path of the flame of the pilot burner 4, the flame conductance, the burner 4, ground connection 49 and ground connection 30 to the negative terminal of condenser 28. When the burner is not in operation, the input circuit passes from switch arm 45 through a back contact 5U, a conductor 5|, and a resistor 52 to ground at 53. The charge on condenser 28 serves as a source of potential for the input circuit. The control electrode '34 is connected to the conductor 44, and the grid bias is, therefore, the potential difference across the

resistors

42 and 43. The resistor 43 is shunted, when the burner is in operation, by a circuit branch which may be traced from one terminal of resistor 43, through a conductor 54, a contact 55, a switch arm 56, and a conductor 51 to the opposite terminal of resistor 43. The grid bias is determined, when the burner is in operation by the conductance of the flame, and when the burner is not in operation, by the resistor 52. Resistor 52 is chosen so that when it is in use, the grid bias value lies between the limits indicated by the dotted lines M and N in Fig. 2. When the burner is in operation, and a flame is present at the pilot burner 4, the grid bias also lies between those limits, and the relay 39 is actuated to raise its switch arms. When no name exists at the pilot burner, the grid bias becomes zero, and the relay drops its switch arms. Also. if the ame electrode 48 becomes grounded, that is, connected to ground 49 through a low resistance path, the grid bias becomes more negative than M, and the relay drops its switch arms.

Since the grid bias is determined by the flame resistance, a scale of iiame resistance abscissae may be placed parallel to the grid bias scale in Fig. 2, and the dotted lines M and N will cross the ame resistance scale at the minimum and maximum values of flame resistance, respectively. The scale Rf in Fig. 2 is such a scale.

When the resistor 43 is in the input circuit, due to the shunt around it being opened at contact 55, it causes the grid bias to become more negative, for a given amount of current in the input circuit, and hence for a given value of flame resistance. Any given point on the Rr, or flame resistance scale in Fig. 2, is, in effect, moved to the left of its normal position with respect to the grid bias scale by the insertion of resistor 43 in the input circuit. The limits M and N will then designate larger values of resistance The effect of the insertion of resistor 43 in the input circuit, is, therefore, a change in the range of flame resistance to which the circuit responds to actuate relay 39, the change being such that both the minimum and maximum values of flame resistance are increased.

The vrelay 39 comprises, in addition to the windings 33 and 4|, switch

arms

58, 59 and 60, which cooperate respectively with front contacts 6|, 62 and 63.

Switch arms

58 and 60 also cooperate with

back contacts

64 and 55, respectively. A contact on which a switch arm is closed whenits associated relay winding is deenergized is referred to herein as a back contact, while a contact on which a switch arm is closed when it is lifted by energization of its relay winding is termed a front contact. In the drawing,

all switch arms are shown in position as though their windings were deenergized.

The transformer secondary winding 25 supplies energizing current to two

relays

66 and 61, through circuits to be described later. Relay 66 comprises a winding 68 and a switch arm 69, cooperating witha front contact 10, in addition to the switch arm 45 and its associated contacts, previously described. Relay 61 comprises a winding 1| and switch

arms

12, 13 and 14, cooperating respectively with

front contacts

15, 16 and 11, in addition to the switch arm 56 and its associated contact 55, previously described.

When the temperature of the room or space to be controlled falls below the value it is desired to maintain, the thermostat I operates to move switch blade I3 into engagement with the contact I4 resulting in closure of an energizing circuit to the transformer primary winding I6 and thereby energization of the transformer secondary windings. The relays are then in the positions shown in the drawing, and, therefore, the input circuit of the electric discharge device 3| is connected through resistor 52. Since the resistor 52 lies within the range of resistance which affects the input circuit so as to cause the relay 39 to raise its switch arms, that action will take place as soon as the cathode 32 has become heated.

Closure of switch arm 58 on front contact 6| will complete an energizing circuit for relay 68 and heater coil 22 of the thermal safety switch |5. This circuit may be traced from the upper terminal of secondary Winding 25, as it appears in the drawing, through winding 88, heater coil 22, a conductor 83, a conductor 18, a conductor 19, switch arm 58, contact 6|, and a conductor 8|) to the lower terminal of secondary winding 25.

Relay 66 being thus energized raises its

switch arms

45 and 69, substituting the resistance between flame electrode 48 and ground for resistor 52, and closing a holding circuit for itself. The connections between the flame electrode 4'8, resistor 52, and the input circuit have been previously described. The holding circuit for relay 66 may be traced from the upper terminal of transformer secondary winding 25, through relay winding 68, heater coil 22, conductor 83, conductor 18, contact 19, switch arm 69 and conductor 88 to the lower terminal of winding 25.

The starting cycle is not permitted to proceed further if there is a high resistance leakage path to ground at the ilame electrode. At this time, -resistor 43 is in the input circuit so that the effective resistance range is higher than normal, as previously explained. Ii such a path exists, the resistance of which lies within the range which is effective to hold relay 39 in', no further action of the relays will take place. The heater coil 22, remaining energized, will, after a predetermined time interval, operate the thermal safety switch to deenergize the system. Thus, a test is made during the starting cycle for the presence of a high-resistance path from the electrode 48 to ground before the burner is started into full operation. i,

If no such leakage path to yground exists, the resistance between ilame electrode 48 and ground will be outside the effective range, and relay 39 will be deenergized whereby it will drop its

switch arms

58, 59 and B0 and thereby cause the closure at contact 64, of an energizing circuit for relay 61. This circuit may be traced from the upper end of secondary winding 25, through relay winding 1|, a conductor 8|, a conductor 8 2, contact Cil 64, switch arm 58, conductor 19, contact 18, switch arm 69 which is then in engagement with contact 10, and conductor 88 to the lower end of winding 25.

Energization of relay 61 causes it to close its from line L2, through primary winding 9, contact 85, switch arm 68, contact 11, and switch arm 14 to line L1. The' energizing circuit for pilot valve 5 may be traced from line L2 through valve 5, contact 11 and switch arm 14 to line L1.

The normal result of opening the valve 5 and energizing the ignition transformer is the appearance of a fiame at the pilot burner 4. If no flame appears, the system continues in the condition described until the heater coil 22 causes the thermal safety switch l5 to deenergize the system. If a flame appears, however, its resistance falls within the range which causes relay 39 to raise its switch arms, opening the circuit through ignition transformer primary winding 9 at contact 65, completing an energizing circuit for the main burner supply valve 3, and completing a shunt circuit around the heater coil 22 of the thermal safety switch. The energizing circuit for the main burner supply valve 3 may be traced from line L2 through valve 3, contact 63, switch arm 60, contact 11 and switch arm 14 to line L1. The shunt circuit around heater coil 22 may be traced from the left end of the coil 22 through conductor 83, contact 62, switch arm 59, contact 16, and switch arm 13 to the right end of coil 22. The burner is then in normal operation.

If, with the burner in normal operation, the flame should go out, the resistance between name electrode 48 and ground would be outside the range which is effective to hold relay 39 in. Consequently, that relay would drop its switch arms, closing the main burner supply valve 3, energizing the ignition transformer primary winding 9,

and opening thershunt circuit around heater coil 22. If the flame should then reappear, relay 39 would again raise its switch arms and the burner would resume normal operation. If the ilame should not reappear, however, the heater coil 22 would operate the thermal safety switch 5 tol deenergize the system.

If the electrode 48 should become grounded at any time during the system operation, relay 39 would drop its switch arms (or fail to raise them, if the ground occurred during the starting cycle) andthe heater coil 22 would be energized to operate the thermal safety switch |5 to thereby deenergize the system.

In Figure 3,1 have illustrated, more or less diagrammatically, a modication of the flame-responsive circuit vof Figure 1 which may be used to operate the same safety control system. this modification, no rectifier is used, the input circuit of the electrcfdischarge device 3| being constructed so as to operate directly from alternating current. This input circuit may be traced from the upper terminal of transformer secondary 24, through conductor 26, resistors 42 and In I 43 in parallel with a condenser 84, conductor 44, switch arm 45, contact 46, conductor 41, electrode 48, the flame conductance, burner 4, ground connection 49, a ground connection 85, a resistor 86, and conductor 31 to the lower terminal of winding 24. Control electrode 34 of valve 3| is connected to conductor 44. As in the circuit of Fig, 1, provision is made for substituting resistor 52 for the flame conductance during the starting cycle, and for` normally shunting resistor 43 when the system is in full operation.

Due to the eflect of condenser 84, the potential between control electrode 34 and cathode 32 of valve 3|, or in other words the grid bias potential, leads the anode-cathode potential in phase by an angle sufciently large that the electric discharge device 3| is biased negatively during the major portion of its effective half cycle. By effective half cycle is meant the half cycle when the anode 35 is positive. The exact angle of lead, and hence the average negative bias, is dependent upon the amount of current flowing in the input circuit, and hence upon the value of the flame conductance. This circuit responds to varying conditions at the flame electrode in the same manner as the circuit of Figure 1. Resistor 86 is used to limit the current which might flow through the control electrode in case of a low resistance ground at the flame electrode 48. Since an alternating potential is applied to the flame electrode, it is necessary to provide a shield 81 for conductor 41, if the latter is of any appreciable length.

Figure 4 shows another modification of the flame-responsive circuit of the Fig. 1 arrangement, which modification is energized from direct current source of potential. This modiilcation shows the use of two separate electronic discharge devices instead of two parts of a double tube as used in the circuits of Figs. 1 and 3. The latter change may, of course, be made in any of the circuits, if found desirable. Similarly, a. double tube may be utilized in the Fig. 3 arrangement, if desired,

A resistor 88 is connected across the direct current source and serves as a voltage divider. The direct current potential maintained across the divider 88 has the polarity indicated. Two

electric discharge devices

89 and 90 are connected between the positive terminal of resistor 88 and a tap 9|. The input circuits of the

electric discharge devices

88 and 90 are supplied from the terminals of resistor 88.

The electric discharge device 88 functions similarly to triode A in Fig. 1, and is a pentode having a heater filament 92, a cathode 83, a control electrode 94, a screen electrode 85, a suppressor electrode 96, and an anode 91. The electric discharge device 90 functions similarly to .triode B in Fig. 1, and is a triode having a heater filament 98A, a cathode 98, a control electrode 98, and an anode |00.

rIhe suppressor electrode 96 is connected directly to cathode 93 through conductor 26. The screen electrode 95 is directly connected to the positive terminal of resistor 88. The

control electrodes

94 and 99 are connected together and to the input circuits of the two discharge devices at conductor 44. The said input circuits may be traced from the positive terminal of resistor 88 through resistor 86,

ground connections

85 and 49, burner 4, the flame conductance, electrode 48, conductor 41, contact 46, switch arm 45,' conductor 44, and

resistors

43 and 42 to the negative terminal of resistor 88. The functions of resistor 43 and resistor 52 are the same as in the circuits previously described. The output circuits of the

discharge devices

89 and 90 are similarv to those of .triodes A and B in the previous circuits, and need no further description. The input circuits operate in a slightly different manner. The variation of grid bias in response to changes in flame resistance is exactly the opposite of that inthe circuits previously described. When the flame electrode 48 of the present circuit is grounded, the grid bias becomes strongly positive, dependent upon the magnitude oi' protective resistor 86. As the flame resistance increases, the grid bias becomes more negative, until at open circuit, the grid is at the same potential as the negative terminal of resistor 88. If .the flame resistance scale in Fig. 2 were reversed, with infinity at the left end and zero at the right, it would indicate the action of the present input circuit. 'I'he effect of this flameresponsive circuit on the relay is the same as that of the previous circuits, as will be understood from a study of Fig. 2. With the flame resistance scale reversed, N now corresponds to the minimum value of flame resistance, and M to the maximum, but the range of resistance is the same, providing the circuit constants are properly proportioned.

Figure 5 illustrates another modification of the flame-responsive circuit of Fig. l, in which modification one stage of amplification has been added. This circuit utilizes one triode of a twintriode discharge device |0| for amplification purposes, the other triode being used as a rectifier. A single triode-pentode tube is used instead of the double triode of Figs. l and 3, or the separate triode and pentode of Fig. 4. The latter change could be made in any of the circuits described herein.

'I'he twin-triode discharge device |0| comprises a triode C, used as a rectier, and a triode D, used as an amplifier. Triode C has a heater filament |02, a cathode |03, a control electrode |04, and an anode |05. Triode D has a heater filament |06, a cathode |01, a control electrode |08 and an anode |09. The rectifier triode C is supplied with energy from transformer secondary winding 23 Ithrough a circuit which may be traced from the lower end of'winding 23 to anode |05, cathode |03, resistor 21 in parallel with condenser 28 and a conductor ||0 to the upper end of winding 23. Control electrode |04 is connected to cathode |03, so that triode C is always con- Y ductive during the half cycles when its anode is positive. This establishes a charge on condenser 28 such that conductor ||0 is negative with respect to anode |03. The resistor 21 has a tap connected to ground as at A conductor ||2 is connected to the positive terminal of resistor 21 and condenser 28, and serves as the positive line of the power supply of an electric discharge A device H3. Another conductor ||4 which is tapped to resistor 21 at any convenient point, which may be, as shown, between the grounded tap and the negative terminal, serves as the negative line of said power supply.

The discharge device ||3 has two sections, a pentode A and a triode B', and has a cathode ||5 and a heater filament ||6 common to both sections. The pentode A also has a control electrode ||1, a screen electrode ||8, a suppressor electrode ||9, and an anode |20. also has a control electrode |2| and an anode |22. The pentode output circuit may be traced from positive line- ||2, through a resistor |23, a

The triode B' terminal |24, anode |20 and cathode H5 to negative line ||4. AThe triode output circuit may be traced from positive line H2, .through a resistor |25, a terminal |26, anode |22, and cathode to negative line ||4. The function of the pentode A' is similar to that of triode A in Fig. 1, while that of the triode B is similar to that of triode B in Fig. 1. The screen electrode ||8 is connected to positive line ||2 through a resistor` |21, and the suppressor electrode ||9 is connected directly to the cathode ||5. The control electrodes ||1 and |2| are connected together by a conductor |28, andthrough a protective resistor |29 to the flame electrode 48. Electrode 48 is connected through

resistors

42 and 43 to .the negative terminal of resistor 21. Means is provided, as in the circuits previously described, to

shunt the resistor 43. l

When the burner control system is first started into operation, the switch arm 45 is resting on contact 50, so that control electrode |08 is connected directly to cathode |01, and amplifier D is conductive, regardless of the conditions existing at the flame electrode 48. Relay Winding |36, accordingly, is energized, and initiates the cycle of the starting operation, during which switch arm 45 is closed on contact 46, thus placing resistors |32 and |30, in series, in the input circuit of ampliiier D. When there is no potential difference across resistor |30, the eiect of the potential across resistor 32 due to flow of output current through it, is to bias the amplifier D to cut off, or to reduce its. output sunlciently so that relay 39 will not be held in. When a difference of potential exists across resistor |30, which potential opposes the potential difference across biasing resistor |32, the negative bias on control electrode |08 is reduced, and, if such reduction is sufficient, the output of amplifier D increases sufliciently to energize winding |36 of relay 39.

The difference of potential across resistor is determined by the relative outputs of sections A' and B of discharge device ||3. The resistances of resistors |23 and |25 are equal. k'I'herefore, when the outputs of sections A' and B are equal, the potentials of terminals |24 and |26 are the same. When the output of section B' is greater than that of section A', the potential of terminal |26 becomes more negative than that of terminal |24, and the difference of potential thus established across resistor |30 opposes the potential across biasing resistor |32 in the input circuit of amplifier D.

The output characteristics of sections A' and B' in response to changes in grid bias potential, are similar to the characteristics shown for triodes A and B in Fig. 2. The change of bias potential in response to a change of ame resistance occurs as it does in the circuit of Fig. 4, in

a manner opposite to that indicated in Fig. 2, such that the dotted line N corresponds to the minimum permissible flame resistance value, and M to the maximum. Adjustment of the points at which ground and tap |I4 are tapped to resistor 21 will serve to regulate the range of permissible flame resistance,

The various modifications of the safety fuel burner control systems I have described herein are effective and relatively simple. They operate in accordance with the resistance of the burner flame, and the permissible range of flame resistance may be readily adjusted to meet various conditions.

While in accordance with the provisions of the statutes I have illustrated and described the best form of embodiment of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

Having now described my invention, what I claim as new and desire to secure by Letters Patent is:

1. A safety system for controlling a fuel burner, comprising in combination, control vmeans which in a first condition permits said to assume said first condition when said outputl currents are substantially equal, and to cause said control means to assume said second condition when said output currents differ more than a predetermined amount.

2. A system as described in claim 1, wherein said control means is biased toward said second condition, and wherein said last-named means includes means to produce a current variable in magnitude in proportion to the difference of said output currents, and means responsive to said variable current to cause said control means to assume said rst condition.

3. An electric circuit for operating a load device in accordance with the magnitude of an impedance, comprising two electric discharge devices having output circuits with different characteristics and a common input circuit including said impedance adapted to control the flow of current in said output circuits, the output currents thereby varying in accordance with `the magnitude of said impedance, and means responsive to the diierence of said output currents to control said load device.

4. An electric circuit as described in claim 3, wherein a common source of energy is provided for said output circuits, wherein each output circuit includes a resistor having one terminal connected to a terminal of said source, the output currents and hence the potentials of the opposite terminals of said resistors thereby varylng in accordance with the magnitude of said currents when the input circuit is in a condition which corresponds to a predetermined range of values of said impedance, and there is substantially no difference in the two currents when the input circuit condition corresponds to a value of impedance outside said predetermined range.

6. The combination of claim 1 wherein a common source of energy is provided for said output circuits, wherein each output circuit includes a resistor having one terminal connected to a terminal of said source, the output currents and hence the potentials of the opposite terminals of said resistors thereby varying in accordance with the iiame conductivity between said electrodes, and wherein said last named means includes a third resistor connected between said opposite terminals, a load device to control said control means, and a third electric discharge device having an output circuit including said load device and an input circuit including said third resistor and adapted to control the ow of current through said output circuit.

HARRY S. JONES.