US2490095A

US2490095A - Burner control apparatus with timing means

Info

Publication number: US2490095A
Application number: US607631A
Authority: US
Inventors: John O Rosche
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1945-07-28
Filing date: 1945-07-28
Publication date: 1949-12-06
Anticipated expiration: 1966-12-06

Description

Dec. 6,: 1949 J. o. RQSGHE 2,490,095

' BURNERCONTROL APPARATUS WITH TIMING MEANS Filed July 28, 1945 are H #9 IN VEN TOR.

Patented Dec. 6, 1949 BURNER CONTROL APPARATUS WITH TIMING MEANS John 0. Rosche, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware A plication July 28, 1945, Serial No. 607,631

16 Claims. 1

The present invention is concerned with improvements in burner control apparatus and more particularly with apparatus particularly designed for the control of oil burners.

An object of the present invention is to provide a burner control system employing a timer which is effective to interrupt operation of the ignition after a timed period and which also is effective to maintain the burner blower in operation a predetermined period of time after the supply of fuel to the burner has been interrupted.

A further object of the invention is to provide apparatus such as set out in the preceding object in which the timer is caused to have a shorter timing period when maintaining the blower in operation following the interruption of fuel than when timing the ignition means.

A further object of the present invention is to provide a system such as set out in the last object in which the'timer is a thermal timer and in which the cooling time of the thermal timer is reduced by reducing the energization of the timer heater when the timer reaches its hot position.

A still further-object of the present invention is to provide an arrangement in which the timing period of the thermal timer when controlling the ignition may be varied without affecting the length of the timing period for the operation of the blower following fuel interruption.

A further object of the present invention is to provide an improved burner control system of the type in which upon a flame failure during operation, the supply of fuel to the burner is immediately interrupted and in which following a predetermined period of time the complete system is shut down in such a manner as to require manual intervention before it can be restarted.

A further object of the present invention is to provide an improved burner control system of the type set out in the preceding object in which provision is made for preheating the oil to a predeperiod after the energization of the blower.

Other objects of the invention will become apparent from a consideration of the accompanying specification, claims, and drawing, of which Figure l is a schematic showing of the improved burner control apparatus associated with portions of the burnerapparatus being controlled,

Figure 2 is a schematic view of one relay employed in the apparatus, and

Figure 3 is a schematic view of another relay employed in my apparatus.

Referring to Figure 1, the burner apparatus to be controlled comprises a blower I0 employed to supply air under'pressure to an atomizing type of oil burner, the nozzle of which is shown in the lower part of the figure and designated by the nmneral M. The numeral II is employed to indicate an oil valve which controls the flow of fuel to the burner. This valve may, for example, be of a solenoid type which is open'whenever energized and closed when deenergized. Also included in the burner apparatus being controlled is a pilot burner gas valve I! which controls flow of fuel to a pilot burner [3. Provided with the pilot burner is some means for igniting the pilot burner whenever gas is supplied thereto, This means might well be an electrical ignition means which is energized simultaneously with the pilot burner gas valve. For purposes of simplicity, this ignition means is not shown. An electrical heater i5 is employed for preheating the oil before it is delivered to the burner. The operation of this heater is controlled by a thermostat 2| having a bimetallic element 22 exposed to the oil and adapted upon a drop in temperature to move into engagement with a fixed contact 23.

The main control for the system in which my apparatus is employed is a thermostat Hi. This thermostat may, for example, be a room thermostat located in the space whose temperature is to be controlled or it may be a boiler water thermostat responsive to the temperature of the boiler water. The thermostat is shown as comprising a bimetallic element II to which is attached a contact arm is adapted to sequentially engage with two contacts is and 20. As indicated by the legend adjacent the thermostat, the bimetallic element l'l moves contact arm l8 to the left upon a drop in temperature. The contact arm [8 is so spaced from contacts 19 and 20 that it first engages contact l9 and then upon a further drop in temperature engages contact 20.

The thermostat I6 controls the energization of a relay indicated in its entirety by the reference numeral 25. This relay comprises a relay coil 25a and a plurality of switch blades 25b, 25c, 25d, and 25e. In order to simplify the showing of the circuit, switch blade 25c has been shown in a different portion of the drawing than switch blades 25b, 25c, and 25d. In order to illustrate the normal relationship between the units of relay 25, the relay has been shown with the elements in their normal relation in Figure 2. The relay switch blades 25b, 25c, 25d, and 25e are biased out of engagement with contacts 253, 25g, 25h, and 25k, respectively. Upon energization of the relay coil 25a, the switch blades 25b to 25s 3 are moved against the bias into engagement with contacts 25; to 257:, respectively.

A thermal safety switch 35 also controls the energization of relay 25. This safety switch may be of any type which at the end of a period of time moves to a circuit interrupting position and remains in that position until manually reset. A typical switch of this type which may be employed is that shown inthe patent to John M. Wilson, 2,290,399. As shown schematically in the drawing, the switch consists of a pair of switch blades 36 and 31, the switch blade 31 being held in engagement with the switch blade 36 against its bias by a bimetallic element 38. An electric heater 39 is located adjacent bimetallic element 38 and is effective when energized to heat bimetallic element 38 to cause it to warp to the right, as indicated by the legend adjacent the element 38. Upon continued energization of heater 39 beyond a predetermined period of time 1 the bimetallic element 38 moves out from under switch blade 31, permitting switch blade 31 to drop away from switch blade 36. When this happens, the switch blades 36 and 31 must be manually reengaged.

The reference numeral 40 is employed to generally designate a further relay. This relay comprises a coil 40a and three switch blades 40b, 40c, and 40d. The latter switch blade is shown in the upper portion of the drawing to simplify the showing of the circuit. The relay isshown with the elements thereof in their normal relationship in Figure 3. The switch blades 40b, 40c, and 40d cooperate with fixed contacts 40e, 401', 40g, and 40h. Switch blades 40b and 400 are normally biased out of engagement with contacts 40c and 40 respectively, being moved into engagement therewith upon energization of relay coil 40a. The switch blade 40d is normally biased into engagement with contact 4071., and is moved out 40 of engagement with contact 40h and into engagement with contact 40 upon energization of relay coil 40a.

The relay 40 is controlled by flame responsive apparatus including an electronic discharge tube 50. This tube is of the-.twin triode type having two anodes and 52, two grids 53 and 54, and a cathode 55. The cathode 55 is associated with both anodes 5| and 52. The anode 5|, grid 53, and cathode 55 constitute a first triode, while the cathode 55, the grid 54, and the anode 52 constitute a second triode. The cathode heater 56 can be connected with any suitable source of power. Since such connections are conventional, they have been omitted for theflpurpose of simplicity. The tube 50 is supplied with power by a transformer 59, this transformer comprising a primary winding 60 and a center tapped secondary winding 6|. The right hand half of secondary 6| is employed as a source of anode potential for the second triode including anode 52. The left hand half is employed in connection with the first triode including anode 5| and the flame detecting elements of the apparatus. The center tap 63 is connected by conductors 64, 65, and 66 to the cathode 55. The pair of resistors 68 and 69 are connected in series across the left hand half of secondary 5|. The resistor 68 is relatively small as compared with resistor 69. In one particular application, the resistor 68 was a 25,000 ohm resistor while the resistor 69 was a 300,000 ohm resistor. The junction of resistors 68 and 69 is connected by conductor III, a condenser II, a conductor I2, and resistors I3 and" to grid 54. This subsequent description, serves to apply to grid 54 an alternating voltage which is out of phase with respect to the anode voltage and hence serves to bias the grid 54 negatively during the positive half cycles of the second triode including anode 52. A condenser I6 is connected between grid 54 and cathode 55 by resistor I4 and conductors II, I8. and 66. The condenser I6 serves to act as the filter for the voltage applied between the grid 54 and the cathode of the second triode and also to delay the response of the tube to the establishment of flame. The left hand terminal of resistor 69 which is at the same potential as the left hand terminal of secondary 6| is connected by conductor 80, resistor 8|, conductors 82 and 83, resistor 84, and conductor 85 to a flame electrode 86. This flame electrode is adapted to project into the path of the flame from pilot burner I3, the pilot burner I3 being grounded at 88. Also connected to the left hand terminal of resistor 69 is a photoelectric cell 90. This cell is of any suitable type comprising an anode 9| and'cathode 92 located connection, as will be more apparent from the 7 within an envelope 93. The anode 9| is connected by conductor 34 to the flame electrode 86 and hence to the left hand terminal of resistor 69. The cathode 92 is connected by a conductor 95 to a ground connection 98. It will be apparent that the flame gap of the pilot flame and the photoelectric cell are connected in parallel between the left hand terminal of the transformer 6| and round. 'Theconductor 12 connected to the lower plate of condenser II is connected to ground at 98. The condenser II is also connected to the grid 53 by conductor I2 and a resistor 99.

The relay 40 is in the output circuit of the second triode. Since it is a relay designed to be operated on direct current, a condenser |0| is connected in parallel with relay 40 to filter out the alternating component of the pulsating current supplied by the triode to the relay 40. A meter I82 is connectedin series with relay 40 to indicate the current supplied by the triode to relay 40. v

A third relay is designated by the reference numeral I III. This relay comprises a relay coil III, a plurality of switch blades H2, H3, H4, and H5. The switch blades H2, H3, and 4 are designed to cooperate with fixed contacts I I6, I II, H8, and H9, respectively. Switch blades H2 and II5 are biased out of engagement with contacts I I6 and I I9 and are moved into engagement with their respective contacts upon energizing of relay coil III. The switch blades 3' and H4 are biased into engagement with contacts III and H8, being moved out of engagement therewith upon energization of relay coil III.

A thermal timer I25 is employed to terminate the operation of the pilot burner as well as to maintain the blower in operation after the oil valve has closed following a normal operation of the burner. This timer I25, as shown schematically in the drawing, comprises a bimetallic element |26 which normally engages a contact |2I, this contact being the cold contact. Associated with the bimetal element |26 is a heater I28. Upon energization of the heater, the bimetal element |26 warps downwardly into engagement with another fixed contact I29, which may be referred to as the hot contact.

Among the switches controlling the energization of heater I28 is a thermostat I35 adapted to respond to the temperature of the oil. This thermostat comprises a bimetallic element I36 adapted to engage with a fixed contact I38. As long as the oil is at the proper temperature, the

contacts I31 and I38 are held in engagement. If the oil assumes too cold a temperature for proper operation of the system, contact I31 is moved by bimetallic element I36 to the left out of engagement with contact I38 until the preheater controlled by thermostat 2| has heated the oil to the desired temperature.

Connected in series with heater I28 are three series connected resistors I40, MI, and I42. A manually operated switch blade I43 is adapted to engage with either of two contacts I44 or I45. both of which are connected to the series connected resistors I40, I4I, and I42. Contact I44 is connected by a conductor I46 to the junction of resistors I40 and MI while the contact I is connected by a conductor I41 to the junction of resistors HI and I42. Thus, depending upon the position of switch blade I43, it is connected to either the junction of resistors I40 and I or the junction of resistors Ill and I42. The switch blade I43 isemployed to control the lengthof the ignition timing and is manually moved between contacts I44 and I45 depending upon the length of ignition timing which is desired. When switch blade I43 is in engagement with contact I44 the timing is short and when in engagement with contact I45, the timing is long, as indicated by legends on the drawing and as is explained in more detail later.

A manually operable switch blade I is adapted to be moved into engagement with either of two fixed contacts I5I and I52. This switch is employed to control whether the oil valve and blower are energized simultaneously or whether there is a delay following the energization of the oil burner blower, in the energization of the fuel oil valve. As shown, the switch blade I50 is in engagement with the contact |5I identified by the legend ND. In this position, as will be more clear from the subsequent explanation, the blower and oil valve are energized simultaneously; in other words, there is no delay in the energization of the oil valve after the energization of the blower. With the switch blade I50 in engagement with contact I52, the blower is energized before the fuel valve so that there is some delay in-the energization of the oil valve following that of the blower. This contact is identified with the legend D standing for delay.

Power is supplied for energization of the relay' 25 by a transformer I55. This transformer consists of a primary winding I56 and a low voltage secondary winding I51. The right hand terminal of the primary winding I56 is connected by conductors I59 and I60 to a line wire I6I. The left hand terminal of primary I56 is connected by conductors I63 and IE4, contact I21, bimetallic element I26, and conductors I65 and I66 to the other line wire I62.

While the values of the various elements employed in the apparatus described are not critical, and in many cases a wide range-of values may be employed, I have found in one particular embodiment the following values to give satisfactory operation. Resistors having resistances of 1500 ohms, 1000 ohms, and 3500 ohms were employed for resistors I48, MI, and I42, respectively. The heater I28 had a resistance of 2650'ohms. The secondary I51 had an output of 24 volts. Referring to the flame detecting apparatus, resistors 69 and 68 had, as previously mentioned, resistances of 300,000 ohms and 25,000 ohms, respectively. Resistors having resistances of 10,000 ohms and one megohm were employed for-resistors 9| and 84. A 7 megohm and a 2 megohm resistor were employed for resistances 98 and 14, respectively. The resistor 13 was a l megohm resistor. A .02 microfarad condenser was employed for condenser 1| and a .25 microfarad condenser for condenser 16. A .5 microfarad I condenser was employed for condenser IOI.

Operation Before starting with the operation of the system in general, the operation of the flame detecting apparatus will be briefly described.

While the operation of this flame detecting apparatus will be briefly given in the present specipreviously noted, the right hand terminal of this resistor is connected to cathode 55. The right hand terminal of the secondary 6| is connected through relay coil 40a, conductor I04, meter I02, and conductor I05 to the anode 52. Hence, during those half cycles in which the right hand terminal of secondary 6| is positive with respect to the center tap, which are the half cycles during which the second triode including anode 52 may be conductive, the left hand terminal of secondary 68 is negative with respect to the cathode so that the voltage impressed by reason of the connection of grid 54 to the left hand terminal of resistor 68 is such as to cause grid 54 to be biased negatively during the conductive half cycles. Also, the condenser 1| tends to assume a negative charge by reason of its inclusion in two grid cathode rectifying circuits. Thus, a circuit may be traced during the half cycle in which the left hand terminal of grid 68 is positive with respect to the right hand terminal (the opposite cycle to the one just considered) as, follows: from the left hand terminal of resistor 68 through conductor 10, condenser 1I, conductor 12, resistor 99, grid 53, cathode 55, and conductors 66 and 65 to the right hand terminal of resistor 68. The efiect of current flow through this path is to cause condenser H to become charged with a voltage of such polarity that the lower plate of condenser H is negative with respect to the upper plate. Due to the rectifying characteristic of the grid 53, current cannot flow in the opposite half cycle. During the same half cycle, current can also flow between grid 54 and cathode 55 through the following circuit: from the left hand end of resistor 68 through conductor 10, condenser 1|, conductor 12, resistors 13 and 14, grid 54, cathode 55, and conductors 66 and 65 to the right hand terminal of resistor 68. Again, this current flow can take place only during the half cycle now being considered. As a result of the two circuits just traced, the condenser 1I tends to acquire a charge of such polarity as to cause the lower plate of condenser 1| to be negative with respect to the upper plate. Since grid 54 is connected to the lower terminal of condenser 1|, grid 54 tends to assume a negative potential with res ect to cathode 55. As previously noted. the fluctuations in the charge across the condenser tend to be filtered out by the bypass condenser 16.

As a result of this negative bias on grid 54, the

a 7 triode including anode 52 is substantially nonconductive.

Upon the establishment of either the pilot flame or the main burner flame. a change in the .charge of the condenser results. Let it be assumed at first that the pilot burner is ignited so that the gap between the pilot burner l3 and the flame electrode 85 is bridged. A conductive path will then be established from the left hand terminal of resistor 69, through conductor 00, resistor 8|, conductors 82 and 03, resistor '84, conductor 85, flame electrode 85, burner I3, ground connections 88 and 98, conductor 12, condenser 1i, and conductor III to the right hand terminal of resistor 69. The current flowing through this circuit flows in such a direction as to cause the lower plate of condenser 1| to become charged positively with respect to the upper plate. During the next half cycle, when the polarity of the voltage across resistor 60 is reversed, current can still flow through the circuit traced but in the opposite direction. Due, however, to the fact that current tends to flow through a flame better in a direction opposite to that of 'flame propagation, the current 'flow through the circuit in the direction traced is greater than that in the opposite direction.

condenser, 1| is to tend to charge the condenser in such a manner that the lower plate tends to become positive with respect to the upper plate. It will be noticed that this charge is opposite to that produced by reason of the flow between the grids 53 and 54 and the cathode. 55. In other words, the effect of'the establishment of the pilot flame is to tend to raise the potential of the lower plate of the condenser 'II in a positive direction with respect to the upper plate. Since it is the lower plate that is connected to grid 54, the potential of grid 54 with respect to cathode 55 is raised. The apparatus is so designed that the rise in potential of grid 54 is such as to cause energization of relay 40 through the following circuit: from the left hand terminal of secondary through conductor I02, relay coil 40a, conductor I04. meter I02, conductor I05, anode 52, cathode 55, and conductors G6, 55, and 54 to the left hand terminal of resistor 5|. The resultant energization of relay coil 400. causes switch blades 40b, 40c, and 40d to move into engagement with fixed contacts 40e, 40], and 409, respectively, switch blade 40d moving out of engagement with contact 4071..

The nhotelectric cell 90, as previously noted, is connected in parallel with the gap between flame electrode 86 and ground. Such a photoelectric.

Consequently, the net eflect of the current flowing through the flame throughas the flame gap. Thus, again; a charge is impressed upon condenser 1| which is of such polarity as to cause the grid 54 to assume a potential sufliciently high as to cause the second triode to be conductive.

As more fully explained in the copending Beam application, the apparatus is designed so as not to be responsive to the presence across either the flame gap or the terminals of the photoelectric cell of an impedance which is not rectifying in character. If such an impedance is connected across the flame gap, the eflect of the circuit through condenser II including the flame gap 01 the photoelectric cell is to impress upon condenser an alternating voltage. Since the left hand terminal of condenser 59 is opposite in phase with respect to the cathode potential to that of anode 52, it will be apparent that this alternating voltage across condenser II is opposite in phase with respect to the anode voltage applied to the second triode including anode 52. Hence, the tendency of this voltage is to drive grid 54 even more negative during the conductive half cycle. When a rectifying impedance is connected across this gap, such as the flame or the photoelectric cell, the flow of current during the half cycle in which the applied voltage would be out of phase with the anode voltage is blocked so that the voltage impressed on condenser I I tends to drive grid 54 more positive rather than more negative.

As will be more apparent from the subsequent description of the operation of the apparatus, it is necessary to check both the pilot flame and the main burner flame. Since a flame electrode is ideally adapted to check the presence of a pilot burner flame, the flame is employed as a rectifying impedance in the case of the pilot burner. A flame electrode, however, is not satisfactory in connection with the main burner flame due to the fluctuations of that flame. By using a photoelectric cell, this objection is overcome. Since a photoelectric cell is likewise asymmetrically conductive, it is possible to obtain the same effect from a photoelectric cell as with the flame electrode. In other words, I employ two flame detecting arrangements, one responsive to the pilot flame, one to the main burner flame and both of which result in the presence of an asymmetrically conductive impedance in the circuit upon being exposed to the respective conditions to which they are designed to respond. Thus, the flame detecting apparatus functions to cause energization of relay 40 whenever either the pilot burner flame or the main burner flame is present.

Referring now to the operation of the system as a whole, the various elements are in the position shown when the controlling thermostat I6 is in its satisfied position, that is, when the temperature to which bimetallic element I1 is subjected is at or above the desired value. Under these conditions, none of the contro1 apparatus, with the exception of transformer I55, is energized. Now let it be assumed that the temperature adjacent bimetallic element I1 drops to cause engagement of contact blade I8 first with contact I9 and then with contact 20. Nothing happens until both sets of contacts are made. When this happens, an energizing circuit is established for relay coil 25a as follows: from the right hand terminal of secondary I51 through conductor I15, safety switch blades 31 and 36, conductor I15, thermostat contact I9, contact blade I8, thermostat 20, conductor 1, contact 40h, switch blade 40d, conductor I10, safety switch heater 39, conductor I80, relay coil 25a, and conductor I8I to the left hand terminal of secondary I51. The establishment of the circuit just traced causes energization of relay coil 25a with a resultant movement of switch blades 25b, 25c, 25d, and 25e into engagement with contacts 25}, 25g, 25h, and 25k, respectively, as previously explained.

The movement of switch blade 25d into engagement with contact 25h establishes a holding circuit'for relay coil 25a independent of thermostat contact 20, this holding circuit being as follows: from the right hand terminal of secondary I51, through conductor I15, safety switch bladesthe new circuit does not include thermostat contact 28. It will be recalled that this was the last to be engaged of the contacts. Thus, upon a subsequent temperature rise, the'relay 25a will remain energized until the temperature has risen sufficiently to cause contact blade I8 to separate from contact I9. In this way, a temperature differential must occur between the energization and the de-energization of relay 25.

The engagement of switch blade 251) with contact 25! results in the following circuit being established to primary winding I56: from line wire I62 through conductors I66 and I84, switch blade 25b, contact 25f, conductors I94 and I63,

transformer winding I56 and conductors I59 and I68 to the other line wire I6I. It will be noted that this circuit does not include the cold contact I21 of" the timer, so that when the timer ment with the fixed contact 25k results in the following circuit being established to the pilot burner gas valve I2. ductors I66, I65, and I85, switch blade 25c, contact 25k, conductors I86 and I88, switch blade II3, contact I I1, conductor I89, gas valve I2, and conductors I98, I9I, I92, and I68 to the other line wire I6I. The energization of the pilot burner gas valve I2 results in gas being supplied to the pilot burner. As previously explained, suitable means are provided to ignite this gas so that under normal conditions the pilot burner is ignited asa result of the establishment of the circuit just traced.

The transformer 59 is initially deenergized so that the flame responsive apparatus is incapable of responding to the presence of the pilot burner flame. At the same time, however, as the circuit to the pilot burner gas valve is established, a circuit is established to the primary 64 of transformer 59 as follows: from line wire I62 through conductors I66, I65, and I85, relay switch blade 25c, contact 25k, conductors I86 and 285, primary 68, and conductors 2I5, 282, 283, I92, and I68 to the other line wire I6I. As a result of the energization of primary winding 68, the electronic apparatus is energized. The cathode heater 56 is heated up so as to render tube 58 efiective after a predetermined period; As-soon as the tube is so heated, the apparatus responds to the presence of the pilot burner flame to cause energization of relay 48 and the subsequent movement of switch blades 48b, 48c, and 48d, to their energized position, as previously explained.

The separation of switch blade 48d from contact 48h interrupts the energizing circuit for relay 25a previously traced. Since a holding circuit through switch blade 2511 has already been established, this does not affect the operation of relay 25a. The purpose of requiring the energizing circuit for relay 25 to proceed through contact 49h initially is to insure that relay 48 is in its deenergized condition before the apparatus-is From line wire I62 through con-l I i 10 started. If relay 48 were accidentally maintained in its energized position with no pilot flame or main burner flame, no checking oi the flame would result. By requiring that the energizing circuit to relay 25a include a contact of relay 48 which is made only when the relay is deenergized, this possibility is prevented. The movement of switch blade 48d into engagement with contact 489 establishes a shunt around the safety switch heater 39 as follows: from the lower terminal of heater 39 through conductor I93, contact 48g, switch blade 48d, and conductor I19 to the upper terminal of heater 39. As a result of this shunt circuit being established,

heater 39 is no longer energized. The establishment of a pilot flame has already been checked so that presumably it is now possible to establish a main burner flame without difficulty. Moreover, as will be apparent later, provision is made for reenergizing heater 39 if the main burner flame is not established.

Theengagement of switch blade 480 with contact 48f causes energization of the burner blower I8 through the following circuit: from line wire I62 through conductors I 66, I 84, and I93, switch blade 25c, contact 259, conductors I91, I98, and 284, blower I 8, conductor I99, switch blade I58, contact I5I, conductor 288, contact 48 switch blade 48c, and conductors L282, 283, I92, and I68 back to line wire I6I. blower I8 is energized, if the temperature of the oil is sufficiently high to cause closure of oil thermostat I35 the fuel oil valve II is likewise energized as a result of engagement of relay switch blade 48b with contact 48c, the circuit to oil valve II being as fololws: from line wire I62 through conductors I66, I65, and I85, relay switch blade 25c, contact 251:, bimetallic element I36, contact I38, conductor 286, oil valve II, conductor 281, contact 48c, switch blade 48b, and conductors 289, 28I, 282, 283, I92, and I68 back to the line wire I6I. The oil valve and blower are now thus both in operation. Since the operation of the pilot burner has been checked prior to the opening of the fuel oil valve and the energization of the blower, it is assured that all of the conditions necessary to the operation of the burner have been established.

In the operation just described, the oil valve and blower are brought on simultaneously. If the switch I58 were in engagement with contact I52, a circuit would have been established to the blower motor I8 as soon as relay 25 was energized, this circuit being as follows: from line wire I62. through conductors I 66, I84 and I93, relay switch blade 25c, contact 25g, conductors I91, I98 and 284, blower I8, conductor I99, switch blade I 58, contact I52, and conductors 2I I, 283, I92, and I68 to the other line wire I6I. Under such conditions, there would have been a delay between the time that the blower was first energized and the fuel oil valve was energized equivalent to the time required for the cathode heater.

56 to warm up and for the apparatus to respond to the presence of the pilot burner flame.

In the above paragraphs, it has been assumed that the oil has been sufliciently warm that the preheater thermostat I35 is closed. If it is not, the circuit traced above to the fuel oil valve cannot be established until the oil has had an opportunity to heat up. If the oil is not warm enough, as soon as the circuit is established to the blower motor, a circuit is also established to.the preheater I5. For example, if the switch I58 is in the delay position, which is the position normally At the same time as act on 11 assumed when it is necessary to employ a preheater, a circuit is established to preheater I following the energization of relay 25 as follows: from line wire I62 through conductors I66, I84,

and I93, switch blade 25c, contact 25g, con-- ductors I91 and I98, oil thermostat contact23, bimetallic element 22, preheater I5, and con' ductors 234, 235, I92, and I60,.back to the other line wire I6I. This energizationof the preheater continues until thermostat 2| is satisfied. Shortly before this temperature is reached, the contacts of thermostat I35 are closed so as to make possible the establishment of the circuit to the oil valve. The oil thermostat I35 also controls the energization of .the timer heater I28. The reason for this is that the time required for preheating the oil may be quite substantial and during this period it is not desirable for the timer to be heating. It is also to be noted that during the period required for preheating, the safety switch heater is shunted by the circuit traced above. 7

Assuming the oil thermostat I35 to be closed,

' the energization of relay 40 results in the energizaticn the heater I28 of the thermal timer I25. The energizing circuit for this heater is as follows: from line wire I62 through conductors I66, I65, and I85, switch blade 25c, contact 25k, bimetallic element I36, contact I38, heater I28, conductor 2| 9, resistor I40, conductor I46, contact I 44, switch blade I43, conductor 220, switch blade. II4, contact II8,-conductors 2 2I, 222, 223, and 200, contact 40 switch blade 40c, and conductors I, 202', 203, I92, and I60 to the other line wire I6I. In tracing the above circuit, it has been assumed that switch blade I43 is in engagement wtih contact I44 to give a short ignition timing period. -If .switch blade I43 were in engagement with contact I45, the above circuit would have included both resistors I40 and MI which would, of course, reduce the energization of heater I28. As a result of the energization of heater I28, the timer I is efiective at the end of a predetermined period of time, dependent in length upon the position of switch I43, to move from engagement with contact I21 into engagement with contact I29.

The movement of the bimetallic element I26 of thermal timer I25 into engagement with the hot contact I29 results in the establishment of the following circuit to relay winding III of relay IIll: from line wire I62 through conductors I66 and I65, bimetallic element I26, contact I29, conductor 22 5, relay coil I I I, and conductors 226, I9I, I92, and I60 to the other line wire IN. The energization of relay IIO as a result of the circuit just described causes switch blades I I2 and H5 -to move into'engagement with contacts H6 and H9 and switch blades H3 and H4 to separate from contacts Ill and H8.

The separation of switch blade H3 from contact II'I results in the interruption of the circuit to the pilot burner gas valve previously traced. This causes interruption of the operation of the pilot burner so that there no longer is a flame between electrode 86 and the pilot burner I 3. Since, however, both oil and air have been sup-- plied for a period of time equivalent to the timing period of the thermal timer, the ignition of the main burner has presumably taken place. Under these conditions, the photoelectric cell 90 responds to the presence of the burner flame and maintains relay 40 energized despite the termination of the. ignition. For the present, it

will be assumed that the apparatus operates in this. manner.

The separation of switch blade contact II4 from contact II 8 interrupts that portion of the previously traced circuit to timer heater I28 which acted as a shunt around either both resistors I and I42 or resistor I42 alone, depending upon the position of switch blade I43. Proceeding from the right hand terminal of resistor I40, the circuit to heater element I28 is now completed through resistors I and I42, conductors 228, 223, and 200, contact 40 switch blade 40c, and conductors 20I, 202, 203, I92 and I to the other line wire I6I. It will be noted that the circuit to heater I28 now includes all three resistors I40, I, and I42 so that the energization of heater I28 is less than that which was the case when the timer blade I 26- was moved from the cold to the hot contacts.- The purpose of this reduction in the energization of heater I28 is to reduce the time required for cooling. As will be more apparent later, this cooling time is employed to provide for continued energization of the blower a predetermined period of time following closure of the oil valve. If the energization of heater I28 were continued at the level employed to move the bimetal blade I26 from engagement with contact I21 to engagement with contact I29, this cooling time would be excessively long.

The movement of switch blade I I2 into engagement with contact '6 results in the establishment of a new circuit to blower motor I0, independent of relay 25, this circuit being as follows: from line wire I62 through conductors I 66, I84, and 230, switch blade I I2, contact H6, conductors I98 and 204, blower l0, conductor I99, switch blade I50, contact I5l, conductor 200, contact 40 switch blade 40c, and conductors 20I, 202, 203, I92, and I60 to the other line wire I6I. The purpose of establishing this circuit is to permit operation of the blower under the control of relay II 0 and timer I 25 following the deenergization of relay 25 as a result of the termination of the call for heat, as will be more apparent later.

The movement of switch blade II5 into engagement with contact II9 results in the establishment of the following circuit for timer heater I28 which is independent of the relay 40 controlled by the flame responsive means: from line wire I62 through conductors I66, I65, and 185. switch blade 25c, contact 25k, oil thermostat I35, heater I28, conductor 2I8, resistors I40, I, and I42, conductors 228, 222, and 233, switch blade II5, contact H9, and conductors 235, I92, and I60 to the other line wire I 6|. The purpose of this circuit is to insure continued energization of heater I28 in the event of a flame failure so as to prevent ignition from being reestablished following such a flame failure.

The system is now in normal operation and has '-gone through a normal starting sequence. Presumably, it will continue in operation until such time as the controlling thermostat I6 becomes satisfied so as to cause separation of contact blade I8 from both contacts I9 and 20. When this happens, the maintaining circuit for relay coil 2511 will be interrupted was to cause deenergization of relay 25. This in turn will result in deenergization of all of the elements connected in series with relay switch blade 25c, namely, the transformer 59 -of the flame detecting apparatus, the oil valve II, and the heater I28 of the thermal timer I25. The delivery of oil to the furnace will .be stopped immediately and'the timer I25 will continues in operation due to the circuit for it previously traced through switch blade I I2, which circuit is independent of any of the switch blades of relay 25. This operation of the blower will continue until the bimetal element I26 ha's cooled sufficiently to cause its separation from fhot contact" I29. When this happens, the relay H is deenergized to cause interruption of the only remaining circuit for the blower. The various elements of the apparatus are now in the position they assumed at the start so that upon a subsequent call for heat by the room thermostat, the apparatus can again be placed into operation.

It was originally assumed that the pilot burner ignited properly so that as soon as the cathode heater of tube 50 warmed up, the relay 40 became energized. If the pilot burner is not properly lighted, relay 0 is not energized. As as result, switch blade 40d is never moved into engagement with contact 40g to shunt the safety switch heater 39. Consequently, the safety switch heater 39 remains energized until the bimetallic blade 88 moves out from underneath 'switch blade 31 to complete separation of switch blades 36 and 91. This in turn causes deenergization of relay 25 to terminate operation. of the entire system. As previously explained, the system cannot again be placed into operation until the safety switch v 85 is manually reclosed.

It was also assumed durin the description of the operation that the main burner flame was properly established so as to continue the energization of relay 40. If this does not occur, the relay 40 becomes deenergized as soon as relay H0 is eflective to deenergize the pilot burner gas valve; It will be obvious that this is the case since as soon as the pilot burner is extinguished, no rectifying path is established between conductor 85 and ground. The energization of relay IIO, it will be recalled, not only resulted in the interruption of ignition but also established a holding circuit for heater I28 through switch blade II controlled by the relay IIO. Thus, despite the deenergization of relay 40 and the resultant interruption of the first traced energizing circuits for heater I28, the heater continues to remain energized to maintain timer I25 in its hot position which in turn results in the continued energization of relay IIO. In the meantime, the deenergization of relay 40 has caused switch blade 40d to move out of engagement with contact 409 to interrupt the shunt circuit around safety switch heater 39. The safety switch thus begins to heat up again. During this time, the oil valve II is deenergized so that no oil flows to the burner. The blower I0 is maintained energized through one or the other of two circuits.

'n the switch blade I50 is in the position shown,

the conductor I99 leading from blower I0 is connected to line wire IGI through switch blade I50, contact I5I, conductors 223, 222, and 233, switch blade II5, contact H9, and conductors 285, I92, and I80. If, on the other hand, the switch blade I50 is in engagement with contact I52, the conductor I99 leading from blower I0 is connected to line wire IGI by switch blade I50, contact I52, and conductors 2I I, 203, I92, and I60. Thus, regardless of the position of the switch blade' I50, the blower continues to operate during this period while the safety switch heater 39 is again heating the bimetallic element 38, This provides for a purging of any oil from the burner before the system is shut down. After a predetermined period of time corresponding to the timing of the safety switch heater, the bimetallic element 38 is deflected to the right to cause separation of switch blades 80 and 31 as previously described. This will in turn cause deenergization of relay 25. As a result, the heater I28 of the thermal timer I25 is deenergized. The blower I0 will continue in operation until the timer I25 has cooled sufiiciently to cause separation of bimetal element I26 from contact I29 to cause deenergization of relay 0. Thus, when there is a flame failure under these conditions, the system provides for a blow out cycle equivalent in length to that of the timing of the safety switch plus the timing of the thermal timer.

If the system is placed into operation satisfactorily but a flame failure occurs during the normal operation, the sequence leading to a safety shut down is substantially the same as that occurring when the main burner flame was not initially established in spite of a successful establishment of the pilot burner flame. Under these conditions, relay is immediately deenergized to cause immediate deenergization of the oil valve. Relay IIO does, however, remain energized under the control of the time switch which in turn continues to be energized by reason of the engagement of switch blade I I5 with contact I I9. The remaining operation is otherwise exactly the same as described in the previous paragraph. It will be recalled from the previous paragraph that the blow out cycle was considerably longer in the event of a shut down followin a flame failure than after a normal operation of the burner. This is a very desirable feature of my apparatus since if combustion has not been established or has failed following a successful establishment of it, there is more likelihood of unburned oil remaining in the burner. By operating the blower motor for a longer period of time, provision is made for blowing out this extra accumulation of oil. Moreover, by having the thermal safety switch 35 and thermal timer I25 proceed through their cycles in sequence, the relative length of their timing periods is not critical.

It will be seen that I have provided an im- 0 proved burner control system in which the utmost of safety is obtained. A check is, made of the establishment of ignition before fuel is supplied to the main burner. A check is then made of the establishment of the main burner flame. In each case, the check is done by electronic apparatus which is .capable of distinguishing between the condition actually being checked and a false condition having similar characteristics. The ignition is turned off at the end of a time period and the timer which serves to time the ignition also provides for a purging operation of the blower to remove any unburned oil before the control apparatus is completely deenergized. If there is a safety shut down because of a failure of the main burner to become ignited, a blow out cycle of extra length is provided.

It will further be seen that my apparatus provides for an extreme flexibility of operation. By a simple switching operation, either one of two lengths of ignition timing can be secured. Furthermore, provision can be made for either opening the oil valve at the same time as the blower or a predetermined period of time thereafter.

While I have described a specific embodiment of my invention for the purposes of illustration, it is to be understood that the invention is limited only by the scope of the appended claims.

I claim as my invention:

1. Burner control apparatus including, a fuel control, a burner blower, a burner ignitionl means, a timer, means to initiate operation of said ignition means, said fuel control, said blower, and said timer, meanscontrolled by said timer for deenergizing said ignition means a predetermined period of time after initiation of control has been caused to assume said fuel interrupting condition to maintain said blower in operation while said timer is returning to its -l starting position so as to give a purging period.

2. Burner control apparatus including, a fuel control, a burner blower, a burner ignition means, a timer, means to initiate operation of said timer upon said fuel control, said blower, and said ignition means being effective, means controlled; by said timer for deenergizing said ignition means a predetermined period of time after initiation of operation of said timer, means to cause said fuel control to assume a fuel interrupting condition when burner operation is not desired, means controlled by said timer eifective after said fuel control hasbeen caused to assume said fuel interrupting condition to maintain said blower in operation while said timer is returning to its starting position. and manually positioned means selectively operable upon change in position thereof to vary the length of said predetermined period of time prior to termination of ignition without varying the length of said purging period.

3. Burner control apparatus comprising a fuel control, a burner blower, a burner ignition means, a thermal timer including means for heating the same, means operative to initiate operation of said ignition means upon said fuel control, said blower, and said timer, means controlled by said timer for deenergizing said ignition means a predetermined period of time after energization of said timer heating means and for decreasing the energization of said timer heating means 'so as to reduce the cooling time of said timer, and means controlled by said timer effective after said fuel control has been caused to assume a fuel interrupting condition to maintain said burner blower in operation while said timer is cooling.

5. Burner control apparatus comprising a fuel control for a burner; a burner blower; burner ignition means; combustion responsive apparatus adapted to respond to the establishment of combustion at the burner being controlled;

means including a main control device for initiating operation of said fuel control, said burner blower, and'sai d burner ignition means, and for maintaining said fuel control and said burner blower inoperation; means controlled by said main control device effective when said control device assumes a condition in which burner operationis not demanded .to cause said fuel I control to interrupt the delivery of fuel; timing means to cause said blower to continue its operation a first predetermined period of time after interruption of the fuel; means controlled by said combustion responsive apparatus effective when said combustion responsive device indicates a failure in combustion also to cause said fuel control to interrupt the delivery of fuel; and means including said timmg means for maintaining said blower in operation for a second predetermined period of time after such a combustion failure, said second period of time being greater than said first predetermined period of time.

6. Burner control apparatus comprising a fuel control for a burner; a burner blower; burner ignition means; combustion responsive apparatus adapted to respond to the establishment of combustion at the burner being controlled; means including a main control device for initiating operation of said fuel control, said burner blower, 'and said burner ignition means, and for maintaining said fuel control and said burner blower in operation; means controlled by said main control device effective when said 4. In control apparatus for controlling aburner fuel control device, a combustion air supplying device, and a burner ignition means; said control apparatus comprising a main switch; a thermal timer having a hot position and an electrical heating'element for causing it to move to said hot position from a normal cold position; means including said main switch and adapted upon closure thereof to cause energization of such a burner control device, combustion air supplying device, and burner ignition means; means controlled by said last named means for causing energization of the heating element of said thermal timer; means controlled by said timer and adapted upon the same moving to said hot position to interrupt the operation of said ignition means and to decrease the energization of the heating element of said timer; and further means controlled by said timer and adapted to maintain said combustion 'air supplying device in operation after deenergization of said fuel control device as a result of opening of said main switch and until said timer moves from said hot position.

control device assumes a condition in which burner operation is not demanded to cause said fuel control to interrupt the delivery of fuel; timing means to cause said blower to continue its operation a first predetermined period of time after interruption of the fuel, means controlled by said combustion responsive apparatus efiective when said combustion responsive device indicates a failure in combustion also to cause said fuel control to interrupt the delivery of fuel; and means to delay the timing operation of said timing means after such a combustion failure so as to maintain said blower in operation for a longer predetermined period of time than after a fuel interruption under the control of said main control device.

7. Burner control apparatus comprising a fuel control for a burner; a burner blower; burner ignition means; combustion responsive apparatus adapted to respond to the establishment of combustion at the burner being controlled; means including a main control device for initiating operation of said fuel control, said burner blower, and said burner ignition means, and for maintaining said fuel control and said burner blower in operation; means controlled by said main control deviceefl'ective when said control device assumes a condition in which burner operation is not demanded to Cause Said fuel control to interrupt the delivery of fuel; first timing means tocause said blower to continue its operation a first predetermined period of time after interruption of the fuel; means controlled by said combustion responsive apparatus effective when said combustion responsive device indicates a failure in combustion also to cause said fuel control to interrupt the delivery of fuel;

a second timing means; and means including said second timing means for delaying the initiation of the timing operation of said first timing means for a second predetermined period of time after such a combustion failure so that said blower continues in operation for a period of time equal in length to the sum of said first and second periods of time.

8. Burner control apparatus comprising a fuel control for a burner; a burner blower; burner ignitionme'ans; combustion responsive apparatus adapted to respond to the establishment of combustion at the burner being controlled; means including a main control switch for initiating operation of said fuel control, said burner blower, and said burner ignition means, and for'maintaining said fuel control and said burner blower in operation; means controlled by said main control device effective when said control device assumes a condition in which burner operation is not demanded to cause said fuel control to interrupt the delivery of fuel; timing means to cause said blower to continue its operation a Y first predetermined period of time after such interruption of the fuel; means controlled by said combustion responsive apparatus effective when said combustion responsive device indicates a failure in combustion also to cause said fuel control to interrupt the delivery of fuel; means including a'thermal safety switch in series with said 'maincontrol switch effective after a second predetermined period of time to move to open position and at the same time to initiate the timing cycle of said timing means so that the blower is maintained in operation following a combustion failure over both said second and said first predetermined periods of time.

18 ignition means being operative; and means including a device responsive to the establishment of combustion for maintaining said safety shutdown device inoperative.

-11. Burner control apparatus for controlling a fuel control, a burner blower, and a burner ignition means; said apparatus comprising: a first relay adapted to be connected to a main control switch and a source of power so as to be energized whenever said main control switch is closed; a second relay; means adapted to be associated with a burner and with the ignition means therefor and to energize said second relay whenever either said ignition means is operative or there is combustion at the main burner; a thermal timer having an electrical heater; means for energizing said heater when said first and second relays are energized; a third relay; means for energizing said third relay when said thermal timer is 9. Burner control apparatus comprising a fuel period of said timer; and means for initiating the operation of said timer simultaneously with said fuel control.

10. Burner control apparatus comprising a fuel control for a burner; a heater for the fuel; a burner blower; burner ignition means; means including a main control device for initiating operation of said burner ignition means, said fuel heater, and said burner blower; means including a device responsive to the operation of said ignition means and a device responsive to the temperature of the fuel for energizing said fuel control when said ignition means is operative and said oil is at a predetermined temperature; means including a timer for rendering said ignition means inoperative at the end of a timing period of said timer; means for initiating the operation of said timer simultaneously with said fuel control; a safety shutdown device effective after a timed period of operation to render said control device inoperative to continue or again to in,- itiate operation of said ignition means; means including said device responsive to the operation of said ignition means for interrupting the operation of said safety shutdown device upon said in its hot position; first circuit means controlled by said first and third relays and adapted to be connected to a burner ignition means to energize the latter when said first relay is energized and said third relay is deenergized; second circuit means controlled by said first and second relays and adapted to be connected to a burner fuel control to cause operative energization of the latter when both said first and second relays are energized; and third circuit means controlled by said first and third relays and adapted to be connected to a burner blower-to cause operation of the latter whenever either of said first or third relay is energized so that following the deenergization of said first relay, said third circuit means is adapted to maintain said blower in operation while said timer is in its hot position.

12. Burner control apparatus for controlling a fuel control, a burner blower, and a burner ignition means; said apparatus comprising: a-

first relay adapted to be connected to a main control switch and a source of power so as to be energized whenever said main control switch is closed; a second relay; means adapted to be associated with a burner and with the ignition means therefor and to energize said second relay whenever either said ignition means is operative or there is combustion at the main burner; a thermal timer having an electrical heater; means for energizing said heater when said first and second relays are energized; a third relay; means for energizing-said third relay when said thermal timer is in its hot position; first circuit means controlled by said first and third relays and adapted to be connected to a burner ignition means to energize the latter when said first relay is energized and said third relay is deenergized; second circuit means controlled by said first and second relays and adapted to be connected to a burner'fuel control to cause operative energization of the latter when both said first and second relays are energized; third circuit means controlled by said first and third relays and adapted to be connected to a burner blower to cause operation of the latter whenever either said first or third relay is energized, and fourth circuit means controlled by said second and third relays and adapted to be connected to said burner blower to cause operation of the latter whenever either said second or third relay is energized; and switching means for selectively rendering either said third or fourth circuit means effective to cause blower operation to be initiated either when said first or when said second relay is energized.

13. Burner control apparatus for controlling,

19 a fuel control, a burner blower, and a burner ignition means; said apparatus comprising: a first relay adapted to be connected to a main control switch and a source of power so as to be energized when said main control switch is closed; a second relay; means adapted to be associated with a burner and with the ignition means therefor and to energize said second relay whenever either said ignition means is operativeadapted to be connected to a burner ignition means to energize the latter when said first relay is energized and said third relay is deenergized; second circuit means controlled by said first and second relays and adapted to be connected to a burner fuel control to cause operative energization of the latter when both said first and second relays are energized; and third circuit means controlled by said second and third relays and adapted to be connected to a burner blower to cause operation of the latter when either said second or third relay is energized so that following the deenergization of said first relay with the resultant interruption of the energization of said second relay, said third circuit means is adapted to maintain said blower in op eration while said timer is in its hot position.

14. Burner control apparatus for controlling a fuel control, a burner blower, and a burner ignition means; said apparatus comprising: a first relay adapted to be connected to a main control switch and a source of power so as to be energized when said main control switch is closed; a second relay; means adapted to be associated with a burner and with the ignition means therefore and to energize said second relay whenever either said ignition means is operative or there is combustion at the main burner; a thermal timer having an electrical heater; means controlled by said first and second relays for energizing said heater; a safety switch connected in series with said first relay and tending to deenergize said first relay at the end of a predetermined period of time; means operative upon energization of said second relay to render said safety switch inoperable; a third relay; means for energizing said third relay when said thermal timer is in its hot position; first circuit means controlled by said first and third relays and adapted to be connected to a burner ignition means to energize the latter when said first relay is energized and said third relay is deenergized; second circuit means controlled by said first and second relays and adapted to be connected to a burner fuel control to cause operative energization of the latter when both said first and second relays are energized; and third circuit means controlled by said first and third relays and adapted to be connected to a burner blower to cause operation of the latter when either said first or third relay is energized so that following the deenergization of said first relay, said third circuit means is adapted to maintain said blower in operation while said timer is in its hot position.

15. Burner control apparatus for controlling a fuel control. a burner blower, and a burner lgnition means; said apparatus comprising: a first relay adapted to be connected to a main control switch and a source of power so as to be energized when said main control switch is closed; a second relay; means adapted to beassoclated with a burner and to energize said second'relay when there is combustion at the main burner; a thermal timer having an electrical heater; means controlled by said first relay for energizing said heater; a third relay; means for energizing said third relay when said thermal timer is in its hot position and for decreasing the energization of the electrical heater of said timer; first circuit means controlled. by said first and third relays and adapted to be connected to a burner ignition means to energize the latter when said first relay is energized and said third relay is deenergized; second circuit means controlled by said first relay and adapted to be connected to a burner fuel control to cause operative energization of the latter only when said first relay is energized; third circuit means controlled by said first and third relays and adapted to be connected to a burner blower to cause operation of the latter when either said first or third relay is energized so that following the deenergization of said first relay, said third circuit means is adapted to maintain said blower in operation while said timer is in its hot position.

16. Burner control apparatus for controlling a fuel control, a burner blower, and a burner igntion means; said apparatus comprising: a first relay adapted to be connected to a main control switch and a source of power so as to be energized when said main control switch is closed; a second relay; means adapted to be associated with a burner and with the ignition means therefor and to energize said second relay whenever either said ignition means is operative or there is combustion at the main burner; a thermal timer having an electrical heater; means for energizing said heater when said first and second relays are energized; a third relay; means for energizing said third relay when said thermal timer is in ,its hot position; first circuit means controlled by said first and third relays and adapted to be connected to a burner ignition means to energize the latter when said first relay is energized and said third relay is deenergized; second circuit means controlled by said first and second relays and adapted to be connected to a burner fuel control to cause operative energization of the latter when both said first and second relays are energized; and third circuit means controlled by said first relay and adapted to be connected to a burner blower to cause operation of the latter when said first relay is energized.

JOHN 0. ROSCHE.

REFERENCES CITED The following references are of record in the file of this patent;

Gille July 10,

Certificate of Correction Patent N 0. 2,490,095 December 6, 1949 JOHN O. ROSOHE It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 7, line 8, for electrode 85 read electrode 86; column 8, lines 61 and 62, for thermostat 20, conductor 117, read thermostat contact 20, conductor 17?, column 10,1ine 36, for fololws read follows column 11, line 25, before the, first occurrence,

insert of;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the casein the Patent Ofiice.

Signed and sealed this 23rd day of May, A. D. 1950.

a [sun] THOMAS F. MURPHY,

Assistant Gammim'oner of Patents.