US2025292A - Circuit controlling device - Google Patents

Description

Dec. 24, 1935. I R. s. LOCKETT 2,025,292

' CIRCUIT CONTROLLING DEVICE Filed Oct. 23, 1933 Patented Dec. 24, o 1935 UNITED STATES CIRCUIT CONTROLLING DEVICE Ralph G. Lock'ett, Wauwatosa, Wis., asslgnor to Cutler-Hammer, 'Inc., Milwaukee, Wis., a cor- Nation of Delaware Application October 23, 1933, Serial No. 694,696

15ClaimS- control means of the aforementioned character.

for automatically eflfecting given circuit commutations.

Another object of the invention is to provide automatic control. means for periodically and sequentially commutating a plurality of electric circuits. f

Another object is to provide automatic control means for effecting a preselectable and pre-adl5 justed periodic cycle of circuit commutations.

Another object is to provide control means of the aforementioned character wherein a single electro-thermally controlled element is utilized to eflect control of aplurality of associated electric 20 circuits.

' Another and more specific object is to provide control means of the aforementioned character which automatically eilects given periodic reversals in the direction of operation of an electric 2 motor.

- Another object 'is to provide novel control means for the reversible driving motors of washing machines and the like whereby like periods of operation of the washer cylinders in reverse di- 30 rections respectively may be insured.

Another object is to provide for utilization of electro thermally responsive timing means for accomplishment of the desirable results aforementioned.

Various other objects-and advantages invention will hereinafter appear.

The accompanying drawing illustrates an em- 35 of the bodiment of the invention which will now be described, it being understood that the embodiment 40 illustrated is susceptible of modiiicationwithout departing from the spirit and scope of the appended claims.

In the drawing, Figure 1 schematically and agrammatically illustrates an adaptation of 5 invention to the control of an electric motor, and Pig. 2 is an enlarged fragmentary perspective view showing in detail the construction of the heating element employed in the elecfro-thermal device illustrated in Fig. 1.

50 Referring to Fig. -1 of the accompanying drawing, there is iilustrateda motor control system for a reversible three-phase alternating current motor 1! which is supplied from lines L L L throughreversingswitchesrandlt. MotorMis 55 shown as being operatively connected with a retatable member I such as the rotary cylinder of a washing machine, and as will be hereinafter described the illustrated system is adapted to provide automatic control for eiiecting given periodic reversals of the motor M.

The forward switch F and the reverse switch R are each triple pole switches, the same being provided with three sets of contacts 2, 3 and 4, and

5, 6 and 1, respectively. Said switches are respectively provided with operating windings 8 and 10 9, said windings being adapted for energization alternately through the medium of a relay l0.

Relay It includes two sets of contacts I I and I2, which are associated with the armature leversl3 and I4, respectively, and further includes a common operating means for said armature levers. Said operating means comprises the winding and core element IS. The contacts I I and I2 are normally biased toward closed position by atension spring 16 which links the lower extremi- 20 ties of the armature levers I l and H, but, as will be apparent, upon deenergization of the winding l5, one or the other of said levers is-held by its associated spring-biased latch, shown at H and II, to prevent closure of one or the other of the two sets of contacts II and I2. The latches l1 and I8 are cooperable with the ilxed latching pins l9 and 20, respectively, and as shown at 2| and 22 said latches are provided with interengageable projections which are adapted upon successive energizations of the winding It to trip the latches I1 and is alternately, the tripped latch being free to drop out upon deenergization of the winding ii to eflect closure of its associated set of contacts, while the tripping latch is held by its latch pin to prevent closure of its associated set of contacts.

Thus assumingthelatch iltobeinlatched position and the latch l'l tube in unlatched position, as shown in the drawing, the contacts I2 40 will be prevented from closing, while the contacts II will be closed. The projections 2| and 22 are arranged so that in the illustrated relative positions of the armature levers l2 and il the projection 2! of latch ll underlies the projection 22 of latch ll whereby upon energisation of the winding II, the projection 2| will lift the proiection 22 to trip the latch l2. The latch I! will thereupon latch itself upon pin ll, while latching of the latch I8 is prevented by the projection 2|.

Thus upon subsequent deenergintion of winding II the armature lever I! will be retained in latched position to prevent closure of the contaotsll,whi1ethe armaturelever Ilwillbefree todropouttoclosethecontacts l2. Alsathe s5 projections 2| and 22 will have reversed their positions, the projection-22 of latch l3 underlying projection 2| of latch l1,'so that upon repeated energization of the winding l6 the projection 22'- will lift the projection 2| to trip the latch i1. Upon deenergization of winding l6, the armature lever I3 will be released to close contacts H and the armature lever l4 will be held by latch l8 to prevent closure of contacts l2; in other words, the elements of relay "I will again assume theirillustrated positions.

Relay I3 is a modified form of the relay disclosed in Patent No. 1,186,561, issued June 13, 1916 to Clarence T. Evans, and inasmuch as the operation of the relay [3 is substantially the same as that of the relay disclosed in said patent, a more detailed description thereof herein is deemed unnecessary.

Energization of the winding I6 is effected by timed impulses controlled by an electro-thermal- .ly responsive device 23 through the medium of relays 24 and 25 and a quick-acting electrothermally responsive device 26.

They electro-thermally responsive device 23 includes a. pivoted contact element 21 which is oscillatable between a pair of stationary contacts 23 and 23. A compression spring 33 mounted between the contact element 21 and an abutment 3| tends to bias said contact element into engagement with the contact 23, but a taut wire 32 restrains movement thereof and normally maintains the same in engagement with the contact 23. The taut wire 32 isstrung with the desired or required degree of tension between the posts 33, 33, the ends of said wire being secured to the posts by clamping nuts 34, 34. As illustrated more clearly in Fig. 2, a high-resistance wire is wound around the taut wire 32 over a covering 36 of asbestos or other non-fusible or heat-proof insulating material to form a heating coil 36. When the wire 32 becomes heated upon passage of current through the'heating coil 36 said wire lengthens,

due to expansion, relaxing its tension sumciently to permit movement of the contact element 21 under action of the spring 33 into engagement with the contact 23. The time required for movement of the contact element 21 is dependent to a considerable extent upon various factors, principally the size of the wire 32, the thickness of the covering 36, the size of the wire in the heating coil 36 and the value of the current supplied to said coil, but it is apparent that this time element may be varied by changing certain characteristics of any of these factors, or by shifting the position'of the contact 23, as by means of a screw I adjustment. As shown in Fig. 1 a loop 31 is provided at one end of the wire '32 to permit the attachment of some tensioning means, such'as, a

spring, or a weight for effecting the required tensioning .of said It is obvious that ad-. lusting means, such as an adjusting screw to which one end of the wire 32 is secured and upon which the wire'is wound upon turning thereof.

could be associated directly with the wire 30 adwlre,passageofacurrentthroughthesameeffects heating thereof, relaxingits tension to perm'it movement of the contact element 33 under action of the spring 43 into engagement with the contact 33. The wire 42 may be tensioned inva manner similar to that heretofore described in 5 connection with the wire 32, a loop 43 being provided for the purpose, and said wire, as shown, may be secured to the posts 44, 44 by the clamping nuts 45, 45. y

The relay 24 is provided with two sets of no l0 mally open contacts 46 and 41 and an operating winding 43. The relay 26 is provided with a set of normally closed contacts 43, two sets of normally open contacts 53 and 5|, and an operating winding 62. A three pole switch 63 serves to con- 1 nect the system to lines 1 ,1), L 1

Assuming the elements of the aforedescribed system to be in the positions illustrated in Fig. 1, operation thereof will be as follows: Upon closure of the line switch 63 the system is con- 2 nected to lines-L U, L, and an energizing circuit for the operating winding 3 of the forward switch F is established from line L through the contacts ll of relay I 3 and throughsaid winding to line L. Response of switch F closes its 25 contacts 2, 3 and 4 to establish connections from lines L L, L to effect operation of the motor M in its forward direction. Circuits are also established upon closure of the switch .63 for the heater coil 36 of the electro-thermally respon- 30 sive device 23 and for the operating winding 43 of the relay 24, the former circuit extending from line L to and through contact 23 and contact element 21 of the thermally esponsive device 23, by conductors 64 and 66,- to and through the 3 normally closed contacts 43 of the relay and through the heater coil 36 and through the resistances 63 and 63, which are connected in parallel, to line L; and the latter circuit also extending through the contact 23 and the contact 40 element 21, by conductor 64 through said winding 43 and the resistances 63 and 63 to line I The e of current through the heater coil 36 heats the taut wire 32 relatively slowly, due to the factors hereinbei'ore described, and as the 45' wire siackens, the contact element 21'-is moved crating winding 43 of, said relay is also provided, said circuit extending from line L through the contacts 43 and conductor 66 through said winding and resistances 63and 63 toline U.

After a predetermined interval of time the wire 32 will have become slackened suiliciently'to permit engagement of the contact 23 by the contact element 21, whereupon an energizing circuit for thewinding 62oftherelay26isestablished.said60 circuit extending from line L through the contacts 43 of relay 24 by conductors 66 and 64. to and through the contact element 21 and contact 23, toand through contacts 41 of relay 24 and throughthewinding 62 tolineL. Relay 26 re- 65 sponds to close its normally open-contacts 63 and 6|, andto open its normally closed contacts 43 thus interrupting the circuit of the heater coil 36, the energizing circuit for the winding 62 being maintained upon separation of the contacts 21 70 and 23byacircultextendingfromlinemthrough the contacts 46 of relay 24 by conductors 66 and 66, through the contacts 63 of relay 26 and through said winding 62 to line L. Closure of contacts and" ofrelayflestablisheaanen-u ergizing circuit for the winding ii of relay i through the taut wire 42 01 the thermal relay 26, which circuit extends from line L through contacts 46 of relay 24, by conductors 55 and 56. through contacts 50 and and through the taut wire 42 and winding l5 to line L. Relay l6 responds, and as hereinbefore described, the projection 2| of latch l'l lifts the projection 22 to trip the latch l8, and upon further movement of the armature lever II the latch l1 engages the pin l9. Separation of the contacts Ii upon response of the relay l0 deenergizes the operating winding 8 of the forward switch F, causing said switch to open its contacts 2, 3 and 4 to disconnect the motor M from lines U, L, L whereupon the motor coasts until reverse operating connections are established by reverse switch R, response of which is effected as follows:

The wire 42 of the electro-thermally responsive device 26 is adapted to heat relatively quickly, whereupon the tension thereoi is relaxed to permit engagement of the contact 39 by the contact element 38 .under the action of spring 46, which establishes a short circuit around the operating winding 48 of relay 24, said circuit extending from line L through contacts 46 of said relay, by conductors 55 and 54, to and through the contact 39 and contact element 28 of thermal relay 26 and through the resistances 58 and 59 to line L.

Upon deenergization of winding 48 relay 24 opens its contacts 46 and 41, interrupting the energizing circuit for the winding 52 of the relay 25 which closes its contacts 49 and opens its contacts 50 and 5|, interrupting the energizing circuit for the winding l5 of relay H), which circuit includes the heater wire 42.

As hereinbefore described, upon deenergization of the winding l5, the latch ll being latched, the armature lever I3 is held to prevent closure of the contacts I l while the latch l8, being tripped, permits release of the armature lever l4 to close the contacts l2, whereupon the operating winding 9 of reverse switch R is energized through a circuit extending from line L by conductor 51, through contacts l2 and through said winding to line L. Switch R responds to close its contacts 56 and 1 to establish connections from lines L L, I.- to eflect operation or the motor M in the reverse direction.

The wire 32 becomes tensioned upon cooling, the cooling time being dependent upon one or more of the factors mentioned herein, and as the wire tightens it moves the contact element 21 into engagement with the contact. 26, whereupon the aioredescribed circuits are again established to effect response'of relay 24 and to effect energizetion of heater coil 36. Thereafter the electrothermally responsive device 23 operates in the manner aforedescribed to effect response of the" relay 25, thereby again establishing an energizing circuit through the heater wire 42 for the opcrating gwinding N5 of relay l6. Response of relay l6 opens contacts l2, interrupting the energizing circuit for the winding 9 of reverse switch R which opens to interrupt the reverse operating connections to'the motor M. Said motor again coasts until the, electro-thermally responsive device 26 operates to drop out relays 24, 25 and I, whereupon the armature lever l3 oi! the latter is released to close contacts ll, efiecting response of the forward switch F, which again establishes said forward operating connections for motor M.

It will be apparent that the aforedescribed system operates automatically to effect given periodic reversals in the direction of operation of the motor M and that the periods of forward and reverse operation of said motor are substantially equal since the duration of the same is determined by the'cycle of movement oi the contact element 21, that is, each successive engagement of the 6 contact element 21 with the contact 29 results in response of the relay l0 through the medium oi! relay 25 and the heater wire 42 of thermal relay 26, and as hereinabove described, the relay I6 is adapted to effect alternate response of the motor switches F and R upon successive operations of said relay.

What I claim as new and desire to secure by Letters Patent is:

1. In a circuit controlling device, in combination, a contact element mounted for pivotal move ment between two circuit-closing positions, thermally responsive means comprising a wire having its end portions'fixed under apredetermined degree of tension and arranged to normally hold 0 said element in one of said positions, means for biasing said element to the other of said positions, an electric heating coil associated with said wire and adapted when energized to provide for expansion of said wire with consequent movement of said element to said last mentioned position thereof, and said element being adapted upon movement thereof to said last mentioned position to, effect deenergization of said heating coil.

2. In a circuit controlling device, in combina- 30 tion, a contact element mounted for pivotal movement between two circuit-closing positions, thermally responsive means comprising a wire having its end portions fixed under a predetermined degree of tension and adapted to normally hold said element in one of said positions, means for biasing said element to the other 01' said posi-' tions, an electric heating coil associated with said wire and adapted when energized to provide for expansion of said wire with consequent movement 40 of said element to the last mentioned position thereof, said element being adapted when in said first mentioned position to effect energization of said heating coil and when in said last mentioned position to eilect deenergization thereor.

3. In a' device of the character described, in combination, a reversible electric motor, switching means for said motor and automatic control means for eiIecting periodic operation of said switching means to provide for operation of said motor alternately in opposite directions for predetermined substantially equal intervals of time, said control means comprising a pivoted contact element arranged for reciprocal movement between two spaced stationary contacts, a taut wire acting against said contact element on one side thereof to normally hold the same in engagement with one of said stationary contacts, a spring adapted to bias said contact elementtoward the other of said stationary contacts, and means adapted upon engagement of said contact element with said first mentioned contact to effect heating of said wire to increase the length thereof whereby movement of said elementunder the action of said spring into engagement with said second mentioned stationary contact is effected.

4. In a device of the character described, in combination, a reversible electric motor, switching means for said motor and automatic control means for efiecting periodic operation of said switching means to provide for operation of said motor alternately in opposite directions for predetermined substantially equal intervals of time. said control means comprising a pivoted contact element arranged ior'reciprocal movement bewire acting against said contact element onone side thereof to normally hold the same in engagement with one of said stationary contacts, a'

said element under the action of said spring into engagement with said second mentioned stationary contact is effected.

5. In a circuit controlling device, in combination, a pair of spaced stationary contacts, a pivoted contact element arranged for reciprocal movement between said stationary contacts, a taut wire engaging said element on one side thereof to normally hold the same in engagement with one of said stationary contacts, a spring biasing said element toward the other of said stationary contacts, said wire having an electrical insulating covering, a helical wire heating coil positioned externally of said covering, said coil being adapted when energized to heat said wire to effect gradual lengthening of the same whereby movement of said element under the action of said spring into engagement with said second mentioned contact is eifected, and means associated with said contact element and adapted in the last mentioned position thereof to effect deenergization of said heating coil.

6. In a device of the character described, the

combination with a pair of spaced stationary contacts, a pivotally mounted contact element arranged for reciprocal movement between said stationary contacts to control separate associated circuits, an abutment, a compression spring located between one side of said element and said abutment, said'spring being arranged to bias said element toward one of said stationary contacts, ataut wire engaging the other side of said element to restrain the same against movement by said spring, a pair of spaced stationary posts towhich said wire is attached and between which the same is stretched to a predetermined degree of tension, and electrical means adapted when energized to heat said wire to increase the length thereof whereby movement of said element into engagement with said one of said stationary contacts under the biasing action of said spring is eifected, said means, comprising a helically wound wire heating coll surrounding said taut wire, a covering of electrical insulating material interposed between said wire. and said heating coil to provide for gradual conduction of heat to said wire, and associated means adapted in the last mentioned position of said element to effect deenergization of said heating coil.

7. In a circuit controlling system, in combination, an element arranged for reciprocal movement between two circuit-closing positions, electro-thermally responsive means adapted to normally hold said element in one of said positions, said 'electro thermally responsive means being adapted 'when energized to effect movement of said element into the other of said positions, and means controlled by said element in said second mentioned position to effect deenergization of said electro-thermally responsive means.

8. In a motor circuit-controlling system, in combination, an element arranged :for recipro- 8,095,292 tween two spaced stationary contacts, a taut cal movement between two circuit-closing positions, electro-thermally responsive means adapted when deenergized to hold said element in one of said positions and when energized to effect movement of said element into the other of said 5 positions, means controlled by said element in said first mentioned position and operable to effect energizationof said'electro-thermally responsive means, and means controlled by said element in said second mentioned position to effect deenergization of said electro-the'rmally responsive means.

9. In a motor circuit-controlling system, in combination, a reversible electric motor, switching means for said motor and automatic control means for effecting periodic operation of said switching means to provide for operation of said 5 motor alternately in opposite directions for pre-- determined substantially equal intervals of time, said controlmeans comprising a contact element arranged for reciprocal movement between two circuit-closing positions, electro-thermally re- ,spo'nsive means adapted upon de-energization thereof to normally hold said element in one of said positions and when energized to effect movement of said element into the other of said positions, means controlled by said element in said first mentioned position to effect energization of said, electro-thermally responsive means, and meanscontrolled by said element in said second mentioned position to effect deenergization oi said electro-thermally responsive means and to also thereby eflect a predetermined operation of said switching means.

, 10. In a motor control system, in combination, a reversible electric motor, a pair of electromagnetically operable control switches to effect operation of said motor in forward and reverse directions selectively, a control relay adapted to render said switches effective alternately upon successive operations of the former, an electrothermally responsive device comprising a pair of spaced stationary contacts, a contact element ar-" ranged for reciprocal movement therebetween, electro-thermally responsive means normally biasing said element into engagement with one of said stationary contacts, means for rendering said element responsive to a thermal action of said former means to effect engagement of said element with the other of said stationary contacts, means controlled by said element and operable upon engagement of the same with said first tioned contact for effecting said thermal action, a d means operative upon engagement of said elem nt with said second mentioned contact 55 for efiec ing operation of said control relay and for si ultaneously interrupting said thermal action.

' 11. In a motor control system, in combination, a reversible electric motor, a pair of electromagnetically operable control switches to effect operation of said motor in forward and reverse directions selectively, a control relay adapted to render said switches effective alternately upon successive operations thereof, an electro-therto heat said wireto effect lengthening of the latter whereby engagement of said element with said second mentioned contact is effected, means including an electro-responsive relay operable upon engagement of said element with said first mentioned contact to eflect energization of said heating coil, means including another electroresponsive relay operative upon engagement of said element with said second mentioned contact for eflecting operation ofsaid controlrelay and de-energization of said heating coil, and associated means operable automatically for interrupting the circuits initiated by said electro-thermally responsive device.

12. In a motor control system, in combination, a reversible electric motor for driving the cylinder of a washing machine or the like, a pair of electromagnetically operable control switches to effect operation of said motor in forward and reverse directions selectively, a control relay for said switches comprising a stationary magnetic core surrounded by an energizing coil, a pair of pivoted armature members arranged to cooperate with the opposite ends of said magnetic core, a

I latch pivoted at the free end of each of said armature members and biased by a spring into cooperative engagement with a stationary latch pin, a contact finger carried by each of said armature members and adapted upon engagement thereof with an associated stationary contact to render one of said control switches effective, said contact fingers and said contact being separated during energization of said coil, the arrangement of said latches and said latch pins being such that upon successive deenergizations of said coil the armature members are released alternately to efiect engagement with their associated contacts, an electro-thermally responsive device comprising a pair of spaced stationary contacts, a contact element pivotally mounted between said contacts, a compression spring located on one side of said element and a taut wire engaging the same on the other side to normally hold the same in engagement with one of said stationary contacts, said wire having a covering of asbestos with a heating coil wound externally of the latter, said coil being adapted to heat said wire to effect gradual lengthening of the same whereby movement of said element by said spring into engagement with the other of said stationary contacts is eflected, a pair of electromagnetically operable relays, one of which is responsive to a circuit through said-element and said first mentioned stationary contact to effect energization of said heating coil, the other of which is responsive to a circuit through said element and said second mentioned stationary .contact to effect operation or said control relay and to interrupt the circuit of saidheating coil, a second electro-thermally responsive device comprising a stationary contact, a pivoted .contact element, a compression spring engaging the latter on one side and a taut resistance wire en- ,gaging the same on the other side to normally cuits of said electromagnetic relaysw is effected to deenergize said control relays 13. The combination with a washing machine of the rotary cylinder type, of an electric motor for driving theme, a pair of electromagnetically operable reversing switches, said switches being adapted to be energized selectively to ef- 5 feet operation of said motor in reverse directions respectively, an electromagnetically operable relay for effecting energization of said reversing switches, said relay having two sets of normally closed contacts, one set for each of said reversing switches, said contacts being adapted to open upon energization of said relay, latching means associated with each of said sets of contacts and tending to hold the latter in open position upon deenergization of said relay, interengageable tripping means associated with the respective latching means whereby the latter are alternately rendered ineffective for holding their associated sets of contacts in open position, means for effecting energization and deenergization of said relay at predetermined intervals of time, said last mentioned means comprising a pair oi spaced stationary contacts, a contact element pivotally mounted between said contacts, spring means associated with said element to bias the same toward one of said contacts, a taut wire associated with said element and arranged to normally hold the same in engagement with the other of said contacts against thebias cf said spring means, electrical heating means associated with said wire to provide for length.- ening of the latter whereby said element is moved by said spring into engagement with said first mentioned contact, means controlled by said element when engaged with said second mentioned contact and operable to effect energization of said heating means, means controlled by said element when engaged with said first mentioned contact and operable to efiect energization of said electromagnetically operable relay and to simultaneously effect deenergization of said heating means, the aforementioned means for energizing and deenergizing said, relay including a stationary contact, a pivotally mounted contact element, spring means associated with said element and tending to move the same into engagement with said contact whereby deenergization of said electromagnetically operable relay is effected, and a taut wire associated with said last mentioned element and arranged to normally hold the same out of engagement with said contact, said wire being adapted to be heated electrically upon energization of said. electromagnetieglly operablerelay whereby said wire is lengthe d to provide for engagement of said last men ioned element with its associated contact.

14. 1, a motor control system, in combination, a reversible electric motor for driving the cyl-' inder of a washing machine or the like, a pair of 0 electromagnetically operable control switches to effect operation of said motor in forward and reverse directions respectively, a control relay for said switches comprising a stationary magnetic core surrounded by an energizing coil, a 5

pair of pivoted armature members arranged to cooperate with the opposite ends of said magnetic core, a latch pivoted at the free end of each of said armature members and biased by a spring into cooperative engagement with a sta- 7o tionary latch pin, a contact finger carried by each of said armature members and adapted upon engagement thereof with an associated stationary contact to render one of said control switches eifective, said contact fingers and said- 6 f g I contacts being during energization of said coiLthearrangementoisaidlatchesand saidlatchpinsbeingsuchthatuponsuccessive deenergizations of said coil the armature mem- -mentwith one of said stationary contacts, said wire having a covering of fibrous insulating material with a heating coil wound externally oi the latter. said coil being'adapted to heat said wire to eflect gradual lengthening of the same whereby movement oi said element by said spring into engagement with the other oi saidstaticnary contacts is eilected, a pair of electromagnetically operable relays, one of which is responsive to a circuit through said element and said first mentioned stationary contact to efiect energization of said heating coil, the other of p which is responsive to a circuit through said element and said second mentioned stationary contact to eflect operation of said control relay and to interrupt the circuit of said heating coil.

15. The combination with a reversible electric motor, of a rotary device connected therewith to be driven thereby, a pair of electromagnetically operable reversing switches, said switches being adapted to be energized alternately to efiect operation of said motor in'reverse directions re-. spectively, an electromagnetically operable relay for effecting energization a: said reversing switches, said relay having two sets of normally closed contacts, one set tor each of said reversing switches, said contacts being adapted to open upon energization said relay, latching means associated with each 0! said sets of contacts and tending to hold the latter in open position upon deenergization oi. said relay, interengageable tripping means associated with the respective latching means whereby the latter are alternately rendered ineilectlve for holding their associated sets of contacts in open position, means for eflecting energization and deenergization of said relay at predetermined intervals of time, said last mentioned means comprising a pair of spaced stationary contacts, a contact element pivotally mounted between said contacts, spring means associated with said element to bias the same toward one of said contacts, a thermally responsive member associated with said element and arranged to normally hold the latter in engagement with the other 01 said contacts against the bias of said spring means, electrical heating means associated with said thermally responsive member to provide for operation-oi the latter, whereby said element is moved by said spring means into engagement with said first "mentioned contact, means controlled by said element when engaged with said second mentioned contact and operable to eflect energization of said heating means, and means controlled by said element when engaged with said first mentioned contact and operable to effect energization of said electromagnetically operable relay and to simultaneously eiiect deenergization of said heating means. g5 RALPH G. IOCKETT.

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