US 3720899 A
The housing of this thermostat contains a normally closed switch arm, and a bimetallic, temperature-responsive disc, which flexes to open the switch arm and interrupt the associated circuit when the ambient temperature exceeds a predetermined value. An indicator pin, which is mounted to reciprocate in an opening in one end of the housing, may be attached at its inner end to an over-center, snap-acting spring which is mounted in the housing, or the pin may carry a flexible spring member engageable in spaced recesses in the housing. When the metal disc opens the switch arm, the arm shifts the indicator pin outwardly to indicating position against the resistance of the over-center spring, or the flexible spring member, where it is readily viewable. The indicator pin must be manually reset, but the switch are reclosed independently of the indicator pin.
Description (OCR text may contain errors)
4 United States Patent 1191 Hire et al.
Appl. No.: 147,739
THERMOSTAT WITH OVERLAOD INDICATOR inventors: Charles John Hire, Pittsford; George C. Luke, lrondequoit, both of NY.
Fasco Industries, Inc., Rochester, NY.
u.s. c1 .1. ..337/348, 337/354 Int. Cl. ..ll0lh 37/70 Field of Search .337/91, 342, 343, 348, 354,
References Cited UNITED STATES PATENTS I451March 13, 1973 FOREIGN PATENTS OR APPLICATIONS 1,130,033 5/1962 Germany ..337 348 Primary Examiner-Harold Broome Assistant Examiner-F. E. Bell Al!orney-Shlesinger, Fitzsimmons & Shlesinger\  ABSTRACT The housing of this thermostat contains a normally closed switch arm, and a bimetallic, temperatureresponsive disc, which flexes to open the switch arm and interrupt the associated circuit when the ambient temperature exceeds a predetermined value. An indicator pin, which is mounted to reciprocate in an opening in one end of the housing, may be attached at its inner end to an over-center, snap-acting spring which is mounted in the housing, or the pin may carry a flexible spring member engageable in spaced recesses in the housing. When the metal disc opens the switch arm, the arm shifts the indicator pin outwardly to indicating position against the resistance of the over-center spring, or the flexible spring member,
where it is readily viewable. The indicator pin must be manually reset, but the switch are reclosed independently of the indicator pin.
8 Claims, 10 Drawing Figures PATENTED MR 1 3 I973 SHEET 10F 3 BY J INVENTORS. CHARLES JOHN HIRE GEORGE C. LUKE ATTORNEYS PATENTEDHAR 13 1975 3, 720 899 SHEET 2 BF 3 FIG. 5 K
F |G 7 INVENTORS CHARLES JOHN HIRE GEORGE C.LUKE
ATTORNEYS PATENTEDHAR 1 3 I975 3. 720.899 SHEET 3 BF 3 INVENTORS HG. 10 CHARLES JOHN HIRE GEORGE c. LUKE 41% MMM A AT TORNEYS THERMOS'IAT WITH ovERLoAD INDICATOR This invention relates to thermostats, and more particularly to a snap-action type thermostat having visible means for indicating whether or not the thermostat has been tripped.
It is customary to employ snap-action type thermostats to prevent undesirable overheating of equip ment, or for that matter to prevent excessive reduction in the ambient temperature of equipment. For example, thermostats of the type employing bimetallic, temperature-responsive discs are employed in clothes driers, heaters, air conditioning and ventilating equipment, etc. to prevent overheating and consequent damage to the equipment. Typically this type of thermostat includes a reciprocable plunger, which is positioned between the bimetallic disc and a normally .closed switch arm that is connected in the power circuit for the equipment. When the ambient temperature of the equipment exceeds a predetermined value, the disc snaps suddenly in a direction to cause the plunger to open the switch arm, thereby to deenergize the circuit.
With certain such prior thermostats, it has been customary to mount the thermostat switch arm in such manner that, once it has been opened by the bimetallic disc, it remains open until it is pushed back to its closed position by a manually operable reset button. In still another type of conventional thermostat, the switch arm is mounted automatically to return to its closed position, when the bimetallic disc returns to its normal position.
These prior thermostats have not been satisfactory, however, for use in, for example, the computer hardware field, where they often are calibrated to operate in rather critical or narrow temperature ranges, with the result that they may be tripped or triggered by transient power surges in the equipment. Very often these surges are not critical enough to warrant the shutting down of the equipment for inspection or repair; but it is nevertheless essential, particularly in the case of thermostat switches that are designed to reset automatically, that some means be provided for indicating that the thermostat had been triggered, and that the equipment had been subjected to a critical temperature.
It is an object of this invention to provide an improved, snap-action thermostat having means thereon for indicating when its switch arm has been tripped.
Another object of this invention is to provide for a thermostat of the type described an indicator, which is viewable from the exterior of the thermostat to indicate whether or not the switch arm therein has been moved from its normally closed to its open position.
A still further object of this invention is to provide a novel thermostat having a temperature-responsive, bimetallic actuator, and a reciprocable indicator movable in one direction by the actuator to indicate operation of the thermostat, and movable manually in the opposite direction to a reset position independently of the actuator.
Other objects of the invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.
In the drawings:
FIG. 1 is a plan view of a thermostat made in accordance with one embodiment of this invention;
FIG. 2 is a sectional view of this thermostat taken along the line 2 2 in FIG. 1 looking in the direction of the arrows;
FIG. 3 is a sectional view taken along the line 3-3 in FIG. 2 looking in the direction of the arrows;
FIG. 4 is a fragmentary sectional view taken along the line 4-4 in FIG. 2 looking in the direction of the arrows;
FIG. 5 is a sectional view taken on an axial plane through a thermostat made in accordance with another embodiment of this invention;
FIG. 6 is an axial sectional view of still another type of thermostat made in accordance with this invention;
FIG. 7 is a fragmentary sectional view taken along the line 7--7 in FIG. 6 looking in the direction of the arrows;
FIG. 8 is a plan view of a thermostat made in accordance with still another embodiment of this invention;
FIG. 9 is a sectional view of this thermostat taken along the line 9-9 in FIG. 8 looking in the direction of the arrows; and
FIG. 10 is a fragmentary sectional view similar to FIG. 9 but showing the indicator in its operative position.
Referring now to the drawings by numerals of reference, and first to FIGS. 1 to 4, l0 denotes generally a thermostat comprising a generally cupshaped base or housing 12 having at diametrally opposite sides thereof a pair of integral, laterally projecting shoulders 13 and 14. Housing 12 is made from a phenolic, ceramic or similar rigid, dielectric material, so that it is electrically non-conductive.
Secured intermediate their ends by rivets 16 and 17 in registering recesses formed in the upper surfaces of shoulders 13 and 14 are two, conventional metal elec tric terminals 18 and 19. Adjacent its inner end terminal 19 is bent to extend axially downwardly into housing 12, and then radially inwardly thereof to form a stationary switch contact 20/Secured by the rivet 16 beneath terminal 18, and extending diagonally downwardly and centrally across the interior of housing 12 to overlie contact 20, is a flexible, electrically conductive switch arm 22. The free end of arm 22 carries a metal contact 23 of silver, or the like, which normally is held by the arm in a closed position in which it is resiliently engaged with the stationary contact 20.
Mounted to reciprocate beneath the arm 22 in a central opening formed in the end 25 of the housing 12 is a cylindrical pin or plunger 26, which also is made from a plastic, dielectric material. A bimetallic, temperatureresponsive disc or actuator 30 is secured over the lower end of housing 12 by a metal cap 32. This cap has a circular', bezel-like flange 33 which overlies the marginal edge of the disc 30 to secure the edge of the disc against axial movement on housing 12, but to allow the disc to flex in known manner in response to predetermined temperature changes.
Secured by a pair of screws 36 over the upper end of housing 12 is a plastic cover 37, which may be made from the same material as housing 12. Cover 37 has an integral, annular boss 38, which has an axial bore 39. Secured by screws 36 between the cover 37 and a pair of diametrally opposed ears 41 in housing 12, is a snapacting spring plate 42. This plate is disposed transversely of switch arm 22 and has therethrough a pair of spaced, parallel slots 43, which separate the plate into a central, flexible portion 44, and a pair of side portions that are bent or crimped slightly intermediate their ends as at 46.
Mounted to reciprocate in the bore 39 of cover 37 is a cylindrical indicator pin 18, the lower end of which is staked or otherwise secured as at 49 in the flexible portion 44 of spring plate 42.
In use thermostat is calibrated, in known manner, to cause the bimetallic disc 30 to snap suddenly upwardly from the position illustrated in FIG. 2, to drive the plunger 26 upwardly against the switch arm 22 to open the switch, whenever the ambient temperature exceeds a predetermined value.
- As the switch arm 22 is swung to open position, arm 22 strikes the flexible, central strap portion 44 of the spring 42, causing this portion to snap suddenly upwardly to its elevated or broken line position in FIG. 4, and to drive the indicator pin 48 upwardly to its indicating (broken line) position (FIGS. 2 and 4). The upper end of pin 48 is now readily viewable by the person servicing the equipment that is protected by the thermostat l0.
If after the switch arm 22 is opened, the ambient temperature drops low enough to cause the disc 30 to return to its original position, illustrated in FIG. 2, then arm 22 will automatically return to its closed position. This return of switch arm 22 to its closed position, however, will not affect the spring 42 nor the indicator pin 48, and pin 48 will remain in its upper indicating position until manually reset or pushed back to its retracted position. It is not necessary to reset or reclose the switch 22 manually to cause the equipment to recommence operation, but the fact that the thermostat has been operated will remain indicated. Consequently the shutdown time of the associated equipment is minimized. This is particularly advantageous in the case of computer circuits, for example, which might be subject to infrequent or transient power surges. In such cases the thermostat 10 would allow the equipment to return to an operative state after a momentary opening of the switch arm 22; but the pin 48 would remain in indicating position so that a service man would know that the equipment had been subjected to an undesirable temperature ambient during some preceding period of operation.
After the indicator 48 has been shifted to its indicating position, it may be returned to its retracted position merely by pushing it inwardly against the resistance of the flexible portion 44 of spring 42. This return movement is resisted because of the snap-acting or overcenter type of movement of the portion 44, so that the operator will be able to feel and/or hear the sudden return of the flexible section 44 from its upper position to its lowered position in FIG. 4. Thus, even where it may be difficult for the service man to see pin 48, he or she will known that the pin has been returned to retracted position.
In FIG. 5, wherein like numerals are employed to denote elements similar to those employed in the embodiment of FIGS. 1 to 4, 50 denotes a modified thermostat comprising a shallow, generally cup-shaped base 56, which is made from a plastic, dielectric material. Two, right-angular terminals 58 are secured, each at one end, to the underside of base 56 by rivets 59, with their opposite ends projecting from the thermostat 52 as at 60.
Secured at one end by one of the rivets 59 in the recess 57 in the base 56, and extending transversely across the center of recess 57, is a flexible switch arm 62. At its opposite, free end, arm 62 has secured thereto a contact 63, which normally is held in resilient engagement with the inner end of the other rivet 59. Base 56 is enclosed in an inverted, cylindrical, metal can or cover 52, which extends beyond the lower end of base 56; and the space in the cover beneath base 56 is filled or potted with an epoxy resin 65, or the like.
In this embodiment the trip-indicator pin 48 is mounted to reciprocate in a central opening formed in the closed end of cover 52. The spring 42, to which the inner end of the pin 48 staked, is seated at its marginal edge on an internal circumferential shoulder 54, which is formed in cover 52 adjacent its closed end. Mounted beneath the spring 42 with its marginal edge seated on a second circumferential shoulder 55 in cover 52 is a temperature-responsive, bimetallic disc 30 of the type employed in the first embodiment. Interposed between disc 30 and switch arm 62 is a thin, disc-shaped insulating layer 66 of flexible, dielectric material.
In use, the external ends 60 of the contacts 58 are connected in an electrical circuit. Under normal conditions the disc 30 will be in the position illustrated in FIG. 5, wherein it urges the insulating disc 66 resiliently against the midpoint of the switch arm 62 to hold the contact 63 thereof in closed position against the rivet 59 shown at the left of FIG. 5. Thus the two terminals 60 are connected electrically to one another. At this time the indicator pin 48 will be in its lowered or retracted (solid line) position, and will not project beyond the closed end of cover 52.
However, when the thermostat 50 is subjected to an ambient temperature equal to, or in excess of, the critical temperature for which it is calibrated, the bimetallic disc 30 will snap suddenly upwardly to its broken line position (FIG. 5), thus driving the indicator pin 48 upwardly to its broken line or indicating position, where it will be readily visible. When this occurs, the free end of switch arm 62, which tends normally to flex to its open position, moves contact 63 away from the associated rivet 59, thereby opening the circuit in which the terminals 60 are connected. Thereafter, when the ambient temperature falls below the critical value for which the thermostat 50 is set, disc 30 will snap automatically back to the position illustrated in FIG. 5, again urging switch arm 62 to closed position. This movement, however, occurs independently of the movement of the pin 48, which remains in its upper or indicating position until manually reset.
Referring now to the thermostat illustrated in FIGS. 6 to 7, 26 denotes an actuator, which is mounted to reciprocate, as in the case of the first embodiment, in a central opening formed in a cup-shaped, dielectric housing 12. As in the first embodiment, a bimetallic disc 30 is held over the lower end of housing 12 by a cap 32, which is secured to the housing so that its bezellike flange 33 overlies the marginal edge of the disc. Also, the switch arm 22 of the thermostat 70 is normally disposed in a closed position, in which its contact 23 is engaged with the stationary contact 20 formed on the inner end of the terminal 19.
Secured over the upper end of housing 12 is an inverted, generally cup-shaped cover 72, which may be made of the same material as housing 12. Secured in cantilevered fashion at one end thereof by a rivet 73 to a radial boss 74, which protects from the inside bore wall of the cover 72 above and adjacent the fixed end of the switch arm 22, is a spring plate 42. Also secured by rivet 73 to opposite sides of plate 42 at its fixed end are two shims 75 and 76 having integral, narrow finger portions 77 and 78, respectively, that engage opposite sides of the central portion 44 of plate 42 adjacent its fixed end. The opposite, free end of the spring plate 42 is disposed above the free end of the switch arm 22, and beneath an indicator pin 48', which is mounted to reciprocate in a central opening formed in the closed, upper end of the cover 72. On its inner end pin 48' has an integral, radially offset projection or toe 80, which overlies the free end of spring plate 42.
When the ambient temperature exceeds the critical temperature for which the thermostat has been calibrated, the center of the disc 30 snaps suddenly upwardly, to drive plunger 26 upwardly against the switch arm 22, thereby opening the latter, and simultaneously causing the free end of the cantilevered spring plate 42 to snap suddenly upwardly to its upper or broken line position illustrated in FIG. 6. During this movement, the spring plate 42 strikes the dog 80 on the indicator pin 48 to shift this pin upwardly from retracted position to indicating position. Thereafter, when the ambient temperature falls below the critical temperature, the switch arm 22 and the bimetallic disc 30 return to closed position, enabling the equipment to continue operation, but the indicator pin 48' remains in its upper, indicating position.
The closed end of cover 72 is provided withan opening 82, which registers with the finger portion 77 of shim 75, so that by inserting a small tool downwardly through the opening 82, and against the shim portion 77, the latter may be bent to adjust the resistance to pivotal movement of the free end of spring plate 42. As in the first embodiment, the base 12, the plunger 26, the indicator pin 48', and the .housing cover 72 are all made from a dielectric plastic material. Moreover, the switch arm 22 is free to move independently of the movement of indicator pin 48, so that it is free to be switched between its open and closed positions by the disc 30 regardless of the disposition of the indicator pin 48'.
In the modified thermostat 100 of FIGS. 8 to 10, a plunger 26 is mounted in a housing 12 for movement to a switch-opening position by a bimetallic element 30 in much the same manner as the plunger 26 of the first embodiment. In thermostat 100, however, the overcenter spring 42 is eliminated; and the bore 39 in cover 37 has in its upper end a radial notch 139, and in its lower end an inclined camming surface 140', which opens at its upper end (FIGS. 9 and on bore 39 just beneath the notch 139, and at its lower end on a counterbore 141 formed in the lower end of bore 39.
Mounted to reciprocate in bore 39 of cover 37 is an indicator pin 148, which is generally semi-cylindrical in cross section, and which has an axially extending, plane, chordal surface 149 facing in the direction of notch 139 and the camming surface 140. Integral with the lower end of pin 148 is an enlargeddiameter, discshaped base 150, which registers with the counterbore 141.
Integral at its lower end with base 150, and projecting above the base in spaced relation to the flat 149 on pin 148 is a resilient reset element or spring 152. On its upper end element 152 has an integral, laterally pro- 5 jecting lip 153, which faces in the direction of notch 139 and surface 140. Element 152 is fastened or molded to base 150 in such manner that it tends normally to assume an inclined position relative to the flat 149 on pin 148, as illustrated for-example in FIG. 9.
Normally, when the switch 22 of thermostat 100 is in its closed position (FIG. 9), pin 148 is in a retracted position (FIGS. 8 and 9) in which its upper or outer end is disposed substantially within bore 39, and its base "150 is spaced axially downwardly or inwardly from counterbore 141. At this time the upper end or lip 153 of element 152 is urged resiliently outwardly from the pin 148, and into engagement with the inclined camming surface 140 in bore 39, whereby the spring element 152 resists outward movement of pin 148.
When the ambient temperature of thermostat 100 causes its disc 30 to drive plunger 26 to its switch opening position (FIG. 10), switch 22 swings open to strike the bottom of base 150 and drive pin 148 outwardly against the resistance of element 152 until the base 150 comes to rest in the counterbore 141. At this time the lip 153 on element 152 is snapped into the notch 139 in bore 39 (FIG. 10) to hold pin 148 in its extended or trip indicating position until it is reset manually.
Thereafter when the temperature ambient changes enough so that disc 30 allows switch 22 to return to its closed position, the pin 148 is nevertheless retained in its extended position by element 152. To reset the pin 148 to its retracted position an operator must press on the outer end thereof with sufficient force to cause the flexible lip 153 to be pivoted or otherwise bent relative to the stern portion of element 152 so as to slide downwardly off the bottom of recess 139, and along the bore wall until the lip is returned to engagement with the camming surface 140 as shown in FIG. 9.By molding the pin 148, base 150 and element 152 in one piece from nylon, or the like, it has been found possible to construct a reset spring 152 requiring a reset pressure of over one hundred grams.
From the foregoing it will be apparent that the thermostats disclosed herein provide relatively simple and inexpensive means for not only protecting equipment against undesirable surges or overloads, but also for indicating when the equipment has been exposed to objectionable temperature ambients. Moreover, the indicating means, as represented by the pins 48, 48 and 148, in no way interfere with the continued opening and closing of the associated switch arms 22 or 62 in response to the operation of the associated bimetallic discs 30. Furthermore, in none of the illustrated embodiments is the means, which resists movement of the indicator pin (spring 42 or 152) designed to carry current, or otherwise to form any part of the circuit in which the thermostat is connected. In addition, once the respective indicator pin 48, 48' or 148 has been shifted into its indicating position, it will be readily visible, under normal circumstances, to indicate that the associated switch arm 22 or 62 has at some previous time been tripped to its open position, at least momentarily, by the associated bimetallic disc 30. Moreover, in order to reset or return one of the pins 48, 48 or 148 to its retracted position, it is necessary for an operator to push firmly against the pin to overcome the resistance in the associated spring 42 or 152, thereby removing any doubt as to whether or not the indicator pin was, or was not, in its trip-indicating position at the time it was reset.
In each of the thermostats 10, 70 and 100 the indicator pinis located in the back or end of the thermostat remote from the associated disc 30, while in the thermostat 50, both the indicator pin and disc 30 are mounted in the front end thereof. Thus, depending upon which thermostat is employed, the indicating pin may be installed and reset from either the front or back of the thermostat. Also, if the reclosing temperature of one of these thermostats i.e., the ambient temperature at which the disc 30 is set to return to its normal position is set low enough, then the associated switch arm 22 or 62 will, once tripped, be held open until reset by the resetting of the associated indicator pin 48, 48' or 148. Moreover, although screws 36 are utilized to secure together the housing 12 and cover sections 37 of several of the illustrated thermostats, it will be apparent that, instead of using screws 36, the associated sections could be secured to one another by, for example, flexible tabs that are formed on one section and bent over the other section.
Having thus described our invention, what we claim l. A thermostat, comprising a housing,
a pair of spaced terminals secured to said housing and extending exteriorly thereof for connection in an electrical circuit,
an electrically conductive switch arm secured to one of said terminals in electrical contact therewith, and having a free end extending into a chamber in said housing for movement in said chamber between a closed position, in which it is engaged with the other of said terminals electrically to connect said terminals to each other, and an open position, in which it is disengaged from said other terminal,
means including a temperature-responsive. element in said housing responsive to predetermined changes in the ambient temperature to cause said free end of said arm to be shifted from one to the other of said open and closed positions, respectivey.
a movable indicator element mounted in said housing to project at its inner end into said chamber, and being movable from a retracted position in which said indicator element is disposed substantially within said housing, to an extended position in which a portion of the outer end of said indicator element projects exteriorly of said housing, when said free end of said arm is moved to said other position,
a resilient, snap acting spring,
means mounting said spring in said chamber to extend between said arm and said inner end of said indicator, but to be spaced from said arm when said arm is closed so that no current will then flow through said spring, and said spring being operat of said indicator element by said arm from one to the other of its two limit positions.
2. A thermostat comprising a housing,
a pair of spaced terminals secured to said housing and extending exteriorly thereof for connection in an electrical circuit,
an electrically conductive switch arm secured to one of said terminals in electrical contact therewith, and having a free end extending into said housing for movement between a closed position, in which it is engaged with the other of said terminals electrically to connect said terminals to each other, and an open position, in which it is disengaged from said other terminal,
temperature-responsive element in said housing responsive to predetermined changes in the ambient temperature to cause said free end of said arm to be shifted from one to the other of said open and closed positions, respectively,
a movable indicator element mounted in said housing for movement from a retracted position in which said indicator element is disposed substantially within said housing, to an extended position in which a portion of said indicator element projects exteriorly of said housing, when said free end of said arm is moved to said other position, and
resilient means interposed between said elements and operative resiliently to resist initial movement of said indicator element from one to the other of its two limit positions, and vice versa,
said indicator element being reciprocable in a bore in said housing,
said housing having therein a pair of spaced recesses opening on said bore adjacent opposite ends thereof, and
a resilient spring member secured to said element for sliding movement thereby in said bore between a first position in which said member is engaged in one of said recesses to resist movement of, said element to its extended position, and a second position in which said member is engaged in the other of said recesses releasably to secure said element in its extended position.
3. A thermostat as defined in claim 2, wherein said element comprises a pin reciprocable in said bore,
said spring member is secured at one end to said pin and has a lip on its opposite end urged resiliently away from said pin into sliding engagement with the wall of said bore selectively to seat in said recesses,
said one recess is a camming surface on said housing at the inner end of said bore and inclined to the axis of said bore, and
said other recess is a transverse notch formed in said housing at the outer end of said bore.
4. A thermostat as defined in claim 1, wherein said snap-acting spring is operative to move said indicator element into its extended position, when the free end of said arm moves to its open position, and resiliently retains said indicator element in its extended position independently of the subsequent movement of said free end of said switch arm to closed position, said indicator element being returnable manually to its retracted position.
5. A thermostat as defined in claim 1, wherein said indicator element is a pin mounted to reciprocate in an opening in one end of said housmg,
means on said housing is engaged with opposite ends,
respectively, of said spring to support the mid-portion of said spring for snap-acting movement in opposite directions axially of said pin, and
means is provided for securing said pin to said midportion of said spring resiliently to be held thereby in said retracted and extended positions, respectively.
6. A thermostat as defined in claim 1, wherein said indicator element is a pin mounted to reciprocate in an opening in one end of said housi s means is provided for securing one end of said spring in said housing adjacent one side of said chamber, said spring having a free, cantilevered end extending into said chamber between said free end of said switch arm and said pin to transmit the movement of said free end of said arm to said pin upon movement of said free end of said arm to its open position.
7. A thermostat as defined in claim 1, wherein said indicator element comprises a pin mounted to reciprocate in an opening in one end. of said houss,
said temperature-responsive element comprises a bimetallic disc mounted in said chamber between said pin and said arm and being normally bowed inwardly toward said arm to hold the arm in closed position, and disposed to flex in opposite directions axially of said pin in response to predetermined changes in said ambient temperature,
a flexible layer of dielectric material is interposed between said disc and said arm to allow movement of said free end of said arm to its open position, when said central portion of said disc is flexed in one direction, and
said spring is interposed between said disc and said pin and is responsive to the flexing of said disc in said one direction to shift said pin suddenly from its retracted to its extended position.
8. A thermostat comprising a housing having an indicator pin mounted therein for movement between a retracted position substantially within said housing, and an extended position in which it projects from said housing to be visible from the exterior thereof,
resilient means in said housing for resisting the movement of said pin from one to the other of its limit positions,
a reciprocable plunger mounted in said housing,
a pair of spaced terminals secured to said housing for connection in an electrical circuit,
a normally closed switch arm having one end secured to one of said terminals, and having a free end projecting between said plunger and said pin, and normally urged resiliently into contact with the other of said terminals, and
a temperature-responsive, bimetallic mounted in said housing adjacent said plunger, and operative, when the ambient temperature of said housing exceeds a predetermined value, to drive said plunger against said arm to move said free end thereof away from said other terminal and against said pin, thereby to open said circuit and to shift said pin to its extended position, and operatlve, when sa1d amblent temperature falls below said predetermined value, to allow said switch arm. to return to its closed position without moving said pin,
said pin being reciprocable in a bore in said housing and having an enlarged diameter base reciprocable in said housing,
said resilient means comprising a flexible member secured at one end to said base, and having a free end projecting into said bore in spaced relation to said pin, and urged resiliently away from said pin into sliding engagement with the wall of said bore,
said housing having therein a pair of spaced recesses opening on opposite ends, respectively, of said bore, and
said member having a lateral projection on the free end thereof releasably engageable in said recesses to hold said pin resiliently in one of its limit positions.