US3890586A

US3890586A - Temperature responsive electric switch

Info

Publication number: US3890586A
Application number: US473596A
Authority: US
Inventors: Ward Barry Krause
Original assignee: Square D Co
Current assignee: Schneider Electric USA Inc
Priority date: 1974-05-28
Filing date: 1974-05-28
Publication date: 1975-06-17
Anticipated expiration: 1992-06-17

Abstract

A thermomagnetic temperature switch which uses the temperature dependent magnetic permeability properties of a preselected number of plates formed of certain alloyed materials to control the magnet flux through the contacts of a reed switch. The temperature switch includes a fluid tight housing for the reed switch. The housing is immersed in the fluid, the temperature of which is to be detected, and positions in a permanent magnet and temperature sensitive plates so the magnet and plates are immersed in the fluid. The number and type of plates used in the switch is determined by the characteristics of the reed switch, the magnet and the desired temperature differential response of the switch. The switch includes a means for varying the response of the reed switch to magnet flux from the magnet to vary the range of operation of the temperature switch to a predetermined temperature.

Description

United States Patent Krause June 17, 1975 TEMPERATURE RESPONSIVE ELECTRIC [57] ABSTRACT SWITCH [75} Inventor: ward Barry Krause Alfixander A thermomagnetic temperature switch which uses the N C temperature dependent magnetlc permeability properties of a preselected number of plates formed of cer- Assigneer Square B p y, a g tain alloyed materials to control the magnet flux throu h the contacts of a reed switch. The tern era- [22] Flled' May 1974 ture sEvitch includes a fluid tight housing for the reed pp 3,596 switch. The housing is immersed in the fluid, the temperature of which is to be detected, and positions in a 521 US. Cl 335/208; 335/217 immanent magne and temperature i Plates 5 I) Int. Cl. HOlh 37/58 g are l g ".f :W The num er an type 0 p ates use in t e SW1 c IS eter- [58] Fleld of Search 335/208 296 mined by the characteristics of the reed switch. the [56] References cued magnet and the desired temperature differential response of the switch. The switch includes a means for UNITED STATES PATENTS varying the response of the reed switch to magnet flux 3 3 [2/1966 Bowyer 335/203 from the magnet to vary the range of operation of the 3549936 3/1972 Masuda et 335/208 temperature switch to a predetermined temperature.

Primary Examiner-Harold Broome Attorney, Agenl, or Firm-William H. Schmeling; Harold J. Rathbun 10 Claims, 7 Drawing Figures 10 84 i as 3 741 "2-: 12 f 108 m is I :4

i I6: I62 2 I72 I82 I64 PATENTEBJUN 17 I975 FIG.5

1 TEMPERATURE RESPONSIVE ELECTRIC SWITCH This invention relates to electric switches and is more particularly concerned with a temperature responsive magnetically operated switch having a preselected temperature differential and range of response.

Temperature responsive switches using a magnetic shunt of temperature dependent magnetic permeability material together with a reed switch and a permanent magnet are well known and are frequently used in domestic appliances and the like as a safety switch or a switch which controls the temperature output of a heating unit of the appliance. As these switches are not re quired to exercise precise control, they are usually inexpensive and unprecise in their operation. The switch according to the present invention is intended to be used in demanding environments, i.e., to sense the temperature of fluids in a diesel powered locomotive, and provides advantages not present in industrial type temperature switches heretofore known. The switch according to the present invention contains essentially no moving parts so it will have excellent repeatability, will be relatively immune to vibration and will have a long operating life.

The switch further is substantially insensitive to variations in ambient temperature and barometric pressure, and is not orientation sensitive. The switch does not have any contained fluids which will eliminate failures due to leakage of contained fluids and simplify the sealing requirements when the switch is mounted in an opening of a receptacle containing fluids. The switch further has a fast response to changes in the temperature of the fluid being monitored and is compact as well as capable of withstanding relatively high temperatures.

It is an object of the present invention to provide a switch having the foregoing advantages.

Another object is to provide a temperature sensing switch having a reed switch, a permanent magnet and a novel means for shunting the flux output of the magnet when the temperature of a fluid surrounding the magnet and means is less than a predetermined value.

An additional object is to provide a temperature responsive switch wherein a pair of flexible reeds having overlapping ends are the only moving operative parts of the switch.

A further object is to provide a temperature responsive switch with a pair of electrically conducting flexible reeds of magnetic material extending in opposite directions and having spaced overlapping ends adapted to engage each other in response to magnet flux through the reeds. The reeds are contained within a fluid tight enclosure that is immersed in a fluid, a permanent magnet that is mounted on an external wall of the enclosure and immersed in the fluid and arranged to induce a magnet flux in the reeds, a means for adjusting the distance between the reeds and magnet for varying the range of temperature variation response of the switch, and a novel magnet shunt formed of a selected number of plates of ferromagnetic material with each of said plates having selected temperature responsive magnetization characteristics and an edge engaging a wall of the magnet so the plates are immersed in the fluid and provide a shunt path for the magnet flux when the temperature of the plates is less than their Curie temperature and wherein the number and characteristics of the plates is determined by the characteristics of the magnet. the flexible reeds and the desired temperature differential of the fluid which will cause the reeds to move between their spaced and engaging positions.

Further objects and features of the invention will be readily apparent to those skilled in the art from the following specification and from the appended drawings illustrating certain preferred embodiments, in which:

FlG. I is an enlarged view partly in cross section of a temperature responsive switch according to the present invention.

FIG. 2 is a cross-sectional view taken along lines 2-2 in FIG. I in the direction of the indicating arrows and showing a reed switch in one of its adjusted positions.

FIG. 3 is a cross-sectional view taken along lines 3-3 in FIG. 1 in the direction of the indicating arrows with certain portions of the wires included in the switch removed.

FIG. 4 is a perspective view of a magnet flux shield as used in the switch in FIG. 1.

FIG. 5 is a cross-sectional view of the mounting and housing for a reed switch as detached from the switch in FIG. 1.

FIG. 6 is a cross-sectional view similar to the crosssectional view shown in FIG. 2 with the reed switch in another adjusted position.

FIG. 7 is an exploded view showing the components of the switch in FIG. I in perspective which provide the means for adjusting the position of the reed switch in FIG. 1.

A temperature responsive switch 10 as shown in FIG. 1 includes an upper housing 12, a lower housing 14 and a mounting flange 16. The lower housing 14 is preferably formed of a zinc alloy die cast material to have a cylindrical upper portion 18 with an annular flange 20 extending outwardly from the front and of the portion 18. Extending rearwardly from the portion 18 is a semicylindrical portion 22 having a closed rear end wall 24 from which a pair of spaced bosses 26 extend, one of which is shown in FIG. 1. The portion 22 has a flat wall 28 which extends between the rear end wall 24 and a rearwardly facing semi-circular wall 30 located at the rear end of the upper portion 18. Extending upwardly into the wall 30 in a manner illustrated in FIGS. 1 and 2 are three spaced notches 34. The walls of the upper portion 18 and the semi-cylindrical portion 22 provide the housing 14 with a fluid tight cavity 36 that has an open front end 38.

The upper housing 12 may also be formed of a zinc alloy die cast material to have side walls defining a rectangularly shaped cavity 40 extending between an open front end 42 and an open rear end 44 of the housing 12. The open front end 42 is closed by a suitable bayonet connector 46 and gasket 48 which are sealingly secured to the front end 42 by four screws 50 which are tightened into threaded openings 52 at the four corners of the front end 42. The connector 46 has a pair of terminals 54 extending rearwardly into the cavity 40. The terminals 54 are connectible in an external control and [or alarm circuit. not shown. Extending outwardly at the rear end 44 is a substantially rectangular flange 56 having four openings extending therethrough and an annular rearwardly facing groove 60 formed in the rear face of the flange 56. The inner peripheral wall of the groove 60 is defined by an annulus 62 that has its front end connected to the inner walls of the cavity 40 by a forwardly facing ledge 64. If desired, the inner walls of the cavity 40 and the annulus 62 may be covered by a liner 66 formed of insulating material.

Positioned at the rear end of the cavity 40 by the ledge 64 is a printed circuit board 68 having suitably located eyelets which are electrically interconnected to provide a convenient means for connecting a pair of wires 70 and 71 extending from the terminals 54 to a pair of

wires

72 and 73 respectively extending in cavity 36. If desired, the switch may be provided with a suitable push-to-test means for testing the integrity of the external circuit including the operability of the components which may be connected to the switch 10. The push-to-test means includes a button 74, a pair of posts 76 and 78 and a torsion spring 80. The button 74 is preferably formed of insulating material to have a cy lindrical portion extending through a circular opening in one of the side walls of the upper housing 12 to provide an end 82 that is external of the housing 12 and a flange 84 on an end which is positioned in the cavity 40. The flange is arranged to engage the wall of the cavity 40 to limit the outward movement of the button 74 in the housing 12. The inner end of the button 74 is provided with a groove 86. The posts 76 and 78 extend forwardly of the printed circuit board and are each connected to wires 70 and 71 respectively. The torsion spring 80 has a convoluted midportion 88 surrounding post 76, an end 90 engaging a wall portion of the housing 12, and an arm portion 92 positioned in groove 86 and extending to engage the post 78. The spring 80 is mounted and arranged to normally urge the button 74 outwardly in the housing 12 and position the arm portion 92 in a separated position from the post 78. When the button 74 is pushed inwardly in the housing 12, against the force provided by the spring 80, the arm portion 92 will engage the post 78 and thus complete a test circuit between the posts 76 and 78.

The mounting flange 16 is formed as a flat metal part having a substantial thickness and includes a central circular opening 94 and four threaded openings aligned with the four openings into which four screws 96 are threaded to secure the rear surface of the flange 56 to the front surface of the flange 16. The flange 16 also includes a pair of openings 98 at its opposite ends which receive suitable screws when the switch 10 is secured to an outer wall of a receptacle or manifold, not shown. that has an opening to receive the lower housing 14 and contains a fluid, the temperature of which is detected by the switch 10.

The annular flange is sized to be received in the groove 60. A gasket 100 positioned between the front side of the flange 20 and the rear surface of the groove 60 provides a sealed connection and thermal barrier between the upper housing 12 and the lower housing 14. The opening 94 is sized to receive the upper portion 18. A gasket 101 positioned between the rear surface of flange 20 and the front surface of the flange 16 provides a sealed connection and thermal barrier between the lower housing 14 and the flange 16 when the screws 96 are tightened in the threaded openings in the flange 16.

The components of the switch 10 included within the cavity 36 are most clearly shown in FIG. 7 and comprise a reed switch 102 and an adjustable mounting for the switch 102. The adjustable mounting includes a pivotal mounting member 104, a pivot pin 106, a torsion spring 108, an adjusting screw 110 and a support 112 for the screw 110 and pin 106.

The switch 102 has a tubular glass envelope 114 and a pair of contacts mounted within the envelope 114. The contacts are formed as electrically conducting

reeds

116 and 118 of magnetic material and extend in opposite directions from the opposite ends of the envelope 114 and have overlapping spaced ends 116A and 118A, respectively, which move into engagement with each other in response to a magnet flux passing through the

reeds

116 and 118. The ends of the

reeds

116 and 118 are hermetically sealed in and extend through their associated ends of the envelope 114 and are connected to the

wires

72 and 73, respectively. Preferably, the connection between the

reeds

116 and 118 and the

wires

72 and 73 is covered with a suitable insulating plastic sealing material so that the switch will not have any electrically conducting metal parts exposed in the cavity 36 which could be contacted by a fluid in the event the housing 16 should cease to be fluid-tight. While in the embodiment described the switch 102 is a conventional switch having normally open contacts, if desired, the switch 102 may be selected to have normally closed contacts or double throw contacts without departing from the scope of the present invention.

The mounting member 104 is formed of a rigid material to have a pair of spaced ears 120 and 12] extending from a side 122 and an inclined surface 124 extending from a top edge 126 to a flat front face 128. The

ears

120 and 121 have

bores

130 and 131 respectively extending therethrough which are aligned on an axis 132. The mounting member 104 also has a concave groove 134 extending in the face 128 over the entire length of the member 104. The groove 134 extends along an edge of the member that is remote from the side 122 along an axis that is spaced and parallel to the axis 132. The pivot pin 106 is rod-shaped and has its midportion secured in the

bores

130 and 131 and its rear end rotatably mounted in a bearing opening 136 that extends rearwardly in a front face of a projection 138 that extends forwardly from the wall 24. The front end of the pin 106 is rotatably received in a suitably located bearing opening in the support 112. The bearing openings for the pin 106 are aligned along an axis that is parallel to the flat wall 28 and displaced from the vertical center of the wall 28 so that when the mounting member 104 and pin 106 are positioned in the cavity 36, the longitudinal axis of the groove 134 will extend in a plane substantially perpendicular to the vertical center of the wall 28.

The torsion spring 108 has a central convoluted portion 142 surrounding the pin 106 and an end 144 positioned in an opening 146 in the support 112 and an end 148 positioned adjacent a surface 150 on the car 120. The spring is positioned between the car 120 and the support 112 and is wound to provide a bias which causes the member 104 to swing about the axis of the pin 106 toward the wall 28.

The adjusting screw 110 has a threaded mid-portion 1S2 threaded in a threaded opening in the support 112 and a tapered tip 154 at its rear end positioned to ride upon the inclined surface 124. The screw 110 extends forwardly through an opening in the printed circuit board to a front portion which has a slotted front end 156 which receives the tip of the screwdriver when the screws 50 and connector 46 are detached from the front end 42 to provide access to the screw 110 so that the screw 110 may be rotated in the opening to adjust the position of the reed switch 102, as will be later described.

A means for inducing a magnet flux in the

reeds

116 and 118 when the temperature of a fluid surrounding the housing 14 reaches a predetermined value includes a permanent magnet 158, a plurality of ferromagnetic plates which are laminated in three

groups

160, 161 and 162, and a bracket 164. The permanent magnet 158 is formed as a rectangular bar to have opposite polarity magnet poles at its opposite ends and a side 166 positioned adjacent the outer side of the wall 28 and centered along the vertical central axis of the wail 28. When the magnet 158 is positioned on the wall 28, the upper end of the magnet 158 is adjacent the wall 30 and the lower end is adjacent a leg portion 168 on the bracket 164. The groups of

plates

160, 161 and 162 are positioned on the three sides of the magnet 158 that are not in contact with the wall 30 respectively, as illustrated in FIG. 2. The plates of each

group

160, 161 and 162 are formed of ferromagnetic material and takes advantage of the temperature magnetization characteristic of the magnetic material of the plates below the Curie point temperature of the material.

The bracket 164 has a leg portion 170 overlaying the rear wall 24 and a pair of openings receiving the pair of bosses 26 which are peened over the portion 170 to secure the bracket 164 to the housing 14. The bracket also includes a body portion 172 extending between the

leg portions

168 and 170 which positions the leg portion 168 so that three spaced openings 174 are vertically aligned with the notches 34 and arranged to receive the projections 176 at the lower ends of the group of

plates

160, 161 and 162 while the notches 34 receive the projections 178 on the upper ends of the group of

plates

160, 161 and 162.

As illustrated in FIGS. 5 and 7, the reed switch 110 is secured in the groove 134, as with a suitable adhesive, to have the longitudinal axis of the envelope 114 and

reeds

116 and 118 extending parallel and spaced from the axis 132. The spring 108 is arranged to bias the mounting member toward the wall 28 to a position determined by the point of engagement of the tip 154 with the inclined surface 124. When the tip 154 is positioned on the portion of the surface 124 adjacent the edge 126, the reed switch will be in close proximity to the wall 28 and the magnet 158. When the screw 110 is adjusted in the support 112 in a direction which causes the tip 154 to move rearwardly on the surface 124, the member 104 will swing like a gate about axis 132 and increase the distance between the switch 102 and the magnet 158.

The notches 34 and the projections 178 position the plates of

groups

160, 161 and 162 so that each of the plates has an edge engaging a surface portion of the magnet 158 and a side wall positioned against the side wall of an adjacent plate. The

plates forming groups

160, 161 and 162 resemble fins which extend from the outer surface of the magnet 158. The material of the plates is formulated with temperature dependent magnetization characteristics in the range of temperature at which the switching function of the switch is desired. At a low temperature, the material of the plates has a high permeability and in effect serves as a shunt or short circuit for the magnet flux output of the magnet 158. As the temperature of the plates of the

groups

160, 161 and 162 increases, because of the rise in temperature of the media being sensed, the permeability of the plates decreases and the plates become less effective to shunt the magnetic output flux of the magnet 158. At a predetermined temperature, depending upon the composition of the ferromagnetic material of the plates which provides the plates with a selected magnetization temperature characteristic, lines of flux, which are previously shunted through the plates, are now allowed to pass through the

reeds

116 and 118. At the preselected temperature, the number of lines of flux passing through the

reeds

116 and 118 is sufficient to cause the reeds to move so that the

ends

116A and 118A which are normally spaced from each other, move into engagement. This is called the pull-in temperature of the switch 10. As the media temperature and the corresponding temperature of the plates decreases, the permeability of the plates will increase, allowing the plates to become a more effective shunt for the magnetic flux. At a preselected temperature which is lower than the Curie point temperature of the plates, sufficient flux will have been diverted from the

reeds

116 and 118 to permit the reeds to move so that ends 116A and 118A separate. This is called the drop-out temperature of the switch 10. A broad range of temperature adjustment of the switch 10 can be controlled by the formulation of the alloy material forming the plates within the

groups

160, 161 and 162. A specific temperature setting within the range can be obtained using the adjusting screw to move the reed switch 102 forward or away from the magnet 158 in a manner previously described. When the switch 102 is positioned so a minimum distance is present between the switch 102 and the magnet 158, a minimal loss of shunting ability of the plates will be required to actuate the

reeds

116 and 118. As the switch 102 is moved away from the magnet 158, the

reeds

116 and 118 will be subjected to a weakened flux from the magnet 158 and thus require that the plates lose more of their shunting ability before the magnet 158 will supply sufficient flux to the

reeds

116 and 118 to cause them to be activated and engage each other. Moving the reed switch 102 away from the magnet 158 thus increases the pull-in temperature of the switch 102. It is well known that magnetic saturation permeability with temperatures of the plates can be varied by varying the alloy composition of the plates. Thus the individual plates forming the

groups

160, 161 and 162 may be selected to have different temperature response characteristics. Also, it is well known tolerance variations may cause the flux output of magnets to vary and the switching response of the reed switches to vary with varying degrees of magnet flux. The tolerance variations of the magnet 158 and the reed switch 102 may be compensated in the switch 10 according to the present invention by varying the cross section of plates that have selected different temperature response characteristics in the

groups

160, 16l and 162 and also varying the position of the reed switch 102 relative to the magnet 158.

Since the shunting ability of the

groups

160, 161 and 162 is dependent on the cross-sectional areas of the

groups

160, 161 and 162, the differential between the pull-in and drop-out characteristics of the switch 10 is dependent upon the cross-sectional areas of the

groups

160, 161 and 162. As shown in FIG. 2, the

groups

160, 161 and 162 each comprise four plates. Therefore a small temperature change of the material of the

groups

160, 161 and 162 will cause the

reeds

116 and 118 to move between their open and closed position and the switch 10 will operate with a small temperature differential response. If the number of plates of the

groups

160, 161 and 162 is reduced, i.e., each group has two plates. a larger temperature change will be required before the

reeds

116 and 118 move between their normally open and closed positions so that the switch 10 will have a larger differential response.

The switch 10 is designed so that the temperature sensing portion of the switch 10, including the lower housing 14, the magnet 158 and the

plates forming groups

160, 161 and 162, are totally immersed in the media to be sensed and are isolated from the ambient temperature surrounding the upper housing 12 of the switch 10. This will cause the switch to respond quickly to changes in temperature of the media being monitored and to be substantially insensitive to ambient temperatures surrounding the housing 12.

If desired, switch 10 may also be provided with a magnetic shield 180 which surrounds portion of the switch immersed in the media, the temperature of which is to be sensed. In the embodiment shown, the magnetic shield 180 comprises tubular member 182 as shown in FIG. 4, formed of magnetic metal having openings 184 therein aligned with the

groups

160, 161 and 162, a disc 186 is secured on the rear end of the tubular member 182. The disc 186 also has an opening 188 therein to permit passage of the fluids into the area within the shield 180 occupied by the

groups

160, 161, and 162 and the magnet 158 so that the groups 160, l6l and 162 will be immersed in the circulating fluid media to increase the temperature response of the switch without subjecting the reed switch 102 to stray magnetic fields which may be present in the environment in which the switch 10 is required to operate.

While certain preferred embodiments of the invention have been specifically disclosed, it is understood that the invention is not limited thereto, as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims:

What is claimed is:

l. A temperature responsive electric switch comprising: a pair of magnetically responsive contacts movable to an operated position in response to magnet flux, a permanent magnet having a pair of opposite polarity magnet poles at its opposite ends and a pair of walls extending between the ends with a first of said walls facing and spaced from the contacts and a second of said walls spaced from the first wall and facing away from the contacts, and a plurality of plates formed of ferromagnetic material having selected magnetization characteristics with each of the plates having an edge engaging the second wall and providing a shunt path for magnet flux when the temperature of the plates is less than the Curie temperature.

2. A temperature responsive electric switch comprising: a pair of normally open switch contacts comprising a pair of electrically conducting flexible reeds of magnetic material extending in opposite directions and hav ing overlapping ends adapted to contact each other in response to a magnet flux through the reeds, a permanent magnet having a rectangular cross section and positioned to have a first side facing and extending in a plane spaced from the reeds, poles of opposite magnet polarity at opposite ends of the magnet arranged to induce a magnet flux in the reeds for causing the ends of the reeds to contact each other, and a second side extending between the poles in a plane not common to the plane of the first side and a stack comprising a selected number of plates laminated together with each plate formed of ferromagnetic material having selected magnetization characteristics and an edge engaging the second side to provide a shunt path for the magnet flux when the temperature of the plates is less than the Curie temperature.

3. The structure as recited in claim 2 wherein the permanent magnet has one side facing the switch contacts and three other sides having an individual stack oflaminated ferromagnet plates positioned thereon to provide a shunt path for magnet flux between the poles of the magnet.

4. The structure as recited in claim 2 wherein the number of plates in the stack is determined by the characteristics of the magnet and the reeds to provide the switch with a predetermined differential response to temperature excursions less than the Curie temperature.

5. The structure as recited in claim 4 wherein the reeds are enclosed within an elongated tubular envelope and including means providing an adjustable mounting for the envelope for adjusting the space between the reeds and the first side of the magnet whereby the range of response of the device to temperature excursions less than the Curie temperature may be adjusted.

6. The structure as recited in claim 4 wherein certain of the plates in the stack are formed of ferromagnetic material having different Curie temperature and magnctization characteristics than other plates of the stack and wherein the number and type of plates in the stack is determined by the characteristics of the magnet and reeds to provide the device with a selected differential response characteristic to temperature excursions less than the Curie temperature.

7. A temperature responsive switch having a first portion constructed to extend through an opening in a wall of a receptacle containing a fluid the temperature of which is to be detected, with first portion comprising: a fluid tight housing having an open end, an electric switch mounted in a cavity within the housing, said switch having a tubular envelope extending along a longitudinal axis of the cavity and switch contacts within the envelope comprising electrically conducting flexible overlapping reeds of magnetic material adapted to contact each other in response to a magnet flux passing therethrough, a permanent magnet mounted on an external wall portion of the housing so as to be immersed in the fluid and having poles arranged to induce a magnet flux in the reeds, a selected number of plates of ferromagnetic flux material, each of said plates having selected magnetization characteristics and an edge engaging a wall portion of the permanent magnet that is immersed in the fluid, said plates providing a shunt path for the magnet flux when the temperature of the fluid is less than the Curie temperature and said number of plates being selected to calibrate the switch for tolerance variations of the magnet and electric switch.

8. The structure as recited in claim 7 including a ferromagnetic shield providing an outer wall for the first portion of the temperature responsive switch that is immersed in the fluid.

9. The structure recited in claim 8 including a mounting for the electric switch which is adjustable to vary of normally open contacts connected in parallel circuit with the switch contacts to provide a test circuit for the temperature responsive switch and a means for adjusting the mounting for the electric switch.

a: a: a:

Claims

1. A temperature responsive electric switch comprising: a pair of magnetically responsive contacts movable to an operated position in response to magnet flux, a permanent magnet having a pair of opposite polarity magnet poles at its opposite ends and a pair of walls extending between the ends with a first of said walls facing and spaced from the contacts and a second of said walls spaced from the first wall and facing away from the contacts, and a plurality of plates formed of ferromagnetic material having selected magnetization characteristics with each of the plates having an edge engaging the second wall and providing a shunt path for magnet flux when the temperature of the plates is less than the Curie temperature.

2. A temperature responsive electric switch comprising: a pair of normally open switch contacts comprising a pair of electrically conducting flexible reeds of magnetic material extending in opposite directions and having overlapping ends adapted to contact each other in response to a magnet flux through the reeds, a permanent magnet having a rectangular cross section and positioned to have a first side facing and extending in a plane spaced from the reeds, poles of opposite magnet polarity at opposite ends of the magnet arranged to induce a magnet flux in the reeds for causing the ends of the reeds to contact each other, and a second side extending between the poles in a plane not common to the plane of the first side and a stack comprising a selected number of plates laminated together with each plate formed of ferromagnetic material having selected magnetization characteristics and an edge engaging the second side to provide a shunt path for the magnet flux when the temperature of the plates is less than the Curie temperature.

3. The structure as recited in claim 2 wherein the permanent magnet has one side facing the switch contacts and three other sides having an individual stack of laminated ferromagnet plates positioned thereon to provide a shunt path for magnet flux between the poles of the magnet.

6. The structure as recited in claim 4 wherein certain of the plates in the stack are formed of ferromagnetic material having different Curie temperature and magnetization characteristics than other plates of the stack and wherein the number and type of plates in the stack is determined by the characteristics of the magnet and reeds to provide the device with a selected differential response characteristic to temperature excursions less than the Curie temperature.

9. The structure recited in claim 8 including a mounting for the electric switch which is adjustable to vary the spacing between the electric switch and the magnet.

10. The structure as recited in claim 9 wherein the temperature responsive switch includes a second portion that is positioned to close the open end of the fluid tight housing and said second portion including a pair of normally open contacts connected in parallel circuit with the switch contacts to provide a test circuit for the temperature responsive switch and a means for adjusting the mounting for the electric switch.