US1958081A

US1958081A - Warp switch

Info

Publication number: US1958081A
Application number: US226395A
Authority: US
Inventors: Frederick S Denison
Original assignee: Honeywell Inc
Current assignee: Honeywell Inc
Priority date: 1927-10-15
Filing date: 1927-10-15
Publication date: 1934-05-08
Anticipated expiration: 1951-05-08

Description

F. S. DENISON May 8, 1934.

WARP SWITCH FiIed Oct. 15 1927 2 Sheets-Sheet 1 /e 55 Q5 4 55 e F7@. 4 1' n venaor FREDERICK 5. DEN/6 o/v BrMfi/M Azworneus 2 Sheets-Sheet 2 F'ALL a: so (BELOW NORMAL) F. :S. DENISON WARP SWITCH Filed Oct. 15. 1927 Ease 0F 30 dsovs NORMAL) RISE OF 150 TQ (NQRMAL) May 8, 1934.

go Tov-m. RANGE 4,4 TEMP..70

. I'm/@2100) FREDERICK S. DEN/8 0N ISA I Auto; eys

III

Patented May 8, 1934 WARP SWITCH Frederick S. Denison, Minneapolis, Minn., assignor, by mesne assignments, to Minneapolis- Honeywell Regulator Company, Minneapolis, Minn., a, corporation of Delaware Application October 15, 1927, Serial No. 226,395

24 Claims.

This invention relates to improvements in warp switches. The main object of the device is to provide means whereby the switch will actas an accurate timing device to positively break the circuit at'the end of a predetermined time interval. Devices of this kind ordinarily have a'single bimetallic element or a pair of such elements deflectable in response to temperature changes. These elements are generally associated with a heater element, as an electric resistance element, which, when heated, is supposed to deflect the switch control element in a certain degree. However, it has been found that due to changes of temperature in the medium surrounding the device, the length of travel of thatbimetallic portion of the device used as a timing control element varies, so that when heat is applied by the coil a premature, or a tardy operation occurs. The present device finds valuable application in heat regulation, in which the device is used to control trial ignition periods at a burner so that the fuel can be shut oiT if trial ignition fails and so that the trial ignition takes place during a predetermined time interval. If the ordinary type of warp switch is submitted to an unusually hot outside temperature or to an unusually cold outside temperature, the circuit control would either be affected prematurely, or not at all, or if at all at the end of a 'longer period than desired.

The present device contemplates a compensation which prevents operation'of a circuit controlling element of the device in response to temperatures other than those set up by the resistance element, therefore making it possible to obtain accurate timing, irrespective of the temperatures other than those set up by the heating element. The present device will not trip as the result of abnormal high or abnormal low temperatures, other than those produced by its heating element.

Objects are, therefore-to provide a warp switch which will trip at a predetermined temperature for which it is set, regardless of the temperature of the surrounding medium; to provide a switch which will neither break its circuit prematurely oftardily; to provide a compensating blade for Figure 2 is a top plan view showing the relation of the tripping control element to one of the switch elements;

Figure 3 is an end View or side elevation of the elements of Figure 2;

Figure 4 is a view similar to Figurel but showing the device with the contacts open, and ready to be closed when the resetting button is pushed upwardly;

Figure 5 is a view similar to Figure 2 but corresponding to the position of the parts shown in Figure 4;

Figure 6 is 'a side View in correspondence to Figure 5;

Figure 7 is a diagrammatic view showing the action of the switch, in reference to temperature changes other than those supplied by the heating element;

Figure 8 is a fragmentary view showing that the switch contact element cannot be closed when the controlling thermostatic element is raised be yond a certain level;

Figure 9 is a side view of the tripping ele merits, corresponding in position to Figure 8;

Figure 1c is a section substantially on line 10i0 of Figure Figure 11 is a section on line 11-11 oi Figure 31, viewed in the direction of the arrow;

Figure 12 is a view of a modification showing one position of the parts; and

Figure 13 is a view of the same modification showing a diiferent position of the parts.

The device illustrated in the drawings, as showing one application of the invention, comprises a base 1, which is adapted to be attached flatly against a vertical surface. lower portion of the plate 1 is a horizontal shelf 2 having a vertical extension 3 at one end. A boss 4 is formed below the shell. 2 and is bored vertically and threaded as at 5. The shelf 2 is thickened at that end opposite the vertical extension as at 6 and a shoulder '1 is provided. Upon this shelf 2 is mounted the elements of a Projecting from the switch, along with means for manually opening the switch, under'certain conditions. The construction of the switch constitutes a feature of the present invention, although the most -.impor-' tant feature is the arrangement of the bimetallic blades of the warp switch to obtain the compensation, above mentioned. Mounted flatly against the upper surface of the shelf 2 is a flexible piece of insulation 10. Overlying this insulation, and secured at one end by a binding post 12, is a metallic strip bent at one end twice at right angles to provide a horizontal portion 16, this horizontal portion being supplied with a contact 17. A spacer 18 is interposed between the insulating strip 10 and the metallic strip 15. Upon the top of the metallic strip 15 is a flexible strip of insulating material 20, this strip being suitably secured at one end as by rivets passing through the metallic element 15. The elements l0, l5 and 20 are, therefore, capable of being flexed in an upward direction by means of a plunger of insulating material, socketed in a slidable element 26, limited in its downward movement and held in position by a plug 27 engaging with the threads 5, see Figure 10. A spring '28 urges the plunger downwardly so that its upper end normally lies below the level of the element 15. The insulating element 10 is provided with an opening 29 which the plunger or button 25 traverses when forced upwardly against the action of the spring 28. A second movable switch element is provided having the following construction: A extension element 30, as a bracket, is secured by one foot upon the top of the insulating element 10 by means of the binding post 31. The upper foot of this element is spaced above the binding post 12, and has a resilient metallic contact arm 33 secured against its top. The arm has at its outer free end a contact 34 cooperative with the con tact 17, by movement in an upward direction. The element is flexible. Secured to the under side of the shelf portion of the member 36 is an arm which is arranged so that its outer end is normally spaced above the element 20 as shown in Figure 1 when the device is set for a timing operation. Upon the top of the element 33 is mounted a bracket having an upstanding extension 36, having its upper ehd formed as shown in Figures 3, 6, and 9, to provide lateral recesses 38 providing shoulders '73, '74 arranged to limit motion of the upper bimetallic element. The upper end of the vertical portion of the bracket 36 is also center-notched or recessed as at 40, to receive a projecting portion of the upper bimetallic element. Suitable insulating material 41 is interposed between the bracket, having the arm 36,\ and the element 33. Suitable conductors 42-43 are connected respectively with

binding posts

12 and 31. It is to be understood that the binding posts are suitably insulated from the shelf 2. The detailed construction of the posts and their connection has notbeen shown, inasmuch as this construction is well known in the art. The arrangement, of course, is such that the current can only pass through elements 15-33 when the contacts 1? and 34 are in engagement.

The arrangement of the switch elements is such that when the upper bimetallic element 49 overlies the arm 36 as shown in Figures 4 and 8, the contacts 1'7 and 34 cannot be brought togather as the result of pushing upwardly on the plunger 25, the only function of the plunger being to throw the arm 36 outwardly sufficiently to allow the upper bimetallic element to again fall to its norma s t position, shown in Figure 1, after it has cooled sufliciently. It will be noted particularly by an inspection of Figure 8, however, that if the bimetallic element has not cooled sufficiently to be under, tension against the upper end of the arm 36, in a manner shown in Figure 4, the contacts cannot be brought together. However, when the upper bimetallic member isexerting pressure downwardly upon the upper end of the arm 36, the upward movement of the plunger 25 will move the element 36 in direction of the arrow, see Figure 4, to release the upper bimetallic element and permit it to assume the position shown in Figure 1.

The main feature of this invention relates to the means now to be described,which provides a compensation for a warp switch, whereby that bimetallic member of the switch which controls the switching mechanism, can be caused to move a certain predetermined distance, and only that predetermined distance, when its heating coil is operating, regardless of the temperature of the surrounding medium or atmosphere, thus providing an accurate timing device.

Attached horizontally to the upstanding portion 3, as at 45, is an elongated bimetallic element 44 having its active side 46, in this instance, facing upwardly. This element 44, in this instance, may be considered the compensating element and is shown as'of greater length than the element 48 which lies above it. This element may also be considered as a supporting member to which is attached a second temperature responsive element which is composed of two sections respectively designated 48-49, having their active sides, 50-51, opposingly faced. The elements 48-49 are connected together by a relatively large mass of metal 52, see Figure 11, which is recessed or slotted as at 53, see Figure 1, to receive the ends of the respective eletween these elements is a heating coil 54 having its terminals 5556 connected to binding posts 5758 carried by a shelf-like projection 60 of the base 1. The heating element 54 has been shown in a diagrammatic manner but as constructed, the coil is composed of a number of turns of fine wire wound upon a flat mica core. This mica core is inserted between mica plates. The upper element 49 of the upper thermostatic section is spaced below the shelf 60 to allow free movement of the bimetallic elements of this upper element when expanded. The coil 54 is inserted between two insulating plates 62, in this instance composed of mica, and of a greater width than the bimetallic elements, see Figure 11. Between the ends of these mica plates, and the vertical face of the block of metal 52 is inserted another insulating member 63 shaped to engage the element 52 and prevent its own lateral movement. The coil is compressively held between the mica plates by means of two metallic strips 65 engaging at opposite sides, and inserted in the slots 53 along with respective bimetallic elements. The strips 65 also assist in conduction of heat from the coil to the bimetallic elements. Although the combined thickness of the strips, mica plates, and coil is less than the normal distance between opposing active faces of the bimetallic elements 48-49, they have been shown, for convenience of illustration, as being the equal in thickness to that distance. The strips 65 actually flex toward one another. The

means of studs 67. These studs have relatively smaller cross-section to lessen heat conductivity at pointof connection, in order to affect the bimetallic element 44-as little as possible, when heat is applied by means of the coil 54.

The outer end of the upper bimetallic element 49 is recessed inwardly to receive the upper end of element or arm 36 and to provide projections 72, adapted in one position to rest upon shoul- 15 ders '73 of the element '36, and in another position to engage abutments or shoulders '74 of the element 36 to limit upward movement. A central projection is also provided, which abuttingly engages the inner surface of the element 36 when the instrument is positioned as shown in Figures 1, 2 and 3, but lies upon the top of the element 36 between the arms '77 when the element 49 is in the position shown in Figures 4', 5 and 6, these figures illustrating the .open position of the switch, while Figures 1, 2 and 3 illustrate the closed position in which the element 49 is set to rise in a timing movement.

,The upper limit position of the member 49 is shown in Figure 9, and, as before mentioned, when the element is in this position the switch cannot be closed by operation of the plunger 25. If these contacts were closed so that the heating element could be raised to a temperature above which it is.desired, the coil would bedestroyed .or injured.

It will be understood that when that feature of the invention relating to the closing of the switch is used, the circuit changes controlled by the contacts 1'734 is also the supply circuit for the coil so that under the conditions above mentioned, the contacts cannot be closed while the element 49 is at any position above the upper end of thearm 36. If, for example, the element was at a level indicated in Figure 8, and the

points

17 and 34 could be brought together, they could be mechanically held together during that time necessary for the element to cool to bring it to the level shown in either Figures 4 and 1. If the points were closed during this interval sufficient current would be supplied to burn out the coil.

In order to adjust the system so that the outer end of the element 49 occupies a proper relation relative to the upper end of element 36, and so that fora predetermined temperature rise, the element 36 can spring inwardly to open the switch. an adjusting screw 80, ,see Figure 10, is

provided. This screw has threaded engagement with a vertically disposed collar or sleeve 61, which, in turn, has threaded engagement with the shelf-like projection 60, see Figure l, at a point laterally of the connecting element 52,- and between that element and the point of attachment 45 of the element 44. The adjusting screw is headed, see Figure 10, and provides a shoulder which engages thetop of the sleeve 81 to positively prevent further adjustment in a downward-direction. The sleeve is held against movement by screw 83. I It will be seen by an inspection of Figures 1 and 3 that further downward adjustment of the outer end of the element 49 would result in unnatural flexing of the elements of the bimetallicsystem. The adjustment, however, can be made in any desired degree in an upward. direction. This is a feature of the in- The arrangement of the tripping element 49 is such that when the contacts are closed, the extension 36 tensionally engages the free end of the upper bimetallic element. However, only a small area of contact is had between the surfaces due to the small area of the, end of the projection 75. It will, moreover, be noted that the bimetallic element 49 is formed to be guided,

as well as limited in its movements, by element 36. When the element 49 rises from its position shown in Figure 1 sufllciently to occupy the position shown in Figure 4, the switch will be opened as the result of the inward springing of the element 36. After this has taken place, the element 36 will lie beneath the element 49, as a stop, and

the switch cannot assume its normal position un- I til the plunger 25 is pushed upwardly to raise

elements

10, 15, 20 and 35, and swing arm 36 outwardly. As before stated, if the element 49 has not cooled sufficiently, and remainsin the position shown in Figure 8, the operation of the button or plunger 25 will not bring the contacts 1'7 and 34 together and, therefore, the coil is protected against over-heating. However, just as soon as the system of bimetallic elements have cooled sufflciently the upper element will drop and tensionally engage the upper end of the member 36 and then upon operation of the button 25, element 49 will fall to the position shown in Figure l holding the

contacts

17 and 34 together.

It will be noticed that the

upper elements

48 and 49 have the mostactive sides facing toward one another, in other words facing the heating element. The compensating element 44 has its active side 46 facing the inactive side of the adjacent element 48. Thus if the element 44 is heated, it will bend in drection away from the other element, while if the other elements are deflected as the result of heat furnished by the resistance element 54, they will move in direction away from the heating element and away from one another. It is understood that the deflection varies inversely as the square of the thickness, and directly as the square of the length, and that as the width of the elements increases a cross bend acts counter to the longtudinal bend. For this reason, the width of the elements is kept at the minimum allowable.

in order to .thoroughly understand how the switch is compensated so that the surrounding temperature will not affect it to make it inaccurate as a timing device, reference will be had to Figure? which illustrates only the action of the dev'ce, in response to temperature changes oher than those produced by the heating coil 54. The object of the diagram is to show that one of the bimetallic elements, in this instance the I element 49, acting as a switch control element, will remain substantially stationary for all changes in temperature except those created by the coil 54. For purposes of illustration, a temperature of 70 has been assumed to be the normal temperature of the inclosure or room in which the warp switch is tooperate. It has been further assumed, for example, that the element 49 open the contacts; when the coil has raised the will move upwardly sufficiently to release the element 36 and allow it to spring inwardly, to temperature of the

bimetallic elements

48 and 49, 50, which rise, added to the normal temperature, gives a total of 120. This, on the assumption that the instrument is designed andset to trip after such a rise in temperature (50), due to heat supplied by the coil. Figure A of the diagram represents thedevice set in correspondence to the position of the parts in Figures 1, 2 and 3, that is, disposed to operate as a timing device to open the

contacts

17 and 34, at the end of a predetermined time interval, when the elements 48-49 have risen 50 in temperature due to action of coil 54. In Figure B it is assumed that there has been a temperature rise of 30 in the room where the device is disposed. It is then obvious that if no compensation were allowed, the device would, when heat was applied to its coil, trip at a rise in temperature oi 20 only because,

if we add the rise of 30 to the normal 70 the' device is heated to 100, and slnce120 is the tripping point (50 plus '70) it is obvious that only 20 would be needed to trip. This would cause premature tripping and would lessen the time so that the device could not be accurate as a timer. However, it will be seen that due to the compensation, see Figure B, the upper element has remained at a constant level because elements 4448 have bent downwardly. Let us further assume that instead or a temperature rise, we have a temperature fall of 30, see Figure C or the diagram. In this instance if it were not for the compensation, a rise in temperature 0! 50- caused by the coil, would not move the tripping element to tripping position, because the temperature has fallen to 40 and this 40 added to the 50 would only make 90, instead of 120 necessary for trippins.

Among the features or the invention, therefore, are the following: The bimetallic element 49 controls the flexible contact element 93 hold the same under tension for release to automatically move to one circuit control position, in this instance to circuit breaking position, see Figure 1. The flexible contact element 93 through its extension 36, acts to latch the bimetallic element 49 to prevent complete re-fiexion, see Figure 4. The flexible element actsv also to limit the flexing movement of the bimetallic element in both directions, see Figures 3 and 9. The button 25 is only adaptedto move both flexible elements in a manner to separate the contacts and, it desired, to hold them separated, when the bimetal lic element is latching the flexible element, see Figure 1. The button cannot, however, cause the contacts to close when the bimetallic element is lying above (not touching) the flexible element. When the bimetallic element is engaging the flexible element, see Figure 4, (the latter acting as a stop) the button can be operated to release the automatically movable circuit control element for movement to reset the device or close the contacts, and also to allow the bimetallic element to assume that normal position, from which it starts to move for its timing cycle.

It is not intended herein to limit the invention to the movement of any particular number oi bimetallic eiementsor to the direction or such movements, or to have an arrangement in-which any particular number of elements act on the switch control element. For example, it is conceivable that the switchcontrol element can be 11c elements so that the differential movement of these elements will cause the tripping. Or one of these elements may engageone portion of one switch control element. In both cases, the arrangement is such that there is a differential movement between the elements, when subjected to outside temperatures which does not permit a tripping to take place. What is broadly claimed is, that differential movement of the blades releases a tripping or equivalent element to perform its function only under influence of heat applied to the coil and not at any other time, although movement of the blades in a certain manner takes place upon rise and fall of the temperature or thesurrounding medium. a

Figures 12 and 13, therefore, show different positions of the parts of a modification of the invention, in which a suitable base of insulation is provided. At the opposite sides of this base are bimetallic elements respectively designated 91--92. To one oi! the elements is pivoted as at 93, a latch 94. In this instance, this latch or trip element has its unpivoted end resting by gravity upon the free end or the other bimetallic element. The active sides of the elements are designated 95. In this instance, they both lie at the left of their corresponding inactive elements. The heating coil is represented at 96. It will be noted that the coil is arranged in opposition to the active 'side' 95 of the element 92. In this form of the device, we have an arrangement whereby the diiferent'i l deflections oi bimetallic ''element is obtaine Forexample, a temperature change in the surrounding medium will cause both elements to deflect always in the -same direction. On a rise of temperature, their outer free ends will both swing slightly to the right. 0n lowering oi. the temperature they will both swing to the left. It will be noted that inasmuch as under these conditions the distance A between the elements remain constant, the free end oi the latch 94 will remain resting upon the element 91. if, however, heat is applied through the coil 96 the element 92 will be deflected to a greater degree, resulting in an increase of distance represented at B, suilicientto allow the latch to fall and make contact at 97 to complete the circuit.

The invention, therefore, in its broader aspects contemplates the arrangement of bimetallic blades so that one or more of the blades coning the device, but in a manner to prevent operation of a switch control element or equivalent device unless heat is applied to the coil, and moreover, providing a device in which the blade movement which finally causes release or tripping, is oi a predetermined length, and only takes place when heat is applied to the coil, and regardless of the degrees of temperature change in the surrounding medium, and regardless of the degree or direction of flexion, due to temperature change in the surrounding medium.

In both illustrated forms or the invention, a constant space is being maintained either between-the free end oi one of the elements, and the switch control arm or latch, or between two 01' the elements and this latch, the arrangement of the elements being such asto obtain what may be called a dii'ierential compensation between the blades, so that the blades are free to move upon change 01.- temperature in the surand only being the controlling means, for obtaining control release or the latch or equivalent device.

I claim as my invention:

1. A warp switch having three bimetallic blades connected together so that one of the blades has an end free to move, a coil for heating two of the blades, said blades having their most active sides faced in a manner to substantiallyprevent movement of the free end of the element, as the result of temperature changes other than those produced by its coil.

2. A warp switch comprising a bimetallic element attached at one end to a support, a second bimetallic element arranged in spaced opposition to the first and attached to the free end of the first, a third bimetallic element arranged in spaced opposition to the second and attached to the free end of the second, and a heating coil between the second and third elements.

3. A warp switch comprising a bimetallic element attached at one end to a support, a second bimetallic element arranged in spaced opposition to the first and attached by one end to the free end of the first, a third bimetallic element arranged in spaced opposition to the second and attached by one end to the free end of the second, and a heating coil between the second andthird elements.

4. A warp switch comprising a bimetallic element attached at one end to a support, a second bimetallic element arranged in spaced opposition to the first and attached to the free end of the first, a' third bimetallic element arranged in spaced opposition to the second and attached to the free end of the second, and a heating coil between the second and .third elements, the active sides of the elements being so arranged that the free end of the third element remains substantially stationary during temperature changes in the medium surrounding the elements.

5. A warp switch comprising a bimetallic element attached at one end to a support, a second bimetallic element arranged in spaced opposition to the first and attached to the free end of the first, a third bimetallic element arranged in spaced opposition to the second and attached to the free end of the second, and a heating coil between the second and third elements, the means attaching the first and second elements having a reduced cross-section to reduce heat conduction, and means attaching the second and third elements being of a character and of a size to increase radiation.

6. A warp switch comprising a bimetallic element attached at one end to a support, a second bimetallic element arranged in spaced opposition to the first and attached to the free end of the first, a third bimetallic element arranged in spaced opposition to the second and attached to the free end of the second, and a heating coil 'betweenthe second and third elements, the most active sides of the elements being so arranged that the free end of the third element remains substantially stationary during temperature changes in the medium surrounding the elements, the means attaching the first and second elements having a reduced cross-section to reduce heat conduction and means attaching the second and third elements being of a character and of a size to increase radiation.

'1. A warp switch comprising a bimetallic element attached at one end'only to a support, ,a second bimetallic element arranged in spaced opposition to the first, spaced posts connecting the free end of the first with one end of the second, said postshaving a small cross-section and being composed of material which is a poor conductor of heat, a third bimetallic element ar-.

elements, said posts having a small cross-section and being relatively poor conductors of heat, a third bimetallic element arranged in, spaced opposition to the second, and. a metallic element connecting the second and third elements, and being of such a size and substance as to increase radiation, and a heating coil arranged between the second and third elements, the free end of the third element being at that end opposite the point of attachment of the first element with its support.

9. A device of the class described comprising three bimetallic members connected in spaced relation, two of the members having a coil for heating them, said members having their most active sides so related that one of the members will remain substantially stationary for all changes of temperature except those created by the heating coil.

10. A device of the class described comprising three bimetallic members connected in spaced relation, two of the members having a coil'for heating them, the most active sides of those members associated with the coil being opposingly faced, and the most active side of the remaining member being opposed to the most inactive side of one of the members.

11. A switch comprising two fiexible contact elements one arranged to move under tension to automatically control the circuit, a bimetallic element having a heating coil'for causing it to flex, said bimetallic element having a free end arranged to hold the said contact element to prevent automatic control, said tensioned element having an extension by means of which motion can be imparted to it as the result of movement of the other element, for releasing the bimetallic element and permitting circuit control movement, the arrangement being such that if the bimetallic element has not cooled sufficiently the contacts cannot be brought to circuit control position.

12. A pair of flexible metallic circuit control elements arranged in spaced opposition and having corresponding ends each separately secured by a binding post, and having free ends cooperative to control a circuit, one of the elements being adapted to move out .of contact with the other, to break the circuit, one of said elements having an extension spaced from the other element when the elements are in circuit making position, a bimetallic member having a heating coil for causing it to fiex, one of said circuit control elements normally tensionally engaging the released to permitmovement of the contacts to another circuit control position, when the bimetallic element flexes sufliciently, and means for moving the other contact element to engage the extension and force the tensioned element to restore tension engagement after such cooling or 6 the bimetallic element.

13. A warp switch including a bimetallic element having an end tree to move under change of temperature, a pair of opposingly spaced flexible contact elements cooperable to control a circuit, one arranged to move under tension to obtain automatic control, and having an extension engageable with the free end of the bimetallic ele- 'ment in a direction lengthwise oi the element to prevent automatic action, the extension and 16 the bimetallic element being so related that when said bimetallic element is flexed sufliciently the extension is permitted to engage with said extension as a stop to prevent return flexing movement, said automatically tensioned element having an extension which is spaced from its companion element when this companion element is in tensioned condition and which approaches the companion element when in tripped position, and means engaging the companion element to move it toward the other element in a manner to prevent closing of the contacts if the bimetallic element is not locked.

14. A warp switch having three bimetallic blades connected end to end in parallel relation, an element associated with the blades to be released when a predetermined movement of at least one of the blades occurs, a heating coil arranged to cause movement of at least one of the blades, the most active sides of the blades being so arranged that movement of the blades by temperatures other than the heating coil will fail to cause a release movement of said associated element. V

15. A warp switch having three bimetallic has an end tree to move, a coil for heating at least one or the blades, a switch associated with and operable as the result of movement of the blades, the most active sides of the blades being so a ranged that although the blades are moved as the result or temperature changes in the surrounding medium, switch operation does not take place except by application of heat through the heating coil.

16. A device of the class described, comprising three bimetallic elements connected end to end and a switch operating element associated therewith to be tripped upon movement of the elements in a certain manner and a heating device associated with the elements, the arrangement being such that the tripping element is not released except as the result of heat applied by the heating device.

17. A warp switch having three bimetallic blades arranged in substantially parallel relation, 8. switch control arm controlled by one of the blades and heating means arranged between two of the blades substantially in contact therewith.

18. A warp switch having three bimetallic blades connected end to end, and bymeans which substantially heat-insulates them, an element associated with the blades to be released when a predetermined movement of at least one oi. the blades occurs, heating means arranged to cause blades arranged so that at' least one of the blades movement of at least one of the blades, the most active sides of the blades being so arranged that the movement of the blades by temperatures other than the heating coil will tail to cause release movement of said associated elements.

19. A warp switch including a bimetallic element having an end free to move under change of temperature, a pair of flexible contact elements cooperable to control a circuit, one arranged to move under tension'to obtain automatic control and having means engageable with the free end of the bimetallic element to prevent automatic action, this means being so related to the bimetallic element that when said element is flexed suiiiciently said means is permitted to engage as a stop to prevent return flexing movement, said automatically tensioned element having means which is spaced from its companion element when this companion element is in tensioned condition, and which approaches the companion element when in tripped position.

20. A device of the class described having a plurality oi. bimetallic elements connected together, and with their active sides so related that a portion of one of the elements is adapted to be substantially immovable during changes in ambient temperature, switch control means with which the last mentioned element is cooperable to control switching action, and means for applying an additional amount of heat to the last 106 mentioned element, and heat absorbing means associated with the bimetallic elements for lengthening the time required to raise their temperature.

21. A device of the class describedincluding a 110 bimetallic blade, and switch control means with which the blade is slidingly cooperable to control switching action, said switch control means having means for limiting blade motion in both directions.

22. A warp switch having three bimetallic elements connected together so that each element has an end free to move, said elements having their most active sides so related as toisubstantially prevent movement of one of said free ends as the result of temperature changes equally atfecting all blades.

23. A warp switch comprising, a bimetallic ele ment attached at one end to a support, a second bimetallic element arranged in spaced opposition to the flrst and attached by one end to the free end of the first, and a third bimetallic element arranged in spaced opposition to the second and attached by one end to the free end of the secend, the active sides of the elements being so related as to substantially prevent substantial movement of the free end of one of the elements as the result of temperature changes equally atfecting all elements.

24. A device of the class described, comprising 5 at least three bimetallic elements so connected that each is free to move relative to the other, the most active faces of said elements being so related as to cause a portion of one of said elements to remain substantially stationary as a result of temperature changes equally aflecting all elements.

FREDERICK s. DENISON.