US2166238A - Movement-producing device - Google Patents

Description

y 18, 1939- 1.. K. DAVIS 2,166,238

MOVEMENT-PRODUCING DEVICE Original Filed April 6, 1935 2 Sheets-Sheet 1 INVENTOR [mm/n ff. fiawls ATTORNEYS July 18, 1939.

HOVEXENT PRODUC I NG DEV I CB Original Filed April 6, 19:55 2 Sheets-Sheet 2 INVENTOR [1km Patented July 1a, 1939 MOVEMENT-PRODUCING DEVICE Lincoln K. Davis, Brockton, Mass.

Application April 6, 1935, Serial No. 15,033 Renewed May 4, 1939 23 Claims.

This invention relates to movement-producing devices.

More particularly, the invention relates to movement-producing devices, such as thermostats, hygrostats'and the like, wherein a change in length of a control element, induced by a predetermined change in the physical condition of a substance or medium surrounding the control element, is employed to eflect movement of an operator controlling a connected device, such as an electric switch. a valve. an indicator, or the like.

In such devices, extreme sensitivity often is .required, in which event a very slight change in length of the control element must be caused to produce a much greater movement of the operator in order that the operator will be moved sufiiciently to actuate the associated device.

It also often is a desideratum in such devices thatthe control element be quickly responsive to change its length in accordance with a change in the physical condition of the surrounding substance or medium, in order that the operator will be moved promptly to effect the operation of the associated device within a short interval after the change in the physical condition of the substance.

In certain instances, it is desirable that the movement of the operator be accomplished by an accelerated movement, or snap-action, so that an acceleration may be given to the associated device, or that a positive indication of the operation will be given, or that an electric switch will be opened 'or closed quickly, and so forth. In order to accomplish this, the movement producing device must have a snap-action" mechanism therein, or must produce this effect inherently by, its construction."

The present invention relates to a device wherein an element,- such as a plate, frame, dish or the like, is formed with'a rigid or confined portion and a flexible or otherwise movable portion, and a second element is attached to the flexible or otherwise movable portion, and is responsive to change its length in accordance with a change in the nature of the surrounding medium to eflect a movement of the first-named element, or a portion thereof.

The invention also concerns a device wherein the element, which changes its length, is .of such form and size and is so attached to the other element that the device is quickly sensitive to changes in the substance or medium controlling its operation.

Another feature of the invention is the provision of a device of the above-described type,

wherein the movable element is so formed or supported as to have a position of instability, and which is movable by the other element from a position of stability into the position of instability from whence it moves, due to its own internal stresses,.to a-second positiofl of stability, whereby to provide a snap-action."

Other and special objects of the invention will appear from the appended description taken in connection with the drawings wherein like reference characters denote like parts.

Although the invention is herein disclosed with reference to several specific embodiments in order to clearly illustrate the same, it is not limited thereto, and the appended claims are to be read as-hroadly as is consistent with the prior art.

In the drawings accompanying and forming. part of this specification,.certain specific disclosure of the invention is made for purposes of explanation, but it will be understood that the details may be modified in various respects without departure from the broad aspect of the invention.

Fig. 1 is an isometric view showing a thermostat constructed according to the present invention,

Fig. 2 is an enlarged fragmentary view of a cross-section taken along-line 2--2 of Fig. 1,

Fig. 3 is a diagram illustrating certain principles of the invention,

Fig. 4 is another diagram illustrating further principles, v

Fig. 5 is a. top plan view showing a thermostat constructed according to the invention and adapted for use as an electric switch,

Fig. 6 is aside elevational view showing the device of Fig. 5,

Figs. 7, 8 and 9 are isometric views showing three modifications of the thermostat respectively, and

Figs. 10, 11 and 12 are enlarged fragmentary views showing respectively modifications of the method of attaching the wires to the frame, and

Fig. 13 is a diagrammatic view representing a section taken along a diagonal of a modified form of thermostat.

.corner portions 4 of the frame, thus permitting fiexure of the frame along its diagonals.

Portions of the sheet from which the frame is formed are cut out to provide a cruciform strap 5 integral with the frame 2, having its arms 8 integrally connected with the mid-portions of the sides of the frame 2 respectively, and its central portion I offset to place the arms 8 under a tensile stress acting to draw the mid-points of opposite sides of the frame! closer together, for a purpose which will hereinafter be pointed out. Each of the arms 6 of the strap 5 is formed with a reduced portion 8 closely adjacent the juncture of the arm I3 and the sideof the frame 2 to increase the flexibility of the arm 6 relative to the frame 2.

Extending along a diagonal of the frame 2 is wire liIa, which is secured at its ends to one face of the frame at opposite corner portions. The wire Illa is attached to. the frame I by suitable 'means, forzexampie, soldering or brazing IM,

and is under tension as will later' be explained. A second wire Ila, having a lesser unit length thermal expansibility than that of the wire Illa, is attached to the other face of the frame at the corner portions in a manner analogous to the first wire Illa, by soidering'or welding Iia. A second pair of. wires lIib and I Ib, similar to the wires Illa and Na, are connected between the other two corner portions for a purpose which will appear hereinafter.

If an initially planar sheet or frame of resilient material, having a discontinuouslystiffened peripheral portion providing two pairs of oppo-- sitely disposed stiifenedsldes and two pairs of flexible portions intermediate the stiffened sides,-

is stressed to force the oppositely disposed stiffened sides close to one another, the sheet or frame will not remain planar but will warp with opposite curvature along intersecting lines extending between opposite flexible portions, to lie in the surface of an hyperbolic paraboloid.

This will be seen from an inspection of the diagram (Fig. 3) wherein the frame 2 is represented diagrammatically as a surface. If opposing forces are applied to the midpoints W and Y of opposite sides respectively of the frame 2 (or at the points X and Z, or at points W, X, Y and Z) the frame will be warped into either the position A'B'C'D' or A"B"C"D", and will lie in the surface of an hyperbolic 'paraboloid, In either of these positions the frame will be curved oppositely along its diagonals which become parahoias, for example, in the position A'B'C'D, the frame is curved in one direction about A'C' and in the opposite sense about BD'. As viewed in the diagram, the frame is concave along BD', and convex along A'C'.

This warping results from the fact that when the sides are forced toward one another, unrelieved stresses are set up in the frame due to the rigidity of the side portions, and the frame is in a position of instability. The unrelleved stresses force the frame to assume the warped position A'B'C'D' or A"BC"D". If the frame is in the position A'B'C'D, for example, and a force is applied which is slightly greater than necessary to return the frame to the initial position ABCD, unrelleved stresses are set up which prevent the frame from remaining in this position, and the frame is moved rapidly through the position of instability ABC!) to the other position of stability Alinllcllnli.

This abrupt change of curvature as the frame passes through the position of instability or "dead center" position permits the unrelleved stress to come into play and move the frame from the position of instability to the second position of stability without the aid of any external force. Thus a snap-action of the frame is provided,

If the thermostat I (Fig. 1), shown inits position of instability, is left free to assume a position of stability, the tension on the arms 8 draws the mid-points of the sides closer together, and the frame 2 is warpedto a position wherein it is curved oppositely along the diagonals. The wires Illa and I la are under the tension initially applied when they are attached to the frame 2. 10

Assuming the thermostat I to be in a position corresponding to the position A'B'CD of the diagram, an increase in the temperature of the air surrounding the thermostat causes the wires Illa and Na to increase their lengths; the frame also expands to increase the distance between the points of attachment of the wires Illa, Ila. Since the wire Illa has a greater unit length thermal expansibility, it will elongate more than the wire Ila, and thus'the stress which the wire Ilia exerts 20 on the frame to bow it along the diagonal BD' becomes less than the stress exerted by the wire Ila, tending to bow the frame in the opposite sense along the diagonal.

The wires Ila and Na are displaced a distance 25 In from a median surface (indicated-by MN in F152) passlngthrough the frame and parallel to its faces, and exert opposite moments of force upon the frame tending to bend it about the median surface; the moment of force in each case 30 is the tension on the respective wire times the distance 1:. -'l'hus, when the temperature increases, the stress on the re Ifla. tending to bow the frame about the diagonal BD' to a posi-' tion A'B'C'D', becomes insuilicient to balance the 35 sum of the stress exerted by the frame 2 and by the wire Ila tending to return the frame tothe position ABCD, and the frame moves toward the latter position. when the temperature increases to a value where the force on the wire IIa -be- 40 comes enough greater than the force on the wire Ila to overcome the stresses in the frame, due to the warping and to the tension of .the wire Ilia, the frame is moved to a position past the position of instability ABCD. As the frame is moved 45 through the position ABCD, the stresses in the frame undergo a reversalin direction, and abruptly reverse the direction of curvature of the frame. A rapid movement of the frame into the position A"B"C"D" is effected by thestresses in the 56 frame and the tension exerted by the wire Ila.

when the temperature decreases, the force exerted upon the wire Illa increases more rapidly than the force on the wire Ila, and the frame is moved back toward the position ABCD, and when the temperature drops to a predetermined value, the frame is moved through the latter position and mapped to the position A'B'C'D'.

The action of the wires I lb and III; is similar to the action of the-wires Ila and Ila, but, since 0 the frame curves along the diagonal AC with a curvature opposite to the curvature along the diagonal BD, the action of the wires lb and Nb, when the temperature increases, is reversed in direction; That is, the wire Ilb expands more as rapidly than the wire I"), and moves the corner portions from the positions A'C' to A"C". when the temperature again drops. the corner portions are returned to the positions A'C. Either the pair of wires Ila, He, or the pair Ilib, iIb, may be 70 omitted and the thermostat will operate satisfactorilybut it is preferably to employ both pairs in order to enhance the force on the frame and to prevent undesirable distortion of the frame due"- t0 unbalanced forces. 7t

Where one or more of the wires has the same expansibility as the frame, that wire or those wires maybe omitted, and the thermostat will operate with a snap action, butin a slightly different manner than where two wires'of two different expansibilities are employed.

In this arrangement (Fig. 13) a frame 1! similar to the frame 2 may be used, and a wire I,

or wire lta, lib of similar expansibilities attached at opposite corners. The wires "a, lib may be attached in'the same manner as the wires in Fig.

I into a position of opposite curvature. The wire I641 may be assumed to exert its tension upon the frame with a'moment having an arm of length 9, equal to the distance'of the wire from the medial plane EF passing parallel to the faces of the frame when the frame is flat.

Assuming the wire lid to have a greater expansibility than the frame It, upon an increase in temperature, the force moment, exerted by the wire, tending to flatten the frame I! becomes less than the moment tending to movethe frame to a position of greater curvatura- As the frame moves toward a position of greater curvature, the length g becomes progressively less, and the force moment tending to. move the frame into flat position becomes progressively less. Also, the internal stresses tending to curve the frame I5 increase as the frame l5 becomes increasingly curved, and hence the resultant force, tending to move the frame to a position of greater curvature, progressively increases, and. an accelerating movement or snap action results.

Assumingthe frame l5 to be in its position of greatest curvature, upon a decrease in temperature, the wire iSa contracts and moves the frame through its position of instability, whereafter the increase in the length a and decrease in internal stress in the frame produce a. progressive increase of the moment tending to flatten the frame, over the moment tending to resist flattening, and the frame is snapped toward flat position.

It will be seen that, where this type of thermostat is used, it will be necessary to provide suitable stops, indicated diagrammatically. at ll, to

.but rock about their midpoints W, x, Y, Z, there is no change in curvature of the frame along the lines WY and KL and there is a maximum change in curvature along the diagonals AC and BD. The curvature along the line, such as DP, extending between a corner D and a point P,

lying between an adjacent corner A and a midpoint X, is less than the-curvature along the diagonal, and hence the portion of the frame along this line DP undergoes a lesser change in curvature than along the line BD when the frame is moved between the positions ABCD and A"B"C"D". Similarly, lines parallel to DP, as for instance RQ, undergo the same change in curvature as DP.

Thus, if wires are connected between points on the frame corresponding to the points-R and Q, a small ch ange in length thereof will produce the same movement of the frame as a greater change in length of a wire connected between B and D,

. as are wires [0a and Ila.

A'thermostat constructed according to the present invention is well adapted for useas a component of a thermally actuated switch.

Referring to Fig. 5, the switch 20 comprises an insulating base 2| having a pair ofupstanding studs 22 each carrying a blade 23 having a notch 24 'for. receiving outer edges of adjacent sides of a thermostat 25. A third blade 26 is secured to the base 2lby a screw 21, and has a notch 28 for engaging the inner edge of the corner of the frame 29 of the thermostat between the edges engaged by the blade 22.

The thermostat- 25 comprises the rectangular metal frame 29 having a cruciform central portion 30 for warping the frame 29 as above described and having portions of each side edge turned'up to constitute stiffening flanges 3i the corner portions of the frame being flexible. The ends of a wire 32 are attached respectively to points on opposite sides of the frame 29, and lying on opposite sides of a line through the mid-points of the sides, a wire 33 having a similar unit length thermal expansibility, is attached between the other two sides in an analogousmanner. The wires 22 and 23 are secured to the frame 29 by rotatable pins 34 with slotted heads by which the tension on the wires can be adjusted.

The. comer of the frame 29 carries a contact button 35 which ispositioned to move between an adjustable contact member 36, threaded into the base 2!, and an adjustable contact member 31 supported from an upstanding terminal 38 secured to the base 2|. Suitable conductors 39, 40 and H are connected to the frame 29, the contact member 36 and the terminal 38, respectively.

The thermally actuatedswitch 20 may be employed to control the opening and closing of an electric circuit in accordance with changes in temperature. When the device is to be used to open an electric circuit upon a predetermined change in temperature, the conductors 38 and N are connected to a circuit and the frame 29 or contacts 38, 31, or both adjusted to bring the contact button 25 into engagement with the contact member 28. The current flows through the frame 28 and the contact button 35 to the contact member 26.

wires 22, II and frame 29 expand at different rates, and, if the frame 28 has a greater expansiwhen the temperature of the atmosphere surrounding the switch increases, the

bility than the wires 32 and 23, then when a.

predetermined temperature is attained, the frame 28 is moved in a direction to cause the contact.

button 28 to move rapidly out of engagement with the contact member 30 and through the position of instability, and against the contact 31. The circuit is broken when the contact button It isseparated from the contact member 36.

When .the temperature again drops to the predeterplated by the present invention, it will be obvious that any of the other forms of the thermostat herein disclosed may be employed in the construction of a switch of the same general form as the one disclosed.

Where it is desired to exert a stronger force upon the frame than is conveniently possible with a single pair of wires of different expansibilities, two sets of wires may be employed, each set including a plurality of wires of the same expansibility and wherein the wires of each set have different expansibilities. v

Referring to Fig. 7, a frame 50 having flanges II formed along the sides to stiffen the sides, except at the corner portions, has attached thereto two wire screens 2 and 53 each being attached to opposite faces of the frame 50. In the screen It, the wires 4 constituting the. "warp" of the screen have similar unit length thermal expansibiiities, and the wires II constituting the "woof" of the screen have similar expansibilities but different from that of the wires N. In the screen 53, the warp 51 has the same expansibility as the "woof" 85 of screen 52, and the "woof" 56 the same as the warp" 54.

The frame when in use is suitably compressed between one or more pairs of equal and opposite foroes applied to sides to warp'the frame in the manner aforementioned.

Instead of a frame, an imperforate plate may be used, as illustrated in Fig. 8. In this form an imperforate rectangular plate .00 has its edge portions stiffened, as by a flange GI except at the corners. A plurality of wires "-69 are connected between pairs of points, of which the points he in opposite sides of a diagonal. The wires 2-85 have a similar unit length thermal expansibility which is different from that of the wires 06-. Similarly the wires 10-13 and the wires If-11 are connected to the other face of the plate. The wires 6245 and III-I3 have the same expansibility, and the wires 66- and ll-I1 have the same expansibility. The plate 60 is compressed between equal and opposite forces applied equally on both pairs of m'idpoints of the sides in a manner analogous to the forces W. X, Y and Z, indicated in Fig; 2 to warp the plate oppositely about its diagonals. When a predetermined temperature change occurs, the wires and plate change their respective dimensions at different rates, and the curvature of the plate is abruptly reversed.

In another form (Fig. 9) a strap or bar may be substituted for the wires, and may be placed under either a compressive stress or a tensile stress. A plate ll is suitably stiffened along a portion of each edge portion by beads II, and a bar or strap 02 having a different unit length thermal expansibility than the plate is secured to the plate I along a diagonal. Similarly, asecondstrap 83, having a similar expansibility to the first strap, may be secured to the other face along the'other diagonal. The plate]. is compressed between equal and opposite forces applied to mid-points of the sides in a manner similar to that above disclosed to warp the plate III. At temperatures below the predetermined operating temperature, the plate is bowed about its diagonals in oppo-- site senses. Upon the occurrence of a predetermined temperature change, the straps I3, 84 and the adjacent portion of the plate change their lengths unequally, and the plate is warped in the reverse sense. A slightly different construction may employ a rod, or other stiff member may be substituted for the strap shown.

amazes permitting the wires to be set close to the frame.

Where the wires are disposed in opposite faces of the flat frame, the center lines will be separated a distance equal to the thickness of the frame plus the sums of the radii of the two wires. The wires may be brought closer together by disposing them on the same side of the frame, but provision must be made so that the center-lines of the wires lie in opposite sides of the median surface of the frame in order to obtain the differential actionof the wires. The differential action may be accomplished with both wires on the same side of the frame by offsetting the portion of the frame to which the ends of the wires are attached, so that the median surface passes between the axes of the wires.

Inone form, (Fig. 10) a frame portion SI is provided with an oifset portion SI, to which a pair of wires 92 is attached at one portion, and

' a wire 98 is attached to another portion. The

frame portion 90 is so formed and positioned with respect to the frame (not shown), that the median surface of the frame passes between the center of the wires 82 and 93. Thus, when a predetermined change of temperature occurs, the wires 92, 93 act to warp the frame (not shown), in the manner described previously. Where wires are attached to the same face of a frame, and they are disposed side by side, as shown, it is preferable to provide two wires 92 having the same unit length thermal expansibility and a third wire 93 having a different expansibility disposed between the two wires 92. Thus, the forces tending to impart undesirable distortion to the frame arebalanced.

Another mode (Fig, 11) of attaching the wires consists in forming a frame I00 with a flat offset portion IM to which the wires are attached. A pair of similar wires I02 are attached to the offset portion IOI and a wire I03 having a different expansibility and,a greater diameter is attached to the frame I00 between the wires I02. The offset portion I 0I of the frame I00 is sufllciently offset, and the relative diameters of the wires I02 and I03 so selected, that the median surface of the frame I00 passes between the plane of the axes of the wires I02 and the axis of the wire I";

As stated in the foregoing, the end of the wires may be attached by various means such as by the pins shown in connection with the thermal switch, by rivets or eyelets, by soldering or brazing, or by other means.

Where the ends of the wires are attached by soldering or brazing, it is often desirable to provide means for adjusting the tension on the wires. This may be done by constructing a frame IIO (Fig. 12) having an ear III to which a wire H2 is soldered. The tension on the wiremay be adjusted by bending the ear III toward or away from the plane of the frame II 0.

Instead of the wire or wires being'connected at both ends to the frame, one end only may be connected to a suitablepoint on the frame, and the other end connected to some point on a member other than the frame; the frame and member will, of course, be suitably mounted and the fixed 76 point so positioned relative to the frame that the effect a movement of the frame upon a predetermined temperature change.

The principle of a warped frame moved between two positions of stability by a control member, which changes its length in accordance with a change in the condition of the medium surrounding the control member, is capable of wide application.

A device constructed according to the principles of the present invention may be employed to indicate the presence of certain gases, or to operate other mechanisms when such gases sur- -round it. A frame and wire structure, construct- 'ed according to the principles above-disclosed,

is provided with a wire or wires having a coating of spongy platinum, or the like, which becomes heated in the presence of certain gases. When the gas or gases surround the wire or wires in suillcient concentration, the wire is heated and effects movement of the frame to indicate the presence of the gases, or operate the associated mechanism.

The invention also contemplates use of the principle of the warped sheet and expansible member in the construction -of hygrostats. A

hygrostat may be constructed in a manneranalogous to the thermostat by substituting hygroscopic members, such as catgut filaments. for eifecting a movement of the frame upon a predetermined change in the relative humidity of the atmosphere surrounding the hygr'ostat. As will be understood, a, change in the moisture content of the filaments changes their lengths, and thereby changes the pull exerted on the frame to move the latter. Thus, the movement of the frame may be utilized to indicate a predetermined change in relative humidity, or to operate associated apparatus.

The present invention provides a movementproducing device wherein an element is moved in accordance with a change in the length of an attached control element induced by a change in temperature, gaseous composition, relative humidity or other changes in the condition of the medium surrounding the control element.

In such a device a very small change in length of the control element effects a correspondingly greater movement of the other element. The control element, being in most cases very small in volume, responds quickly to change its length when the predetermined stimulus occurs. This is especially true in the case of the thermostat, inasmuch as the wire is of small cross-section, and is in contact with the frame over a'small cooled in accordance with a change in tempera- I ture.

The device, in most of its forms, inherently provides a snap-action" which is desirable in many instances such as where it is desired to indicate that the device has operated or where a rapid movement of the frame into the final position is desired, or the like.

One advantage possessed by a device constructed' according to the present invention is thateach of the various elements of the device is relatively complete within itself. and hence each element may be bent, drawn, formed. heat-treated or otherwise operated upon separately prior to attachment to the other elements. Thus, one element may be given a treatment which might adversely affect the other elements were they so formed and joined that the treatment could only be carried out upon all of the elements simultaneously.

Other advantages possessed by the device are that it is simple and inexpensive to construct,

' rugged and reliable in operation, and lends itself to a large number of modifications and adaptations.

It will be understood that while the various forms of the invention have principally been 11- lustrated as using wires of small cross-section and capable of exerting a tension only, the wires may be replaced by rods capable-of exerting either a push or a pullupon the. associated member or members. v

It is also to be understood that where the term diagonal" is used it is not to be limited to mean a line joining two corners of a geometrical figure, but is to be understood as designating a line which is non-parallel with respect to the sides of a figure, or more particularly, non-parallel to the rigid portions of a frame or sheet element.

While certain novel. features of the invention have been disclosed and are pointed out in the annexed claims, it will be understood that various omissions, substitutions "and changes may be made by those skilled in the art without depart- 'ing from the spirit of the invention.

What is claimed is:

1. A resilient flexible element rigid along discontinuous portions and initially stressed to curve it oppositely about lines extending between nonadjacent flexible portions, and a second element secured to said first element at portions lying on opposite 'sides of oneof the lines about which said first element is, curved, said second element being responsive to a predetermined change in temperature to change its length, and effect an abrupt change of curvature of said first element.

2. A resilient frame initially stressed to warp it oppositely about two intersecting lines'extending along the surface between points on the periphery thereof, and a second element connected between portions of the periphery lying on opposite sides of one of the lines of curvature and responsive to a change in length to reverse the direction of the 4 along its diagonals and having rigid portions ex- 0 tending between the diagonals, means for warping said element in opposite senses about its respective diagonals, a second element having a different thermal expansibility than said first element,

- and attached to said first element at portions on opposite sides of a diagonal, said second element being responsive to a predetermined change in temperature to effect a change of curvature of .said first element about its diagonals.

4. In combination, a resilient element having spaced rigid portions, and stressed to flex it about its diagonals, and a second element connected between spaced portions of said first element on opposite sides of a diagonal, said second element having a thermal expansibility such that, upon a predetermined temperature change, said first element is moved through a position of unstable equilibrium to a position of fiexure different from the ilexure of its initial position.

5. A device for producing a snap-action movement upon a change in the condition of the medium surrounding the device, comprising an element having flexible diagonal portions, and rigid portions extending between the diagonal portions, said element being initially stressed to warp it 6 I amazes .to effect an abrupt change of curvature of said V first element. 1

6. A thermostat comprising a resilient element having rigid side portions and flexible along its diagonals and curved about its diagonals in opposite senses, a second and thermally 'expansible element, and means connecting said expansible element to said resilient element for effecting an abrupt change of curvature of said first element upon a thermally-induced predetermined change in the length of said expansible element.

7. A thermostat comprising a resilient element having spaced, stiffened side portions and stressed to curve it concavely about one diagonal and convexly about the other diagonal, a plurality of elongate elements having different thermal expanslbilities respectively, and means connecting each of said elongate elements to portions of said resilient element on opposite sides of a diagonal, said elongate elements being of such expanslbility relative to each other and to the resilient element as to produce an abrupt change of curvature of said resilient element upon a predetermined temperature change.

8. A thermostat comprising an element having two pairs of oppositely disposed rigid portions and flexible portions therebetween, means stressing said element to curve it oppositely along lines extending between said flexible portions, a second element having a unit length thermal expansi- .bi1ity difierent than that of the first element, and

means attaching said second element to portions of said first element lying on opposite sides of one of said lines of curvature, said second element being stressed to effect an abrupt change of curvature of said first element when a predetermined change in temperature effects a predetermined change in the relative stresses on said elements.

9. A thermostat comprising a plate-hire member having at least two angularly disposed rigid portions and flexible portions intermediate said rigid portions, means for initially stressing said element to warp it about lines extending between the flexible portions, and a plurality of members secured between portions of said first member lying on opposite sides of oneof said lines of curvature, said second-named members being stressed whereby, upon a predetermined change in temperature, they alter the angularity of said-rigid portions to effect an abrupt change of curvature of said plate-like member about the lines of curvature.

10. A thermostat comprising a plate-like element of flexible material, means stiffening spaced portions of said element against bending, means warping said element about lines extending between opposite non-stiffened portions and a second element secured continuously to said first element and extending between portions of the first element lying on opposite sides of one of said lines of curvature, said second element being active upon a predetermined temperature change to move the first-named element through a position of instability to a position of different curvature from the initial position. 7

11. A thermally actuated snap switch, comprising an element having relatively rigid portions relatively movable to varying positions of singularity, means establishing a toggle action of said relatively movable portions, a contact carried by a movable portion of said element, and a second and homogeneously thermally expansible element connected to at least one of said movable portions, and active upon a thermally induced change in the relative dimensions of said elements to effect a snap-acting actuation of said contact-carrying portion, and 'means for passing an electric current through at least one of said thermostatic elements.

12. A thermostat comprising an originally planar element having alternate rigid and fiex ible peripheral portions, means urging at least two of said rigid portions toward one another to warp said element about intersecting lines of curvature, and means for abruptly reversing the direction of curvature of said element upon a predetermined change in temperature including a pair of elongate elements having non-similar thermal expansibllities, each of said elements being secured at its ends to portions of said firstnamed elements on opposite sides of one of said lines of curvature, the axes of said elongate members being spaced from a medial surface passing parallel to the faces of the first-named element, whereby the differential forces applied to said first-named element by said elongate members upon a change in temperature bend said first-named element about said lines of curvature.

13. In combination, a thin rectilinear frame member formed of flexible resilient material stiffened along discontinuous portions to provide rigid side portions and flexible corner portions, means stressing said member to curve it in the opposite senses along its diagonals, an elongate member connected at its end to portions on one face of the rectilinear member lying on opposite sides of a. diagonal thereof, a second elongate member connected at its ends to the other face of said rectilinear member, the thermal expansibilities of said elongate members, relative to said frame member, being such that, upon a predetermined change in temperature, said elongate members exert a stress upon said rectilinear member tochange the curvature thereof about its diagonals.

14. A snap-acting device comprising a resilient snap-acting frame initially stressed to warp it about intersecting lines extending along the surface defined by the frame element, and an element connected between portions of said frame lying on opposite sides of at least one of said lines of curvature, said frame and connected element being responsive to a relative change in their dimensions to effect an abrupt change of curvature of said frame.

15. A snap-acting element comprising a frame having relatively rigid rectilinear portions connected by relatively flexible, resilient connecting portions, and means stressingsaid frame to warp it into a position wherein said rigid portions lie substantially in a hyperbolic paraboloidal surface, whereby said frame has at least one position of stability to which the frame tends to return if warped therefrom.

16. In a measuring instrument, a first member of substantially homogeneous material, a second member of material substantially homo geneous but dissimilar to the material forming said first member, said second member being secured to said first member at spaced points and unsecured between said points, the stresses on greases ssidmembersbeingsoarrangedthatsaidfirst member, upon relative change in length through a given value, will snap from one position to another.

17.-In a snap-acting device, in combination,

a first element having at least a portion thereof I connected at the corners thereof, a force-applying member foryapplying atom to opposite-sides of said frame ,to distort said frame, and an active temperature-responsive element secured between relatively movable portions of said frame.

19. In ameasuring instrument, a quadrilateral frame having comparatively stifi. sides articulately connected at the corners of the frame, and a transverse strap connecting at least two sides of said frame, the tension of said strap beingincreased when said frame is moved from a limiting position.

20, In a movement-producing device, in combination, a first element, means establishing a displacement stress characteristic of said first element varying from zero internal displacing stress and maximum normal displacement through maximum internal displacing stress to zero internal displacing stress and zero displacement, and a second element-attached to at least one movable portion of said first element and exertinga stress moment upon said first elegmentwhichisaeroatthedisplacementcorreand relatively movable sponding to .the maximum displacing stress of said first element, and which varies with the displacement of the point of attachment between said elements and the effective relative dimensions of said elements, whereby, upon a-predetermined change in relative dimension of said elements, said first element is moved through a position of unstable equilibrium, and into a position of relative stability, with a snap-action.

21. A movement-producing device comprising an initially planar element having relatively rigid peripheral portions, with relatively flexible portions therebetween, means subjecting said element to forces tending to urge opposite of said rigid portions towards one another, whereby said element tends to warp out of flatness into con-' formation with a substantially hyperbolic paraboloidal surface.

22- .A measuring instrument including a frame having quadrilaterally arranged, relatively rigid portions, connecting portions Joining the ends of adjacent of said rigid portions and permitting relative movement of said rigid portions, a flexible portionconnecting opposite of said rigid portions intermediate the ends thereof, said rigid portions and saidfiexible portions together approximately defining a surface, and means stressing said flexible portions whereby to cause said frame to warp oppositely about intersecting lines on said surface.

23. A measuring instrument including a. frame having quadrilaterally arranged, relatively rigid sides, relatively flexible corner portions connecting the ends of adjacent sides, a cruciform strap portion having each of its intersecting arms extending between points intermediate the ends of opposite sides of the frame, and means stressing said strap whereby to cause said frame to assume an anticlastic form.

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