US2338474A - Thermostat - Google Patents

Description

. H. M. WILSON THERMOSTAT Jan. 4, 1944.

Filed Dec. 4, 1940 2 Sheets-Sheet l FIGZ.

J 1944- H. M. WILSON 2,338,474

THERMOS'I'AT Filed Dec. 4, 1940 2 Sheets-:Sheet 2 FIG] 0. 8 a 2 &

Patented Jan. 4, 1944 UNITED STATES PATENT. ()FFI CE THERMOSTAT Harold M. Wilson, Attleboro, Mass, assignor to Metals & Controls Corporation, Attleboro, Mass, a corporation of Massachusetts Application December 4, 1940, Serial No. 368,472 29 Claims. (or. 200-113) This invention relates to contact-actuating mechanisms, and more particularly to structures employing thermally-responsive means for such actuation.

Among the objects of this invention may be noted the provision of means for giving a differential to a control device; the provision of means for preventing electrical contacts in switch structures from remaining at or near the point of zero contact pressure; the provision of means for giving electrical contacts or other controlled elements more positive action; the provisionofthe combination of a prime mover with a thermallyresponsive element which is responsive to heat controlled by the prime mover; the provision of apontact-operating means by which the speed of opening the contacts is increased, and by which the contact pressure is increased when closed; the provision ofa thermostatic mechanism which has a positive temperature differential induced by the gradual motion of it constituent parts; the provision of a simple thermostatic switch which is less subject to chattering, arcing, continuous low contact pressure, etc.; the provision of improved means for separating and closing electrical contacts; the provision of a thermostatic structure utilizing more than one gradualacting thermostatic element. for the purpose of producing a single device having a positive temperatur e differential; and the provision of an economical, simple and improved thermostat. Other objects will be in part apparent and in part pointed out hereinafter. I

The invention accordingly comprises the elements and combinations of elements, features of construction. and arrangements of parts which will be exemplified in the structures hereinafter described. and the scope of the application of which will be indicated in the following claims.

In the drawings in which are illustrated several possible embodiments of this invention:

Fig. 1 shows schematically a simple device in which the principles of the present invention are embodied in a thermostat;

Fig. 2 shows an alternative form of the Fig. 1 thermostat Fig. 3 shows an alternative arrangement of parts utilizing the principles of this invention:

Fi 4 shows another circuit and embodiment of the present invention, the latter in perspective;

. Fig. 52

Fig. 7 is an end view of the embodiment Fig. 5;

Fig. 8 is a cross-section along the lines 8'-8 of Fig. 6;

Fig, 9 is a schematic wiring diagram using the embodiment of Fig. 5;

Fig. 10 is an alternative form of the thermostatic element of the present invention;

Fig. 11 is still another form; and,

Fig, 12 i an additional form of the thermostatic element.

Similar reference characters indicate corresponding parts throughout the several views of the drawings.

In the thermostatic art, thermostats may be divided broadly into two classes: Those having a gradual motion in response to temperature change, and those having a snap motion in response to temperature change. The gradual type has the objection that arcing on contact separation is maintained longer than is desirable, par- 'ticularly on slow rates of temperature change.

Also it tends to chatter when subject to vibration. To overcome these faults, which give rise to static radio interference, burning of contacts, erratic performance, and other deleterious efl'ects, it has usually been customary to employ a so-called snap-action thermostat which has a positive temperature differential and abrupt contact action. The differential is generally obtained either by using with the thermally-responsive member a magnet, an overcenter toggle, or other snap mechanism; or by using a thermally-responsive structure in the form of a disc or other device inherently so shaped that its-change of shape with temperature is a discontinuous function of the temperature. These snap-acting thermostats are, generally speaking, more costly. The pre ent thermostat has the advantages of the gradual-acting .type, such as low cost, ease of design and calibration, plus, to a large extent, the advantages of the snap-type such as freedom from chattering and arcing, a controllable differential, ability to handle heavier loads, etc.

My invention, however, goes further than the field of thermostats. There are many devices, such as, for example. pressure-operated bellows, motor-operated switches, etc., in which the slow motion or other, behavior provided by the control element (bellows or motor, etc.) gives rise to some of the same troubles as are found in the gradual-acting thermostat. It is the purpose of my invention, therefore, to provide a means which can be used in various ways to improve the control of electrical contacts by prime movers,

etc., which provides a positive differential, which is economical and easy to apply,whlch will make it possible for slow-moving devices to open and close electrical switch contacts without detrimental arcing; which will prevent the lingering of the contacts at a point of zero contact pressure when the contacts make, and quicken the contact opening on the contact break, which employs a simple thermally-responsive element in its construction without the use of additional overcenter toggles, springs, releases, detents, etc., and which is rugged and has long life. For the purpose of illustrating and explaining my invention, I have used as embodiments of the principles thereof, thermostats and thermostatic elements. Obviously, in the place of the main control thermostatic element a cam with a follower could be substituted, or a pressure-actuated beilows, orother contact-moving means, depending on the application. The secondary element which provides the novel features of this invention would act in the same way as described below in my thermostats.

Basically, the thermostat embodiment of my invention employs a main thermostatic element which predominates and provides the temperature-controlling means, and a second which is mounted and arranged to be heated above its ambient temperature by heat induced by current flow when the contacts or the thermostat are engaged. The direction of movement of the second element is suchthat when the contacts are brought into engagement the immediately ensuing heating of the second element acts to promptly increase the contact pressure. This results in the first element having to move through a temperature differential to relieve this contact pressure before it can open the contacts. Immediately upon the contacts opening the heating of the second element stops and it cools. This cooling immediately increases the contact opening, thus giving a positive and quick action.

Referring now to Fig. 1, numeral I indicates schematically a thermostatic composite metal element, made for example of bimetal, fastened by one end to a support 2. Upon the other, or movable end of the composite metal element, is fastened a second thermostatic composite metal element 3, as at 4 by welding or riveting, etc. In this instance, element 3 is so constructed and arranged that the passage of electrical current therethrough heats it to a temperature above its ambient temperature, whereas element I is heated little or none at all by this current. Element 3 is also so proportioned that for the same temperature change its movement is small in proportion to that of element I. Composite metal element I is, therefore, the controlling element for a temperature change affecting both elements. Thermal element 3 carries a contact I on its end 3. Contact I cooperates with stationary contact 8 mounted on support 9. When heated, element I in this case moves in the direction indicated by arrow I0, and element 3 moves in the direction indicated by arrow II.

The operation of the device is as follows: Assuming that contacts I and 8 have just touched and that electrical current passes through both I and 3, then element 3 heats rapidly and I is heated little or none at all. The heating of 3 presses and holds the contacts I and 8 together. The closing of the contacts I and 8 energizes the heating means which the thermostat controls, which in turn raises the temperature of since it is above its ambient temperature.

the whole thermostat until element I moves far enough to separate contacts I and 3. The instant this occurs, the heating of element 3 by the current ceases and element 3 cools rapidly The direction of the i rriovement of composite metal element 3 on cooling is such as to rapidly increase the contact separation. If element I were used alone',, it might reach and linger at such a temperature that contact I was just barely touching contact 8; that is, a point of zero contact pressure, or at a point where the contacts were separated but arcing. However, with my thermostat, in order to separate the contacts, element I has to overcome the added tendency to bend of element 3. Then, when the contacts do separate, the unbending of element 3 as it cools to its ambient temperature, quickly and positively separates the contacts sufliciently far so that no further arcing or chattering can take place. Element 3 is preferably designed with a low heat mass so as to heat and cool very rapidly, eiiectively accelerating the separation of the contacts. This facilitates the above action by making the motion of element 2 rapid in its contact-opening action, and speeds its pressureincreasing action on contact-closing.

On the cooling part of the cycle, element I eventually cools down and moves contact I into engagement with contact 8. The contact pressure then is promptly increased by theaction or element 3 as described above. This completes the cycle.

In this embodiment I have shown element 3 as being thinner than element I. The purpose of this is to increase its electrical resistance so that under the influence of the electric current passing through both elements, 3 will heat more than I. This can be accomplished also by using a narrower strip, a strip of smaller cross-section, or a material having higher specific electrical resistivity.

Fig. 2' shows schematically an alternative method of using composite metallic elements I and 3, suitably made of bimetal, wherein bimetal 3 is fastened to the support 9 and carries contact 3 on its free end 5. Contact 1 is now III) carried by the end 6 of bimetal I. Both bimetals inthis instance move in the same direction (arrow I2) when heated. In this embodiment, when heat is first applied under the control of contacts' I and 8, bimetal 3 functions in the same manner as before, and tends to press contacts 1 and 8 together, Bimetal I eventually overcomes the movement of bimetal 3 and separates contacts I and 8. Immediately, bimetal 3 cools because of its -low heat mass, etc., thus sepa-. rating the contacts further with a quick positive motion.

Fig. 3 shows schematically an arrangement similar to Fig. 1, but with the inclusion of an auxiliary heater I3 0! low heat mass .in the electrical circuit to assist bimetal 3 in heating. Numeral I4 indicates a source of electrical power and I5 is a load to be controlled by the thermostat.

Fig. 4 shows schematically an arrangement of parts and a heating circuit wherein no current flows through the composite metal elements. In this embodiment I have shown a perspective view of the thermostatic elements in order to better illustrate the construction of the thermostat. Here bimetal strip I is fastened to the support 2 as in the other embodiments, and strip 3 is fastened by any well-known means, as before, to the end of strip I, with movement in the same relationship as in Fig. 1. Electrically insulated from strip 3 but securely carried thereby, is an electrically conducting, bridging-type, contact-carrying bar l6, with contacts IT and I8 carried at its ends.

Bar |6 serves as a bridging bar across the two circuit-controlling contacts l9 and 28. It will be observed that the bimetal elements I and 3 are now not included in the electrical circuit, but that the electrical contacts still control the heating of the bimetal elements. Numeral 2| represents an auxiliary heater of low heat mass, whose purpose is to heat the bimetal element 3. Numeral 22 represents the controlled heater means to whose heat the bimetal strip is responsive. When electrical current flows in the circuit, heater 2| heats bimetal 3 rapidly to move bar |6 in a downward direction to increase the contact pressure. Meanwhile bimetal I responds to the heat from heater means 22, and moves in a contact-opening direction, but must overcome the bend of bimetal 3 to separate the contacts I! and I8 from their respective cooperating contacts l9 and 2|]. When this happens, the heat leaves heater 2| rapidly because of its low heat mass, and bimetal 3 also cools rapidly because of its low heat mass. Consequently, the contact-carrying arm I6 is separated from contacts l9 and 28 rapidly, thus accomplishing a quick and positive breaking of the circuit. On the contact-closing part of the cycle, the instant the contacts touch, the contact pressure is promply increased by the action of bimetal 3, as has been heretofore described. This completes the cycle.

Figs, 5, 6, '7. and 8 show various views of an embodiment in which the features and principles mentioned above have been carried out in a further practical and useful way. The thermostat is designed to be used with commercial electric flatirons in such a manner that the composite metal control element is near the sole plate of the iron when the thermostat is installed, and thus is responsive totemperature thereof. Obviously, however, the thermostat can be used for other applications to control the heating of any object.

Referring to Fig. 5, the base 23 of the thermostat is made of sheet metal with flanges 24 to give it rigidity. Securely fastened to base 23 at 25 is a bracket 26. The fastening 25 may be by welding or riveting. Bracket 26 has some flexibillty for a purpose described later. Mounted on a shoulder 21 formed on bracket 26 is a bimetal member 28. Member 28 may be riveted or welded to 21, or fastened in any other way to give a rigid fastening. To the movable end of member 28 is secured transversely a second bimetal member 29. Member 29 is electrically insulated in the customary manner from member 28, by means of mica or other suitable washers and insulators indicated at 30. Riveted or welded'to the ends of bimetal member 29 are twomovable contact buttons 3| and 32. of the frame 23, but insulated therefrom by appropriate mica washers, etc., 30a, are stationary contact buttons 33 and 34 which cooperate respectively with buttons 3| and 32. Connected to buttons 33 and 34 are the respective screw terminals 35 and 36. Screwing into an extruded, threaded hole 31 in frame 23 is an adjusting screw 38. An extension 39 of screw 38 passes through a hole 48 in bimetal member 28, and is rotatably secured by means of an abutment 4|,

Mounted on the end' reduced portion 42, collar 43, and riveted-over head 44 to the free end of bracket 26. Thus, the screw motion of adjusting screw 38 will forcibly bend the bracket 26 toward or away from the frame 23. This bending of bracket 26 is sufficient to move the end of bimetal 28 to or away from the frame 23, and thus provide a simple temperature adjustment.

and serves to limit, in cooperation with the stop screw46, the motion of screw 38. Stop screw 46 is threaded into an extruded hole 41 in the frame 23, and is locked in place by means of nut 48. Screw 46 also serves to limit the motion of bimetal member 28 toward the frame 23 in the event member 28 becomes very cold. If this movement toward the frame were not stopped, bimetal member 29 might be strained out of shape and calibration by being forced against the frame too far by'member 28. It will be understood that screw 46 is adjusted to prevent such deformation of bimetal 29. Extensions 49 of bracket 26 serve to mount the thermostat on the inner surface of the sole plate of an electric fiatiron; Connections are made as shown in Fig. 9: Terminal 36 (and thus contact 34) is connected to one side of a source of electrical potential E. The other side of E is connected to one side of the fiatiron heating element 58'which heats the sole of the iron. The other side of element 58 is connected to terminal 35, and thus to contact button 33. I

Bimetal member 28 is made thin, narrow, and short in order to be heated rapidly above its ambient temperature by the passage of electrical current through it. and also to cool rapidly due to its low heat mass. Member 28 is so mounted that on being heated it moves away from the frame 23. Member 29 is so mounted that on being heated, the ends carrying-contacts 3| and 32 move toward the frame, thus tending to close the respective contacts.

The operation of the device is as follows: When the fiatiron is cold and. first connected to the source of potential E, contacts Ill-33 and 32 34 are closed. Current flows through the heating element 50 and through bimetal member 29. bimetal member 29 heats, it tends to bend and force the contacts harder together. Meanwhile member 28 responds to the heat of the iron and gradually the free end of member 28 moves away from the thermostat frame. As it does so, it begins to relieve the stresses 29 until eventually the contact-opening motion of member 28 overcomes the contact-closing tendency of member 29. When the respective contacts separate and current ceases to flow, bimetal member 29 cools rapidly (due to its low heat mass and the fact that it is above its ambient temperature) and separates the respective contacts with a rapid, positive movement.-

On the cooling part of the cycle, the iron cools and with it bimetal member 28. This cooling of member 28 brings the contacts together, but when they touch, current flows, bimetal memer 29 heats rapidly, and the contacts are forced together rapijdlyzand positively by the quick bending of memer s The differential of the thermostat can be controlled by varying the amount of deflection of the secondary element. For a fixed differential thermostat this can be done, for example, by properly proportioning the length and/or thickness of the secondary element; or, if current is to flow therethrough, its resistivity to current A stop plate 45 is force-fitted onto the stem of adjusting screw 38 built up in member ilow can be changed to produce the proper temperature change and resulting movement. In a variable differential thermostat, the differential can be adjusted by making the secondary element adjustable, in; its length, or by spring-loading it, or by controlling the current flow through it by means of a variable resistor in series or shunt with the .bimetal element.

As examples of these last-named means, Figs. 10, 11 and 12 may be referred to. In Fig. 10, the bimetal 311 has been slotted as at and is adjustably fastened to the end of bimetal I by screw 52. In this case the contact 8a is made long enough so that change in the effective length of bimetal 3a by sliding it back and forth will not prevent contact 1 making contact with contact 8a.. In Fig. 11, a spring 53 with a return bend as shown is fastened to the end of bimetal I. An adjusting screw 54 passes freely through the upper part of the spring 53 and threads into the end of bimetal I. A bent-over portion 55 of spring 53 bears against bimetal 3. Adjustment of screw 54 causes the spring 53 to bear more or less heavily against the bimetal 3, thus changing its thermal movement. In Fig. 12, an adjustable resistance 56 has been shunted across bimetal 3 by means of flexible pigtails .51 and 58. Then when current is passed through the thermostat, as for example in the manner shown in Fig. 9, changing the adjustment of the resistance 56 changes the eiTective heating and thus movement of bimetal 3. thus changing the differential of-thc complete thermostat.

It will be observed that by the addition of the second element, the complete thermostat has a positive temperature difierential. It will be understood that this differential is controllable as it is in a snap-action thermostat, and that the contact-opening and closing takes place in a positive manner, regardless of the-slowness or lingering at any point of the main element. It-ls obvious that while composite metal strips have been used as embodiments, other slow-acting thermally-responsive elements may be used in such way that the same principles are utilized.

The action may be accentuated when using two composite metallic thermal elements if the second of these elements has a smaller thermal lag or inertiathan the other; that is, if it responds more quickly to changes in ambient temperature,

but at the same time has a smaller sensitivity or movement per unit temperature change. This second element then responds more quickly than the main element to a relatively rapid change in temperature which becomes available upon the closing of a pair of contacts, due either to the in herent heating caused by a flow of current through the second thermostatic element, or from an auxiliary electrical heater in relatively close spaced relationship to the second thermostatic elementl The difference in thermal inertia between the two elements also allows the second element to cool much more rapidly than the main element upon cessation ,of this source of heat when the contacts open.

One element is arranged to predominate. It will be the one which has the greatest sensitivity or movement per unit temperature Change. The element which predominates will thus determine the direction of contact travel, while the magnitude of this travel will depend upon the motions of the two elements.

Variation in the relative movement of these two elements may be obtained by varying the material and the dimensions of the thermostatic elements. For example, the movement of the end of a straight strip of bimetal is proportional to the square of its length and to its deflection constant, that is, its sensitivity or movement per unit temperature change, and is inversely proportional to its thickness, for a given tempera ture change. The relative movement may likewise be varied by imposing stress upon one or both of the elements; for example, by means of loading springs, Weights, etc.

It will be noted that the secondary thermo static element, upon initiation of a given operation by the primary thermostatic element, acts to reinforce the operation initiated by the primary thermostatic element.

In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As many changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first memberand adapted to accelerate the movement of said means in one direction after the first member initiates said movement, said two thermostatic members only, operating said means.

2. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and tending to hold said means in its final position at the end of movement of said means in one direction, said two thermostatic members only, operating said means.

3. Control means, a thermostatic strip adapted to operate said means, a second thermostatic strip associated with the first strip and adapted to accelerate the movement of said means in one direction after the first strip initiates said movement, said two thermostatic members only, operating said means.

4. Control means, a thermostatic strip adapted to operate said means, a second thermostatic strip associated with the first strip and tending to hold said means in its final position at the end of movement of said means in one direction, said two thermostatic members only, operating said means.

5. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement and tending to hold said means in its final position at the end of movement of said means-in the other direction, said two thermostatic members only, operating said means.

6. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and having different temperature response during operation than said first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement, said two thermostatic members only,

operating said means.

7. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and having difierent temperature response during operation than said first member and tending to hold said means in its final position at the end of movement of said means in one direction, said two thermostatic members only, operating said means.

8. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and having difierent temperature response during operation than said first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement and tending to hold said means in its final position at the end of movement of said means in one direction, said two thermostatic members only, operating said means.

9. Control means, a thermostatic member adapted to operate said means, a thermostatic strip associated with the thermostatic member and having a smaller thermal lag and smaller sensitivity than said member and adapted to accelerate the movement of said means after said member initiates said movement, and tending to hold said means in its final position at the end of movement of said means in the other direction, said two thermostatic members only, operating said means.

10. Control means, a thermostatic member adapted to operate said means, a second thermostatic member mounted on the first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement and adapted to tend to hold said means in its final position at the end of movement of said means in the other direction, said two thermostatic members only, operating said means.

11. Electrical contacts, a thermostatic member adapted to operate said contacts, a second thermostatic member associated with the first member and adapted to accelerate the contact-separating movement after the first member has opened the contacts, said two thermostatic members only, operating said contacts.

12. Electrical contacts, a thermostatic member adapted to operate said contacts, a second thermostatic member associated with the first member and tending to increase the contact pressure after the first member has closed the contacts, said two thermostatic members only, operating said contacts.

13. Electrical contacts, a thermostatic member adapted to operate said contacts, a second thermostatic member associated with the first member and adapted to accelerate the contact-separating movement of the first member after the first member has opened the contacts, and adapted to increase the contact pressure after the first member has closed the contacts, said two thermostatic members only, operating said contacts.

14. Electrical means to control the fiow of cursociated with the first means and tending to hold.

said electrical means in final position when said electrical means has been moved in one direction by said first thermostatic means, said two thermostatic means only, operating said electrical means.

16. Electrical means to control the flow of current, thermostatic means to operate said electrical means, and a second thermostatic means associated with the first means and adapted to accelerate the movement of the electrical means in one direction after said movement has been initiated by the first thermostatic means, and adapted to exert pressure upon said electrical means to hold said electrical means in final position when said electrical means has been moved in the other direction by said first thermostatic means, said two thermostatic means only, operating said electrical means.

1'7. Electrical means to control the flow of current, thermostatic means to operate said electrical means in response to ambient temperature, and a second thermostatic means mounted on the first means and adapted to accelerate the movement of the electrical means in one direction after said movement has been initiated by the first thermostatic means, and tending to hold said electrical means in final position when said electrical means has been moved in the other direction by said first thermostatic means, said two thermostatic means only, operating said electrical means.

18. Contact means adapted to control the fiow of current, thermostatic means to operate said contact means, and a, second thermostatic means, responsive to the flow of current, to accelerate the circuit-opening operation of said contact means after said first thermostatic, member has initiated said operation, and adapted to increase the contact pressure after the contact means are closed by the first member, said two thermostatic means only, operating said contact means.

19. Control means, thermostatic means adapted to operate said control means, and a second thermostatic means adapted to reinforce the operation of said control means after said operation has been initiated by said first means, said two thermostatic means only, operating said contact means.

20. A thermostat comprising contacts, a main contact-moving means and a second contactmoving means, said second contact-moving means comprising a bimetallic element adapted to be heated by the flow of current when the circuit controlled by said contacts is closed and arranged to tend to move said contacts in a. contact-closing direction when heated and in a contact-opening direction when cooled so as to accelerate the motion of the contacts on the opening of the contacts and to quickly increase the contact pressure on the closing of the contacts, said two contactmoving means only, moving said contacts.

21. A thermostat comprising two temperatureresponsive elements, one of said elements being arranged to oppose the then action of the other element, said one and said other elements having different temperature-response characteristics, relatively movable contacts, the position of which is determined by the two thermally-responsive elements only, and means for adjusting the position of at least one of said thermally-responsive elements.

22. A thermostat comprising two temperatureresponsive elements, one of said elements being arranged to oppose the then action of the other element, said one and said other elements having difierent temperature-response characteristics. relatively movable contacts, the position of which is adjusted by the two thermally-responsive elements only, and means for adjusting the temperature differential of said thermostat.

23. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement, said two thermostatic members only, controlling said means, and means for varying the operation of said second thermostatic member.

24. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement, and means for varying one of the efiective dimensions of said second thermostatic member.

25. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement, and means for opposing the action of said second thermostatic member to adjust the operating characteristics of said second member.

26. Control means, a thermostatic member adapted to operate said means, a second thermostatic member associated with the first member and adapted to accelerate the movement of said means in one direction after the first member initiates said movement, and means for subjecting said second thermostatic member to a varying amount of heat.

27. Control means, thermostatic means adapted to operate said control means, a second ther- 5 mostatic means adapted to reinforce the operalo )characteristics of said second thermostatic means.

28. A thermostat comprising relatively movable contacts, a main contagtlmovingmeansand a second contact-moving mgans movement of said second contact-moving means tQlfldinBitO oppose movement of said first contact-movingmeans, said main and said second contactgmov ing means having different temperatore esgopse characteristics. said two contact-moving means only, moving said contacts, and means for adjust; ing the operating characteristics of said second contact-moving means.

29. A thermostat comprising contacts, a main thermostatic contact-moving means and a second thermostatic contact-moving means, said second contact-moving means comprising a bimetallic element adapted to be heated by the flow of current when the circuit controlled by said contacts is closed and arranged to tend to move said contacts in a contact-closing direction when heated and in a contact-opening direction when cooled so as to accelerate the motion of the contacts on the opening of the contacts and to quickly increase the contact pressure on the closing of the contacts, said second contact-moving means having a smaller thermal lag and a smaller sensitivity than said main contact-moving means, said two contact-moving means only, moving aid contacts.

HAROLD M. WILSON.

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