US3246104A

US3246104A - Electrical switch having an external wrap-around resistance heater

Info

Publication number: US3246104A
Application number: US93571A
Authority: US
Inventors: James E Shockroo
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1961-03-06
Filing date: 1961-03-06
Publication date: 1966-04-12
Anticipated expiration: 1983-04-12
Also published as: GB1005782A; NL275585A

Description

prll 12, 1966 .1 E, SHQCKROO 3,246,104

ELECTRICAL SWITCH HAVING AN EXTERNAL WRAP-AROUND RESISTANCE HEATER Filed March 6, 1961 2 Sheets-Sheet l .f E TWENTE mn az "I I 2 7 /Msl/Ar/o/v In verz taf', James E AS'zaclroo,

ZJ MJQMLA' April 12, 1956 J. E. sHocKRoo 3,246,104

ELECTRICAL SWITCH HAVING AN EXTERNAL WRAP-'AROUND RESISTANCE HEATER Filed March 6, 1961 2 Sheets-Sheet 2 In van on rfa m es E Sie oci'roa,

Aar/gy.

United States Patent O 3,246,104 ELECTRICAL SWITCH HAVING AN EXTERNAL WRAP-AROUND RESISTANCE HEATER James E. Shockroo, Norton, Mass., assigner to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Mar. 6, 1961, Ser. No. 93,571 3 Claims. (Cl. Zim-122) This invention relates to thermally responsive switches and with regard to certain more specific features, to thermostatic switches inl which heater means are provided by resistance wire or the like. The thermostatic switches of the instant invention are especially adapted for (though not limited to) protection of electric motors, and are generally mounted on the motors so as to be subject to both the heat of the motor and to the current actuating the motor to thereby provide so called inherent overheat protection. This invention is particularly concerned with such types of thermostatic switches which employ external or wrap-around resistance heaters. Such thermostatic switches, for example, which employ external wrap-around resistance heaters are those which are especially designed for repair shop installation on motors, wherein the repair shop may assemble the protector to provide the required rating for a particular motor. Other examples of motor protectors employing a wrap-around resistance heater are disclosed in the application of William A. Broadley and Edward P. lastram, entitled Electrical Switch Means, Serial No. 803,282, tiled March 3l, 1959, and assigned to the assignee of the instant application.

In the operation of electrical switch structures which embody coacting electrical contacts, the likelihood may well exist that arcing from one contact to another will occur. This phenomenon and its deleterious effects are well known in the art.

It has been found that such wrap-around external heaters describred above, have a tendency to create a magnetic field and damaging magnetic flux effects (resulting from the turns of the heater) within the switch. When a thermostatic switch (especially one of the snapacting disc type) is employed in a magnetic field, any arcing which may be created by the quick break of contacts, is intensely magnified because of its presence in the magnetic field. In certain orientations of the contacts in the magnetic field, the arcing may be directed toward the snap-acting thermostatic element, and exposure thereof or its nondevelopable dimpled portion (responsible for its snap action) to the arcing, will tend to destroy, or at least deleteriously affect, the temperature calibration of the switching device. Furthermore, when such switches are operative in a magnetic field, the arc sometimes formed upon opening of the contacts is frequently pushed (or directionalized) by the magnetornotive force so as to strike along the surface of the thermally responsive element which not only may change the calibration of the switch, but it may change inconsistently. Moreover, physical deterioration and annealing due to the burning action of the arc on the thermally responsive element also tends to change its operating characteristics, particularly if these affect the nondevelopable area responsible for snap action. In certain orientations of the electrical contacts in the magnetic field (created by the external wrap-around resistance heater) the arcing may be directed for striking the housing. The housing, if formed of a conventional phenolic or resinous material (such as a thern'losetting resin) may eventually deteriorate due to such intensified arc strikes and build up undesirable gases in the interior of the switch. When the phenolic housing deteriorates, short circuiting of the contacts can 3,246,104 Patented Apr. 12, 1966 occur due to the formation of a low current resistance path created by the deteriorated housing material. Such short circuiting may, in some cases, result in an explosion because of the gases created, and continuous current ow through the low current resistance path between the contacts created by the material of the deteriorated phenolic housing.

It is accordingly one object of the instant invention to provide thermostatic switches, of the class described, and methods of making the same, which will obviate or at least minimize the arcing problems described above.

It is another object of the instant invention to provide an improved thermostatic switch, and a `method of making the same, which provides resistance heater means so constructed as to inhibit, or at least minimize, the creation of damaging magnetic ux within the switch.

It is yet another object of the instant invention to provide thermally responsive switches and methods of making the same, which switches can be easily and quickly assembled, for example, by a service shop, to provide a thermostatic switch having the required ratings for protection of a given motor.

It is yet another object of the invention to provide a thermally responsive protective switch which will provide for total protection of a motor.

Among the further objects of the invention may be noted the provisions of a novel, thermostatic electrical switch which is simple and economical in construction, simple and economical to manufacture, and which is effective and dependable in operation.

Other objects will be in part apparent and in part pointed out hereinafter.

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

In the accompanying drawings, in which one of the various possible embodiments of the invention is illustrated FIG. 1 is a top plan view of a thermostatic switch according to this invention;

FIG. 2 is a side elevation of the switch shown in FIG. l;

FIG. 3 is a bottom plan view of the switch shown iny FIG. l;

FIG. 4 is a plan View taken on line 4-4 of FIG. 2;

FIG. 5 is a sectional View taken on line 5 5 of FIG. 1; and

FIG. 6 is an exemplary schematic wiring diagram for the thermostatic switch illustrated in FIGS. 1-5, in combination with a split-phase electrical motor.

Dimensions of certain of the parts as shown in the drawings have been modied and/or exaggerated for the purposes of clarity of illustration.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation.

Referring now to the drawings, an improved thermostatic switch embodying this invention is shown at numeral 10. Exemplary switch 1li may be of the type which is electrically connectable with both the main and start windings of a split-phase electrical motor to provide so called complete or total protection for both the start and main windings, separately as well as both of them combined, against abnormal conditions which can cause overheating and burnout of the windings, such as, for example, described in a copending application of James E. Shockroo and Walter H. Moksu, entitled Thermally Responsive Switches, Serial No. 841,237, led September 21, 1959, and assigned to the assignee of the instant application.

Switch includes a switch housing or base member 12, open at one end and formed of one of the conventional electrically insulating plastics, such as a moldable, phenolic, resinous material. Base member 12 may also be formed of a ceramic material. Base 12 is provided with a pair of open-ended slots 14 and 16, (see FIG. 4) which slots cooperatively intert with and respectively mount electrically conductive terminals 18 and 2t). Each of

terminals

18 and 20 are respectively provided with a

deformable projection

26 and 28.

Projections

26 and 28 are respectively received within slotted

openings

22 and 24, provided by base 12, and are deformed or twisted thereabout to tightly

mount terminals

18 and 20 on base 12.

Thermostatic switch 10 includes a pair of

elongated terminal elements

30 and 32. As best seen in FIG. 5, each of

elements

30 and 32 respectively mount

electrical contacts

34 and 36.

Terminal elements

30 and 32, and

contacts

34 and 36, are mounted and received within suitable slots and apertures provided by base 12, as shown. Contact

members

34 and 36 may, for example, take the form of rivets which are respectively secured in electrically conductive relation to

terminal elements

30 and 32.

Contacts

34 and 36 are aligned for respective mating engagement with a pair of spaced

contacts

38 and 40, carried by and electrically connected to a snap-acting thermostatic plate member 42, as shown. Thermostatic member 42 comprises a dished bimetallic snap-acting thermostatic disc of the type shown and described, for example, in the U.S. Spencer Patent No. 1,448,240 or in the U.S. Vaughan et al. Patent No. 2,317,831. The operation of this type of snap-acting thermal element is well known in the art. Thermostatic element 42 is provided with a centrally located aperture within which is received a portion of adjusting screw 44. Thermostatic element 42 is loosely mounted on adjusting screw 44 between shoulders or abutments, such as that at 46 and at flanged end 48, all as described in the aforesaid patents.

As best seen in FIGS. 4 and 5, thermostatic element 42 is guided for assembly into base 12, and once assembled therein, is prevented from rotating relative to the base through cooperation of

upstanding walls

50, 52 provided by the base 12, and ear 54 of the thermal element 42. Screw 44 is received in threaded engagement within a threaded open-ended aperture 56, provided in base 12, as best seen in FIG. 5. As thus mounted, relative rotation between the thermal element and housing is inhibited, but relative rotation between disc 42 and the adjusting screw 44 is permitted. Thus, screw 44 may be rotated to adjust the operating temperature of the thermostatic disc 42 without rotating the disc itself.

Thermostatic switch 16 also includes an electrical resistance type heating element 58 disposed within switch housing 12. Heater element 58 is mounted on and electrically connected, as by welding, at or adjacent its ends 60 and 62, respectively to electrical terminals 18 and 2t), as best seen in FG. 4. Heater 58, when mounted on the base and electrically secured to

terminals

18 and 20, `as described above and shown in FIGS. 4 and 5, is suspended directly above the thermostatic element 42 and in good heat-transfer relation or thermally conductive juxtaposition therewith. A strip of electrically insulating material 64 (as best seen in FIG. 5) is disposed intermediate internal heater 58 and thermostatic element 42.

When the switch 10 has been assembled as thus far described, the open end of base member 12 is closed with a cover member 68 formed of electrically insulating ma- 4 terial such as, for example, a ceramic or resinous material and a layer of electrically insulating adhesive tape 74) is wrapped about and binds the cover 68 and base 12 together. Thereafter, a length of magnet Wire or electrically conductive wire forming an external wrap-around resistance heater, generally referred to by numeral 72, is disposed about casing 12 and cover member 68. The wire forming the wrap-around resistance heater 72 is of the type having an electrically insulating cover or outer sheath, and such wire may, for example, comprise a copper wire. Base 12 and cover member 68 are provided with pairs of aligned and spaced

apertures

76 and 78. Aperture 76, as shown, is round, while aperture 78 is preferably elongate or ovate in form. The external heater wire is doubled back on itself to form two integral lengths or

strands

80 and 82 which remain electrically connetced in series With each other through the bight portion S4, as best seen in FIG. 1.

The external heater 72 is formed by ythreading one of the lengths 80, `82 through 4the round aperture 76 and disposing the bight portion 84 on the base, as shown. Thereafter, the two lengths or strands 80 and `82 are wound or wrapped, side by side, around the unit (including base 12, cover 68 and insulating tape 70) making one complete layer and 'forming alternate heater turns or

loops

80 and 82, in the manner illustrated in the drawings. The free ends of lengths S0 and 82 are then passed through the elongate opening 7 8 to retain the external wrap-around -heater 72 in place about the switch 11i. 'The insulation is then removed .from the free end of strand or length 82, leaving an exposed, uninsulated portion of wire 86. Portion 86 is electrically connected to

terminals

30 and 18, as by soldering or crimping, etc., thereto. The external heater 72 is thus electrically connected to both Athe start circuit terminal 30 and vmain winding circuit terminal 18, when the switch ltl is connected in a .circuit such as that shown 1in FG. 6 to be described in greater detail below.

The doubling of the external heater wire to form an integral set of double strands or lengths, and Wrapping them in `the manner described above to form the external heater, advantageously provides for a reverse flow of current in the alternate turns (81) a-nd 82). The reverse cu-rrent tlow provides a degaussing or flux neutralizing etect, which tends to preclude or inhibit the creation of a magnetic field or damaging magnetic flux within the switch 10. After the thermostatic switch 10 and external heater '72 have been `assembled in the manner describe-d above, the entire assembly may be wrapped with a layer of glass silicon adhesive :tape (not shown rin the drawings) and the entire assembly installed on a motor winding of a motor to be protected. Installing .the thermally responsive switch directly adjacent the motor winding ladvantageously provides for inherent protection, and the switching device can quickly sense temperature variations of the motor windings to be protected.

Heater element 58 has been illustrated as being of a serpentine fonm and formed of a round wire. It should be understood :that although a round wire is illustrated in the drawings as being exemplary of a material of which the internal and external heating elements may -be made, that this may have other cross-sectional configurations, such as rectangular or polygonal and may also be a strip, ribbon or the like. The terms wire and wire-like are; used herein as characterizing each of these heater forms..

The thermostatic switch 10, described above, is particularly suited for assembly by service or motor repairl shops. Electric motors of different makes, although of similar ratings, are not exactly alike. For this reason, in the past each manufacturers model has required a protector tailor made to match its particular thermal characteristics. This tailoring process has created a technical problem for motor servicemen who wanted to apply a protector to a motor not originally equipped with one. Formerly, only motor Imanufacturers Ihaving the necessary test equipment could tailor protectors, to

match motor characteristics. It is now possible, by determining the selection of Ithe heater(s) (both internal and external) based on the size of the wire used in the motor winding to be protected, for the service repair shops to assemble tailored overheat inherent motor protectors for specific motors tnom a kit provided for this purpose. Such a service shop kit may, for example, include several base assemblies (eign, comprising a base 12, a pair of terminals 30 and 312,

electrical contacts

34 and 36, and a thermostatic element 42) and a selection of internal heater subassemblies of diierent ratings (eg. comprising a heater 58 and terminals 18 and 20). With this kit, the service shop can now select the proper parts and assemble its own service Shop inherent motor protector tailored to meet specic motor requirements.

Investigation has shown that there is in `general use, a limited number of sizes of wire ya-nd materials commonly used in start windings. It is thus possible to supply the service shops with a table designating the appropriate heater element to be employed as a protective start winding heater (c g. internal heater 58) fora given motor start winding fonmed of wire of a particular size and material. The external heater 72, which is electrically connected in series writ-h the main winding, for example as in the circuit of FIG. 6, can also be selected in relation Ito the size and material of the motor main winding. Generally, the main winding heater is formed of a wire one size smaller than `that of the motor main winding so as to provide for anticipation.

Thus, the motor repair shop serviceman, to assemble a motor protector yfor protection of a particular motor, proceeds as follows:

(l) selects .the proper base assembly (comprising base,

disc 42, etc), as described above;

(2) selects the proper `size internal thea-ter 58 (in accordance with a predetermined size relation to the stant winding of the motor, as described above);

(3) inserts and deforms

tabs

26 and 28 within and about the

respective apertures

22 and 24 provided inthe base, so as to mofunt the heater assembly;

(4) thereafter, cover member 68 is applied and the insulating tape 70 is disposed about the cover and base; (5) selects the proper size wire (from a line of standard gauge wires usually in stock) i-n accordance with a predetermined relation to the size of 'the motor main winding; and

(6) mounts or wraps the external wrap-around resistance heater about the switch 10, in ilux neutralizing relation to the switch, in the manner described above.

ASelecting the thermostatic element 42 subassembly, internal heater 58 and external wrap-around 'heater 72 in accordance with the size of the motor windings to be protected, advantageously eliminates the necessity of carry ing an overly large inventory of protector pants by the serviceman, and penmits tailoring of a moto-r protector to a specific moto-r without the necessity lfor employing expensive and complex test equipment, as formerly required.

It is to be understood that similar reference characters as employed in the specification indicate corresponding parts throughout the several views of the drawings.

Referring now to the circuit diagram of FIG. 6, thermostatic switch 10 is schematically illustrated in an exemplary circuit with a motor 100 having an auxiliary start or phase winding 102 (indicated at SW) and a main or running winding 104 (indicated at MW) One power supply line, L1, is connected by conductor 106 to terminal 32. The other side of the supply line, L2, is connected by conductor 108 to the common connection between the start winding 102 and the main winding 104 of the motor generally indicated by the numeral 100. The other end of the start winding 102 is connected by a conductor 110 to a conventional start winding disconnect switch, indicated schematically by reference numeral 112,

6 and then by conductor 114 to terminal 20. The other end of the main or run winding 104 is electrically connected by conductor 116 to the free end of length 80 of external wrap-around heater 72.

As thus connected, it will be observed that the current in the starting winding flows from L2, one side of the power supply line, through conductor 108, start winding 102, conductor 110, switch 112, conductor 114, terminal 20, internal start winding heater 58, to terminal 18, thence through segment 86 of length 82 (of external wraparound resistance heater 72) to terminal 30, to contact 34, to contact 38, through thermal element 42 to contact 40, to Contact 36, then through terminal 32 t0 conductor 106 to L1, the other side of the power source.

Current through the main or run winding 104 flows from L2, conductor 108, through the main winding 104, conductor 116, serially through length 80, to bight portion 84 (of the doubled lengths 80, 82) serially through length 82 (the direction of flow now being reverse to that through length to terminal 30,contact 34, contact 38, thermostatic element 42, contact 40, contact 36, terminal 32, and then by conductor 106 to L1, the other side of the power source.

From the above, it will be Observed that internal heater S8 is electrically connected in series with the start winding and external wrap-around resistance heater 72 is electrically connected in series with the main winding 104. The operation of the thermostatic device 10, in the exemplary circuit of FIG. 6, as described above is as follows:

Under normal operating conditions of the motor 100, the heat generated by the current passing through the thermally responsive element 42 and the internal and

external heaters

58 and 72 will not he suicient to raise the temperature of the thermally responsive element 42 to the point where it snaps (if it is of the snap-acting type) or moves a distance (if it is of the creep-acting type) to open the

electrical contacts

38 and 40 to deenergize `the motor. If, however, the motor is so heavily overloaded that the rotor cannot turn, then the heavy starting and run winding currents will influence their

respective heaters

58 and 72 as well as the thermal element 42, to cause the latter to move to open the circuit to both windings, thus protecting the motor windings from overheating. If, on the other hand, the motor has been running and the start winding is not in the circuitand then the motor becomes overloaded, or there is a locked rotor condition with the main winding only in the circuit (which may have resulted from open circuit sta-rt switch 112 failure or open circuit in the reversing switch) the excess current in the run winding 104 will raise the temperature of the external wrap-around heater 72 and (according to the calibration of the thermally responsive element 42) the effect of heater 72 on the thermal element 42 will, after a time, lbe suiiicient to cause the latter to move to open the circuit before the windings are damaged.

Another abnormal condition is that condition where the motor is running, but at such a low speed that both the start winding 102 and the run winding 104 are in the circuit, which might result from a start switch failure in the closed position or low voltage which prevents reaching a switch-over speed. If the motor should operate in this fashion, then it will be observed that the relatively heavy current in the starting winding again has a rapid direct influence on the thermal element 42 (both because it traverses it and also because this current rapidly heats internal heater 58) to rapidly heat it to the contacts-separating point. This rapid heating of the thermal element to the contacts-separating point is, of course, also aided by the heat which is derived through external wrap-around heater 72 by the run winding current flowing therethrough. Thermostatic switching device 10, in the circuit illustrated in FIG. 6, is also effective in the condition which involves a locked rotor with the start winding 102 only in the circuit, such as might result from an open main winding circuit or an open circuit in a reversing switch. In this condition, the relatively heavy current in the start winding again has a rapid direct influence on the thermal element 42, as described above, heating it quickly -to the contacts-separating point. It should be understood that the thermally responsive switch of the instant invention is susceptible to diverse applications in motor protective circuits other than that illustrated by way of example in FIG. 6. For example, the switch of the instant invention could be employed with multispeed, dual voltage, or other electric motors or energy-translating devices having more than two windings.

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 and methods 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, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

I claim:

1. A thermostatic switch adapted for protection of an energy-translating device having at least one winding; said switch comprising a switch housing; a pair of electrical contacts disposed within said housing; a thermally rcsponsive device disposed within said housing and operatively connected to move said contacts into and out of engagement at predetermined conditions; a wire-like resistance heater doubled to form two lengths and wrapped in loops in side-by-side relation around said housing externally thereof in heat-transfer relation to said thermally responsive device whereby any magnetic ux formed by current flow in one length will cancel any magnetic ux formed by the said current ow in the other length such that there will be no resultant ux caused by said resistance heater to push any current arc, flowing between the contacts on movement into and out of engagement thereof, toward said thermally responsive device or said housing so that there will be no deleterious effect on the calibration of said thermally responsive device and no deterioration of the housing caused by said resistance heater; the planes of said loops being substantially parallel to the direction of `contact movement; and said resistance heater being electrically connectable with said one winding and having a wire size selected according to the size of the wire used in said winding to provide a predetermined size relationship therebetween.

2. A thermostatic switch adapted for protection of an energy-translating device having at least two windings; said switch comprising a switch housing; a pair of electrical contacts disposed within said housing; a thermally responsive device disposed Within said housing and peratively connected to move said contacts into and out of engagement at predetermined conditions; an electrical heater element disposed within said housing in heat-transfer relation to said thermally responsive device and electrically connectable in series with one of said winding, said heater element having a preselected size, bearing a predetermined relation to the size of said one winding; a wire-like resistance heater doubled to form two lengths and wrapped in loops in side-by-side relation around said housing externally thereof in heat-transfer relation to said thermally responsive device whereby any magnetic flux formed by current flow in one length will cancel any magnetic flux formed by the said current ow in the other length thereby inhibiting the creation of damaging magnetic ilux within said housing which ux would push any current arc, flowing between the contacts on movement into and out of engagement thereof, toward said thermally responsive device or said housing so that there will be no deleterious eiect on the calibration of said thermally responsive device and no deterioration of the housing caused by said resistance heater; the planes of said loops being substantially parallel to the direction of contact movement; and said resistance heater being electrically connectable in series with the other of said windings and having a wire size selected according to the size of the wire used in said winding to provide a predetermined size relationship therebetween.

3. A thermostatic switch adapted for protection of an energy-translating device having at least one winding; said switch comprising a switch housing; a pair of electrical contacts disposed within said housing; a thermally responsive device disposed within said housing and operatively connected to move said contacts into and out of engagement at predetermined conditions; a wire-like resistance heater doubled to form two lengths and wrapped in loops in side-by-side relation around said housing externally thereof in heat-transfer relation to said thermally responsive device whereby any magnetic flux formed by current flow in one length will cancel any magnetic flux formed by the said current flow in the other length such that there will be no resultant flux caused by said resistance heater to push any current arc, flowing between the contacts on movement into and out of engagement thereof, toward said thermally responsive device or said housing so that there will be no deleterious effect on the calibration of said thermally responsive device and no deterioration of the housing caused by said resistance heater; the planes of said loops being substantially parallel to the direction of contact movement; and said resistance heater being electrically connectable with said one Winding.

References Cited by the Examiner UNITED STATES PATENTS 1,513,210 10/1924 Banan 200-122 1,828,321 10/1931 Hesse 200-122 1,969,955 8/1934 Thomas 200-122 2,279,214 4/ 1942 Veinott 200-122 2,354,529 7/ 1944 Ludwich 200-122 2,458,225 1/1949 Trilling et al 338-63 2,855,495 10/1959 Grant 219-35 2,996,590 8/ 1961 Shockroo et al, 200-1122 3,023,350 2/1962 Broadley et al. 318-221 3,041,567 6/1962 Baker 338-63 BERNARD A. GILHEANY, Primary Examiner. RICHARD M. WOOD, E. JAMES SAX, Examiners.

Claims

1. A THERMOSTATIC SWITCH ADAPTED FOR PROTECTION OF AN ENERGY-TRANSLATING DEVICE HAVING AT LEAST ONE WINDING; SAID SWITCH COMPRISING A SWITCH HOUSING; A PAIR OF ELECTRICAL CONTACTS DISPOSED WITHIN SAID HOUSING; A THERMALLY RESPONSIVE DEVICE DISPOSED WITHIN SAID HOUSING AND OPERATIVELY CONNECTED TO MOVE SAID CONTACTS INTO AND OUT OF ENGAGEMENT AT PREDETERMINED CONDITIONS; A WIRE-LIKE RESISTANCE HEATER DOUBLED TO FORM TWO LENGTHS AND WRAPPED IN LOOPS IN SIDE-BY-SIDE RELATION AROUND SAID HOUSING EXTERNALLY THEREOF IN HEAT-TRANSFER RELATION TO SAID THERMALLY RESPONSIVE DEVICE WHEREBY ANY MAGNETIC FLUX FORMED BY CURRENT FLOW IN ONE LENGTH WILL CANCEL ANY MAGNETIC FLUX FORMED BY THE SAID CURRENT FLOW IN THE OTHER LENGTH SUCH THAT THERE WILL BE NO RESULTANT FLUX CAUSED BY SAID RESISTANCE HEATER TO PUSH ANY CURRENT ARC, FLOWING BETWEEN THE CONTACTS ON MOVEMENT INTO AND OUT OF ENGAGEMENT THEREOF, TOWARD SAID THERMALLY RESPONSIVE DEVICE OR SAID HOUSING SO THAT THERE WILL BE NO DELETERIOUS EFFECT ON THE CALIBRATION OF SAID THERMALLY RESPONSIVE DEVICE AND NO DETERIORATION OF THE HOUSING CAUSED BY SAID RESISTANCE HEATER; THE PLANES OF SAID LOOPS BEING SUBSTANTIALLY PARALLEL TO THE DIRECTION OF CONTACT MOVEMENT; AND SAID RESISTANCE HEATER BEING ELECTRICALLY CONNECTABLE WITH SAID ONE WINDING AND HAVING A WIRE SIZE ELECTED ACCORDING TO THE SIZE OF THE WIRE USED IN SAID WINDING TO PROVIDE A PREDETERMINED SIZE RELATIONSHIP THEREBETWEEN.