US2022518A

US2022518A - Electric heater

Info

Publication number: US2022518A
Application number: US520576A
Authority: US
Inventors: John H Payne
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1931-03-06
Filing date: 1931-03-06
Publication date: 1935-11-26
Anticipated expiration: 1952-11-26

Description

Nov. 26, 1935.

J. H. PAYNE ELECTRIC HEATER Filed March 6, 1931 2 His At oorne g.

Patented Nov. 26, 1935 UNITED STATES- PATENT OFFICE 1 General Electric Com New York pony, a corporation of Application March 6, 1931, Serial No. 520,576

10 Claims. (Cl. 21%) My invention relates to electric heaters, more particularly to electric heating pads and the like, and has for its object the provision of an improved electric heater of this character where- 5- in a suitable heating elementand also a controlling thermostat are embedded in a body of flexible, water-proof, air-tight, heat-conducting and electrically-insulating material, and also one which for all practical purposes is indestructible, and has for a further object the provision of a simple and efiicient method of making a heater of this character.

In carrying my invention into effect in one form thereof, I form in a body of flexible, waterproof, air-tight, heat-conducting, electricallyinsulating and coalescent material, such as rubber, a retaining groove for receiving a resistance conductor, and also a cavity or chamber for receiving a controlling thermostat. This body of material, which preferably will be of sheet form, may be prepared by molding it to the desired shape, the groove and cavity being formed during the molding process; preferably the groove will be formed into a series of sinuous convolucomparatively large area of the sheet, and the cavity will be molded so as to be included in a length of the groove. After the sheet has thus been prepared, a suitable heating resistance conductor is threaded or otherwise placed in the groove provided for it in the sheet of rubber, the I conductor thereby assuming a sinuous form, the convolutions of the conductor being distributed substantially uniformly over the sheet; then a suitable thermostat is placed in the cavity provided for it in the sheet, the thermostat then being electrically connected with the conductor. After this, a second sheet of rubber is assembled with the first sheet so as to cover the conductor and thermostat, and then is coalesced with this sheet, as by vulcani zing, so that the two sheets are united into one body with the resistance con-,

ductor and thermostat embedded therein; if desired, and it is generally preferably to do so, a third sheet of' rubber may be arranged so as to cover the side of the first sheet opposite to that side in which the conductor and thermostat have been received, and coalesced with the other sheets during the vulcanizing step. This adds thickness and strength to the finished pad. Preferably before the sheets have been assembled and coalesced suitable reenforcing members will be arranged adjacent theend turns of the convolutions of the conductor and adjacent the thermostat so that they too will be embedded in I131 tions distributed substantially uniformly over a,

tary body formed by the subsequent coalescing operation.

For a more complete understanding of my invention reference should be had to the accompanying drawing, in which Fig. 1 is a perspective 5 view of an electric heating pad formed in accordance with my invention; Fig. 2 is a perspective view of a sheet of rubber used in making the pad of Fig. 1, in which sheet a suitable groove for receiving a resistance heating element and a suit- 10 able cavity or recess for receiving a controlling thermostat for the conductor are provided, and also illustrating one method of inserting the resistance element into itsgroove; Fig. 3 is an enlarged fragmentary transverse sectional view of 15 the sheet of Fig. 2 and showing the resistance element mounted in its groove; Fig. 4 is an expanded view showing the positions various elements of the electrical heater of Fig. l occupy relatively to each other; Fig. 5 is a fragmentary view illustrating the manner in which an electrical supply conductor provided for the heating pad is secured thereto; Fig. 6 is a perspective view of a contact clip used to connect the resistance conductor to the supply conductor; Fig. 7 is a fragmentary view in section illustrating a method whereby the rubber sheets can be coalesced so as to form a single substantially homogeneous body; Fig. 8 is an expanded view of a thermostat for the heating pad of Fig. 1 made in accordance with my invention and illustrating the positional relation between the various elements of the thermostat; and Fig. 9 is a vertical section of the assembled thermostat.

Referring to the drawing, I have shown my 35 invention in one form as applied to an electric heating pad. As shown, the electric heating pad (Fig. 1) comprises a flexible, water-proof, airtight, heat-conducting and electrically insulating body ID in which a suitable resistance heating 4 element (not shown in Fig. 1) and a controlling thermostat therefor (also not shown in Fig. 1) are embedded, and a suitable twin supply conductor ll connected to the heater. As has been pointed out in a previous portion of this specifl- 45 a cation, the heater preferably will be of flat sheetllke form and the resistance-conductor preferably will be formed into a plurality of convolutions distributed over a, comparatively large area of the sheet. The thermostat will be included in the circuit of the resistance conductor so as to maintain .a substantially constant temperature throughout the heater when the latter is in use. While the sheet I: may have any suitable shape, 5

I have shown it for the purposes of illustration to be of rectangular form. v

In forming the electric heater of Fig. l in accordance with my invention, a body 12 (Fig. 2), preferably in sheet form and made of a suitable flexible, water-proof, air-tight, heat-conducting, heat resistant and electrically insulating coalescent material, such as rubber containing a suitable anti-oxidant, is provided with a suitable groove 13 in one surface thereof for receiving a resistance heating element H, and also in the same surface with a suitable cavity or recess l5 for receiving a suitable controlling thermostat i5 (Figs. 4, 8 and 9). As shown, the sheet I2 has ti e same shape as the finished pad (Fig. l) i. e., it is rectangular in form. lhis sheet of rubber may be prepared in any suitable way, but preferably will be shaped by molding, the conductor receiving roove it the thermostat cavity 15 being for ed during the molding operation.

1, (Figs. 2 and 4) the groove !3 for receiving one conductor i4 is formed into a series of convolutions Zl comprising a plurality of substantially parallel lengths Ha. arranged lengthwise of the rectangular sheet 12 and distributed over a comparatively large area of the sheet; the

groove is started at substantially one corner portion of the sheet, the starting point being designated by the numeral i8, and as shown its parallel convolutions have a length substantially equal tot-he full length of the sheet and are formed to cover substantially the full width of the sheet. The thermostat cavity or recess 15 is formed in substantially the central portion of the sheet, and the cor olution 20, which normally would pass t ough this central portion throughout substanas s hand end of the sheet, as viewed in ct, therefore, the convolutions are iii and 22, each series como convolutions having lengths equal to nil ngth of the sheet g substantia y one ie sheet, and the two series being connected convolution 20. It will be obs arrangement of the convolutions "des i r the thermostat cavity l5 arranged e portion of the sheet. It will also iat a length 23 of the groove exe last convolution 24 of the series end portion, of this series so as to join the 'inostat cavity l5, and from this cavity anngth 25 oi substantially L-shape bends left-her portion of the series 2 i as a as to e; I to the corner of the sheet e groove started. It is to be underinvention is not limited to the spenent olutions shown, but that ements can be used, it beever, that the convolution; -ai area of the sheet. Furtn ot necessary that the groove start and f the sheet as shown. The

ostat cavity 15 is cular portion 26 the tl and. '7) recei i6 and a second for accommodating binding posts 27 of the ther mostat. The several lengths of the convolutions on each side of and adjacent the cavity l5 are curved to correspond with the shape of the cavity.

Preferably the groove l3 will be under-cut so that overhanging portions 28 (Fig. 3) will be provided at the mouth of the groove whereby when when the thermostat is assembled with the sheet pattern will be arranged so as to provide a sheet 2- of rubber of the form shown in Fig. 1 with the under-cut grooves and cavity provided therein.

After the sheet 12 has thus been formed with its conductor retaining groove [3 and thermostat cavity I5, the electrical resistance heating element I4 is inserted in the groove l3 so as to extend throughout the entire length of the groove. Preferably, the element should be covered with a suitable adhesive that will harden quickly, such as rubber cement, before it is placed in the groove, so that when it is placed or inserted therein the cement together with the overhanging portions of the groove will effectively secure the element in the sheet. The element I4 may readily be inserted in the groove by means a:

of.a suitable pencil 32 (Fig. 2) through which the element is passed and in which the adhesive is placed. It will be observed by reason of this arrangement that as the element is led from a suitable supply source, as for example a spool 33, and is passed through the pencil, the entire outer surface of the element will be covered with the adhesive. It will also be observed that if the pencil point be placed in the initial portion of the groove and if the end portion 34 of Ti the element be held temporarily in some suitable fashion, .as by the fingers, and then if the pencil point be drawn through the groove by some suitable means, as by the fingers, the element will be inserted or threaded in the groove. this manner the element may readily be inserted throughout the full length of the groove and will be deposited or laid beneath the overhanging portions 2B of the ve, these portions being suiiiciently yi ding to allow the pencil point to be drawn along length of the groove so as to deposit the elci t i:- "ieath them. It will be understood the, e m .rhanging portions and have been previously ap- .he sheet efiectively secure the l and thence is directed from the under side of the sheet'through the second hole 3' back to the upper side of the sheet. This arrangement separates the ends of the element and also tends to secure them to the sheet.

While any suitable heating element ll may be used, I prefer to use a resistance conductor having a comparatively low specific resistance and one which will not be attacked by rubber. For this purpose, a conductor formed of nickel may be used. Moreover, the resistance characteristic of nickel is such that the resistance of the conductor rises rather rapidly as the temperature increases. Consequently, the power or wattage input decreases as the pad is heated from room temperature to its operating temperature. This is desirable both because the initial high wattage input quickly brings the pad to its operating temperature, and the lower wattage input after the pad has been heated gives economical operation. In a specific pad it has been found that the wattage input decreases by at least 20% when the pad is heated from a room temperature of 20 C. to an operating temperature of C. It is not necessary to use pure nickel for the resistance conductor, but a nickel alloy may be used, such as a nickel-copper alloy having a relatively high nickel content of say 95%. The heating element may be formed by winding the nickel resistance conductor in the form of a helix on a flexible cord-like supporting member Ila. Preferably, a non-extensible cord, such as cotton cord, will be used.

After the heating element has been laid in the groove and-its end portions securui, the thermostat I6 will be inserted in its cavity IS, the

overhanging portions 30 of the cavity serving to secure the thermostat. Preferably, the resistance conductor will have been threaded in the groove so that a loop or bight' of the conductor will be formed about the cavity l5; after the conductor has been laid, this loop will be severed so as to form two relatively long lengths 40 (Fig. 4), which lengths afterwards will be connected with the binding posts 21 of the thermostat so as to include the thermostat in series with the conductor. After the thermostat has been inserted in its receptacle the lengths M) of the conductor will be secured to the binding posts 21. The binding posts, as shown, are turned uponrthemselves so that the lengths 40 can be pinched between the return-bend and the body of the posts. Pref erably, the surfaces of the posts in contact with the lengths 40 will be covered with a suitable soft metal, such as solder or tin, so that when the lengths are pinched the nickel resistance element will be pressed into the soft metal coating by the resiliency of the supporting cotton body Ila. This insures a good electrical contact between the resistance conductor and the thermostat.

After the conductor and thermostat have thus been assembled with the sheet I! and have been connected in circuit with each other, the twin of the insulating fibrous filling or rope surrounding the conductors. The bared portions of the supply conductors are secured to the respective 43, which as shown (Fig. 6) are cylindrical members broken so as to have overlapping edges whereby the members can be clamped or squeezed about the conductors to secure them. Preferably, the inner surfaces of the clamping members will be lined with a soft metal, such as solder or tin, so that when the members are clamped-about the conductor the nickel resistance element of the heating element II will be pressed by its cotton supporting body into the soft metal linings; this insures a good electrical contact between the element I l and the clamping members. After the supply conductors have thus been secured to the resistance conductor ll, a portion-approximate ly one-half-of the insulating fibre 42 of the supply conductor is secured to the upper side of the rubber sheet l 2 by means of a suitable tab H, while the remaining portion of the insulating fibre is secured to the under side of the insulating sheet by meant of a similar tab 45. These tabs preferably will be formed of fabric rubber and will be arranged with their fabric sides adja-. cent the rubber sheet i2. The tabs will be secured to the sheet by means of a suitable adhesive, such as rubber cement.

After the tabs 44 and 45 have thus been applied to secure the insulating fabric, a suitable reenforcing member 46 having a substantially triangular shape is arranged in the corner of the sheet I! where the supply conductor l l is connected so that the right angle of the member as the upper tab 44, and is secured to the sheet i2 in any suitable manner, as by means of a suitable adhesive, which may be rubber cement. Then a suitable tape 4'! (Fig. 5) is wrapped around the connected end portion of the supply conductor ll adjacent. the sheet l2. Both the reenforcing, member 16 and the tape 4'! will be formedof unvulcanized rubber so that during the subsequent vulcanizing operation, referred to in a previous portion of this specification, these members will flow about and effectively seal the connected end of the supply conductor i i.

In order to prevent displacement of the end turns of the conductor and in order to strengthen the heating pad at these portions, suitable reenforcing strips 48 formed from some suitable mechanically strong material and one which is not thermally plastic, such as vulcanized rubber,

are placed over the end turns of the convolutions.

may be placed over each end turn, I prefer to form the reenforcingstrips so that they will have a length suflicient to cover all of the end turns at one end of the sheet l2. As shown in Fig. 4, these strips'have a length substantially equal to the width of the sheet i2, and, moreover, the end portions of these strips are curved inwardly so as to cover the end turns of the first and last convolutions formed in the pad. These reenforcing strips preferably will be secured to the pad by means of a suitable adhesive, such as rubber cement.

Likewise, suitable reenforcing members 50, shown as of disc form, are secured to the opposite sides of the sheet I! over the thermostat I. These discs preferably will be formed of fabric rubber, the fabric side being placed adjacent the sheet; and will be secured to the sheet in any suitable manner, as bymeans of an adhesive, which, as before, may be rubber cement.

After the reenforcing members l6, l8 and 50 have been applied to the sheet I2, suitable sheets of coalescent rubber and 52 having areas substantially equal to the area of the sheet I2 are placed on the opposite surfaces of the sheet so as to substantially cover the areas of these surfaces.

The sheets I2, SI and 52 are then coalesced so as to form a single flexible body in which the resistance conductor, thermostat and the reenforcing members are embedded. This operation may be efiected by vulcanizing the sheets. To effect this vulcanizing operation, the assembled sheets may be placed in a suitable pressure device (Fig. 7) for applying a suitable pressure to the members during the vulcanizing heating step. This pressure device comprises two plate members 5i and 55 provided with recesses 54a and which when the members are placed togather define a chamber having the same shape that the finished pad is to have, including a portion arranged to define the teatlike projection (Fig. 1) embracing the connected end portion of the supply conductor and which portion, as has been pointed out, will be formed by the vulcanizing of the

members

46 and 41 during the vulcanizing step; By means of this vulcanizing process the sheets I2, 5i and 52 and also the members 45 and H are coalesced into a single homogeneous body.

The reenforcing members 41 eflfectively prevent displacement of the end turn of the resistance element during the vulcanizing step. It will be understood that during this step the rubber tends to expand somewhat when the temperature is raised, whereas the cotton cord on which the resistance conductor is wound is practically non-extensible; as a result of this tendency it is possible that the end turns of the element would be pulled from the groove I3 if the members i8 were omitted.

The members 18 and the groove I3 also overcome any tendency of the conductor to become displaced by any flowing action of the rubber sheet 12 during the vulcanizing operation. As is well understood by those skilled in the art, it is possible that the rubber will tend to flow somewhat during the vulcanizing step because of the difference in thickness of the central portion and the remaining portions of the sheet I2; if the groove I3 and members 48 were omitted it is quite likely that some of the turns and lengths of the heating element would become displaced by reason of this tendency of the rubber to flow.

The resulting heating pad is practically a single sheet of flexible rubber with a resistance heating conductor and a controlling thermostat embedded therein, and also reenforcing members positioned at those portions of the pad where if they were omitted it is possible that the pad would be weakened. The resistance conductor is firmly anchored in place so that it is impossible for any of its turns to become separated from each other or displaced.

Moreover, the rubber forms a water-proof and air-tight casing around the resistance conductor, thermostat and reenforcing members; also a substantially waterand air-tight seal is arranged about the connected end portion of the supply conductor II.

The non-extensible cord Ha on which the resistance conductor is wound prevents lengthwise stretching of the pad; this protects the heating element and also strengthens the pad.

Furthermore, by reason of the fact that the rubberbody of the pad is a good conductor of heat, it is not likely that the portions of the body adjacent the resistance conductor will deteriorate.

It will be understood in view of the fact that the thermostat I6 is subjected to considerable pressure during the vulcanizing proces it must have considerable mechanical strength. I have provided a thermostat having a comparatively great mechanical strength and also having a fairly simple and inexpensive construction.

Referring to Figs. 8 and 9 it will be observed that the thermostat I6 comprises a pair of casing members 80 and BI. These members are arranged substantially alike, and each, as shown,-

is of circular form and is provided with a depressed annular portion defining a channel 62 so that when the members are placed together an annular chamber 620 (Fig. 9) is defined. Each depressed annular portion is defined by a central circular upraised portion 63 and a peripheral upraised flange portion 64 provided on its associated casing member. This latter flange portion, it will be observed, has provided at its upper edge a substantially flat horizontal flange portion 65 projecting outwardly therefrom and this in turn is provided with another flange portion 56 projecting upwardly therefrom and extending through an arc of approximately 150. At one end of these latter flanges are arranged the binding posts 21. As shown, these posts comprise enlarged portions having their ends folded on themselves.

On one of the casing members, as for example the member 60, is mounted a curved thermostatic bar 61. This bar is shown as a bimetallic bar made of two strips of metal having dissimilar tempera-ture coefficients of expansion.

such as brass and invar, these strips being securely welded or brazed together lengthwise. As shown this bar has a curvature substantially the same as the channel 62 "provided in the member and is arranged in an upright position in the channel. The bar is secured at one end to the member in any suitable manner, as by brazing, and has its opposite end free to move circumferentiaily upon distortion of the bar due to temperature changes. On this free end is mounted a contact, preferably formed of silver, this contact being secured to the bar in any' suitable manner, as by welding.

0n the other casing member is mounted a silver contact III secured to the casingin any suitable manner, as by welding.

The members El and SI are assembled with their flanges 65 facing each other, and the bimetallic bar 61 and contact III are so arranged on their respective members that when the members are assembled the contact II is secured between the free and fixed ends of the thermostatic bar, and bearing against the free end of w the bar so as to hold the bar under an initial tension. The members and SI are secured by means of a suitable eyelet II passed through central apertures 12 provided in the members. A suitable insulating bushing 13 is arranged in these apertures about the eyelet, this member being provided with a flange arranged to be interposed between one head Ila of the eyelet and the casing SII; an insulating washer II Is interposed between the other head Ilb of the eyelet and the casing 6|.

Interposed between the flanges of the members S0 and GI is a suitable insulating washer I5, and interposed between the upraised portions 63 is a similar washer IS. The washer I5 Is 82- cured by tions it. 11, which maybe formed of mica, is placed between the bimetallic bar and the memb r 61, the member 11 being placed so that the contact 10 is received between its ends. This arrangement ofthe members 60 and ii is important because any pressure applied to the sides of the members, as during the vulcanizing step, merely forces the members into firmer contact with the insulating

washers

15 and 16. Consequently, it is quite unlikely that any of the rubber softened during the vulcanizing step will gain admittance to the interior of the thermostat casing, in spite of the fact that the rubber will be under considerable pressure.

It will be observed that the casing members 60 and iii are arranged for limited relative rotative movement, the-limits of this motion being defined by the cooperating ends of the flange portions 66. This limited motion is provided so that the tension in the thermostatic bar 61 can be varied whereby the setting of the thermostat can be adjusted.

It will be observed by reason of the foregoing arrangement that when the two conductor portions 40 (Fig. 4) are secured to the binding posts 21 the thermostat is connected in circuit with the conductor I4. The circuit through the thermostat, it will be observed, is traced from one binding post 21, as for example that carried by the member 60, through the member 60 to the bimetallic bar 61, and thence through the contact 10 and easing 6| to the other binding post 21.

It will be understood that in the operation of the heating pad, the thermostatic bar 61 is distorted when a change: in temperature occurs, whereby the bar tends to break the circuit through the thermostat when the temperature of the pad increases, untilfinally when a predetermined high temperature is reached the contacts'will be separated and the heating circuit broken. As the pad cools, the bar will tend to close the heating circuit and will close this circuit upon the occurrence of a predetermined low temperature. In this manner the thermostat operates to maintain a substantially constant temperature throughout the pad; it is to be noted that by reason of the fact that the rubber body of the pad is a good conductor of heat, the temperature in that portion of the pad adjacent the thermostat is substantially the same as the temperature in the remaining portions of the pad. As a result, the thermostat. responds to the temperature of the pad body itself.

It will be understood that the thermostat will be adjusted initially, i. e., before it is assembled with the sheet l2, by moving the casing members 60 and BI relatively to each other so that it will operate to maintain any suitable temperature throughout the pad, the members being secured in their adjusted position by means of the eyelet 1|.

It is to be understood that while a single resistance conductor and controlling thermostat thereforhave been shown, several resistance conductors and a controlling switch may be provided so that different .pad operating temperatures can be secured by arranging the conductors in various electrical circuits. In this case, thermostats may be provided to limit the temperature rise of thepad. Also, instead of only one thermostat several thermostats may be provided in the several circuits of the various elements.

means of the cooperating flange por- I An arcua-te shaped insulating member .tor formed into a series of convolutions com- While I have shown a particular embodiment of my invention, it will be understood, of course, that I do not wish to be limited thereto, since many modifications may be made, and I therefore contemplate by the appended claims to cover 5 any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric heater comprising a resistance 10 conductor, a flexible, substantially homogeneous, heat conducting and water-proof electrically insulating body formed of flexible rubber embedding said resistance conductor, a thermostat electrically connected in'the circuit of said resistance 15 conductor embedded in said body of rubber so as to control said circuit responsively to the tem perature of said body and reenforcing members on opposite sides of said thermostat embedded in said body of rubber.

2. An electrical heating pad comprising a body of flexible rubber, a resistance element formed into a plurality of convolutions embedded insaid body of rubber and reenforcing members positioned on the end turns of said convolutions so as 25 to strengthen said pad.

3. A flexible, electricalheating pad comprising a sheet-like body of flexible rubber, a heating element embedded in said body of rubber formed into a plurality of elongated convolutions, an insulated electrical supply conductor attached to said body and electrically connected to said heating element and reeniorcing strips embedded in said body adjacent the end turns of said convolutions and at the point of attachment of said supply conductor to said pad.

4. An electrical heating pad comprising a flexible body of rubber, a resistance element embedded in said body of rubber, said resistance element being formed into a plurality of substantially parallel convolutions extending lengthwise of said body, a controlling thermostat embedded in substantially the central portion of said body and connected to control said resistance conductor, reenforcing members embedded in said 45 body crosswire thereof and over the end turns of said convolutions and reeniorcing members embedded in said body on the opposite sides of said thermostat.

5. An electrical heating flexible body of rubber, a resistance conductor embedded in said body so as to be distributed throughout a substantial portion-thereof and so that its ends are brought to points adjacent each other, an electrical supply conductor secured at 55 one end to the ends of said resistance conductor, said rubber body having a portion embracing the connected end portion of said supply conductor and a covering of flexible rubber for said supply conductor integrally united withjsaid flexible rub 60 ber body. I

6. An electrical heating pad comprising a sheetlike body of flexible rubber, a resistance conducprising a plurality of substantially parallel lengths embedded in said rubber body so as to be distributed over a substantial area of said body, a controlling thermostat connected to said resistance conductor and embedded in said body of 7 rubber, and means for reenforcing the portions of said pad covering the end turns of said convolutions and said thermostat.

7. An electric heater comprising a resistance element, rubber sheets vulcanized together so as 75 device comprising a 50 to embed said resistance element between them, and a thermostat embedded between said rubber sheets, said thermostat comprising superposed casing members and sealing means interposed between them arranged so that an application or" pressure to said sheets forces said casing members into firmer sealing relation with said sealing means.

8. An electric heating pad comprising a resistance element, sheets of rubber coalesced together so as to embed said resistance element between them and a thermostat connected elec trically with said resistance element and embedded between said sheet-s, said thermostat comprising control means including a temperature responsive element and a pair of superposed casing members each having a channel formed therein and secured together so that said channels define a closed chamber for said control means, and sealing means interposed between said casring members.

9. An electric heating pad comprising a resistance element, rubber sheets vulcanized together so as to embed said resistance element between them, and a substantially flat thermostat for controlling said resistance element embedded between said vulcanized sheets, said thermostat comprising temperature responsive control means and a pair of substantially flat superposed casing members defining a chamber for housing said control means, and sealing means interposed between said casing members arranged so that an application of pressure to the sides of said casing members forces said members into firmer sealing relation with said sealing means.

10. An electric heater comprising a resistance conductor formed mainly of nickel, and a flexible, substantially homogeneous, heat conducting and water-proof electrically insulating body formed of rubber embedding said resistance conductor.

JOHN H. PAYNE.