US 2938992 A
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Description (OCR text may contain errors)
May 3l, 1960 R. E. CRUMP HEATERS USING coNDucTIvE wovEN TAPES Filed April 18, 195s r' l| f| ufl rroeA/Exs.
United States Patent O HEATERS USING CONDUCTIVE WOVEN TAPES Ralph E. Crump, Granada Hills, Calif., assignor to Electrolilm, Inc., North Hollywood, Calif., a corporation of California Filed Apr. 18, 1958, Ser. No. 729,297
9 Claims. (Cl. 219-4'6) This invention relates to improved kelectric heating units which are in certain respects improvements on the type ofheating element disclosed and claimed in copending applications Serial No.` 596,064, led July 5, 1956, now abandoned, by Arthur N. Heath on Sheet Or Layer Form Heating Elements, and Serial No. 644,070 filed March 5, 1957, now Patent No. 2,884,509, by Arthur N. Heath on Heating Element Containing a Conductive Mesh.
In the above mentioned applications, there has been disclosed a unique type of electric heater unit in which the electrically resistive heater element takes the form of a closely woven mesh. At least some of the strands forming this mesh are of electrically conductive material, and offer sufficient resistance to develop heat when current is passed through the mesh. Preferably, the heater unit includes a layerof non-conductive material, such as a suitable resinous plastic material reinforced by and irnpregnating a sheet of cloth, and bonded to one side of thev woven mesh to hold the various strands of the mesh in fixed relative positions, and to insulate the mesh. For best results, and to provide an ultimate product of maximum usefulness, two such insulative layers are provided at the two opposite sides respectively of the mesh.
The general object `of the present invention is to provide a heater of the above discussed type which is especially designed so that its mesh-form heater element may be manufactured continuously as an elongated'thin tape or strip of material, which can then be cut to any desired length and be applied with maximum facility to any of numerous different uses. Particularly contemplated is a tape of this type which isso constructed that, after it has been cut to a desired length, both of the power leads to the `tape can be connected to the same end of the tape. This feature has proven in practice to be of very great importance, since in many of the situations in which the present type of heater can be used, the two ends of the tape 'must necessarily be separated a substantial distance from one another, so that it becomes very inconvenient to connect the power lead's `to these two different ends of the tape. Y
In order to allow connection ofk the two power leads to a common end of the tape, l so form the tape as to have two conductive areas extending longitudinally along the tape, withk a non-conductive area of the tape vextending longitudinally between the two conductive areas. In forming a heater unit from this tape, the two power leads are connected to the two conductive areas at a rst end of the tape, and these two areas are electrically connected together at the other end,` so that current ows rst in one direction along a first of the conductive areas and then in a reverse direction along the other area.
The tape includes some non-conductive strands', typically iiber glass, in addition to the conductive strands. These non-conductive strands form the non-conductive intermediate area of the tape, and also may be woven through the `two conductive areas to assist in holding the conductors in accurately predetermined relation duringY "ice manufacture, and to thus give the tape an accurately predeterminable and controlled resistance characteristic; The conductors may in some cases extend only longitudinally of the tape, but for optimum results they preferably also extend transversely of the tape, at the con'- ductive areas, so that the transverse strands can function as potential equalizers for conducting equalizing current transversely between corresponding portions of different longitudinal conductors. This potential equalizing function is particularly useful if one of the conductor circuits becomes damaged and partially opened at one location, in which case the transverse conductors allow a transverse flow of current as necessary to allow the current to pass the damaged location. l
The power connections to the conductive tape, and the cross connection between the two conductive areas, may be formed by bus elements engaging the conductors of the tape, and each confined between the tape and one of the insulative outer layers.` These bus elements may take the form of thin metal bus bars or screens lying against the tape at the desired terminal locations, to contact the tape conditions over an extended area.
The above and other features and objects of the present invention will be better understood from the following detailed description of the typical embodiments illustrated in the accompanying drawing, in which:
Fig. l is a plan view of a first form of heater unit constructed in accordance with the invention;
Fig. 2 is an enlarged fragmentary section taken on line 2-2 of Fig. l;
Fig. 3 is a greatly enlarged fragmentary section taken on line 3-3 of Fig. 4, and showing only the woven mesh portion of the unit, without the insulative layers;
Fig. 4 is an enlarged fragmentary plan view of the woven mesh of Fig. l; i I
Fig. 5 is a view corresponding to Fig. 1, but showing a variational form of the invention;
Fig. 6 is a section taken on line 6 6 of Fig. 5.
Fig. 7 is an enlarged fragmentary plan view of the woven mesh in another variational arrangement.
Referring iirst to Figs. l to 4, the heater unit 10 shown in those gures takes the form of a thin laminated composite sheet-like unit, typically applied to the surface of a carrier member 11. This carrier member may be any of a wide variety of types of parts, which it is desired to heat, or to radiate heat from, as for instance an aircraft part which is to be heated to prevent icing, an electronic component or device which is to be heated to render it unsusceptible to ambient temperature fluctuations, or a pipe or tube carrying a chemical which should not lose its heat (e.g., liquid sodium). 1n the Fig.`l form of the invention, it may be assumed that the unit 10 is bonded directly to carrier part 11, and this carrier part may be either rigid or flexible as desired.
The composite heater v10 includes an inner elongated strip of woven tape or mesh 13, having two spaced parallel and desirably .identical conductive areas 14 and 15 adjacent its two parallel opposite side edges 16. Be tween the two conductive areas 16, the tape has a nonconductive area 17, extending along the entire length of the tape. At one end of the heater 10, the two conductive areas 16 are electrically connected together by a bus element 1-8, while at the other end `of unit 10, areas 16 are connected respectively to a pair of individual buses 19 and 20, to which the two leads 21 of a power supply cord 22 are connected. Cord 22 may have a conventional plug 23 connected to its end, and adapted to be plugged into a receptacle represented at 24, which receptacle is connected to a power source represented at 25. The power supplied to theunit by source 25 may typically be l2() v. A.C. The wovenkmesh 13 is confined between and bonded to a pair of Vlayers 50 of insulative material, which eectively 11 or any other part (except for the previously mentioned connection to power source The mesh or inner heating element 13 includes a number of elongated strands or wires 26a and 26h made of an electrically conductive material, and forming the electrically conductive areas 16 of the tape. Also, the woven mesh includes a number of elongated strands 27a and 27b which are woven together across the entire width of the tape or mesh 13, and which are `formed of a' less conductive and preferably electricallyinsulative material. The conductive strands 26a and 2Gb may be formedl of any suitable material such as a nickel chrome alloy, tungsten alloy, alluminurn alloy, stainless steel, Monel` metal or the like. The insulative strands 7.7./.1 and' 27h are in most instances preferably formed of glass fibers, but may also be formed of Y'asbestos or any other fibrous material able to withstand the high operating temperatures with each of the strands 27a and Z719 being formed of a large number of very iine glass fibers grouped together to form in essencev a single strand. The-strands 26a, 27a and 27h are woven together in any conventional or suitable weaving pattern, so that the various conductive wires 26a andv 2Gb are interwoven and interloclted with respect to each other and with respect to the conductive strands 27a a-nd 27h. Preferably, the conductors and non-conductors extend both longitudinally of the mesh 13 and transversely thereof with the longitudinal conductors being in contactwith the transverse conductors. As will be understood, the non-conductive strands 27a and 27b hold the conductors 26a and 26b in predetermined spaced relationship, `so that hte overall mesh formed by 26a, 26h, 27a and 27h caribe given an accurately predeterminable and controlled electrical resistance and heating characteristic.
With more specific reference to the particular weave pattern illustrated in Figs. 1 to 4, `it is noted that in this arrangement the non-conductive strands designated 27a extend directly longitudinally of the woven mesh 13, that is, directly parallel to the opposite side edges 16 of the mesh or tape. These longitudinal non-conductive strands are spaced unformly and rather closely across the entire width of mesh 13 (that is, between the two opposite side edges 16). The non-conductive strands designated 27b in Fig. 4 extend directly transversely of strands 27a.v and therefore directly transversely of the tape 13. These non-conductors 27h are spaced uniformly and closely along the entire length of the woven mesh 13, and each of the transverse conductors 27b desirably extends across the entire width of the mesh, from one of the edges 16 to the other edge 16. Preferably, all of the transverse strands 27 of the mesh are portions of a single continuous strand, which first extends across the tape in one direction, is then doubled back at one of the edges 16 (as at 28 in Fig. 4) to extend across the tape in the opposite direction, and then is doubled `back at the opposite edge 16 to return again in the first mentioned direction, etc., so that a single strand may forrn all of the transverse non-conductors for the entire length of the tape. As will be apparent, each of the strands 27a or 27b is woven alternately over and then under successive strands extending in the opposite direction, so that the strands are all effectively integrated into the overall woven tape, as shown in Fig. 3 and 4.
In the Figs. l to 4 arrangement, the conductive strands 26a extend longitudinally of the tape 13, and parallel to side edges 16, and are spaced uniformly and closely across the width of each of the two conductive areas 14 and 15. That is, for conductive area 14, the strands 26a are provided between the left hand edge 16 in Fig. l, and a line 29 which defines the inner edge of conductive area 14. Similarly, for conductive `area 15, the strands 26a are provided at closely spaced and uniformly spaced locations between the right hand edge 16 in Fig. l, and the inner edge 30 of conductive area 15. These edges 29 and 30 of the two conductive areas extend parallel to insulate mesh 13 from parth i one another and parallel to side edges 16 of the tape,
and are spaced apart a substantial distance to provide the non-conductive central area 17, The width of area 14 is desirably the same as the width of area `15. Pref-- erably, one of the longitudinal conductors 26a is pro-- vided for each of the longitudinal non-conductors 27a,l across the widths of the areas 14 and 15, with each of' the conductors 26a being woven in and out with respect to the transverse strands in unison with, and along with,
step with the first two strands, that is, when the first twoy longitudinal strands extend upwardly over a particular transverse strand, the second two longitudinal strandsy extend under that transverse strand, and vice versa.
The transverse conductors 26b of each of the nonconductive areas 14 or 15 may comprise portions of a. single elongated wire, which is woven alternately in op-y posite transverse directions in somewhat the same man-- ner as has been previously described in connection with transverse non-conductive strands 27h. More specifically, the wire forming the transverse strands 26b of conductivey area 14 can first extend in one transverse direction, and then be doubled back at one of the side edges 16 (see 31 in Fig. 4), following which the wire extends in the reverse transverse direction to the location of inner edge 29 of the conductive area, at which location the wire is doubled back (as at 32 in Fig. l) to return toward edge 16, etc., for entire length of the tape. As the wire 26h extends transversely, it may on each run be woven over and under the longitudinal strands in unison with an associated one of the transverse non-conductive strands or runs 27b. At each of the points at which one of the transvers conductors 26b crosses one of the longitudinal conductors 26a, the transverse and longitudinal conductors are preferably in direct electrical contact so that the conductors of each of the areas 14 and 15 form together a woven conductive mesh, with the wires of this mesh being held in predetermined relation by the associated non-conductive strands 27a and 27 b.
Desirably, the total cross sectional area of the longitudinal non-conductors 27a within each conductive area 14 or 15 is greater than the total cross sectional area of the longitudinal conductors 26a in that same conductive area, and is preferably several times as great. The same is also preferably true of the relationship between the cross sectional areas (taken in any longitudinal plane within area 14 or 15) of the transverse non-conductors and conductors. The individual wires 26a and 2Gb may be as small as .0002 square inch in cross sectional area (.04 circular mills), and .0005 inch in diameter. Nonconductive strands 27a and 27b may have cross sectional area of .0005 to .060 inch (total cross section of all glass fibers forming one strand). The meshsize of the heating element many range between about l0 and 400 of the glass strands per inch.
The two insulative layers 50 between which conductive mesh 13 is conned, may be formed of any of various materials capable of providing suitable protection to the heater unit against electrical contact with nearby parts. Suitable and preferred insulating materials may be selected from the general class of plastics, elastomeric or non-elastomeric, including such thermosetting resins as the phenol formaldehyde, urea formaldehyde, melamine modified phenol formaldehyde or urea formaldehyde, epoxide, polyester and polysiloxane resins, and such elastomers as the heat resistant silicone rubbers and neoprene. The properties and particularly the thermal resistance of all such plastics being known, it is but a matter of selecting among them one or a combination of materials However, the in and out weaving of' the second ltwo longitudinal strands is of course out of' assegna capable of retaining such properties as chemical Vor body stability, dimensional stability, and iiexibility if desired, under the. temperature conditions to which the insulation may be subjected by the mesh heating element 13.
I prefer to reinforce either or both insulating layers 50 by the use of woven fabric such as Woven glass. In practice the insulating layers 50 may be formed by impregnating the fabric with any ofthe above mentioned plastics and laminating type resinous materials, and then applying the impregnated fabric and the plastic material to opposite sides of element 13 prior to curing. The reinforcing fabric may be impregnated with the appropriate laminating plastic by any of various possible methods, such as spraying a solvent solution of the resin (either catalyzed or uncatalyzed, depending upon the resins used) onto one or both sides of the fabric, or by dipping the fabric in the resin and then suspending the fabric vertically to drain off excess resin, or by'knife coatingthe fabric with the resin at an appropriate and uniform thickness. Any'such methods may be used to produce impregnated fabricswhich may be air dried or forced hot air dried for an appropriate time to evaporate any solvents or to partially set the resins prior to a iinal cure which may occur during -the laminating process. After drying, the fabric maybe stored for subsequent use in the manufacture of thelaminated heating unit. If desired, each of the insulative layers 50 may include more than one of the sheets of plastic impregnated reinforcing fabric, in order to increase the overall strength of the composite unit.
The three terminals or bus elements 18, 19 and 20 may be Iformed of conventional metal screen or mesh, made up entirely of conductive` metal wires Woven together to produce a mesh which has considerably greater electrical conductivity and less resistance than the heater mesh 13. The buses 18, 19 and 20 may be formed of any of the same conductive metals which may be utilized as the conductors 26a and 26b in heating element 13, but the wires of screens 18, 19 ,and 20 are Ydesirably considerably larger in diameter `than conductors 26a and 26h, and are desirably considerably closer together than the conductors V26a and 26b`of heater 13. The mesh size of screens 18, 19 and 20 may typically range between about 30 and 400 per inch'. The twoscreens 19 and 20lie across the upper Vsides of iirst'ends of conductive areas 14 and 15, between the mesh' and the upper insulative layer 26, and are in direct electrical contact with the conductors 26a and 26h of element 13 `for conducting electricity thereto. Screen 18 is similar but extends across the entire width of the woven tape, to form an electrical connection between second ends of the two areas 114 and 15. Preferably, the screens, 18, 19 and 20 are soldered to conductors 26a and 2Gb prior to the bonding together of the insulative layers 26. Also, the wire leads are soldered to the two buses 19v and 20', so that current will ow from one of these buses along a iirst of the conductive areas 14 and 15, then across the bus 18, and then back along the second conductive area. Before layers 50 are placed about the inner heater parts, an uncured resin may be applied as at 131 about the buses 18, 19 and 20 and associated portions of the mesh 13, to completely encapsulate and protect the terminal areas. This resin may be of the same composition as that used in layers 50, and maybe cured at the same time as those layers. Preferably, layers 50 have peripheral edge portions at 32 extending laterally beyond the tape and terminals at all points, to form a continuous peripheral area at which layers 50 are directly bonded together.
Alfter theparts 13, 26, 18, 19, 20' and 131 have all been assembled in the relation illustrated in Figs. l and 2, these parts may be heated under pressure to a temperature capable of curing the resin, or other curable or hardenable material contained in layers 50 and regions 131, to bond these, layers 50 tightly together and to the intermediate heater element 13 and bus assemblies. This bonding together of the laminates or layers may be accomplished by means of a heated hydraulic press, f or example of the type used'in the making of plywood. This press may typically be adapted to compress the various layers together at a pressure that may be upwards of 500 pounds per square inch, and which is sufficient to compact the layers to very thin hard form. During such compression, the resin or other curable material is heated at an appropriate curing temperature, say upwards of 350 Fahrenheit, over a suitable period of time, typically 15 minutes, to thermally polymerize or set the resin to form a composition that will remain dimensionally stable and resistant to the heat which the element is designed tc` generate.
If desired, the laminating process may be performed by means of the known vacuum bag technique, in which the various layers to be bonded are placed in a plastic, rubber or metal foil bag from which the air is evacuated. The evacuated bag is then placed in a controlled ternperature oyen to cure the resins and form the laminating bonds at reduced pressure. This method may be utilized to fasten flexible laminated heating elements onto parts requiring heating of the layers in deformed or non-planar shapes which can not be formed practically by hydraulic press lamination.
As will be apparent, the pressure curing of the resin in layers 50 and regions 131 will force that resin into the interstices or apertures formed between the strands 26a, 2611, 27a, and 27b of heater element 13, to tightly bond the resin to the strip 13, while the two layers 50 are being correspondingly bonded tightly together about the periphery of element 13, and are being bonded to the buses at the ends of element 13.
It is contemplated that where the carrier part 11 is of a type permitting bonding of the heater unit 10 directly thereto during formation of the heater unit, such direct bonding will be effected by placing the part 11 in the hydraulic press or other laminating equipment together with the parts 13, `26, etc. Aof unit 10. The laminating process then cures the resin Aofthe under layer 50 while it is directly in contact with part 11, to tightly bond the resin and unit 10 to part A11 over the entire area of unit 10.
Figs. 5 and 6 illustrate another f orm of the invention which may be identical with that of Figs. 1 to 4 except for the arrangement of the conductors in woven mesh 113, and the formation of the bus elements 118, 119 and 120. Speciiically, the mesh 113 includes longitudinal conductors 126:1, correspondingto those shown at 26a in Figs. 1 to 4, but does not include transverse conductors such as those shown at 26b in Figs. 1 to 4. The arrangement of the glass strands, .insulative layers 150, encapsulating resin 231, and power leads y121 may all be substantially the same in Figs. 5 and 6 as in the rst form of the invention.
In Figs. 5 and 6, the bus elements 118, 1'19 and 120 may be thin sheet metal shim stock, of copper, stainless steel, or the like, with this shim stock being doubled to the Fig. 6 U-shaped configuration, to electrically engage both sides of tape 113. Preferably, the shim stock is precoated with solder, and after being deformed to the Fig. 7 condition, is heated to melt the solder, while part 118, 119 or 120 is tightly clamped against the tape, to thus form a conductive solder connection between the bus elements and the conductors of tape 113. The encapsulating resin 231, and the insulative outer layers 150, are then applied in the same manner as in Figs. l-4.
Fig. 7 is similar to Fig. 4, but illustrates fragmentarily the woven mesh portion of another form of the invention. This third form is identical with that of Figs. 1-4, except for the'deletion of the longitudinally extending conduc tors 26a` of Figs. 1 4. That is, the mesh 213 of Fig. 7 has only transverse conductors (226b),- corresponding to conductors 2Gb of Figs. l-4. A s in the rst form of the invention, all Vof these transverse conductors (in each of the two conductive areas) are` portions of a single wire which is woven back and forth as in Fig. 4, and whose ends are in electrical contact with the bus elements (see bus 218 in Fig. 7). Thus, current flowing through the heater of Fig. 7 must follow a very circuitous back and forth path, to give to the element a very high electrical resistance.
In all of the forms of the invention, the total overall thickness of the unit 10, including mesh 13, 113 .or Z13, and layers 50, 150 or 250 (but not taken at the locations of buses 18, 19, 20, etc.) may typically be between about .015 inch and 1A of an inch. The insulative layers 50, 150 and '250 may typically be between about .001 inch and 1A; of an inch. The number of longitudinal conductors present in each form of the invention is preferably between about and 100 per inch, and the number of transverse conductors is preferably between about 0 and 50 per inch. In the case of both the longitudinal and transverse conductors, fit may be desirable in some cases 4to provide conductors at only alternate non-conductive bers, or at every -second or third non-conductor, or even less frequently where necessary to achieve a desired overall result.
In using any of the three forms of heater, the user merely connects plug 23 into power socket 2.4, so that current flows through the conductive areas 14 and 15 in opposite directions, to produce heat. The overall resistance of these areas is such as to develop a desired amount of heat at a particular predetermined operating voltage. Since the two power leads 21 (or 121) are connected to the same end of unit (or 110), the unit may be very long if desired, and may be applied to a carrier part of virtually any shape. For instance, if the part 11 is left olf of the Fig. 1 arrangement, and the resin and other parts are chosen to give the tape flexibility, the heater unit may be wrapped as a long tape about a pipe, or may be wrapped about a work piece of virtually any other shape. This would not be as easily attained if the two leads 21 came out of opposite ends of the elongated tape like unit.
As an example of a particular unit made in accordance with the present invention, a steel plate of approximately .050 inch in thickness was degreased and sandblasted to provide surface irregularization. Two layers of approximately 60 mesh close weave glass cloth impregnated with phenolic resin were then laid on the steel plate, over which was applied a mesh heating element such as that shown at 13 in Figs. 1-4. The heat or strip had one conductor 26a-or 26b for each principal glass strand 27a or 27b, and had approximately 40 of each per linear inch. Each principal glass strand 27a or 27b included a large number of glass bers. The cross sectional area of the longitudinal glass ibers was about 20 times as great as the cross sectional area of the longitudinal conductors 26a, and the same was true of the transverse glass and conductive strands. The 'conductors 26a and 26h used in the example had a diameter of about .002 inch, and were formed of constantan while the `composite cross sectional area of all of the glass fibers in each strand 27a or 27b was about .000006 square inch.
Over the strip 13, there was applied another layer of the resin-impregnated glass cloth, with terminal screens of 400 mesh Monel metal `being positioned as shown at 18, `19 and 20 in Fig. l, and 'being soldered to the conductors of the heater strip 13, and encapsulated in phenolic resin as at 131. All of these layer and the base plate were then covered on both sides with double thicknesses of uniformly textured iblotter paper and placed in a hydraulic press whose flat platens were heated to 350 F. and which were then closed upon the layers at a pressure of approximately 500 p.s.i. for approximately minutes.
As a second example, the foregoing procedure was employed using, in place of the impregnating and encapsulating resin mentioned before, a phenolic-epoxide resin sold by Narmco Resins and Coatings Co. as Conolon 506. Also, the. heater strip 13 was preimpregnated with uncurde Conolon 506 resin prior to the lamination process, to assure thorough bonding of the strip 13 to the other parts. The fabric utilized in this example -was formed of glass iibers and conductive strands of substantially the same sizes, mesh, etc. as in the irst example, except that no transverse conductors 26b were used (see Figs. 5 and 6). The bus terminals were made as shown in Figs. 5 and 6.
A third example was made using as the insulating layers a silicone resin impregnated onto a glass fabric. The heating unit was a common weave as shown in' Fig. 4, but included one of the conductors 26a or `2Gb (.002 inch diameter) for every third principal glass strand 27a or 27b (in areas 14 and 15). The heater strip was pre-treated with Dupont Volan to enhance the adhesion of the resin to the glass fibers. This unit was not applied over a rigid backing plate, but was laminated in such a manner that it produced a free standing and flexible unit which was subsequently bent to fit-over a part of non-planar shape, and was secured thereto.
Any of the described laminations, after being bonded as described, may Ibe coated with an insulating varnish that favors preservations and protection of the product, and enhances its appearance.
1. An electrical heating element comprising an elongated thin electrically conductive woven mesh tape composed of elongated strands of material extending in at least two different directions and each closely interwoven in a predetermined pattern with a series of the other strands, some of said strands of material being electrically conductive and offering sufficient resistance to develop heat upon passage of electricity therethrough, said conductive strands being arranged to form two conductive areas extending longitudinally of the tape and spaced laterally apart and insulated from one another for an extended distance, others of said strands being relatively non-conductive of electricity and being interwoven with said `conductive strands and forming a nonconductive area elongated longitudinally of the tape and located laterally between said two conductive areas said conductive strands including wires running continuously back and forth transversely of and at opposite sidse of said non-conductive area.
2. An electrical heating element as recited in claim l, including means for connecting two sides of an electric power source to said two conductive areas respectively at a first location along the tape, and cross connecting means for electrically connecting said two conductive areas together at a second location along the tape spaced longitudinally of the tape from said first location and so that current from said source -will flow in a first direction longitudinally of the tape along one of said conductive areas and then through said cross connecting means to tlow back along the other conductive area in the second longitudinal direction for return to said source.
3. Anelectrical heating element as recited in claim l, including a layer of non-conductive insulating material bonded to and insulating at least one side of said woven mesh tape.
4. An electrical heating element as recited in claim l, in which some of said conductive strands extend longitudinally of the tape and are interwoven with a series of the non-conductive strands extending transversely of the tape.
5. An electrical heating element as recited in claim l, in which said conductive strands extend both longitudinally and transversely of the tape and are interwoven with one another and with the non-conductive strands along said two conductive areas, but are discontinued at said intermediate non-conductive area, said non-conductive strands extending both longitudinally and transf 9 versely of the tape throughout both said conductive areas and said intermediate non-conductive areas.
6. An electrical heating element comprising an elongated thin electrically conductive woven mesh tape composed of elongated strands of material extending in at least two different directions and each closely interwoven in a predetermined pattern with a series of the other strands, some of said strands of material vbeing electrically conductive and offering suflicient resistance to develop heat upon passage of electricity therethrough, said conductive strands being arranged to form two conductive areas extending longitudinally of the tape and spaced laterally apart and insulated from one another for an extended distance, others of said strands being relatively non-conductive of electricity and being interwoven with said conductive strands and-forming a non-conductive area elongated longitudinally of the tape and located laterally between said two conductive areas, said conductive strands including wires running back and forth transversely of and at opposite sides of said non-conductive area, rst connector means for connecting two sides of an electric power source to said two conductive areas respectively at a Ifirst location along the tape, cross connecting means for electrically connecting said two conductive areas together at a second location along said tape spaced longitudinally of the tape from said rst location and so that current from said source will ow in a. rst direction longitudinally of the tape along one of said conductive areas and then through said cross connecting means to flow back along the other conductive area in the second longitudinal direction for return to said source, and two layers of non-conductive insulating material bonded to and insulating the opposite sides of said woven mesh tape.
7. An electrical heating element as recited in claim 6, in which said Ifirst connector means and said cross connecting means are 4buses contacting said conductive strands and `confined between the tape and said nonconductive layers.
8. An electrical heating element as recited in claim 6, in which some of said conductive strands extend longitudinally of the tape and are interwoven with said wires running transversely of and at opposite sides of said non-conductive area.
9. An electrical heating element as recited in claim 6, in which said conductive strands extend both longitudinally and transversely of lthe tape and are inter-woven with one another and with the non-conductive strands along said two conductive areas, but are discontinued at said intermediate non-conductive area, said non-conductive strands extending both longitudinally and transversely of the tape throughout both said conductive areas and said intermediate non-conductive areas.
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