Heating Element Assembly

Abstract

There is provided an electrical resistance heating assembly for heating a fluid moving along a pathway in which the resistance element or elements are disposed along a serpentine or oscillatory wave path, which wave path is characterized by decreasing frequency in the direction of flow. Insulators are arranged in staggered, parallel rows at the points of reversal of direction of the resistance element or elements.

Parent Case Text

BACKGROUND OF THE INVENTION AND PRIOR ART

This application is a continuation-in-part of U.S. Pat. application Ser. No. 344,848, filed Mar. 26, 1973, now abandoned.

Description

The present invention relates generally to an electrical resistance heating element or assembly, and more particularly to the configuration of the heating element itself and the variation which occurs therein along the path of the fluid medium being heated thereby.

The use of electrical resistance heating elements for heating a fluid medium, such as air in electric clothes dryers, is well known. The U.S. Pat. to Fedor No. 3,651,304 shows a type of such heating element. To this end, a type of resistance element such as that disclosed in the aforesaid U.S. Pat. No. 3,651,304 is particularly well adapted. This element is a thin strip of apertured, foil-like material. It has been found that best results in terms of efficiency of power utilization and durability are achieved with an elongated electric resistance heating element when the temperature differential between the extremities of the element is maintained at a minimum, whether the device is operated at red heat or above or below red heat in the so-called "black heat" range. For heating a gaseous medium electrically, operation in the black heat range is preferred because less energy is lost by radiation, which energy is very inefficient for heating a gas. Heat transfer by convection is preferred.

It has been found that where the pathway traversed by the fluid medium while in contact with an electrical resistance type heating element is more than a few inches, e.g., 4 to 6 inches, the rate of heat transfer from the heating element to the fluid decreases moderately from the fluid inlet end toward the fluid outlet end. Where the heating element is operated at a wattage which is normally sufficient to maintain the heating element at a preselected temperature (for example, red heat in a quiescent gas state), movement of a gas across the heating element under such circumstances, while creating a black heat condition adjacent to the inlet, has insufficient delta T adjacent the outlet to prevent this portion of the heating element from persisting at red heat. This is evidence of an unduly high temperature differential in the heating element with attendant thermal stress. In heating a moving gas, operation of any portion of such heating elements at red heat is undesirable because it is inefficient in heating a gas and it adversely affects the durability of the heating element, requiring replacement more frequently than is otherwise necessary.

The present invention solves the foregoing problem of undue temperature differential in the heating element regardless of the power input level by providing a serpentine or oscillatory wave form path for the electrical resistance heating element which decreases in frequency of undulation as it approaches the exit end where the fluid leaves the heating chamber at an elevated temperature. In this way, power inputs normally productive of red heat in a quiescent gas system can be used in a moving gas system without experiencing red heat in any portion of the electrical heating element. Moreover, in fluid heating systems where it is desired to operate with the heating element at red heat or higher in a moving fluid stream, the present invention solves the problem of undue temperature differential from one portion of the heating element to the next.

It has also been found that where insulating elements are disposed at the points of reversal of direction of a heating element following an oscillatory wave form path, whether of uniform frequency or increasing frequency, there is a problem of local overheating and relatively large temperature differential within the heating element itself as it changes direction around the insulator. This condition obtains whether the heating element is operated intentionally in the red heat range or in the black heat range. Moreover, the portion of the insulator facing upstream is maintained relatively cooler by the flow of fluid or gas. However, on the downstream side of the insulator, the contact area between the insulator and the heating element is normally shielded and local overheating of the resistance heating element can occur. This again induces local overheating, which promotes chemical erosion of the element at these points and unnecessary loss of power by radiation.

In the preferred embodiments of the present invention, a solution is provided for these problems wherein generally rectangular, prismatic blocks of refractory material are utilized and angularly related to the direction of flow of the gases so that in the area of contact between the heating element and the insulating facing upstream, there is a "wiping effect" of the gas over the surface which tends to prevent local overheating of the resistance heating element in this region. On the downstream side, contact between the electrical heating element and the insulator is kept to a minimum, and because of the disposition of the block-shaped insulators, eddy currents are presumably generated which loop about behind the insulator and tend to cool the downstream side of the resistance heating element.

The preferred embodiments of the present invention also provide a box-like structure for supporting the insulators and the heating element thereon, enabling ready installation of the device and, if necessary, easy replacement of the heater assembly.

BRIEF STATEMENT OF THE INVENTION

Briefly stated, therefore, the present invention is in the provision of a resistance heating assembly for heating a fluid moving along a pathway and including a plurality of insulators arranged in staggered, parallel rows at the points of reversal of direction along an oscillatory wave path. A flexible elongated electrical resistance material of any convenient form (e.g. thin, narrow metal strips, wire, or apertured strips of metal) is provided and supported by the insulators at intervals along the length of said material, the material being formed as a fluid intercepting grid of maze along an oscillatory wave path of decreasing frequency which crosses and recrosses several times the direction or pathway of fluid flow. Means are provided for supporting the insulators along the wave path and electrical terminal means communicating with the extremities of the electrical resistance material are also provided.

The present invention also contemplates, but is not limited to, an insulator comprising a generally rectangular, prismatic block of refractory material, e.g., a refractory metal oxide composition, having a laterally extending acircular projection from each of the short sides, respectively, and a pair of symmetrically disposed, shouldered, recesses of like geometric configuration extending inwardly from the same longitudinal side of the block. These insulators are preferably disposed in angular relationship to the path of fluid movement so as to provide for maximum contact on the upstream side of the insulator, minimum contact on the downstream side, and cooling on the downstream side such as by the effect of eddy currents generated by the air stream passing over the edge of the rectangularly prismatic block form insulator.

The heating assemblies of this invention are adapted for disposition in a receptacle defining a fluid conduit in which a fluid, e.g., air, flows, for example, as in a clothes dryer.

The present invention also contemplates a unitary structure composed of a box-like frame for holding insulators arranged in staggered parallel rows at the points of reversal of direction along the oscillatory wave path. A preferred embodiment of the elongated flexible electrical resistance material is that which is disclosed in the aforementioned U.S. Pat. No. 3,651,304. The disclosure of the aforementioned patent is, therefore, incorporated herein in its entirety by reference thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood by having reference to the annexed drawings, wherein:

FIG. 1 is an illustration in perspective showing a preferred embodiment of the present invention and illustrating a use of a "folded" apertured thin electrical resistance element supported in a framework along an oscillatory wave path of decreasing frequency in the direction of gas flow.

FIG. 2 is a top elevation of a resistance heating assembly in accordance with the present invention.

FIG. 3 is a side elevation of the resistance heating assembly shown in FIG. 2.

FIG. 4 is a plan view of a ceramic insulator useful in accordance with the present invention.

FIG. 5 is a diagrammatic illustration of the gas flow on the upstream and downstream sides of a pair of insulators.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring now more particularly to FIGS. 1-3, inclusive, illustrative of a preferred embodiment of the present invention, there is provided a box-like frame, generally indicated at 10, formed of longitudinal angle beams 12, 14, 16, and 18, which are parallel to each other and extend between corresponding corners of rectangular end frames 20 and 22 to which the ends of angle beams 12, 14, 16, and 18 are secured by any suitable means, for example, as by spot welding. End frames 20 and 22 may be made adjustable in one direction if desired to facilitate assembly and take up manufacturing tolerances. For example, two relatively movable U-shaped portions may be used to provide such an adjustable rectangular end. After a proper dimension is established between the beams 12 and 14 and between the beams 16 and 18, the ends of the cooperating U-shaped portions may then be secured by spot welding. Longitudinal beams 12 and 14 lie in a first common plane and are each provided with acircular apertures 24 in opposite and aligned relationship to receive and nonrotatably retain correspondingly configured projections hereinafter more particularly described extending from insulators 26. In like manner, longitudinal beams 16 and 18 also lie in a second common plane parallel to the first plane and are provided with oppositely disposed apertures 28 which, when the beams 16 and 18 are properly assembled as shown in FIG. 1, lie in staggered relation with the apertures 24 and beams 12 and 14. Thus, insulators 26 seated between beams 12 and 14 are parallel to each other in a row and the insulators 26 seated between beams 16 and 18 are also parallel to each other in a second row and in staggered relation to those lying between beams 12 and 14. The longitudinal axes of all the insulators 26 are parallel to each other although the major faces thereof may be, and desirably are, angularly related to each other in the opposing rows.

In a preferred embodiment of the present invention, the heating element is preferably a thin strip of apertured foil-like electric resistance material 30, such as disclosed in U.S. Pat. No. 3,651,304 disposed within the framework 10 along an oscillatory wave path of decreasing frequency in the direction of fluid flow. The element 30 traverses back and forth between insulators 26 on opposite sides of the framework 10 forming a corrugated grid and changing direction around successive staggered insulators 26 in a serpentine or oscillatory path. As will be best seen in FIG. 2, the distance between the nodes on each side of the framework 10 gradually increases from the gas inlet end 32 in the direction toward the exit end 34. In a specific embodiment, the spacing between successive insulators 26 disposed between longitudinal beams 12 and 14, for example, increases by an increment of 0.62 inches.

The spacing between insulators 26 extending between longitudinal beams 16 and 18 also increases by a like increment successively. There results, therefore, when the heating element 30 is wound along an oscillatory wave path such as shown in FIG. 2, a situation where the location of the nodes evidences a decreasing frequency of oscillation. In such an arrangement, when gas is moved in a direction shown in FIG. 1, persistence of red heat in a portion of the heating element 30 near the exit end 34 is not experienced. Where the frequency of oscillation is constant from one end of the device to the other, or where the frequency of oscillation increases in the direction of gas flow and the temperature at which the device is operated is at red heat under static conditions, when air or other fluid to be heated is passed through the apparatus in the direction shown in FIGS. 1 and 2, the portion of the heating element contacted by the cooler gas will be cooled from red heat into the black heat range. However, that portion adjacent the exit end will remain at red heat because the temperature difference between the now heated gas and the heating element is insufficient to enable cooling of the element from its red heat condition.

As best shown in FIGS. 1 and 2, and where it is desired to have the electrical terminals 36 and 38 at the same end of the assembly, it is convenient to provide a folded or reversing pathway for the heating element 30. Thus, a strip having a width which is less than half the depth of the assembly is used and woven in an outwardly extending segment 40 and a return segment 42 joined together by a direction reversing segment 44. This configuration of the heating element is termed herein as a folded heating element which extends outwardly and then returns along substantially the same but laterally displaced oscillatory path. As will be explained hereinafter more particularly, the preferred configuration of the insulators 26 is such that it is especially adapted for a folded path by having upper and lower recesses for receiving and retaining the electric resistance heating strip 30. The ends 54 and 56 of the outwardly extending reach 40 and the return reach 42, respectively, are suitably secured to electrical terminals 36 and 38, respectively, by any suitable means such as welding.

To improve the structural strength of the box-like frame 10, there may be provided cross ties, such as cross tie 46, extending between longitudinal beams 12 and 14 and cross tie 48 extending between longitudinal beams 16 and 18. These are suitably secured to the respective longitudinal beams by any suitable means, such as weldments 50 and 52, for example.

To aid in mounting the assemblies of the present invention in a system where a moving gas stream is to be heated, e.g., an electric clothes dryer (not shown), there may be provided on the end frames 22 and 20 suitable mounting means. For example, the end frame 22 may be provided with a centrally located projection 58 which may be pierced to receive a metal screw, for example. Also, the end frame 20 may be provided with laterally extending portions 60 and 62 for seating of the device on suitably located projecting retaining rods in the apparatus where the assembly of the present invention is to be utilized.

FIG. 4 shows an elevation on an enlarged scale of an insulator 26 which is particularly useful in accordance with the present invention. As will be seen from FIG. 4, the insulators are of generally rectangular prismatic configuration. Adjacent one of the longitudinal edges 64 and projecting from each of the short sides thereof 66 and 68 are laterally extending acircular projections 70 and 72, respectively, which are adapted to fit easily into correspondingly configured and angularly disposed slots 24 and 28 in the longitudinal members, for example longitudinal members 12 and 18. The laterally projecting lugs or projections 70 and 72 being acircularly shaped and fitting into rectangularly shaped, angularly disposed openings in the longitudinal beams are therefore nonrotatably retained therein, and the angular disposition of the insulating members 26 is fixed thereby. The insulating members 26 are also provided with inwardly extending recesses 74 and 76 extending inwardly from the edge 64. The recess 74 is provided with shoulders 78 and 80, and the recess 76 is provided with shoulders 82 and 84. The width of the recesses 74 and 76 is such as to accommodate the width of the electrical resistance heating member 30 for seating engagement of the outwardly extending segment 40 on the shoulders 78 and 80, and for seating engagement of the return segment 42 on the shoulders 82 and 84. The fit between the projection 70 and 72 on the insulators 26 and the correspondingly configured openings 24 and 28 in the longitudinal members is loose, but sufficient to prevent substantial rotation of the insulators 26. The angular disposition of the insulators 26 relative to the gas stream is between 90.degree. and 180.degree. and a suitable specific angular disposition for the insulator with respect to the direction of movement of the gas being heated is 45.degree..

As shown in FIG. 5, when the insulators 26a and 26b are disposed at an angle of approximately 45.degree. relative to the direction of gas flow and the heating element 30 reaved across the shoulders of the recesses 74, for example, on the upstream side of the insulators where the contact area of the heating element with the insulators is at a maximum, the gas being heated tends to flow across the face of the heating element in a "wiping" action and minimizes any tendency for local overheating of the heating element in the region of contact with the insulator. On the reverse side or downstream side of the insulator, the contact with the insulator is maintained at a minimum, and the orientation of the insulators is such that they act as air foils to create low pressure areas and to thereby create eddy currents or turbulence indicated by the reversing arrows in FIG. 5, which again tend to sweep the very small area of contact with the insulators and minimize the chances for local overheating thereof. Recesses 74 and 76 also permit air to flow through and further aid in cooling to prevent local overheating.

The extremes of the range of angles for disposition of the insulators 26 are the last satisfactory. At 90.degree. disposition, the downstream side experiences maximum shielding from cooling gas and local overheating or development of red heat in the shielded heating element is experienced. At 180.degree. to the direction of gas flow, there is minimum turbulence which is useful in effecting the downstream side of the heating element 30 as it changes direction around the insulator 26 and, again, localized development of red heat may occur. A disposition at 45.degree. to the direction of gas flow minimizes the shielding effect, maximizes turbulence, and minimizes sharp bends which can become points of thermal stress and reduced durability. Thus, the middle portion of the range 90.degree. to 180.degree. is preferred where the desirable effects are most pronounced.

In the preferred embodiment of the invention, the heating element itself acts as an air foil to promote turbulent flow or "wiping" action of the element in downstream locations wherein the element is blocked by an insulator. A characteristic of expanded metal foils, employed as heating elements according to U.S. Pat. No. 3,651,304, is the fact that the lattice structure forming diamond-shaped openings includes interconnected webs which are oriented at an angular to the plane of the foil, and that angle is a function of the degree to which the metal is expanded. In FIG. 5, webs 95 define openings through the heating element and are oriented at 45.degree. to the plane of the element. It may be noted that the portion of the element on the upstream face of the insulator 26a has webs which are oriented at 90.degree. to the air stream, thus tending to block air flow through the recess 74. However, this orientation adds to the air foil effect of the insulator 26a to promote element cooling eddy currents on the downstream side of the insulator 26a. It may further be noted that the portion of the element on the upstream face of the insulator 26b has webs which are oriented parallel to the air stream, thus reducing the air foil effect of the insulator 26b but permitting flow through the recess 74 to aid in cooling downstream portions of the element. The net effect of this relationship is to minimize heat differentials along the length of the heating element. Furthermore, the various web orientations encountered by the air stream tend to promote a degree of overall turbulence which may tend to eliminate localized hot spots in the heating element.

When leads are connected to the binding posts 86 and 88 and current permitted to flow in the circuit formed by the electrical resistance heating element 30, heat is developed in the heating element, preferably at a level which is at red heat. When gas is moved over the surface of the heating element 30, the gas at the inlet end 32 being relatively cooler traverses a higher density of the heating elements in the region where the frequency of oscillation along the wave path is considerably higher and reduces the temperatures of the heating element 30 to within the black heat range where most efficient utilization of applied power is realized. Thus, the watt density or watt output of heat per unit of length of the assembly is greater, and the temperature of the gas more rapidly increased. As the temperature of the gas continues to increase, the watt density of the electrical resistance heating element 30 is decreased (or the frequency of oscillation decreased) so that the rate of change of temperature of the gas more nearly accommodates the rate at which heat may be transferred to a gas with a continuously decreasing delta T relative to the heating element 30. It is believed that by this mechanism, maintenance of the entire heating element at a temperature just below red heat may be realized without any portion of the exposed grid experiencing elevation of the temperature locally to red heata through inability to discharge sufficiently heat units by convection to the moving gas.

Also, because of the disposition of the insulator element with the major areas of contact between the heating element 30 and the insulators 26 being angularly disposed to the direction of flow of the gas on the upstream side, local overheating due to static conditions on the upstream side is heat minimized by a "wiping action" of the gas as it passes over the angularly related surface against which the electric resistance heating element 30 rests. On the downstream side, where contact between the heating element 30 and the insulator 26 is at a minimum, removal of heat units from otherwise shielded portions of the heating element 30 is accomplished by eddy currents or turbulence of the gas occurring behind the trailing edge of the insulator 26, as well as the recesses 74 and 76 provided therein.

The effect of the oscillatory wave path of decreasing frequency is to minimize the opportunity for development of red heat in the heating element near the exit end 34 while providing a maximum surface for efficient exchange of heat between the heating element 30 and the moving gas stream in a relatively short gas conduit while at the same time enabling operation of the heating element at a temperature just below red heat for maximum efficiency of heat transfer and durability of the heating element.

The amplitude of the wave form is generally less than the maximum width of the frame structure. This prevents shorting out of the electrical heating element against metallic fluid conduit defining elements. It also allows fluid to pass externally of the heating element grid as it changes direction around the insulators to provide a cooling effect on the heating element at these points of high thermal stress.

Principal advantages of the present invention in respect of efficient heating and durability of the heating element are achieved with the decreasing frequency of oscillation of the wave pattern from the inlet end toward the exit end of the pathway traversed by the moving fluid independently of the configuration of the insulators. Thus, tubular insulators, rod type insulators, bar type insulators or specially configured insulators may be used in any resistance element heater assembly where there is a decreasing frequency of oscillation in the direction of fluid flow as herein described with realization of such principal advantages. Best results are achieved when the insulators, positioned as herein described, are configured in the preferred manner set forth. The manner in which decrease in frequency is effected is also unimportant as long as the watt density at any given points is not so high as to result in the persistence of a region or regions of red heat. Thus, the change in frequency may be irregular, although for convenience, the change in frequency is preferably at a uniform decreasing rate along the moving fluid pathway from the inlet toward the outlet.

Claims

What is claimed is:

1. A resistance heating assembly for heating a fluid moving along a predetermined pathway and including:

a. a plurality of insulators arranged in staggered parallel relationship at the points of reversal of direction along an oscillatory wave path;

b. an elongated electrical resistance material supported by said insulators at intervals along the length of said material, said material being formed as a grid along an oscillatory wave path of decreasing frequency;

c. means for supporting said insulators along said oscillatory wave path; and

d. electrical terminal means communicating with the extremities of said electrical resistance material.

2. A resistance heating assembly in accordance with claim 1 wherein the insulators are nonrotatably mounted in said support means.

3. A resistance heating assembly in accordance with claim 1 wherein the support means includes a box-like framework.

4. A resistance heating assembly in accordance with claim 3 wherein the box-like framework includes longitudinal members extending between the corners of parallel rectangular frame members and defining first and second pairs of longitudinal members, the members of the first pair lying in one plane and the members of the second pair lying in a second plane.

5. A resistance heating assembly in accordance with claim 4 wherein the longitudinal members are apertured, and the insulators are provided with laterally extending projections for interlocking coaction with said perforations in the longitudinal members.

6. A resistance heating assembly in accordance with claim 5 wherein the laterally extending projections are acircular.

7. A resistance heating assembly in accordance with claim 1 wherein the insulators are generally rectangular prismatic blocks of refractory material.

8. A resistance heating assembly in accordance with claim 1 wherein the insulators are generally rectangular prismatic blocks of refractory material having:

(1). a laterally extending acircular projection from each of the short sides, respectively; and

(2). a pair of symmetrically disposed shouldered recesses of like geometric configuration extending inwardly from the same longitudinal side of the blocks.

9. A resistance heating assembly in accordance with claim 1 wherein the insulators are generally rectangular prismatic blocks of refractory material disposed with their major faces lying at an angle to the direction of movement of the fluid, and the major area of contact of the electrical resistance material with each of said insulators faces in an upstream direction.

10. A resistance heating assembly in accordance with claim 1 wherein the insulators have an acircular cross section, and the major area of contact with the elongated electrical resistance material faces in an upstream direction.

11. A resistance heating assembly in accordance with claim 1 wherein the insulators are generally rectangular prismatic blocks of refractory material having:

1. a laterally extending acircular projection from each of the short sides, respectively; and

2. a pair of symmetrically disposed shouldered recesses of like geometric configuration extending inwardly from the same longitudinal side of the block;

said insulators having their major sides disposed at an angle to the direction of movement of the gas, and the major area of contact between the insulators and the elongated electrical resistance material faces upstream, and the minor area of contact between the electrical resistance material and the insulators faces downstream.

12. A resistance heating assembly in accordance with claim 1 wherein the electrical resistance material is a thin strip of apertured foil-like electrical resistance material retained and supported on said insulators at intervals along the length of the strip, and said strip is formed as a grid along a folded serpentine path having an advancing portion and a returning portion laterally displaced with respect to each other and connected by a reversing portion.

13. A resistance heating element in accordance with claim 12 wherein said insulators have recesses which are dimensioned for contacting the marginal portions only of the advancing portion and the returning portion, respectively, of said strip as it changes direction in passing around said insulators along said serpentine path.

14. An insulator comprising a generally rectangular prismatic block of refractory material having:

a. a laterally extending acircular projection from each of the short sides, respectively; and

b. a pair of symmetrically disposed recesses of like geometric configuration extending inwardly from the same longitudinal side of the block, each of said recesses having a pair of shoulders therein.

15. A resistance heating assembly for heating a fluid moving along a predetermined pathway and comprising:

a. a box-like framework including longitudinal members extending between the corners of parallel rectangular frame members and defining first and second pairs of longitudinal members, the members of the first pair lying in one plane and the members of the second pair lying in the second plane;

b. a first plurality of insulators extending between and nonrotatably supported by the longitudinal members of said first pair;

c. a second plurality of insulators extending between and nonrotatably supported by the longitudinal members of said second pair and in alternating staggered relation to the insulators of said first plurality;

d. a thin strip of apertured, foil-like electrical resistance material retained and alternatingly supported by insulators of said first and second pluralities of insulators at intervals along the length of said strip, said strip being formed as a grid along a folded serpentine path having an advancing portion and a returning portion connected by a reversing portion.

e. electrical terminal means communicating with each extremity of said strip;

said insulators having first and second shouldered recesses dimensioned for contacting the marginal portions only of the advancing portion and the returning portion, respectively, of said strip as it changes direction between the insulators of said first and second pluralities.

16. A resistance heating assembly in accordance with claim 15 wherein the longitudinal members are provided with acircular apertures to receive correspondingly geometrically configured ends of the insulators.

17. A resistance heating assembly in accordance with claim 15 wherein the longitudinal spacing between alternating insulators in the first and second pluralities of insulators increases in the direction of fluid movement.

18. A resistance heating assembly in accordance with claim 15 wherein the insulators are formed of refractory material.

19. A resistance heating assembly in accordance with claim 15 wherein the insulators are formed of ceramic material.

20. A resistance heating assembly comprising:

a. a plurality of insulators arranged in staggered parallel relationship at the points of reversal of direction along a serpentine path, said insulators having axially spaced first and second shouldered recesses;

b. a thin strip of apertured, foil-like electrical resistance material supported by said insulators at intervals along the length of said strip, said strip being formed as a grid along a folded serpentine path having

1. an advancing portion supported by the shoulders in the first recesses of said insulators,

2. a returning portion supported by the shoulders in the second recesses of said insulators, and

3. a reversing portion of said strip;

c. means for supporting said insulators along said serpentine path; and

d. electrical terminal means communicating with each extremity of said strip.

21. A resistance heating assembly for heating a fluid moving along a predetermined pathway and including:

a. a plurality of insulators arranged in staggered parallel relationship at the points of reversal of direction along an oscillatory wave path;

b. an elongated electrical resistance material supported by said insulators at intervals along the length of said material, said material being formed as a grid along an oscillatory wave path, said insulators and said resistance material defining areas of mutual contact, with major portions of mutual contact facing in the direction of the wave path;

c. means for supporting said insulators along said oscillatory wave path; and

d. electrical terminal means communicating with the extremities of said electrical resistance material.