US3607446A

US3607446A - Thermopile and method of making

Info

Publication number: US3607446A
Application number: US680895A
Authority: US
Inventors: Alfred Sugarman; Han Chang; Alvin H Kasberg
Original assignee: Nuclear Materials and Equipment Corp
Current assignee: Intermedics Inc; Nuclear Materials and Equipment Corp
Priority date: 1967-11-06
Filing date: 1967-11-06
Publication date: 1971-09-21
Anticipated expiration: 1988-09-21

Abstract

Method of making a thermopile 61 composed of a large number of wire elements in whose practice continuous wires 37 and 41 of dissimilar thermoelectric material are lapped in opposite directions. The wires form an array of regions of intersection. An electric arc is passed along this array while the portions of the wires near the intersection are cooled. The arc produces welded joints 58, 59 and at the same time separates the wires at the joints from the continuum of wire to produce thermocouples 63. A thermopile assembly 80 formed by embedding the wires in insulating sheets 71, 73 before they are welded and wrapping the sheet with the wires in them in a cylinder 91. Apparatus for making the thermopile including a mandrel 11 with opposite running helical grooves 13, 15, 17, 19 for winding the wires 37, 41, channels 13, 25, 27, 29 along the arrays of intersection of the wires and chill blocks 51 for engaging the wires near their array of intersections to cool them.

Description

United States Patent 715,265 12/1902 v Heil lnventors Alfred Sugarman Monroeville;

llan Chang, Apollo; Alvin H. Kasberg, Murrysville, all 01 Pa.

Nov. 6, 1967 Sept. 2 1 197 1 Nuclear Materials and Equipment Corporation Apollo, Pa.

Appl. No. Filed Patented Assignee References Cited UNITED STATES PATENTS Primary Examiner-Benjamin R. Padgett Assistant Examiner-Stephen U. Lechert Attorney-Hymen Diamond ABSTRACT: Method of making a thermopile 61 composed of a large number of wire elements in whose practice

continuous wires

37 and 41 of dissimilar thermoelectric material are lapped in opposite directions. The wires form an array of regions of intersection. An electric arc is passed along this array while the portions of the wires near the intersection are cooled. The are produces

welded joints

58, 59 and at the same time separates the wires at the joints from the continuum of wire to produce thermocouples 63. A thermopile assembly 80 formed by embedding the wires in

insulating sheets

71, 73 before they are welded and wrapping the sheet with the wires in them in a cylinder 91. Apparatus for making the therrnopile including a mandrel 11 with opposite running

helical grooves

13, 15, 17, 19 for winding the

wires

37, 41,

channels

13, 25, 27, 29 along the arrays of intersection of the wires and chill blocks 51 for engaging the wires near their array of intersections to cool them.

PATENTED SEPZI I971 3.607.446

SHEU 1 BF 3 I9 IS IS PATENTED SEPZI I97! SHEET 2 [IF 3 FlG.2A.

' FIG.2B.

FIG.4.

PATENTED SEPZI i971 SHEET 3 BF 3 FIG.5A.

FIG.7.

mmisq zz kzwmmbo OUTPUT VOLTAGE THERMOPILE AND METHOD OF MAKING BACKGROUND OF THE INVENTION This invention relates to the generation of electrical power for use in regions of the earth where commercial electrical power is not readily available or in outer space and has particular relationship to the generation of power in the milliwatt range, for example to electrical generators which deliver 100 milliwatts of power. Power generators include a source of heat, for example a radioisotope source, and a thermoelectric converter. Where the generator operates in the milliwatt range, the thermoelectric pile is made up of a large number of thermocouples formed of pairs of wire elements of dissimilar thermoelectric properties. To attain the desired voltage these thermocouples are connected in series. It is an object of this invention to provide such a thermopile made up of a large number, typically of the order of several hundred, thermocouples.

In accordance with the teachings of the prior art, wires of dissimilar thermoelectric properties are lapped on a rectangular card or core or metal band and the intersecting ends are formed into junctions by resistance welding or plating. The wires are then severed at the joints by a cutter to form a plurality of thermoelectric elements. Where a pile of a large number (several hundred) thermoelectric couples is to be produced this procedure is not practical. The wires cannot readily be removed from the card, core or band; where an attempt is made to remove them the wires become tangled or knotted and reassembly of the pile with parts cut away is required. This difficulty is increased where the wires are of small diameter. The forming of the joints or junctions also presents severe problems particularly where the wires are of small diameter or of high resistivity. Firm electrical connection at the joints is indispensable and this necessitates that the wires be metallurgically joined at the joints. But heat adequate to produce a firm electrical connection at a joint of two wires is frequently so high that the wires near the joint are burned away.

It is an object of this invention to overcome the difficulties and disadvantages of the prior art in the making of thermopiles formed of a large number of thermocouples. It is a further object of this invention to provide a method of making a thermopile of a large number of wires in whose practice the junctions of the couples of the piles shall be metallurgical joints constituting firm electrical connection of the individual wires and the wires at or near the joints shall be sound. It is also an object of this invention to provide thermopiles made by this method. Also it is an object of this invention to provide novel apparatus for practicing this method.

SUMMARY OF THE INVENTION This invention arises from the discovery that if the individual wires at, or near, an intersection of a pair of wires are adequately cooled, the wires at this intersection may be metallurgically joined by welding and at the same time severed. A pair of separate thermoelectric junctions are thus formed in a single stroke.

In accordance with this invention, a thermopile with a large number of thermocouples is formed by lapping continuous wires of dissimilar thermoelectric properties in opposite direction to produce an array of intersections, each between a wire of one thermoelectric property and a wire of another thermoelectric property. A welding arc is moved along the array impinging on the intersections and, at the same time, the wires at or near the intersections are cooled. Typically the welding arc is of the TIG type. The are produces fusion welds of the wires at the intersection and in addition severs the joints so that two junctions are produced at each intersection.

In accordance with this invention the wires are lapped on a cylindrical mandrel which has oppositely running helical grooves to accommodate the respective wires of the dissimilar thermoelectric properties. For cooling purposes the mandrel is composed of a highly thermally conducting material, typically copper or silver. Channels, parallel to the axis of the mandrel, are provided along the intersections of the oppositely running grooves so that the welding operation may be effectively carried out.

The cooling is effected by chill blocks which extend over the mandrel and have projections or edges engaging the wires at or near the intersections in cooling relationship.

After being wound on the mandrel the wires of each thermoelectric property are secured to electrically insulating sheets which leave exposed short sections of the wires, at the intersections. These sections are fusion welded. The sheets and wires form a thermopile assembly which is rolled into a cylinder that may be used in a generator with facility.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of this invention, both as to its organization and as to its method of operation, together with additional objects and advantages thereof, reference is made to the following description taken in connection with the accompanying drawings, in which:

FIG. l is a view in perspective showing apparatus in accordance with this invention used in the winding of wire in the practice of this invention;

FIG. 1A is an enlarged view of the section IA of FIG. 1;

FIG. 2 is an exploded view in perspective showing use of apparatus in accordance with this invention in the making of a thermopile from wound wires;

FIGS. 2A and 2B are enlarged views showing the formation of junctions or joints of wires of dissimilar thermoelectric properties in the practice of this invention;

FIG. 3 is an exploded view similar to FIG. 2 but showing the insulating sheets to which the wires of dissimilar thermoelectric properties are secured;

FIG. 4 is a plan view of a thermopile formed with the apparatus shown in FIG. 3;

FIG. 5 is a view in end elevation of the pile shown in FIG. 4;

FIG. 5A is an enlarged view of section VA of FIG. 5;

FIG. 6 is a view in side elevation ofthe pile shown in FIG. 4;

FIG. 7 is an exploded view in perspective of the pile shown in FIG. 4 assembled in a cylinder; and

FIG. 8 is a graph showing the actual operation of the pile shown in FIG. 7.

DESCRIPTION OF PREFERRED EMBODIMENTS AND PRACTICE The apparatus shown in the drawings includes a cylindrical mandrel 11 composed ofa material having high thermal conductivity. A plurality of pairs of

grooves

13 and 1S and 17 and 19 extend around the surface of the mandrel in helices. The grooves !l3 and 15 have a clockwise, or left-hand, direction of rotation, as viewed from one end of the mandrel, as they advance along the surface and the

grooves

17 and 19 have a counterclockwise, or right-hand, direction of rotation viewed the same way. Typically the starting points for the grooves l3, 15, 17 and 19 are displaced by around the cross section of the mandrel. The pitch of the helices may be set so that there are a selected number of arrays of intersections of the helices; typically, four linear arrays of regions of intersection 21 of the clockwise and counterclockwise grooves 13 thru 19 spaced by 90 around the periphery of the mandrel 11 are shown. The mandrel II is provided with

channels

23, 25, 27, 29 (FIG. 3) along these arrays.

The apparatus further includes means for rotating the mandrel 11 which typically is a lathe 31. The mandrel 11 is mounted between the chuck 33 and spindle 35 of the lathe 31 and is rotated as the chuck 33 is driven.

A wire 37 of desired thermoelectric properties is wound from a reel 39 on the mandrel 11 as the mandrel is rotated. The wire 37 is deposited in one of the grooves, 13 or 15 for example. The grooves 13 or I5 are dimensioned to accommodate the wire 37 readily; the depth of the grooves should be somewhat greater than the diameter of the wire so that the wire fits neatly in the grooves.

The wire 37 is then secured to the mandrel 11 so that it remains tightly wound on the mandrel. If, as is typical, the thermopile is to include couples connected in parallel in double redundancy, a second wire (not shown) of thermoelectric properties similar to wire 37 is wound on this mandrel in the

other groove

15 or 13 and secured tightly wound to it. Next a wire 41 of dissimilar thermoelectric properties from wire 37 is wound in the

cross grooves

17 or 19 on opposite rotation grooves. In this case also a second wire of same thermoelectric properties as wire 41 may be wound in

grooves

19 or 17, as the case may be, where the pile is to include couples connected in double redundancy. The

wires

37 and 41 intersect in arrays along the

channels

23, 25, 27 and 29.

With the

wires

37 and 41 wound on, and secured to, the mandrel 11 the mandrel is removed from the lathe 31. The procedure just described may be carried out with enameled

wires

37 and 41, the enamel insulating the wires electrically from each other. It is now necessary to prepare the wires on the mandrel 11 for welding. For this purpose the mandrel is mounted securely between the

brackets

43 and 44 of a supporting fixture 45 and the segments between the channels 23 thru 29 are covered leaving exposed only the regions of intersection of the

wires

37 and 41. The regions of intersection are then sandblasted to remove the enamel from the exposed portions of the wires and the sandblasted areas are thoroughly cleaned.

The apparatus includes a plurality of chill blocks 51 (only one shown) cooperative with the mandrel 1 1 on the fixture 45. Each chill block 51 is composed of a material of high thermal conductivity, such as copper, and is of generally C-shape having legs 53 whose tips 55 are beveled. The chill blocks 51 are secured to the mandrel l 1 and extend across each quadrant of its surface (where there are four arrays of regions of intersection). The length of each chill block 51 is such that it extends over the

wires

37 and 41 on the surface of the mandrel 11 with the tips of the bevels 55 on the legs 53 engaging the

wires

37 and 41 at or near their intersections. The chill blocks 51 are secured to the mandrel 11 so that this engagement is firm assuring effective transfer of heat between the intersections of the wires and the blocks 51. With the chill blocks 51 secured to the mandrel 11 and the chill block mandrel assembly in the supporting fixture 45, the wire intersections along each of the arrays are fused into junctions and separated. For this a TlG welding electrode 56, (FIG. 2A) appropriately connected to a welding supply, is set into welding relationship with an array 57 of intersections at one end of each

slot

23, 25, 27, 29 in its turn. An arc is fired between the electrode and the wires and the arc is moved along each array in fusion-welding relationship with the array 57. The heat of the arc causes the

wires

37 and 41 at the intersection to be metallurgically joined and in addition separates the wires at each joint into two separate pairs each having a welded tip of junction 58 and 59 (FIG. 2B). The chill blocks 51 conduct away the excess heat from the joints preventing damage to, or burning of the wires at, or near, the joints. By welding and separating in this manner the arrays of intersections along

adjacent slots

23 and 25, for example, a pile 61 of series-connected thermocouples 63 are formed; each couple has a length substantially equal to onefourth the circumference of the mandrel and this length determines the electrical resistance of the couple. Where the mandrel has only a single slot a pile may be formed of couples each of which has a length substantially equal to the circumference of the mandrel. The pile so formed may be mounted on or secured to a frame or cloth and used in any desired manner.

Where the

wires

37 and 41 are bare,

sheets

71 and 73 of electrical insulating material may be secured to, or disposed on, each array of wires as shown in FIG. 3. in this case wire 37 is first wound tightly on a mandrel 11 and secured to the mandrel. Then, insulating sheets 71 having a width such that they cover the turns of wire 37 between

adjacent slots

23 and 25, and 27, 27 and 29, and 29 and 23 are disposed on or secured to the layer of wires. The sheets 71 may be adhesive tape with adhesive on both faces and they are adhesively secured to the

wires

37 and 41. The

sheets

71 and 73 may also be sprayed on. Between the sheets 71 short sections of wire 37 extending over the slots 23 to 29 are exposed.

The wire 41 is then tightly wound on sheet 71 with the mandrel 11 rotated in the opposite direction during the winding. Where there is adhesive on the outer face of tape 71 wire 41 is secured to this adhesive. The

slots

17 and 19 are covered by the sheets 71 but the winding of wire 41 is guided by the exposed short sections of the wire 37. In addition the lathe 31 is set so that the pitch of the wire 41 is properly maintained. The winding of the wire 41 is started so that the intersections of

wires

37 and 41 extend along slots 23 thru 29.

Next with wire 41 secured to the mandrel 75, sheets 73 similar to sheets 71 are disposed on wire 41. The joints between

wires

37 and 41 are then formed by arc welding as described above.

Where the thermopile is to be used in a vacuum, typical adhesives readily available decompose when subjected to the treating-out temperature during evacuation (700 C.). In this case the tape-pile assembly made as disclosed above is first sewed with quartz thread and then the adhesive is dissolved away. Usually the pile is secured to the tapes by stitches along the junctions and near the center of the pile.

A thermoelectric assembly in accordance with this invention is shown in F105. 4 thru 6. The pile 61 is secured to the

tape

71, 73 which insulates the

wires

37 and 41 of the couples 63 from each other. Terminals 81 are welded to the end wires 83 at both ends (one end not shown).

As shown in H0. 7 the assembly 80 is wrapped into a cylinder 91. The protruding junctions are then encased in ceramic potting compound (typically aluminum phosphate Al(PO:;).-;) which has high thermal conductivity. The compound is formed into bases at 93 at the ends which taper out from the cylinder 91. Ceramic discs 95 are brazed (brazing compound Cu 15.6 Ti 66.3 Ni 18.1) to the bases 93 and metal discs 97, typically, of columbium coated with copper are brazed (same compound) to the ceramic discs 95. The terminal 81 that is connected to the negative end of the pile 61 is grounded; the other terminal is ungrounded.

The heat source is applied to the one end disc 97, which does not include the terminals 81. The other end disc is connected to the heat sink.

The following description of actual making of a thermopile assembly may aid in the understanding of this invention.

A copper (sulfur copper) mandrel 11 was placed in a lathe 31 for which the centering hole 99 on one end of the mandrel was made. The mandrel 11 is 1.592 inches in diameter and 22 inches in length with two left-hand

helical grooves

13 and 15 and two right-hand

helical grooves

17 and 19 each with Vainch pitch and a /a-inch lead. The grooves are 000710.001 inch by (100710.001 inch.

One pair of 3% mil chromel wires 37 were fed into each of the two sets of helical grooves at about 12 rpm, after which a l fii-inch wide fiber glass tape 71 is sprayed with a rubber cement compound and applied over the grooves and wires. The above winding procedure is then repeated with 3% mil constantan wire which is wound over the fiber glass tape 71 in the opposite direction, and again covered with a l%.-inch fiber glass tape 73 coated with a rubber cement.

The copper chill blocks 51 are then screwed on and the fixture is now ready to accommodate the welding process.

The welding is carried out with a tungsten inert gas welder using a l/l6-inch tungsten electrode 56 with nitrogen purging gas at about l amp welding current. No slope control or time control is used. The electrode is held in the hand and the ceramic cup, from which the shielding gas flows, is rested on the edges of each consecutive pair of blocks 51 so that the electrode tip-wire distance can be adjusted and be maintained steady with the hand. After the arc is fired at the extreme of the mandrel 11 with the high frequency stabilizer, the arc is drawn down the length of the channel 23 that is machined in the surface of the

mandrel

25, 27, 29. This procedure is repeated in each of the three remaining channels so that all the thermocouples are welded. It has been found that the are power may be varied over a reasonable range without affecting the soundness of the welding.

Upon completion of the welding, the blocks 51 are removed and the fiber glass tapewire-fiber glass tape assembly 80 is peeled from the surface of the mandrel. The rubber cement causes the thermocouple wires to adhere to the glass tape while not sticking to the copper mandrel. The assembly may then be sewn together and the adhesive dissolved away.

The following brief summary may further aid in the understanding of this invention.

This invention relates to the thermopiles for radioisotope generators operating in the milliwatt range; a typical generator operates at 100 M.W. In making this thermopile it was necessary to provide a (1) method for joining the

thermocouple wires

37 and 41; (2) a means of insulating each of the thermocouple wires from each other. 7 V

In typical practice in accordance with the teachings of the prior art, a thermopile consisting of 1,300 thermocouples made from Tophel Special and Cupron Special wires of 2 mils diameter was made. The hot and cold junctions were made by twisting, dipping, and brazing the wire ends. The entire thermopile was potted in silicone rubber. The thermopile was designed to operate with a cold side of 100 F. (38 C.) and a hot side of 400 F. (204 C.) to produce 1.55 milliwatts. The making of this thermopile presented severe difficulties and consumed excessive time and the pile was very costly.

in the practice of this invention it was necessary to provide a thermopile to operate with a cold-junction temperature at 200 C. and a hot-junction temperature of 600 C., to generate 100 milliwatts.

This invention is described in detail above. In the specific practice of this invention, a solid l'h-inch copper cylinder 1 1, 22 inches in length on the surface of which are threads or

helical grooves

13 and 15 and 17 and 19. The

threads

13, 15, 17, 19 are two 0.005 inch by 0.005

inch grooves

13, 15 for 0.003- inch wires 37 in double redundancy, threaded in the righthand direction with a 18 inches advance and two of the

same grooves

17, 19 in the left-hand direction with the same ad vance. The grooves are started 90 from each other and meet on four lines parallel to the axis of the cylinder on which is machined a l/ 16-inch wide by 0.010-inch

deep channels

23, 25, 27, 29 in which the wires are welded. To make the thermocouples, the wire 37 (or wires, when redundancy is desired) are wound in the

grooves

13, 15 in one direction for one leg of the thermocouple and 49 in the other direction for the other leg of the thermocouple. The winding can be by hand or with a lathe 31 in which the wires are fed through a guide 101 into the grooves as the cylinder turns on its axis and the feeder follows a particular groove. The wires are then welded along their intersections.

The apparatus includes the chill blocks 51 which engage the wires near their intersections in cooling relationship. It is important that the blocks 51 and cylinder 11 are flush along the cylinder surface and channel edges to present a uniform heat ,sink to the weld bead and to prevent the head from flowing between the cover and cylinder. It is also important for all the wires to be together at the weld area and not less than l/65 inch behind the weld area to prevent the occurrence of broken and unwelded wires.

Welding is carried out by the T16 process in which a fine tungsten electrode is held about 1116 inch above the center of the section of wires bridging the l/ l 6-inch channel and moved in the axial direction, after an arc is fired, at a rate of about 30 feet/minute.

The blocks 51 are hollow so that

tape

71, 73 can be placed over the bare wires and not interfere with the thermal contact among the block, wires and cylinder 11. After the welding is completed, the covers can be removed and the wires peeled from the mandrel with the

tape

71, 73. In this way the positions of the thermocouple legs are not altered in handling.

The factors important in achieving a high degree of reliability in the practice of our invention are:

1. Good thermal contact between the blocks 51,

wires

37, 41 and cylinder 11.

2. A flat heat sink that cools the beads uniformly.

3.

Grooves

13, 15, 17, 19 in which the wires are wound, that are large enough to keep the wires in place and together but small enough to allow the cover to exert a minimum of pressure on them.

4. Uniform but small wire tension.

5.

Channels

23, 25, 27, 29 that are deep enough so that wire-metal contact is not made and narrow so that the bead of molten metal does not cool before making contact with the heat sink.

Important features of this invention are:

1. That a pile of a large number of series connected thermocouples may be made by welding the wires to each other, thereby eliminating a braze material.

2. That the thermocouple 63 may be readily insulated from each other so that the thermopile 61 can operate at the temperature capability of the thermocouple material.

3. That wires of any gauge, over a reasonable range, can be welded together to make a thermopile.

4. That any reasonable number of wires can be welded to each other.

5. That a pile of any reasonable number of units of thermocouples connected in series can be fabricated.

6. That any reasonable size of thermopile can be made.

FIG. 8 shows the characteristics of a thermopile according to this invention operating between hot-junction and coldjunction temperatures of 435 C. and 50 C. and 500 C. and 50 C., respectively. This pile delivered 15 milliamps at about 6.7 volts at the lower hot-junction temperature and 15 milliamps at 9.8 volts at the higher temperature. This thermopile was composed of 416 couples of Tophel Spe cial wire (Ni Cr 10) and Cupron Special wire (Cu 60 Ni 39.5 Mn 0.25).

We claim as our invention:

1. The method of making a thermopile composed of a large number of thermoelectric elements which comprises lapping continuous wires of dissimilar thermoelectric material in opposite directions so that the wires cross over at least once producing at least one array of regions of intersection of said wires, impinging an electric are on said regions of intersection and at the same time conducting away the heat transferred by said arc through said regions from the portions of the wires forming said regions, both, to produce fusion at each region of intersection and said fusion severing the wires forming said intersection to produce at each said region at least one thermoelectric element having a fusion joint as a junction and the dissimilar wires remaining connected to said joint as com ponents the wires of each thermoelectric material being insulated from contiguous wires of dissimilar thermoelectric material except at the fusion joints.

2. The method of claim 1 wherein the wires cross over at least twice producing at least two arrays of regions of intersection on each of which array an arc is impinged.

3. The method of claim 1 wherein the wires are lapped by being wound on a mandrel of high thermal conductivity material.

4. The method of claim 1 wherein heat is conducted away from the portions of the wires forming the regions of intersection by a chill block which engages said wires near said portions in cooling relationship.

5. The method of claim 1 wherein the wires of dissimilar thermoelectric properties are enameled whereby contiguous wires of each thermoelectric material are insulated from contiguous wires of dissimilar thermoelectric material by enamel on the surface of at least a wire of one of said materials.

6. The method of producing a thermopile composed of a large number of thermoelectric elements which comprises winding a wire of one thermoelectric material on a mandrel in a helix in one direction, covering said wire with electrically insulating material leaving exposed short sections of wire of each of the turns of said helix, then winding another wire of another thermoelectric material dissimilar from said one material on said mandrel over said insulating material in another helix in a direction opposite to said one direction, such that said other wire forms crossover regions with said first-named wire at the exposed short sections of said firstnamed wire covering said other wire with additional electrically insulating material, fusing the wires of said one and other thermoelectric material together at said crossover regions to form fused joints at said regions, and said fusion severing said wires at said fusion joints to produce at each said region at least one thermoelectric element having a fusion joint as a junction and the dissimilar wires remaining connected to said joint as components.

7. The method of claim 6 wherein the mandrel has at least one channel extending in an axial direction along the surface on which the wires are wound and the wires are wound so that the crossover regions extend along said channel.

8. A thermopile comprising a first plurality of wires of a first thermoelectric material covered with a first layer of insulating material having exposed short sections of said wires along at least one length of said insulating material, a second plurality of wires of a second thermoelectric material, dissimilar from said first material, covered with a second layer of insulating material disposed over said first layer of insulating material and having exposed, short sections of said wires of said second plurality along at least one length of said insulating material. said last-named short sections fused to said first-named short section and severed to produce at each end section at least one thermoelectric element having a fusion joint as a junction and the dissimilar wires remaining connected to said joint as components.

9. The thermopile of claim 8 wherein the wires and insulating material are in the shape of a cylinder.

Claims

2. The method of claim 1 wherein the wires cross over at least twice producing at least two arrays of regions of intersection on each of which array an arc is impinged.
3. The method of claim 1 wherein the wires are lapped by being wound on a mandrel of high thermal conductivity material.
4. The method of claim 1 wherein heat is conducted away from the portions of the wires forming the regions of intersection by a chill block which engages said wires near said portions in cooling relationship.
5. The method of claim 1 wherein the wires of dissimilar thermoelectric properties are enameled whereby contiguous wires of each thermoelectric material are insulated from contiguous wires of dissimilar thermoelectric material by enamel on the surface of at least a wire of one of said materials.
6. The method of producing a thermopile composed of a large number of thermoelectric elements which comprises winding a wire of one thermoelectric material on a mandrel in a helix in one direction, covering said wire with electrically insulating material leaving exposed short sections of wire of each of the turns of said helix, then winding another wire of another thermoelectric material dissimilar from said one material on said mandrel over said insulating material in another helix in a direction opposite to said one direction, such that said other wire forms crossover regions with said first-named wire at the exposed short sections of said first-named wire covering said other wire with additional electrically insulating material, fusing the wires of said one and other thermoelectric material together at said crossover regions to form fused joints at said regions, and said fusion severing said wires at said fusion joints to produce at each said region at least one thermoelectric element having a fusion joint as a junction and the dissimilar wires remaining connected to said joint as components.
7. The method of claim 6 wherein the mandrel has at least one channel extending in an axial direction along the surface on which the wires are wound anD the wires are wound so that the crossover regions extend along said channel.
8. A thermopile comprising a first plurality of wires of a first thermoelectric material covered with a first layer of insulating material having exposed short sections of said wires along at least one length of said insulating material, a second plurality of wires of a second thermoelectric material, dissimilar from said first material, covered with a second layer of insulating material disposed over said first layer of insulating material and having exposed, short sections of said wires of said second plurality along at least one length of said insulating material, said last-named short sections fused to said first-named short section and severed to produce at each end section at least one thermoelectric element having a fusion joint as a junction and the dissimilar wires remaining connected to said joint as components.
9. The thermopile of claim 8 wherein the wires and insulating material are in the shape of a cylinder.