Document zo8poyyQXLkkQypX3Vj9zNVa7

FILE NAME:McGraw-Edison (ME) DATE: DOC#: ME0 DOCUMENT DESCRIPTION: WDZZZ ZZZZ April 16, 1963 M. L. MANNING 3,085,315 METHOD OF CONSTRUCTING INSULATING BARRIER FOR STATIONARY INDUCTION APPARATUS Original Filled Aug. 6, 1952 2 Sheets-Sheet 22 S Za A %S..SS.S M AEIE NVentor MELVIN MANNING April 16, 1963 M. L. MANNING 3,085,315 METHOD OF CONSTRUC INSULATING BARRIER Original Filed Aug. 6,FO1R9S5T2ATIONARY INDUCTION APPARATUS 2 Sheets-Sheet 2 NVENTOR MELVIN L. MANNING 4 *r2 Aana;/ -- -- - - United States Patent Office Patented Apr3.,0168,5,1391653 sad 3,085,315 trated as shown in FIGURE 1 by dot-and-dash lines, A core to low voltage winding (CL) barrier envelope 12 METHOD OF CONSTRUCTING INSJLATING BARRER FOR STATIONARY ENDUCTION stirrounds core leg 11 in close proximity thereto. Axial APPARATUS spacers 13 may be positioned around the outside of CL Melvin L. Manning, Upper St. Clair Township, Allegheny 5 barrier 12 in spaced arrangement where they are held County, Pa., assiglior to McGraw-Edison Company, a in the course of the winding of the concentric layers 14 corporation of Delaware and 5 of a helically wound low voltage winding. Spacers Original application Aug. 6, 1952, Ser. No. 32,837, now 13 may be held against barrier 12 by a suitable silicone Patent No. 2,827,616, dated Mar. 8, 1958. Divided and this application July 15, 1957, Ser. No. 675,792 adhesive or other means. The conductors employed in 0 winding the low voltage winding layers 14 and 15 may be 10 Clains. (C. 29-155.57) wire which is served with a glass silicone resin bonded This application is a division of application Serial No. tape insulation prior to winding. In other cases, such 302,887, filed August 6, 1952, now patent No. 2,827,616. conductors may be served with other materials such as This invention relates to a new high temperature trans flexible asbestos or glass cloth silicone tape or "Teflon' former construction or the like. More particularly, it tape, Insulation in the form of a cylindrical envelope pertains to an improved method of constructing insu 16 may be interposed between the layers 14 and 15 by lating barriers for stationary induction apparatus in which using, for example, silicone rubber ply material similar silicone and fiber insulating materials are combined. to that employed in the construction of CL barrier 12, High temperature transformers and other high tem A further series of axial spacers 17 may be held against perature electrical induction constructions have been an 20 and spaced around the exterior of the low voltage wind objective of various manufacturers for some time. The ing coil comprising layers 14 and 15. A further insu work in that direction received added impetus upon the lating (HL) barrier envelope 18 between the low voltage widespread introduction of silicone materials a few years coil 14-15 and a high voltage coil 19 may be provided ago. Despite the recognition of the electrical qualities under my new construction entirely of silicone and fiber of such materials, a number of drawbacks attended use matrix materials. HL barrier 18 may comprise a combi of them. One manufacturer despite the availability of nation of more of the interleaved plies than the number such silicone materials provided heat-resistant glass cylin used for the CL barrier as shown in FIGURE 6. Barrier ders for insulating barriers. 18 insulates the low voltage windings from the high volt I have discovered a new method of constructing in age windings and must withstand the relatively high volt sulating barriers for high temperature transformers which 30 age potentials and thermal increases to which it is sub enables the remarkable properties of silicone materials to jected. A further series of axial spacers 20 of solid or be fully employed for insulation and support purposes to molded U shapes may be fastened at uniform intervals provide a transformer or the like with a Class-H rating. around and to the exterior of HL barrier 18. Alternate At the same time, I am enabled to shield the leads and spacers 21 in the series of spacers 20 may be of key windings by means of my new method of construction in 35 stone shape, if desired, to permit keying engagement with a manner which preserves the advantages thereof without and movement during assembly for lateral spacers 22. risk of creepage or breakdown in that area. Other ad The high voltage coil 19 may comprise a plurality of vantages of my new method of construction are that it double layer horizontal pancake windings 23 in vertically provides protection against local dielectric and thermal stacked arrangement as shown in FIGURE 2, respective weakness points and permits efficient construction practices ones of said such windings being interconnected electri to be employed. cally by crossover conductors 24. The radial spacers 22 Other objects and advantages of this invention will be may be made of a mechanically and electrically strong apparent from the following description and from the material like porcelain. Axial spacers 13, 7 and 20 may drawings, which are illustrative only, in which be made of Class-H insulating material in the form of FIGURE 1 is a plan view, partly schematic, of low and solid or channel-shaped spacers. As an example, I have high voltage windings, barriers and spacers of a high discovered that glass cloth or asbestos cloth bonded with temperature dry transformer embodying my invention; silicone resin materials may be molded or otherwise FIGURE 2 is a view in elevation of the structure shown shaped and cured to provide axial spacers of markedly in FIGURE taken along line II-II of FIGURE 1; superior mechanical and dielectric strengths. The annular FIGURE 3 is a somewhat detailed view of the portion pressure plates and tie rods which respectively bear end of the structure shown in FIGURE 1 adjacent the low wise on the respective windings and lateral spacers 22 have voltage winding leads and shielding, taken along the line not been illustrated as such features will be well under III-III of FIGURE 1; stood by those skilled in the art. The vertical ducts 25 FIGURE 4 is an illustrative dissected view of a form able silicone resin laminate employed in my new con 5 5 between the axial spacers 13, the vertical dicts 26 between the axial spacers 17, the vertical ducts 27 between the struction; axial spacers 21 and the lateral ducts 28 between the FIGURE 5 is an illustrative dissected view of a flexible lateral spacers 22 afford access to the turns of the respec silicone rubber and fiber ply material employed in my tive windings for whatever cooling or ambient fluid may new construction; FIGURE 6 is a section through a core to low voltage 60 be employed. In the case of a sealed dry-type transformer embodiment, a gas like nitrogen may be used to fill the winding barrier in my new construction showing the sili gas space within a transformer tank in which my new cone materials of FIGURES 4 and 5 in an interleaved construction is placed. combination; and In construction the embodiment shown in FIGURES 1 FIGURE 7 is a diagrammatic showing of one end of and 2, one mode of making the new barrier envelope is a transformer embodying my invention in the course of illustrated in connection with making a barrier 18 and being constructed. other barrier 12 may be similarly made. In the trans Referring to the drawings, a power transformer 10 is former or the like being constructed, any entire or other shown in FIGURES 1 and 2 in sufficient detail to illustrate partial barriers, whether cylindrical or arcuate, may be the new construction features in one embodiment of my 70 constructed in accordance with my invention. In that invention. In that embodiment, a cruciform laminated invention, one or more sheets 29 of a formable silicone core leg 11 of a conventional magnetic core may be illus resin and fiber laminate is combined in interleaved man 8,085,315 3. 4. ner with one or more plies 30 of flexible silicone rubber of the embodiment of the new transformer which is and fiber material. Generally, in preparing a cylindrical shown. In my invention, cylindrical insulation plates 34 envelope barrier in accordance with this invention, each which are arcuate and may be preformed of interleaved kind of material is formed into the desired cylindrical layers of laminate like laminate 29 alternating with inter shape beginning with the innermost layer. Thus, a cylin leaved layers of silicone rubber material like ply 30 are der of the laminate may be provided and the ends over provided and inserted in place as shown in FEGURES 1 lapped and fastened by a silicone adhesive, for example, and 3 generally in the course of the construction winding or by Class-H tapes of sufficient strength, enough turns as on lathe 31. The surface of the insulation exposed to being made in the laminate layer to provide a due share of the lead or leads is preferably faced with a silicone rubber the total insulation of the new combination. Then one or 10 coated glass cloth or asbestos cloth. more layers of the silicone rubber and fiber material may In Such construction, as an example, the innermost be formed in cylindrical shape around the laminate and plate 34 may be pinched between and held between the the outermost end of said material fastened again by top turns of layer 14 and the adjacent axial spacers 13 silicone adhesive or other means. In some cases, the on the other side of that plate 34. On the other hand, laminate material may have a silicone rubber layer affixed if keep-back insulation of a Class-H category is interposed to one or both surfaces of the laminate before the laminate between the plates 34 above the topmost turn of the is formed. Around the silicone rubber and fiber material, layers 14 and 15, and, on each side of leads 33 in a another one or more layers of laminate material may be wound against the exterior of the silicone rubber and fiber circumferential direction, as shown in FIGURE 3, then the plates 34 may respectively be held between such material layer or layers to complete the interleaving. As 20 keepback insulation 35 and the axial spacers 13 and 23 shown in FIGURE 6, barrier 12, for example, may be respectively. The height of the plates 34 above the bar made in the form of a right cylinder, the innermost layer riers 12 and 18 guards against creepage and any possi of which as shown is a layer of formable laminate Sur bility of short-circuiting at the outlet shown of the taps rounded by two layers of silicone rubber and fiber in turn surrounded by a single layer of laminate again, the entire 25 33 relative to parts of the transformer at different voltage potentials. construction comprising a single barrier envelope with Laminate sheets such as sheets 29 may now be obtained markedly superior features, both dielectrically and me as a commercial article having a fiber cloth matrix of an chanically, to any insulation heretofore provided. inorganic material like heat-cleaned glass or asbestos. The construction of a new barrier envelope may be 30 Such fibrous matrix in the case of glass, for example, may be in part a staple cloth 36 woven of short lengths performed in a winding lathe such as those used in the transformer industry. For example, as illustrated in FIG of glass filaments in the manner of a staple vegetable URE 7, a barrier such as HL barrier 18 is shown being fiber cloth. Laminate 29 also preferably employs a fiber formed in place. The CL barrier has already been formed matrix cloth 37 woven of continuous glass filaments placed in juxtaposition to the cloth 36. These cloths 36 and is molded in a lathe 31 which rotates in the direction of the arrow. If desired, a steel cylinder may be used and 37 are impregnated with a silicone resin which enters as the innermost mold to initiate the construction of bar the interstices of the cloth and wholly covers them to rier 12 thereagainst in which event a mold release com form, when solidified, the laminate sheets 29. In place of such fiber matrix, it may also be possible to use such pound, for example, may be used between the exterior of the mold cylinder and the interior cylindrical surface inorganic fibers in the form of a felted cloth or matte of the CL barrier 12. In the partially completed con 40 highly resistant to electrical, mechanical and thermal deterioration. The silicone resins may be of a kind such struction shown in FIGURE 7, the axial series of spacers 13 has been positioned, the low voltage winding 14-15 as those sold by Dow Corning Corporation under the has been wound on the lathe around the spacers 13, the trade identification "DC 2104.' A silicone material lami nate of suitable character for employment in my invention spacers 17 have been positioned and the lathe is in the midst of winding the HL barrier 18 as aforesaid with a is one such as that made by Mica Insulator Company of Schenectady, New York, having the trade designation strip 29 of laminate. Enough layers of silicone laminate "E-724 Lamacoid." In preparing laminate 29, usually and silicone rubber material will be utilized in the new interleaved relation to provide the necessary mechanical from 40 to 60 percent of the weight thereof will be the and dielectric strength required for the high-low voltage 50 partially or In applying wholly cured and solidified silicone the laminate 29, it is preferable to resin. place barrier envelope service in the particular transformer or the staple cloth 36 on the inside of the bend and the con the like under construction. When such winding of bar rier 18 is completed, that barrier, as is the case of barrier tinuous fiber cloth 37 on the outside, generally with the warp of both cloths parallel to the longitudinal axis of 12, will have sheets 29 and 30 interleaved throughout. the insulation envelope or cylinder being constructed. Preferably, the sheets 29 and 30 are of a width equal to the entire height of the transformer windings plus the Suitable silicone rubber plies such as the sheet 30 are additional height equal to the keep-back insulation length also now commercially available on the market for em ployment in my invention. Such a silicone rubber ma desired at the respective axial ends of the transformer terial may comprise a cloth of inorganic fiber like glass or the like. or asbestos, thoroughly heat-cleaned to remove organic The new construction of this invention also effects a GO binders, which cloth forms a matrix for a silicone rubber marked improvement in taking care of lead outlets par coating. One suitable silicone elastomer for such coat ticularly in the case of the more interior windings such ing purpose is "Silastic," a trade name of Dow Corning as the low voltage coil 14-15 shown in the illustrated Corporation, which corporation also makes a suitable embodiment. Leads 32 from the high voltage coil 19 silicone rubber and fiber matrix material suitable for the do not generally present any particular problem in the sheets 30. The fiber glass which may be utilized in the case of an embodiment such as that shown because of construction of the new insulation components of this in their presence in the vicinity of the exterior of the trans vention should have characteristics suitable for satis former where they can, for example, be readily conducted factory electrical, mechanical and thermal service in a upwardly through an insulating tube, which, again, may Class-H transformer. Certain of the glass products be made of tubular silicone material now available on the termed "Fiberglas' by Owens Corning Fiberglas Corpora market. On the other hand, conductor leads 33 are generally connected not only to the ends of the low voltage coil 14-15 but also to intermediate points therein and all of the leads or taps have to have an outlet from the interior tion are suitable in making up the matrices for silicone sheet materials like sheets 29 and 30. In my employment of such new insulation in a trans former or the like, I have also found it useful to have the dielectric constants of the interleaved sheet materials 3,085,315 5 6 like sheets 29 and 30 of about the same dielectric con stant, which may be in the neighborhood of 3.5. There ceeded by means of the present invention without any by, the gradient through the materials in series because observable deterioration. In constructing the new trans of their interleaved relation appears to be relatively uni form and the voitage strength throughout the new insula former or the like, it will be understood that other insulat ing materials that may be used for other parts of the tion structure will also be generally uniforin and foci of 5 transformer than those described herein will be of suit weakness for breakdown in the course of use are avoided. able electrical and mechanical characteristics for the in tended purpose. The invention is applicable not only The use of various filler materials such as titaniura ii oxide or zinc oxide may be employed in the manufacture to dry but also to wet transformers or the like and to of the silicone rubber material, as will be well understood ventilated and sealed transformers and the like. O Various changes may be made in the practice of the by those in the art to whom this inventicn is disclosed, to achieve relative equality of the dielectric constants of present invention without departing from the spirit there the component laminate and silicone rubber materials of or the scope of the appended claims. I claim: in the composite insulation. Other ways may be emi ployed to harmonize the respective constants of the two 1. In the method of constructing a coil assembly of sheet materials. high heat resistance for a high temperature electrical In constructing the new transformer the laminate may transformer, the steps of winding a cylindrical first elec be formed cold for bent diameters exceeding about {} trical coil, forming at least partially cured silicone resin impregnated sheets having an inorganic filler and silicone inches whereas formerly it was believed that such kind of material had to be heat formed. I find, however, that rubber sheets having an inorganic filler into an insulating not only may such material be cold formed but that barrier of concentric, radially staggered silicone resin im pregnated and silicone rubber tubes supported on and such cold forming for laminates not in excess of about 32 of an inch in thickness does not produce any tendency surrounding said first coil, winding a second electrical to craze it, particularly when used in cooperation with a coil supported on and concentric with said barrier, im silicone rubber material like the sheets 30 having a thick 25 pregnating said coils and said barrier with a silicone warnish, and heating said coils and said barrier, whereby ness between about 36 of an inch and 32 of an inch. In some cases the laminate, particularly for Smaller di said coils and said tubes are mechanically bound together ameter uses or thicker sheets, may be heated to about and the mechanical and dielectric strength of said barrier is materially increased. 165 C. for a relatively few minutes generally not in excess of four. 2. In the method of constructing a coil assembly of It appears that the new composite insulation forms a 30 high heat resistance for a high temperature electrical unitary mass which functions ideally as the supporting transformer, the steps of winding a first electrical coil framework for the windings of the new transformer and upon a rotatable cylindrical mandrel, bending while provides dielectric characteristics and strength between the various parts of the transformer at different potentials to rotating said mandrel stiff but formable, flat, at least partially cured silicone resin impregnated sheets having an enable a new transformer to function in the Class-H cate 3 5 inorganic filler and silicone rubber sheets having an inor gory without the disabilities of prior transformers in Such ganic filler into an insulation barrier of radially staggered, category. The sheet materials 29 and 30 so employed may in the case of the laminate be partly or fully cured concentric silicone resin impregnated and silicone rubber tubes Supported on and Surrounding said coil, winding a whereas in the case of the silicone rubber and fiber matrix second electrical first coil concentric with and supported material it will generally be in a partly cured state. Cur from said barrier to completic an assembly of concentric ing of the insulation after completion of the winding as coils having a tubular insulating barrier therebetween, semblies may be performed at at temperature at about removing said coil assembly from said mandrel, im 250 C. Prior to such curing, the winding assembly may pregnating said coil assembly with silicone varnish, and be dipped into a silicone type of varnish, as one example, heat curing said coil assembly. to complete the mechanical binding of the new trans 3. In the method of constructing a heat resistive tubular former windings and assembly. One advantage of such insulation barrier for a high temperature electrical trans a varnish is that its high gloss reduces any opportunity former, the steps of winding an electrical coil upon a rotat for the settlement of dust or other contaminant which able mandrel securing an edge of a silicone resin im might constitute a leakage path and another is its high pregnated rigid but bendable laminate sheet having an craze resistance. Thereafter, if not earlier, any mold inorganic fiber cloth filler relative to said coil and said cylinder which has been employed will be removed before mandrel so that said edge is parallel to the axis of said the completion and performance testing of the entire mandrel and the warp of Said cloth is also parallel to said transformer or the like. axis, rotating said mandrel while retaining said sheet A study of the advantages of my new composite insula in tangential relation relative to a projected cylinder con tion and cooperation in a transformer or the like leads centric with said mandrel to roll said sheet into a tube to the belief that the interleaved substances complement Surrounding said coil, and repeating the last two steps to one another in the course of use to a marked and unusual roil additional silicone resin impregnated sheets into tubes extent. For example, the silicone rubber and fiber matrix Surrounding said coil and build up a tubular insulating material appears to at least hinder the aging of the silicone barrier surrounding said coil having sufficient flexural resin and fiber matrix laminate and also to cooperate in strength to withstand the forces resulting from electrical dissipating any tendency to craze or crack which the short circuit on said coil. laminate might otherwise have. Further, the heat con 4. In the method of constructing a heat resistive tubular ductive properties of the silicone rubber material appear insulation barrier surrounding the winding of a high ten to be superior to those of a silicone laminate such as that perature electrical transformer, the steps of winding a described with the consequence that the thermal lag of cylindrical electrical coil upon a rotatable mandrel, the one material like sheet 29 is materially aided if not Securing an edge of rigid but bendable, flat silicone resin overcome by the thermal conductivity of sheet material impregnated laminate sheet having an inorganic filler rela like sheets 30. Thereby the new transformers or the tive to said coil and said mandrel so that said edge is like can be consistently worked and operated at higher parallel to the axis of said mandrel and said coil, rotating lievels and/or for longer times as in the case of varying said mandrel while retaining said sheet in tangential rela tion relative to a projected cylinder concentric with said load cycles or load demands on transformers, without detriment. mandrel to roll said sheet into a tube surrounding said The hot spot tempcrature standard of 180 C. set some coil, holding said silicone resin laminate sheet in tubular from, repeating the securing and rotating and holding years ago for Class-H insulation can be materially ex steps to roll additional silicone resin impregnated lami 3,085,315 7 S nate sheets into tubes surrounding said coil and build up sulating member and said radially inner and outer cylin drical portions of said coil assembly to effect final curing a tubular insulation barrier surrounding said coil, and heating said coil and said tubular insulating barrier to thereof. 9. In the method of constructing a high temperature effect curing thereof. 5. In the method of constructing a coil assembly having transformer winding of high heat resistance and high high heat resistance for a high temperature electrical thermal endurance upon a rotatable mandrel, the steps of securing an edge of a flat, rigid but formable, silicone transformer, the steps of winding a first electrical coil upon a rotatable cylindrical mandrel, securing an edge of stiff resin impregnated sheet having an inorganic filler relative but formable silicone resin in pregnated laminate flat sheet 10 to said mandrel and parallel to the axis thereof, rotating said mandrel while retaining said sheet in tangential rela having an inorganic filler relative to said coil and said mandrel so that said edge is parallel to the axis of said tion relative to a projected cylinder coaxial with said mandrel, rotating said mandrel while retaining said in mandrel to roll said sheet into a tube surrounding said mandrel, forming a silicone rubber sheet having an in sulation sheet in tangential relation relative to a projected cylinder coaxial with said mandrel to roll said sheet into organic filler into a second tube surrounding said first a tube surrounding said first coil, repeating the last two 5 tube, repeating the above steps to build up a tubular in Sulating member of radially staggered silicone resin im steps to roll additional silicone resin laminate sheets into tubes surrounding said first coil and build up a tubular pregnated and silicone rubber tubes in said winding hav insulating barrier having sufficiently high flexural strength ing sufficient mechanical strength to withstand the forces resulting from electrical short circuit on said winding, to withstand the forces resulting from electrical short cir cuit on said first coil, winding a second electrical coil 20 and supporting a cylindrical portion of said transformer coaxial with and supported from said barrier to complete winding upon said tubular insulating member in coaxial an assembly of electrical coils having a tubular insulating relation thereto. 10. In the method of constructing an electrical wind barrier therebetween, and removing said assembly of elec ing of high heat resistance and high thermal endurance trical coils from said mandrel. 6. In the method of constructing a tubular insulation 25 for a high temperature electrical transformer upon a barrier of high heat resistance and high thermal endurance rotatable mandrel, the steps of securing an edge of a stiff but formable, flat, silicone resin impregnated laminate for an electrical transformer, the steps of winding a cylin drical first electrical coil, disposing elongated insulating sheet having an inorganic filler and high flexural strength spacers extending parallel to the axis of said coil at cir 30 relative to said mandrel so that said edge is parallel to the axis of said mandrel, rotating said mandrel while re cumferentially spaced apart positions about the circum ference of said coil, bending stiff but formable, flat, silicone taining said sheet in tangential relation relative to a pro resin impregnated laminate sheet having an inorganic jected cylinder concentric with said mandrel to roll said sheet into a tube slirrorInding said mandrel and repeating filler into tubular form circumjacent said axially extend ing spacers, holding said silicone resin impregnated lami Said steps to roll additional silicone resin laminate sheets nate sheet in tubular form, forming silicone rubber sheet 35 into tubes surrounding said tubular insulating member in mandrel and build up a said winding of sufficiently having an inorganic filler into a tube surrounding said high mechanical strength to prevent collapse under the silicone resin laminate sheet in tubular form, repeating the bending, holding, and forming steps to build up a forces resulting from electrical short circuit on said wind ing, and Supporting a cylindrical portion of said electrical tubular insulating barrier of interleaved silicone resin laminate and silicone rubber tubes, and winding a second 40 winding upon Said tubular insulating member in coaxial cylindrical electrical coil coaxial with and supported from relation thereto. said tubular barrier to complete an assembly of electrical coils having a tubular insulating barrier therebetween. References Cited in the file of this patent 7. In the method of constructing a coil assembly hav 45 UNITED STATES PATENTS ing high heat resistance and high thermal endurance for 1,278,993 Parks ----------------- Sept. 17, 1918 a high temperature electrical power transformer, the steps 1,535,094 Bingay ---------------- Apr. 28, 1925 of forming a silicone resin impregnated laminate sheet 1,641,272 Horelick --------------- Sept. 6, 1927 having an inorganic filler into a first tube, forming a 1,726,100 Da Costa -------------- Aug. 27, 1929 silicone rubber sheet having an inorganic filler into a sec 50 1,833,221 Leidy ----------------- Nov. 24, 1931 ond tube surrounding said first tube, repeating the last 1,836,948 Anderson -------------- Dec. 15, 1931 two steps to build up a tubular insulating member of in 2, 195,233 Boyer ----------------- Mar. 26, 1940 terleaved silicone resin impregnated laminate and silicone 2,527,236 Whitman --------------- Oct. 24, 1950 rubber tubes, and supporting a cylindrical portion of said 2,595,729 Swiss et al. ------------- May 6, 1952 coil assembly upon said tubular insulating member in 2,734,150 Beck ------------------ Feb. 7, 1956 coaxial relation thereto. 8. In the method of constructing a heat stable coil as OTHER REFERENCES sembly for a high temperature electrical power trans Rubber Age, vol. 58, No. 5, February 1946, pages former, the steps of winding a radially inner cylindrical 579-84. portion of said coil assembly, forming an at least partially 60 Electrical Manufacturing, The Gage Publishing Co., cured silicone resin impregnated laminate sheet having an 1250 Sixth Ave., New York 20, N.Y., October 1951, inorganic filler into a first tube surrounding said radially pages 134-137, and 266; and June 1950, pages 100-103, inner portion, forming a silicone rubber sheet having an 200, 202, and 204. inorganic filler into a second tube surrounding said first Modern Plastics, 122 E. 42nd St., New York 17, N.Y., tube, repeating the last two steps to build up a tubular insulating member of interleaved silicone resin and March 1946, pages 160-162, 192 and 194. Chemical & Metallurgical Engineering, August 1944, silicone rubber tubes, supporting a radially outer cylin pages 135 and 136, McGraw Publishing Co., Inc., New drical portion of said coil assembly upon said tubular in York, N.Y. Sulating member, and heating said assembled tubular in