US1721503A - A copvporation oe - Google Patents

Description

w. H. PRIESS ,721,503

CONDENSER AND PROCES S July 23, 1929.

Filed April 50. 1920 4 Sheets-Sheet l July 23, 1929. w. H. PRIESS 3 CONDENSER AND PROCESS Filed April :50. 1920 4 Sheets-Sheet 2 July 23, 1929. w. H. PRIESS CONDENSER AND PROCESS 4 Sheets-Sheet 3 Filed April 30. 1920 jar/37% July 23, 1929. w. H. PRIESS I CONDENSER AND PROCESS Filed April 50, 1920 4 Sheets-Sheet 4 a 4 @(JLQ 'b -M' lulu-Infill IIIIII IA IIIIIII Patented July 23, 1929.

WILLIAM: H.

raise CIALTY APPARATUS COMPANY, OF BOSTON, MASSACHUSETTS, A CORPORATION OF NEW YORK.

CONDENSER AND PROCESS.

Application filed April 30, 1920. Serial No. 377,977.

This application is a continuation in part i of my application Serial No. 332,421, filed October .42, 1919.

One of the first steps in the p ocess ofmaking condensers consists in .nbling alternate sheets of conducting and dielectric materials to build up a condenser stack of the required height, these sheets of material being secured together by insulation having suitable properties for this peculiar class of work. Notwithstanding great care in selecting these materials and in assembling them, it is a very common experience to have a con denser break down during test or while in use and it has been definitely determined that a very important cause of such break-clowns is the presence of voids, gas, or an excessive amount of insulating material between the sheets of material that make up the condenser.

It is an important object of the present invention to devise a process of making condensers which will reduce to a minimum imperfections and faults of the character above mentioned. In other words, the invention aims to devise a process of building condensers which will eliminate an unnecessary thickness of insulating material between the layers of conducting and dielectric materials that make up the condenser and which will also reduce to a minimum the presence of voids, blisters or bubbles in the condenser. It is also an object of the invention to devise a process of making condensers which will produce a condenser that is a firm mechanical mass, that will have a very low dielectric loss and in which the leakage between successive conducting elements of the condenser will be reduced to a minimum. A further object of the invention is to improve condenser constructions both from an electrical and also from a mechanical standpoint.

The various features of the invention will be readily understood from the following description when read in connection with the accompanying drawings, in which Figure l'is a View, partly in vertical cross section and partly in side elevation, showing the more important parts of a machine for coating the dielectric elements;

Fig. 2 is a side elevation showing the construction of a condenser stack;

Fig. 8 is a front elevation of a baking stand;

Figs. and 5 are plan. and. side views, re

spectively, of the baking stand shown in Fig. 3;

Fig. 6 is a plan view, partly in cross section, of another form of baking stand;

Fig. 7 is a plan view of a condenser after it has been baked;

Figs. 8 and 9 are vertical cross sectional views at right angles to each other of a condenser after it has been baked, and showing the elements enlarged;

- Fig. 10 is a perspective View of a finished condenser Figs. 11 and 12 are plan and side views, respectively, of a baking clamp;

Fig. 13 is a view similar to Fig. 12 but shows a condenser secured in, the clamp;

Fl l.-lisa front elevation showing a step in the process of manufacturing of a modified construction of condenser, and the apparatus used;

Figs. 15 and 16 are side and plan views of the construction shown in Fig. 14;; and

l? and 18' are plan. and side views, a holy, of the finished condenser.

Assuming that the mica or other dielectric elements to be used in the condenser have been made by splitting the mica sheets to the desired thickness and cutting them out to the required shape and dimensions, andalso that the tin foil or other conducting elements of suitable shape and size have been provided, the next step is to coat the mica elements with vari'iisli or other adhesive insulating material. it is desirable that this coating shall be very thin, and that it shall completely cover both surfaces of each mica sheet. This coating operation may be performed in any suitable manner and with any suitable insulating material, but in order to obtain the desirable results ust mentioned I prefer to perform the operation in the coating machine illustrated in Fig. 1.

This machine comprises upper and lower rolls 2 and 3, respectively, mounted in suitable bearings and geared together. The rolls are yieldingly forced together by springs and he lower roll dips in a body of varnish or other adhesive material in a dip pan t. Varnish is supplied to this clip pan from a suit able tank 5 and the overflow is caught in an outer pan or reservoir 6 and conducted by a pipe 7 to a tank 8 where it is saved; The surfaces of the rolls at opposite sides of their act ve areas are grooved to prevent the var- .nislt from. world into the l rings. In

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using this machine the mica sheets are fed between the rolls and thus each receives a very thin but substantially uniform coating of varnish covering its entire area. -Any one of a great variety of insulating varnishes may be used, but I prefer to use a copal varnish containing a benzol or similar solvent.

The mica films ejected from between the rolls drop on to a conveyor belt 10 which delivers them to a hopper 11 through which a current of air is blown by a fan 12. Due to the fact that the coatings of varnish are very thin and that the solvent is very volatile, the mica films dry 'very quickly and the length andrate of travel of the conveyor 10 are made such that the mica films are sufficiently dry by the time they are delivered to the hopper 11 so that they do not stick together. The fan 12 also exerts a further drying action on the lilms so that the greater part of the solvent in the coating is eliminated before the films are deposited in the hopper. Both the rolls and the conveyor are driven from a motor 13 by connect-ions readily understood from an inspection of the drawings.

After the dielectric elements have been coated and partially dried, as above described, they are next assembled with sheets of tin foil, or other conducting material of the proper size and shape, to form a condenser stack. The condenser stack may either be built up in the form of a tall stack that later is split up into units of the required height, or the stack may be built up in the form of individual units, each unit stack being kept separate from the others. Fig. 2 illustrates diagrammatically a portion of a stack, the two sets of foil elements being indicated. at C and D, respectively, and the mica elements separating them at M. The elements are here shown separated from each other for the sake of clearness.

After the assembly of the stack has been completed, the stack is next heated as by baking for the purpose of driving out the solvent in the varnish, eliminating any excess of varnish between the elements, and driving out any gas or vapor that may be pocketed between the elements of the stack. A further result accomplished in the baking operation when certain varnishes are used is the oxidizing or chemical changing of the solid constituents of the varnish. For this purpose the condenser stack preferably is transferred to a baking stand of the general character illustrated in Figs. 3, 4t and 5.

Assuming that the condenser stack has been assembled in units, as above described. those units indicated at S are placed one above the other in a stand whichcomprises a base 16 and two posts 17 and 18 rising from the opposite sides of the base and threaded near their upper ends. The individualunits S are separated from each other by separator strips 19 of brass or other suitable material having holes drilled in them so that they slide freely on the posts 17 and 18.

For the purpose of clamping this stack in the stand and compressing the stack, a pressure plate 20 is placed on top of the stack, a. spring plate 21 is superposed on this pressure plate and another heavier plate 22 is placed on top of the spring plate 21. Nuts 2323 are then threaded 011 the posts 17 and 18 and are tightened up to apply pressure through the plate 22 and spring plate 21 to the entire stack. The spring plate 21 preferably con sists of a steel plate bowed or sprung, as best shown in Fig. 3, and of such thickness that the proper pressures are obtained. A pressure of approximately 600 pounds per square inch of condenser is ordinarily used. Usually the nuts 2323 are turned down before the baking operation is begun until the spring plate is practically flat. The baking stand with the stack of condenser units clamped therein is then placed in an oven and baked for several hours at a temperature of approximately 110115 C.

The condenser shown in the baking stand in Figs. 3, 4 and 5 is of a somewhat different form from that shown in Figs. 2 and 7, since the former type of condenser is provided with holes 25 extending vertically therethrough which later receive pins by which the finished condenser is secured to its support. In bak-,

ing this type of condenser separator plates 19 are used which are drilled to receive pins 26 that extend through the holes 25 25 in the condenser. These pins are held in position by friction and as the stack is compressed the pins are forced downwardly by their contact with the spring plate 21 through holes provided for them in the base 16, as shown in Figs. 3 and 5. These pins serve a useful purpose in the laking operation since they support the elements of the condenser against lateral slip or displacement.

\Vith the type of condenser shown in Figs. 2 and 7, I prefer to use a baking stand in which four posts 28 of the cross sectional shape-shown in Fig. 6 are located, respectively, at the four corners of thestack, the posts being grooved longitudinally to receive the corners of the mica sheets M. Pressure is applied to the stack in the same way as in the construction previously described but the engagement of the posts 28 with the corners of the mica sheets support these sheets against lateral slip or displacement.

lVhile, as above stated, the greater part of the solvent in the varnish coating of the dielectric elements is eliminated before these elements are assembled to form a condenser stack, a certain amount of solvent still remains in the lower layers of the varnish coating. Furthermore, the coating is thicker than is necessary. The baking of the stack while it is maintained under a heavy spring pressure squeezes out the surplus varnish which softens under the relatively high degree of heat to which it is subjected. 'At the same'time this comparatively high heat vaporizes the solvent together with any other vaporizable elements, such for instance as traces of water, which may be pocketed between the sheets, and the heat, together with the pressure, cooperate to reduce to a mini mum gas, vapor, and the thickness of the adhesive remaining between the sheets. In other words, the heat tends to diltuse any gases or vaporizable materials present between the sheets while the pressure tends to squeeze out these materials from between the sheets of the stack. lVhen separator plates 19 made of brass or similar material are used, as dis tinguished from insulating material, they serve not only to separate the units and give the stack additional mechanical rigidity, but also to conduct heat into the interior of the stack and thus to facilitate the baking operation. The even is ventilated during the baking operation for the purpose of carrying oil the solvent vapors and also, when an oxidizable varnish such as copal varnish is used, to provide an ample supply of oxygen to eficct this oxidation.

As the baking operation progresses there is a tendency to relieve the pressure on thestack due to the expansion of the rods 17 and 18 and also due to a reduction in the height of the stack caused by the elimination of excess adhesive materials from between the elements of the stack. This tendency is countor-acted to a substantial degree by the spring plate 21. I find that better results, however, can be obtained by tightening up the clamp as the baking operation progresses to more or less restore the initial compression. For instance, in one type of condenser manufactured according to this process, the baking operation is conducted at about 110l15 C. and requires a period of approximately twelve hours. At the end of the first three hours, and again at the end of nine hours, the baking stand is removed from the oven long enough to tighten up the nuts E2323 and these nuts are tightened up a third-time when the baking operation has been completed and immediately after the baking stand is removed from the oven, and, consequently, while the stack is still at baking temperature. In other cases only one readjustment of the pressure is made and in such cases this readj ustment usually made at substantially the end of the baking operation. It will, of course, be understood that the temperature at which the baking operation is conducted, the duration otthis operation, and the readjusting of the pressure, will depend upon the nature of theadhensive material used in the condenser, the nature of the service for which the condenser is designed, and other considerations. I

I prefer to use a dielectric in the condenser of the above described process which will form a condenser of superior electrical qualities. I find mica the best for his purpose. Mica is also non-porous and also non-inflammable. Hence under compression and baking, the adhesive, which is a poor dielectric, does not penetrate the dielectric as in fibrous dielectrics, nor is there danger of carbonizing the dielectric when baked. as in dielectrics of organic composition. Upon compression and baking, the greater part of the adhesive is forced into the margins where it serves as a protector against dirt and moisture and to a very small degree only forming part of the dielectric, sutlicient only to hold the sheets together.

Aft-er the baking operation has been completed and the pressure on the stack has been restored, above described, immediately fol lowing the removal of the baking stand from the oven, the rods 26, if used, are withdrawn and the stack is allowed to cool while still maintained under spring pressure. Usually this cooling action is hastened by directing a blast of air against the projecting ends of: the toil elements. lVhen the temperature of the stack has been reduced to substantially that of the room, the pressure on the stack is relieved and the condenser units are removed from the bakin stand. It the separator plates 19 have not been used but the stack has simply been baked as one tall unit,'this unit or stack is next split up into units of the desired height. I

I regard the use of a spring in the application of pressure as especially important. The very desirable results produced by this process apparently are due to a more thorough elimination of gas and vapor from between the elements of the condenser and to a reduction in the thickness of the varnish or other adhesive remaining between the condenser elements after the baking operation has been completed.

Another factor contributing to the superior results produced by this process is the elimination of the greater part o1 the solvent from the varnish coating on the dielectric elements before these elements are assembled to form a condenser stack. This leaves only enough solvent in the coating to cause the coating to soften and become tacky during the baking operation, which result is desirable at this time for the purpose of adhesively securing all the sheets of material firmly together, and since only a relatively small quantity of solventstill remains in the coating, the elimination of the solvent can be made much more complete than would otherwise be possible. A further advantageous result produced by this process is the sealing of the marginal spaces between adjacent dielectric elements. It will be seen from an inspection of Fig. 7 that the part of each conducting element lying on a mica sheet is surrounded on three sides by an inactive margin of mica; that is, an area of mica that is not subjected to the dielectric strain. This margin is covered with varnish during the coating operation, as above described, and the compression to which the condenser stack is subjected during the baking operation results in squeezing additional varnish out from between the overlapping sheet and into this marginal area.

This varnish completely seals the space between the inactive margins of adjacent dielectric elements, as indicated at E, Figs. 8 and 9. It will be understood that when a condenser is in use there is a tendency for the current to discharge from the edge of one of the conducting elements across the inactive margin to the edge of the next adjacent conducting element. Ordinarily a comparatively free path is left for this discharge over the surface of the mica. The sealing of these marginal spaces, however, by varnish introduces a solid wall of insulating material in the path of this discharge and thus reduces very substantially the surface leakage that otherwise would be present in the condenser.

In other words, this construction reduces the electrical loss in the condenser.

After the baking operation has been completed, the condenser will be found to be a firm, mechanical mass capable of withstanding very rough usage. The sealing of the edges, as above described, contributes to this result and also is of advantage when the condenser is to be used in a damp location.

After the condenser has been baked its manufacture may be completed in any desired manner. According to the process which I prefer at present, the condenser units are next tested and any units which fail to withstand the tests are discarded. The units which sucsessfuly come through the testing operation, however, next have the projecting ends of foil trimmed off and suitable clips, such for instance as those indicated at and 31in Fig. 10, are secured to the opposite edges of the condenser in contact with the respective sets of foil elements. In order to reduce still further the electrical losses in the condenser and also for the purpose of improving its mechanical structure, it is preferable under some conditions to repeat the baking operation. Before this baking is begun, however, I prefer to coat the edges of the condenser sheets with varnish in order to seal any cracks in it or to check any tendency to split that may have been started by the clipping opera tion. This second baking need not be continued as long as the first-but usually is from six to eight hours at a temperature from 110 to 115 C. Theexact natureof the change produced in the condenser by this second baking is difficult to determine but probably is due to some change produced in the exceedingly thin film of varnish that still remains between the adjacent elements of the condenser. 1

have determined, however, by actual test that the electrical characteristics of the condenser are improved by this second baking and that they can be still further improved by a third or fourth baking. The degree of improvement, however, decreases with successive bakings and usually it is not advisable from a manufacturing standpoint to carry the condenser through more than two of these heat cycles. As shown in Fig. 10, the sheet metal clips or clamps 30, 31 are formed with integrally projecting terminal connectors, which are preferably perforated as shown to facilitate the making of circuit connections with the completed condenser, such connectors projecting away from the stack, as shown.

For the purpose of compressing the condenser stack during the baking of the condenser after the clips have been secured to the condenser, I prefer to use a novel form of baking clamp illustrated in Figs. 11, 12 and 13. This clamp comprises a base 33 having a threaded post or pin 34 secured thereto at one end. Another screw threaded post 35 is secured to the opposite end of the base by a pivot 36. A spring plate 37 has a hole through one end to fit loosely on the pin 34 and is notched at its opposite end to receive the swinging pin 35, the upward movement of this spring on the post 34 being limited by a nut 38 threaded on this post and adjustably secured in position by a setscrew 39. The condenser is clamped between this spring plate 37 and the base 30 by pressure exerted on the spring through a nut 40 threaded on the pin 35. In order to enable this apparatus to exert pressure uniformly over the entire surface of the condenser, a plate 41 is reinovably secured to the upper surface of the base 33 by means of a screw 42, this plate being shaped to fit the lower face of the condenser between the clips. A similar plate 43 is secured tothelower face of the spring 37, this plate being provided with a locating pin 44 and a threaded stud 45 that project through the spring plate and being held in position by a nut 46 threaded on the stud 45. This plate fits the upper surface of the condenser between the clips.

The apparatus is very convenient for use in baking condensers after the clips have been applied to them whether such baking is for the purpose of improving the electrical characteristics of the condenser or is for the purpose of baking on a. label or cover slip, or for any other reason. It will be seen that after the nut 38 has been adjusted at the proper height to take a condenser of a given thickness, it is unnecessary to change the adjustment of this nut again until another size of condenser is to be baked. The condenser is slipped between the plates 41 and 4", the pin 35 is swung into its upright posit-ion. as shown in Fig. 12, and the nut 40 is quickly turned down to straighten. spring plate 37, as

shown in Fig. 18, thus applying a fairly heavy pressure to the condenser. After the baking operation has been completed the condenser can be quickly removed by reversing the ope *ations above described. A further advantage of this construction is that the plates 41 and 43 can be quickly changed to accommodate different sizes and styles of condensers.

While the foregoing description has referred to the coating of the dielectric elements of the condenser, it would be possible to coat the conducting elements instead or to coat both sets of elements wit-h anin'sulating material more or less adhesive. In some cases, however, it is not practicable to coat the conducting elements because they are too thin and flimsy and it is necessary to coat one set only of these elements. It is not regarded, therefore, as a departure from this invention to coat the other elements of the condenser if desired, nor is it regarded as a. departure to use an adhesive material other than varnish, this material being mentioned specifically be cause it is preferred for the particular type of condenser specifically described herein.

A somewhat different form of the invention is shown in Figs. 17 and 18 and certain steps in its process of manufacture and in the apparatus preferably used in this process are illustrated in Figs. 14, 15 and 16.

In this form of the invention the coating and assembling operations are performed in the same manner as above described. Any desired number of the units S are then placed in a baking stand having parts 16, 17 and 18 like the baking stand shown in Figs. 3 and 4. In this arrangement, however, each unit S is located between two metallic spring plates normally convexly curved or bowed with reference to the condenser, as clearly shown in Fig. 15, one of these plates being designated at and the other at 51. It will be seen that the plates are alike in construction and that each plate is provided at opposite ends with projections extending at substantially right angles to the plate and designed to fit over the opposite sides or edges of the condenser unit. Ears 52 and 53 also project on these plates and have holes punched in them. The parts 50 and 51 consist preferably of a metal which can be easily bent or shaped to form the intermediate spring portions and the rigid angular projections. Any suitable sheet metal is adapted for this purpose, such as steel or phosphor bronze. In assembling the parts 50 and 51, the spring portions are arranged on opposite sides of the stack with their convex surfaces facing the same and in parallelism with the sheets composing the stack. They are then compressed. flat against the stack and rigidly secured at opposite sides as hereinafter described to maintain the sheets together under spring compression.

prises two parts which can be cheaply made and easily assembled to maintain the stack under good compression and at the same time allow the leads to be brought out at opposite ends of the stack, the stack projecting at opposite ends beyond the clamp. The individual units with their respective pairs of plates are separated by plates 19, the entire stack of units being set up in the baking stand, as shown in Fig. 15. This baking stand also is provided with a pressure plate 22 mounted to slide on the rods 17 and 18 and arranged to bear on the top of the stack of units. Any suitable means may be provided to compress the stack, that shown consisting of a square threaded screw 54 having a ball ended connection with the pressure plate 22 at its lower end and threaded through a yoke or plate 55 which is removably secured to the upper ends of the rods 17 and 18. The hands 56 is secured to this screw and affords a convenient means for turning it. It will be seen from an inspection of Fig. 14 that with this arrangement the entire stack can be compressed until the springs 50 and 51 are straightened out, thus compressing the units S with a spring pressure in the same way that pressure is applied to them in the arrangement shown in Figs. 3 and 4.

The heating or baking, and if desired also, the re-heating or subsequent baking operations are performed in the same manner as above described, but after the baking oper ation has been completed the projecting ends of the spring plates which have now been brought into overlapping relationship at opposite sides of each unit, are permanently secured together by soldering or welding. Preferably this securing operation is performed by spot welding, as indicated at 57, Fig. 18. The condenser terminals 58, 59 are shown in Figs. 17 and 18 connected to the foil-sheets projecting from the stack. As shown in Fig. 17, the integral parts 52, 53, formed with holes, constitute means for securing the finished condenser in place.

This process results in enclosing the condenser stack in a casing which because of its resilient nature and the manner in which it is secured on the stack, exerts a permanent spring pressure on the stack over substantially its entire active area. After the securing operation the units may be removed from the baking stand.

The connections to the opposite sets of conducting elements can be made in any convenient manner as, for instance, by soldering leads 58 and 59 to these conducting elements, respectively, with a suitable soft solder. The punched cars 52 and 53 are provided to facilitate the securing of the completed condenser to its support. This construction provides a very compact and efficient condenser especially adapted for locations where the condenser is subjected to rough usage.

The expressions mica and foil sheets are used herein to include substantially equivalent materials, both being substantially nonporous and non-impregnatable, and uncompressed under even the high pressure employed in accordance with the invention.

hat is claimed as new is:

1. That improvement in the process of making electrical condensers which consists in applying a thin film of varnish to substantially all parts of both sides of sheets of dielectric material, drying said varnish more or less, then building said sheets into a stack with sheets of conductor placed between successive sheets of dielectric, compressing said stack in the direction of its height under resilient pressure, and baking said stack while under said resilient pressure.

2. That improvement in the process of making electrical condensers which consists in coating both sides of the dielectric elements with a film of varnish covering substantially all parts thereof, and of substantially uniform thickness, drying said varnish more or less, building said sheets into a stack with conducting sheets interposed between suecessive sheets of dielectric, applying spring pressure to saidstack in the direction of its height, baking the same while under said spring pressure, and maintaining ventilation in the baking chamber to supply oxygen for chemical changes in the varnish and. to carry off evaporated solvent.

3. That improvement in the process of making electrical condensers which comprises the steps of coating the dielectric elements of the condensers with varnish, drying the coated elements sufficiently to eliminate the greater part of the solvent in the coating, assembling said elements with conducting elements to form a condenser stack, compressing said stack, baking the stack while so compressed, adjusting the pressure on the stack during the baking operation, and maintaining said stack under compression while it cools.

4. That improvement in the process of making electrical condensers which comprises the steps of covering the opposite sides of each dielectric element of a condenser with a thin coating of varnish, drying the coated elements sufliciently to enable them to be conveniently handled, assembling said elements with conducting elements to form a condenser stack, compressing said stack in the direction of its height with a resilient pressure, baking the stack for several hours at a temperature of over 100 C. while the stack is so compressed, adjusting the pressure on the stack after the baking operation has progressed several hours and while the stack is .at baking temperature, and maintaining pressure on the stack while it cools.

5. That improvement in the process of making electrical condensers which consists in placing a condenser stack between two spring plates that engage the opposite ends of said stack, said plates normally tending to assume a convexly curved shape with refercnce to the stack and having parts adapted to be brought into engagement at opposite sides of the stack, applying pressure throu h said plates to compress the stack thereby straightening the plates against the ends of the stack and bringing said parts into engagement, and then fusing said parts together while the plates hold the stack so compressed.

6. The hereindescribed process of making electrical condensers which consists in applying thin uniform coatings of insulating resinous material over both sides of mica sheets, at least partially drying said sheets, building said sheets into a stack with conducting sheets and then heating said stack for several hours at a temperature of the order of 100 C. and under a pressure of the order of hundreds of pounds per square inch to modify the physical characteristics of the material.

7. The improvement in the art of making mica and foil sheet condensers, which consists in coating the mica sheets with a more or less adhesive insulating liquid; drying the coatings on such mica sheets more or less at ordinary temperature; assembling such mica sheets with the foil sheets in a stack; applying a high mechanical compression to the faces of the stack; subjecting the compressed stack for a substantial time to a sufliciently high temperature under oxidizing conditions to soften the coatings between the micas and foil sheets and to vaporize more or less of the solvent in said coatings; said compression being suiticiently high to force out excess softened coatings from between the foil sheets, and thereby reduce the length of the stack; and more or less restoring said high compression when the coatings of reduced thickness are in softened condition.

8. The improvement in the art of making mica and foil sheet condensers, which consists in coating the mica sheets with a more or less adhesive insulating liquid; drying the coatings on such mica sheets more or less at ordinary temperature; assembling such mica sheets with the foil sheets in a stack; applying a high mechanical compression via a spring plate to the faces of the stack; subjecting the compressed stack for a substantial time to a sufficiently high temperature to soften the coatings between the micas and foil sheets and to vaporize more or less of the solvent in said coatings; said spring plate compression being sufficiently high to force out excess softened coatings from between the foil sheets and therebyreduce the length of the stack; and more or less restoring the compressing flexing of said spring plate when the coatings of reduced thickness are in softened condition while subjecting the insulating material to taining sufficient pressure on the faces of the stack during said heat treatment to force out excess softened coatings from between the mieas and foil sheets; applying a metal clip to the stack; coating the edges of the stack sheets with insulating liquid; and subjecting the clipped and edge-coated stacks to heat treatment under temperature conditions similar to those of said first heat treatment.

In testimony whereof I have signed my name to this specification.

lVILLIAM H. PRIESS,

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