US1897818A - Insulating body - Google Patents

Description

Feb. 14, 1933. PAS-EN ET AL 1,897,818

INSULATING BODY Y Filed April 9, 1951 3 Sheets-Sheet l L hwy@ Feb. 14, 1933. P. PAscHEN E'r AL INSULATING BODY s sheets-sheet A 2 Filed April 9, 1931 Patented Feb. 1.4, 1933 UNITED STATES PATENT OFFIC PAUL PASCHEN, HANS RITTER, GEORG STAUBER, WILHELM GEBHARDT, AND KARL SCHMIEDEL, F NUREMBERG, GERMANY, ASSIGNORS TO SIEMENS-SCHUCKERT- WERKE AKTIENGESELLSCHAFT, OF BERLIN-SIEMENSSTADT, GERMANY, A. CORPORA- TION OF GERMANY INSULATING BQDY Application led April 9, 1931, Serial No. 528,846, and in Germany April 14, 1930.

Our invention relates to insulating members and more particularly to an insulating member made of ceramic material, for ex-l ample, of porcelain, for electro-technical purposes having a wall or walls for separating and insulating from one another two objects of differential potential. The insulating member is composed of several parts, which form component portions of said separating and insulating wall. These portions are according to the invention joined dielectrically tight to one another, i. e. they are joined to one another in such a manner that the dielectric strength at the joint is substantially the same as at the other parts of the Wall; one of the principal features and great advantages of the invention is, that at the joint the thickness of the separating wall is not increased. The joint used according to our invention extends through the wall, i. e. from one side of the wall to the other, as distinguished from an overlapping joint usually employed in the art in joining insulating members used for the aforementioned separation purposes. Accordingly the definition of the jointpas extending through the wall used in the annexed claims should bel understood in the light of the foregoing explanation.

Our invention will be described with reference to the accompanying drawings, illustrating examples for carrying youtthe invention.

Fig. 1 shows a view of a high-tension instrument current transformer, partially in section.

Fig. 2 shows a longitudinal section through the same instrument transformer taken on the plane A-B of Fig. 1.

Fig. 3 shows a high-tension instrument potential transformer in longitudinal sec'- tion.

Figs. 4 and 5 show another constructional 'form of a measuring transformer, in two vertical sections at right angles to each other.

Figs. 6 and 7 show a high-tension instrument potential transformer, in two vertical sections at right angles to each other.

Fig. S shows a suspension-type insulator in longitudinal axial section;

Fig. 9 shows a high-tensi0n condenser in longitudinal axial section; and

Fig. 10 represents a graph, showing the dielectric strength of insulating material joints as a function of the joint thickness. The different modifications shown and described are merely examples of the types of electric device in which our invention is of particular advantage, and should not be understood as limiting the invention to these types.

Referring to Figs. 1, 2, on the supporting insulator of tubular shape 1 with the foot 2, the metallic base 3 is fixed. On this base is mounted the laminated three-legged iron core 4, the middle leg of which passes through the opening 5 of the annular porcelain receptacle or casing 6. In the annular space inside the receptacle the low-tension winding 7 is placed, and on its outside, the high-tension winding 8. The connecting leads 9 of the winding 7 are led out through the tubular insulator extension 10, the axis of which is parallel to the axis ofV the annular receptacle 6 and to the axis of the supporting insulator l. The insulator 10 is disposed inside the tubular supporting insulator l. One of the ends 11 of the winding 8, is electrically connected with the iron core 4 at which the angle-shaped member 12 is provided, serving as the terminal for that winding end; the other winding end 13 leads to the angle-shaped terminal 14, which is also fastened to the iron core 4, but is insulated therefrom by interposed insulating material l5.

The annular receptacle 6 consists of two parts', of the annular receptacle proper hav- 90 from actual tests,

ing a U-profile and of a flat annular cover 60, the parts being joined together in a dielectrically tight manner. For this purpose at the joint 61 the two parts are carefully plane ground. The grinding must be so fine that the joint is extremely close and constitutes practically a uniform surface-to-surface contact throughout the joint width. It is most essential to make the joint so close that its-thickness in the direction at right angles to the joint surfaces, caused by possibly remaining minute uneven portions in the surfaces, amounts to not more than 1/ 100 to 1/200 mm. We have found that at such closeness of the joint its dielectric strength suddenly rises and rapidly increases with further reduction in thickness. It is therefore better to make the joint still thinner than 1/200 mm., and for instance a thickness of from one to two microns is preferable.

ln Fig. 10 is shown a graph which is made and which shows that above a thickness of 0.02 mm. the joint has a relatively small influence upon the dielectric strength. Below a joint thickness of 0.01 mm. however the dielectric strength very rapidly increases to values, which are of the order of that of the high class insulating materials themselves. We have found in some instances with a joint thickness which was with certainty below 0.005 mm. break downs only at from 122-18 kilovolt occur. The material was a glass plate 4 mm. thick joined by a butt joint according to theff' present invention. At one place wher'gathe' joint was practically a continuous close contact for an appreciable distance, vthe joint broke down only at 30 kilovolt, at which also the solid portion of the glass plate broke down. These tests have proven that with a suiiciently finely finished joint, the dielectric strength of the material itself can be attained for the joint, or at least a strength which is of the order of the material strength.

It is advisable to join together the surfaces of the parts of the insulating member meeting at the joint by means of a very thin layer, or film of material of great dielectric strength, such as resin, asphalt, tar etc. The film has the object of filling out possibly remaining inequalities in the surfaces of the vments, as set forth in the claims, as distinjoint. lt will be noted that while in this particular modification the joint surface extends along one of the surfaces of one of the component elements of the joint (namely along the inner annular surface of rin the joint nevertheless extends through the wall of the casing formed by the two eleguished from an overlapping joint. Also,

as stated in the annexed claims, the two component elements of the joint abut against one another-in 'the sense that pressure exerted on one element will increase the pressure on the joint surfaces, which would not be true if for instance annular cover 60 were inserted into casing 6, forming a joint parallel to the cylindrical wall of the casing.

It is particularly advantageous to tighten dielectrically the joint of two such component parts by means of artificial resin capable of being hardened. For this purpose the surfaces of the joint are, for example, dipped into a solution of resin or into melted resin, the parts beingpreferably warmed at the same' time. To harden the resin, the parts are then put together and, with the joint surfaces forcibly pressed against each other, are placed in a heated chamber, where they remain a length of time depending upon the degree of hardness desired. The resulting product obtained with this method is of the higher quality, the thinner the resin film inthe joint, i. e., the better the parts of they insulating member fit together, and the slower the hardening process is carried out. After the hardening is finished, the wall of the insulating body, as tests show, possesses along the joint the same dielectric strength as at the integral wall portion. According to tests carried out, a joint in a wall of about l cm'. thickness at the joint, has a puncture strength of 40 to 50 kv. and more, i. e., it has the same puncture strength as the integral wall portion. The resin not only tightens the joint dielectrically, but also makes the connection of the parts of the insulating member mechanically so strong that it is able to offer an extremely great mechanical resistance.

So-called plastic shellac is also particularly suitable for tightening the joint dielectrially. It is produced by saponification of natural shellac or artificial shellac, the shellac molecule or a part of these molecules being decomposed into components, which form a thick liquid mass, which dissolves the molecules which were not decomposed. This plastic shellac may according to requirements be hardened to a greater or lesser degree. The hardening takes place quickly at about 180 deg. C., but the resulting joint is in that case not as good as it might be. For joining, the surfaces are dipped into the shellac mass, then placed upon one another and, with or without the application of pressure, put into a chamber having a relatively low hardening temperature, 100 deg. C. or less. Here again, the resulting product is all the better, the slower and the more careful the hardening of the shellac is'carried out.

The great dielectric and also mechanical strength obtained with this method of joining the parts of the insulating member, permits the placing of the joint, if desired, into the electrically highly stressed portion of the insulating member, without danger of breakjoint is practically just as ing down. There is no necessity for making the wall 'of the insulating member thicker at the joint, as the dielectric strength of the reat as that of the insulatin member itsel For the a ove named reasons, joints of component parts of insulating bodies .of widely var ing forms may be placed, where it is most avorable for the assembly of the insulating member and of the electrical apparatus, and where it permits the most favorable shape for the individual parts of the insulating member, and not where it is furthest removed from dielectric stresses as is customary in the present day art.

This is particularly important with porcelain as insulating mategial, as here ver complicated forms may be subdivide into simple component elements which are easily shaped and do not suffer any distortion in firing. In this manner, it is also possible, as shown in Figs. 1 and 2, to produce porcelain receptacles or casings of very great dielectric strength which are adapted to the shape of, and come into close contact with the windings of an electrical'apparatus. Owing to these electric qualities and to the small s ace which such an article requires, parts of) electrical apparatus having a very great difference in potential can be placed very close to one another in a small s ace. With the current transformer shown in lgs. 1 and 2, this' has the special advantage that the mean length of the path in the iron core 4 can be made very short, a feature which according to the known laws ensures great accuracy of measurement with such a current transformer. s

To avoid low discharges, the insulating casing 6 in Figs. 1 and 2 is provided both, inside and outside with a conducting lining which also covers the edges of the joint 61. The lining should extend also over at least a part of the length of insulator 1Q, containing the connecting low otentlal wires, Over the electrical partso the instrument transformer, a cover 16 1s placedand provided with openings 17 for air circulation which cools the windings and the insulating casing.

In F ig. 3, the insulating casing of a potential transformer is composed of two approximately symmetrical halves 62, 63. 61 is the joint through the Wall of the vassembled casing, in this case forming a true butt joint. Into one half 63, part 18 of the high-tension winding is placed, and the other half 62 contains the other part 19 of that Winding.` The connecting wlres 20, 21 of the high-tension windin are led out through insulators 22, 23. he insulators may be formed integral with the pertaining part of the casing, as shown in the left hal of Fig.l 3, but may also consist of a se arate Ielement joined to the casing in a die ectri- 'a dielectrically tight joint 29. 4 is cally tight manner by the butt joint 610. The numeral 24 indicates the low-tension winding.

The space inside of the insulating members, 62, 63 not occupied by windings and leads may be filled with oil or insulating compound to revent corona occurrence in these spaces. he vertical insulator sleeves may then serve at the same time as expansion vessels for the filling compound.

In Figs. 4 and 5, corresponding substantially to Figs. 1 and 2, the supporting in- ,sulator consists ofv a pot-shaped insulating member 25, onto the bottom of which the insulating casing 26 is joined in a dielectrically tight manner. The joint is indicated by 27. 28 is the cover of the casing, which is also joined to the body 26 of the casin by ere again the iron core, 7, 8 are the two windings. The threaded bolts 30, which also serve as connecting conductors for the winding 7, pass through the joint 27 By means of nuts 31 the insulating parts 25, 26 can be tightly drawn together at the joint 27.

Whilst in Figs. 5 and 4 the electrically active transformer parts are positioned on the outer side of the pot-shaped supporting insulator 25, in Figs. 6 and 7 they are accommodated inside the pot. Here, again, 4 is the iron core, 7, 8 are the two windings, 28 is the cover of the casing, 26 is the part of the casing with a U-shaped profile. The casing has openings 260 directed downwardly' in the drawings. The wires for connecting up the winding 7 are passed through these openings. The part 26 is integral with the insulator 32; the bottom of the part 32 is provided with an opening for introducing the sheets for the iron core 4, the sheets being placed alternately from the top and the bottom. The opening is closed dielectrically tight through the plate 33 by means of the joint 34. A special feature of this form of instrument transformer is that its dimensions and height are particularly small.

In Fig. 8, a suspension insulator of the so-called cap type is illustrated. 35 is the stem with the head 36. The cap consists of the pot 37 and the cover 38 threaded into the pot and provided with the suspension eye.39. The insulating member is composed of the supporting cup 40, the cover 41 and the bell 42. At the joints 43, 44, the insulating parts are ground and tightened dielectrically against each other in the manner described above. The parts 40, 41 are mcchanically held together by the parts 37 and 38, a pad 45 being placed between the pressing and pre ised parts. The parts 42 and 40 are clamped together on stem 35, by head 36 and the tightening nut 46 threaded onto stem 35.

This embodiment of the invention shows that for suspension insulators the insulating member-'can also be subdivided in the manner the most favorable for assembling and for the purpose iu question, and that through the subdivision simple, easily producible porcelain parts are obtained, instead of the otherwise complicated forms of insulators used heretofore.

kThe advantage of the insulator shown iu Fig. 8 consists in the existence of only compression stresses in the insulating material and in that it is impossible for the stem to fall out of the insulating member and for the insulating member iu the cap to become loose. As the joints 43, 44 have the same dielectric strength as the wall of the insulating member, they may without hesitation be positioned at places which are exposed to great electric stresses.

A further application of the invention is shown in Fig. 9. The drawings illustrate a high-tension condenser with porcelain as a dielectric and in which a large capacitance is to be accommodated in a small space. By introducing joints the very complicated insulating member is here subdivided ink a series of simple trays or shields 47 as in the bottom part, or of flat bottle-shaped members 48, as in the middle part, or of short pieces of tube with broad flanges 49 at their ends, as in the upper part. The joints are indicated at 50. Here again, the joints are made dielectrically tight in the described manner. The separate parts of the insulator arev mechanically held together by the threaded bolt 51 and the nuts 52.

The bolt 51 is passed through the leadingin insulator 53 to the one pole 54 of' the condenser. lVith exception of the surfaces of the joints themselves, the component parts of this ventire insulating structure are covered, as shown by the heavy contour lines, inside and outside with electrically conducting coatings of, for example, graphite and waterglassor the like, adhering closely to the insulating material. The inner coatings are connected to thelbolt 51, and the outer coatings to the metal housing 55 which is in connection with the other terminal 56 of the condenser.

The condenser according to Fig. 9 has the advantage over known condensers also built up of parts having the form of .plates or trays, that the whole surface of the individual elements, of which the assembled insulating body is composed, including the joint edges, can be utilized for placing the coatings, whereas in the known condensers the joint edges must remain free, and the width of these free or blank zones,particu larly in high-tension condensers, very great,

so that for a certain capacitance much more' -insulating material 1s required. Another considerable advantage consists in the avoidmg of surface leakage currents over the assasi edges of the component insulator parts, because the entire insulating body, in spite of its complicated form, forms a completely closed vessel which separates the inner and outer coatings. As the dielectric of' the condenser consists of porcelain, which material advantages in cases in which complicated forms of insulating elements are required. lVhen' windings, conducting coatings, supporting parts and the like are to be introduced into the hollow spaces of the members after the baking, the invention is'of inestimable value. If an insulating member consisting of several parts is suitably assembled, it will always be possible to avoid a complicated threading through of` wire for windings, and to ensure that all supporting parts are well cemented-in and that no parts of the insulating member are subject to mechanical tension stresses. Furthermore, our invention also renders it possible to use superior dielectrics, such as porcelain, in fields of electrical engineering where the prior art has usually employed oil, insulating compound, layers of brous material and the like for insulating purposes.

l/Ve claim as our invention:

l. A body of electric insulating material, having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least two component portions/having a joint extending through the wall and having the contacting joint surfaces suiiiciently finely finished to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint, and to impart to said joint a dielectric strength ofthe order of that of the adjacent material.

2. A body of electric insulating material,

having a wall for separating two spaces.

containing objects ofdiferent electrical potentials, said wall comprising at least two component portions having a joint extending through the wall and having the contacting joint surfaces finely inished to form substantially a uniformly close surface-tosurface contact throughout the extent of the joint, said joint being not thicker than 1/ 100 3. A body of electric insulating material, having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least two separate portions abutting against one another to form a joint extending through the wall, and having their contacting' surfaces sulficiently finely finished to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint and to impart to said joint a dlelectric strength of the order of that of the adjacent material, and a film of dielectrically highly resistive binding material disposed between said j ointy surfaces.

4. A body of. electric insulating material, having a Wall forv separating two spaces containing objects of different electrical potentials, said wall comprising at least two separate portions abutting against one another to form a oint extending through the wall, and having their abutting surfaces sufficiently finely finished to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material, and a. thin film of artificial resin disposed between the jointl surfaces.

5. A body 0f electric insulating material, having a wall for se arating two spaces containing objects of different electrical potentials, said wall comprising at least two separate portions abutting against one another to form a joint extending through the wall, and having their abutting surfaces sufficiently finely finished to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material, and a thin film of plastic shellac disposed between the joint surfaces.

6. A body of electric insulatlng material, having a wall for se arating two spaces containing objects of different electrical potentials, said wall comprising at least two separate portions abutting against one another to form a joint extending through the wall, and having their abutting surfaces finely finished to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint, and a thin film of dielectrically highly resistive binding material disposed between the joint surfaces, the thickness of the complete joint being not greater than l/lOO mm.

7. A body of electric insulating material', having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least two separate portions abutting against one another, and having their abutting surfacescontaining objects of different electrical potentials, said wall comprising at least two separate portions abutting against one anot er to form a joint extending through the wall, and having their abutting surfaces sufficiently finely finished to form substantially a uniformly close surface-to-surface contact throughout' the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material, the entire surface of the joint being located in one plane. j

9. A body of electric insulating material, having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least two component portions forming a joint extending through the wall and having their abutting joint surfaces sufficiently finely ground to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material.

10. A body of electric insulating material, having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least two component portions forming a joint extending through the wall and having their abutting joint surfaces sufficiently finely ground to form substantially a uniformly close surface-to-surface contact throughout the ex-4 tent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent mate ial, and a thin film of dielectrically highl resistive binding material disposed between the ground surfaces.

1l. A body of electric insulating material, having a wall for separating two spaces containing objects of different electrical poten-l tials, said wall comprising at least two component portions forming a joint extending through the wall and having their abutting joint surfaces sufiiciently finely ground to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material, the entire joint surface being located in one plane, and a thin film of dielectrically highly resistive binding material disposed between the joint surfaces.

12. A body of electric insulating material, having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least'two 'component portions forming a joint extending through the wall and having their abutting joint surfaces finely ground to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint, the entire joint surface being located in oneplane, and a thin film of dielectri- 10 through the wall, and' having their abutting joint surfaces? `'sufficiently finely finished to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material.

14. A body of porcelain having a wall for separating two spaces containing objects of ,different electrical potentials, said walll comprising at least two component portions orming a joint extending through the wall, and having their abutting joint surfaces sufficiently finely ground to form substantially a uniformly close surface-to-surface contact throughout the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material.

15. An electrical insulating body of ce ramic material having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least two separate portions abutting against one another to form a joint extending through the Wall, and having the abutting joint surfaces finely ground to form substantially a uniformly close surface-to-surface contact throughout the extent of .the joint, and a thin film of dielectrically highly resistive binding material between the joint 4 surfaces, the thickness of the joint being not more than 1/100 mm.

16. An electrical insulating body of ceramic material having a wall for separating two spaces containing objects of different electrical potentials, said wall comprising at least two separate portions abutting against one another to form a joint extending lthrough thei wall, and having the abutting joint 'surfaces finely ground to form substantially a uniformly close surface-to-surface contact throughout the extent of the-joint, and a thin film of'dielectrically highly re sistive binding material between the joint surfaces, the thickness of the joint being not more than 1/ 100 mm., said ground joint sur-- faces extending in one plane.

17. An electrical insulating body of ceramic material having a wall for separating two spaces containing objects of different 69 electrical potentials, said wall comprising at least two separate portions abutting against one another to form a joint extend'- ing through the wall, and having the abutting joint surfaces finely ground to form substantially a uniformly close surface-tosurface contact throughout the extent of the joint, and a thin film of dielectrically highly resistive binding material between the joint surfaces, the thickness of the joint being not more than 1/100 mm., the transverse extent of thel joint surfaces being approximately in parallelto the lines of electric stresses, and a conducting coating on at least one side of said wall, covering said joint.

18. A body of electric insulating material, having the form of a casinor for elec-- trically insulating objects of different potentia'ls disposed inside and outside the casing, said casing comprising at least two component parts assembled to form a joint runnin through the casing wall, the abutting joint surfaces being sufficiently finely ground to 'form a plane, substantially uniformly close surface-to-surface contact, throughout the extent of the joint and to impart to said joint a dielectric strength of the order of that of the adjacent material.

19. A body of electric insulating material, having the form of a casing for electrically insulating objects of different potentials disposed inside and outside of the casing, said casing comprising at least two component parts abutting against one another to form a joint running through the casing wall, the abutting joint surfaces being sufficiently finely ground tol form a plane, substantially uniformly close 'surface-to-surface contact throughout the extentof the joint and to impart to said joint a dielectric strength of the older of that of the adjacent material, and a thin film of dielectrically highly, resistive binding material disposed between the joint surfaces.

20. A body of electric insulating material, having the form of a casing for electrically insulating objects of different potentials disposed inside and outside of thev casing, said casing comprising at least two component parts abutting against one another to form a joint runmng 'through the casing wall, the abutting joint surfaces being finely ground to form a plane, substantially uniformly close surface-to-surface contact throughout the extent of the joint, anda thin film of dielectrically highly resistive binding material disposed between the joint surfaces, and forming a joint not thicker than 1/100 mm.

21. A body of electric insulating material, having the form of a casing for electrically insulating objects of different potentials disposed inside and outside of the casing, said casing comprising at least two component parts abutting against one another to form a joint running through the casing wall, the abutting joint surfaces being sufficiently finely ground to form a plane, substantially uniformly close surface-to-surface contact throughout the extent of the joint and to iso impart to said joint a dielectric strength of the order of that of the adjacent material, and a conducting coating on at least one side of the casing wall, covering said joint.

22. An electrically insulating body of ceramic material having the form of a hollow ring, for electrically insulating objects of different potentials disposed within thewalls of said ring and outside thereof, said ring comprising at least two component parts abutting against one another to form a joint running through the `ring walls, the abutting joint surfaces being finely ground to form a plane, substantially uniformly close surface-to-surface contact throughout the extent of the joint, ,and a thin film of dielectrically highlyl resistive binding matea joint at least as close as 1/10() mm., and a thin film of dielectrically highly resistive binding material, disposed between the joint surfaces for equalizing the remaining minute uneven portions in the joint surfaces. v

24. An instrument transformer having an annular casing of ceramic electrically insulating material, a4 first winding disposed" within the annular casing portion, and a second winding disposed concentrically with the first winding, but outside of one of the casing walls, and a magnet core extending centrally through said casing, for magnet- `ically interlinking said two windings, said casing comprising at least two component portions forming a joint running through the casing wall, the abutting joint surfaces being finely ground fiat to produce asufficiently close and uniform' surface-to-surface contact, to form a joint at least as close as 1/100 mm., anda thin film of dielec trically highly resistive binding material, disposed between the joint surfaces for equalizing the remaining minute uneven portions in the joint surfaces.

25. An instrument transformer having an annular casing of ceramic electrically insulating material, a first winding disposed within the annular casing portion, and a second winding disposed concentrically with the first winding, but outside of one of the casing walls, and a magnet core extending centrally through said casing. and an insulating sleeve on said casing extending substantially in parallel to the casing axis, and forming an outlet for said first winding, said casing comprising at least two component portions forming a joint running through the casing wall,the abutting joint surfaces being finely ground flat to produce a sufiiciently close and uniform surface-t0- surface contact, to form a. joint at least as close as 1/100 mm., and a thin film of dielectrically highly resistive binding material, disposed between the joint surfaces for equalizing the remaining minute-uneven portions in the joint surfaces.

26. An instrument transformer having an annular casing of ceramic electrically insulating material, a first winding disposed within the annular casing portlon, and a second winding disposed concentrically with the first winding, but outside of one of the casing walls, and a magnet core extending centrally through said casing, and an insulating sleeve on said casing, extending substantially in parallel. to the casing axis, and forming an outlet for said first winding, said casing comprising at least two component portions forming a joint running through the casing wall, the abutting joint surfaces being finely ground flat to produce a sufficiently close -and uniform surface-tosurface contact, to form a joint at least as close as 1/100 mm., and a thin film of dielectrically highly resistive binding material, disposed between the joint surfaces for equalizing the remaining minute uneven portions in the joint surfaces, and a tubular base of insulating material for supporting said casing and surrounding said insulator sleeve.

27. An electrical insulator of ceramic material comprising an annular casing of U- shaped cross-section for electrically insulating from one another objects disposed inside of the casing and outside thereof, and an annular cover for said casing resting upon the wall ends of the casing, the contacting surfaces of said two elements being finely ground in oneplane to produce a sufficiently close and uniform surface-to-surface contact to form a joint of a dielectric strength similar to that of the integral wall portions of said two elements.

28. An electrical insulator of ceramic material comprising an annular casing of U- shaped cross-section for electrically insulating from`l one lanother objects disposed inside-of the casing and outside thereof, and an annular cover for said casing resting upon the wall ends of the casing, the contacting surfaces of said two elements being finely ground in one plane to produce a sufficiently close and uniform surface-to-surface contact toform a joint of a dielectric strength similar to that of the integral wall portions of said two elements, and a thin film of dielectric:l highly resistive binding material disposed between the joint surfaces for equalizing the remaining minute uneven portions in the joint surfaces.

In testimony whereof we aiix our signa,-

tures.

PAUL PASCHEN. HANS RITTER. GEORG -STAUBER WILHELM GEBHARDT. KARL SCI-IMIEDEL. v

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