US1703410A - Electric transformer - Google Patents

Description

Feb. 26, 1929 1,703,410

F. s. SMITH ELECTRI C TRANSFORMER Filed March '7, 1922 2 Sheets-Sheet l INVENTOR ATTORNEY Feb. 26, 1929. 1,703,410

F.5 SBVTH ELECTRIC TRANSFORMER Filed March 7, 1922 2 ShetS-Sheet 2 sNvENToR Y ATTORNEY Patented Feb. 26, 1929.

UNITED STATES PATENT *'oFFicE.

FRANKLIN S. SMITH, OF PHILADELPHIA, PENNSYLVANIA, ASSIGNOR TO THE PROD- UCTS PROTECTION CORPORATION, OF NEW YORK, N., Y., A CORPORATION OF4 DELAWARE.

ELECTRIC TRANSFORMER.

Application led March 7', 1922. Serial N'o. 541,749.

My invention relates to electric transformers, particularly the high voltage windings thereof and the means for supporting and insulating the same, and it has for one of itsv objects to provide anovel and thoroughly practical construction wherebyv a fluid dielectric and cooling medium within the tank of the transformer is brought, in a simple, de-- pendable and efficient way, in contacting` or impinging relation to the metal'of the windings of the transformer for the purpose of lowering the temperature gradient between the windings and the circulating cooling medium. The mechanical features of my conr struction are particularly well suited and adapted for use in connection with a gaseous dielectric and cooling medium under pressure i greater than atmospheric pressure, and in such coacting relation I achieve many thoroughly practical and important advantages.

Another object of the invention is to provide a solid dielectric for association with the high voltage winding in which the parts employed are of a character that can be readily V standardized and may be readily molded or stamped and formed, thereby facilitating quantity production.

Another object of the invention is to provide a high voltage transformer construction in which the dielectric strength between turns of the high tension winding between coils, between the end coils and core,bet'ween the high and low voltage windingsfand between the winding and coie is much greater r than heretofore has been attained, as far as I am aware, in any transformer structure of like volume, capacity and voltage.

Other objects are to provide a high voltage winding structure in which the coils yare spaced and supported= by solid dielectric parts or members of such configuration. that the said solid dielectric is in series with fluid dielectric iii the path of higher' voltage gradi- F cnt, and in which the path of surface leale age on said solid dielectric is in a path lower voltage gradient.

Other objects will be in part obvious or in part pointed out hereinafter. l

The invention accordingly consists in the features of construction, combinations of elements, and arrangements of parts as will be l exemplified in the structure to be hereinafter described and the scope of the application of which will. be indicated in the following claims.

In order that the invention may be readily understood and its advantages from a practical standpoint fully appreciated, 'reference may be had to the accompanying drawings in which certain convenient forms of embodiment thereof are shown.

In the drawings:

Fig. 1 is a view partly in top plan and partly in transverse section showing certain mechanical featurescf a transformer embodying my invention, taken on the line 1-1 of Fig. 2;

Fig. 2 is a view partly in elevation and partly in central vertical section; Fig. 3 is a view partly in plan and partly in transverse section taken on the line 3-3 of Fig. 4 depicting a modified form of transformer construction embodying the invention;

Fig. 4 is a View partly in elevation andl partly in central vertical section of the transformer shown in Fig. 3; and

Fig. 5 is a fragmentary view of a portion of a coil of a high voltage winding showing y may be and preferably is built up of suitable laininations, as is clearly shown in Figure 1, and. may be of any suitable form or shape; conveniently and illustratively I have shown the core 4 to be of the shell type and accordingly it has two vertically extending legs 4 and 4b joined by the horizontal or transversely extending portions 4c and 4B, with a middle or central vertically extending leg 4 joining the portions 4 and 4. About the middle leg 4t are the windings. The low voltage winding 10 is wound upon a tubular solid dielectric support 11 which incloses the central leg/1t of the core and is supported upon or by the said core. The adjacent turns (clearly shown p as indicated at 12 in Figure 1, are formed,

and these are preferably also utilized, as will be made more clear hereinafter, in the circulation of thefiuid dielectric and cooling medium which, as above ynoted, .is preferably a gas under pressure greater than atmospheric` the transformer in the path offhigher voltage For the purpose of supporting the coils of the high voltage'winding of the transformer, l have provided a support eon'iprising the tubular part or section 15 of solid dielectric material having at its lower and upper ends outwardly-extending annular disks 25 and 26 of solid dielectric material which are arranged substantially at right v angles to lthe axis of the said tubular part or section 15. These disks are each provided with annular flanges at their inner edges which titl closely against the outer lower and upper surface portions of the tubular part 15 and are secured thereto. Preferablyv the said tubular part or section 15 .and the horizontally extending annular disks 25 and 26 are formed of molded bakelite or of bakelized paper, hot pressed.

l However, it will be understood that theseparts may be constructed of other suitable material. y y

The high tension winding is preferably made up of a plurality of coils 20 extending about the tubular insulating; member 15 and hence about the central leg 4f of the core; the coils 20 are spaced-from one another in an axial direction byparts 16` which are situated between the adjacent coils of the high voltage winding. The parts 16, which I shall for convenience designate as (washers, are of sinuous form, as is clearly indicated in the several figures of the drawings, so that the oppositey edges of the wires of vthe coils -20 touch the washers at intervals only. The sinuous parts 16 of solid dielectric are in series with the fluid dielectric within'the tank (not shown) of gradient while the leakage along the `surface of the said solid dielectric between theV points of contact thereof with the opposing coils is in a path of lower voltage gradient. This is due to the fact that the annular parts 16 of dielectric material, previously referred to as washers', are of corrugated or sinuous formation so that the points of contact of oppsing sides oredges of adjoining coils 2O with an interposed washerare arranged in staggered relation to each other, as shown in Fig. 2. By reason of such relationship it is apparent that the leakage between adjoining coils along the said interposed washer of dielectric material must be along inclined paths which are of greater length than the vertical vdistance between the said coils with the result that the relation -of the voltage gradients is as just stated. The inner tubular portion or part 15 of the support for the high voltage windings is provided with perforations through which the Huid dielectric and cooling medium, preferably gas, may pass. 'lhese perforations, in the construction shown, are circular and may be regarded asVv arranged in groups of three, the axes of the perforations of each selected group being located withrespect lto each other at the three corners of equilateral triangles. The diameters of the perforations lare-of such length that a line drawn from a point outside of any such group of openings cannot pass straight through such group without vcutting at least oneof vthe perfolations. It follows, therefore, that'the leakage along lthe surface of the tnbe'15`cann0t be in straight paths or lines. 'It is to be understood, however,l that perforations of other configuration and lrelative position may be employed.

The bottom .coil of thehigh voltage winding is supported upon the upper side of the annular disk 25 of solid'dielectric while-the bottom side of the disk 26 contacts with the top edges of the uppermost of the coils of the high voltage winding.

Theopposite edge portions of the 'flat wire of the high vltagecoils are bare and rounded, and the opposite .sides thereof are coated with enamel, or .may be-spaced or separated by bakelized paper or other suitable material. f

The high voltage winding' structure is supported from the cross-Wise extending angle iron members 2 at vthe upper part of the core `:tructure and angle iron members 3 at the bottom part of the'core structure` by means of X-shaped 1 mbers-30 and 31. The ends of the bars on one side of the X-shaped members 30 and 3l are secured to the horizontal flanges of the angle' ironl members 2 and 3 respectively, while the ends of fthe said bars at the oppositeI sides of said X-shaped members are inengagement with the outer faces of the disks 25 and 26.' In order to prevent displacement lateralrly of the-disks-Q and 26 with respect to :the supports 3() and .31 vthe disks are provided with raisedportions 32 veachof which. has a recess 33 therein into which `the, ends of the bars at the inner sides of the said X-shaped members 30.and 31 extend, as is shown in Figs. 2 and'4 of theI drawings. As is clearly shown in Figure 1, I preferably employ these X-shaped members in'pairs and in Figure 1 will be seen an X-shapedmember 31 on each side of the upper transverse portion4 of the core, and as is clear from Figure 2, there isan X-shaped imember 30 on each side of the lower transverse portion 4 of the core. l As already hereinabove noted, the fluid dielectric and cooling medium which I prefer to use is a gas under pressure greater than atmospheric pressure and'with suoli a gasy ymagnetic circuit, space thevarious parts more closelyy together, lessen the aniountof copper necessary and thus materially diminish the iron and copper losses and achieve better regulation of the transformer. As illustrative of how such advantages as the above are achieved, I might point out, forexample, that the space (see Figure 12) between vthe uppermost coil 20 of the `high tension winding and the upper transverse portion 4e of the transformer core is occupied by the member 26 which is of solid dielectric mate- 'rial and a layer of gaseous dielectric under pressure. These two dielectrics are thus combined in series and the total dielectric stress is distributed between them inversely as their respective permittivities.

'lhepermittivity of solid dielectric material such as the bakelite of the member 26 is relatively high being on the order of 6, while the permittivity of the gaseous dielectric'under pressure is relatively low being substantially unity. Due to suoli characteristics as these and due to the serial arrangement of the two dielectrics, I ain enabled to space the uppermost coil 2() of the high tension winding much more closely to the portion leef the transformer core than would otherwise be possible and, moreover, I am enabled also to use a much smaller thickness of solid dielectric, in such a relation, than would otherwise be necessary for the solid dielectric is made to assume a'relatively small Afraction l of the total stress while the great dielectric fl. similar series relation betwen solid dielectric and the gaseous dielectric undei' pressure with resultant features of action and advantages like those above-mentioned will be seen tojexist for insulating the high tension winding from the middle leg 4f of the transformer core, and by way of further example will also be seen to exist for insulating adjacent coils 20 of the high tension winding from each other. Thus, I am enabled to achieve better insulation between parts of the construction which are of different potentials, and I am also enabled to achieveV great compactnessof construction with other advantages thatwill be clear in view of what Has hereinabove been set forth.

As further conducive to a clearer understanding of certain other features of my invention, I might note that the gaseous dielectric under pressure coacts with the solid dielectric in still a further high advantageous manner in that leakage along the solid dielectric material is greatly reduced. Considering, for example, the sinuous solid dielectric membersl that extend between the adja- .cent coils 20 of the high tension winding (see the right-hand portion of Figure 2) it might first be pointed out, as has already above been noted, that the construction of the members 16 greatly lengthens the leakage path from one coil 20 to an adjacent coil 20, that is, the leakage path along the curved or inclined portion or portions of the sinuous members 16. This feature alone achieves many advantages, but in coaction with the gaseous dielectric under pressure the latter acts to prevent` leakage even along such a lengthened inclined path and due to this action of the gaseous dielectric under pressure, I yam again enabledL much more closely to space adjacent coils 2() of the high tension winding than would otherwise be possible.

Moreover, due to the conrmation of the sinnous members A16, the leakage path available is along inclined paths which are of greater length than the spacing between the coils and hence is in a path of lower voltage gradient. Thus, leakage between adjacent coils of the high tension Winding is effectively prevented while the spacing between the coils is made much smaller than would otherwise be possible. The coaction ot the gaseous dielectric under pressure with the tubular member l5 of solid dielectric material and about which are supported the coils 20, will be better understood in view of the foregoing, but it may at thisl oint be noted that leakage along the mem er l5 between parts such as coils 2O that are of different potential is effectively cut down, while the perforations 2l in themember l5 insure that any leakage paths between two points of substantial spacing are longer than, the shortest"- distance between these two\ points.

It may also be noted at this point that the members 30 and' 31 which support the high voltage winding structure are so shaped that ',the leakage path from the winding structure to the core structure (such as'the angle iron members 3 or2 to which the members 30 and 3l respectively are secured) is longer than the shortest distance. between the winding structure and the angleiron members, and

the above-noted feature ofthe gaseous dielecof heat fromv such portions and makes it possible to effectively 'guard against abnormal temperature rises.

rlhe construction as shown in Figs. 3 to 5 inclusive is identical with that shown in Figs. l and 2 of the drawings `except for certain features of construction, such as the provision in the laterally and outwardly extending washers 16 of openings 35 at intervals therein, which openings are situated practically or approximately one above the other,

' Y as shown in Fig. 4.

ln the construction, as shown in Figs. to inclusive, the respective coils 20 of the high `voltage winding are subdivided; that is, they are divided into two parts a and 2Gb separated from each other by an annular passageway 38 in which is situated a strip 40 of solid dielectric of corrugated oi' sinuous forni and which is of general circular fform, as

clearly shown in Figs. 3 `and 5. The outer wire. of the inner subd1vis1on 20h of each complete coil 2O situated between the washersI 16 extends between the opposing adJaceutl ends of the annular strip 40 and continuous forwardly upon the outer;A side of said strip to form the inner turn of the outer subdivision 20"L of each coil as indicated at 42 in Fig. 5. lt will be noted that the outer vturns of wire of the inner subdivisions of the coils n l and the inner turns of wire of the outer subdivisions of the coils contact at intervals only with the said strip Ll0, Ias is clearly shown 1n 4the-drawings. l

By reason of the prov1s1on of the open-v ings 35,Which are situated the same distance radially from the axis of the tubular part l5 as are the annular spaces 38 between the in-` ner and outer subdivisions of the coils 20, vertical ducts 36 are provided through the high Voltage winding as shown. The corrugated or sinuous form of the separating strip between the subdivisions 20"L and 20b of the coils 20 will be seen to. substantially unobfluid dielectric and cooling medium within the tank of the transformer can freely pass through the vertical ducts 36 and withdraw heat from the coil subdivisions with which it is thus brought into intimate thermal contact. This medium, as above noted, is preferably a gaseous dielectric under pressure greater than atmospheric pressure.v

It will thus be seen that by reason of the employment of such features as the serial relation of solid dielectric material with a gaseous dielectric `under pressure for insulating the high tension winding from the horizontal portions 4 and le of the transformer core and the X-shaped members 30 and 3l for spacing and supporting the high voltage winding structure in substantially the manner indicated in the drawings, a constructionl isv provided in which the distances between the end coils and the horizontal portions of the core are less than would be possible if the spaces between said coils and the said horizontal portions of the core were brid ed by solid dielectric material. As has a ready been described hereinabove and is clearly indicated in the several figures of the drawings, the solid dielectrics 25 land 26 are in series with the tluid'diel'ectric between said end coils and said horizontal portions of the core. The coaction and advantages of this serial relation where the fluid dielectric is a gas under pressure will be clearjand readily understood in view of what has hereinabove been set forth. lt will be seen, therefore, that on account of such features of coaction as above-noted and in which the high dielectric strength of the fluid dielectric and cooling medium where the latter is a gas under pressure, and the form of constructionLhereinbefore described and illustratively shown in the drawings, the core dimensions in the paths of magnetic iuX are decreased. rlhe decrease in the core dimensions not only makes possible the use of less iron in the magnetic circuit but also eects a decreaseiin the iron losses. Also. the .closer spacings made possible between other portions of the transformer construction which are of different, potential, all as have already hereinabove been pointed out, makes possible the achievement of such advantages as lessening the from thel fact that a lower -temperature gradient between the end coils and the Huid dielectric and cooling medium. is obtained than would be the case if the"` spaces between the end coils and the horizontal portions of' the core were bridged by solid dielectric.

It will be understood that the transformer as illustrated is adapted to be inclosed and supported within a hermetically sealed casing or tank, which, however, is not shown in the drawings.

In constructing the high voltage winding structure one of the annular disks 25 or 26 will first be secured to the tubular part 15. If it be assumed that the disk 25 is iirst secured to the part 15 then the bottom coil of thewinding will be wound upon the part 15, then a washer 16 will be placed in position upon said coil with the part 15 projecting therethrough, after which the next adjacent coil will be wound ripon the part 15 and so on until the required number of coils have been wound after which the annular disk 26 will be placed in position upon the topmost coil and secured at its inner edge to lthe upper outer end portion of the part 15.

As many possible embodiments may be made of the above invention, and as many changes might b-e made in the 'embodiment above set forth, it is to be understood that all matter hereinbefore set forth is to be interpreted as illustrative and not in a limiting sense.

Having-thus described my invention, what I claim and desire to secure by Letters Patent is:

1. In apparatus of the character described, in combination, a high voltage winding for a transformer made up of a plurality of coils positioned one above the other to form substantially a stack, and a member of solid dielectric material extending substantially lengthwise of said stack of coils for holding the latter in substantial alinement, said member being provided with perforations shaped and positioned so that a line drawn along the surface of said member in the general direction of the axis of said coils and between any two spaced points is greater in length than the straight line distance between said two points, whereby the leakage path along vthe surface'of said member and in a general direction along which said coils make eifective a potential'diiference is other than in a ystraight line.

2. In apparatus of the character described, in combination, a high voltage winding for a transformer made up of a plurality of coils positioned one above the other to form substantially a stack, and a member .of solid dielectric material extending substantially lengthwise of said stack of coils for holding ,the latter in substantial alinement, said member being provided with perforations shaped and positioned so that a line drawn along the surface of said member in the general direction of the axis of said coils and between any two spaced points is greater in length than the straight line distance between said two points, whereby the leakage path along the surface of said member and in a general direction along which said coils make effective a potential difference is other than in a straight line, anda gaseous dielectric under pressure contacting said member and filling said perforations.

3. In apparatus of the characttfdescribed, i

in combination, a high voltage winding for a transformer made up of a plurality of coils positioned in spaced relation one above the other, and insulating means between adjacent coils comprising solid dielectric material and a gaseous dielectric under pressure, said solid dielectric material and said gaseous dielectric under pressure being combined so as to be in series in the path of higher voltage gradient between adjacent coils and so that surface leakage along said solid dielectric and between adjacent coils is in a path or paths of lower voltage gradient.

4. In apparatus of the character described,

a transformer made up of a plurality of coils Aarranged in spaced relation one above the other, means interposed between adjacent coils comprising a corrugated member of solid dielectric material, said member acting to hold adjacent coils in spaced relation and contact therewith only at intervals, and a gaseous dielectric under pressure filling the space between the corrugations of said solid dielectric member and adjacent coils whereby said corrugated imember and said gaseous dielectric under pressure are in series in the path of higher voltage gradient between adjacent coils and whereby any surface leakage between adjacent coils and along said solid dielectric member is yin a path or paths of lower voltage gradient and whereby said adjacent coils may be closely spaced.

5. lThe high voltage winding of an electric transformer, comprising a plurality of coils situated one above the other, a tubular member of'dielectric material upon which said coils are wound, said tubular member being provided with perforations, the centers of which are coincident with the apices of substantially equilateral triangles and the diameters of which are of such relation to the altitude of the said triangles that a straight line cannot be drawn, except for a limited dislOl) tance, along the surface of the said tubular member from one point to another, and annular washer-like members of dielectric material situated between adjacent coils and contacting at intervals therewith to support the said coils in spaced relation to each other.

6. The high voltage winding of an electric transformer, comprising a plurality of coils situated one above the other, a tubular member of dielectric material upon which said coils are wound, said tubular member being provided with perforations, the centers of which are coincident with the apices of substantially equilateral triangles and-the diameters of which are of such relation to the altintu-de of the said triangles that a straight line cannot be drawn along the surface of the said tubular member from one point to another, and annular washer-like sinuous members of dielectric material situated between-the said coils and supporting the same in spaced relation with respect to each other.

7. lin apparatus of the character described, in combination, a high voltage winding of anv electric transformer made up of a plurality of coils positioned in spaced relation one above the other, a substantially tubular member of solid dielectric material upon which said coils are wound, substantially washershaped members ofsolid dielectric material positioned between adjacent coils for holding the latter in spaced relation, said members being corrugated and extending in a direction away from said substantially tubular member of solid dielectric material, said last-mentioned member having apertures therein forming communications between the interior thereof and the spaces between the corrugated members and coils spaced thereby,'and a gaseous dielectricv under pressure filling said spaces and said apertures.

8. ln apparatus of the character described, in combination, a transformer core having a leg for receiving a winding and having a portion extending substantially transversely thereto, a high voltagefwinding about saidl core leg, and means comprising a substantially X-shaped member for supporting the end portion of said winding from said transversely extending portion of said core, whereby the leakage path betweenthe end portion of said winding and along said X-shaped member is materially greater than the shortest straight line distance between the end portion of said winding and said transversely extending portion of said core.

9. llhe high voltagewinding of a transformer, comprising a pluralityof coils situated one above the other each of which coils is subdivided into inner and outer portions, said portions beingseparated f rom each other, a sinuous member substantially circular in outline situated in the annular space between the said inner and outer portions of the said coils, sinuous washer members of dielectric material situated between the said coils and 4supporting the same in spaced relation to each other, and solid dielectric means for supporting the said coils and spacing means.

l0. The high voltage winding of a transformer, comprising a tubular part of dielectric material having annular disks situated at the upper and lower end portions thereof, coils wound upon the said tubular part, which coils -are situated one above the other, and each of which is subdivided into inner and outer portions, which portions are separated by an annularspace, sinuous strips of dielectric material situated in said spaces and contactingat intervals with the turns of the said windings being concentric and the primary winding being located inwardly of the secondary winding, a tubular support for the secondary winding, the said support inclosing the primary winding and being lspaced 'therefrom and-also being provided with perforations, and annular corrugated washerlike members of dielectric material located between the secondary windings, the alternate groovesand ridges formed by said corrugations extending radially from the'axis of the said transformer.

i2. ln apparatus of the character described, in combination, a transformer' core having a leg for receiving a winding and having a portion extending substantially transversely thereto; a high voltage Winding about said core leg; means for insulating said winding from said transverse portion of said core, said means comprising solid dielectric material and a gaseous dielectric under pressure in series with said solid dielectric material across the` space between the end portion of said windingr and said transverse portion of said core; andv means extraneous of the space between the end portion of said winding and said transverse portion of said core for holding said winding relative to the latter.

i3.- ln apparatus of the character scribed, in combination, a high voltage winding having relatively adjacent portions thereof lengthwise of its axis at different potentials, and a member of solid dielectric material extending along said portions of the winding for mechanically holding the latter, said member being provided with perforations shaped and positioned so that a line drawn along the surface of said member in the general direction of the axis of said winding and between any two spaced points is greater in length than the straight line distance between said two points, whereby the leakage path along the surface of said member and in a general direction along which said portions of the winding make effective a potential difference is other than in a straight line.

14. ln apparatus of the character described, in combination, a high voltage windinghaving relatively adjacent portions thereof lengthwise of its axis at different potentials, and a'member of solid dielectric material extending along said portions of the de- Y winding for mechanically holding the latter, said member being provided with perforations shaped and positioned so that a line drawn along the surface of said member in the general direction ofthe axis of said winding and between any two spaced points is greater in length than the straight line distance between said two points, whereby the leakage path along the surface of said member and in a general direction along which said portions of the winding make effective a potential difference is other than in a straight line, and a gaseous dielectric under pressure contacting said member and filling saidfperforations. y y

15. In apparatus of the character described, in combination, a high voltage winding of an electric transformer made up of a plurality` of coils positioned in spaced relation one above the other, a substantially tubular member of solid dielectric material about which said coils extend, and insulating means between -adjacent coils comprising solid dielectric material and a gaseous dielectric under pressure, said solid dielectric material and said gaseous dielectric under pressure being combined so as tobe in series in the path of higher voltage gradient between adjacent coils and so that surface leakage along said solid dielectric and between adjacent coils is in a path or paths of lower voltage gradient, said tubular member being provided with perforations shaped and positioned so that a line drawn along the surface of said member in the general direction of the axis of said coils and between any two spaced points is greater in length than 'the straight line distance between said two points.

1G. In apparatus ofthe ch'aracter described, in combination, a transformer core having a leg for receiving a winding and having a portion extending substantially transversely thereto; a high voltage winding about said core leg and having its end port'ion spaced from said transverse core por; tion; and means for insulating said winding from said transverse core portion and for supporting said winding from said core, said means including a member of solid dielectric material resting against the end of said winding and having a lesser thickness than the spacing between said end portion of the winding and said transverse core portion, a gaseous dielectric under pressure interposed between said solid dielectric member and said transverse core portion so that said two dielectrics are in series, and means secured to said core and engaging said solid dielectric member extraneously ,of the space between the latter and said transverse core portion for holding said winding in place.

17. In a transformer, in combination, a high Voltage winding, a low voltage member in the form of a low voltage winding, a core member with which said windings coact, said high voltage winding being spaced from `both of'said members; and means for insulating said high voltage winding from one of said members, said means including a layer of solid dielectric material and a layer of gaseous dielectric under pressure in the space between said high voltage winding and said one of said members. v18. In a transformer, in combination, a transformer core member, a coil member about said core member, another coil member about said core member and adapted to operateat a Voltage relatively higher than that of said first-mentioned coil member and lspaced from both of said other members, and means for insulating said relatively higher voltage member from one of said other members, said means comprising solid dielectric material and a gaseous dielectric under pressure, both of said dielectrics being in the space between said members to be insulated and being combined so as to be in series in the patli of higher voltage gradient between said members to be insulated and so that 'surface leakage along said solid dielectric between said members-is in a path or paths of lower voltage gradient.

In testimony that I claim the foregoing as my invention I have hereunto signed my name this 4th day of March, 1922.

FRANKLIN S. SMITH.

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