US1957982A - High voltage terminal construction - Google Patents

Description

May 8, 1934- F. s. SMITH HIGH VOLTAGE TERMINAL CONSTRUCTION Filed. Deo. 5, 1931 v IN lav/Q24 ATTORNEYS Patented May 8, 1934 man voLrAoa mamar. coNsraUcnoN immun' s. smith, New neven, com

Application December 3, 1931, Serial No. 578,724

12 Claims.

This invention relates to-ele'ctric terminal construction and more particularly to high voltage terminal construction.

One of the objects of this invention is to provide an electric terminal which is of a rugged, efficient and thoroughly practical construction. Another object of this invention is to provide an electric terminal construction which may be simply, rapidly land economically produced.

Another object is to provide an electric terminal construction in `Which-the possibility of a damaging breakdown of any part is substantially reduced to a minimum. Another object of this invention is to provide a practical and efficient l5 electric terminal construction in which there are obtained maximum insulating properties with a minimum amount of insulating material. Another object is to provide a high voltage terminal in which outside influences will not aiiect its action or disturb its high insulating properties. Other objects will be in part obvious or in part pointed out hereinafter.

As conducive to a clearer understanding of `certain features of my invention, it vmight at this point be noted that in high potential terminal construction, it is economically impractical to produce a single insulating sleeve for insulating the high potential conductor from the low potentialsupporting structure. Not only would such a construction necessitate the handling of large and cumbersome parts but'it would also require extra large machinery for producing them. Particularly is this true where phenolic condensation product or other moldable solid dielectric material is employed. As a commercial expediency, it is found advisable to make such insulating sleeves in a plurality of sections interconnected by threaded fittings which afford suicient strength to the assembly. It has been found that 43 metallic fittings are necessary to give the required strength to the assembly and that as a consequenceof their use, these fittings asume a potential dependent upon that of the electrostatic eld in which they are situated; these fittings are in effect electrostatic capacities and they may be made to assume an undesired capacity, depending upon outside conditions, and thus disturb the action of the terminal and cause it to break down at voltages which it normally can safety withstand. One of the outstanding objects of this invention is to provide a simple, practical and efficient terminal construction in which outside disturbing influences, such as neighboring metal parts, electric circuits or apparatus, or high voltage conductors or the like, are dependably prevented from having any eiect upon the terminal'.

The invention accordingly consists in the features of construction, combinations of elements, and arrangements of parts as will be exemplified in the structure to be hereinafter described and o0 the scope of the application of which will be indicated in the following claims.

In the accompanying drawing, in which there is shown one of the possible embodiments of this invention, the single gure is a vertical central 6., sectional view of the high potential terminal construction related, by way of example, to a transformer, the latter being indicated diagrammatica ly.

Referring to the drawing, there is shown at 10 7n a gas-tight container which,I for the purpose of illustration, may be cylindrical and has an integral lower end 11 and an upper peripheral portion 12. The container is closed by a closure 13 which may be joined, after the apparatus has been mounted therein, to the upper edges 12 of the container 10 as by lapping the edges 12 over the closure and welding theedge portion to the closure 13 to form a gas-tight connection or seal. Carried by the tank or container and preferably by the closure 13 and preferably axially thereof is a high voltage terminal construction generally denoted 14. The container 10 is then filled with a gaseous dielectric under pressure, for example nitrogen, or with a mixture of nitrogen and Ahelium if better dissipation of the heat generated by the enclosed electric apparatus is desired, and this gas is under a pressure on the order of 15 atmospheres.

Within the container 10 is the electrical apparatus, illustratively a transformer diagrammatically shown at 15; the transformer has a high potential winding 16 and a low potential winding 50, and it may be insulated in any suitable way, preferably in coaction with the above mentioned gaseous dielectric. One end of the high voltage winding 16 is preferably grounded to the container 10, as at 1'?, but it is to be understood that it may be treated the same as the other terminal of the winding and that I do not Wish to be limited to the grounded arrangement.

For illustrative purposes the high potential terminal construction 14, which leads 'the other end of the winding 16 out of the casing 10, is shown as comprising an internally threaded metallic collar 18 intertting a hole provided therefor within the central portion of the shell 13. A gas-tight connection or fit is assured preferably by welding the two together as shown at 19. Interthreaded with the collar 18, as at 18, is an externally threaded metal bushing 20, preferably of brass, having an accurately machined cylindrical bore and rounded lower and upper end portions 21 and 22. The threaded joint 18e is sealed in any suitable way.

Threadedly engaging the upper portion of bushing 20 is a tapered insulating housing portion or sleeve 23 preferably made of phenolic condensation product and having lower and upper internally threaded portions 24 and 25. Part 23 is preferably of circular cross-section. The physical dimensionsof the sleeve 23 are such as to withstand the dierences in pressure between the 1nside and the outside thereof and the threaded portions thereof and of the parts interthreaded therewith are of such dimensions as to assure a gas-tight nt; any suitable cement may be employed if desired.

The upper threaded part 25 of the insulating member 23 directly engages the lower externally threaded portion of a second metallic bushing 26. The part 26, like the part 20, is accurately internally bored and is preferably provided with rounded end portions; it is held coaxially wlth part 20 by the insulating part 23. The upper threaded end portion of the part 26 engages a second tapered insulating sleeve or housing member 27, preferably also of molded phenolic condensation product, its upper end portion terminates in an inside tubular supporting member 28 with which it is interthreaded and sealed, as at 27e; member 28 carries an outside connecting member 29 by which a conductor of the circuit in which winding 16 isto b e included, may be connected, and member 28 carries also a corona or linx-control cap 30.

Contacting the adjacent ends of insulating sleeves 23 and 27 and electrically and mechanically engaging the interconnecting conducting nipple 26, is a metallic annular ange 45, the purpose of which will be more fully described hereinafter. Flange 45 may be threaded onto connectng bushing 26.

Within the passage formed by the connected insulator sleeves 23 and 27 and preferably coax-v ially therewith and spaced therefrom, is a high voltage conductor in the form of rod or tube 31 of round cross-section which is tightly tted into a bored-out portion of the inner tube-like member 28 at the upper end of the structure, The lower end of the high voltage conductor 31 preferably terminates in a swiveled connection 32 and a bent arm portion 33 which may be ilexible if desired, and which leads to the high potential end of winding 16. The high voltage conductor 31- is preferably in the form of a round hollow rod to which is connected or within which is enclosed the high potential conductor 34 connected to the high potential end of the high voltage windingv 16.

Coaxial with high potential conductor 31 and equally spaced thereabout is an electrically conductive tube 35 whose upper end mechanically engages and interts with a reduced inner portion of the metallic bushing 26. Tube 35 is thus carried in depending relation by bushing 26. and its inner surface has the same radius as the unreduced inner part of bushing 26. The lower end of the tube 35 terminates in a iared end portion 36 which is recessed at 37 to receive the upper edges 0f an insulating CUD 38 in the center of whose bottom, as by a bushing 31, the lower end of conducting tube 31 is mechanically supported and by which is also supported the swiveled connection 32.

Thus there is assured an equal spacing between the high potential conductor 31 and the outer concentric tube 35; the conducting tube 35 is, moreover, held concentric with the inner surface of the lower bushing 20. While the potential of the lower bushing 20 is maintained at the potential of the tank to which it is mechanically and electrically connected, the potential of the innermost conductor 31 is maintained at the maximum transformer potential, being connected to the high potential end of Winding 16. The intervening conducting tube 35 is given a fixed potential of an intermediate value as by connecting it through conductors 39 and 15 to an intermediate point 40 on the high potential winding 15.

Since the inner portion of the terminal structure generally shown at 14 directly leads into the container 10, the gaseous dielectric under pressure enclosed within the container extends Within the terminal structure and thus extends between the high potential conductor 31 and the surrounding tube 35 and between the tube 35 and the bushing 20. The relative distances between these parts, taken in conjunction with the gaseous dielectric under pressure used, are such that two fast discharge gaps are formed. These fast gaps are so constructed that upon the application of a potential higher than that which the electric apparatus-enclosed within the tank can safely withstand, there is an instantaneous breakdown across vthe gaps, the breakdown occurring without being preceded by corona.

More particularly, the ratio of the radius of the inner surface of bushing 20 to the radius of tube 35 is equal to or less than 2.718 and the same relation exists between the radii of parts 31 and 35 or of parts 31 and 26. This relation insures breakdown without being preceded by corona.

To better understand certain features of my invention it might be noted at this point that, the higher the voltages which are to be led into or out of the casing 10 and hence to or from the high voltage apparatus contained therein, the greater should be the dimensions of the solid dielectric material employed at the high voltage terminal even though the gaseous dielectric under pressure coacts advantageously with the solid dielectric material in minimizing the amount of the latter that need be employed; the solid dielectric material, preferably of molded phenolic condensation product, might readily assume proportions which make it uneconomic to mold and which might entail some diiculties in molding. It is therefore desirable to sectionalize the solid dielectric material and, as above noted, I'have illustratively shown such material as comprising two sections, namely members 23 and 27, though it is to be understood that more than two sections, connected suitably as by metallic bushings like the member 26, may be employed if desired. I am thus enabled economically to mold sections of smaller dimension.

But by reason of certain features of my invention I am enabled, for a given high voltage, materially to decrease the length of the individual sections and hence to decrease the overeall length of solid dielectric material employed and thus further to achieve greater economy and other advantages in manufacture.

In operation, a sectionalized high voltage terminal of the character here dealt with is subject to disturbing extraneous influences which oftentimes in practice can be and are of a character vastly-to lower the safe operating voltage of the terminal even though that terminal, as initially calcuated or designed, may have adequate voltage-withstanding capacity with even an appropriate factor of safety, when uniniiuenced by such extraneous factors.

For example, in the dielectric eld or flux that extends from the upper connecting member 29 or 28 down to the low potent'al or grounded bushing 20, the metallic connecting bushing 26 forms an isolated capacity and assumes a. definite potential; uninuenced by outside factors, the high voltage terminal construction functions as desired and as intended. But the designer or constructor cannot foresee what circumstances of use the construction may be put to in actual practice; the terminal might, for example, be placed in reasonably cose proximity to other apparatus such as, for example, electrical apparatus or machinery, conductors, grounded frames, supports or the like, and when this occurs, the electrical capacity of the connecting bushing or bushings 26 becomes of a different value from what it should normally have or assume and its potential is likewise made to be of a sometimes totally different Value than was initially intended for it. 'Accordingly the voltage gradients throughout the terminal construction are changed, certain parts thereof are made to be overstressed, and sometimes breakdown occurs at voltages at which the terminal otherwise would unquestionably safely withstand.

' Such detrimental action. however, :'s prevented from occurring as a result of certain features of my invention. The connecting bushing 26, as above already described. is connected to the midpoint of the high tension winding 16 of the transformer and thus has its potential definitely and positively determined and extraneous influences are thus prevented, some having such effects as those pointed out above. But, furthermore, I am thus enabled to avoid increase in the length of the solid dielectric members employed for. to avoid the effects of outside influences otherwise would necessitate substantial increase in the over-all dimensions of the terminal construct'on.

In order to achieve further diminution in the dimensions of the solid dieectric sections of the high voltage terminal construction and further to prevent such extraneous iniiuences as those mentioned above from affecting the desired action of the construction, I bring into co-action with the corona shield or flux control member 30 the iiux control members 44 and 41. Member 44 is frusto-conical in shape and is secured. as by screws 46, to the flanges 45a and 45h of member 45 which, being in mechanical and electrical connection with bush'ng 26, is maintained at the same potential as the latter and thus the shield or control member 44 is likewise maintained at this intermediate potential, illustratively assumed to be the mid-potential.

Member 4i is preferaby in the form of an annulus of round cross-section and may conveniently take the form of a tube as shown in the drawing; it is supported concentrically with respect to the terminal construction 14 by a suit- .able number of posts 42, also preferably of metallic tubing, which are mechanically and electrically connected to the member 41 at their upper ends and to suitable studs 43 connected as by weld'ng to the casing 10-13. Member 41 .is thus at the same potential as the casing and in the illustrative embodiment is therefore grounded.

Member 44, being frusto-conical in shape and preferably made of sheet metal, has progressively varying diameters that are graded with respect to the position it occupies intermediate of the corona cap or control member 30 and the grounded annulus 41. All three members are shaped preferably to avoid sharp exposed edges and, by way of example, the upper and lower ends of the member 44 may be spun or rounded over as at 44 and 44".

In action, members 30, 44, and 41 act to draw the dielectric flux away from the exterior surface of the solid dielectric members 23 and 27 and thus greatly relieve the stress to which the latter are subjected exteriorly, greatly minimizing the possibility of flash-over along the outer surface of the solid dielectric members. These control members 30, 44 and 4l are spaced from each other sufficiently to prevent direct breakdown of the air path that extends therebetween.

But member 44 is maintained at a fixed potential which is commensurate with the position or region which it occupies in the extraneous dielectric eld extending from the member 30 to the member 4l; illustratively it is approximately at the mid-position of this eld and hence is maintained at the mid-potential. This arrangement results against detrimental disturbance by extraneous influences of the dielectric field or flux that exists extraneously of the solid dielectric parts and upon the intended maintenance of which absence of surface flash-over depends. Thus the extraneous dielectric field is positively controlled and fixed and in fact graded, and I am enabled additionally to utilize shorter lengths of exterior surfaces of the various sections of solid dielectric material.

Solely by way of illustration and not by way of limitation, it may be assumed that the potential at which the apparatus Within the casing l0 functions, illustratively the high tension winding 16 of the transformer, is 150,000 volts; t .e casing l0 and hence the bushing 22 and flux control member 41 are at ground potential while the connector 29 and corona cap 30 at the upper'end of the terminal construction 14 is at 150,000 volts. Tubular conductor 35, connected to the midpoint of the winding 16 is thus at substantially 75,000 volts and thus the potential of the connecting bushing 26 and also of the ux control member 44 is 75,000; these parts are main'- tained positively at this intermediate potential, a potential which will be seen to be similarly proportioned to the total potential as is the position of the parts 26 and 44 in the dielectric eld extending from the upper end of the terminal construction to its lower or grounded end.

As above pointed out, the radii of the concentric ,u

cylindrical members 3l, 26-35, and 22 are proportioned with respect to each other so that the member 31 with the inside cylindrical face of parts 26 and 35 forms a fast discharge gap while the member 35 with the bushing 22 forms another i fast discharge gap, these gaps being adapted to breakdown instantaneously and without the breakdown being preceded by corona. They are safety gaps which protect the apparatus served by the high voltage terminal and they are, moreover; in series as will presently be seen. The

crit-ical breakdown voltage of these two gaps in series is a voltage an excess of which,wou1d damage the apparatus served by the terminal construction.

For example, let it be assumed that the apparatus functions normally at 150,000 volts and is intended to be safeguarded against potentials whether' due to transients, surges, or other factors that are in excess of 200,000 volts. Should such a damaging potential be impressed upon the conductor leading to the connector 29, the two fast gaps in series instantaneously breakdown and discharge the high potential energy to ground.

More specifically, the breakdown, under these abnormal conditions, occurs from the central tubular conductor 31 to the connecting sleeve or bushing 26 or to the tubular conductor 35 which is concentric with the inside surface of the bushing 26 and then the fast gap between the tubular member 35 and the bushing 22 breaks down; since the bushing 22 is grounded, the two gaps which breakdown instantaneously and simultaneously, thus shunt the damaging high potential to ground and away from the apparatus served by the high voltage terminal.

These manifold advantageswill be seen to be achieved With a construction which is at once simple, inexpensive, compact and reliable, and it will thus be seen that there has been provided in this invention a construction in which the various objects hereinbefore set forth, together with many thoroughly practical advantages, are successfully achieved. f

As many possible embodiments may be made of the mechanical features of the above invention and as the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinabove set forth or shown in the accompanying drawing is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. In apparatus of the character described, in combination, a casing having therein a high Voltage apparatus that has a plurality of taps of different voltage; and a terminal construction associated with said casing, said terminal construction including a plurality of tube-like solid dielectric members arranged end to end interconnected and alternated with a plurality of metallic members; and means comprising concentric conductors. one being received within the other and each connected to and supported in depending relation by one of said metallic members, whereby said conductors extend coaxially into said casing and are spaced from the Walls of said tube-like dielectric members, said conductors having electrical connection to respective taps having voltages substantially proportional to the remoteness of said metallic members from said casing.

2. In apparatus of the character described, in combination, a casing having therein an electrical apparatus which has a plurality of taps of different voltages, said casing having therein a gaseous dielectric; and means for making connection to a high voltage tap from the exterior of said casing, said means including a metallic bushing sealed into an opening of said casing, a plurality of tubular insulating housings arranged end to end, the end of one of said housings being connected to said bushing, a conductor supported from the remote end of the most remote tubular housing and held thereby substantially concentrically with said bushing, said conductor being in electrical connection with the high voltage tap of said apparatus, metallic sleeve-like means interconnecting adjacent ends of said tubular housings, a conductor extending alongside of said first-mentioned conductor and supported at its one end by said metallic sleevelike means, said second-mentioned conductor being in electrical connection to a tap of intermediate voltage, and means insulatingly supporting said two conductors one from the other.

3. In apparatus of the character described, in combination, a casing having therein an electrical apparatus which has a plurality of taps of different voltages, said casing having therein a gaseous dielectric; and means for making connection to a high voltage tap from the exterior of said casing, said means including a metallic bushing sealed into an opening of said casing, a plurality of tubular insulating housings arranged end to end, the end of one of said housings being connected to said bushing, a conductor supported from the remote end of the most remote tubular housing and held thereby substantially concentrically with said bushing, said conductor being in electrical connection with the high voltage tap of said apparatus, a metallic sleeve-like member connecting adjacent ends of adjacent housings and disposed interiorly thereof, a ilux control member extending about said housings in a region adjacent said metallic sleeve-like member, and means connecting said metallic sleeve-like member and said ux control member to a tap of intermediate voltage.

4. In apparatus of the character described, in combination, a casing having therein an electrical apparatus which has a plurality of taps of different voltages, said casing having therein a gaseous dielectric; and means for making connection to a high voltage tap from the exterior of said casing, said means including a metallic bushing sealed into an opening of said casing, a plurality of tubular insulating housings arranged end to end, the end of one of said housings being connected to said bushing, a conductor supported from the remote end of the most remote tubular housing and held thereby substantially concentrically with said bushing, said conductor being in electrical connection with the high voltage tap of said apparatus, a metallic sleeve-member connecting adjacent ends of adjacent housings and disposed interiorly thereof, a flux control member extending about said housings in .a region adjacent said metallic sleeve-like member, said two members being in electrical connection with each other, and a metallic tubular member spaced about said conductor and in electrical connection with said metallic sleeve-like member and in electrical connection with a tap of intermediate voltage.

5. In a high voltage terminal, in combination, a conductive member having an opening therein and through which a high voltage conductor is to be passed; a high voltage conductor passing through said opening; a tube-like housing of solid dielectric material of a single layer and supported by said member and about the opening thereof and supporting said conductor at its remote end; and conductive means in the form of an annulus having a cross-section along a radius thereof that is H-shaped and having one vertically extending leg of said H-Shaped cross-section positioned interiorly of said housing and the other vertical leg of said H-shaped section positioned externally of said housing, said conductive means being`positioned intermediate of the ends of said housing, and said interior and exterior vertical legs of said conductive means fixing the voltage gradients respectively along the interior and the exterior of said housing.

arranged in end to end relation and metallic means connecting adjacent ends of adjacent solid dielectric members; said metallic means being in vthe form of` an annulus of substantially H-shaped section along a radius thereof and having one vertical leg of said H-shaped section position interiorly of said housing and the other positioned exteriorly thereof, and at least one of said vertical legs being in mechanical connection with the adjacent ends of the adjacent solid dielectric members; and means positively iixing the potential of said metallic connecting means at a value intermediate of the potential difference to 'which said supporting means is subjected.

7. In a high voltage terminal, in combination, a conductive member having an opening therein through which a high voltage conductor is to be insulatingly passed; a high voltage conductor extending through said opening; a metallic bushing secured to said member; and means for supporting said conductor with respect to said bushing, said means including a plurality of tubular sections o solid dielectric material arranged in end to end relation, one of said sections being secured to said bushing, a metallic sleeve-like member securing adjacent sections together, means mounting one end of said conductor in the end of a section remote from said bushing and holding said conductor in spaced relation to the inside wall of said bushing, said sleeve-like-member and the side walls of said sections being spaced from said conductor; flux control means about said supporting means and positioned intermediate of the ends thereof; and means maintaining upon said sleeve-like member and said ux control means a potential intermediate of the potentials of said conductor and said bushing.

8. In a high voltage terminal, in combination, a conductive member having an opening therein through which a high voltage conductor is to be insulatingly passed; a high Voltage conductor extending through said opening; a metallic bushing secured to said member; and means for supporting said conductor with respect to said bushing, said means including a plurality of tubular sections of solid dielectric material arranged in end to end relation, one of said sections being secured to said bushing, a metallic sleeve-like member securing adjacent sections together, means mounting one end of said conductor in the end of a section remote from said bushing and holding said conductor in spaced relation to the inside wall of said bushing, said sleeve-like member and the side walls of said sections being spaced from said conductor; a conductor extending from said sleeve-like member and spaced from both said first-mentioned conductor and said bushing, said two conductors and said bushing being shaped to provide two fast discharge gaps in series having a critical voltage less'than that which the apparatus served by said high voltage conductor can safely withstand, and means connected to said second-mentioned conductor for impressing upon said sleeve-like member a iixed potential.

9. In a high Voltage terminal, in combination, ya conductive member having an opening therein through which a high voltage conductor is to be insulatingly passed; a high voltage conductor extending through said openingya metallic bushing secured to saidA member; and means for supporting said conductor with respect to said bushing, said means including a plurality of tubular sections of solid dielectric material arranged in end to end relation, one of said sections being secured to said bushing, a metallic sleeve-like member securing adjacent sections together, means mounting one end of said conductor in the end of a section remote from said bushing and holding said conductorin spaced relation to the inside wall of said bushing, said .sleeve-like member and the side walls of said sections being spaced from said conductor, the radii o f said conductor and said sleeve-like member being proportioned to form a fast discharge gap; and means in' electrical connection with said sleeve-like member and proportioned with respect'to said bushing to form therewith a second fast discharge gap, said gaps Ain series having a critical break-down voltage less than that which the apparatus served by said conductor can safely withstand.

10. In-a high voltage terminal, in combination,

a conductive member having an opening therein through which a high voltage conductor is to be insulatingly passed; a high voltage conductor extending through said opening; a metallic bushing secured to said member; and means for supporting said conductor with respect to said bushing, said means including a plurality of tubular sections of solid dielectric material arranged in end to end relation, one of said sections being secured to said bushing, a metallic sleeve-like member securing adjacent sections together, means mounting one end of said conductor in the end of a section remote from said bushing and holding said conductorin spaced relation to the inside wall of said bushing, said sleeve-like member and the side walls of said sections being spaced from said conductor, a tubular conducting member extending concentrically about said conductor and being in electrical connection with said sleeve-like member at its upper end and being spaced from said bushing adjacent its lower end; and means for impressing upon said tubular conductor a fixed potential.

11. In a high voltage terminal, in combination, a conductive member having an opening therein through which a high voltage conductor is to be insulatingly passed; a high voltage conductor extending through said opening; a metallic bushing secured to said member; and means for supporting said conductor with respect to said bushing, said means including a plurality of tubular sections of solid dielectric material arranged in end to end relation, one of said sections being secured to said bushing, a metallic sleeve-like member securing adjacent sections together, means mounting one end of said conductor in the end of a section remote from said bushing and holding said conductor in spaced relation to the inside wall of said bushing, said sleeve-like member and the side walls of said sections being spaced from said conductor, a tubular conducting member extending concentrically about said conductor andbeing in electrical connection with said sleevlike member at its upper end and being spaced from said bushing adjacent its lower end, the radii of curvature of said conductor and said tubular conducting member and the radii of curvature of the latter and of said bushing being proportioned to form fast discharge gaps.

12. In a high voltage terminal, in combination,

a conductive member having an opening therein through which a high voltage conductor is to be insulatingly passed; a high voltage conductor extending through said opening;` a metallic bushing secured -to said member; and means for supporting said conductor with respect to said bushing, said means including a plurality o1' tubular sections of dielectric material arranged in end to end relation, one of said sections being secured to said bushing, a metallic sleeve-like member securing adjacent sections together, means mountingone end of said conductor in the of a section remote from said bushing and holding ysaid conductor in spaced relation to T113 `the inside wall of said bushing, said sleeve-like member and the side walls oi'said sections being spaced from said conductor, a tubular con-y ducting member extending concentrically about said conductor and being in electrical connection with said sleeve-like member at its upper end and being spaced from said bushing adjacent its lower end, the radii of curvature of said conductor and said tubular conducting member and the radii of curvature of the latter and of said bushing being proportioned to form fast discharge gaps, and means connected to said tubular conducting member for .iixing the potential oi said sleeve-like member at a value which it would assume in the normal voltage gradient along said solid dielectric tubular sections.

FRANKLIN S. SMI'I'H.

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