US2732419A

US2732419A - wilson

Info

Publication number: US2732419A
Authority: US
Publication date: 1956-01-24
Anticipated expiration: 1973-01-24

Description

Jan. 24, 1956 w. R. WILSON 2,732,419

ELECTRICAL BUSHING Filed Jan. 20, 1950 2 Sheets-Sheet 1 PRIOR Al? ig-3 27 Fl .4. F?

3/32 F r 2 r' 5 *1 "I 20% a: i -24 g g 407 39 J0, o Inventor:

W Walter R. Wilson,

z b a I His Attorney Jan. 24, 1956 w. R. WILSON ELECTRICAL BUSHING 2 Sheets-Sheet 2 Filed Jan. 20, 1950 O L @w R w n H is Attorneg.

United States Patent ELECTRICAL BUSHING Walter R. Wilson, Drexel Hill, Pa., assignor to General Electric Company, a corporation of New York Application January 20, 195%), Serial No. 139,746 3 Claims. (Cl. 174-31) This invention relates specifically to electrical bushings and more generally to phenomena occurring at the boundary .of solid and fluid dielectrics in an alternating electric field.

In order to provide suitable electrical connection between tank-enclosed electrical equipment and the external Circuits to which such equipment is connected, it is nearly universal practice to use an insulating bushing assembly which passes through the top or side of the tank or enclosure in which the equipment is contained. Such a bushing assembly comprises a center conducting memher, an insulating structure concentric of the conductor, and a symmetrically-located metal grounding sleeve by means of which the bushing assembly is maintained in position with respect to the tank casing.

in the case of oil-immersed electrical equipment, the portion of the bushing assembly which projects inside of the tank or enclosure is usually either wholly or partially immersed in oil.'

One of the serious problems which is encountered in the operation of bushings associated with oil-immersed current-interrupting contacts, such as the contacts of a power circuit breaker, is the fact that the outer surface of the bushing insulator which extends into the oil frequently becomes coated with a carbon deposit. Deposited carbon is an electrical conductor or at least a semi-conductor, and the dielectric strength of the bushing is materially reduced, particularly when abnormal amounts of water are suspended in the oil, and may result in arc-over or other electrical failure of the apparatus during normal operation if the carbon deposit is not removed by frequent servicing.

The carbon which is deposited on the surface of the bushing insulator comes from carbonized and other cracked oil products which are created by the electric are which occurs when the circuit breaker interrupts current. Examination indicates that alternating-currcnt arcs produce carbonized oil particles having diameters ranging from less than 0.1 micron to about microns, with most particles having a diameter under 0.5 micron. Chemical analyses reveal that in addition to carbon the particles contain either Within or about themselves appreciable percentages of acids and solids representing various stages .of decomposition of oil. When these particles are in proximity to' the oil-immersed insulator portion of a bushing assembly which also includes an eenrgized A.-C. conductor, the particles line up like strings of beads along the electric fiux lines and gradually move toward the insulator surface.

The deposition of the carbon particles on the insulating surface is due largely to non-uniformities or distortions in the alternating current electric field surrounding the insulating surface of the bushing. These distortions in the electric field may be produced in a number'of different ways. In the case of glazed ceramic bushing insulators, such as porcelain insulators, one cause isthe non-homogeneity of thebushing porcelain or glaze such as is produced by gas or airpockets the porcelain, or other Patented Jan. 24, 1956 foreign particles therein which have different dielectric properties from the porcelain itself.

Still another factor in causing motion of carbon particles toward the bushing insulating surface is the interparticle force between the carbon particles themselves, since they have different dielectric properties from the oil and bushing insulator, causing a distortion in the electric field which causes deposited carbon particles to attract additional particles.

A further important factor in causing a deposit of carbon on the bushing porcelain is what is known as the image force. This force draws the carbon particles toward the boundary between the oil and the surface of the bushing by virtue of the higher dielectric constant of the surface material which results in an increased electric field intensity on the side of the particle toward the bushing.

in my copending application, No mber 30, 1946, now Patent Ea nco Serial No. 713,249, filed Number 2,523,082, and

the same assignee as the present application,

cred one solution to the problem of avoiding siiion due to the image force.

Closely associated with the image force, and the factor with which this application is most concerned, is the effect upon carbon deposition of the orientation of the electric field surrounding the bushing with respect to the bushing insulating surface. I have discovered that the orientation of the electric field with respect to the bushing insulator contour is a very important consideration in determining the amount of carbon deposited on the bushing. Numerous tests which I have made indicate that when the e ectrostatic field surrounding the bushing has its lines of force parallel to the oil-immersed bushing insulating surface, the collection of carbon is most severe, and when the lines of force approach a perpendicular direction to the insulating surface of the bushing, the carbon collection is almost eliminated, even though the intensity or strength of the field may be as great or greater than in the area where the collection of carbon is severe.

This phenomenon is in large part accounted for by the fact that the image force by which the carbon particle is attracted to the insulating surface is greatly increased When the electric flux lines are parallel to the bushing insulating surface. This is more fully explained hereinafter in connection with Figs. 2 and 3 which graphically illustrate the relation of the electric fiux direction to the intensity of the image force which attracts carbon particles to the bushing insulating surface.

A further explanation of the pronounced effect of electric flux-to-insulating surface orientation on carbon deposition may be found in the relation of the strings of carbon particles to the electric flux lines.

As has been mentioned hereinbefore, the carbon particles are attracted to each other by a high inter-particle force and will line up like strings of beads along the flux lines. If the electric flux is perpendicular to the bushing insulating surface, the string of particles will also beperpendicular to the insulating surface. In such case, only the first particle in' the string will touch the insulating surface, and this particle is held by the attraction to the next particle more strongly than by the insulating surface. Therefore, if the string of particles is carried off by oil currents, it will take the first particle clean surface.

On the other hand,

angle with respect to the insulating surface, with the line of carbon particles lying along the flux-.lines-at to-insulator attractive W with it, leaving a.

if the flux lines are at an acutev thesame' angle with respect to the insulating surface, the particle force component or image fo U will then be heldby the intense image force. Thus, if the flux-insulator surface angle is gradually reduced, at some stage the image force on the second particle will pull that particle to the surface. In like manner, successive particles of the string will be drawn to the surface like a rope being laid on a fioor. In the most extreme situation, where the flux lines are parallel to the insulating surface, with the strings of carbon particles lying along the flux lines, there will be no tendency of particles in the same string to exert an interparticle force tending to pull away particles from the insulating surface. Each particle of the parallel string closest to the insulating surface will adhere individually to that surface, and a high resistance is offered by the carbon particles of such a parallel string to removal from the insulating surface by oil agitation.

Accordingly, it is an object of my invention to provide a new and improved construction for oil-immersed bushings which will restrict to a minimum the amount of carbon deposited on the insulating surface of the bushings. It is a further object of my invention to provide a new and improved method for minimizing carbon deposits on the oil-immersed insulating surfaces of electrical bushings. It is still a further object of my invention to provide a new and improved bushing insulator contour for oilimmersed bushings which will provide an improved characteristic of the electric field about the bushing with respect to the bushing insulating surface with a consequent reduction in amount of carbon deposited on the bushing insulating surface.

It is another object of my invention to provide in combination with a bushing a metal shield member which will cause such an orientation of the electric field with respect to the bushing insulating surface that a minimum of carbon will be deposited on the bushing insulating surface.

In accordance with these objectives, my invention provides a new and improved construction for oil-immersed bushings such that a normal to the insulating surface of the bushing forms a minimum angle with the direction of dielectric stress or, expressed differently, the lines of dielectric stress form an angle as closely approximating a perpendicular relation to the insulating surface of the bushing as can be obtained. In accordance with my invention, I may achieve this desired orientation of the electric flux with respect to the bushing insulating surface either by modifying the contour of the bushing insulator so as to adapt it to a given electric field, or by modifying the electric field to adapt it to a given bushing insulator contour.

The features of this invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and use, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing.

Fig. 1 represents a conventional bushing construction in accordance with the prior art.

Figs. 2 and 3 represent schematic illustrations of the attractive forces between a carbon particle and an insulating surface with a parallel electric field and a perpendicular electric field respectively.

Fig. 4 represents a new and improved bushing insulator contour in accordance with my invention.

Figs. 5 and 6 represent modified bushing insulator contours in accordance with my invention.

1 Figs. 7 and 8 represent bushings having shielding means in accordance with my invention to obtain the desired flux-insulating surface orientation.

Referring now to the drawing, there is shown in Fig. 1 the oildmrnersed portion of a conventional bushing assembly 1 having an insulating member 2 with a convoluted outer surface to increase the creepage path from the conductor to ground. This convoluted efiiect on the conventional bushing insulating surface is obtained by' having a plurality of approximately annular-shaped petticoats spaced apart from one another longitudinally of the main body portion of the bushing insulator. Thus, bushing insulator member 2 is represented as having eleven petticoats numbered 3 to 13 inclusive from the upper to the lower end of the insulator. A metal mounting member or ground sleeve 14 is provided as a standard part of the bushing assembly at the upper end of the insulator member 2, with respect to the view shown in the drawing, for connection to the casing or tank surface through which the bushing passes. Metal sleeve member 34 is at ground potential. The bushingassembly 1 in cludes a center conductor 15 which is provided at its lower end with a terminal washer 16. Voltage equalizers 17 of conventional construction are also provided between the outer surface of conductor 15 and the inner surface of bushing insulator member 2. It will be noted that bushing insulator member 2 tapers inwardly from its upper edge to its lower edge in accordance with the typical construction of such bushings.

Although the main body portion of bushing insulator member 2 tapers inwardly from its upper to its lower surface, with respect to the view shown in Fig. 1, so that the radius of the main body portion decreases due to the inward taper, the outer diameter of the annular petticoats remains substantially the same. Due to this fact, the differential between the outer diameter of the annular petticoats and the outer surface of the main body portion of the bushing insulator becomes increasingly greater in proceeding from the upper to the lower edge of the bushing. Thus, annular petticoats 3, 4, 5, 6 and 7 at the upper portion of the insulator member 2 do not protrude very far out from the main body portion of the insulator with the result that the undulations between successive annular petticoats 3-7, inclusive, are not very large. However, there is a pronounced differential between the diameter of the main body of the insulator member 2 and the outer diameter of annular petticoats 8-13, inclusive, since the differentials become increasingly greater proceeding from petticoats 8 to 13.

It will be noted that particularly for petticoats 7-13, inclusive, there is a considerable distance longitudinally or axially of the bushing insulator 2 between adjacent petticoats. As will be explained hereinafter, it is principally in this region between petticoats that excessive carbon deposits occur.

The electric field about the bushing is indicated by the equipotential lines which appear to radiate outwardly from the center conductor 15. Points in the field lying along the same equipotential line are, as the word equipotential denotes, at the same potential with respect to center conductor-to-ground voltage. The voltage value 'of the field at the various points lying along the respective equipotential lines is indicated by the percentage values at the end of the line. Thus, the respective lines are labeled 90%, 80%, 70%, etc. indicating the percent of the total voltage drop from the conductor to ground reached at the particular equipotential line.

It is well known that the lines of electric fiux are perpendicular to the equipotential lines, and these lines of electric flux are indicated by the arrows adjacent the outer surface of the insulator member 2. it will be noted that. particularly at the upper section of the insulator member 2, the flux lines, as indicated by the arrows, lie parallel orapproximately parallel to the main body portion of the insulator member 2 between the annular petticoats 3 to 10, inclusive. As I have indicated hereinbefore, numerous tests which I have made indicate that when the lines of electric flux are parallel to the bushing insulating surface, as shown in Fig. 1, the collection of carbon is most severe.

It can be seen that a conventional bushing such as that shown in Fig. 1 offers many surfaces which are substan tially parallel to the lines of electric flux, these surfaces principally being between the annular petticoats. 7

There are shown in Figs. 2 and 3 a comparison of image force between a carbon particle and an insulating surface with a parallel and a perpendicular electric fluxto -insulator-relation, respectively. Fig. 2 shows a carbon particle 18 touching the exposed surface 19 of a bushing insulator. The dotted lines with arrows represent electric flux lines in the region where the carbon particle is located. In this case the field flux is parallel to the insulating surface. It will be noted that there is a heavy concentration of flux lines just inside the surface of the insulator, causing a more intense stress on the side of the carbon particle adjacent the insulating surface. This stress causes the carbon particle to adhere tightly to the insulating surface.

The relative image force for a perpendicular field is shown in Fig. 3. A carbon particle 20 is shown touching the surface 21 of a bushing insulator. The dotted lines with arrows represent electric flux lines in the region where the particle is located. In this instance, the field flux is perpendicular to the insulator surface. It will be noted that there is no high flux concentration at the insulator surface, as is the case with the parallel field. Therefore, there is not nearly as much stress tending to hold the carbon particle 20 to the insulator surface 21 as there is force tending to hold carbon particle 18 to insulator surface 19. Thus, 'Figs. 2 and 3, taken together illustrate the fact that the image force tending to hold carbon particles to an insulating surface is much greater where the electric flux is parallel to the insulating surface than when the fiux is perpendicular to that surface.

Referring now to Fig. 4, there is shown the oil-immersed portion of a bushing assembly 22 having an insulating member 23 constructed in accordance with my invention to minimize the carbon deposits on the surface of the bushing. It will be noted that the configuration of the equipotential lines about the bushing is substantially the same as that in the case of the conventional bushing of Fig. 1 since the distribution of the electric field about the bushing is dependent chiefly upon the geometry of the conducting parts of the bushing and to a lesser extent on the proximity of other conducting surfaces such as the tank casing. However, I have arranged the contour of the bushing insulator 23 in Fig. 4 in such manner that the outer insulating surfaces of the bushing are not paral lel to the electric flux lines as in the conventional bushing insulator of Fig. lbut rather in which substantially all of the surfaces are inclined more than 45 degrees to the direction of the electric flux lines, with many of the insulating surfaces approaching the ideal condition where the insulating surfaces are perpendicular to the electric flux lines. As I have indicated hereinbefore, when the lines of force approach adirection which is perpendicular to the insulating surface of the bushing, the carbon collection is to a large degree eliminated, even though the intensity or strength of the field is as great or greater than in the area where the collection of carbon is severe.

The bushing insulator member 23 of Fig. 4 surrounds a center conductor member 24 rigidly attached to a bottom washer 25 of metallic conducting material. Voltage equalizers 26 of conventional construction are shown between the inner surface of bushing insulator member 23 and the center conductor 24. The equipotential lines are marked 90%, 80%, 70%, etc., to indicate the relative potential of the various points in the field about the bushing with respect to the total conductor-to-ground potential drop. A metal support member or ground sleeve 27 is attached to the enclosing structure, such as a transformer tank, at the surface where the bushing passes through the cover or wall of the enclosure, and serves to hold the bushing in position with respect to the enclosing tank. The insulator member 23 is provided with petticoats 28-42, inclusive, having .a contour which gives the desired flux-insulator surface angle.

In accordance with my invention, the outer contour of the bushing ofFig. 4 is arranged so that the amount of bushing insulating surface area which is parallel to the electric flux lines is reduced to a negligible minimum.

The major portion of the surface of the bushing insulator 23 is so oriented with espect to the electric'field that a normal at substantially any point on the interface between the oil surrounding the bushing insulator and the surface of the bushing insulator 23 forms an angle less than 45 degrees with the direction of dielectric stress, or path followed by the lines of electric flux, with a substantial part of the insulator surface area being perpendicular to the direction of dielectric stress, so that the normal to the insulator surface area in such regions is coincident with the direction of dielectric stress.

In designing a bushing insulator contour in accordance with my invention, the contour is made to conform with a known fixed electric field. That is, knowing the geometry of the grounding sleeve 27 and the enclosing tank structure with respect to the conductor 24, it is possible to determine and plot with a relatively high degree of accuracy the electric field present between the conductor and all grounded surfaces in proximity to the conductor.

Knowing the electric field, it then becomes a matter of applying my invention to designing a particular bushing insulator contour which will have the desired relation to the direction of dielectric stress, this direction being the path followed by the lines of electric flux.

It will be noted that the petticoats on the outer surface of the bushing at the upper end of the bushing near support member or grounding sleeve 27 and in the region between zero and 20% potential drop where the dielectric stress gradient is relatively low, are spaced closely together. Furthermore, all of the petticoats 28 through 35, inclusive, in this region of relatively low dielectric stress, are all of substantially the same size, with rather shallow indentations or valleys between adjacent petticoats.

However, it will be noted that unlike the conventional bushing of Fig. l, the bushing of Fig. 4 has no space longitudinally of the insulator surface between adjacent sides of adjacent petticoats. It is this space between petticoats on the conventional bushing insulator which provides most of the surfaces parallel to the electric flux, and, consequently, the surfaces most heavily coated with carbon deposit. Thus, on the bushing of Fig. 4, adjacent sides of adjacent petticoats form a circular line of intersection around the periphery of the bushing. For example, the lower side of petticoat 37 and the upper side of petticoat 38 intersect along a circular line around the outer periphery of the bushing, although this intersection appears as a point in the cross-sectional view shown in Fig. 4. There is no space longitudinally of the bushing between adjacent sides of adjacent petticoats.

In designing a bushing insulator of the convoluted type having petticoats to increase the leakage path from the conductor to ground, it is impossible to have both sides of the various petticoats in the ideal position with respect to the electric field, that is, in a position such that the electric flux lines are perpendicular to both the upper and lower surfaces of each petticoat. Therefore, it is necessary in most instances to compromise by having each of the sides of each of the respective petticoats offset to some extent from the ideal perpendicular relation with respect to the electric flux lines. It will be noted that in most instances, the lower edge of each respective petticoat is more closely in a perpendicular relation to the electric flux lines than are the upper sides of the respective petticoats. This is particularly noticeable in the case of the lower sides of petticoats 37, 3S and 39.

There are shown in Figs. 5 and 6 modified bushing insulator contours in accordance with my invention, distinguished from the structure of Fig. 4 chiefly by the fact that the contours of Figs. 5 and 6 have deep valleys between adjacent petticoats, with the valleys having shallow openings. The respective sides of the various petticoats are angularly positioned in a manner similar to that of the bushing insulator of Fig. 4 in that the electric flux makes a minimum angle of 45 degrees with substantially all of the insulator surfaces. However, the angles of the sides of the various petticoats are so adjusted with respect to one another that deep valleys with shallow openings are produced. Test data indicate that such a construction restricts the amount of carbon which comes in contact with the insulator surface, and thereby reduces the amount of carbon deposited.

There are shown in Figs. 7 and 8 modified bushing constructions in accordance with my invention in which unconvoluted insulating members are used. These structures are characterized by the fact that instead of modifying the insulator contour to adapt it to the field,v I apply the principles which I have discovered as to the relation of the electric field to carbon deposition by using suitable means to modify the field to adapt the field to the insulator contour. It can be seen that this is just the reverse of the procedure followed in the structures of Figs. 4, 5, and 6 where the insulator contour was changed to meet a fixed field condition.

There is shown in Fig. 7 the oil-immersed portion of a bushing assembly 43 having an insulator member 44, shown in phantom view, which surrounds a central conductor 45. A metal mounting member or ground-sleeve 46 is suitably attached to bushing insulator member 44 and serves as the means by which the bushing structure is connected to the coveror side of the transformer tank or other enclosing means in which the oil-immersed portion of the bushing is contained. In the particular bushing structures illustrated in both Figs. 7 and 8, equalizer members are not used as in structures of Figs. 1 and 4. However, such equalizer members may be used if desired, although their use changes the characteristics of the field and must be taken into consideration when attempting to obtain any particular orientation of the electric flux lines with respect to the bushing surface.

It will be noted that the outer surface of bushing insulator member 44 is smooth and is not convoluted in the manner of the insulators of Figs. 1, 4, 5 and 6. In order to change the direction of the electric field at the upper portion of the bushing and to prevent the parallel alignment of the fiuX lines with respect to the bushing surface which would otherwise occur in that region, I provide a shielding means 47. The shield 47 is cone-shaped with the upper part of the cone being rigidly attached to a cylindrical-shaped mounting member 48 which is rigidly held in position with respect to the insulator by any suitable means such as one or more set screws 49. The cone-shaped shield 47 illustrated in Fig. 7 is made of wire mesh in order to facilitate circulation of oil in the The equipotential lines will align themselves approximately parallel to the contour of the shield 47, and to obtain the desired configuration of equipotential lines which results in the electric flux lines being as nearly as possible perpendicular to the insulator surface, the cone-shaped shield 47 may be given the appropriate angle to obtain such a configuration. The electric flux lines, as is well known, are perpendicular to the equipotential lines.

There is shown in Fig. 8 a further modification of my invention which differs principally from the construction of Fig. 7 in that a rounded metal shield is used in place of the wire mesh screen 47 of Fig. 7. In Fig. 8 there is shown the oil-immersed portion of a bushing assembly 5t) having a insulator member 51, shown in phantom view, which surrounds a. central conducting member 52. A metal mounting member or ground-sleeve 53 is attached to the insulator member 51 at the surface where the bushing passes through the side or wall of the tank and is the means by which the bushing structure is held in position with respect to theftank or other enclosure in which the oil-immersed portion of the bushing is contained. In

order to properly orient the direction of electric stress at the portion of the insulator 51 in proximity to groundsleeve 53 in particular, a conical shield member 54 is rigidly attached by any suitable means to the lower portion of metal mounting member or ground-sleeve 53. The outer edge of conical shield member 54 is rounded in such manner as to obtain the desired orientation of the electrostatic field. Since the electric equipotential lines will substantially conform to the curved contour of the shield 54, shield 54 can be bent at such a radius of curvature as will cause the equipotential lines to have the desired configuration at which the electric flux lines are as nearly as possible perpendicular to the bushing surface.

It will be seen that in the construction of Figs. 4, 5, and 6 where I modified the bushing insulator contour to conform in the desired manner to the electric field, and also in the constructions of Figs. 7 and 8 where I modify the electric field to conform in the desired manner to the bushing insulator contour, in both cases seeking to achieve as nearly as possible a perpendicular relation between the electrostatic flux lines and the bushing insulator surface, I have provided a new and improved means for minimizing carbon deposits on bushings immersed in oil. This improved method will result in improved operating characteristics of oil-immersed bushings while in operation, as well as longer life and less frequent servicing of such bushings.

While there have been shown and described particula embodiments of my invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the invention, and therefore, it is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination with tank-enclosed, oil-immersed electric apparatus, an electrical insulating bushing assembly which passes through a surface of said tank comprising an electrical conductor, a convoluted ceramic insulating member surrounding said conductor, a metal grounding sleeve surrounding said insulating member in the region where said insulating member passes through said tank surface, at least part of the bushing insulating surface contained in said tank being immersed in said oil, an electric field surrounding said insulating member when said conductor is energized, the normally-immersed part of said insulating surface throughout substantially its entire extent being so shaped with respect to said electric field as to define interface surfaces disposed at an angle of not less than 45 degrees to the direction of electrical stress of said field.

2. In an electrical insulating bushing for use with oilimmersed electrical apparatus, said bushing including a ceramic insulating member having a convoluted outer surface, said insulating member surrounding an electrical conductor connected to said apparatus, said apparatus being contained in a metal enclosure which is at ground potential, said insulating member and conductor passing through a surface of said enclosure so that a portion of said insulating member is positioned interiorly of said enclosure, an electric field surrounding said conductor and said insulating member when said conductor is energized,

. said enclosure being at least partially filled with oil, the

portion of said insulating member within said enclosure being at least partially immersed in said oil, the method of minimizing carbon deposits from said oil on said insulating member which comprises orienting said electric field with respect to said insulating member so that the direction of electrical stress at the interface between the surface of said insulating member and said oil makes an angle of not less than 45 degrees at substantially all surfaces of said insulating member immersed in said oil.

3. An electnical bushing for use with oil-immersed electrical apparatus, said bushing including a ceramic insulating member having a convoluted outer surface, said convoluted insulator surface normally being at least partially immersed in oil, said insulator surrounding an e1ectrical conductor, said insulating member and said conductor normally being surrounding by an electric field when said conductor is energized, the normally oil-irnmersed portion of said insulating member having its convoluted contour so shaped with respect to the electric field surrounding said insulating member that substantially the entire interface between the surface of said insulating 1 member and the oil in which it is immersed is defined by surfaces disposed at an angle of not less than 45 degrees to the direction of electrical stress of said field.

References Cited in the file of this patent UNITED STATES PATENTS 996,878 Nichols July 4, 1911 10 Aichele May 15, 1917' Fortescue et a1 Apr. 19, 1921 Faccioli Apr. 22, 1924 iackson Mar. 29, 1927 Miner -1 Sept. 11, 1928 Nicholas Apr. 17, 1945 FOREIGN PATENTS France Dec. 20, 1924 OTHER REFERENCES llicholas, i. 'i..: Trans. A. i. E. E. vol. 68, part 5! (i949), pages 1264-5.

(Copy in Science Library.)

(1949), page 1269.

(Copy in Science Library.)