To all whom it may concern:

Be it known that we, CHARLES LE G. FORTESCUE, a subject of the King of Great Britain, and a resident of Pittsburgh in the county of Allegheny and State of Pennsylvania, and GEORGE I. GILCHREST, a citizen of the United States, and a resident of Wilkinsburg, in the county of Allegheny and State of Pennsylvania, have invented a new and useful Improvement in Insulators, Of which the following is a specification.

Our invention relates to electrical insulating bodies or structures and particularly to such insulators as are employed for supporting high-voltage lines or conductors.

One object of our invention is to provide an insulator, of the above indicated character, that shall be simple and durable in construction and of such shape and conformation as to utilize substantially the maximum dielectric strength of the air or other insulating medium between conductors of considerable difference of potential.  Another object of our invention is to provide a support for electrical conductors that shall be protected from adverse influences, such as rain or dust, in a manner to insure substantially a minimum degree of disturbance to the dielectric field in which it is disposed.

A further object of our invention is to provide a condenser type insulator of such construction as to cause the mechanical and the electrical stresses imposed thereon to extend in substantially coincident lines, thereby obtaining an insulator of minimum size and weight and of maximum electrical and mechanical strength.

Heretofore, in the design of insulators, more consideration has been given to the distribution of potential stresses caused by leakage currents than to stresses caused by the dielectric field.  Corrugated or long leakage-path insulators have been designed to compensate for the stresses caused by leakage, but, in many instances, they have so disturbed the dielectric field, by reason of ionization of the surrounding air, as to nullify the surface resistance thereof, resulting in "flash-over" at voltages below the voltages at which insulators of the proper shape and of the same weight and quality of material break down.

Attempts have been made to increase the puncture strength of insulators by providing the well known superposed or chain petticoat insulators, but, unless compensated for, as is done in our improved insulator, the ionization of the air between the individual members of such an insulator causes high frequency oscillations and streamers over the same and thereby increases the stress, to which they are subjected, to a higher value than the resistance provided by the additional members.

The design of an insulator should be a compromise between the leakage current distribution and the dielectric field distribution, since, in every instance, each of these is a factor in the actual distribution of potential. That is, the surface leakage resistance should be proportional to the difference of potential caused by the dielectric field.

The members of a built up insulator should be of such design and arrangement and severally of such capacitance as to cause the distribution of the electrostatic stress to conform to the natural form of the field between the conductors as the same would be if no insulator were present.

As disclosed in patent No. 1,129,520, granted to the Westinghouse Electric and Manufacturing Company, February 23, 1915, upon an application filed by K. C. Randall, S. W. Farnsworth and C. Le G. Fortescue, the concentration of potential stresses, in the vicinity of insulators, may be avoided by having the insulators of such shape as to conform to the electrostatic flow lines of the field in which they are disposed.

In practicing our invention we utilize the above mentioned principle in shaping the body portions of the several members comprising our insulator, and further provide the same with protecting barriers or petticoats that are of such shape as to conform to equi-potential surfaces of the respective parts of the field in which they are severely disposed. That is, all points on the surfaces of the petticoats are substantially at right angles to the lines of the field at these points. 

By properly constructing the several juxtaposed members of insulating material, and by providing conducting bodies in proper relation thereto, in accordance with the principles set forth in Patent No. 1,129,468, granted to the Westinghouse Electric and Manufacturing Company, February 23, 1915, upon an application filed by C. Le G. Fortescue, we obtain an insulator comprising a series of condensers of such capacitance that the potential gradient conforms to the natural field between the conductors and in which the liability of fracturing the insulator adjacent to the positive charge, whereat the surface tension is normally the greatest, is substantially eliminated.

The advantages resulting from the above indicated features, as well as those obtained by having the electrical and mechanical stresses substantially coincident, and other features will be hereinafter more fully pointed out.

Figure 1, of the accompanying drawing is a view, partially in section and partially in elevation, of an insulator embodying our invention; Fig. 2 is a fragmentary view, similar to a portion of Fig. 1, showing a modified form of petticoat therefore, Figs. 3 and 4 are fragmentary views of a modified form of pin support for the same, and Figs. 5 and 6 are diagrammatic views illustrating certain characteristics of our improved insulator, to be hereinafter explained.

Referring to Fig. 1, a clamp 2, mounted on a cap member .3, or any other suitable means, such as a tie wire, as shown in Fig 5 secures a high-potential conductor 1 to the uppermost member of a plurality of superposed nested members 4 of insulating material, such as porcelain. The members 4 are of substantially bell-shape, having body portions 5 that conform, in contour, to the static field lines which extend from the conductor 1 convergingly toward a supporting pin 6, one end of which is disposed in a partially screw-threaded recess 7 in the lowermost member 4. Petticoat portions 8, on each of the members 4, are of such shape as to be substantially, at all points on their surfaces, at right angles to the static field surrounding the insulator. Coaxial centrally disposed recessed projecting portions 9 and bodies 10 of cementitious material or of conducting material, if it is so desired, disposed there between constitute the condenser elements of the assembled insulator.

The relation of the pin 6 to the conductor 1 so directs the static field lines as to cause the same to extend in substantially the same direction as certain mechanical stresses, in a manner to be hereinafter pointed out, and the projecting portions 9 are of such shape as to resist these stresses in a manner permitting desirable latitude in the selection of certain electrical characteristics, and at the same time insuring maximum mechanical strength.

The flow line conforming body portions 5, when assembled, constitute a multi-part main body portion of the complete insulator that substantially conforms to the static field lines throughout the space separating the conductor and pin. This, as explained in Patent No. 1,129,520, fully utilizes the dielectric strength of the air.  When exposed to adverse weather conditions of exterior installation or to an interior installation where globules of water, particles of dust or other extraneous matter are likely to settle on the insulator surfaces, these conditions are not disturbed and the "flashover" point will not be lowered unless the deposited matter becomes of sufficient quantity to short circuit the condenser elements.

As shown in Fig. 6 the static field lines extend undisturbed from the conductor 1 to the pin 6, by reason of the fact that, since the petticoats 8 conform to equi-potential surfaces and extend into the field to a region where it is comparatively weak, there is substantially no tendency toward the concentration of stresses at any particular point. This principle, as applied to curved or directed field lines, was evolved by further research of the principles set forth in Patent No. 1,129,520, and has been demonstrated to be substantially true by tests in which insulators, constructed in accordance therewith, showed no corona at, and could be continuously operated within, two percent of their breakdown voltages.

To the extent that the dielectric equi-potential surface petticoats do not disturb the field, they approach the characteristics of a metal under the influence thereof, and that the use of metal petticoats is herein comprehended, is illustrated in Fig. 2. Metal petticoats 11 may comprise extensions of the conducting bodies 10 and have inner edges disposed between the members 4 or they may be constructed and mounted in any suitable manner. Some of the advantages attending the use of metal are durability, protection from heat caused by arcing and mechanical protection.

An advantage of the dielectric petticoats 8, when exposed to rain or mist, is the leakage resistance provided thereby in the preferred embodiment of our invention, the leakage path of each of the members 5 is of substantially equal resistance, but, by reason of the greater washing action of rain on the top petticoat and the consequent probability of greater deposits of dust forming on the lower petticoat, the leakage path is so formed as to provide a slightly and gradually increasing resistance from the upper end to the center of the insulator.

The above arrangement provides a substantially constant leakage current, under wet and dry conditions, in the following manner: 

Letting I = total current from conductor to ground.

I_C = current through the insulator.

I₁ = leakage current over the insulator.

Then at all times

                I = I_C + I₁.

in dry weather, I_C greatly dominates I₁, so that substantially

                  I = I_C.

When the insulator becomes wet, the greater portion of the leakage current becomes shunted by way of the petticoat path which is so proportioned, as hereinbefore pointed out, that I₁ is related to I_C in substantially inverse ratio as under dry conditions, so that substantially

                  I = I₁.

Our improved insulator thus operates substantially as well under the most severe as under normal weather conditions.

In order to eliminate the fracturing stresses normally existing adjacent to the positive charge, the members 4 are so proportioned and so inter-disposed with the conducting material 10 as to constitute a series of condensers that may be severally of any desired capacitance.

The cap member 3 constitutes one terminal plate of the condenser, the other terminal plate 12 of which may be conveniently formed by providing the surface of the recess 7, of the lowermost member 4, with a coating of the aforesaid conducting material, or in a manner shown in Figs. 3 and 4.

As shown in Fig. 3, the metal pin 6 is of such shape as to substantially fill the recess 7, a body of cement 13 being disposed between the pin and the member 4. The same electrical result would obtain if the pin 6 were of wood and the cement 13 of conducting material.

Fig. 4 shows another method of providing the lowermost member 4 with a terminal plate by means of a metal sleeve 14 which embraces the pin 6 and engages the walls of the recess 7. In the above manner, conductors of high potential may be supported without undue stresses in the component parts of the supporting means. We prefer to use, for the material 10, a cement having the same coefficient of expansion as the members 4. If the material 10 is conducting material, it may consist of finely divided metal, such as type metal, suspended in a good adhesive binder, such as cement, or it may be of any other suitable substance which also serves to hold the members 4 together.

It will be understood that the specific inductive capacity of cement or similar material differs from that of porcelain to such degree that the bodies of materials constitute a condenser even though metallic plates or other conducting material are not employed.

As is well known, the mechanical forces to which a transmission line insulator may be subjected are, in all instances, capable of being resolved in vertical and horizontal components.  The vertical components of a force, in order to be transferred from the line conductor to the support or pin for the insulator, must pass through the body of the insulator.  By referring to Fig. 6, the pin 6 will be observed as being the mechanical support for

the insulator to which the line conductor is attached. This pin 6 possesses the form of a hyperboloid of revolution. As brought out in Horace Lamb's Hydrodynamics published by Cambridge University Press, a vertical force transmitted through a body which is supported upon a pin that is a hyperboloid of revolution will resolve itself into mechanical stresses that follow the contour of ellipsoids of revolution. These ellipsoids have their axes perpendicular to the axes of the hyperboloids. The lines 8 of Fig. 6, therefore, represent various hyperboloids of revolution which

form a confocal system.  The dotted lines of Fig. 6 represent segments of ellipsoids of revolution which constitute another confocal system.  The equal potential surfaces are the confocal hyperboloids, and the flow lines or lines of maximum electrical, as well as mechanical, stresses are the confocal ellipsoids.  Since the mathematical equations representing the electrical stresses are identical with those representing the mechanical stresses that are set up in a non-elastic medium and under the circumstances indicated above, it is apparent that the electrical stresses coincide with the mechanical stresses in the insulator of our invention.

Our insulator, as shown in Figs. 1 and 5, is made up of sections, the adjacent surfaces of the body portions of which extend substantially at right angles to the "flow lines" or lines of the electrostatic field.  The dividing lines of the adjacent nested portions of the body members 4 are, accordingly, substantially at right angles to the substantially coincident electrical and. mechanical stresses. The surfaces are thus so arranged that the several sections withstand, to a maximum degree, the various forces to which they are subjected without tendency to relative movement of the insulator parts. This arrangement permits the employment of multi-part insulators designed to withstand given mechanical stresses in which considerable latitude in the predetermined capacitance is permitted. The capacitance may be predetermined by selecting the dimensions of the several parts without affecting the mechanical strength of the insulator, as a whole, since the relations of the parts are not changed.

Another advantage of an insulator constructed in accordance with the above described principles, is that foreign particles do not end to accumulate on the flow-line surfaces thereof. Since the stresses are substantially tangential to all points on these surfaces, there are no component forces tending to attach such particles which, as a matter of fact, are attracted toward the equi-potential surfaces. By this agency, our improved insulator will not readily become deposited with dust or other matter at the portions whereat it is most desirable that it be clean.

The arrangement of the body portions of the insulator sections to conform substantially to the "flow lines" of the electrostatic field operates to keep them free from dust or other foreign materials which tend to collect on all exposed portions. The dust particles become charged upon entering the electrostatic field and flow in the direction of the current.  Since the "flow lines" are everywhere tangential to the outer surfaces of the body portions of the insulator sections, the particles do not collect thereon but are carried to the adjacent surfaces of the petticoats.  The particles travel in both directions since alternating currents are employed in the transmission lines.

The deposited dust upon the petticoats is substantially immaterial since, when the dust is dry, it does not affect operating conditions of the insulator. Dust on the upper surfaces of the petticoats is washed away upon the occurrence of rain.  The dust on the inner surfaces of the petticoats is not subject to moisture and accordingly, it does not affect the operation of the insulator under normal conditions.  It is essential, however, that the body portions of bowl-shape be maintained free from dust particles or other foreign materials since these portions are exposed more or less to moisture and the operation of the insulator is correspondingly affected.

It will be noted that the petticoat portions are of relatively thin material while the main body portions to which they are joined are of relatively thick material. There is an abrupt change in cross-sectional area at the junction of the petticoat and body portions.  This arrangement insures that, in case of fracture of the petticoat portion, the main body portion is not damaged thereby since there is little or no tendency for the fracture to extend into the latter portion.  The advantage of this arrangement lies in the fact that, in case of partial destruction of the insulator by fracture of the petticoat portions, the main body portions are intact and insure that the insulator is, at all times and under all conditions, effective to prevent current from flowing from the line conductor to the supporting pin of the insulator. In the insulators of the prior art, the fracture of the petticoat portions substantially invariably effects the destruction of the main body portion and the insulator is substantially ineffective to afford protection from the voltage of the line conductor. While we have shown and described particular forms of our invention, many changes may be made therein without departing from the spirit and scope thereof, as indicated in the appended claims.

 We claim as our invention:

 1. An insulator for disposition in an electrostatic field of predetermined form comprising a body portion having its exposed surface shaped to conform substantially to the flow lines of said electrostatic field having a uniform potential gradient and a petticoat portion having its contour shaped to conform substantially to an equi-potential surface of said electrostatic field.

2. An insulator for disposition in a dielectric field of predetermined form comprising a plurality of body portions having exposed surfaces shaped to conform to the flow lines of said field having a uniform potential gradient, and petticoat portions having their contours shaped to conform substantially to equi-potential surfaces of said field.

3. An insulator for disposition between two zones of different potential in a dielectric field of any predetermined form comprising a plurality of juxtaposed members of insulating material, each so shaped as to cause the mechanical stresses therein to be substantially coincident with the electrical stresses in the electrostatic field between said zones.

4. An insulator comprising a plurality of coacting members so shaped as to cause the mechanical stresses therein to be substantially coincident with the electrical stresses of the electrostatic field in which it is disposed.

5. An insulator for disposition between two members of different potential comprising means for directing the electrostatic field therebetween to have a predetermined substantially bowl shape and a plurality of superposed insulating members having body portions all of which cooperate to form a subdivided single structure substantially conforming to said field.

6. An insulator for disposition in an electrostatic field of predetermined shape, said insulator comprising a plurality of juxtaposed members of insulating material, each having a body portion shaped to cooperate with the other body portions to conform substantially to said field of predetermined shape and a laterally projecting barrier portion shaped to conform substantially to an equi-potential surface in the said field and bodies of material disposed with respect to the said members to constitute a system of condensers of predetermined capacitance.

7. An insulator for disposition between two members of different potential comprising means for directing the electrostatic field there between to have a substantially bowl shape and a plurality of superposed nested insulating members having portions cooperating to form a subdivided single main body structure substantially conforming to said field of bowl shape, and petticoat portions conforming to equi-potential surfaces In said field.

8. An insulator for disposition between two members of different potential comprising means for directing the electrostatic field therebetween to have a substantially bowl shape and a plurality of superposed nested insulating members of substantially bell shape each having two concentric annular flanges at the open end thereof, the inner of said flanges cooperating to substantially form the bounding surface of said field of bowl shape and the outer of said flanges constituting petticoats for the insulator.

9. An insulator for disposition between two members of different potential comprising means for directing the electrostatic field there between to have a predetermined substantially bowl shape and a plurality of superposed nested insulating members of substantially bell shape each having two concentric annular flanges at the open end thereof, the inner of said flanges cooperating to substantially form the bounding surface of said field of bowl shape and the outer of said flanges constituting petticoats having equi-potential surfaces.

10. An insulator comprising a plurality of superposed nested members of substantially bell shape certain outer surfaces of which cooperate when in assembled position to form the bounding surface of a main body structure of a predetermined substantially bowl shape substantially conforming to an electrostatic field between the ends of the insulator.

11. An insulator comprising a body member shaped to conform substantially to the electrostatic field in which it is disposed, a petticoat member shape to conform substantially to an equi-potential surface in said field, means whereby the electrical and mechanical stresses in the said body member are caused to be substantially coincident and bodies of material related to the said body member to constitute a condenser.

12. An insulator for disposition in an electrostatic field of predetermined shape between two conductors of different potential comprising a plurality of body members having surfaces cooperating to conform substantially to said electrostatic field of predetermined shape extending between said conductors, and surfaces substantially at right angles to the said body surfaces at their junctions and conforming substantially to equi-potential surfaces between said conductors.

13. An insulator for disposition between two members of different potential comprising means for directing the electrostatic field there between to have a predetermined shape and a plurality of superposed insulating members having body portions all of which cooperate to form a subdivided single structure substantially conforming to said predetermined shape.

14. An insulator c6mprising a plurality of juxtaposed members cooperating, when in assembled position, to constitute a single composite structure having a body portion of substantially bowl-shaped tapering from a relatively narrow portion at one end to a relatively wide portion at the other end, each of said members having a petticoat portion which at its junction with the body portion extends substantially at right angles thereto.

15. An insulator for disposition in an electrostatic field having a uniform potential gradient comprising a plurality of juxtaposed members of substantially bowl shape each having a surface cooperating with similar surfaces on all of the others to form the bounding surface of a single composite regular structure, the outline of which conforms substantially to the electrostatic field in which said insulator is disposed.

16. An insulator comprising a plurality of juxtaposed members each having a surface cooperating with a similar surface on the others to form the bounding surface of a single composite regular structure, the outline of which conforms substantially to the electrostatic field in which it is disposed, and each of said members having a petticoat portion of substantially bowl shape, said petticoat portions all converging in the same direction.

17. In an insulator, the combination with a plurality of conducting members adapted to be of different potential, of a plurality of sections of insulating material having body portions conforming substantially to the electrostatic field between said members and having petticoat portions integral therewith, each of said sections varying abruptly and materially in cross-sectional area at the junction of the corresponding body portion and petticoat portion whereby fractures of the latter portion are substantially prevented from affecting the former portion.

18. In an insulator, the combination with a plurality of conducting members adapted to be of different potential, of a plurality of members haying body portions which conform substantially to the flow lines of an electrostatic field having a single potential gradient between said members and having petticoat portions whereby particles tending to collect on said body portions are attracted toward, and collected upon, the surfaces of said petticoat portions.

19. An insulator for disposition between two zones of different potential comprising a plurality of juxtaposed insulating members and means for so shaping the electrostatic field between said zones that the lines therein coincide substantially with the mechanical stresses in said members.

20. The combination with two spaced members of different potential and of such shape as to cause the dielectric field there between to assume a predetermined form, of a plurality of insulating members, certain of which have body portions conforming substantially to said field of predetermined form and certain of which have petticoat portions projecting into said field in such directions as to offer substantially minimum disturbance to said field.

In testimony whereof, we have hereunto subscribed our names this 31st day of Jan., 1918.

CHARLES LE G. FORTESCUE.

GEORGE I. GILCHREST.