US2264428A - Article of ceramic or vitreous material - Google Patents

Description

Dec. 2, 1941.

A. o. AusTlN ARTICLE OF CERAMIC OR VITREOUS MATERIAL V Original Filed June 22,l 1955 2 Sheets-Sheet l 2, mvENToR Dec- 2, 1941- A. 0. AUSTIN ARTICLE OF' CERAMIC OR VITREOUS MATERIAL:

Original Filed June 22, 1955 2 Sheets-Sheet 2 IBB Patented Dec. 2, 1941 ARTICLE OF CERAMIC OR VITREOUS MATERIAL Arthur O. Austin, near Barberton, Ohio Original application June 22, 1935, Serial No.

Divided and this application October 26, 1939, Serial No. 301,428

8 Claims.

My invention relates to improvements in articles of ceramic or vitreous material. One object of the invention is to eliminate dielectric breakdowns due to internal faults, checks or flaws in porcelain or other vitreous insulators.

Another object is to eliminate the dielectric weakness due to internal voids formed in the casting process when applied to a dielectric article.

Another object is to provide insulators with improved electrical characteristics for operation under fog or severe line conditions.

Another object of the invention is to reduce the amount of material required in the construction of insulators without impairing their electrical properties.

Other objects and advantages will appear from the following description.

The invention is exemplified by the combination and arrangement of parts shown in the accompanying drawings and described in the following specication, and it is more particularly pointed out in the appended claims.

This application is a division of my prior application Serial No. 301,450, filed October 26, 1939, as a substitute for my application Serial No. 27,897, iiled June 22, 1935. Serial No. 301,450 has been granted as Patent No. 2,224,853.

In the drawings:

Fig. 1 is a fragmentary, vertical, sectional View of a kiln or furnace for iiring or burning ceramic materials.

Fig. 2 is a part section, part'elevation of an insulator.

Fig. 3 is a section of an insulator showing an internal fault.

Fig. 4 is a part section, part elevation of a high voltage bushing.

Fig. 5 is a section of an insulator containing a space between the solid dielectric.

Fig. 6 is a part section, part elevation of the dielectric member for a high voltage bushing.

Fig. 7 is a part section, part elevation of a multi-part bushing.

Fig. 8 is a section through a sleeve or baille for a bushing.

Fig. 9 is a section through a sleeve or baille for a bushing.

Fig. 10 is a section through a sleeve or baffle for a bushing.

Fig. l1 is a part section, part elevation of a suspension or strain insulator.

Fig. 12 is a part section, part elevation of a pin insulator.

Fig. 13 is a part section, part elevation of a high voltage condenser.

Fig. 14 is a part section, spark plug.

Fig. 15 is a part section, pin type insulator.

Fig. 16 is a part section, radio tower insulator.

Fig. 17 is a section of a hollow brick or tile.

The invention has several distinct advantages which apply particularly to insulators or vitreous materials. Parts of the process may be applied very effectively to other materials such as tile, brick, enamels, and other ceramic materials. The method is particularly valuable as it eliminates losses from internal defects in high voltage insulators and permits the construction of designs having greatly improved results not possible with the methods now in use.

The advantages of the method will be more readily seen by first discussing several of the figures.

Fig. 2 shows the porcelain for a bus insulator. The dielectric body I 0 has recesses Il for the insertion of metal fittings used for supporting the bus insulator and the bus or conductor. In the casting operation slip is poured into the molds or forms in the usual way. As the water is absorbed by the molds or forms a layer of solid material builds up. However, since the water increases the volume occupied by the solid material a void or space I3 and I4 will remain after the piece is formed.

The size of the pocket or space I3 can be part elevation of a part elevation of a part elevation of a readily regulated by the amount of water in the slip or by draining out excess slip after the wall reaches the desired thickness. The hole I2 through which the excess slip is drained is easily sealed by a small amount of slip which is allowed to remain, by glaze, or by inserting a plug which will form a tight seal during the ring opera tion. Most insulator sections are such that a porous center or core will be obtained in the casting operation. This is caused by the shrinkage in volume of the entrapped slip due to the absorption of water. A large shrinkage in the volume of the entrapped slip may permit the walls to collapse due to atmospheric pressure unless the casting operation is carried out in a partial vacuum or at an increased temperature.

In the casting process there is a tendency for the line material to form a dense outer layer or shell with the coarser particles toward the center. This tends to limit the casting operation for insulators, but with my improved process it can be used to advantage. One method of preventing the collapse of the walls is to start the casting operation with slip having nely ground material and then finishing the operation by feeding in material which has coarser particles. The slip having the coarser particles, although containing considerable water, will have reduced shrinkage. This reduced shrinkage on the part of the material in the center of the piece will support the outer Walls and prevent collapse of the piece during the forming or drying operations.

The Wall thickness at different points may be controlled by the porosity or absorption of the adjacent surface of the mold, and by pressure or vacuum. The collapse of the walls of cast pieces due to the reduction in volume of the entrapped slip is easily prevented by casting and hardening in a partial vacuum. This eliminates the necessity of venting the piece. Since the pockets or porous centers of insulators do not constitute a Weakness withmy process, the casting method may be used to advantage for reasons which will appear later.

In firing or burning most porcelain insulators a temperature of l300 to 1500 C. is reached. At this temperature the gas is greatly expanded in accordance with well known laws. InV porcelain or other body compositions used for insulators it is necessary to produce a dense body in order that the insulators will have the necessary dielectric' strength. The density' is Yproduced by a shrinkage of the body composition during the firing operation. The fusion and melting of the materials making up the body composition is such that the pore spaces are largely eliminated or sealed. Since the sealing of the pore spaces takes place at or near the highest temperatures reached in the firing operation, it is evident that the density of gas in any pore space or in any pocket formed by the walls of the material making up the insulator will be determined by the temperature at the time the pore spaces are sealed.

Since the density of air or gas varies inversely as the absolute temperature and directly as the pressure, the density of the air or gas in pores orpocketswhich are sealed at high temperatures can be readily determined for other temperatures. The value of controlling the density of the air or gas in the pore spaces or in pockets formed by the dielectric is readily seen by the following. A temperature of 1350 C. corresponds to a temperature of 1623* C. above absolute zero. A normal operating'temperature of 20 C. corresponds to a temperature of 293 C. aboveabsolute Zero. Since the densityA will be inversely' proportional to the absolute temperature Where the pressure remains the same, the density of the air or gas in the pore spaces at 13.50 C. compared tol that at 20 C. will be 29%623 or 18.2%.

Gas or air at normal temperature and atmospheric' pressure of approximately 14.7# per square inch has a dielectric strength of approximately 30,000 volts per centimeter. Since the dielectric strength is proportional to the density a reduction ofthe density to 18.21% will reduce the dielectric strength accordingly or to 5,460 volts per centimeter. This reduced dielectric strength forthe included air or gas generally constitutes a dielectric weakness. This dielectric weakness may cause the insulator tol fail on dielectric test, under operating conditions, or constitute a limitation-upon the manufacturing processes and design of insulators. With my improved method this weakness is eliminated by controlling the density of air or gas in the furnace before the pore spaces or walls of the insulator are sealed so that the desired density will be obtained in the pore space or in sealed pockets for normal operating temperatures.

In order to obtain a density in the gas or air corresponding to normal air density at 14.7# it will be necessary to seal the'pores with an absolute pressure of approximately 81#, or a gauge pressure of 66.3#- By closing the pore spaces at 81stL absolute pressure the dielectric strength of the spaces in the ware at 20 C. will be increased from 5,460 volts per centimeter to 30,000 volts per centimeter.

Since porcelain or other ceramic materials used for insulators have good mechanical strength it is evident that insulators may be so designed and fired that the pore spaces and air or gas pockets may be used to advantage in increasing the dielectric strength of the insulator and in improving the electrical performance particularlyv for high voltages. To increase the density of the air or gas in the pore spaces to ten times that corresponding to a pressure of 14.7# per square inch and a temperature of 20 C. will require an absolute-pressure in the kiln at the sealing temperature ofl 810# absolute. By maturing the insulators at this pressure and regulating or controlling the pressure in the kiln in accordance with the absolute temperature the insulators will becooled without subjecting them to stress due to anunbalanced pressure on the walls forming the pockets. As ceramic materials are very strong in compression an excess of pressure over that called for by the absolute temperature usually may be applied to advantage while the ware is cooling. Y

Due to improved characteristics the maturing of the ware at increased pressure is of considerable economic importance. Insulator designs may be readily made in which an air density many times normal may be used without exceeding safe working limits for the material. Increasing the density of the rair or gas in the pore spaces and sealed pockets to ten times normal will increase the dielectric strength from 30 kv.

per centimeter to-300 kv. per centimeter; Since most ceramic dielectrics such as porcelain -or glass will fail at values usually vless than kv.

per centimeter, it is readily seen that the use of pockets or pore spaceV under increased density of the air or gas can be used to increase the dielectric strength and improve the reliability. While this in itself is a very great advantage, there are other advantages which may be even more important. Y

With my improved method it is possibleV to improve the dielectric strength by properly located pockets and Ypore spaces so as to reduce the strain in the solid dielectric at points of. high flux density. In addition the location of pockets makes it possible to substitute air or gas spaces for the solid dielectric s o as to control the breakdown of free air adjacent to electrodes or metal parts attached to the insulators. p

One advantage in increasing the densityl of the air or gas in the pore spaces or pockets is seen from a discussion of the following.

When the insulator is placed under dielectric stress the air may become ionized and act as a conductor. This is more readily seen by reference to Fig. 3. In Fig. 3 electrodes I5 and I6 are placed on opposed sides of a dielectric member I1 which has an internal fault I8, and which has been fired inthe usual way. Dielectric members sufch as porcelain or glass have much higher specific inductive capacity than air, therefore the resistance to the passage of electrostatic fiux through the dielectric is much less than through air. This concentrates the stress upon the fault I8. Since the density of the air or gas in the fault I 8 is relatively very low compared to normal atmospheric density, the air is readily ionized and acts as a conductor. The discharge in the fault is somewhat in the nature of that of a dischargey in a Geisler tube or in a tube at reduced air density.

Under alternating current a discharge tends to flow under each alternation. The heat generated by discharges in similar defects is responsible for the destruction of some insulators particularly where the insulator is subjected to stress at increased voltage and frequency. A further disadvantage is that the benefit due to the wide separation of the electrodes I and I6 is lost due to the fact that the capacitance is mtaerially increased due to the flow of current in the fault I. This tends to start a streamer or discharge I9 from the electrode I5 at a much lower voltage particularly when the member I5 is positive or allows the piece to fail by puncture.

The shunting effect of streamers which is materially increased due to an increase in the charging current as in Fig. 3 is very effective in causing puncture or in reducing the flashover voltage of bushings and other types of insulators particularly where the Working stresses are high or Where the insulators are working under fog conditions.

The air spaces or porous cores, which generally constitute a limitation for the casting method as applied to insulators, can be used to advantage by applying my improved method of firing. This will be more readily seen by reference to Fig. 5. In Fig. 5 dielectric members 20 and 2| are separated by an air space 22. Electrodes 23 and 24 are used to apply voltage. Neglecting fringe effects the dielectric fieldsor lines of force passing through the dielectric between the two electrodes will be proportional to the voltage and the specific inductive capacity of the dielectric, and inversely proportional to the thickness of the dielectric or separation between electrodes. Porcelain has a specic inductive capacity of approximately six times that of air. It is therefore seen that the inclusion of an air space 22 will greatly reduce the flux passing through the dielectric. This reduction in dielectric ilux makes it possible to greatly increase the voltage before shunting streamers 25 will start over the surface. However, since the stress upon the dielectrics in series will be inversely proportional to their specific inductive capacities and directly as to their thickness, it is readily seen that the air space will carry a large part of the electrical stress or strain. It is therefore necessary that the dielectric strength of this air space be sufficient so that it will not break down under the applied voltage.

With one-third of the distance between the electrodes lled with air the relative capacitance will be reduced from 2 to 0.75. This will result in a reduction in the electrostatic flux of over 73% and permit of a mu-ch higher applied voltage before shunting streamers will start from the electrodes. However, it is evident that since the relative stress carried by the dielectrics will be proportional to the thickness and inversely proportional to the electrostatic capacity, the air gap will carry approximately three times as much stress as the solid dielectric. It is therefore evident that unless the dielectric strength of the air or gas space is increased, the substitution of air or gas for the dielectric will produce a dielectric Weakness which may cause failure. While it is readily seen that the inclusion of the air space has very great economic possibilities when applied to the design of insulators, it is necessary to provide sufficient dielectric strength in this air space. This dielectric strength is readily obtained by firing the insulator under an increased pressure at the time the walls surrounding the air space or pockets become impervious or are sealed.

Fig. 4 shows the principle applied to a high voltage bushing. In the bushing the dielectric member 26 has an outer shell and an inner tubular member enclosing a pocket 21. The bushing is equipped with metal mounting ange 29 of the usual type and a conductor 29. The design is such that it may be readily formed by a casting method. The pocket 21 provides an effective space of low flux carrying capacity. This reduces the electrical stress upon the solid dielectric members and the tendency for streamers to flow over the outer surface is greatly reduced. This makes it possible to raise the arcing or operating voltage of the insulator. The pocket is particularly effective in reducing the shunting streamers or corona over the surface below the flange, which tends to cause radio interference. The inclusion of the pocket therefore makes it possible to operate a bushing of a given size or diameter at a higher voltage.

While in many cases the pockets containing air or gas are readily formed in the casting process and constitute a limitation with the present methods, there are cases Where it may be desirable to form a part or all of the pocket or porous space by means of cores which may be made up in various ways. These cores may be made of absorbing material such as paper pulp, or of material which will soften during the forming operation and permit shrinkage of the walls without causing defects. The cores may be made of wax, a combination of ceramic materials mixed with organic materials such as wood, wood fibre, cork dust, meal, powdered waX, or various materials depending upon the results desired.

In Fig. 4 it may be desired to form a portion of the space very carefully, which is accomplished by using a core. This core is located by a pin 28. By making this pin of material similar to that used in the body composition and coating it with a glaze and impregnating it with wax it will support the core during the forming operation. During the ring operation it will fuse to the wall and form a tight seal. In other words the pins 28 act as chaplets in supporting the cores as in ordinary foundry practice. The portion projecting outwardly from the wall is readily removed by chipping or grinding.

In some cases pre-formed ceramic inserts or cores are placed in the piece during the forming operation, in which case means for locating the core may be omitted. In other cases small locating pins or needles can be used which are readily withdrawn during or after the piece is formed and the holes filled with glaze or slip which will form a tight seal during the firing operation.

In order to accomplish the desired results a special form of kiln or furnace shown in Fig. 1 has been devised. This kiln has a metal jacket 30 and heads 3l and 32. These are so joined by bolts and other means that pressure may be controlled within the interior at all times. The

chamber is lined withsuitable refractoryl material 33. The refractory lining makes it possible to develop high temperatures in the interior and cut down the ylo'ss due to radiation. Various typesy of construction may be used depending upon the natureof the ware and pressures desired or part of the process to be carried out. A duct 34 is connected to the jacket of they kiln at suitable points. This duct is lined with refractory material 35 so-as to reduce radiation and protect the metal.

In one method of operation air is forced in through a metal tube 36 which preferably is placed in the duct 34. A fuel such as gas is allowed to enter through the member 31. The gases ofl combustion are allowed to escape through the member 38 by regulating the valve 39. The incoming `air and gases are'regulated by valves connected to suitable sources of supply.

The arrangement permits of a circulation of the gases of combustion. The gaseswhich escape through the member 38 pre-heat thev incoming air which enters through the member 36, thereby conserving some of the heat. Porcelain bodies are made of fine materials and are relatively very dense so that gases enter and escape rather slowly unless there is some change in pressure. However, the pore space usually `is from to 30% of the volume, consequently if the pressure is made to fluctuate-it is possible to produce a material breathing action. This has the advantage in that combined and free vcarbon in the body composition can be readily oxidized or removed as against normal firing conditions where there is little or no fluctuation in pressure. This fluctuation in pressure can be accomplished by raising the sealing in order to provide suflicient dielectric strength in the pockets or' spaces in dielectrics it is generally advisable to develop the highest temperature by electric heat. InFig. l this is accomplished by closing the valves 39, 4i), and 4i. The resistance elements 42 and 43 are then heated by suitable transformers 44 and 45. Connections are made to the heating elements through suitable leads 46 which are insulated from the jacket and form a tight seal so that pressure may be maintained. With this arrangement the desired pressure may be maintained by opening Valve 41 which is connected toa suitable source of pressure. If desired special gas from the container 48 may be allowed to produce the necessary pressure;

The pressure may be `iluctuated in order to produce a breathing action up until the time the sealing of the pore spaces in the body takes place. The rapidity with which the pressure may be changed'will depend upon the density of the body composition and the amount of gas or air which must pass through the walls in order to equalize the pressure without damage to rthe articles. However, it is evident that rby causing the pressure to fluctuate a breathing action may be set up which may be used to advantage. This breathingaction may be used toy change the natureof A relatively slight changein theatmosphereiin the pockets or pore spaces, and.

consume combustible material in the body or pockets.

During the firing operation both free and combined water may be given off from the ware. This water vapor may constitute an appreciable part of the gas'in the pockets orpore spaces at the time thevitrication or sealing ofthe pores takes place. This may not constitute any serious difficulty where the density of the gas and the amount of Water vapor in the pocket is sufficiently low so that condensation will not take place in the pores or on the Walls of the pocket forv the normal operating temperature. The presence of water vapor may be serious in other cases, however. It is evidentV that where the density of the atmosphere in the pocketis increased condensation of Water vapor may take'place on the inner wall under normal working temperatures. While this condensed moisture may be used in certain cases to generate heat, the presence of the moisture may constitute a'serious disadvantage where high dielectric strength and the control of the electrical eld is desired. Water has a very high flux carrying capacity compared to dielectrics such as porcelain or glass and to air or gastherefore aflm of moisture condensed upon the surfaces may produce a material change in the electrostatic eld. However, with my improved process Vit is possible to change the atmosphere in the pore spaces or pockets by fluctuating the pressure in the ring chamber before the sealing action takes place. After the free and combined water has been released from the material making up the structure of the insulators the atmos-V phere surrounding the objects may be changed and the pressure fluctuated soy as to cause a breathing action and the scavenging of the entrapped vapor.

Where it is desired to ll thepockets or pore spaces with a specialgas the pore spaces, pockets and the atmosphere in the kiln may be exhausted or operated at a materially reduced pressure, after which the special gas such as nitrogen, helium, carbon dioxide,v or mixtures may beadmitted. The fluctuating pressure therefore may be used not only to control the chemical action taking place within the body but to control the nature of the atmosphere within the pockets or pore spaces.

Where the ware being fired contains considerable carbon or combustible material either in the body or in the cores or spaces the gases of combustion may form suchV a` large proportion of the included. gas that scavenging is not necessary. In this case the percentage of oxygen may be increased in the furnace atmosphere so that the oxidation can be accomplished without changing the atmosphere in the kiln, It is evident that the methodY maybe used to generate heat in articles being burned. This allows heat to be developed where most needed. By incorporating or including combustible material in the articles being fired it is possible to reduce the time of burning and to obtain greater uniformity, as well as other advantages.

Where it is desired toV produce a rapid change in the atmosphere the lin-e 60 is connected to an exhauster or vacuum pump. The temperature is maintained in the kiln and the rate of exhaustion isl such that the ware will not be damaged by a sudden change in temperature or pressure.

While high pressures'have the advantage in vthat there will be less tendency for the forming of bubbles or pore space in the glaze or body of the dielectric, there is a possibility that a high vacuum may be used in the sealing process where the dielectrics are such that they will not be damaged. Materials which have received the biscuit fire or which have been previously fired or formed generally can be raised to high temperatures in a much shorter time so that the material may be subjected to a high vacuum at the high sealing temperature without detrimental effects. Dielectrics such as porcelain mad-e up of several different parts may be fused together in this way and pockets sealed by glaze or fusible material. Glass insulators having pockets may be sealed by using a plug of fusible glass or glaze to seal an opening at the high temperature. Where the material is such that the time and high vacuum will not be detrimental, the density in the kiln may be lowered by connecting to a suitable exhauster. Where the materials are such that a very high vacuum can be reached during the sealing process, suflcient dielectric strength may be obtained in the spaces. The high temperature in the kiln or furnace is used to expand the gas, which greatly facilitates the obtaining of the high vacuum in the pockets or pore spaces in the material.

Owing to the diiculty of providing a uniform distribution of heat at a high vacuum and the difficulty of obtaining a sufficiently high vacuum so as to develop the necessary dielectric strength in the pocket, pressure generally is preferable to a high vacuum at least for the early stages of heating or curing.

In order to reduce heat losses due to the circulation of gases in the duct 34 the sand 49 is allowed to iiow into the passage and shut oiT the circulation. With this arrangement it is seen that the temperature and pressure both can be controlled and the preliminary heating accomplished by combustion. A pyrometer 50 indicates the temperature and a window I also may be used with an optical pyrometer or for inspection. A pressure gauge 52 is used to indicate the pressure within the chamber.

Several different pieces of ware illustrating the application of the method are shown such as the bushing part 53. A bushing 54 which has been formed in two parts but which is being forced together by a weight 55 during the firing process is shown located on the same refractory shelf. 56 shows a large sleeve or baille used for bushings. 51 shows a suspension insulator disc, and 58 a pin type. The ware is placed on the shelves or in saggers while the head 32 is disconnected from the shell and in lowered position. The head with ware and supporting refractories is raised by the hydraulic ram 32 and the head 32 secured to the case 30 so as to form a tight seal.

The improved method of manufacture has a number of important advantages when applied to insulators as will be seen by reference to the following figures:

Fig. 6 shows a high voltage bushing having a. pocket or space 6| filled with air or gas under pressure. The design is such that it may be made in two parts consisting of an outer sleeve or case 62 and an inner tubular member 63. The outer and inner members are fused together during the firing operation at 64 and B5, or to the shoulder at 65', so as to make a sealed pocket 6l. The hol-e 66 through the inner member allows for the passage of the lead, and a suitable flange may be placed around the straight portion or shank 61 of the outer sleeve.

The low ux carrying capacity of the space 6| reduces the stress on the solid dielectric members 66 and 62. In addition the corona point is materially raised, thereby increasing the working range or voltage of the bushing. The outer sleeve or case 62 is readily formed by casting, throwing, turning, or by any other suitable method. The inner member 63 may be formed by casting, throwing, or by extruding in the form of a tube, after which the tube may be turned or shaped as desired. Pieces of ware formed by the casting method generally have a higher ring shrinkage than pieces formed by the usual wet process. It is therefore easy to form a tight joint at the points 64 and 65 during the firing operation where the pieces have been previously glazed or iluxed over these surfaces. The greater shrinkage of the outer cast member causes it to shrink upon the inner so that the glazed joint will form an air-tight seal. Where shrinkage cannot be depended upon to produce a tight joint butt or tapered joints can be used to insure a tight seal. As the pieces are soft and readily distorted at the maximum temperature without producing a fault, considerable latitude is possible in making a tight joint even where the outer member has a much higher ring shrinkage than the inner member.

The gas tight pocket formed by glazing or fusing is of much value for some insulators as a relatively high air or gas pressure may be provided in the pocket so as to insure a high dielectric strength. The methodhas the advantage in that all organic insulation such as paper, varnished cambric and oil may be eliminated in the bushing and at the same time a bushing of high dielectric strength and rating for a given size can be produced.

Fig. 7 shows a multi-part high voltage bushing of the oil-lled type in which the dielectric members are made by the improved process. The bushing consists of an upper cone 6B with weather sheds and a lower sleeve 69 with an intermediate insulating baille system 1|). The bushing is provided with a tubular conductor 1 I, an expansion chamber 12, and a mounting flange 13 in accordance with the usual practice. 'I'he lower sleeve 69 is provided with a pocket 14. This pocket reduces the dielectric flux and the tendency for streamers to start from the flange 13 over the surface of 69. By providing a sullicient density of the air or gas in the pocket 14 the dielectric strength is materially increased. The pockets may be large or small depending upon the results desired.

In bushings it is common practice to wrap the conductor with paper, varnished cambric or other insulating material which has a lower flux carrying capacity than the porcelain. The use of a dielectric material which has a lower flux carrying capacity reduces the stresses, tending`I to cause streamers to form over the outside of the bushing. However, paper and varnished cambric are subject to deterioration and it is advisable to replace them by insulating members which have the necessary dielectric strength but which at the same time have a low flux carrying capacitance.

The member 16 is composed of several parts which include annular pockets. The several members are so made that they can be formed into one piece by fusing during the firing process.

By vitrifying ata sufficiently high pressure in the kiln it is possible to provide a high dielectric strength Vfor the air or gas in the spaces between'the walls. Itis necessary to provide good dielectric strength, otherwise the air or gas in the pockets will become ionized and break down under the electrical stress. The space between separate insulating systems is filled with oill or other suitable insulating liquid or compound.

The upper cone is provided with an annular groove or pocket 16 to allow the insertion of a metal ring projecting from the flangg'IS. This tends to screen the electrostatic field, tending to cause a Adischarge over the surface starting from thel` flange. The arrangement also tends to reduce the stress upon the thin section of the insulating member 69. The several parts are held together by springs acting through the conductor |I. The several springs preferably are used `with some air space between. This reduces the magnetic flux set upv by the current and allows a jumper '|8 to make contact between the-tubular conductor or current carrying systern and the walls of the expansion chamber. If desired the helical spring members 19 can be used for carrying all or part of the current. The members 'I9 are forced into lan annular groove formed by a rib or wall 80 of the expansion chamber and the conductor 1|. 1

It is evident that a wide range` of insulating members or baliles may be used to advantage with my improved method. Fig. 8 shows one form of baille in which the spaces 8| and 82 are provided. The longitudinal ribs 83 and 84 preferably are staggered so that the electrostatic capacity will be reduced between the inner surface 85 and the outer surfacell. Members of this kind'may be formed by the extruding process and the openings 8| and 82 closed while the material is` soft, or by any other convenient method such as fusing or slipping.

Fig. 9 shows a different form of baille in which a simple tubular member 8l is provided with annular spaces 88, the spacing flanges insuring the proper location of the tubular member `89 with respect to the central member 81. The inside diameter of the member 9i! is less at the ends so that the space 9| will be properly located.

The whole system may be formed by the usual methods used in forming ceramic wares and fused together in the kiln. By using body compositions for the outer members which have slightly higher shrinkages than the inner members it is not necessary to provide very close fits as the clearance will be taken up during the firing process. The glaze on the pieces will provide for any small irregularitiesv and insure gastight pockets. f

Fig. 10 shows a cross section of an insulating sleeve or baffle ofy somewhat different construction. A gas-tight pocket is formed by the outer sleeve 92 and the inner sleeve 93 at the ends of the outer member 92. Dielectric spacing members'94 are used to maintain the proper relation between the inner and outer members, and to provide additional mechanical strength. The members 94 preferably are of light construction so that the amount of solid dielectric between the two main members will be relatively small. By glazing the surfaces of the inner and outer members or the spacer members 94 it is possible to fuse several members together during the firing operation. -In placev of tubular spacing jmembers A94 other shapes of course may be usedA If'desired the space may-be filledwith fused or loosely packed dielectric pieces such as porcelain sand or other material. The amount of solid dielectric may be reduced by using a filling, part of which will burn out during the ring operation. Glass may be used for some of the baffles or insulating members, the pockets being sealed while the density and temperature is controlled.

Fig. 11 shows a suspension or strain insulator produced, by the improved method. The insulator is composed of a dielectric member 95 having a head 96 for attaching'a cap Sl. The pin 98 is inserted in a suitable recess inthe head of the insulator. -The pin transmits stress to the lhead by means of cement or thru resilient members B9. The flange of the insulator is provided with an Yannular pocket |00 near the axis of the insulator,

and an outer annular pocket |0l. YA small annular pocket V|02 is provided in the head of the insulator to increase the dielectric strength at this point. The large outer pocket |90 reduces the stress tending to cause streamers to flow from the edge of the cap and pin over the surface. The wide separation between the upper and lower surface of theinsulating iiange with a pocket giving low capacitance reduces the current in charging streamers starting over the surface and raises the corona point. Since streamers have a very high negative coeflicient of resistance a reduction of the current in these streamers materially reduces the shunting effect. The low stress allows an increase in the Voltage at which corona will start, and the low capacitance between the two faces materially increases the arc-over voltage for the insulator. The improved performance is of great economic value as it increases the effective arcing voltage andtime lag for a given length or size of insulator. In addition to increasing the length efficiency of an insulator string composed of one or more sections, the reduction in surface charging current improves the performance of the insulator for fog conditions or for conditions where the insulator is effected by accumulations of dirt or conducting material.

As the insulators made by the improved process may be used for very severe conditions where leakage current is relatively high, special means are provided so that the cap 91 and pin 98 will not be damaged by the leakage current. Spiral guard rings |03 and |04 are placed in contact between the cap and upper face of the insulator, and between the pin and lower face of the insulator. lThese guard rings are in direct contact with the cap and pin so that any discharge or leakage streamer starts from the outer edge of the guard members rather than from the main members. By making the replaceable guard members of bronze or other suitable material corrosion will have little serious effect upon the insulator. Byusing these guard members it is possible to use steel caps and pins or malleable castings. Without the guard rings .the heavy leakage current may cause the removal of the zinc, after which the metal parts will rust and shorten the life ofthe insulator. The improved construction makes itV possible to operate the insulators under much more severe conditions than is possible with the ordinary types which have a high relative capacitance between'the rtwo faces particularly when the faces are wet by fog or have conducting deposits. The wide separation betweenthe twoy faces without including large masses of 'solid dielectric reduces the capacitance and the shuntin'g eiectof the streamers.` The thinner walls reduce the absorption of heat, which reduces the condensation during fogs or periods of high humidity. This is of great advantage over that where heavy solid parts are used.

Since the process makes it possible to design and manufacture insulators of greatly improved electrical characteristics, these can be used to improve the insulation of suspension insulator strings so as to provide more insulation for a given length of insulator. The improvement in the insulation is particularly valuable where the improved insulator can be used to reduce the length of the string and clearance in the tower. By increasing the effective fiashover and corona point for the insulators subjected to the greatest electrical stress it is possible to materially increase the operating voltage for the string without producing over-stress or radio interference. The method can be used to increase the ratio of flashover voltage to the voltage duty for the several members making up an insulator so as to improve the string efficiency. An improved stress distribution or gradient in the string may be used together with the improved flashover voltage for some of the members to further increase the efficiency. The process makes it possible to increase greatly or decrease the capacitance through the head of the insulators which in turn can be used to improve the gradient or stress upon the several members.

The methods which are used for the formation of the insulator in Fig. 13 may be used for the forming of suspension members as shown in Fig. 1l. In this case the head portion is formed with all or part of a material which has a greater flux carrying capacity. Such compositions are readily made which include av large portion of titanium oxide.

The dielectric strength through the head of the insulator should bear some relation to the flashover voltage of the disc or member. Since the invention permits of the design and manufacture of insulators having higher effective flashover voltages, it will'be seen that it may be advisable to increase the dielectric strength or puncture voltage for the heads of insulators which are under maximum stress. This may be accomplished by the use of glands or pockets at the points of greatest stress such as that in the corner of the pin hole. By using a suitable pocket |02 the flux tending to break down the inner corner of the pin hole is materially reduced. This construction makes it possible to reduce the size of the head or increase the dielectric strength and thereby improve the design.

Various methods are available and a wide range in design may be used depending upon the results desired. The pockets in the insulator may be entirely filled with loose material such as that in By partially forming the piece and then placing cores or inserts in position the material may be formed over same so as to provide an enclosed pocket. As previously explained this material may be melted out if in the form of wax, or burned out during the firing operation. In some cases freshly formed cast or formed inserts having a composition similar to that in the main body may be used. This form of insert is particularly applicable to cast ware, a modified method being used in the production of sanitary ware.

Fig. 12 shows a multi-part pin insulator of improved properties manufactured by the improved process. The insulator consists of a top |06 and a shell or center |01. The top has a pocket |08. The pieces are so formed that they will be fused together at |09 during the firing operation. The shape Aof the pieces is also such that an additional pocket ||0 is provided between the two parts. It is evident that the pockets will greatly reduce the electrostatic flux passing between the conductor which fits in the upper groove and the tie wire groove |2, and the pin which fits into the pin hole ||3. This reduction in stress will materially increase the voltage at which the insulator may be operated without producing streamers or radio interference. Since there is a wide separation between the two surfaces with the intervening space, the tendency for streamers to form over the surface will be greatly reduced. This is accomplished without producing excessively thick or heavy parts with their high thermal stress. By properly designing the insulator the pockets may be of such form that considerable resiliency and relief will be provided for diiferent rates of expansion in the outer and inner portions of the insulator. In addition the electrical properties of the insulator will be materially improved. The pockets in the insulating members may be applied to any oneorall of the various members as desired. The method is particularly applicable to large multi-part insulators and to certain types of post insulators.

.Due to the low capacitance and the negligible dielectric loss in the pockets, insulators using the pockets can be used at much higher frequencies and voltage Without serious heating. When used at high frequencies the insulators are designed so that the pockets carry a large part of the dielectric stress so that the hysteretic losses in the solid dielectric will be reduced to a small figure and therefore constitute a small loss.

In Fig. 12 the top member may be readily formed by a casting process, and the center or shellby any of the usual wet process methods of forming. Since the firing shrinkage of the cast piece is normally much higher than that for the center formed by the usual wet process method, the top will shrink onto the center and form a good bond between the two. While fused insulators have very desirable electrical properties due to the elimination of cement or conducting material, they frequently cause trouble due to dielectric breakdown.

When red in the ordinary way pockets formed in the glazed joint between the two pieces have low air density and dielectric strength for reasons previously explained. However, with my improved process this weakness is eliminated and pockets or spaces in the joint may be used to advantage to obtain dielectric strength and to avoid dunting.

Fig. 13 shows an electrostatic condenser having a head IM and a flange ||5 with a pocket IIS. With my improved process it is possible to make use of large and difficult shapes formed very readily by the casting process. With my improved process it is not necessary to match the curing temperatures or temperature of vitrication very carefully where it is possible to maintain or control the density in the pore space within the dielectric member subject to electrical stress. In Fig. 13 the head of the condenser is formed with an outer and inner wall of dense material such as a normal insulator porcelain. The intervening space lll is filled with a composition which has a high specic inlductive capacity suchrv as a composition .containing a large percentage of titaniumoxida Where the walls are rather thin which surround the core the capacitance of the condenser may l:be greatly'increased by the, use of suitable core i Vmaterial even though vit is porous. By matching compositions which have proper `maturing temperatures the entire head may be made up of a -material having high specific inductive capacity as well-as the flange portion H5. Frorntheprevious explanation it is apparent that a high specific inductive capacity in the weather sheds is a vdecided disadvantage. However, the inclusionof the pockets such as H6 reducesV the eiect of .the yhigh capacitance and the process therefore difficult to form and costs several tim-es that of `othercompositions used for insulators', it is vdesirable to confine the material having high specic inductive capacity to the head H4 or yportion which will increase the capacitance of the. condenser or insulator, and to use ordinary porcelain or ceramic material having the usual characteristics for the remaining portion H5.

It is comparatively easy to-form a condenser insulator having a core by removingv the slip .when the desired wall thickness is obtained and replacing same with the special composition. Another method is to form the outer layer ,of dense material in any of the usualforming processesus-ed iny the manufacture of wet processinsulators after which the layer of special material having high Ilux capacity-maybe formed inplace, or if desired the whole head and flange H5 may be formed of the material having the high flux carrying capacitance, or the flange H5 with pocket may be formed with regular material of lower specific inductive capacity joined to the head composed entirely of high capacitance material. The pocket may `be formed rby using a suitable insert or by any one of several methods.

It is evident that the general construction shown in Fig. 1-3may be used-for thesus-pension insulator of Fig. 1.1 so as toincrease-the capacitance of the` insulator without reducing the flash- ,over voltage or corona point. The improved gradient will make it possible to increase the string eiliciency. With the'improvedprocess the head and core may have considerable pore space without aiecting the dielectric strength. The high capacitance and good dielectric strength therefore may be produced. without'necessarily l forming an entirely densefmass. Itis evident that the elimination of discharge Iinv the pore spaces will decrease the losses at-highfrequency and therefore materially reduce the power factor.

Fig. 14 shows a spark plug, the dielectric member of which is formed by the' improved process. The improved process makes it possible toA provide a spark plug having greatly improved characteristics. Fig. 14 showsV only one of a: number of different forms toY which the process'fmayfbe -applied for obtaining an improved result. In Fig. 14 the dielectric |2|) having a pocket |2|lis sealed against the shoulder of the vgland |22 Aby means of a gasket |23. The upper-portionvofi-the nut has an annular ring |24 which' is crimped against a resilient member |25. The pressure exerted insures a tight contact between theporcelain and the gasket |23 at all times. Theuse ofthe resilient member |25 makes it possible to takefup differential expansion and' alsolimits `vthe stress lwhich*maybeplaced upon the dielectric member.

In the operation `of. sparkr plugs it is advisable to maintain a vhigh insulation both inside and outside in order to withstand the high voltage impressed upon the plug. 'Ihe high frequency or oscillating potential which is placed upon the plug not only requires high dielectric strength in the. plug but tends to cause the flashover of leither lthe inner or louter end. The flashover of the surfaces is aggravated due to accumulations of moisture or dirt. The outer end is also Vlikely to flashover if high voltages are applied at high altitudes where the air density is low. The high pressureon-the inside of the cylinder ktends to prevent discharge over the inner surface between the ring points starts at a lower value and is less effective. However, by using the improved process to form a plug with a pocket it is possible to greatly increase the effective arcing lvoltage over the outside of the plug as well as on the inside when-the center member is negative. The reduced stress upon the surface due to the pockets and wide separation between inner and outer surfaces tends to reduce the effect of a shunting streamer and thereforethe plug may be subjected to higher effective operating voltages without discharging over the surface. This higher voltage permits of wider gap spacing so there is less danger of missing due to the wide separation of points or from oil preventing discharge.

Due to the fact that shunting streamers tend toireduce the arcing voltage over the outside of the ordinary plug, the efciency is improved by the use of small inner members. This greatly concentrates the stress upon the inner surface and owing to thesmall cross section of electrode the heat conduction is limited. Where a small member is used the arcing between the points causes a rapid burning away and lengthening of the gap so that plugs may fail after a short period of operation. However, the improved construction permits of larger internal members without producing low flashover voltage. The large members maybe used to control the conduction of heat and temperature of the insulating surfaces, and in addition provide large electrodes which will give longer life. The pocket may extend well down into the plug so that the exposed inner surface near the gasket |23 will operate at a higher temperature and therefore tendfto remain in better condition. This may be further improved bythe use of a pocket |21. It is therefore'seen that the pocket |2| not only makes it possible to obtain much better electrical characteristics for the spark plug but the temperature of the surfaces may be controlled by the use of pockets which will change the heat conduction and temperature of the adjacent surface.

The method also makes it possible toy form the porcelain or dielectric members for spark plugs by a casting method. Any porous portions or spaces within the plug will not be detrimental but may be usedto advantage with the improved process. With methods heretofore in use the porous spaces or voids within the dielectric members constitute a weakness and are therefore a limiting factor in the use of the casting process. In the casting process it is relatively easy to grade the composition throughout the Wall, therefore, it is possible to change the composition for the inner and outer portions of the spark plug similar to that which may be used for the condenser in Fig. 13. By Varying the casting composition the surface subjected to high temperature of fiuxing can be of one composition backed up by a composition and structure giving the necessary dielectric strength. With the improved process it is possible to build spark plugs which will permit a much wider range in the size and shape of the firing points |28 and |29, as Well as for increased potentials.

Fig. 15 shows a Din type insulator in which the process has been used to form an insulator having improved characteristics. The insulator may be made in a single piece by casting so as to provide a pocket |30 or the portions on the two sides of the pocket may be formed and fused together in the firing operation or while the material in the two parts is still plastic. However, the essential feature is the inclusion of the pocket between the head |3I and the pin hole |32 so as to reduce the capacitance and surface stress. The very effective reduction in the electrostatic flux due to the large pocket |30 which separates the two surfaces makes it possible to operate the insulator at a higher voltage without the formation of streamers over the surface. Insulators having these characteristics are particularly eiiicient where the frequency and voltage are high as well as for normal transmission work. The improved characteristics make the insulators valuable for carrier current circuits, for fog conditions, or where it is desired to use an insulator of minimum size. The pocket |30 may be formed in any convenient way as previously described, or if desired the pocket may be filled with relatively coarse particles which are fused together where they corne in contact so as to provide increased mechanical strength with resiliency.

The use of the pocket is particularly valuable in preventing the destruction of insulators under operating conditions. When the sun shines upon the insulator heat is readily transferred to the inner portion due to the large outer surfaces and conductivity of the solid material. In the ordinary types of insulators the inner portion becomes almost as hot as the outer portion. If a cloud obscures the sun or a fall of rain strikes the insulator while hot, the outer portion is suddenly cooled and contracts upon the inner heated and expanded portion. The stress thus set up causes the destruction of many insulators after a few years operation. By including a pocket the conduction of heat between the outer and inner prtions is greatly reduced, therefore the temperature and expansion of the inner portion is very much less than where the pockets are not used. Therefore if the outer portion is suddenly cooled the stress will be very much less so that the danger of cracking is materially reduced.

Fig. 16 shows an application of the process to an improved type of insulator used for sectionalizing or supporting radio towers, or for high tension strain insulators. The dielectric member |33 has a pocket |34. The dielectric member is cemented into the flange or supporting yoke |35. A central tension member |36 is provided with an enlarged end |31 and a threaded end |38 for attaching to tower leg or tension member. The metal weather shed |39 is provided with a washer |40 which permits of alignment. The inner tubular portion of the dielectric surrounding the bolt |36 prevents discharge between this member and the inner surface adjacent to the flange. Where the dielectric member consists of two conical portions similar to that normally used for bus insulators and bushings, a discharge may readily take place when lightning strikes the radio tower. A discharge starting from the member |36 may puncture the outer wall of the dielectric member. The heat of the discharge may cause an explosion which will result in the failure of the insulator. owever, with my improved process it is possible to provide greatly increased dielectric strength so that the danger of an internal discharge and puncture of the outer shell is removed.

Due to the great increase in dielectric strength and reduced flux it is possible to provide a much longer effective leakage path over the outside of the insulator for a given diameter. The double conical members with pockets made by the improved process have many applications such as that for spacers, for supporting center member in concentric conductors, for the insulation of bus bars, bushings, and many other items.

Portions of the process may be used to advantage in the firing of various types of ceramic materials such as tile, brick, stoneware, table ware, refractories, and sewer pipe. The application of the process to other materials will be evident from the following.

Fig. 17 shows a hollow tile or brick in the unred state. The hollow shell |38 is formed in any suitable manner, after which the combustible material |39 is included. A paper or other form |40 is used to locate and hold the end |4| in place. Any suitable method can be used which will permit a good bond between the members |40 and 4| such as slipping, fusing or casting of the head.

The small amount of material required will permit of the use of much better types of material and a great reduction in weight for handling. By using a fluctuating pressure there will be a considerable breathing action due to the space inside the brick or tile. By controlling the atmosphere fuel may be consumed on the inside of the piece if desired so as to produce a more uniform temperature throughout the mass. By this means it is possible to greatly reduce the time of firing and use higher grade material for the brick or tile. Since the chemical condition of the atmosphere in the kiln and pore space can be readily controlled, it is possible to control the color of the brick or ceramic ware much more readily than where large solid masses are burned without the fluctuating pressure.

Due to the small amount of material the heat required for burning is necessarily reduced. The member |42 may be readily burned out during the firing process and will tend to furnish heat for the burning of the material. The use of combustible material within the piece is not necessary but with the nuctuating process it can be used to advantage as there will be but very small consumption of the material until a fluctuating pressure is produced. By using an increased pressure it is possible to raise the temperature rapidly even where the material contains free Water and is quitel dense, as the bursting action due to the formation of steam Withinthe piece can bje prevented. The increasedpressure also controls the evaporation of water thereby reducing checking kdue to thermalstress.

The fluctuating pressure has material advantages "in controlling the temperature and uniformity of heat distribution in the firing of ceramic material such as table ware. By varyingr the pressure before the sealing takes place it is possible to produce either an oxidizing or reducing condition throughout the body. It is also possible to provide a more uniform composition at different-points in the kiln atmosphere insuring uniformity in color. .Where the iinishing is carried on at increased pressure the tendency for bubbles to form in the glaze/and other Adefects will .be materially reduced. Due to the greater density of the atmospheric Vconvection currents Will be more effective in transferring heat, which isamaterial advantage particularly in very large kilns. It is evident that it is comparatively easy to build kilns of fairly large size by usingl steel jackets vvhere the flucy tuating pressure is vnot very large. It is evident that continuous kilns may be used by providing suitable locking chambers at the entering and discharge ends.

The improved process has several advantages and can be used for producing many types of insulators such as bushings, link strains, X- strains, hollow post insulators, pot-heads, X-ray casings, and many other types of ceramic methods which cannot be applied at present but which have material advantages. It is evident that the control of pressure and breathing action can be used to material advantage yin the production of brick or tile as a large portion of the heat may be produced in the Ware, thereby making it possible to greatly reduce the time of firing and amount of fuel required for same. By controlling the density Within the chamber it is evident that some dielectric members containing pockets may be Vsealed under a high enough vacuum so that lthe pocket Will have the desired dielectric strength.

The addition of a small amount of Whiting to the porcelain mix increases the toughness of the Ware, but tends to increase the firing loss due to the formation of bubbles or bloat at the high temperature. The increased pressure during firing not only reduces the size and formation of the bloat but prevents electrical weakness in the small blebs or spaces. Where the outer Wall (or surface) of the insulator is impervious due to glaze or iluxing, the body can be fired at a reduced temperature in order to obtain greater toughness as the necessary dielectric strength in the porous structure can be obtained by controlling the density Vat the time of sealing or firing.

It is evident that the advantage of the processmay be obtained even Where the density in the pockets is controlled by forcing in air or gas under pressure, or exhausting after the insulators have been removed'frointh'e kiln. The vent such as I2 in Fig. 2y can be sealed by any suitable material. One method is to use gold or. platinum fused to the insulator around the'vent, so that solder may be used for closing the vent. While it is possible to control the density in the large pockets by this method, the density in closed pockets such as I 4 which do not connect with any` duct system` must be obtalnedin the Cil firing operation. I-t is also difficult to placca vent so 'that the vent will not introduce a dielectric Weakness. `i's particularly true vvherel the air density ina pocket is such as to constitute a Weakness..V With my improved process fit is possi-ble to provide Walls of solid dielectrico-f good strength so that high effective insu lation vv-ill be obtained even though the density iin the pocket m'aynot be such as to develop the highest 'dielectric strength. Difficult sealing operatings are avoided and the density in all parts of the structure can be provided for regardless of number, location, and size.

I claim.:

1; In an insulator, a one piece body of fused dielectric material having therein a pocket which is completely enclosed and hermetically sealed by the fused material of said .body and Which is filled With gas having a `density'approximating the density `of airiat Anormal atmospheric pres- 'sure.` A`

2'.v In van insulator, a one piece body of fused dielectric materia'lhav-ing means for increasing the Fdielectric strength thereof comprising an opening Within said body which is completely enclosed and hermetically sealed by the fused materia-lof the. `body and which contains gas `having ra density greater than the density of air at normall atmospheric pressure. Y 3. I-n combination with an electrode having an. opening'therethrough, a conductor extending through: 'said opening and a bushing insulator separating. said conductor from said electrode and comprising a one `piece shell of fused dielectric material having therein a pocket completely enclosed and hermetically sealed by the fused 'materialiof saidfshell, -said pocket containing gas lia-vinga pressure at least approximately lasV great as normal Vatmospheric pressure.

4. An. electric insulator comprising a body of fused dielectric material, a pair of electrodes separated by `said body and mechanically connected thereby, and means for minimizing the charging currents over vsaid body and for raising the corona point of said insulator comprising a pocket in said-insulator completely enclosed and h'ermetically sealed by the fused material of said body,.v said pocket yhaving-'a lower specic inductivercapacity therein than said 'fused material and :having -a resistance to electrical 'discharge therethrough at least as great ras that of air at approximately atmospheric pressure.

5. In an insulator, a body `of fused dielectric material having therein an opening 'completely surrounded andhermeti'cally sealed by the fused material of said body' land having a speciiic indu'ctiyefcapa'city less 'than the inductive capacity of 'said Efused'materia'l and? a resistance lto electrical discharge `therethrough at least as great as that of lair at Vappronimately atmospheric pres- Sure.

6. An insulator comprising spaced terminal fittings and a dielectric member mechanically connecting Vsaid fittings for transmitting mee chanical load between said fittings, said dielectric member having a dense outer portion and a porous` inner portion, the material ofY said member being vitriiied to seal the inner portion, the interstices ,of said porous portion being lled With gas having a density-greater than the densityy of air at atmospheric pressure. Y

'7. An insulator comprising a dielectric body having a flange thereon and an hermetically sealed'pocket in said'flange; and gas-in' said-pocket under pressure at least approximately as great as atmospheric pressure.

8. An insulator comprising a body of dielectric material having a boss at one side thereof and a pin opening at the other side thereof and having a laterally extending ange, said body having an hermetcally sealed opening therein for increasing the corona and flashover voltage of said insulator and for minimizing charging current thereover for any given voltage, and gas in said opening at a pressure at least approximate- 1y as great as atmospheric pressure the Wall of 5 said opening being the continuous unitary material of said body.

ARTHUR, O. AUSTIN.

Images