US1743526A - Lightning protection - Google Patents

Description

J. M. CAGE 1,743,526 LIGHTNING PROTECTI 0N Filed Dec. 27, 1926 5 Sheets-Sheet l fda Jan. 14, 1930.

62 Jm/fz/wf Mor/wy.

Jan. 14, 1930.

J. M. CAGE LIGHTNING PROTECTI ON Filed oec. 27, 192e s sheets-sheet 2 fly 5.

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LIGHTNING PROTECTION Filed Deo. 27, 1926 3 Sheets-Sheet 5 PTNT ...envi/inn, en

rifhis inyenion has to do Wih the pioeotion iighiningg of enr'h mens, end hoehe sincin'es o1 obiecs of any eiesci'ipon, Whe'bher or not fhey may 1oe upon oi' in connection with ihe ground, ai een, in the ein, etc. The invention Ycheiac'ei'i lcniy Contenipiutes, as will ne nndesooei from what foiloTJs, the protection of any such aree o1' body by lightning pi'eyenicn in e. novel inennei based upon dissipation or transfer o1 the flesh-causing charge; ont ef; ehe sein nine, i conceinphies and aioids proteeion also in inzinnei's that are oid and Weil known.

@he theoy oi"l lipfntni,l y rod piotec'-ion has long been undeiscood to be nfo-foii; 'he a lightning rod has two functions. One of these functions is the dissipation of Cnage energy to enosphei'e, thus somewhat o decease che ifinhiiiy ci he poteniel gredien between 'che cioni and rod huil-:Eine np to ihe discharge poin; the ohei 'in c* oi is to ziLciect and caifiy o ground e flash tha may ocl i Within neinjhhoihooi'i of Jche rod. rens in been prnesiceiiy inefiec ,ye mssinntion or ii'ansiief; oi the non d hw vns i Cess f sfchin dis i t iev have in heir pi h; e. d if ii pi' snco'essfnfibY telic .nfi cniry it to 'i'ouni' such lightning rod of what is known onfinry en" on surrounding o ec The exact conse o1.' n i builds up the noeniei cionds o1 eel-h and dond, en conse? he hgh-*nine finish, nient i iinpoi' ce se fev: y eonceineC.e itis generally unlcieisooii ha potentiel diiieiences hemveen cion-fi and earth, foi instancei may be the iesni of causes in; which inducion oi influence, cloud and a- EGEITEIHG PROTEGTST Serial No. 157,056.

ohjects, che sion so the distance-the potentiel gindien eieeiio stress or electric field inten, iinniiy heconiing' high enough to break down air resistenoe and cause the iiasn. The pi'innuy object of niy invenion is to ieniovf,

o1* ieahicebelow the danger poin, the ineCate cause of boh the lightning Hash and the secondary effect.

ifi/*hen n flash occurs he poentiai diei'eiwe at the points at opposite ends of che iiash is zihnos insiennneonsiy substnn't-aiiy equal ized et those poi-nis, he Hash formingA n pe'h of low iesisencc between cloud ann eni'th .ihei'e is cons-:equeniy an inlniedizio of charge from suii'oundine' areas or poins io `the point of dis-change. Surrounding; p en en earth eren, for instance, have e ci densL y perhaps neef'1 point where the di. large oecnl'ieii. qneniy, ai nii such surrounding point is nnnedine release, so 'o speak, of the previensiy bound hy the cloud chaig that z'elcase constitnes the Secondary e which often-times causes disastrous resnis. rhe action of this secondary eifec on oil tanks, for instance, is weil known frein cxpeience; the cniren io'w set up in fiifieient parts oi" an oil teni: or reservoir on herb' of oii may he sn ient7 even though the iightning sifoke be at e consider-(foie fhsninceiv to nix also permit a ready flow of the charge from or throngh the shielding systems; but such shielding system must necessarily take the charge and therefore does not reduce the liability of a lightning flash occurring. Thus, such a system deals with conditions occurring during or after a lightning flash, and with conditions caused by such flash, whereas the present invention deals with conditions precedent to the flash for the purpose of preventing the flash itself.

Thus, in a typical instance of protecting oil storage tanks and reservoirs against the influences of lightning, it has been proposed to set up tall towers, or other structures, primarily for the purpose of taking a light* ning flash; or to provide a series or net work of conductors over a tank or reservoir, to protect in some degree against the secondary edect. ll either of tl ese means, however, prevent the occurrence of lightning flashes or remove the canse of the eco-ndai'y effect; in fact, the incre provision of what amounts to lightning rods may have the effect under certain circumstances of causing a lightning discharge to take place close to the tank; and although lightning rods or towers may effectually carry the discharge to ground, the secondary effect may cause damage. And if adequate shielding` provision can be made to take care of the secondary effect, there is always a chance that the lightning discharge may strike the tank or reservoir itself, instead of the rods or towers; because, as l have said, neither the shielding system nor the rod system will prevent lightning strokes.

ldy invention is aimed primarily at the prevention of lightning flashes and secondary effects by way of removing their causes; by way of removing or neutralizing the charges which tiltimately causo the lightning flash,`

r the secondary effect, and removing those charges before the potential. gradient becomes r"- fi crocs. Although l shall explain my intention as applied particularly to the protection of an oil tank or reservoir, and the immediately surrounding earth area, it will be understood from what l in the followii detailed c sciiption,tl1atiny system is applcble as well to the protection of other bodies or structures which do not stand upon l g of the same;

Fie. 3 is a diagrammatic elevational sect-ion showing the application of the system to an oil reservoir;

Fig, l is a diagrammatic elevation showing the relation between a cloud and a protected area;

Fig. 5 is a diagrammatic plan showing a variant form of charge dissipating element, as itV may be arranged about the object to be protected;

Fig. 6 is detail elevation showing the charge dissi'iiating element of Fig. 5.

Generally speaking, my invention characteristically resides in the provision of a charge dissipating or charge transferring element of some horizontal citent, preferably elevated well above the level ofthe area or object being proteced and connected into that object or area in such a manner that the change of the obj ect or area is freely transferable to the dissipating or transfer element fast as the element transfers or dissipates the charge. The di eating or transferring element is characteristically one of such relative size, form and location as to cause concentration of the lines of force and thus to cause localized increase of the potential gradient, electric stress-or electric field; and further of such formation, that it will, in that localized field of increased potential gradient, act to dissipate or transfer the charge at a. total rate suilicient to keep the charge on the protected area or object from building up to the danger point. This last mentioned formation, for dissipation or transfer, may be attained by using wires preferably of small diameter (short radius of curvature) or, better practically, by using wire for concentration and equipping that wire with sharp points for dissipation. o

For reasons that l hereinafter touch upon it may be preferred, wherever the nature of the situation permits, to arrange the dissipating and transferring element above and peripherallyY aroundthe in'nnediate objector area to be protected; but this is not n cessarilv the case, as will hereinafter appear,

The particular manner in which the elevated charge transferring element is connected into the or area is variable to snit particularl circumstances. Such connections, for instance, may be effected through ground connections of both area or body and the elevated element; or may be effected by direct conductor connections between the area or body and the charge-transferrin0' element; or by both. so made that they are effective to pass the charge from the whole of the area or body to the chargetransferring element at a rate at least equal to the rate of dissipation'or transfer which is designed to be great enough, at a comparatively low potential gradient, to pass ofi" the charge at least as fast as it tends to build up, so to prevent a charge from ln any case, these connections are reaching anyz dangerous intensity In cases,

Where the body or area to be protected is ofa dielectric or poorly. conducting nature, the connections may be so niadeinto itv atfdiilerent points as to be effective in carryingaway its Whole charge, or substantially its Whole charge, to the charge dissipatingor transfer ring element.

From WhatiI have said,aitivill be seen that the invention is, applicable in various inanners to the protection of variousv specific things, and in the following detailed' description, in connectionv with the accompanying drawings, I shall attempt to set forth a. few

typical instances, without, however, limiting'V the invention or its application or use, to such speciiic or mentioned instances.

In Figs. 1 and 2 I show, for instance, an

oil tank T, of usual typical forni, resting such as used for fencing, will do the Work very Well, although heavier Wire such as used for trench entangleinents ina-y be used. The only requisite is that the Wire or other charge transferring or dissipating element shall be of such forni (sinall diameter or radius of curvature) as to cause concentration of the lines of force, thus causing ionic discharge to readily take place. Thisionic discharge inay be greatly accelerated and increased by` the placing of frequent properly distributed points along the length ofthe charge transferring Wire or elenient. As to the number and spacing of points and the spacing of the Wires, When niore than one is used, I shall speak later.

An important thing in connection with the charge transferring` eleinent is that it is suitably and adequately connected into the protected area or body or object; and this inay be done in a variety of nianners. In addition to being connected into the protected area or. object, it is of course always adviasable that the elevated charge transferring element will also be grounded to take advantage o'f ground Water conductivity. In sonic installations the grounding and the connection into the protected area or body or object inay be, in practice, one and the saine thing; it may be that the connection into sucl area, body or object niay be by way of ground connections; but it is Well to keep in mind the two possible distinct functions of such ground connections, as the system is capable of application in seine situations Where thereisno ground connection at all, and it is also capable of functioning forl charge transfer Without. a ground connection. However, in inany cases Where protected objects or structures are upon the earths surface, the connections will involve` ground connectionswill involve connections to the earths surface, and to ground below the surface, to protect the earthY area as Well as the structure or body, to reinove the lightning causing charge built up in the earth area as Well as in the body or structure.

rlllius, in the typical instance of protecting an oil body, the poles or torers 10 inay all be thoroughly grounded, as by providing tliezn with grounds G placed below the Water level lll Where theground connection is highly effective. rllhe tank itself may also be similarly grounded, at G1; and thus through the several Grounds the tank and poles vvill not only be grounded. but Will be interconnected. rlhe poles or towers may typically be of inetal, and Wire or wires 12 nia-y be siinply strung uponl theni in electrical connection with thein or they niay be of Wood orany other suitable material, and concluetors may be strung up the poles to provide the connections.

In addition to, orl in substitution for, the grounded interconnections, the poles and tank Inay be interconnected by suitable inetallic conductors as indicated at 13b and 13. rIhe conductors 13 niay run to the tank itself, or niay, for instance, run to a pipe la' that contacts with the tank. The pipe le, are the several such pipes whichI usually connnunicate With the tank, inay themselves forni the ground connection for the tank, particularly if they are deepl Ienough or extended enough tol inake good grounds. In situations Where the earth is relatively moist or the Water level YV is close to the surface, the connections to ground also forni adequate connections into the earth surface or area, the Water level being a comparatively good conductor. However, where the earth is dry, or the Water level rather deep, it is Well to provide conductors at or near the surface to carry olf the surface charge. rlhus there inay be radiating conductorsSO laid near the surface E. These inay extend outwardly to any desired extent to connect in the earth surface in any desired areas; and they inay be occasionally ground-Y ed as at G2y to take advantage of the Water level conductivity inv gathering in the earth surface charge. Pipes leading freni the tank, on or near the surface, inay constitute part, or Wholly, the conductors 30. Such conductors 30, Whatever their nature, typify s surface conducting systein so spread upon or near the surface as to feed in the earth surface charge frein substantially the Whole ofthearea being protected g so that the charge is fed in even though the surface be a relatively poor conductor.

It will be seen from what has now been de scribed, that the charge transferring or dissipating element is connected into the earth area desired to be protected, and also connected into the tank itself. If the substance contained in the tank is of such dielectric strength that itwill hold a charge and prevent that charge passing sutliciently rapidly to the tank and thus to the dissipating element, then connections may also be made into the body of liquid in the tank in such a manner as to connect intimately into that body of liquid at various points and thus take off its charge. For instance, spaced floats l5 may rest at the surface of the oil and they may support conductors 16 which are connected to the bottom of the tank in any suitable manner; or are connected in any suitable manner to any conductor going to the'dissipating element or toground, or both. These conductors may preferably be bare wire or rods and the floats themselves may also be conductors. However, any conductive system that is in more or less intimate contact with the body of oil or other substance may be used. For instance, a screen may be floated on the liquit surface or screens may be placed at spaced levels in the tank. It is not positively known at present whether such conductors to the oil itself are necessary for the protect-ion et the oil. electric strength; but my experimental data seems to indicate that most petroleum oils are sufficiently mobile under the charges and time intervals involved, to pass the charge off from the oil at a suliicient rate to keep the oil fully protected. However with particularly viscous oil such conductors may accelerate removal of the charge. Thus, the question as to whether it is necessary or desirable to use such intimate connections with an oil body depends upon how fast it is necessary to pass the charge off from the oil; and that of course depends upon how fast a lightning causing charge will, under any circumstances, build up in the oil. However, and whether or not such connections are necessary o-r desirable for the protection of oil, such connections are again illustrative of the general proposition involved in my invention that wherever a body or substance to be protected is of a nature that can be regarded as cli-electric or poorly conductive, that body or substance may be intimately electrically co-nnected by conductors with the .dissipation element.

In Fig. 3 I show atypical application of the system to lthe protection of an oil reservoir R. Such reservoirs may be simply earth reservoirs, with or without a roof 40; or they may have concrete bottom and side walls, as shown at C, usually reinforced as indicated at I. The connections to groundsG1 may' Petroleum oils have a certain dif thusy run directly to the reinforcement and the concrete, as may also the connections 13a that go to the towers or poles l0. However, if a reservoir has not such a reinforced concrete lining, a net work of conductors, or spaced conductors, may be placed upon the walls and floor to be the conductive equivalent of the reinforced concrete lining.

The poles or towers 10 in this case are shown set on concrete bases 20. There may be connections to the ground G and conductors l?)a may be connected from the reservoir to the towers. Conductors 13a may be placed close to the surface and may be, if desired, extended as at 30a to perform the same service as before described. The roof e0, if it be not in its own structure well connected to parts connected to the wires 12, may be especially so connected; and if the roof is not of metal conductor wires 4l may be laid across and in contact with it and connected into the charge transferring system at any convenient poin Y Y Suppose now that for whatever reason it may be, opposite charges tend to build up in a cloud overhead and in the earth area in which the tank or reservoir is located, and thereforetend torbuild up in the tank or reservoir. The charge in the cloud is usually negative and the earth charge positive, although frequently these signs willv be reversed. For the purpose of this illustration the cloud will be assumed negative, but the argument will be equally strong and identical in terms with the cloud positive. A stress eX- ists between the two opposite charges, and

.the lines of force are comparatively highly concentrated on the elevated wire or wires of the dissipating system, the surface charge density on that wire or those wires being substantially higher than is the charge density elsewhere. lCorrespondingly, the potential gradient or electric field in the immediate vicinity of the wire or wires is substantially increased. The charge on the cloud is more or less uniformly distributed. This condition is indicated in the drawings in 4. The sharp points on the wire or wires are located in what may be termedl a zone of concentration of the lines of force, i. e., a high electric field, or a Zone of comparatively increased potential gradient; and the silent discharge activity-the ionization activity at each point, is thus comparatively enhanced and increased. lt is a characteristic feature of my system that the change carrying' element on which the points are mounted, or with which they are associated, is of such a nature (for instance, a comparatively small wire rather than a broad sheet of metal) that the lines of force are so concentrated and the po- 'ino tential gradient locally so increased as to diferencebetween the cloud and earth, or to protected body, is Well below the flash-over point, typically less than say half the flash over gradient; and that the number of points so associated With such concentrating element is sufficient, at such potential gradients, to dissipate and transfer by ionization the total charge at least as fast as that charge tends to build up.

rlhe typical utility of having the points mounted on comparatively small Wires, with the points spaced apartfar enough to get little or no interference between them, and with the Wires spaced far enough apart so as to get little or no interefernce between their concentrated elfects on the lines of force, Will now become apparent; and so also the utility of having the dissipat' ig system elevated and comparatively Wide-spread horizontally and thoroughly connected into the obj ect or body b ing protected and also, in most cases, thoroughly connected into a comparatively Widely ent-ending earth surface area. The charges built up on cloud and earth respectively are usually spread over a considerable extentareas of the order of a circular mile. rlhe feeder system Whichkfeeds the earth charge to the dissipating system as fast as dissipation talres place; andthe Wide spread of the dissipating system as for instance, extending completely around an earth area, or around a large reservoir) makes that dissipating system, so to speak, more effectually ccover the charged cloud area. Thus, the ionization current, which is dissipated from the dissipating system is most readily transferred through atmosphere as a return flow current to the under surface of the cloud, the return currentbeing more effectively distributed over the under surface of the cloud, causing eli'ectual discharge of the cloud as Well as discharge of the earth. t is apprehended that the cloud (that area of cloud that is concerned in a single charge formation) must be discharged as Well as the earth area or object. Consequently it is of importance that my dissipating system be so spread as to effectually cover a charged cloud area with the ionic stream that is dissipated from the points and that flows bach to the cloud along the lines of force. rl`he peripheral, encircling arrangement hascertain advantages. With that kind of arrangement it -is relatively easy to have, along any one side of a protected body, enough -ioints to tale care of the total necessary dissipation; so that a cloud drifting over a protected area from any direction Wil-l begin to be discharged immediately it begins to approach, carrying its earth .charge along with it, so to speak. And also there is, in a peripheral arrangement, an action of concentration of charge from a sur-rounded area much as a static charge on a disk tends to go to its periphery. The concentration of charge, and the ionic -action of the point-s, on a peripheral Wire is found to be much higher than on points and Wires not so arranged.

Generally speaking, any operation which would transfer this earth charge bac-lr to the charged cloud will orevent a lightning stroke. The theory of lightning causation generally accepted is that electrical energy is transferred from cloud to earth mainly by falling rain. Any means or any operation which Will elfectually afford a path for a return current to the cloud will equalize, or substantially equalize, the potentials of cloud and earth or body and prevent a lightning flash. Thus any means of conductance between the earth or body and cloud will have the desired effeet. F or instance, a conductor cable suspended to cloud height by a balloon may have that effect but there seemsto be practical dificulties in the Way of such mode of protection. For instance, Wind conditions may maire such a means impracticable; and there would a-lvvays be the liabilityof a charged cloud another elevation discharging through a flashover into the suspended balloon, much as flashes talre place from cloud to cloud or to rods.

My invention fundamentally makes use of the atmosphere as the dissipating or transferring path and dissipation or transfer by that path is substantially unaffected by Weather conditions. ln transferring through the atmosphere as a path the current lioiv may be looked upon as caused by atmospheric ionization caused by the relatively high potential gradient set up at the zone of concentration o-f the charge. The ionization, hoivever, may be caused in other manners; or th ionization at the discharging points may be assisted by other means. For instance, it is of course Well knovn that air ionization may be set up by flame and chemical action, splashing, etc. rlhus the requisite ionization, and the consequent establishment of a path for dissipation and transfer of the charge may be up by ionization by any suitable means, and data to the present time seem to indicate that the discharge from the sharp points of barbed Wire, for instance, takes place at a sufficiently high rate at comparatively loiv potential, hence no such help seems at the present time to be necessary. lhile the rate of discharge per point increases as the poten tial difference between earth and cloud increases increases the ionization creases-there seems to be sucient ionization and therefore sutlicient dissipation and transfer at comparatively louT potentials. Hou'- ever, if it be desired to increase the dissipation and transfer at the lower potentials, ionization may be started or augmented at such lower potentials by some suitable means.

ln the practical use of my system there are certain controlling factors that are taken into account. lilithout going into a large amount of detail I may say that the most reliable iig TEO (G. C. Simpsons, Proc. Roy. Soc. 1909- g Phil. Mag. 30, l, 1915), as to the rate electrical energy transfer from cloud to enth s em to indicate that in heavy thunderlterms the transfer is at the rate of l2 microaniperes per acre of earth area, and the maximum observed, so far as I am aware, has been 4.0 inicroamperes per acre. rIhus, for instance, from a cloud area s ay one mile in diameter, or approximately 500 acres, the maximum observed energy transfer would be at the rate of about 20,000 microamperes. On the assumption that all of that energy transfer might under circumstances be more or less concentrated in a small earth area or ina single bedy or structure, the dissipation or retransfer rate should be at least as high as the last figure and preferably, for safety, should be substantially higher.

:ExperimentalY results and data indicate that the dissipation and transfer rate from a single vpoint,such as the sharp point of a wire barb, elevated and isolated from bodies or structures that would interfere with its action, can be ,asV high as 40 microamperes at a potential or potential gradient substantially less than that corresponding to the iiash-over point. Thus 500 points would, on that basis, take care of the maximum observed rate of transfer from a Circular mile of cloud. But it is desirable to have what may be called a factor of safety;l to provide a sufficient number of points to take careof even an exceptional transfer requirement at a potential gradient always much less than that of the iiashover point, say at' less than half the liash over gradient; consequently in a practical installation the number of point-s will be much larger than the comparative figures given above. For instance, in protecting an oil reservoir where the encircling wire system is ene-half miie in circumference, ordinary barbed wire may be used. Provision of an excess of the discharge points does not hurt the system as a whole, even though the discharge points may be so close together that the discharge or transfer from eadh individual point may be somewhat less than if the points were spaced further apart. For instance, within the potential gradients observed by me the spacing between points would seem to be about sixl inches for maximum eiiiciency at all points; and if a number of parallel wires with points are used, the spacing, for maximum efficiency, ietween the parallel wires should be somewhat larger. However that has only to do with the individual eiiiciency of each point; although the individual efficiency at each point in ordinary barbed wire where the bai-bs are spaced at about four inches may be lower than the maximum attainable point efficiency, the presence of the excess Vpoints raises the totaleificiency of the system. As to the question. of the number of parallel wires forming the peripheral or encircling concentration and discharge system, however, other factors may come into play. For instance, in some cases, it may be desirable t employ only a single barbed wire, as the potential and therefore the transfer rate of the charge concentrated in and discharging from a single wire may be greater than if the concentration were spread on numbers of parallel wires. However, for mechanical reasons, and because a single wire might by accident or deterioration fail, it will usually be wise to use two or more such wires.

'I he results of investigations indicate that the rate of transfer from the concentrating and transferring system depends among other things upon the height of the system above the area or object being protected. The greater the elevation of the system, the higher the rate of dissipation and transfer, other things being equal. Consequently, the protective system will ordinarily be placed upon towers, or otherwise suitably supported at considerable elevation, or as high as s practicable above, the uppermost parts of the protected object or area so as to increase correspondingly the potential gradient in the zone of concentration at the dissipating points. Also, to equalize the eld concentration and flux, the outer wires, or outer parts of the wire system, might be relatively lower than the inner or central wires or parts; or if desired, the outer wire or wires, or any of the wires, might be arranged elevated above the others to get an earlier discharge from the higher wires.

In Figs. 5 and 6 I show a physical modification of the system wherein, instead of using continuous encircling wire or wires, I may arrange a series of towers a each having a concentrating and dispersive system of wires and points at its top, as shown at 3l. Such towers may be electrically connected as hereinbefore described, and of course electrically connected to the tank T in' some one of the manners hereinbefore described. In Fig. 5 conductors 32 are shown running from the towers to the tank. The towers will be so placed and spaced that they form a wide spread system that covers the cloud area and from which the earth charge will be dissipated and transferred as before explained.

From what has been said it will now be readily gathered that my system involves characteristically the transfer or neutralization of lightning-causing charges, causing the energy of such charges to be dissipated or spread over comparatively large space and long time period. The actual amount of electrical energy in a lightning Hash is not large; high concentration in time and space is its destructive characteristic. The potential energy of such a flash is spread out both as to area and as to time; and the energy is dissipated in frictional resistance of the air to ionic movement between earth and cloud.

From what has now been said itivill be seen that my system or" protection is essentially different from Whatever protection may have been aii'orded by lightning rods and the like. Although it has been recognized that dissipation ot charge tal-ics place from the point or points oit a lightning` rod, such rods have not been capable, on account of their small superficial arca presented to the cloud, of taking advantage of any pract'cal amount oi" concentration of the lines of torce or materially increasing the potential gradient. The horizontally extending Wire system, isolated by elevation above the body from Which it takes its charge, has greater capacity than a small vertical rod-it collects more lines ot torce; and the dissipating` points, or other formation, then acting in tie zone of concentration of the Wire, further increases the concentration locally. Furthermore, with a given amount of ionic flow between earth and cloud, a spreading' of that ionic flow apparently increases the eiective flash-over gradient. And neither is my system to be compared to the action that may be supposed to take place in a forest or area surrounded by trees, which is not self-protective, as has been observed in large numbers ot instances. The trees themselves are not suiiiciently good conductors to take and transfer the energy or" the charge.

In the following claims the word body is used in a broad and inclusive sense except Where otherwise indicated, to include the body of Jthe `protected earth area as Well as a bot y such as an oil tanlr or other structure upon an earth area, or a body not support d upon the earths surface.

l claim:

l. A lightning prevention system, embodying' substantially exclusively an elevated conductor encircling the earth area to be protected and connected into that area to receive its charge, the size ot the encircled area being sucl relatfrve to the dissipating capacity of the elevated conductor as to keep the earth charge Within that area 'from buil ling' up to the danger point as that passes into the encircling' conductor, the conductor being` at a substantially uniform elevation throughout its lengt-h and provided with charge dissipating points ot substantially uniform elevation spaced along the length of the conductor at spacings substantially tar enough apart to avoid dissipation interference with each other.

2. A lightning' prevention system, embodying' substantially exclusively an elevated conductor encircling the earth area to be protected and connected into that area to receive its charge, the horizontal spread of said encircling conductor being proportioned to the dissipating capacity or" the elevated conductor for limiting the earth charge Within that area to a sate maximum, the conductor being at a substantially uniform elevation throughout its length and provided with charge dissipatng points of substantially uniform elevation spaced along the length of the conductor at spa-cings substantially tar enough apart to avoid dissipation interference with each other.

ln Witness that I claim the foregoing l have hereunto subscribed my name this 17th day of December 1926.

JOHN M. CAGE.

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