US1164757A - Lightning-arrester. - Google Patents

Description

E. 0. SCHWEITZER & N. J. CONRAD.

LIGHTNING ARRESTER.

APPLICATION FILED MAR. 20. 1915.

Patented Dee. 21, 1915.

3 SHEETS-SHEETl Q ff? E. 0. SCHWEITZER & N. J. CONRAD.

LIGHTNING ARRESTER. APPLICATION FILED MAR, 20, I9I5.

1,164,757', Patented Dec. 21, 1915.

3 SHEETS-SHEET 2.

E. o. SCHWEITZER A N. 1. CONRAD.

LIGHTNING ARRESTER.

APPLICATION FILED MAR..20, 1915.

Patented Deo. 2l, 1915.

3 SHEETS-SHEET 3.

UNITED sTATEs PATENT OFFICE.

EDMUND 0. SCHWEITZE-R AND NICHOLAS J'. CONIIAD, OF CHICAGO, ILLINOIS,

ASSIGNOBS TO SCHWEITZER AND CONRAD, 0F CHICAGO, ILLINOIS, A COR-PO- naTIoN or ILLINOIS.

LIGHTNING-ARRESTER.

Specification of Letters Patent.

Patented Dec. 21, 1915.

Application med umn 20I 1915. serial No. 15,783.

`exact description, reference being had to the accompanying drawings, forming -a part of this speciication.

This invention lightning arrester. y

The present invention is a further advance along the line of development pointed out yin our co-pending application Serial No. 866,214, filed October 12th, 1914. In that application we ,have disclosed a horn gap lightning arrester in which the movement` of the dynamic arc along the horns of theuarrester lauomatically includes resistance in se-V provides an improved ries with the arc but externally thereof,

while preserving atv all times the original gap for relieving a second discharge.

The plain horn gap lightning arrester as known and constructed according to the prior art is inherently defective, but it has such great practical advantages, being simple, inexpensive, of rugged construction and requiring no attention after once being in-, stalled, that it persists in the ield in spite of its disadvantages.

Thehorn gap lightning arrester, by flashing over, relieves at once the excessive po-I tential on the line, but the relief is heroic; instead of discharging only the potential in excess of the normal line potential, the horn gap arrester practically short circuits the line and discharges the entire potential on the line. This action is effective in relieving excessive potential, but it is needlessly severe. However, the action is due to the characteristics of this type of apparatus. The initial discharge occurs across the point of least resistancel and the arc whichis then established is itself the only resistance in the circuit. It 'canrbe seen at once that this is like a fusible plug in a boiler, which, when it lets go, always allows a greater dischargeof pressure than is necessary.

We have conceived the idea of building a horn gap arrester so that `the tendency` will be to discharge only the excessivev pressure and not the line pressure. Specifically we l' do this"by providing a series of discharge l paths ofditferent discharge capacities, the

ones of lowest discharge capacity coming ,into operation as soon as the pressure shows a given predetermined excess over li'ne pressure. `(.lhe paths of greater discharge capacity lcome successively into operation in case the first path is insuiicient to relieve the pressure. This is analogous to a series lof pop valves of successively increasing discharge capacity, set at successively increasingv discharge pressure. For instance, if a oney inch pop valve` set at 100 pounds per square inch cannot relieve the accumulated excess pressure, a one and one-half inch valve set at 102 pounds per square inch will then-come into play and, if these are still insuiicient,.then a two inch valve set at 105 pounds pressure may come into play, and so on, successively, until the excess pressure, no matter how great, is entirely relieved. Such an arrangement will retain always the pressure at which the first one is set to discharge, One manner in which we can construct the apparatus for embodying this idea is toprovide a series of spark gaps made into a horn gapform, or otherwise, and allow the potential to discharge through resistances, the setting of the gap determining the excess of pressure required to cause the discharge, and the resistance in the circuit of the gap determining the discharge capacity thereof. The short gap, having high resistance in series, is set to break down first, and the next gap will tend to break down thereafter, and so on, successively; but we counteract the tendency to break down and discharge which occurs in the gap adjacent the one'which is discharging, by a powerful blowout arrangement, the activity of which is augmented by an increased discharge. This blowout arrangement, while it is particularly adapted for use in this form of arrester, is'also very valuable in other forms, and we have therefore illustrated a number of modifications in which it is used and are describing it as it is employed in a variety of forms of arrester. We have also found the blowout arrangement to be applicable to the horn gap arrester as previously cony structed.

bated, and some of'which are unknown and fact that the discharge to be dealt with vaof the arc itself between the electrodes.

ries within limits of enormous range. If the gap is set for a low value discharge, the dynamic current is likely to hang on and cause damage, either by its continued eX- istence or by the sudden breaking of a large current. If the gap is set for high values then the lesser disturbances on the line, which are not relieved, are likely to cause dam-.we `We have found a simple solution for this difficulty. We construct the arrester to embody the above-mentioned blowout arrangement, so that the arc is compelled to move out and break itself regardless of the quantity or rate of discharge or the point of formation. At the same time that the arc moves the current forming the arc is caused to decrease. This we'accomplish by sectionalizing one or both of the electrodes and inserting resistances between the sections, or by depending solely upon the lengthening Being thus enabled to prevent formation of an excessive discharge and to break any discharge that does form, we can set the arrester at a relatively low value and thus cope with any situation. We secure the positive break or blowout by confining the space within which the arc forms to a degree sufficient to cause either a bomb or explosive effect or a ,chimney effect, or a combination of the two. To this end we construct the electrodes in such manner that they form a confined space between them at the points where the arc forms. This is accomplished specifically by forming one electrode in the shape of a central core and the other in the shape of an outer generally cylindrical shell surrounding the core in whole or in part. Incidentally we secure a number of marked advantages, all of which form part of the present invention. The above arrangement prevents disturbance from wind and gives a greater uniformity of discharge and among other advantages it provides greater electrostatic capacity and hence offers greater ease of discharge for currents 'of high frequency. It offers also a simple and Workmanlike mechanical solution of the problem of construction.

I'n order to set forth with more particularlty the nature of o ur invention, so that the same may be readily understood, conthe art, reference is here made to the appended drawings-which form a part of this specification.

Figure 1 is a vertical sectional elevation of one form of arrester. Fig. 2' is an elevation of the central electrode or core with the outer casing and the bands thereon broken away to expose the interior connections. Fig. 3 is a vertical sectional elevation of a slightly different form of construction with the connections shown diagrammatically. Fig. Li is a fragmentary elevational view of a cone of insulation with the electrode formed of converging strips of metal. Fig. 5 is a plan view of one of the resistance units 13 shown in Fig. 2. Fig. 6 is a vertical sectional view of another form in which our invention may appear. Fig. 'i' is a detail View illustrating the manner in which the core is built up. Fig. '8 is a vertical sectional elevation of a form in which the core is built up of annular segments of alternate resistance material and conducting material. Fig. 9 is a similar view of a form in which the shell and core are both sectionalized and in which an adjustable annular air gap is provided. Fig. 10 is a vertical".

sectional elevation of another form of arrester having adjustable spheres for determining the paths or air gaps for a current of different frequency. Fig. 11 is a vertical sectional elevation of an inverted form of arrester. Fig. 12 is a plan view of the inner electrode as viewed on the line 12-12 of Fig. 11. Fig. 13 is a diagram showing a number of arresters connected in series; and

Fig. 14 is a similar diagram showing a number of arresters in series, the arresters havl the shell. The shell is supported on insulators at three or more points in order to steady the same.

The core electrode 2 is formed of an upper conical part, which may be formed of sheet metal, as shown in Figsfl and 2, and a lower conical part 4, which comprises a number of metallic bands 1l, mounted` upon a. body of insulation. Suitable resistances are connected between the bands so that Awhen an arc is formed between the shell 1v and the upper b and 11, which is nearest to the, shell, sufficient resistance will be in- Fig. 2 illustrates one manner in which the core 2 may be built up. In this form the upper cone 9 is made of sheet metal secured to a lower cone or frustum 12, of insulating material. The .lower cone 12 is hollow and contains within it a number of resistancebearin units 13, which correspond to the metal ands 11 fastened on the outside of the cone. 4The core electrode 2 is supported by the metallic pin or bracket 14, which is secured to a piece of channel iron, both of which are connected to a suitable ground.

- The resistance units 13 may be formed, as

we have shown them, of different diameter disks of insulating material, Iwhich are set loosely within the hollow cone of insulation 12. The disks of insulation 13 each contains a serpentine groove 16 vhaving terminal clips 17 and'18 at the end of the groove, and a powdered or granular resistance material fills the groove. These disks of insulation 13 are made of porcelain or other refractory material and they may be made of a uniformlsize so that the same will be interchangeable for replacement and repairs. The upper cone 9 is secured to the lower cone 12 by means of brackets 19 fastened by screws 20. In the form shown in Fig. 3r the upper cone 9 is also formed of sheet metal and it is secured lto the lower cone by means of a flanged plate or cap member 21.

A pair of through bolts 22 secures the cap 21 tothe pin 14. In this form of construction the lower cone 10.is constructed of a number of independent sections held together by the bolts 22 and each bearing its own metal band electrode 11. These sectio'ns contain the same form of resistance material as that shown in Figs. 2 and 5, or they may be constructed with any other suitable form of resistance included in the body thereof. The plate 21 is made of insulating material or it may be made of metal and the bolts 22 suitably insulated therefrom. In Fig. 4 we have shown the upper cone 9 as constructed of insulating material bearing a number of metal strips 23 converging into a common point at the top. These strips 23 in this form of construction will define the path of the arc and will prevent the same from traveling sidewise as the arc rises. Attention is called to the form of air gap provided in the structure of. Fig. 1. This gap is converging for the purpose of providing the selective discharge paths of different capacity which we have mentioned above.

The air gap between the upper band 11 andtheshell electrode is the shortest, but the resistance between this band and ground is the greatest, so that itv presents a discharge path most easily overcome but of least capacity. If this path is insufficient to relieve the accumulated static, the next path requiring greater break down pressure but having greater discharge capacitybreaks down and so on down the rowv of electrode bands '11, each in turn requiring increased break down voltage but having increased discharge capacity. Hence the arc tends to break downward due to electrical conditions. iVe have provided mechanical con- ,ditions tending to force the arc back, holding the same at a point where the discharge capacity is low.

The Varcr` suddenly heatsy the air in the annular ga'p between the two surfaces. This causes a sudden expulsion of part of the same and a chimney effect or draft then occurs, which blows the arc upward, tending always to confine the arc to a path of low discharge value or to break the arc completely.

We contemplate closing off, either completely or partially, the bottom of the annular air gap space in order .'to secure to a greater .or less extent the bomb or eX- plosion effect which we shall describe more fully in connection with Fig. 6.'

It is to be noted that we also contemplate sectionalizing the upper cone 9 andinserting resistance between the sections, the resistance between the ring having the shortest air gap and ground being the critical resistance at which line surges and similar disturbances will be damped out, and of such amount that the discharge occurring through this path will not materially lower the line potential.

The form of arrester shown in Fig. 6 comprises an outer shell electrode 1 and an inner core electrode 2 in which the annular space between the two electrodes is open to the atmosphere only at one end. The otherend of the gap is closed ofi' by an annular ring of insulation 23. The shell 1 is supported in a block of insulation 24 which is supported from an insulator 25 by means of, suitable brackets 26, clamped to the insulator 25. The core 2 is connected to the ground wire by means of a bolt 27 which is screwed into a socket formed in the shell of the core. The core electrode 2 is built up of a number of sections 28, separated by strips of insulation 29, to sectionalize the electrode. Each of the sections 28 is provided with extending flanges 30 which will tend to carry the arc over the insulation 29 without burning or overheating the same.. The sections 28 are clamped together but are insulated from each other and the hollow sliell which isthus built up is filled with a powdered or granular resistance material, the paths through which are determined by the washers 29 of insulation. These washers are made to extend inwardly in order to dividedinto parts which thread together for convenience in assembling. We have shown this form of arrester as provided wlth a roof or cover 31 supported from the shell 1 by suitable brackets 32. This roof or cover is constructed of sheet metal, asbestos, porcelain, Wood or any other sultable material. This forln of arrester may be constructed to have the annular space forming the air gap open to a certain degree at the bottom end, by removing the insulating ring 23 to allow a draft of air to pass up through the holes 33 and between the space of insulation 24 and the core 2. Thus a combined eXplosion and chimney effect will be secured`inasmuch as rapid movement of air upon the initial formation of a spark will be impeded tosuch a greatextent that the arc will` be blown upward, and thereafter the movement of the 'air into the lower part ofthe air gap will be so slow as not to be materially impeded.

In Fi g. 8 we have illustrated another form of arrester. In this form the outer shell electrode is composed of a bowl 1 of metal fastened by means of brackets or clips 33 to the insulator 34 upon which the same rests. of a-number of sleeves of resistance material and conducting material arranged alter- `nately and strung upon a rodor bolt 35 which passes downto the insulator 34 and connects with the pin 36, which is grounded. Alternate sleeves l37 are composedl of metal or conducting material, while the sleeves clamped between thefxm, 38, are constructed of a material having a high specific resistance.l The vouter electrode 1 hasA suitable openings at the bottom for draining the same. A bolt 35 which forms the `central core'of the core electrode and which is connected to ground through the pins 36 is insulated from all of the sleeves 37 and 38 except the lowermost one byy means of a tube or sleefe of insulation `39.

In the for of arrester shown in Fig. 9,

|both electrodes have been sectionalized and trode 1 is supported in anyconvenient manner on the 'insulator 34 and'consists of an upper flaring portlon which' is composed of the alternate strip of resistance material 40 and the conducting strip 41 therebetween. The lower part of the shell electrode is formed of a cylindrical portion 42 of metal, such as copper. The lower portion 'of the core electrode is also formed of a metal The central core electrode 2 consists' morev frequencies,

The resistance material forms sleeve 43 between which and the shell 42 the initial discharge is formed. The core electrode 2 is constructed of alternate resistance and conducting sections 38 and 37, clamped together by means of the bolt 4() which in this case is shown as a fiber rod. The lowermost section 43 of the core electrode is connected to the supporting stem 3G by means of the bolt or stud 44 passing up through the insulator 34. We have illustrated in this form of arrester the manner in which the air gap, across which the spark is initially formed, can be adjusted. A plurality of concentric sleeves45 formed of conducting material may be slipped inside of the sleeves 42to decrease the air gap to any desired extent. In this manner the arrester can be set forany standard voltage by inJ serting or removing the proper sleeve. It is to be noted that this form of adjustment can also be applied to the arrester shown in Figs. 1, 6 and 8, previously described.

In Fig. 10 we have shown a form of ar rester in which the outer shell electrode l comprises an upper flaring portion and a `lower elongated tubular portion 46 which is open 'at the bottom. The inner core electrode is constructed of alternate copper and resistance sleeves clamped together in the manner described in connection with Figs. S and 9. The shell electrode 1 is secured to the insulator 34 by means of suitable clamping brackets 47. A porcelain or other cover 48 is supported above the open flaring end of the shell electrode by means of the bolt 49, which serves to clamp the sections of the core electrode together. This form of arrester also provides paths which are of different impedance, so that selective paths for V'diferent frequencies of current are provided. In this manner the upper adjustable sphere electrode 5U can loe said to offer a suitable path for the discharge of one or while the adjustable sphere 51 offers a path suitable for a diii'erent frequency. v

In Fig. 11 we have illustrated one manner in which the arrester can be built in the inverted form. In this case the outer shell electrode 1 is inverted, that is, it has'the ilaring portion opening downward so that the arc formed between the outer and the inner electrodeI 2 will be moved downwardly instead of upwardly as in the previously described structures. IThe electrode 2 may be constructed as described in connection with any of the foregoing embodiments or it may be constructed as shown,that is, of a number vof bars of copper 52, between which and at will on a plurality of insulators 55 and is connected to the line. The action of this form of arrester is broadly the same as that shown in Fig. 1. The first discharge forms between the lowermost bar 52 or 53 and if the pressure is relieved the arc is extinguished by the explosion effect. If the discharge cannot be relieved at once, adjacent sections will discharge. The greater the discharge 'the greater will be the tendency to blow the same out. l

In Fig. 13 we have shownk one manner in which a number of arresters of the type previously described may be connected 1n series. After a discharge occurs and an arc is once formed it is subject to being extinguished at each one of the gaps and inasmuch as the action of this type of arrester is quick and certain, the -,arc will 4be very speedily extinguished. It. is well known that the potential across theindividualgaps of the arresters shown in Fig. 13 will not be uniform and that the greatest efficiency is not obtained by such arrangement. p

We have shown in Fig. 14 an equalizing connection between a series of arresters so lthat the potential caused across all of the gaps is uniform.

IVe also contemplate connecting the vari-v ous forms of arresters, above described, in other circuit relations to meet different operating conditions. The operation of the blowout means used in the various forms of arresters shown is essentially the same, namely, that due to the vexpansion of the air caused by the heat of the arc a sudden movement of the air occurs which tends to carry the arc along with it and either hold the arc to a low discharge value or to break the same completely.

le wish it tp be understood that the difference between the explosive effectwhich we have heretofore mentioned, and the chimney effect, which has also been mentioned, is'largely a matter of degree. In both cases we depend. upon a greater pressure upon one side of the arc, caused by the expanding air, than upon the other, with the natural consequence that the expanded air moves in the direction in which there is the least resistance, or in which the pressure is the least. IVhen the air gap is completely 'closed at the bottom the difference in pressure is caused by the expanding air being unable to move out at the bottom, and hence is all land the shell 3. If this path can relieve the excess pressure, the arc does not strike downwardly and hence-*cannot cause any disturbance in the line, as the resistance prevents excessive discharge. The chimney effect or the bomb effect, if the air space is closed off, then breaks the arc which is formed. If this path alone will not sutiice, the adjacent paths will be called into play by the arc striking downwardly until the pressure is relieved. The tendencyof the blowout arrangement is to hold the arc to the paths of low discharge value and to destroy the arc completely. The greater the arc the greater will be the blowout tendency. Hence a very effective means tending to confine the discharge to the high resistance paths and for thereby preserving the line potential is provided.

In the forms illustrated, i/n which the lower end of the airgap is closed, the tendency is to keep the arc out or to drive it out by the expansion of the gases due to the heat of the are. The initial formation of the arc causes such a sudden expansion of the air as to constitute practically an explosion. This will be evident when it is recalled that air if being heated from room temperature to the heat of the electric arc will be expanded to about 60 times its former volume at a given pressure. Consequently the tendency Vto drive the arc outward is very great and the inclusion of resistance and breaking of the arc thereafter is performed Very quickly. This, however, is not dangerous to the line, inasmuch as theresistance which is included in the sectionalized electrodes is invariably included in the circuity before the arc itself is extinguished or broken. As a consequence no matter how rapid the movement of the arc outward, the arc will never be broken until such a resistance has been included as will prevent line disturbances, for the movement ofthe arc outward, though rapid, is not commensurate in speed with the sudden increase Ain resistance caused by actual breaking of the arc, such as occurs `in the ordinary vhorngap lightning arrester or similar type of arrester in which noresistance is included in the circuit previous to breaking the arc. We consider that this form of arrester is broadlynew and desire that the claims be construed with this in mind.

Numerous changes and modifications will tures will suggest themselves forl meeting particular conditions, and we consider such changes, modifications and adaptations to come within the spirit and scope of our invention, and we desire the present specification and appended claims to beconstrued with this in mind.

Nhat we claim as new and desire to secure by Letters Patent is the following:

1. The method of breaking an arc existing between two electrodes which consists first, in partially confining the gases about said electrodes, second, allowing the gases to expand in a given direction, third, moving the arc with said expanding gases, fourth, and increasing the resistance of the circuit in which the arc is formed simultaneously with the movement of the arc.

2. The method of breaking an arc which consists in first, compressing the gases at the point of formation of said'arc, second, decreasing the pressure upon one side of said arc, third, causing the compressed gases about said arc to move toward said side, fourth, thereby carrying the arc with said gases, fifth, and simultaneously causing greater resistance to be included in the circuit of which the arc forms a part but lexterior to the arc.

3. The method of breaking an arc which consists in the following: first, compressing the gases about said arc, second, decreasing the pressure upon one side of said arc, third, causing the compressed gases to move said arc with them, fourth, and simultaneously with the movement of 'the arc inserting re? sistance in the circuit externally of said arc.

4. The method of breaking an arc which consists in first, partially confining lthe gases in which said arc is formed, second, allowing expansion of the gases heated by said arc in a given direction, third, movingsaid arc.

bodily or in part by the movement of said expanding gases, fourth, and thereby driving said arc to a path of increased circuit resistance. p 4

5. The method of breaking an arc which consists in first, partially confining the gases in which said arc is formed, second, causing the expansion of the gases to blow the arc along, third, and simultaneously increasing i the ohmic resistance of the electrodes between which the are exists. Y

6. The method of breaking an arc which consists in first, partially confining the gases in which said arc is formed, second, Jcausing the expansion of the gases to blow the arc along, third, and simultaneously increasing the ohmic resistance of the circuit in which said arc is excluded, extraneously of said arc.

7 The method of breaking an arc which consists in first, partially confining the gases -in which said arc is formed, second,` allow'- ing said gases to expand, third, to blow the arc along, fourth, and causing the arc by its movement to include greater ohmic resist ance in its own circuit. A

8. The method f breaking an arc which consists in first, partially confining the gases in which said arc is formed, second, allowing the gases to expand, and third, -blowing the arc along between the electrodes, fourth, in-

. an outer shell electrode inclosing said inner part of said inner electrode comprising a plurality of metal sections connected together by resistances and the upper portion of said inner electrode comprlsing a conical conducting section.

12.'In combination a pair of electrodes with an explosion concentric chamber formed therebetween, one of said electrodes being sectionalized and resistances connected between said sections.

13. In combination a pair of concentric electrodes with anexplosion chamber formed therebetween, one of said electrodes comprising a plurality of metal sections connected byresistances.

14. In combination a pair of concentric electrodes one of said electrodes comprising a plurality of symmetrical sections, and resistances connecting adjacent sections of said electrode.

15. As a means for breaking an arc, a single central sectionalized electrode and an inclosing shell electrode surrounding said central electrode, said electrodes having an annular space between them forming an air gap, said space being open'to the atmosphere at one-end, said central electrode being conductively continuous.

16. In combination a pair of electrodes having a gap therebetween, one of said electrodes being sectionalized, resistances connected between said sections, said sectional electrode and said other electrode forming a tending electrodes having a gap therebetween, one of said electrodes being sectionalized, resistances connecting said sections, said electrode having an air gap of a length greater at the end of less resistance than at the end of greater resistance of said electrode.

18. The method of relieving excessive pressure which consists in causing the accumulated excessive pressure to discharge in a spark or arc through pathsrequiring successively greater pressures to cause the sarne to operate and having successively greater discharge capacity and simultaneously creating a tendency to drive bodily the arc or spark to the paths of least discharge capacity and then breaking said arc or spark.

19. The method of relieving excessive pressure which consists in causing the accumulated excessive pressure to discharge in a continuous arc or spark through paths requiring successively greater pressure to cause the same to operate and having successively greater discharge capacity and simultaneously creating a tendency commensurate with the discharge occurring to contine the arc or spark to the paths of least discharge capacity and then breaking said arc or spark.

20. The method .of relieving excessive pressure which consists in starting the discharge or arc through a path having relatively less break-down strength and relatively less discharge capacity, then causing said discharge or arc to move bodily to an adjacent path having greater discharge capacity, said movement being progressive 1n accordance with the quantity of excess pressure to be relieved, then creating a blow-out force. tending to drive said discharge or arc bodily to a path of relatively less discharge.

capacity and then breaking the discharge or arc.

2l. The method of relieving excessive electrical pressure4 which consists in initiat*` ing the discharge or arc through a path of relatively less discharge capacity and rela tively less breakdown strength then bodily movingthe discharge or arc to an adjacent path of relatively greater discharge capacity and relatively greater breakdown strength,

then moving said discharge or arc to a path of less discharge capacity and then lengthening the arc and finally breaking the same. 22. The method of relieving excessive electrical pressure which consists in initiating a discharge orarc through a path of relatively less discharge capacity and relatively less break-down strength, then bodily moving the discharge or arc to an adjacent path vori relatively greater discharge capacity and relatively greater break-down value,

and simultaneously conning or partially means tending to drive the discharge from the gaps of greater discharge capacity to the gaps lof less discharge capacity, said gaps being positioned adjacent each other so that the spark or arc can move bodily from one gap to another without cessation.

24. In combination a plurality of juxtaposed spark gaps, said gaps requiring successively greater voltages to break down the saine and having successively greater discharge capacity, said gaps having means tending to drive the spark from the gaps oi greater discharge capacity to the gaps oi less discharge capacity, said means acting in degree commensurate with the size of the spark and a common flaring portion for breaking the spark or iow of current.

25. In combination a plurality of adjacent superposed spark gaps, the upper spark gaps being of relatively less length than the lower spark gaps and being of graduated length, the longer spark gaps having relatively greater discharge capacity, said gaps having means tending to drive the spark from the gaps of greater discharge capacity to the gaps of less discharge capacity and a common Haring or blow-out portion common to all of said gaps.

In witness whereof, we hereunto subscribe our names this 18th day of March` A. 1915.

Witnesses EMIL W. STEEGE, FRANK WV. SxELDxNG.

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