US1144028A - Protective device. - Google Patents

Description

E. E. F. CREIGHTON.

PROTECTIVE DEVICE.

APPLICATION HLED MAR.19,1913.

Patented June 22, 1915.

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E. E. F. CREIGHTON.

PROTECTIVE DEVICE.

APPLICATION HLED MAR.19, 1913.

Patented June 22, 1915.

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UNITED sTA'rns 'PATENT OFFICE.

ELMER E. F. CiBiEIGH'ION, 0F SCHENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION NEW YORK.

PROTECTIVE DEVICE;

7 'o all 'w11 om, t may concern Be it known that I, ELMER E. F. Cnmen'roN, a citizen of the United States, residing at Schenectady, inthe countyof Schenectady, State of New York, have invented certain new and useful Improvements in .Protective Devices, of whichrthe following is a specication.

My invention relates to devices for protccting circuits from. abnormal voltages, lightning and similar disturbances, andl more especially to protective devices for circuits of low normal operating voltage.

The protective device most commonly used for telephone circuits, signal circuitsand similar systems of low normal operating voltage is a form of spark gap lightning arrester comprising essentially two electrodes separated by a very short air gap. The air gap has an appreciable time lag, as it will not break down the very instant an excess voltage appears, and is furthermore liable to be bridged by dust, dirt and by the metal carried from the electrodes by a heavy discharge.-

The object of my invention is to provide a protective device which does not have the defects of the usual spark gap lightning arrester and which will remove abnormal voltages and transient-disturbances without dangerousdelay and before the abnormal voltage rises to dangerous value.

My invention comprises certain novel features set forth more particularly in the appended claims, but the invention itself will be better understood in connection with the accompanying drawings, which merely for purposes of illustration show some of the various forms in which my invention may be embodied; and in Which- Figure 1 is a plan view of one form of a lightning arrester embodying my invention; Fig. 2 is a longitudinal section of the form of arrester shown in Fig. 1 with the electrodes and some of the interior parts shown inelevation; Fig. 3 is a longitudinal section of an arrester shown in Fig.- 2 in which loss of the vacuum in the devicey short circuits the electrodes and permits the defective arrester to be detected-by simple tests; Fig. 4 is a longitudinal section of a modification, in which a desirable form of seal is shown and the electrodes are so arranged that the arcing takes place in a sort of circular horn I Specification of Letters Patent.

Application led March 19, 1918. Serial No. 755,529.

gap, the electrodes also having fusible disks on them; Fig. 5 shows a transverse section of the form of arrester shown in Fig. 4 on the line 5-5; Fig. G is a longitudinal -section of a modification in which the discharge occurs between electrodes which are far removed from the seal and are so mounted that the heatof discharge does not aiiect the seal; .Fig 7 is a longitudinal section of another form of arrester iii which the discharge takes place at such a distance from the seal that the heat of the discharge does not affect the seal; and Fig. 8 is a longitudinal section of a' form of arrester in Which the. discharge takes place in a horn gap mounted in the vacuum chamber.

In the particular form of device shown in Fig. 1 the ari-ester is mounted upon a porcelain base 1 provided at each end withA terminals 2 by means of which connection is made to the conductors of the circuit to be protected. A ground connection or terminal 3 mountcdncar the middle of the porcelain base 1 engages and holds in position a metal tube 4 which is closed at each end in any suitable way to forni a vacuum chamber which contains the electrodes of the spark gap. By suitable connections the line electrodes mounted in the tube are connected to the terminals 2 and when an abnormal voltage or other disturbance appears upon either conductor of the circuit a discharge will occur from the corresponding electrode to the metal tube 4, which forms the ground electrode. The metal tube is exhausted to that degree of vacuum which is most effective in permitting discharge of abnormal voltage to ground and preventing a flow of current at the normal voltage. By making the vacuum chamber of metal I am able to dispense with a special ground electrode,l maintain the vacuum more nearly constant during rdischarge on account of the heat radiating capacity of the metal tube 4, materially reduce the size of the vacuum chamber, as the heat of discharge will not crack or injure the metal tube 4, and in general produce a much more rugged and desirable device than would be possible with a vacuum chamber of glass.

In order to prevent the arrester being partially disabled by dust and dirt collecting between the terminals 2 and thc tube 4 and forming a conducting bridge between them I provide means for protecting the ends of Y between the terminals, but the preferred constructionl is 'that shown in the drawing, 1n

V 1.0 which each end of the tube is inclosed in an,

connections can be established between the terminals and-the tube except through the -electrodesinside of the tube. Various forms of construction may be adopted to protect theend of the tube and shield it from the insulating cap 5, preferabl made of porcelain and extending along t e exterior of the of metal plates set in t tube 4 `to such a 'distance that the tube is thoroughly protected from contact with the terminal 2." f

The electrodes inside ofthe tube 4 are connected to the terminals 2 through end connections 6 preferably made in the form e insulating caps 5 concentric with the axis of the tube 4. These metal plates are connected t'o the outer end of heavy metal leadin in wires 7 which extend longitudinally o the tube and carry on their inner ends electrodes 8 preferably made in the form of disks mounted concentric with the tube 4. The electrodes 8 are spaced -away from the walls ofthe metal tube 4to form a spark gap through which the normal voltage of the line cannot maintain a discharge, even at the reduced pressure ofthe residual gases in the' tube 4. A dangerous voltage, particularly 1f transient or oscillating, will break down the dielectric between the electrodes and the walls of the tube 4 and `discharge through the tube 4 and the terminal 3 to ground.

In order to prevent spattering of molten metal from one electrode to the other in case of a heavy discharge from one line to ground and in general to protect the electrodes from each other, I provide between them a porcelain spacer 9 preferably made in the form shown in Fig. 2 and of such a size that it offers suiiicient cooling surface to prevent molten metal from one electrode spattering to the other electrode and short circuiting the arrester by establishing a conducting bridge'between the other electrode and the tube, or between the two electrodes. The electrodes 8 are supported and positioned in the tube in anysuitable'way,

preferably by means of porcelain or other' insulating supports 10, which may be made in various forms, but are preferably made as shown in the drawings in the form of disks fitting into the tube and having a central projection or hub through which the leading-in wires extend.

Since under some vconditions a heavy discharge current may flow through the leading-in wires 7 and :the electrodes 8, some form of seal must be used which will .not leak when subjected to variations in temperature and which .will not give oif gases and thereby deteriorate the vacuum of the tube dustand dirt-which might form a bridgevable vitreous or aurons 4 if the discharge is' heavy enough to heat the leading-in wires. I provide such a seal by filling the tube adjacent the electrode support 10 with a hard insulating material, which need not-be air-tight, but which will not give 0H any gases .when heated and which. can be fixed inY osition in the tube. For this purpose I pre er to use some suitorcelain like material made in the form o a stopper 11 and placed immediately lbehind the electrode support 10. This stopr will not give oif ases when heated, s elds the remainder o the seal from the heat of the arc and acts as a support to prevent displacement of the vis. cous material forming the remainder of the seal. The stopper 11 is backed up and rendered air-tight by a vfilling of soft wax 12.

which is impervious to air and always makes an air-tightj'oint, `both withthe wallsof the |tube and with the stopper 11, thereby sealing up all of the'pores 1n the stopper 11 and preventing the entrance of air into the tube 4. This layer of wax 12 preferably consists of pitch or other soft material which will 'adhere firmly to the tube and also to the stopper 11. This layer 12 is in turn backed up by another layer 13 of a wax which is more firm and solid than the soft pitch 12 and which also makes a tight joint with the walls of the tube and thelead.- ing-'in wire 7. A seal constructed as just described is not opened or destroyed by variaf tions in temperature either of the tube or of s the leading-in wire 7, as the soft wax 12 and the firmer wax 13 will always make an airtight joint with the tube and leading-in wire under all conditions of temperature to which l the device will ordinarily be subjected. The stopper 11 is strong enough mechanically to revent distortion of the wax layers 12 an 13 by pressure, and as a result I obtain a seal which will not leak' under any conditions to which the arrester will ordinarily be subjected.

The protective device constructed as above described is exhausted to produce in the metal tube a partial vacuum of such a value that lightning and similar abnormal disturbances are removed from the line with minimum delay and resistance. The desired vacuum may easily be obtained by pumps, such as are commonly used in the art, and after the tube is exhausted it may be hermetically sealed in any suitable way. In the opening or connecting the tube to the pump is sealedby means of a drop of solder 14 when the vacuum in the tube is of the proper value. In some cases it may be preferable to exhaust the metal tube 4 through a small tube made of solder which can then be melted and sealed oit' in the same manner as the glass tubulature of anincandescent lamp.

In many cases I prefer to first fill the tube with some suitable gas and then exhaust it to the required vacuum, so as to leave in the tube a vresidual atmosphere of that gas. I prefer to use as a residual'gas some of the monatomic gases, such as argon, neon, or helium, especially the inert atmospheric gases, such as argon or neon, although a residual atmosphere of hydrogen air or nitrogen will give satisfactory results'. The proper pressure of the residual gas depends upon the nature of the gas in the tube and vmined by connecting in parallel between the leads of the same circuit the protective device and an outside spark gap formed by needles which are. adjustable with relation to each other. Sufficient high frequency Ipotential is applied to the circuit to cause oscillating disruptive discharges through either the protective device or the outside spark gap, or both. In practice I prefer to start with the needles far apart so that all the discharges go through the arrester and gradually bring them together until a majority of discharges pass through the needle gap. This test, while not the best way of determining absolutely the equivalent needle gap, is rapid, gives consistent results, and is accurate enough, especially for purposes of comparison. To secure the desired result the frequency of the applied potential must be of the order of that of the spark discharge of a condenser, and in practice I obtain the desired high frequency potential by a condenser discharging through gaps. The `frequencies obtainable by any mechanical or similar interrupter are too low to be of value for this purpose. That value of outside needle gap in parallel, which just causes the protective device to take the majority of the discharges, is vthe equivalent needle gap referred to in connection withthe test above described: With the gas in the protective device at atmospheric pressure, this equivalent needle gap is relatively high. As the protective device is 'exhausted the equivalent needle gap diminishes and passes througha minimum `known as the critical pressure of the spark gap when the pressure of a residual atmosphere of hydro` gen in the protective device is between 8 to 18 mm.' of mercury. The pressure can be considerably higher than this Without causing an objectionable increase in equivalent i needle gap, but further exhaustion beyond critical pressure lcauses the equivalent needle gap to increase very rapidly. This pressure seems to be substantially-constant and substantially independent ofthe length of the spark gap when the gap isbetween about one-half mm. and one cm. in length. In an arrester embodying my invention I prefer to maintain the pressure of the residual gas within this pressure range or a little greater than the critical pressure of the spark gap over which the arrester has the minimum equivalent needle gap.

A disruptive spark discharge furnishes favorable path for di'scharges to ground and is the kind of a discharge most suitable for relieving a system of lightning, highfrequency oscillations and similar abnormal potentlals and frequencies. A brush discharge carries but little current and does not relieve the line of very much energy. Byheating up the electrodes the brush discharge may actually hinder the formation of the spark discharge and the relief of the system, especially where the arrester is used on comparatively low voltage vdirect current systems. I so space the electrodes that at the preferred pressure the spark dis-` charge occurs almost simultaneously with the appearance of a brush discharge and 4at a voltage somewhat above the normal voltage of the system to be protected. This preferred pressure is above the critical pressure at which the spark potential is a minimum. With brass electrodes, which I 'preferto use, the proper length of gap is about 1 mm. which at the pressure of minimum -equivalent needle gap gives an arrester which breaks down with practically no brush discharge and'which has a spark potential very slightly above the minimum spark potential.

A lightning arrester should permit the passage of transient over-voltages and very high frequency oscillations with a minimum time lag. By high frequency I mean oscillations and oscillatory discharges of the frequency of lightning or of the spark discharge of condensers, probably from vonehalf million to several million cycles per second. The time lag must be so small that the transient over-voltage will not have:

time to puncture the insulation. The'spark discharge should occur without appreciable time lag in responseto the application of a transient high voltage -of minimum amount over the normal potential of the system. I reduce the time lag to an unobjectionable amount by adjusting the gas pressure and the .spark gap without seri* ously affecting the minimum equivalent needle gap. I have found that at pressure above 10 mm. of mercury the time lag is not enough to be objectionable, but that it is serious at lower pressures, increasing very rapidly as the pressure decreases. I, therefore, make the pressure such and so space the electrodes that the time lag is unobjecnear the minimum, and spark potential is reasonably low.`

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Through various causes the vacuum in the protective device may-be lost, 1n whlch case it is desirable that some wa be provided for -detectin the defective evice. In accordance wit my invention I provide means whereby the electrodes 8 are automatically bridged or short circuited as soon as the vacuum is lost, whereupon the defective arrester may easily be located by simple tests. The electrodes 8 may be short circuited 1n various ways but the construction which I prefer is that shown in Fig. 3, 1n whlch a short circuiting or bridglng member 15 1s pivotally mounted upon a sgmcer 16 corresponding to the spacer 9 o F1g.'2 and 1s automatically swung into short cu'cultmg position whenl the vacuum is lost by some suitable means which responds to the 1ncrease in pressure in the tube 4. A sultable device for swingin the short circuitmg member 15 is a resi ient diaphragm .17, as shown in Fig. 3, and normally held 1n the position shown by the pressure of the atmosphere but biased to move outward and swing the bridging member into engagement with electrodes 8 as soon as the pressure 1n the tube is increased by loss of vacuum.

The tendency to extinguish the are between the electrodes and the tube may be increased by -positioning theelectro'des eccentrically with relation t`o the tube so that the are occurs in a sort of circular horn gap in which it is automatically lengthened and ultimately extinguished. One form of arrester in which the extinguishing effect of the horn gap is secured is illustrated in Fig. 4, in which the tube 18 contains electrodes 19'n1ounted eccentrically to a ground ring 20,.which asbest shown in Fig. 5, is electrically connected between the electrodes to ground. The electrodes 19 and the ground ring 20 are carried in a yoke 21-made of orcelain or other` suitable insulating material'and itting the tube 18 so as to Ahold the parts in proper relation. Stoppers 22 are mounted adjacent the yoke 21 and are made of some suitable porcelain like material which can be baked in the tube and which will not give oif gases when heated. The Stoppers 22 are backed up by layers 23, 24 and 25 of diii'erent kinds of waxes which form air-tight joints with the tube 18 and also with the leadin -in wires 26, which are connected at their inner ends to the elec# trodes 19 and at theirouter ends to connection plates 27 imbedded` in the porcelain end blocks 28..

The vacuum used in the form of arrester shown in Figs. 4 and 5 is preferably the same as that used in arresters of the type shown in Fig. 2. Under some conditions I accentuate the arc extinguishing feature of the circular horn gap formed between the eccen trically dispose electrodes 19 and the ground ring 20 by so proportioning the space around the electrodes that upon a severe and long continued discharge t e residual gases in the arrester'expand to such an extent that their pressure is very greatly increased and their arc extinguishing effect is correspondingly increased. The arc extinguishing eiect of the arrestersshown in the other res of the drawin may be increased in t e same way if consldered advisable, although I prefer to so proportion the arrester shown in Fig. 2 that the pressure of the residual gases 1s mcreased but slightly during the usual discharge.

Discharges through the protective devicev grounded to protect the apparatus connected to it. In addition the continuous and heavy discharges heat the electrodes and the adjacent portions of the arrester so hot that sufficlent gases are evolved to lower the vacuum,

and in general the arrester is deteriorated to such an extent that it should be removed and replaced by a new one. In order to automatically ground the system when crossed wlth a hlgher potential system, and to facilltate detection and removal of the arresters which have been injured, I provide means whereby the electrodes are automatlcally bridged and short circuited and are also connected to ground when the electrodes are heated to a certain extent. The preferred means for securing this result, as apphedto'the particular form of arrester shown 1n Fig. 4, comprises fusible metal films 29 secured tothe faces of the electrodes 19 and made in `the form of disksof less diameter than the electrodes. The arc or discharge from the edge of 'the electrode 19I does not touch the fusible disk, which is heated only by conduction of heat from the electrode. As the disks 29 are in intimate contact with the electrodes 19 a rise in temperature of the electrodes will cause the disks 29 to melt and the molten metal will run down and form a conducting bridge between both electrodes and the ground electrode, which in this case is the ground ring 20. Whenvthis has occurred the short circuited arrester may easily be detected by suitable tests and then replaced.l The fusible disks 29 may obviously be applied to the electrodes 8 of the arrester shown inFig. 1, if desired, inwhich case the molten metal connects both electrodes to the grounded tube 4.

In some forms of arresters designed to withstand severe and continuedl discharges 1,144,028 y vw it is desirable to protect as far as possible the waxes .and otherv lmaterial of the seal from undue heating by the action `of the arc between the electrodes. I attain this result i by placing the electrodes at a considerable distance from the sealy and connecting them to the leading-in wires through metallic connections so proportioned as to have a very Vthe discharge. 4In this arrester the tube 30,

exhausted'to a suitable degree of vacuum, is

provided with two electrodes 31, one connected to line and the other to ground. Each electrode is mounted on a radiator 32 which exposes a very considerable surface to the residual gases in thetube and is also quite close to the tube so that as the radiator becomes hot the heat is quickly reinovedcfrom it. Each radiator 32 is connccted to a leading-in Wire 33 which extends through a stopper 34 of suitable refractory material. Each stopper is backed up by a seal comprising a layer of suitable wax 35 contained in chambers formed around the leading-in wires by suitable constrictions 36 in the tube 30, the wax being held in place in the constrictions by the Stoppers 34. The leading-in wires may' beponnected to line and to ground by terminals 37 on the ends of the tube V30. The radiators 32 hold the electrodes 31 as far as possible from the seals and no heat can be transmitted Jfrom the electrodes to the leading-in wires of the seal except by passing through the radiators 32. Another device for securing the same result is shown in Fig. 7, in which an evacuated glass bulb 38 contains an electrode 39 mounted to discharge to an electrode 40 at a point which is as far as possible from the ends ,of the electrode 40 which are embedded in the wax-41 of the seal. In this form of arrester the discharge takes place as far as possible from the seal and practically none of ythe heat generated at the electrodes reaches the seal because of the great distance through which it must-be transmitted and the amount of radiating surface of the electrodes 39 and 40.

A modification in which the extinguishing action of the horn gaps is secured is shown in Fig. 8. An evacuated glass bulb 42 contains a ground electrode 43 which coperates with two line electrodes 4-4 so positioned with relation to the ground electrode 43 as to form therewith horn gaps in which the 55 arc may rise vertically and be extinguished.

To prevent the possibility .of'arcs following the electrodes 44down to the seal and there-l by injuring the seal,'I coat both of the electrodes near the seal with' a protective coating 45 of refractory insulating `material soA that the arc cannot reach the electrodes at points close enough to-the seal to injure it. Iii this form of arrester the electrodes are sealed `into the glass stein of the vessel 42, but the seal between the electrodes and the stem is not vrelied uporrto hold the vacuum.

The seal is made tighty by filling the stem with suitable wax 46..v

My invention may 'be embodied in many other ways than those described and I, theref ore, do not limit my invention to the precise arrangements disclosed-except in so far 'as it is limited by the scope of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is 1. A protective device comprising a chamber and spark gap terminals spaced apart within said chamber and surrounded by aluid at a pressure which renders the equivalent needle gap of said spark gap a minimum.

2. A protective device comprising a chamberand spark gapterminals therein, Said chamber containing fluid 'at a pressure which renders the equivalent needle gap of said spark gap a minimum, and said terminals being spaced to have a low spark potential at the pressure existing in said chamber. v

3. A protective device comprising spark gap terminals, and ay partially evacuated chamber inclosing said terminals and containing a monatomic gas at a pressure between 8 min. to 20 mm. of mercury.

4. A protective device .comprising spark gap terminals, and a` partially evacuated chamber inclosiiig said terminals and containingV gas at a .pressureggreater than the critical pressure of saidv gap but low enough to render the equivalent needle gap substantially a minimum'. g

5. A protective device comprising spark gap terminals separatedby a gap of about 1 mm. and a chamber inclosing said spark gap and containingA gas at a pressure of 8 to 2O mm. of mercury.

6. A protective device comprising a sealed metal chamber containing an inert gas below atmospheric pressure, and an electrode in said chamber spaced away from the walls of said chamber to form a spark gap for discharge of abnormal voltages.

7. A protective dev-ice comprising .an evacuated chamber having a'metal wall connected to ground, and twodisk shaped electrodes mounted face to face and concentric with said tube to form spark gaps therebetween, the distance between the edges of said electrodes and said metal wall being spaced away from the walls of said tube" o..

form spark gaps, and an insulating barrier between said electrodes,

9. A protective device comprising an evacuated chamber having a metallic wall, v

two electrodes mounted in said chamber and spaced away fromsaid wall t'o'form spark gaps therewith, and an insulating barrier. between said electrodes proportioned to prevent the formation of la conducting bridge between said electrodes:

10. A protective device comprising an evacuated chamber in the form of a metal tube closed at each end, disk electrodes mounted concentric with said tube and spaced away from the tube to, form spark gaps. and an insulating cylindrical spacer loosely positioned in said tube between said electrodes to prevent theformation of a conducting bridge between said electrodes.

11. A protective device comprising an evacuated chamber in the form of a metal tube, disk electrodes mounted concentric with said tube, and a porcelain spacer between said electrodes having a cylindrical middle portion fitting loosely in said tube and smaller portions which project from the ends of said spacer. y

12. In a protective device, the combination of an evacuated vessel and spark gap electrodes mounted in said vessel, of means responsive to variations in the vacuum in said vessel for varying the resistance to the passage of current between said electrodes.

13. A protective device comprising an evacuated vessel, spark I gap electrodes mounted in said vessel, and means actuated byan increase in the pressure in said vessel to establish a conducting path between said electrodes.

14. A protective device comprising an evacuated vessel, spark gap electrodes mounted in said vessel, a metallic bridging member normally out of electrical contact with said electrodes, and means responsive to an increase in pressure in said vessel for bringing said metallic bridging member into connection with said electrodes.

15. A protective device comprising an.

evacuated vessel, spark gap electrodes mounted in said vessel, a bridging member for engagingsaid electrodes and normally out of contact with said electrodes', and means actua-tedby an increase in pressure 1n said vessel for bringing said bridging member intol engagement with said electrodes and thereby short-circuiting the de- 16. In a protective device, the combnation of an evacuated vessel, spark gapelectrodes mounted in said vessel, and leadingin wires which extend through the Walls of `said vessel, of a seal comprising a refractory stopper adjacent each electrode for shieldf 17. In a protective device,

trodes mounted in said vessel, and leadingin wires which extend through the walls of l 15 the combination of an evacuated vessel, spark gap elecsaid vesselof aV seal comprising refractory Stoppers of porcelain like material mounted adjacent each electrode for shielding said seal from the heat of arc discharge', and a layer of soft impervious adherent wax to the walls of said vessel and to the leading-in 85 wires and impervious to air immediately be' hind and in contact with said stopper and adhering to said stopper.

18. In a protective device, the combination of an evacuated vessel, spark gas elec- .j

trodes mounted in said vessel, and leadingin wires which extend through the walls of said vessel, of a seal comprising a ,plurality of layers of wax-like material impervious sel and to said leading-in wires, the wax in said layers being of different degrees of hardness.

19. In a. protective device the combination of an evacuated vessel, spark gap ,100

'to air and adherent to the walls of said ves- 95 terial adjacent the electrode, a layer of soft impervious adherent wax immediately adjacent said stopper and in contactvwith the Walls of said vessel and with said leading-in wires, a second layer of adherent hard wax adjacent the first layer, and a solid insulat` 11 ing end piece engaging said second layer of lwax and the walls of said passage to form a closure for the opening into said vessel.

20. In a protective device the combination of a metal tube, spark gap electrodes mounted in said tube, and leading-in wires which extend longitudialy of said tube, of a seal comprising a solid stopper of refractory in-` sulating material adjacent each electrode, a plurality of layers of 'adherent wax impervious to air behind said vstopper and engaging the walls'of the tube and the lead-iY ing-in wires, said Vlayers being of different density, and an end piece of solid insulating material having a central opening for the passage of the leading-in wire and fittingA into the end of the tube to engage the wax mounted in said vessel, and leading-in Wires a leading-in wire which projects from 45 which extend intosaid tube from the ends, of a sealV for saidleading-in wires comprising a stopper of solid refractory insulating` material adjacent each electrode, a porcelain end` piece -for each end of the tube having a central opening for the leading-in wire .and a viscous adherent wax filling for the tube between said lstopper and said end piece.. n

22. A protective device comprising an evacuated chamberhaving metal walls, an electrode mounted in said chamber, a ground terminal on the metal wall of' said chamber, a line terminal mounted adjacent the Wall of said chamber and connected to said j' electrode, and an insulating shield substanlsaid chamber to form an insulating Wall between said terminal and said chamber.

24. A protective device comprising an evacuated cylindrical chamber having metal walls, an electrode in said chamber having a leading-in'wire projecting from the end of said chamber, a terminal mounted outside said chamber near the end thereof and con-A nected to said leading-in wire, and an insulating cap for the end of said chamber,

- said ycap having a central opening for said leading-in wire and extending along the outside of said chamber from the end of said chamber to a point beyond said -termi-l nal.

25. A protective devicel comprising an evacuated ,cylindrical chamber having metal walls, an electrode in said chamber having ends of said chamber, a porcelain end cap having side walls which surround said chamber for a considerable distance from the end and a central opening for said leading-in wire, an end connection secured to said leading-in wire and recessed in said porcelain cap, and a terminal provided with an extension for engaging' said connection and mounted adjacent said porcelain'end cap to bring the side walls of said porcelain cap between said terminal and the metal Walls of said tube.A

26. A protective device comprising an inisulatingv base having a recess therein, an

evacuated metal chamber fitted in lsaid recess, a ground connection for said chamber near the middle thereof, electrodes 1n said chamber having leading-in wires which project from the ends of said chambers, insulating end caps having side walls which inclose a considerable `portion of the end Walls of saidchamber and are provided with a central opening through which said leadingin wires project, and terminals electrically connected to said leading-in wires and mounted on said base adjacent the side walls of said insulating caps.

27. A protective device `comprising an evacuated metallic tube closed at the ends, electrodes inside of said tube, terminals mounted adjacent each end of said tube and electrically connected to said electrodes, and

.insulating caps encircling the ends of said tube adjacent said terminals and extending along said tube beyondsaid terminals to prevent electrical connection between said terminals and saidtube.

In witness whereof, I have hereunto set my hand this 17th day of March 1913.

` ELMER E. F. CREIGHTON. Witnesses:

BENJAMIN B. HULL,

HELEN ORFoRD.

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