US2400456A - Spark gap electrical apparatus - Google Patents

Description

May 14,1946. M. E HAINE ETAL: v 2,400,455.

' SPARK GA P ELECTRICAL APPARATUS Filed Dec. 18', 1943 i 2 Sheets-Sheet 1 Negative I 0.0 Supp/y Oscillator Load Nejat/ve I L v WW y Oscil/azofi Tr/p 4 a Lo d +c J F/GZ.

Negative I l p y T 3 Oscillator" I V. I (v/p Load +0 5 iucmuzr. mm 11mm JOHN IILLAR IEEK ATTORNEYS May 14,1946, M. E; HAINE; ETAL 2,400,455

SPARK GAP ELECTRICAL APPARA TUS Fil ed Dec. 18, 1943 2 S heats-Sheet 2" f /6,6 "1 aonnunmnmm ATTIORNL'YS Patented May 14, 1946 CsPAn-K GAP ELECTRICAL APPARATUS Michael fEdwajrd "Haine and John Millar Meek, Sale, England, assignors to Metropolitan-Vicker's Electricalfflompany Limited, London, England, a compa'n'yo'f Great Britain Application Dece ber 18, 1943, Serial No. 514,863

In Grea-t Britain July 11, 1941 '21 Claims. (01. 250 s7) This invention relates t o spark ga electrical apparatus adapted under predetermlnable control to be tripped as nearly as possible at a predetermined instant, but more particularly at a plurality of accurately though not necessarily equally spaced time intervals at recurrence irequencies which may be of the order of several thousand per second. H I I r The present applicationis closely related to copending United States application Serial 7 No. 514,864, invented by Michael Edward Haine, and

copending United States application Serial No.

514,865, invented by Michael Edward Haine, John Millar Meekand John Drummond Craggs, both filed December 18, 1943. v b r I V v The primary object of the invention is to provide a trippable spark gap device w hich shall be reliable in operation as stated hereinafter, and the achievement or this presented a considerable problem which the features of the present invention as hereinafter pointed out in the appended claims have solved to an important extent as has been proved inpractice.

Part of the original secret problem was to employ atri ppable spark gap to provide heavy current substantially square-wave form or very steep-fronted impulses of substantially constant amplitude of the order of several hundred amperes each at recurrence frequencies of from one hundred to several thousand per second, the

impulses being sharply defined so as not to overlap one another and being in some cases of the order of one or a few micro-seconds duration each. For the solution of this problem it was suggested to us'before the date of the present- I are three spheres fixed in line, with the middle one biased so that the voltage is evenly divided across the two gaps in series, the normal voltage between each gap being just too small to break down either gap. To cause the gap to break down an additional voltage or voltage pulse, whose amplitude is a few per cent of the normal voltage existing acros the two gaps in series, is applied to the middle sphere whereby one of the gaps becomes over-volted and break'sidown, resulting in that the full potential becomes applied to the other gap which then also breaks down immediately afterwards, this second spark being the one of primary 'importance and being used for the very high stressing of, for example, an insulator. Which of the two gaps breaks down first, of course, depends on the relative polarity of the tripping voltage applied to the middle sphere. An important advantage of employing the middle sphere is'dueito the 'fact that it has a high and constant circuit impedance 'to it's surroundings so that only a small amount of energy 1 is'required to raise itspotential to the required value to initiate the main breakdown.

I The mechanism of breakdown of this threeball gap seemed to us the most promising with respect to the main object in view as above stated, after consideration of other known arrangements as follows.

Another method (see A. Kohler, Archiv. f. Elek. vol. 30, 1936, page 528) used to trip a spark gap which is stressed slightly below its critical breakdown stress, is to illuminate the cathode of the gap with the light from another smaller spark in the vicinity, when the photo emission at the cathode will cause the main gap to spark over. See inter alia W. Rogows'ki and Wallraff, Zeits. f, Physik. 9'7, 758, 1935; H. 'J. White, Phys. Rev. 1936, vol. 49., page 507; C. Brinkmann, Zeits. f. Physik. 111, 737, 1939; and J. M. Meek, Proc. Phys. Soc. 52, 547, 822, 1940.

Disadvantages of these previously used methods for the predetermined initiation of the breakdown of a spark gap are the general uncertainty in operation, the accurate adjustments required, and the care necessary in keeping the electrodes free from dirt, etc, And, moreover, itis difficult, though essential, to prevent the spark degenerating into an arc. A y

We are also aware of the particular spark gap devices disclosed in British Specification No. 267,929 (the application for the patent not being accepted) of which specification see Fig. 3, di-

' agrarnmatically illustrating. a lightning arrester arrangement for the protection of a transmission line or other part of an electric power transmission system, comprisingtwo metal spheres each having two perforations all inline and each containing a pointed electrode electrically connected to the sphere through a choke reactance, the two points of these electrodes extending into the adjacent perforations of the spheres and facing each other: the one point electrode isv connected to the transmission line or the like and the other is connected through a resistive impedance to earth.

We are further aware of British Specification No, 434,572 which describes and claims a spark gap lightning arrester for the protection of elec trical transmission and like systems, the arrangement being a difierent form of that described in Specification No. 267,929 just above referred to.

In connection with the protection of power transmission line systems, we are also aware of prior British Specifications Nos. 123,064 and 149,618, wherein are included spark gaps Of somewhat similar form to those concerned in the present invention, but with important differences, as will hereinafter appear.

With any protection system on a power transmission line system involving the spark gaps disclosed in the two prior specifications mentioned in the preceding paragraph, and in two other prior specifications earlier herein mentioned, the arrangements are similar to that of the present invention, but only in so far as that the gaps must normally withstand a high voltage until triggered. However, the essential operative difference between said previously disclosed spark gap systems and that of the present invention arises after breakdown of the gap. In the case of the present invention, after the breakdown of the gap the condenser or capacitative network is essentially such (and the appended claims as concerned are to be interpreted accordingly) that the spark discharge occurs in one or a very few microseconds, and thereafter since the remaining or follow-up current is insufficient to maintain ionisation within the gap, the latter ceases to conduct and the gaseous ions are recombined and cool rapidly until in a few milliseconds the gap can withstand again the full voltage. In con tradistinction to this the transmission line protector gap, at breakdown, passes an initial high surge current and thereafter it passes a power line frequency current since in effect it essentially provides a low impedance path for the latter to earth, and the effect of this current is the formation of a hot spot on the cathode electrode of the gap, and hence an arc is formed which must be extinguished or suppressed before the gap will cease to conduct, and this extinction or suppression is an impermissibly slow process with respect to the present invention. It can be achieved on the one hand by reducing the current to so small an amount that the arc can no longer be maintained: this has been achieved in protector gaps by the use of material having non-linear voltage resistance characteristics, for example the substances known under the British Trade-Marks Thyrite and Metrosil. On the other hand, the extinguishing of the arc could be obtained by cooling it artificially such as by means of a blast of air, a method per se well known for circuit breakers, or by causing the arc to spread over a large area, also in per se well known manner, so that it becomes stretched until it is unable to maintain itself. This is achieved in all the arrangements illustrated in the aforesaid prior British Specifications Nos. 123,064 and 149,618 by the provision of arcing horns. Arcing horns are entirely superfluous with regard to the primary object of the present invention qua operation. Since they inherently provide a nonuniform field they could only be used with the spark gap of the present invention, namely only for the protection of the charging source against short circuit or overload, if arranged and disposed so as not impermissibly to impair the cssential uniformity of field. In the arrangements diagrammatically illustrated in Figs. 6 and 8 of British Specification No. 123,064 and in Fig. 5 of Specification No. 149,618 it is possible that the presence of the arcing horns would not greatly impair the uniform field between sphere gaps. It will be clear that the arrangements of all the other figures of these two prior specifications involve spark gaps which are arcing horns and are thus useless for the purposes of the present invention and are excluded from its scope.

When, as a subsidiarily claimed feature of the present invention, an air blast is used, though it is for the purpose of cooling, the object is for preventing, after a desired breakdown of the gap, the premature subsequent breakdown at too low avoltage.

We are of the opinion that the dimensions of at least the main electrodes of the aforesaid prior spark gaps for use for the protection of power transmission lines are at least five, and probably ten, times the dimensions of any form of gap for any of the uses according to the present invention: see, for example, Electrotechnische Zeitschrift, July 1934, pa e 736.

The dissimilar impedances essential in connection with the arrangements shown in the aforesaid British Specifications Nos. 123,064 and 149,618 are superfluous with respect to the claimed scope of the present invention.

Furthermore, in those of the appended claims which involve tripping spark gap electric systems or installations, power transmission lines are excluded, whilst in these claims a condenser or capacitative network is to be interpreted as meaning a specially provided condenser or a specially provided capacitative network (which latter may be what is known as an artificial transmission line) for the essential purpose of shaping the voltage or current wave, and the phrase does not mean a transmission line conveying power from a generating station or sub-station or the like, and the appended claims are to be inter preted accordingly.

Furthermore, since the present invention in operative condition essentially requires that the discharge current in the gap must very rapidly die away after each spark to avoid the latter degenerating into an arc, there must be a high impedance between the power supply and the gap (as is provided by the charging impedance) to limit the current after the condenser or network has discharged.

Some of the spark gaps per se shown in the four prior British specifications above mentioned, on paper, qua form, are somewhat similar to spark gap apparatus which we have evolved with the objects in view as hereinbefore and hereinafter stated, and one aspect of our invention may be said to reside in the selection of such arrangements appropriately modified when used in particular circuits, but we are of the opinion that these prior arrangements as disclosed and as would be used for protection are unsuitable for the purposes in View, namely the very accurate and reliable control in point of time of the breakdown of a spark gap requiring in continued use no adjustment of the gap length and being independent, over a considerable life of the device, of other conditions likely to be encountered in use. Our invention is not concerned with lightning arresters for and when used in power transmission line systems and such are herein specifically disclaimed.

According to another aspect of the present three-sphere gap upon the considerable-increase in the depression ofthe sparkovervoltage: the greater this depresinvention, the spark gap thereof can be con- .namely by reason of there normally being between these main electrodes a substantially "uniform'field, whilst at least one trigger electrode is provided which (or each of which) ises'sentially a slim member arranged and provided in relation to the'main electrodes (which may be 'metal spheres or the like, including plates or part spheres) in such manner as not normally to cause a field which will appreciably interfere with the normal field between the main electrodes. Preferably the main-electrodes have substantially'spherical ends fac'ing each other-{whilst one of them is formed 'with an axial Tperfior-ation towards or into or through which extends the trigger electrode in'the form of a bluntly pointed wire. This trigger electrode, whilst being by reason of the external circuits biased to a i shorter gap between the trigger electrode and that main electrode with which it is associated. An important feature of the unitary structure spark gap device per se of the present invention resides in the specific provision of means or material which effects, continuously or momentarily, irradiation of the gap adjacent the trigger electrode such that the full double breakdown of the gap takes place, every time, with substantially no time lag, and with substantial certainty. It is true that when the triggering voltage pulse is applied to the trigger electrode, corona will be set up around the latter, but we are of the opinion that, on the one hand, this occur too fortuitously for regular controlled breakdown of the spark gap, so that, on the.

other hand, the addition of a separate and distinct source for ionising gas at the electrodes is required. In the original apparatus which we constructed the trigger electrode was mounted within the associated main electrode by means of solid insulating material. This arrangement gave satisfactory results, and we have concluded that this was due to the stressing of, the gas by reason of the presence of this solid insulating material to produce the corona ionising the gas. It is per so common knowledge that the initial ionisation required to cause the breakdown of a spark gap maybe obtained by various specific means, more particularly ultra violet or X-radiation or radiation from radio-active material, and the present invention in some of its aspects includes the specific provision of these means or materials in substitution for or together with solid insulating material or like corona-producing arrangements. The utilisation of cosmic rays solely is unsuitable for our purposes, though its uncontrollable effect cannot, of course, be excluded.

The success of the spark gap set "forth in the preceding paragraph depends in part, we believe,

sion thegreater is the tolerance allowable on-the adjustment of the gap necessary for accurate control of breakdown, and this is a feature of major importance. Thus,- consider any spark gap which, under normal'conditions and with no tripping voltage, wouldhave a breakdown voltage of 100 kv. Now, supposethat with the aid of'the tripping device one can cause the gap to spark over at only kv., thus giVing a ten per cent depression of sparkover voltage, then the application of the tripping'device will cause the gap to break down when it has any voltage between 90 *kv. and kv. upon it. Thus all other factors being constant,'one canwork over this range with no adjustment of'the .gap. How'ever,'in practice, due to change in atmospheric and barometric conditions, to dirt on the electrodes, etc.', thenormal breakdown potential of the gap may vary by five or six .per cent, or even more, so that for reiiable operation'onecan onl work between the limits 'of about 90 kv. to 94 kv. This requires accurate adjustment of the gap initially and from time to time. We have arranged for the controlled tripping device to cause a depression of the sparkover voltage of thirty or fifty per cent of the normal breakdown voltage of the gap. This is an important advantage of the present invention when used for the tripping of an impulse generator apparatus where we have found a great improvement in reliability is obtained, also for spark-operated time sweep circuits for high speed cathode-rayoscillographs or in the synchronous tripping of the impulse generator with the oscillograph time sweep and vice versa. The invention might also be used for the controlled triggering in otherwise per se known manner of spark gaps for certain known photographic purposes. In the photography of objects moving at very great speed, such as projectiles, it is necessary to make use of very short exposures generallysuch that the object travels no appreciable distance during the exposure which may be of the order of cnemicrosecond. By reason of this extremelybrief exposure time, it becomes necessary that the illumination provided for efiecting the exposure should be correspondingly bright, and it is also necessary to provide for controlling very precisely the mom'entof the occurrence of the flash. The foregoing photographic method has long been well known and used. The spark gap and also the gap in association with the charging circuits in accordance with the present invention may advantageously be used in connection with this method of photography since the triggering can be very accuratel controlled, whilst the spark provides a very bright flash of short duration with a minimum of afterglow.

-.'I'he most important use, however, is for the recurrent discharging of a circuit. For such application an additional advantage is obtained by virtue of the fact that, as the gap length is set appreciably or even considerably in excess of the normal sparkover spacing, the tendency for the spark to develop into an arc is avoided or reduced. In such a circuit we have, in our experi ments, run the gap at recurrent frequencies of several thousands per second under conditions where normally it was extremely 'diific'ult t'o'prevent arcing through. v

The slim trigger electrode serves to provide when suddenly excited for a very brief period or a succession of periods a distortion of the normal electric field in the main gap, and this distortion may be stated to be equivalent to the brief formation of a virtual point at the electrode with which it is associated, resulting in a localised concentration of the field, and this will effect ionisation of gas leading cumulatively to a discharge which rapidly spreads across the main gap, that is between the cathode or input electrode and the trigger electrode and thence to earth via the output electrode which is the one having the trigger electrode associated with it.

The initial onset of ionisation requires, as hereinbefore stated, the specific provision of ambient ions or electrons before the field distortion or concentration can cause the main spark breakdown of the gap. As is per se common knowledge stray ions necessary for the purpose can be produced by ultra violet radiation or X-radiation, and the invention (in some of its aspects) involves these radiations (giving photo ionisation) being pro; duced by soft X-rays or ultra violet rays emanating from the pre-breakdown corona discharge which is greatly augmented by reason of the in tense electric stress due to the presence of a solid insulator adjacent the trigger electrode and the main electrode with which it is associated. It

will be appreciated per se that if the gap between a wire and a surrounding metal cylinder be partly filled b a solid insulating material between which and the wire there may be left a small gap, and if this insulating material has a dielectric constant greater than unity, then when a high voltage is applied between the wire and cylinder the voltage stress will be augmented in the gap between these metal members. Ionisation of the gas in this gap will thus occur at a lower voltage than would be the case in the absence of the insulating material and the suddenness of the application of the voltage ensures the corona being formed without time lag after the application of said voltage. It will be appreciated furthermore that adequate creepage distance must be allowed at the end of the solid insulator in order to prevent surface sparkover taking place; thus in those forms of the invention employing such an insulator (as in the preferred forms), the insulator must terminate a sufiicient distance from the ends of the main electrode and of the trigger electrode disposed therein.

The theory briefly indicated above on which we at present consider our invention is in part based, but to which we do not bind ourselves, will be hereinafter further considered.

One of the important uses of the invention, as hereinbefore mentioned, requires that the spark gap shall on breakdown pass a relatively large discharge current of about 50 amperes, with a low voltage drop across the gap of the order of 200 volts, that is low in relation to the applied gap I potential which may be of the order of 10 to 20 kv., each discharge being required to take place during a very brief period, for example one or a few microseconds, and at very accurately spaced recurrence intervals up to relatively great frequencies of, for example, 3,000 or even 5,000 per second, the instant of breakdown being always accurately determined by the tripping voltage pulse. Considerations which then arise are, inter alia, (1) the reliability of operation throughout a reasonable life of the device, that is in respect of erosion and corrosion of the electrodes, I and changes which may be made in the ambient gas, (2) avoidance of a tendency to are with relation to the requisite recovery properties of the device after each main discharge, this being a factor otherwise limiting the recurrence frequency.

The main use of the invention, more specifically, is for the modulation of a generator or oscillator producing ultra high frequency oscillations in pulses of very short duration and considerable peak power. The results achieved have been of successful importance. The generator or oscillator may thus be considered as a resistive load circuit into Which the energy stored in the charged condenser or network passes through the trigger spark gap of the present invention and the electrical connection of the oscillation generator is preferably such that it produces the oscillation pulses only during the instant or instants during which the spark gap breaks down.

The invention also includes further features which will be apparent on perusal of the appended claims.

In connection with the appended claims for triggerable spark gaps per se, we are aware of the following two prior British specifications:

No. 505,529, the arrangement shown in Fig. 5 of which has some resemblance to an enclosed spark gap according to the present invention, but there is the essential difference that in any arrangement according to thi specification, as will be clear to those skilled in the art, the cold cathode is specifically and deliberately constructed in such a way that during operation at a few hundred volts maximum it will provide thermionic heat as a result of glow discharge in the rarified gas or vapour at a pressure of a few centimetres at the most whereby to provide in effect a thermionic emitting cathode. The construction of the cathode is such that it will heat up vary rapidly. In any case, the result is that the voltage drop in the gap is materially lowered; and there can be no spark, a spark being essential in any spark gap device of the present invention even although the gas pressure thereof may be somewhat sub-atmospheric. Any arrangement disclosed in this prior specification is excluded from the scope of any relevant appended claim which is to be interpreted accordingly.

No. 296,067 relates to apparatus for producing high frequency alternating currents by means of spark gaps for therapeutic and like purposes, and appropriate ones of the appended claims are to be interpreted as excluding nitrogen alone at 400-700 mm. mercury pressure enclosed in a sealed chamber. The expression nitrogen alone is not intended to exclude the presence of small traces of impurities ordinarily found in commercial nitrogen.

In the present specification and appended claims spherical or like as applied to an electrode is to be understood as meaning an electrode active surface of large effective radius of curvature (including infinite) in relation to that of the slim or point-like electrode (as will be well understood by those versed in the science of electric sparks) with respect to the operational purposes stated elsewhere herein.

In the accompanying drawings:

Figs. 1, 2 and 3 are diagrams of difierent circuit arrangements with spark gap devices and other features according to the invention.

Fig. 4 is a purely diagrammatic View of a spark gap per se in accordance with the invention.

Fig. 5 is an elevation, half in section, of an enclosed and preferred form of the spark gap according to the invention, and

Fig. 6 is an underneath plan view of the device shown in Fig. 5.

Fig. 7 is a View of alternative means which may be used for reducing erosion of the tripping electheoscillator load 2.

trode of the spark gapaccordingto the invention: this alternative per se forms-the subject of copending United States application Serial No. 514,864 aforesaid.

Figs. 1,2 and 3 will nowbe described;

The cathode Ia of the sparkgap device I has associated with it and the feeding and current limiting choke coil L an artificial transmission. line in series with the gap anclload 2 a shown in Fig. 1 of'the accompanying drawings, in which case the breakdown of the spark gap device I would cause a negative voltage pulse to appear across the valve oscillator, shown as the load 2, and it would be necessary to connect the valve cathode to the anode 3 of the spark gap deviceand the valveanode to earth. it is found in practice that difficulty is experienced due to. the anode of the spark gap device and the-valve cathode rising to the high negative pulse potential. This difficulty arises mainly by reason of providing requisite insulation in connection with the means providing the recurrent tripping voltage pulse to the trigger electrode 4.

spark gap device is earthed, as is one side of The artificial line 5 is, however, fully insulated from earth. It will be seen that the breakdown of the spark gap device I in this circuit will produce a voltage pulse of positive polarity across the oscillator load, so that the cathode of the said oscillator valve will be connected to earth. This is found to be of advantage when a triode. valve oscillatorv as is per se known is required'to be modulated.

In the case of the oscillator being a magnetron, such as is being used in connection with the invention, it is very desirable to earth the anode of the magnetron, and in this case it is expedient. to make use of a phase reversing pulse transformer 2a as shown in Fig. 3. A pulse'transformer is one capable of transmittingshort pulses of voltages from primary to secondary with no appreciable distortion of wave shape.

It should benoted thatthe use of the modified circuits of Figs. 2 and 3 in'noway, affects the 4 t will be .well understood by those skilled the art that by a point electrode there isnot necessarily herein involved one havinga very sharp point, the requirement being aIslim'trigger electrode of such shape in relation tothe relatively large spherical or like shape of the, main electrode forgiving the requisite main field, as to cause asuflicient distortion and concentration of this .field whereby the depression of the. spark-over voltage .is of the order of thirty per cent and upwards, accordingto varying conditions of use. With regard to the life and stability of the device as, determined by electrode wear, we havefound that superior results are obtained. when the electrodes or parts thereof are formed of molybdenum or tungsten.

Referringnext to Fig. 4, the spark gap device according to the invention comprises a cathode Iztwhich is conveniently a sphere at least on that part thereof which faces the anode 3 which as shown comprises a tubular member but having a substantially spherical left-hand end wherein there is an aperture 3a through which extends the rounded, end of the slim trigger electrode 4 which in the arrangement diagrammatically shown in Fig. 4 is held within and spaced from the tubular anode 3 by the insulating sleeve 4a which serves thexfurther purpose hereinbefore stated. The cathode Iahasa terminal or connector to, theanode-ihas'a terminal or connector 3: and the trigger 4 has its terminal or connector 4. The insulator-4a may assume various other forms, conveniently that shown at 41) inFig,/5 tobe hereinafter; described. As hereinbeforeindicated the presence of the insulator 4a causes thepotential gradient to be augmented at'the surface of the trigger electrode 4 for a given trigger voltage pulse so that the corona is suddenly produced along with the arrival of the tri ger voltage pulse. The radial width or the surface spark-over distance of the insulator "4 must be large enough to obviate surface spark- .over on the applicationof the trigger pulse. At

3" is shown a pipe going to the interior of the electrode 3, and at 3" is a pump for causing gas found that nitro-peroxide-was rapidly formed,v

causing the gap to are. We tried various gases of good chemicalstabil-ity, particularly nitrogen and inert gas, especially.argon, at variouspressures-andwith varying results. It was found that in general argon individually was not very satisfactory at high triggering pulse frequencies, due to the fact that the gas atoms attain a metastable state and recover therefrom slowly, that is to say the atoms recover in a. time which may be as long as 0.001 or even 0.1 second. A filling of nitrogengave poor results, probably. due to the. formation ofmetastable atoms a1so,' Whilst a filling of hydrogen gave good operating results the life of the. device was shortened by reason of too rapid wear of the electrodes, particularly the trigger electrode, possibly due to the forma- V tion-Yin the discharge of unstable hydridesof the electrode materials, which subsequently splitjup,

depositing the metal on the tube walls and on the electrodes in powder form.

We are awarethat in'Geiger counters the .presence of a quantity of oxygen with the argon filling causes a rapid quenchingi; that is, recoveryfrom themetastable state: see Journal of The Franklin Institute, May and June 1941.

We have found by research that very good operational results are obtained when the gas filling comprises argon with from aboutone to fiveper cent oxygen content at pressures from 15 to pounds per square inch. Even better results may be expected with such a' mixture at greater pressures when it may be preferableto use metal containers instead ofglass ones. This feature per se forms the subject of aforesaid United States copending-application Serial No. 514,865.

It will be appreciated that when our spark #gap device is enclosed ina sealedchamber.itsoperasparking is verymaterially reduced.

Accordingtothe at present preferred ccnstructionalform of the invention shown in Figs. 5 and 756,- the sparkgap I is enclosedwithintherelacomprising per se a pressed molybdenum spherical portion fixed by the portion lb to the stem lc which passes through the seal id at the top of the bulb.

The molybdenum anode 3 is spot welded to metal straps 3b which are spot-welded to metal posts 8 one of which as shown passes through the seal 9 and is exteriorly connected to an insulated stranded conductor in which is soldered to a tab ll of an external contact member l2 fxed to the tubular insulating cap 13'. The conductor I8 is continued from the tab I l, as shown at Ilia, to another contact member iZa spaced 90 from the contact member 12.

The trigger electrode is in the form of a rod 4 which passes through the seal 9 and is connected exteriorly to the insulated stranded conductor 24 connected to the external contact member i which is symmetrically spaced from each of the contact members l2 and I211. The insulation 419 between the trigger electrode 4 and the anode 3 is in the form of a glass tube fused to the top of the seal 9.

With further reference to the theory of operation of spark gap devices in accordance with the present invention, it may be explained that, as is per se known, the voltage gradient required to cause breakdown of the gap with uniform field is that which causes an electron avalanche, which crosses the gap to the anode and is of such intensity that the radial field produced by the positive-ion space-charge in the avalanche system is of the order of the interelectrode field since the space-charge forms an efiective point on the anode surface. A positive streamer then develops from anode to cathode to form a conducting filament bridging the gap.

The voltage required to cause the propagation of the positive streamer across the gapis much lower than that required to cause the electron avalanche which leads to the initiation of the streamer, and corresponds roughly to that for breakdown between a positive point and an earthed plane. The field distortion produced by the positive space-charge in the avalanche is simulated in the spark gap device of the invention by the field distortion produced by the trip pulse on the trigger electrode, and the breakdown voltage of the gap is thereby reduced by an amount approximately given by the difference in breakdown voltage for a sphere-to-sphere gap and a gap of the same length between a positive point and an earthed sphere. Thus for a gap of '1 cm. between spheres each of 1.3. cm. diameter, the breakdown voltage in air atnormal atmospheric pressure is 29 kv., whereas for the positive pointto-sphere gap of the same length the breakdown voltage is 12 kv. approximately, which is a depression of sixty per cent.

We have found that the magnitude of the po-' tential pulse applied to the point electrode also affects the depression of the breakdown voltage of the main gap. It not only creates a radial field at the point and initiates a positive streamer, but it also augments the main field across the gap. Thus the higher the applied pulse voltage the greater is the depression, other conditions being unchanged.

Referring to Fig. 7 of the accompanying drawings, the arrangement therein illustrated has for its object the reduction of the erosion of the tip of the slim trigger electrode 4. In preliminary explanation of this, it will be appreciated that tively thick walled glass bulb l, the cathode ia in'the case of spark discharges it is thecathodic electrode which suffers the more from erosion: in the caseof the'electrode Ia (Figs. 1-4) since this is of relatively large size the erosion is not of importance compared with that arising at the tip of the slim trigger electrode 4 since it is the secondary spark from the latter to the anode 3 which causes the erosion, this secondary spark carrying the whole current .of'that of the main and all important spark between the cathode la and the electrode 4. In the arrangement shown in Fig. 7 there is on the one hand connected between the anode 3 and earth a resistance 3c or inductance which is relatively low in value but is, say, ten times higher than theimpedance of the load 2 (Figs. 1-3) through which the current of the sparks passes on tripping. On the other hand there is connected between the tripping electrode 4 and earth the auxiliary spark gap 40 which is adjusted to spark over at a voltage slightly in excess of the trip voltage. It will be appreciated that the operation of the arrangement shown in Fig. '7 depends on the relative impedances of the various electrodes. The electrod 4 normally has a very high impedance to' earth, which may even be infinite: the impedances of the sparks are relatively very low. On tripping a spark takes place between the cathode la and the tip of the tripping electrode 4, immediately followed by the secondary spark from the latter to the edge of the perforation in the anode 3, so that approximately the voltage of cathode la appears across the relatively low resistance 30 whereby the current in the spark from the electrode 4 to the anode 3 is limited while nearly the voltage of the cathode la remains on the electrodes 3 and 4. Immediately following the "secondary spark, therefore, is a spark across the gap 40, the current in which tertiary spark is many times that of the secondary spark; thus the erosion of the tip of the tripping electrode 4 may be reduced, for example, a thousandfold. It will be appreciated that the spark gap 4c'may be made robust.

In general for any given electrode spacing it is, of course, the uniform field gap which requires the highest voltage for breakdown. The spark gap arrangement according to the invention will 0perate withvoltage pulses of either polarity in conjunction with either polarity of the high volt- .age electrode of the main gap. However, the best 7 operation for any given polarity of high voltage electrode is obtained when a pulse of opposite polarity is applied to the fpoin electrode, and the greatest depression of breakdown voltage occurs for a positive voltage pulse when the high voltage electrode is of negative polarity. The fact that the depression is not so great for a negative pulse as for a positive pulse may be explained, since the'negative streamer does not propagate as readily as a positive streamer, and the voltagev requiredto cause breakdown between a negative point and an earthed plane is about twice that for a positive point, although it is still considerably below thatfor the uniform field.

It will be appreciatedthat in the arrangements of spark gap illustrated in the drawings with the active end of the trigger electrode 4 approximately coincident with the active surface of the main electrode 3, the potential at which the electrode 4 is normally biased is the same, or

nearly the same, as the normal potential of the prising in combination, a circuit having'capacitytherein and means for connecting it to a source of current, an impedance connected in said-circuit for charging said capacity to a high potential, a load connected across said circuit through which the charged capacity is dischargeable at predet'erminable instants of time, a spark gapdevice with which said load is connected in series, said for applying a normal potential bias to said trig ger electrode, said trigger electrode being so formed ahdlocated that when normally appropriately biased there is so little disturbance of the substantially uniform field between said main electrodes that breakdown between them "is avoided with appreciable tolerance, and when a triggering pulse of sufficient magnitude is applied to said trigger electrode breakdown is caused due to the appreciable distortion of said field.

2. A tripping spark gap electric system accord-' ing to claim 1, wherein at least one of said trigger electrodes is located sufiiciently near to one of said main electrodes that on the application of a triggering pulse to said trigger electrode a main spark occurs between the other of said electrodes and said trigger electrode followed immediately by another spark in series between said trigger electrode and said main electrode with which it is associated.

3. A tripping spark gap electric system according to claim 1, wherein a portion of said trigger electrode has a relatively small radius of curvature which is directed towards one of said main electrodes.

4. A tripping spark gap electric system according to claim 1, wherein one of said main electrodes has a perforation, and said trigger electrode extends from behind towards said main electrode and towards the other of said main electrodes whereby a radial gap is provided between said trigger electrode and the edge of said perforation.

5. A tripping spark gap electric system according to claim 1, wherein said main electrode with which said trigger electrode is associated has a circular perforation, and said trigger electrode is of small cross-section and has a rounded active end portion.

6. A tripping spark gap electric system according to claim 1, including additional means for irradiating the space between at least two of said electrodes.

7. A tripping spark gap electric system according to claim 1, including means for providing ionizing radiation in the space between at least two of said electrodes by the application of said triggering voltage pulse.

8. A tripping spark gap electric system accord-.

ing to claim 1, including means to augment the gradient at the surface of said trigger electrode for a given trigger voltage pulse, comprising solid insulating material located between said trigger gering voltage pulse a corona discharge which emits ionizing radiations to cause sparkover.

10-. A tripping spark gap electric system according to claim ,1, including means for'causing a fiow.

of a gaseous medium between said electrodes for thepurpose of preventing, after a desired breakdown between them, a premature subsequent breakdownat too low a voltage.

11. A tripping spark gap electric system accord ing to claim 1, wherein one of said main electrodes has a perforation toward whichsaid trigger electrode extends, and'is tubularand provided with means for admitting a gaseous medium to its in- I terion to flow around said tri 'ger'electrode and through said perforation; I

12. A-tripping-spark gap electric system accord-' ing to claim 1, including a sealed chamber enclosing said electrodes and containing an inert gas mixed with a small proportion of a suitable quenching gas.

13. A tripping spark gap electric system according to claim 1, including a sealed chamber enclosing said electrodes and filled with argon.

14. A tripping spark gap electric system according to'claim 1, including a sealed chamber enclosing said electrodes and filled with an inert gas mixed with oxygen in the range of proportions of from one to five percent, of oxygen.

15. A trippable spark gap device, comprising as a unitary structure, input and output main electrodes having spherical surfaces and fixed apart appreciably beyond their normal sparkover spacing in relation to a predetermined voltage to be normally applied across the gap, and at least one fixed slim trigger electrode associated with one of said main electrodes and so formed and located that when normallyappropriately biased there is so little disturbance of the substantially uniform field between said main electrodes that when the normal voltage is applied breakdown is avoided with appreciable tolerance, and when a triggering voltagepulse is applied breakdown is caused due to the appreciable distortion of said field, and a gaseous medium above atmospheric pressure enveloping said electrodes.

16. A trippable spark gap device, comprising as a unitary structure, input and output main electrodes having spherical surfaces fixed apart appreciably beyond their normal sparkover spacing in relation to a predetermined voltage to be normally applied across the gap, and at least one fixed trigger electrode associated with one of said main electrodes and so formed and located that when normally appropriately biased there is so,

little disturbance of the substantially uniform field between said main electrodes that when the normal voltage is applied breakdown is avoided with appreciable tolerance, and when a trigger?- ing voltage pulse is applied breakdown is caused due-to the considerable distortion of said field, a gaseous medium above atmospheric pressure enveloping said electrodes, and mean for causing the gas in at least one of the spark gaps to be irradiated by ionizing radiation.

17. A trippable spark gap device according to claim 16, wherein said means for irradiating said spark gap by ionizing radiation comprises solid insulating material located between said trigger electrode and said main electrode with which it is associated' 18. A trippable spark gap device, comprising as a unitary structure, input and output main electrodes having spherical surfaces fixed apart appreciably beyond their normal sparkover spacing in relation to a predetermined voltage to be normally applied across the gap, and at least one fixed slim trigger electrode so formed and located that when normally appropriately biased there is so little disturbance of the substantially uniform field between said main electrodes that when the normal voltage is applied breakdown is avoided with appreciable tolerance, and when a triggering voltage pulse is applied breakdown is caused due to the considerable distortion of said field, and a gastight chamber enclosing said electrodes and containing a gaseous medium.

19. A spark gap device according to claim 18, wherein said slim trigger electrode is associated with one of said main electrodes and is directed towards the other of said main electrodes.

20. A spark gap device according to'claim 18, wherein one of said main electrodes has a perforation, and said trigger electrode comprises a rod which extends from behind said main electrode and into said perforation and towardsthe other of said main electrodes, so that a radial gap is provided between said trigger electrode and said perforated main electrode.

21. A trippable spark gap device comprising, as a unitary structure, input and output main electrodes having spherical surfaces fixed apart appreciably beyond their normal sparkover spacing in relation to a predetermined voltage to be normally applied across the gap, and at least one fixed slim trigger electrode associated with one of said main electrodes and so formed and located that when normally appropriately biased there is so little disturbance of the substantially uniform field between said main electrodes that when the normal voltage is applied breakdown is avoided with considerable tolerance, and when atriggering voltage pulse is applied breakdown is caused due to the appreciable distortion of said field, said electrodes being exposed to the atmosphere, and a terminal member for each of said electrodes for 20 connecting it to an appropriate circuit.

MICHAEL EDWARD HAINE. JOHN MILLAR MEEK.

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