US2290526A - Spark gap - Google Patents

Description

SPARK GAP W. E, BERKEY ETAL Filed April .16, 1941 July 21, 1942.

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ATTORNEY Patented July 21, 1942 r SPARK GAP William E. Berkey, Forest Hills, and Joseph Slepian, Pittsburgh, Pa., assignors to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application April 16, 1941, Serial No. 388,846

27 Claims.

Our invention relates to electric protective devices embracively designated at lightning arresters, and is more particularly directed to devices of this type employing as one of its component parts one or more insulated spark-gaps in series. Lightning arresters of this kind are quite common, and have a series-gap part, either alone, or in combination with other component parts, such as quench gaps, or valve-elements that limit the surge current, a series gap-device being, generally, a device comprising a pair of electrodes separated by a clear gaseous space, and is the only device now known which has a suillciently consistent breakdown voltage for use in practical lightning arresters.

The function of the series-gap to which our invention applies is usually three-fold; first, under normal conditions it keeps down the voltage gradient across the remaining parts of the arrester; second, when a surge of certain characteristics occurs, the series-gap breaks down, shunting the surge current through the arrester to ground; and third, after the surge energy has been substantially dissipated, the series-gaps interrupt the residual or so-called power-flow current fiowing through the arrester after the restoration of normal voltage conditions across it. Accordingly, the arrester which would afford the greatest degree of protection to the electrical circuit or conductor to which the arrester is applied would have a gap that would consistently break down at a potential well below the insulation level of the line or apparatus being protected, upon the occurrence oLany destructive or undesirable surge, but which would still consistently have a sufllcientlyhigh normal-frequency breakdown to prevent unwanted operation under normal line conditions.

The performance of an arrester with respect to its breakdown is represented by what is known to the art as its impulse ratio. This ratio is obtained by dividing the breakdown voltage under a surge of standardized characteristics by the crest breakdown voltage under normal-frequency potential. Our invention seeks to improve this ratio for arrester gaps while at the same time retaining the efilcacy of the gap with respect to the other functions desirable in such a device.

The breakdown voltage of the gap is generally acceded to be dependent upon the time rate of the application of potential, and the gap is, therefore, said to have a time lag. This time lag may be assumed to be composed of two components: first, the build-up time, and secondly, the random or stray time. The build-up time is that required for one electron to cross the gap under breakdown gradients, and is the minimum time in which a spark can form. In short gaps of one millimeter spacing, the build-up time is in the order of 1 seconds, with an uncertainty factor of 4. The random stray time is directly related to the mean time elapsing until an electron is emitted from the cathode. It seems probable that in short spark gaps, photo-electric emission, high field or cold emission at the cathode provides the starting electrons.

Spark gaps have, therefore, been artificially ionized, as, for example, by a radiant or radioactive source, in order to reduce the time lag of breakdown of a gap. The prior art has also sought to create a supply of electrons suitable for encouraging breakdown by providing sharp point on the cathode to distort and intensify the local electrostatic field about the points. The latter method has taken the form of somehow roughening the surface of the electrodes of the gap to provide projections serving as the necessary points.

We have found that a critical size of projections or particles give a maximum improvement in breakdown characteristics. If one or two relatively large particles are used, then the surge breakdown voltage will be low and consistent, but there will be an objectionable lowering of the normal-frequency breakdown potential due to excessive ionization from the particle tip. However, we have discovered that, on the whole, if many smaller particles are used, the normalfrequency breakdown voltage is not appreciably affected when compared to the surge breakdown voltage which is lowered and stabilized. As an explanation of this phenomenon, assume that one electron will initiate a discharge when a gap is subjected to a given surge gradient and that a time lag of 10- seconds is desired. Then there must be an electron current of 10 electrons per second passing into the gap when the static breakdown potential is reached to give a delay of 10" seconds. Now if this electron current comes from one or two points, then we have experimentally found that the normal-frequency breakdown potential is lowered objectionably. But if the 10 electrons per second are scattered over an area containing, say 1000 discharging points, then the current from each point is only 10 electrons per second, and this condition does not lower the normal-frequency breakdown volta e.

It is an object of our invention to provide a spark-gap or lightning arrester element, whose impulse ratio is considerably improved, that is. lowered by the use of small discrete particles of such quantity and size as to occupy an inappreciable volume of the gaseous space or gap between the two metallic electrodes which are the terminals of a discharge; the impulse ratio of a spark-gap being defined as the ratio of the breakdown voltage under a standard surg test wave front to the breakdown voltage under normal-frequehcy voltage. The particles act as a pre-lonizing means which has a large effect for encouraging surge voltage breakdowns and little effect on normal-frequency voltage breakdowns.

It is an object of our invention to provide a spark-gap with a pre-ionizing means for reducing its impulse ratio, the pre-ionizing means taking the form, in our preferred embodiments, of small particles which reduce the surge breakdown voltage to a greater extent than the extent to which they reduce the normal-frequency breakdown, so that the impulse ratio of the spark-gap is lowered.

In preferred specific forms of our invention, the spark-gap comprises a, pair of spaced electrodes having substantially a clear gaseous gap or space between them and a multiplicity of discrete, small, angular particles in the gaseous gap, in such quantity and volume as to inappreciably change the physical dimensions of the gaseous gap between the electrodes as compared to those of the gaseous gap without the particles therein.

In certain forms of our invention, the improved impulse ratio is obtained by securing or binding extremely small chips or particles on the surface of one or more of the metallic electrodes of the spark-gap, the particles or chips being preferably insulating or semi-insulating, although they could also be metallic, and initiating a discharge at the points of the particles or chips, the breakdown flashing around high resistivity particles to the electrodes proper. Refractory insulating or semi-insulating, that is, relatively high resistivity, chips or particles do not wear down, as a roughened metal surface or metal particles will, and have a considerably greater effect on the surge breakdown voltage than on the normal-frequency breakdown voltage so that a spark-gap of relatively consistent performance and improved impulse ratio is obtained.

In other preferred forms of our invention a spark-gap with an improved impulse ratio is obtained with particles which are loosely dusted in the gap in the course of its assembly or manufacture, that is, the particles need not be secured mechanically to an electrode or electrodes. This makes it possible to considerably reduce the cost of the spark-gap since the securing of the particles or chips to the electrodes constitutes a difllcult production problem.

It is a general object of our invention to provide a form of spark-gap element usable for overvoltage or surge-protection, which has a relativelyconsistent and very low impulse ratio, the spark-gap including spaced metallic electrodes providing a clear gaseous gap therebetween and a pinch or so of small discrete angular particles loosely contained, that is, mechanically free, in the gaseous gap of the spark-gap.

With the foregoing and other objects in view, our invention consists in the elements, combinations and structures set forth in the following description, taken in conjunction with the drawing, in which like reference numerals refer to like parts, and in which:

Figure 1 is a cross-sectional view of one form of series-gap embodying our invention;

Fig. 2 is a similar view of a modified gap structure;

Fig. 3 is a similar view of still another form of gap structure;

Fig. 4 is a curve indicating the impulse ratio, and 60 cycles normal-frequency breakdown voltage, of a gap such as shown in Fig. 1, with different sized particleson the electrode surfaces: and

Fi 5 is a cross-sectional view on a somewhat enlarged scale, of another form of series-gap embodying our invention.

Our invention is applicable to any appropriate type of spark-gap, and for this reason, we have shown four different forms of spark-gaps. As shown in the different embodiments, the seriesgap device I comprises two main electrodes which are preferably made of brass or other low-resistance material having good arc-interruptin properties. The electrodes are separated by an annular or ring-shaped insulator preferably of porcelain. For lightning arresters, it is usually desirable that the height or axial length of the spacer-ring be considerably greater than the arc-gap or gas space between the gap electrodes, and to this end one or both of the main electrodes can be dished inwardly.

In the embodiment shown in Fig. 1, the electrodes 2 and 4 are both of the same dished form, each being centrally perforated and turned outward, away from the other electrode, so as to leave a rounded annular portion 6 which, in conjunction with the similar portion 8 of the opposed electrode, provides the main gaseous gap In across which it is desired to have a breakdown upon the occurrence of a surge.

The gap of Fig. 2 shows a construction that is somewhat more economical than the seriesgap of Fig. 1. In Fig. 2, the outside electrodes l2 and It may be of the same form and shape as the electrodes of Fig. 1, but the porcelain spacer is now formed of two parts l6 and I8, one on each side of a fiat, centrally-perforated electrode 20 therebetween. This construction provides two narrow aseous gaps in series-one 22 between the dished electrode I2 and one side of the fiat electrode, and the other 24 between the opposite side of the flat electrode 20 and the other dished electrode l4.

As a further indication of the general adaptability of the principles of our invention, Fig. 3 shows still another modification of a series-gap. In this embodiment, the main electrodes are somewhat similar to the outside electrodes of Figs. 1 and 2, but, however, are not centrally perforated, being instead continuous and somewhat spherical.

The essential feature of our invention consists in providing the spark-gaps of any of the above or other embodiments with a multitude of preionizing sources which will serve as a source of spark-producing electrons, the sources being obtained from a multitude of insulating or semi-insulating, or conducting particles.

In accordance with our invention, we secure to the surface of the electrodes minute particles or chips 26 having a size measured in a few thoussandths of an inch, the size being determined by the mesh of a wire screen through which they pass. Such fine chips are secured to the arcing portion of the electrodes by any suitable means. For example, a very thin coating of a thermoplastic adhesive may be applied over the arcing portions of the electrodes, which are then dipped into a mass of particles of desired size while the adhesive is soft. After cooling, any surplus grains may be removed in order to provide as far as possible a single layer of chips on the surface of the electrode, which does not appreciably alter the straight-line clear distance of the gaseous gap between the electrodes.

Water-glass may also be employed for binding the chips to the surface of the electrodes. In such case, a very thin coat of water-glass is applied to the active or most stressed portions of the electrodes which are then dipped into the insulating chips. When the binder dries. the chips will be held securely in place.

As another alternative, a low-firing vitreous glaze can be used, in which case the chips should be of a porcelain or similar substance having physical characteristics that will not change at the firing temperature of the glaze.

Still another alternative method for embedding the chips on the surface of the electrodes can be had by softening the arcing portions thereof and then pressing the electrode into a flat mass of chips while the surface of the electrode is still soft. Subsequent cooling of the electrode will then firmly secure the chips onto its surface.

The chips themselves are irregular in shape and will provide points projecting outward from the electrode surface. Many thousands of these chips can be secured to an electrode surface portion of approximately one square inch.

In practice, one or more of any of the abovedescribed series-gap devices are utilized in series with one or more lightning-arrester elements as diagrammatically indicated at 28, to form a complete lightning arrester which is generally connected between an electric circuit, or conductor 30, which is to be protected and the ground 32. The number of series-gaps employed in the arrester is dependent upon the normal-frequency voltage, enough such gaps being stacked to withstand the expected normal stresses.

When a voltage is applied to the series-gap incorporating the chips, the electrostatic field about the points of the chips is intensified and serves, in efiect, to discharge electrons in suflicient numbers into the gaseous gap at the proper time and in the proper area to give a minimum time lag of breakdown. We have found that the diminution of voltage under a surge is different from that under normal-frequency potential, the decrements depending on the size of the chips. In Fig. 4 is shown an impulse ratio curve for porcelain chips of average grain size varying up to approximately 16 mils for a spark-gap such as shown in Fig. 1 having a minimum gas space of about 60 mils, which is about the customary spacing for series-gap devices. It may be observed that there seems to be a critical size for which the lowest impulse ratio is obtained, and we have found this to be true with particles of substances other than porcelain, as, for example, silicon carbide and rutile. The general shape of impulse ratio curves for the latter substances substantially follows that of Fig. 4, and this holds true for any of the series-gaps shown herein.

We have found by actual experiment that the minimum ratios obtained with porcelain, silicon carbide, and rutile, respectively, for a series-gap such as shown in Fig. 2, occur with particles passing through mesh of mil openings for porcelain, approximately /2 mil openings for silicon carbide, and 2 mil openings for rutile. We consider the fact that a critical size of particle gives a minimum impulse ratio to be an important discovery in this field. The small size of the particles or chips renders a mass of them in the form of a powder or dust.

The particle size depends primarily on the substance or material of which the particles are composed, a certain range of sizes for each particular material reducing the impulse ratio, the reduction depending on the size of the particles.

As the size is diminished within the range, tit-e impulse ratio decreases until a minimum is reached, after which the impulse ratio increases. Different materials produce minimum impulse ratios at different sizes of particles, but the improvement in the impulse ratio begins with particle sizes of as much as 20 mils with some materials.

We have further found by experiments that the chips should preferably be of an insulating or semi-insulating material, that is, poor conductors, and by these terms we mean a material surface is not completely covered but provided with numerable crevices between particles. When flashing over the chips, the discharge goes through these crevices. Porcelain, rutile, and silicon carbide, as well as other substances, have been found to yield improved results.

Referring more particularly to the form of our invention illustrated in Fig. 5, the spark-gap comprises electrodes 40 and 42 disposed on each side of a porcelain spacer ring 44, the electrodes being, preferably, of brass or other metal having good arc-interrupting properties. The electrode 42 is preferably a flat disc, and the electrode 40 is preferably a symmetrically dished disc gradually approaching the electrode 42 and having the apex of the dished portion spaced therefrom to provide a minimum gaseous gap or space 46 between the electrodes. "The electrodes 40 and 42 and the spacer ring 44 define a substantially closed gaseous chamber 48, which includes the minimum gap-space 46, between the electrodes.

In accordance with this form of our invention, a pinch or so of very small particles or chips in the form of a dust is indiscriminately placed within the gaseous chamber 48, while the sparkgap is being assembled. The particles can be added in any suitable way and can be dropped into the gas chamber. We have effectively added the particles by first dipping a very small camel's hair brush into a mass of the loose dust particles, and then painting one or both electrodes by brushing the dipped brush lightly over one or both of the electrodes. The particles are loose inside the spark-gap and are not mechanically bound to the spacer ring or to the electrodes, as in the aforedescribed embodiments.

For the embodiment shown in Fig. 5, we prefer to use particles which are of a size so fine that they do not settle readily but have a tendency to float or be suspended in the chamber 48. When the normal frequency voltage is placed across the gap, the created electrostatic forces cause such particles to be attracted to the electrodes, producing a multiplicity of pre-ionization points therein. We have found that the portions of the electrodes at the minimum gaseous gap 46 become covered with a layer of these particles of dust and are held thereto by the electrostatic forces. If a breakdown or spark occurs, the sound or air motion accompanying it stirs up the 1%" inside diameter, 2

dust and recoats the electrode surface again. Large size particles which also decrease the impulse ratio are not as desirable as very fine particles which have a certain percentage which always seem to float in the air within the sparkgap. The number of particles is, of course, considerable but all of them are not necessary for obtaining the improvement in the impulse ratio of the gap. However, the total volume of particles should be small and should be limited so that substantially a single layer only precipitates on the electrodes at the minimum gaseous gap 46. Some excess of particles is not objectionable since the excess particles, including those on top of the layer, are blown or forced by the electrostatic forces toward the spacer ring 44, as indicated at 50.

While we have indicated that very fine particles are preferable, larger particles, which improve the impulse ratio, can also be used in the form shown in Fig. 5 if there is some assurance that the bottom electrode will always be the oathode on a discharge or breakdown. In service, however, this is not generally the case so that it is desirable to use particles of the finer sizes, which will float in the gaseous chamber of the spark-gap and be electrostatically attracted to both electrodes.

In a series of tests on a spark-gap similar to that of Fig. 5, having a porcelain spacer of about outside diameter, and a height of .47", and a minimum gaseous gap of 60 mils, we have found that particles of less than 200 mesh would improve the impulse ratio regardless of which electrode was made the oathode, and the following table is an indication of average results we have obtained with particles of different size and different materials.

From this table it may be observed that silicon carbide particles of 600 mesh were very effective for reducing the impulse ratio. As observed under a high power microscope, the 600 mesh particles used had an average diameter of about .12 to .25 mil with the largest ones ranging up to about .63 mil, and the smallest ones ranging to below .08 mil.

Alumina particles of the size mentioned improved the impulse ratio when the lower electrode was the cathode, but did not have much effect on the impulse ratio when the upper electrode was the cathode. The alumina particles appear to be less eiiicient for the same particle size and shape than some of the other materials. We have observed that the edges of alumina are less sharp than those of the other materials, so that their ionizing effect might be less.

Rutile particles of less than 3 mils average particle size but greater than 200 mesh were effective for reducing the impulse ratio regardless of which electrode was the cathode. Porcelain particles also improved the impulse ratio when the lower electrode was made the cathode.

Our tests indicate that the silicon carbide particles seem to be the most effective, not only because they improve the impulse ratio, but also because of their ability to be attracted to and stick to the upper electrode.

It is, of course, extremely difficult to measure the exact quantity of dust particles placed in the the minimum gap-space 48, leaving a sufficient number in the high field region which lower the impulse breakdown of the surge voltage to a greater extent than the normal-frequency voltage.

In making the herein described tests, the average rate of rise of the surge test voltage was about 50 k. v. per microsecond; the normal frequency voltage was a cycle source; and the impulse values listed were the average of a number of repeated runs.

Where the particles are secured to the surface of the electrode, it is important that the thickness of the binder be very small and less than that of the size of the grains to the end that the arc will transfer from the chips to the electrodes. That is, the particles do not solidly coat the electrode surfaces but are scattered and distributed thereover so that the surface is not completely covered but provided with numerable crevices between particles. When flashing over the chips, the discharge goes through these crevices to the metallic electrodes.

Comparative tests with our new gap devices have shown impulse ratios of relatively great constancy for a given gap, considering the performance of prior devices, and much more consistent values were obtained compared to the values that usually are obtained with gaps without pre-ionizing chips.

In the preferred forms of our invention the atmosphere within the spark-gaps is air at atmospheric pressure except during breakdown or flashover.

While we have described our invention in manners which appear now to be the best modes of application thereof, it is obvious that many changes may be made and different forms of gaps employed. For example, an electrode may be provided with small insulating stubs projecting into the gaseous gap, the stubs being painted with a conducting paint having a large number of particles, such as those described.

This application is a continuation-in-part of our application, Serial No. 239,910, filed November 12, 1938.

We claim as our invention:

1. A series-gap device comprising a spacer ring, a pair of metallic electrodes disposed one on each side of said ring, said electrodes being separated by a gaseous gap, and capable of becoming the terminals of a concentrated arc-discharge including a portion of said gaseous gap, and a multitude of distinct and separate sharp angular particles in said gaseous gap for pre-ionizing said portion of said gaseous gap under the influence of a voltage gradient by concentrating the electrical stress at a multiplicity of points in said gaseous gap whereby its impulse ratio is lowered, said particles being small and inappreciably changing the extent of the said gaseous gap, said particles being of a size between .0005 and .02 inch and numbering in excess of-1000.

2. A spark gap comprising a pair of spaced electrodes of good conducting material, at least one of said electrodes being provided with a multitude of small chips 01' porcelain of approximately 5 mil size.

3. For a lightning arrester and the like, a series-gap device comprising metallic electrodes separated by a gaseous gap, and capable of becoming the terminals of a concentrated arc-discharge and having adequate arc-interrupting qualities, means comprising semi-conducting material on at least one of said electrodes tor concentrating the electrostatic field in a multitude of scattered points on the surface of at least said one of said electrodes, the distance of the furthest edges of said points to said one electrode being in the order of a few mils, whereby a low impulse ratio for said gap device results. F

4. For a lightning arrester or the like, a seriesgap device comprising metallic electrodes separated by a gaseous gap, and capable of becoming the terminals of a concentrated arc-discharge, a multitude of small scattered particles adapted to be secured onto a surface of at least one of said electrodes to provide projections above said surface, and a thin binder for securing said particles to said electrode.

5. The device of claim 4 wherein said particles are of insulating material, and provide paths of higher resistance through the points to at least said one of said electrodes than through said binder under arc-discharge conditions whereby the points initiate the are which finally is established between the electrodes.

6. A series-gap device comprising a spacer ring, a pair of metallic electrodes disposed one on either side of said ring, said electrodes being separated by a gaseous gap, and capable of becoming the terminals of a concentrated arc-discharge and having adequate arc-interrupting qualities, at least one of said electrodes having a centrally dished portion approaching the other electrode to provide, in eflect, an arc-gap, means to concentrate the electrostatic stress at a plurality of points in said arc-gap comprising small, poorly conducting, particles adapted to be secured to at least one of said electrodes, and a thin binder for securing 'said particles to the last said electrode, whereby a gap is obtained whose impulse ratio is a factor of the size of said particles.

7. The structure of claim 6 wherein said particles are porcelain.

8. A spark-gap comprising a pair of metallic electrodes separated by a gaseous gap, and a single layer of particles scattered on a surface of at least one of said electrodes, and secured thereto, said particles being poor conductors.

9. A spark-gap comprising a pair of metallic I electrodes separated by a gaseous gap, and substantially a single layer of particles on a portion of the surface of at least one of said electrodes,

said particles being poor conductors.

10. A spark gap comprising a pair of metallic electrodes separated by a gaseous gap, and sharp, angular particles having a resistivity considerably higher than that of said electrodes, whereby the particles may be deemed insulating or semiinsulating, and comparable in electrical conducting properties to the class of substances comprising porcelain, silicon carbide, and rutile, attached on the surface of at least one of said electrodes.

11. The gap of claim 10 characterized by said particles being of a size in the order of 20 mils or less.

12. The gap 01 claim 10 characterized by said particles being of a size in the order of 20 mils or less, and numbering in the order of 1000 or above.

13. A spark gap comprising a pair of metallic electrodes separated by a gaseous gap, and a plurality of sharp angular chips or particles attached on, and distributed over a surface of at least one of said electrodes, said particles being poor conductors of electricity.

14. A spark gap comprising a pair or metallic electrodes separated by a gaseous gap, at least one electrode having on one of its surfaces a multitude of about 1000, or considerably more,

small, angular protuberances, substantially all of said protuberances extending .02 inch or less beyond said one surface toward the other of said electrodes, said protuberances being scattered over said one surface with portions of said one surface uncovered, so that an impulse ratio for the gap results which is dependent on the size and quantity of said protuberances, the size and quantity being such that substantially a minimum ratio is obtained.

15. The gap of claim 14 wherein said protuberances are semi-insulating protuberances.

16. A metallic spark gap electrode having a multitude of sharp particles distributed over its surfaces, said particles being of a size lying between .0005 and .02 inch.

17. A series-gap device comprising a spacer ring, a pair of metallic electrodes disposed one on either side of said ring, said electrodes being separated by a gaseous gap, said electrodes being capable of becoming the terminals of a concentrated arc-discharge and having adequate arcinterrupting qualities, at least one of said electrodes having a centrally dished portion approaching the other electrode to provide, in effect, an arc-gap, means to concentrate the electrostatic stress at a plurality of points in said arcgap, comprising small semi-conducting particles on one of said electrodes, whereby a gap is obtained whose impulse ratio is a factor of the size of said particles.

18. A gap-device of the type described for an excess-voltage protective device, comprising, in combination, a spacer means, a pair of electrodes of an arc-discouraging metal, maintained by said spacer means in insulated spaced relation, said electrodes being separated by a gaseous gap and having electrode-portions capable of becoming the terminals of an arc-discharge; and a dust in said gaseous gap for producing pre-ionizing electrons at a multitude of points in said gap only when stressed electrically, for lowering the impulse ratio of the gap-device by lowering the surge voltage breakdown in greater proportion than the normal-operation voltage breakdown as compared to these breakdowns without the aforesaid multitude of pre-ionization points, said dust being in small quantity and occupying an inappreciable part of the gaseous gap without said dust.

19. A device of the type described for an excessvoltage protective device comprising a spacer ring, a pair of metallic electrodes disposed one on each side of said spacer ring, said electrodes being separated by a gaseous gap and having electrode-portions capable of becoming the terminals of a concentrated arc-discharge, said spacer ring and said electrodes having surfaces cooperating to define, in effect, the boundaries of a gas-filled chamber, and a pinch of loose and discrete small angular refractory particles of relatively high resistivity in said gas-filled chamber, said particles being of a size in the order of a few mils or less, and not mechanically attached to the said surfaces defining said gasfilled chamber, said particles being in such quantity as to occupy an inappreciable part of said chamber.

20. A series-gap device of the type described, comprising a pair of separated metallic electrodes having surfaces capable of becoming terminals of an arc-discharge therebetween, said device further comprising insulating means associated with said electrodes for providing therewith a gas-filled chamber, in effect, in which said surfaces are exposed. and pre-ionizing means comprising a multitude of discrete small angular particles in said gas-filled chamber for pre-ionizing the gaseous gap between said surfaces in such a manner that surge voltage breakdowns are lowered to a greater extent than'normalfrequency voltage breakdowns as compared to their respective breakdowns without said preionizing means, said particles being in such small quantity as to occupy an inappreciable part of said chamber, and being mechanically free in said chamber, said particles being of a size in the order of less than 20 mils, and numbering in excess of 1000.

21. A series-gap device of the type described, comprising a pair of separated metallic electrodes having surfaces capable of becoming terminals of an arc-discharge therebetween, said device further comprising insulating means associated with said electrodes for providing therewith a gas-filled chamber, in effect, in which said surfaces are exposed, and pre-ionizing means comprising a multitude of discrete small angular particles in said gas-filled chamber for pre-ionizing the gaseous gap between said surfaces in such a manner that surge voltage breakdowns are lowered to a greater extent than normal-frequency voltage breakdowns as compared to their respective breakdowns without said pre-ionizing means, said particles being in such small quantity as to occupy an inappreciable part of said chamber, and being mechanically free in said chamber, said particles being very fine particles of the class consisting of silicon carbide, rutile, porcelain and alumina.

22. A series-gap device of the type described, comprising a pair of separated metallic electrodes having surfaces capable of becoming terminals of an arc-discharge therebetween, said device further comprising insulating means associated with said electrodes for providing thereith a gas-filled chamber, in effect, in which said surfaces are exposed, and pre-ionizing means comprising a multitude of discrete small angular particles in said gas-filled chamber for pre-ionizing the gaseous gap between said surfaces in such a manner that surge voltage breakdowns are.

lowered to a greater extent than normal-frequency voltage breakdowns as compared to their respective breakdowns without said pre-ionizing means, said particles being. in such quantity as to occupy an inappreciable part of said chamber, said particles being rutile particles passing through a 200 mesh sieve, and averaging about three mils.

23. A series-gap device of the type described, comprising a pair of separated metallic electrodes having surfaces capable of becoming terminals of an arc-discharge therebetween, said device further comprising insulating means associated with said electrodes for providing therewith a gas-filled chamber, in effect, in which said surfaces are exposed, and pre-ionizing means comprising a multitude of discrete small angular particles in said gas-filled chamber for pre-ionizing the gaseous gap between said surfaces in such a manner that surge voltage breakdowns are lowered to a greater extent that normal-frequency voltage breakdowns as compared to their respective breakdowns without said pre-ionizing means, said particles being in such quantity as to occupy an inappreciable part of said chamber, said particles being silicon carbide of a size in the order of 600 mesh.

24. A series-gap device comprising a spacer ring, a pair of metallic electrodes disposed one on either side of said ring, said electrodes being separated by a gaseous gap, and capable of becoming the terminals or concentrated arc-discharge, said electrode having arc-interrupting qualities, at least one of said electrodes having a centrally dished portion directed toward the other electrode, for providing, in effect, an arcgap, said spacer ring and said electrodes cooperating to define, in effect, a gas-filled chamber, and pre-ionizing means for said arc-gap, said pre-ionizing means comprising discrete, angular particles mechanically loose and free in said gap, and adapted to be easily displaced therein by gas-fluid waves in said chamber, accompanying an arc-discharge, said particles being of a size in the order of a small fraction of an inch for reducing the impulse ratio of said gap as compared to the impulse ratio of said gap without said particles.

25. A spark gap comprising a pair of spaced electrodes having arcing surfaces in an atmosphere consisting of air having initially suspended therein a finely divided dust.

26. A spark gap comprising a pair of spaced electrodes having arcing surfaces in an atmosphere consisting of air having initially suspended therein a finely divided dust comprising silicon carbide of a size in the order of 600 mesh.

27. A series-gap device comprising a gas-filled chamber, said series-gap device having a pair of separated electrode means having portions capable of becoming the terminals of an arcdischarge in said gas-filled chamber, each of said means being relatively insulated with respect to the other, said pair of means having their aredischarge terminal portions separated by a gaseous gap, and pre-ionizing means comprising a multitude of discrete small angular particles in said gas-filled chamber for pre-ionizing the gaseous gap between said terminal portions in such a manner that surge voltage breakdowns are lowered to a greater extent than normal-frequency voltage breakdowns as compared to their respective breakdowns without said pre-ionizing means, said particles being in such small quantity as to occupy an inappreciable part of said chamber, said particles being of a size in the" order of 20 mils or less, said particles being a refractory substance different from that of said terminal portions, and providing a multitude of uncovered angular edges in said chamber.

WILLIAM E. BERKEY. JOSEPH SLEPIAN.

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