US2449397A

US2449397A - Electric spark gap

Info

Publication number: US2449397A
Application number: US575418A
Authority: US
Inventors: Richard W Lamphere
Original assignee: American Bosch Arma Corp
Current assignee: Ambac International Corp
Priority date: 1945-01-31
Filing date: 1945-01-31
Publication date: 1948-09-14
Anticipated expiration: 1965-09-14

Description

Sept. `14, 1948. R. w. LAMPHr-:RE 2,449,397 j ELECTRIC VSPARK GAP Filed Jan. 31, 1945 Patented Sept. 14, 1948 N, unirsi) STATES PATE i `ELECTRIC SPARK GAP Richard W, Lamphere, Wlbraham Mass., as-

signor to American Bosch Corporation, Spring-, field, Mass., a corporation of New York Application January 31, 1945, Serial No. 575,418 'c claims. (c1. 25o-ansi The present inventionrelates to electric spark gaps, and particularly to those for oscillatory electric circuits, such as have `been used in electric ignition systems of the capacity `discharge type for internal combustion engines, for` example,.as shown `in Fig. 1 of Patent 2,030,228 and also in lightningarresters, and in electronic and other electrical devices.

Under the condition that the applied voltage across the gap is increased from its initial value to the breakdown value` at an infinitely slow rate of l about l volts per second or less, the breakdown Voltage between two separated electrodes forming a spark gap in space, hereintermed a steady state breakdown voltage, dependson the gap distance between the electrodesthe` composition and` density yof the atmosphere in thel gap, `the size and shape of the electrodes at the two points between which the spark occurs, andthe distribution of the bound electrical` `charges in the space surrounding the gap. However, the value ofy the breakdown voltage `is materially affected by the steepness or slope of the wave front and` by the shape of the wave front of the applied voltage when it is increased in `value at a much higherrate measured in kilovolts per micro-second and herein termed the transient breakdown voltage. The charge, mass, cross sectional area, and geometrical lposition of all free charged particles within the gas in a sphere of` diameter having the two electrodes as poles, in addition to the foregoing factors affecting the steady state breakdown voltage, affect the transient breakdown voltage, and likewise the work function of the electrode material expressed in electron volts as represented by the kinetic energy whichlan electron within `the material` must have, `in a direction normal to the surface of the electrode,

system to provide reliable ignition, but, as a result thereof, increases the altitudeat which the sys- Ina` temwill operate onaircraft"without electrical :flash over. v

` `In such sparkgaps in general,` it has beencustcmary to assist instabilizing or rendering uniform `the transientbreakdown voltage ofthe gap by sealing `the spaced electrodesthereof in an:- envelope, usually of glass for insulating purposes,4 tovthereby reducevariations in performance `due to surrounding atmospheric conditions resulting;` from such variablesasaltitude, temperatureand humidity, and also by using `certain metals and alloys for `the electrodes with diierent gap separation and with different gases and admixtures; thereof at diierent` pressuresin theenvelope. i In` i certain cases alsocer,tain activating agents, and

certain shapes of'electrodes, with `different shape and size of electrode shields and envelopes therefor have :been used. `Certain of such features are shown in Patent 2,122,932. Notwithstandingsuch features, these spark gaps deteriorate and change in performance in theoperative `periodloetween overhauls'of the -devices or apparatus with which such spark gaps are associated;` particularly `in the case ,ofA spark1 gaps installed `in proximity to. grounded devices.` The grounded device lcauses distortion of the electric.fieldbetweenthe electrodes vforming the; gap,` and vdeflect the sparks, inthe gap -tothe inside wall of the envelope.`

This lowers the breakdown` voltage across the gap and rendersit unstable or erratic inperforrnance.V

Furthermore, the` sparks so `deiiected quickly cause ametallic deposit on the inside oi the en-` i velope, lowering thebreakdown voltage still more.` with still further instability and erratic perform-i ance of the spark gap. i i

In accordance with my invention, the operation of spark gaps in general invarious kinds of` oscillatory circuits or` electrical devices is imore or lessl stabilized throughoutv the `operative period` thereof whetheror not some or` all ofthe afore-l mentioned'featuresof prior spark gaps are used I accomplish Vthis bylzprovidingfmeans forlmaintaining a substantially uniform voltage gradient i betweenthe two electrodes of the gap, irrespective of the proximity of outside grounded devices.. The deectionofthespark discharges is thereby` reducedor eliminated, and` the distribution of` point.` electrical charges in thespaceoutsiolethe.`

envelope is stabilized or the variation `of their feffectisrend-ered negligible..` `Such means withinr my invention maycomprise a` `conducting or, semi-conducting coating,` Aor a high resistance` such las a winding orlwotherwise electrically shunted across theelectrodes of the gap. "Preferably thiscoating or other resistance is arranged,`

outsidedzheA envelope, of the spark gap, whether along the outside surface thereof or` on a suit-` able suppprtspacedtherefrom, `so as tobe un-` NToFFICE affected by the heat or the chemical action from the spark discharges, Still more preferably the outside of the envelope is completely covered with a suitable semi-'conducting coating extending to the outwardly projecting terminals of the electrodes forming the gap, and the envelope is symmetrical and coaxial with respect to the gap so that the electric field is uniform and the voltage gradient between the electrodes is substantially constant throughout. This condition is then maintained even though the potential of points outside of the envelope, and near thereto, are caused to assume values which otherwise would distort or deflect the field between the electrodes and thereby render the gradient nov longer constant. Also, preferably, good electrical contact between this semi-conducting coating and the electrodes forming the gap is made by a relatively high conducting coating which is arranged on the outside surface of the envelope around the terminals projecting therefrom, and which extends from both terminals to a place overlapping the semi-conducting coating to reliably connect the latter to both electrodes. Furthermore, these coatings are mechanically protected by an outside layer of varnish, shellac, or the like, or by an adniixture of the semi-conducting material therein.

During the period of time that the voltage applied to the gap is increasing, a displacement current ows between 'one electrode and the grounded device, the other electrode of the spark gap being at ground potential prior tothe breakdown of thegap. The magnitude of this displacement .current is preferably made negligibly small, say %V or less 'as compared to the conduction current through the coating, so that the potential at all rpoints on the 'surface to which the coating is applied will be determined almost entirely by the conducting coating rather than by -f the grounded device in proximity to the gap.

- These and other objects and advantages `of my invention will appear from the following description of its preferred form in application, as an example, to a spark `gap for a capacity discharge type of ignition system 'for an internal combustion engine `for aircraft. In the drawings,

Fig. 1 is awce'ntral section, partly in elevation, of this preferred form of spark gap; and Fig. 2 is an end "elevati-on `of the `spark gap with portions of the superposed coatings removed.

Referring to the drawings, the reference numeral I `designates `an yenvol-orne or container of uranium glass. Its tungsten electrodes 2 and 3 have their proximate faces spaced about 3 m-m. apart to form a gap 4 therebetween, and the tungsten rods of stems and 6 project in a sealed manner through the envelope to form the outside terminals 'I and 8 of the spark gap. The shields `9 and I'D are of thin metal to assist in preventing eroded electrode material from depositing on the envelope in such manner as to form a leakage path in a shuntwith the gap, the walls of which are arranged symmetrically and coaxially with respect to the gap 4 and lwith'the uppermost and lfowermost parts of the shields 9 and I0 atleast as distant as the axial length of the gap 4. Before the sealing 'tubulatio'n at I'I isl'rfus'ed off lto seal the envelope, and before the envelope Vis evacuated and then filled with argon, atleast 98% pure, to an absolute pressure of V30 to 'i5 cms. of mercury, a few drops of radio-active solution, as a verydilute solution of Aradium bromide in water, is introduced within the envelope. The order of magnitude of the'transient breakdown -vloltage of such a gap when new as used with an aircraft type of capacity discharge igni-` tion systems is around '2000 crest volts. However, gaps of this gen-eral kind may be designed for a transient breakdown voltage of from 500 to 5000 crest volts depending upon the size and type of the transformer used in the circuit of the spark gap.

A spring connector I2 tightly gripping the rod 'I :of the spark gap furnishes electrical connection to the electrode 2, and a suitable terminal (not shown) generally of a disconnectible type furnishes connection to the rod 8 and therethrough to the electrode f3.

Both'ends of the spark gap, with the connector I2 installed as shown in the drawings, are separately dipped into a highly conducting solution such as a colloidal suspension of graphite in water. Thus the outside of envelope I around the end thereof and over terminal 8 is coated in a circular area, as shown at I3, with the Aquadag, and likelwise the other end including the connector I 2. This provides a good electrical connection between the respective rods 1, 8 and the surrounding end portions of the envelope. After the Aquadag has dried, the entire spark gap with connector i2 is then immersed in a solution of semi-conducting material Ifl, of a high resistance conductive coating material, over which is placed a coating of shellac. The highly conductive coating reliably connects the semi-conducting coating to both terminals 'I and 8 to form therewith a high resistance shunt electrically connected across the yelectrodes 2, 3. After that semi-conducting coating has dried, the spark gap and connector I2 are given a coating of insulating varnish I5 of heator air-drying type to protect the coatings underneath from rubbing off or from so readily being damaged. The terminal area of rod 8 is scraped or otherwise `cleaned off, as indicated in Fig. 1, to provide good electrical connection with the connector that may be installed at that `end of the spark'gap.

It will be understood that many metal-oxide glazes adherent to glass, and suspensions of charcoal in varnish, may be used for the semi-conducting coating with good results. So also sprayed metal or reduced platinum oxide of suit-v able kind may be used for the highlyconducting coating around the terminals 'I and 8, and shellac of suitable kind may be used instead of the varnish. Furthermor'evthe high resistance coating maybe directly admixed with the outside protective coating so `that the two coatings are thus combined into one. However, the coatings as above specified are preferred at this time. Furthermore, it will be understood that other suitable materials and dimensions of the envelopes, gaps, electrodes, `stems and shields may be used, and likewise 'other mixtures and pressures of gas within the envelope.

The resistance of the 'completed coating between the terminals 1, 8 can be very high, ofthe order of 10 megohrns or more, and stili be adeduate'l'y conducting to maintain a reasonably uniform electric field across the gap 4. The exact value ofthe maximum permissible resistance for satisfactory operation depends in each case upon the geometry of the spark gap, the location of the nearby grounded bodies and the rate of use of `voltage applied to the gap, and can be predetermined in 'well known manner. With such uniform iield, or substantially so, the voltage gradient is substantially uniform between the electrodes 2, 3, irrespective of the location with respect to the spark gap of outside grounded devices, as for instance the grounded metallic radio-shielding harness in lwhich the spark gap and other devices are contained. The deiiection of the rapidly-recurring spark discharges across the gap fl is thereby reduced or eliminated, and the distribution of point electrical charges in the space outside the envelope l is stabilized or their effect on the spark discharges is rendered negligible. In this way little or no metallic deposit forms on the inside surface of the envelope resulting from `erosion of the electrode or electrodes, and therefore there occurs no substantial decrease in the electrical resistance of the'gap throughout its useful life such as would render the gap erratic or unstable. Accordingly, the transient breakdown voltage ci the gap is maintained substantially constant, thereby stabilizing or rendering more uniform the action of the electric discharge in the circuit or devices with which the spark gap is associated, and also reducing the voltage required in the system to provide reliable ignition for aircraft engines while even operating at high boost or lean mixture and at high temperatures of operation, and also increasing the altitudes at which the system will operate without electrical fiashover. Such conditions obtain throughout the entire operating period between overhauls of 450 hours or more.

A 500 hour test on a large number of externally coated spark gaps as speciiically described herein, and a substantially similar number of uncoated but otherwise identical spark gaps, using about 2000 crest volts at a thousand sparks per minute, showed for the coated spark gaps about deviation in breakdown voltage against an average maximum deviation of about for the uncoated spark gaps, and a reduction in conductivity from about one megohm resistance to a practically innite value of resistance for the coated gaps. Such deviation as occurred with the uncoated gaps may result in the absence of ring at some spark plugs of aircraft engines.

It will be understood that many modifications and changes may be made in the structures and use of the spark gaps herein disclosed without departing from the spirit of the invention as covered by the broad terms of the appended claims.

Having thus described the invention, what I claim is:

1. An electric spark gap comprising an envelope, electrodes mounted within the envelope, said electrodes being suiciently closely spaced to each other to provide a spark gap, and a, semiconducting coating externally of and entirely enclosing said envelope, said coating being sufficiently conductive to conduct bound charges, but not suiiiciently conductive to short circuit the gap, said semi-conducting coating being electrically connected with at least one of said electrodes.

2. An electric spark gap comprising an envelope, electrodes mounted within the envelope, said electrodes being sufficiently closely spaced to each other to provide a spark gap, and a semiconducting coating externally of and entirely enclosing said envelope, said coating being suiciently conductive to conduct bound charges, but not sulciently conductive to short circuit the gap, and relatively higher condu-cting means connesting said semi-conducting coating with at least one of said electrodes.

3. An electric spark gap comprising an envelope, electrodes mounted within the envelope, said electrodes being suiciently closely spaced to each other to provide a spark gap, and a semi-conducting shunt externally of and entirely enclosing said envelope, said shunt being sufficiently conductive to conduct bound charges but not suiiiciently conductive to short circuit the gap and relatively higher conducting means connecting said semi-conducting shunt with at least one of said electrodes.

4. An electric spark gap comprising an envelope, electrodes mounted within the envelope, said electrodes being sufficiently closely spaced to each other to provide a spark gap, and a resistor externally of and entirely enclosing said. envelope, said resistor being suiiiciently conductive to conduct bound charges, but not sufficiently conductive to short circuit the gap and relatively higher conducting means connecting said resistor with at least one of said electrodes.

5. An electric spark gap comprising an envelope ci insulating material, electrodes mounted within the envelope, said electrodes being sufficiently closely spaced to each other to provide a spark gap, and a semi-conducting coating externally of and covering the entire surface of said insulating material, said coating being suiciently conductive to conduct bound charges, but not suiiiciently conductive to short circuit the gap, said semi-conducting coating being electrically connected with at least one of said electrodes.

6. An electri-c spark gap comprising an envelope of insulating material, electrodes mounted within said envelope, said electrodes being sufficiently closely spaced to each other to provide a spark gap, and a semi-conducting coating externally of and extending over the entire surface of said insulating material, said coating being suiciently conductive to conduct bound charges, but not suiiiciently conductive to short circuit the gap, and relatively higher conducting means electrically connecting said semi-conducting coating with at least one of said electrodes.

RICHARD W. LAMPHERE.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,860,210 Spanner et al May 24, 1932 1,897,587 Pirani Feb. 1.4, 1933 FOREIGN PATENTS Number Country Date 30,745 Holland Sept. 15, 1933 118,827 Switzerland Feb. 1, 1927 417,192 Great Britain Oct. 1, 1934 684,526 France Mar. 1KB, 1930