US3789168A

US3789168A - Ignition distributor device with insulative shutter type rotor

Info

Publication number: US3789168A
Application number: US00322342A
Authority: US
Inventors: E Meyer; I Carter; I Scott
Original assignee: Chrysler Corp
Current assignee: Old Carco LLC
Priority date: 1973-01-10
Filing date: 1973-01-10
Publication date: 1974-01-29
Anticipated expiration: 1991-01-29
Also published as: BR7400108D0; AR201936A1; AU6440974A; DE2400798A1; IT1003310B; FR2213423B3; JPS49101740A; SE397709B; DE2400798B2; FR2213423A1; CA998724A; ES422092A1; ZA739218B; NL7317431A; GB1443592A; DE2400798C3

Abstract

An ignition distributor device for use with vehicle engine ignition systems equipped with electronically advanced spark timing angle controllers has a stationary high tension distribution electrode, a plurality of stationary output electrodes spaced from the distribution electrode, and a rotary dielectric shutter element disposed between the distribution electrode and the output electrodes. The shutter has a window therein dimensioned to permit conductive association of the distribution electrode with only one of the output electrodes at a time. The output electrodes are readily removable from and insertable in the distributor cap and also provide a cable termination for the ignition spark plug cables received in the cap.

Description

United States Patent [191 Meyer, Jr. et al.

[ IGNITION DISTRIBUTOR DEVICE WITH INSULATIVE SHUTTER TYPE ROTOR [75] Inventors: Earl W. Meyer, Jr., Birmingham; Ivor W. Carter, Detroit; Ian J. C. Scott, Madison Heights, all of Mich. [73] Assignee: Chrysler Corporation, Highland Park, Mich [22] Filed: Jan. 10, 1973 [21] App]. No.: 322,342

[52] US. Cl 200/19 R, 200/19 DC, 200/19 M, 123/148 E [51] Int. Cl. H01h 19/60 [58] Field of Search. ZOO/19 A0, 24, 166 BC, 19 R, 200/19 DC, 19 M; 174/138 F; 123/148 E, 148 R, 148 A, 148 DC, 143 C, 146.5 A, 148 AC; 339/223 [56] References Cited UNITED STATES PATENTS 3,375,812 4/1968 Koda ZOO/19 M X 3,463,134 8/1969 Zechnall et a1. 200/19 R X Jan. 29, 1974 3,594,518 7/1971 Bates 200/19 M 3,542,976 11/1970 Moray 200/166 BC X 3,613,653 10/1971 Irvin, Jr. et al 123/148 DC X Primary Examiner-J. R. Scott Attorney, Agent, or Firm-Jess P. Santo [5 7] ABSTRACT An ignition distributor device for use with vehicle engine ignition systems equipped with electronically advanced spark timing angle controllers has a stationary high tension distribution electrode, a plurality of stationary output electrodes spaced from the distribution electrode, and a rotary dielectric shutter element disposed between the distribution electrode and the output electrodes. The shutter has a window therein dimensioned to permit conductive association of the distribution electrode with only one of the output electrodes at a time. The output electrodes are readily removable from and insertable in the distributor cap and also provide a cable termination for the ignition spark plug cables received in the cap.

15 Claims, 13 Drawing Figures IGNITIONDISTRIBUTOR DEVICE WITH INSULATIVE SHUTTER TYPE ROTOR FIELD OF THE INVENTION This invention relates to vehicle ignition systems, and, more particularly, to a novel ignition distributor device for sequential distribution of high tension ignition pulses from the secondary winding of an ignition coil to the respective ignition firing devices of a multiple cylinder internal combustion engine.

BACKGROUND OF THE INVENTION The conventional automotive ignition distributor employs a contact breaker assembly which is mounted on an adjustable timing plate within the distributor housing and is operated from a multiple-lobed cam on the distributor rotor shaft driven from the engine cam shaft. The breaker points are connected in interrupting circuit relation with the ignition coil, which is connected through the ignition switch to the vehicle storage battery when the breaker points are closed. Deenergization of the coil produced by the opening of the breaker points causes the collapse of the magnetic field therein and induces a burst of high tension energy, which is supplied from the coil secondary through a rotary or sliding electrical contact in the distributor cap to the distributor rotor switch arm. The latter is a blade-like electrode driven-by the rotor shaft past a circular array of stationary output electrodes, which are permanently affixed to and located in the distributor cap and are connected to the engine cylinder spark plugs in the order in which the plugs are to be fired.

The angular position of the timing plate, rotor cam and rotor switch arm are adjustable relative to the distributor rotor shaft to permit the spark timing or occurrence of the firing event of the spark plugs to be varied independently in accordance with engine speed and with engine load as manifested by engine vacuum in order to obtain desired engine performance characteristics over a wide range of engine operating conditions. Timing advance for changing engine speed is accomplished by a centrifugal type governor mechanism, which is located within the distributor housing and mechanically simultaneously adjusts the angular position of both the breaker operating cam and the distributor rotor blade or switch arm on the distributor rotor shaft. Vacuum advance is accomplished by a vacuum actuator, which is operated from a source 'of engine vacuum, such as the vacuum advance port of the carburetor, and is coupled through a mechanical link to the distributor timing plate to adjust its angular position relative to the distributor rotor shaft.

The distributor rotor switch arm and breaker cam are set in a predetermined angular position relative to each other on the distributor shaft such that the high tension coil energy generated by the operation of the breaker points appears at the rotor switch arm when it is just opposite a stationary output electrode connected to a selected plug to be fired. Unlike the governor advance structure, which is of a synchronous character, the modulation of the spark timing by the vacuum advance structure is of a non-synchronous character, in consequences of which the position of the rotor blade may be shifted slightly from or relative to the output electrode at the moment of transfer of the high tension energy to the output electrode. Accordingly, in the conventional distributor structures, the combined width of the distributor rotor blade and the facing electrode portion of an output terminal is made of sufficient arcuate extent so that the transfer of high tension energy will occur while some portion of the blade is still adjacent the selected output electrode. In addition, the distributor cap is made of a diameter to provide sufficient spacing between adjacent output electrodes located at the outer periphery thereof to prevent arc flashover and misfiring from the wide bladed rotor switch arm to the next adjacent output electrode.

The above described misfiring problem is particularly acute and severe with ignition systems in which all timing changes are accomplished electronically or without mechanical linkages, and especially where it is attempted to use the distribution electrode assembly of a conventional distributor with such ignition systems. In such cases it would be necessary to further increase the width of the rotor switch blade to accomodate the shift of the blade relative to the output electrodes for the non-mechanically accomplished engine speed timing advance. In addition it would be further necessary to increase the diameter of the distributor cap to increase the arcuate spacing between adjacent output electrodes, resulting in a distributor structure of considerably enlarged dimensions and proportions.

Accordingly, the present invention has for its general object to provide an ignition distributor structure specially suited for use with internal combustion engine ignition systems equipped with electronic or other spark timing advance controls which do not mechanically advance the distributor.

Another object is to provide an ignition distributor for an internal combustion engine in which the high tension energy from the ignition coil is supplied to the distributor input or distribution electrode through a stationary, non-rotating electrical connection.

A related object is to provide a high tension distribution electrode assembly for an engine ignition distributor using a stationary high tension distributor electrode and stationary output electrodes.

Another object is to provide an ignition distributor in accordance with the foregoing objects having a distributor cap and rotor structure of minimum size and dimensions.

Still another object is to provide a push-in, readily removable electrode terminal serving as an output electrode and terminal for the distributor and as a connector for a spark plug cable for an ignition distributor in accordance with the foregoing objects.

The manner of accomplishment of the above and other objects, together with the advantages and other features of the distributor structure of the present invention will appear more fully from consideration of the following detailed description made with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic electrical circuit diagram of an ignition system for an internal combustion engine;

FIG. 2 is a vertical sectional elevation view with parts broken away of a distributor in accordance with the invention;

FIG. 3 is a fragmentary view of one of the output timers of FIG. 2 with the ignition spark plug cable removed therefrom;

FIGS. 4 and 5 are plan views taken respectively in the direction of 4-4 and 55 of FIG. 2;

FIGS. 6 and 7 are enlarged fragmentary views of details of FIG. 3 taken in direction 6-6 and 77 thereof;

FIG. 8 is a perspective view of an output electrode and terminal used in the distributor of FIG. 1;

FIG. 9 is an enlarged plan view of a stamping of which the distributor electrode and cable termination is formed;

FIGS. 10 and l l are enlarged side and top plan views, respectively, of the output electrode and terminal of FIG. 5; and

FIGS. 12 and 13 are opposite end views taken in the directions l212 and l3l3 of the output electrode and terminal of FIG. 11.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIG. 1 of the drawings, a distributor structure is schematically represented at 10 for the ignition system of an internal combustion engine 12 having a plurality of spark plugs 14, which are successively energized in a predetermined order by high tension electrical energy directed thereto from the distributor. In addition to the distributor and spark plugs, the ignition system further includes an ignition coil 16 having a primary winding 17, one side of which is connected through the conventional off-run-start (O-R-S) contact ignition switch 20 to a direct current source including the vehicle storage battery 22, shown connected as a negatively grounded system. The negative or other side of the primary winding of the ignition coil is connected to ground through an interrupter device 24 operated synchronously with the engine, as from the camshaft 26 thereof. The interrupter device may be of a mechanical character as the breaker points of the conventional distributor or of an electronic character as shown in copending U.S. Pat. application Ser. No. 148,354 filed June 1, 1971 of common ownership herewith. Interruption of the magnetizing current in the primary winding of the ignition coil 16 causes the collapse of the magnetic field therein and induces a high tension voltage, which is stepped up by the coil secondary winding 18 and is supplied from the output terminal 23 of the coil to the input or distribution electrode of the distributor The distributorwith which the present invention is concerned is specially suited for use with electronic ignition systems of the type in which the spark timing advance also is accomplished electronically as in copending U.S. Pat. application Ser. No. 322,243, filed Jan. 9, 1973 of common ownership herewith. As shown in FIG. 2 the subject distributor 10 comprises a metallic non-magnetic lower housing 34, an engine driven distributor shaft 36 extending through and journalled for rotation in the housing, and a distributor cap 38 detachably mounted by spring clips 40 to the open upper end of the housing. In accordance with the invention, the cap includes a distributor electrode assembly, which includes a stationary distribution ring or input electrode 44, a plurality of stationary output electrodes 78, and a dielectric shutter element 48 carried on the rotor shaft 36.

Cap 38 is a double-wall, inverted cup-shaped member molded in one piece of dielectric insulating material, such as glass-filled alkyd resin, and has a domelike crown portion 50, an axially extending outer annular side wall portion 52 and a coaxial cylindrical inner wall portion 54, which is spaced from the outer wall portion. Extending axially outwardly from the crown portion 50 of the cap are a plurality of integrally formed tubular truncated conical tower-like projections including a centrally located input terminal housing 56 and an even number of radially outwardly located; output terminal housing 58. The outwardly located towers 58, of which eight are provided for an eight cylinder engine, are equally angularly spaced about the upper peripheral portion of the circular cap, which has a plurality of arcuately spaced circular convex ribs 59 thereon formed by the intersection of the towers 58 with the cylindrical outer surfaces of the cap, as indicated in FIG. 3.

Integrally formed on the cylindrical outer wall 52 of the cap is a pair of diametrically opposed, outwardly located snap catches 60, (only one of which is shown) for receiving the upper end of the spring clips 40, which are pivotally mounted to the lower housing member 34, as shown. A plurality of axially extending rectangular shaped ribs 62 having intersecting planar surfaces to form sharp edges are provided about the inner surface of the outer cylindrical wall 52 of the cap to attenuate and disrupt conductive films deposited by the electrically charged atmosphere within the distributor cap. As shown in FIG. 2, the cylindrical inner wall 54 of the cap is spaced inwardly of the outer wall 52 and has a number of gusset-like strengthening, dimensionally stabilizing and locating webs 63 extending between the inner surface of the inner wall 54 and the inner surface 64 of the top of the cap.

Molded in, and extending axially through the bottom wall or floor 68 of each of the output terminal housings or towers 58 is a T-shaped opening 70, which extends into the annular space 53 formed between the outer and inner walls 52, 54 of the cap and is in registration alignment with a T-shaped dovetail slot 74 formed in the inner surface of the outer wall 52 of the cap, as shown in FIG. 7. The outwardly located, flat back side 76 of the opening and slot 74 is formed at a slight I inclined angle of about one or two degrees to the vertical as shown in FIG. 2 for ease of insertion and ready retention of a removable blade-like output terminal electrode 78 secured to the spark plug cable 80.

As shown in FIG. 2 the central tower or output coil terminal housing 56 has a brass insert terminal 82 molded therein, the upper end of which is adapted to receive a snap-in ribbed connector terminal on one end of an insulated cable (not shown) from the secondary high-tension output side of the ignition coil. At its lower end, the terminal 82 extends centrally through the top wall into the interior of the distributor cap and a centrally located opening in the aforementioned distribution ring element 44 to which the terminal is swaged as shown. I

The distribution ring 44 is composed of electrically conductive material and is a one-piece, inverted, conical, dish-shaped member, which has a plurality of equally angularly spaced and radially projecting electrodes 84 of generally square or rectangular ,-crosssection integrally formed about the lower circumferential periphery thereof. The electrodes 84, which are received in complementary-shaped notches shown at 66 in FIG. 3 and formed in the lower end or edge of the inner wall 54 of the distributor cap, extend radially outwardly a slight distance therebeyond into the annular space 53 between the outer and inner walls 52 and 54 of the cap and terminate a short distance from a corresponding one of the aforementioned output electrode terminals 78 located oppositely therefrom.

The shutter element 48 is the only rotary part of the high tension distribution apparatus of the present invention. In distinction to the electrically conductive rotor arm of the conventional distributor structure,

which is electrically connected through a rotary slip connection to the high tension input or coil terminal of the cap, the shutter element 48 is electrically and mechanically disassociated from the input coil terminal. As shown in FIG. 2, the shutter element is an integrally formed, one-piece, dish-shaped member molded of dielectric insulating material and has a circular plate portion 86, an axially extending annular flange or ring portion 88 having a cut out portion in the form of an elongated aperture or window 90 therein, and an axially centrally located hub portion 92.

I-Iub portion 92 extends axially on opposite sides of the plate portion 86 and is molded about an externally splined or serrated cylindrical insert ring 94 by which the shutter device is received on and suitably secured, as by a pin 96, to the upper end of the distributor rotor shaft 36. On opposite sides of the plate portion 86 are respective sets of equally angularly spaced webs 98 and 100 extending between the hub portion 92 and the plate portion 86 for reinforcement and rigidization of the dielectric shutter.

The rim portion 88 of the shutter 48 extends axially upwardly into the annular space 53 between the outer and inner walls 52 and 54 of the distributor cap an axial distance which is several times greater than the radial gap distance between an electrode 84 on the distribution ring 44 and the adjacent output terminal electrode 78 to prevent flashover or arcing from the distributor electrode upwardly around the rim of the shutter to 76 the associated output electrode. As shown in FIG. 2, the rim portion 88 of the dielectric shutter 48 intervenes or is interposed between the

aforesaid electrode elements

78 and 84 and prevents ionization and breakdown of the interelectrode air gap except in the region of the aperture or window 90 where the aperture or window in the shutter intervenes to expose an output electrode to an adjacently located one of the finger-like electrodes of the distribution ring.

It will be observed that the arcuate or circumferential extent of the shutter window 90 is less than the angular spacing between adjacent ones of the output electrodes. The arcuate extent of the window 90 should accommodate the entire timing shift between the maximum retard and advance conditions of the engine spark timing control and is selected so that in the maximum retard condition of the engine timing, as typified by the position of the shutter in FIG. 3, the input electrode 84b next adjacent the window-exposed input electrode 84a will be spaced by the insulating portion of the shutter a sufficient distance from its associated corresponding output electrode 78b, to avoid flashover of energy around the leading edge 90L of the window to its associated corresponding output electrode 78b. Similarly the height or axial extent of the annular flange or rim portion 88 of the dielectric shutter 48 should be of sufficient distance to prevent flashover or arcing between the distribution electrode and the output electrodes. With the voltage levels and gradients prevailing in automotive ignition systems, the aforementioned distances should at least be equal to and preferably greater than five-eighths inches to prevent such flashover as might otherwise occur between the identified points.

In the illustrated form of the shutter, the window 90 has an angular extent which is less than the angular spacing between adjacent output electrodes 78 in the distributor cap, and subtends an angle 0w of approximately 36 mechanical degrees of distributor shaft rotation. As the latter is driven at one-half of engine crankshaft speed, the window o'pening corresponds to 72 mechanical degrees of engine crankshaft rotation. Since the total timing shift specified for such engines is composed of the engine governor advance, which is in the order of 45 maximum, and of the vacuum advance, which is in the order of say 20 maximum, it will be seen that the arcuate extent of the window 90 will accommodate the total timing shift of 65 from the TDC position of a piston in a cylinder of the engine.

The combination output electrode and cable termination 78 is a one-piece element formed of a fiat T- shaped brass stamping 100 indicated in FIG. 9 as having a pair of oppositely laterally extending

arms

112, 114 and a longitudinallyextending stem or blade por tion 116, which is subsequently drawn and folded or pleated, as in a progressive die, about its longitudinal axis of symmetry 102 to form a raised ridge portion 1 l8.

Arms

112, 114 form the split ferrule portion of the termination and are bent from the plane of the stamping as shown in FIG. 12 to form a trough-like or V- shaped pocket in which is received the ignition cable 80 from which the insulating material has been stripped back to expose the central current carrying core 81 thereof. The current carrying core 81 of the cable is folded back over the outer insulating sheath 83 and is received with the cable between the

arms

112, 114, which are then crimped about the cable firmly gripping and establishing electrical contact of the central current carrying core 81 with the terminal as shown in FIG. 8.

Blade portion 1 16 is of a substantially fiat configuration and has an integrally formed raised ridge or rib portion 118, which is equidistantly located between the longitudinally extending sides 120, 122 and is shown as extending the entire length of the blade portion. The blade portion thus has a substantially T-shaped crosssectional configuration and its side edges adjacent the end thereof opposite the ferrule or cable gripping portion of the terminal are deflected slightly upwardly a few degrees out of the horizontal plane of the blade as shown in FIGS. '10 and 13, for ease of insertion and re tention of the blade in the floor opening and wall slot 74 in the distributor cap.

A conventional insulating boot 124 is slipped over the ignition cable and terminal assembly which is inserted into an output tower 58 of the distributor cap with the blade portion extending through the opening 70 and into the interior of the cap to a depth limited by a ledge formed interiorly of the cap. The openings 70 in the floor of the output towers are aligned with the radially projecting electrodes 84 of the distributor ring element 44 so that the ridged portion 1 18 of each of the terminals 78 faces opposite a corresponding one of the distribution ring electrodes and is spaced therefrom by a uniform air gap.

It will be appreciated that the described terminal provides a simple, inexpensive and reliable termination for an ignition cable as well as a readily removable and insertable push-in pull-out output electrode for a distribution cap. in contrast with current production forms of distributor caps in which the output electrodes are permanently affixed therein, should the subject output electrode be damaged, it can be readily replaced without replacement of the entire cap structure. The instant cable termination and output electrode eliminates the additional interconnection heretofore employed between the distributor output electrodes and ignition cable terminations, the combined functions of which are provided by the integrally formed, one-piece output electrode and cable termination which is adapted to be produced by simple stamping, bending, folding, and drawing techniques.

What is claimed is:

'1. An ignition distributor device for sequential distribution of successive pulses of high tension electrical energy from the secondary winding of an ignition coil to a plurality of ignition spark plug firing devices of an internal combustion engine equipped with an electronic variable spark timing angle controller, said distributor device comprising a housing, a stationary electrically conductive distribution electrode within said housing and adapted to be connected to said high tension secondary winding of said ignition coil, a plurality of stationary output electrodes each adapted to be connected to a different ignition firing device and uniformly angularly disposed within said housing and spaced from said distribution electrode by a uniform spark gap therebetween, and an insulating element 'adapted to be rotatably driven within said housing at an angular rate related to engine speed and disposed in said spark gap to prevent conductive association of all but one of said plurality of stationary output electrodes with said distribution electrode during the firing of the one of said ignition firing devices associated with the said one of said plurality of stationary electrodes.

2. An ignition distributor device in accordance with claim 1 wherein said distribution electrode comprises a plurality of electrically commonly connected electrodes each disposed adjacent to a different one of said plurality of output electrodes connected to said ignition firing device.

3. An ignition distributor device in accordance with claim 2 wherein said rotatable element is a shutter composed of dielectric, electrically insulating material.

4. An ignition distributor device in accordance with claim 3 wherein said dielectric shutter includes a discrete portion which is of low dielectric property relative to the major portion of the shutter and subtends an arc of lesser angular extent than the angular spacing between an adjacent pair of stationary output electrodes connected to said ignition firing devices.

5. An ignition distributor device in accordance with claim 4 wherein the arc subtended by said discrete por-- tion of said dielectric shutter is at least equal to the total timing shift to be accommodated by the engine from the maximally retarded firing position thereof.

6. An ignition distributor device in accordance with claim 5 wherein the discrete portion of said dielectric shutter element is a cut-out portion therein.

7. An ignition distributor device in accordance with claim 6 wherein the discrete cut-out portion of said dielectric shutter element is an elongated slot in the form of a window in the shutter.

8. An ignition distributor device in accordance with claim 3 wherein said distributor device includes a rotor shaft journalled for rotation in said distributor and driven at a rate proportional to engine speed and wherein said dielectric shutter element is mounted on one end of said rotor shaft and comprises a cup-shaped member having an annular rim portion extending axially into the spark gap space between said distribution electrodes and said output electrodes.

9. An ignition distributor device in accordance with claim 8 wherein said cup-shaped dielectric shutter element has an elongated, circumferential arcuately extending opening in the said annular rim portion thereof of a length determined by the total timing shift to be accommodated by the engine.

10. An ignition distributor device in accordance with claim 9 wherein said distributor device further includes a removably attachable cap on the end of the distributor housing adjacent the dielectric shutter element and wherein said distribution electrode is centrally attached to said cap and wherein said plurality of output electrodes for said ignition firing devices are equally angularly spaced about the outer periphery of the cap.

11. An ignition distributor device in accordance with claim 10 wherein the arcuate extent of the opening in the shutter element is less than the angular spacing between adjacent ones of said output electrodes in said distributor cap.

12. An ignition distributor device in accordance with claim 10 wherein the output electrodes at the outer periphery of the cap extend axially from the exterior into the interior thereof to confront the respective elec trodes of the distribution electrode and wherein the respective electrodes of the distribution electrode extend radially toward and are spaced from the outwardly located output electrodes by said uniform spark gap.

13. An ignition distributor device in accordance with claim 12 wherein the axial length and extent of said shutter rim portion is several times greater than the width of the spark gap between the distribution electrode elements and the output electrodes to prevent arc flashover when the dielectric portion of the shutter element intervenes therebetween.

14. An ignition distributor device in accordance with claim 10 wherein each of the output electrodes at the outer periphery of the distributor cap is readily removably insertable in the cap and also provides an integral a termination for an ignition spark plug cable conductively affixed thereto.

15. An ignition distributor device in accordance with claim 10 wherein each of the output electrodes at the outer periphery of the distributor cap is readily removably insertable in the cap and provides an electrical termination for an ignition spark plug cable conductively affixed thereto, each of said output electrodes comprising a unitary electrically conductive terminal element having a ferrule portion at one end thereof electrically connected to and carried on one end of an ignition spark plug cable and having an electrode portion integral with said ferrule portion and extending into the interior of the distributor cap in spaced relation with said distribution electrode.

Claims

1. An ignition distributor device for sequential distribution of successive pulses of high tension electrical energy from the secondary winding of an ignition coil to a plurality of ignition spark plug firing devices of an internal combustion engine equipped with an electronic variable spark timing angle controller, said distributor device comprising a housing, a stationary electrically conductive distribution electrode within said housing and adapted to be connected to said high tension secondary winding of said ignition coil, a plurality of stationary output electrodes each adapted to be connected to a different ignition firing device and uniformly angularly disposed within said housing and spaced from said distribution electrode by a uniform spark gap therebetween, and an insulating element adapted to be rotatably driven within said housing at an angular rate related to engine speed and disposed in said spark gap to prevent conductive association of all but one of said plurality of stationary output electrodes with said distribution electrode during the firing of the one of said ignition firing devices associated with the said one of said plurality of stationary electrodes.

5. An ignition distributor device in accordance with claim 4 wherein the arc subtended by said discrete portion of said dielectric shutter is at least equal to the total timing shift to be accommodated by the engine from the maximally retarded firing position thereof.

12. An ignition distributor device in accordance with claim 10 wherein the output electrodes at the outer periphery of the cap extend axially from the exterior into the interior thereof to confront the respective electrodes of the distribution electrode and wherein the respective electrodes of the distribution electrode extend radially toward and are spaced from the outwardly located output electrodes by said uniform spark gap.