US3531680A

US3531680A - Self-starting arc lamp

Info

Publication number: US3531680A
Application number: US618438A
Authority: US
Inventors: Raymond E Paquette
Original assignee: Varo Inc
Current assignee: Varo Inc
Priority date: 1967-02-24
Filing date: 1967-02-24
Publication date: 1970-09-29
Anticipated expiration: 1987-09-29
Also published as: GB1193095A; NL6715786A; DE1639092A1; DE1639092B2; DE1639092C3

Description

Sept 29, 1970 E, PA UETTE 3,531,680

SELF-STARTING ARC LAMP File d Feb. 24, 1967 2 Sheets-Sheet 1 Fig) 2 1 I INVENTOR.

RAYMOND E. PAQUETTE BY Law 7 ATTORNEY Sept. 29, 1970 .R. E.'PAQUETTE SELF-STARTING ARC LAMP 2 Sheets-Sheet 2 Filed Feb. 24, 1967 INVENTOR. RAYMOND E. PAQUETTE K L LwL ATTORNEY United States PatentO 3,531,680 SELF-STARTING ARC LAMP Raymond E. Paquette, Saratoga, Calif., assignor, by mesne assignments, to Varo, Inc., Garland, Tex., a corporation of Texas Filed Feb. 24, 1967, Ser. No. 618,438

' Int. Cl. H05b 31/06 U.S. Cl. 314-34 21 Claims ABSTRACT OF THE DISCLOSURE A self starting short arc lamp in which a consumable bridgewire is connected across the arc ends of the elec trodes to fuse when normal arc sustaining power is applied to the electrodes to first establish and thereafter maintain the arc. The free length portion of the bridgewire is looped for urging the ends of the free length portion into resilient contact with the electrodes, beyond the electrode tip, to increase the environmental survival capability of the arc lamp and to stabilize the are under all operating conditions.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to self starting short arc lamps and, more particularly, to a short are lamp which is started by the application of normal arc sustaining power to the electrodes to consume a bridgewire disposed across the arc gap and connected to the electrodes, and in which the free length portion of the bridgewire is shaped and its ends positioned for withstanding shock and vibration and for providing a stable are when operated under extreme environmental conditions.

Description of the prior art Self starting short arc lamps utilizing a bridgewire across the arc gap, which fuses (explodes) when normal arc sustaining power is applied to the arc lamp to start the arc, are fully described in U.S. Letters Pats. 3,256,459 and 3,274,427, assigned to the same assignee as the present invention. As disclosed in these patents, such self starting lamps have recently come into use for applications where only a single use of the lamp is contemplated, as in the missile and rocket field.

Short arc lamps without a self starting feature require the initial application of a very high voltage starting pulse, which is typically 15,000 volts and which is normally supplied by a starting or a trigger pulse power supply, to partially ionize the gas between the electrodes to flash the arc. Once started, the arc is maintained by the application of power at a low voltage, typically around 12 volts, normally provided by an arc sustaining or main power supply. The requirement of a starting pulse power supply and the necessary interconnections with the arc sustaining power supply results in a complex, expensive, bulky and heavy package. The bridgewire short arc lamps described in the above-referenced patents overcome these undesirable features for one shot applications by making it possible to start the arc lamp simply by connecting the same to the arc sustaining power supply. The normal arc sustaining power first consumes the bridgewire to generate the arc and thereafter maintains the arc. The bridgewire described in these patents, whether of uniform or non-uniform cross section, usually makes contact directly with the apexes (tips) of the electrodes and is essentially of linear (straight line) configuration and coaxial with the electrode axes.

Even though this straight line bridgewire arc lamp performs very satisfactorily and is well suited as a self starting feature, its survival capability and its arc stability when exposed to a very severe shock and vibration (acceleration) environment has been found wanting in certain respects. The enormous shock and vibration usually experienced during the launching stages of missiles, rockets and the like, places a very severe requirement upon the fragile bridgewire which is usually about 6 mils (thousandths of an inch) in diameter, occasionally resulting in bridgewire breakage. Further, upon exploding the bridgewire under vibration conditions, the wire may have been displaced from the apex, the hottest point of the electrode, so that, when the center of the wire is actually consumed, the ends may stick out sideways from the accommodation slot in which the wire normally rests and form an electrode to which the arc can wander, thereby causing a slight arc instability.

SUMMARY OF THE INVENTION It is therefore a primary object of the present invention to provide a self starting short arc lamp of the bridgewire type which has improved shock and vibration survival capability.

It is another object of the present invention to provide a self starting short arc lamp of the bridgewire type in which the arc is stable even under severe environmental conditions.

It is a further object of the present invention to provide a bridgewire-type self starting short are lamp which has improved environmental survival capabilities and improved arc stability.

It is still another object of the present invention to position the extreme ends of the free length bridgewire portion in such a manner that they are resiliently urged into contact with the electrodes near the apex, and that any displacement of these ends increases their proximity to the hottest portion of the electrodes, namely the extreme electrode tip.

Briefly, one embodiment of the bridgewire short are lamp of this invention accomplishes the stated objects by utilizing a bridgewire which, along its free length portion, is looped in spring fashion to resiliently urge the ends against the exit point of the accommodation slots in the electrodes. Further, the exit points of the accommodation slots in the conical end of the electrodes are positioned so that the bridgewire will have to pass through the apex of the electrode upon insertion and removal. The looped shape of the bridgewire within the arc gap not only provides a resilient support of the bridgewire, but also assures that the ends of the free length bridgewire portion are accurately positioned at predetermined portions on the conical surface of the electrode arc ends in close proximity to the electrode apex and that, after firing, the severed ends cannot leave the accommodation slot without passing through the hot apexes where they are captured by fusion to thereby close the slot and provide a good are end to the electrode.

Further objects and advantages of the present invention will become apparent to those skilled in the art to which the invention pertains as the ensuing description proceeds.

The features of novelty that are considered characteristic of this invention are set forth with particularity in the appended claims. The organization and method of operation of the invention itself will best be understood from the following description when read in connection with the accompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view, partly in elevation and partly in cross section wtih parts broken away, of a self starting short are lamp constructed in accordance with this invention;

FIG. 2 is a partial view taken along line 22 of FIG. 1;

FIG. 3 is an enlarged view partly in elevation and partly in cross section, of the bridgewire and the adjacent electrode portions of the arc lamp shown in FIG. 1;

FIGS. 4A, 4B and 4C are enlarged elevational views, similar to the one shown in FIG. 3, showing three different bridgewire configurations, including the optimum configuration;

FIG. 5 is a view, similar to the one shown in FIG. 3, of an alternate embodiment of this invention in which the free length portion of the bridgewire has a single loop configuration;

FIG. 6 is a view taken along line 66 of FIG. 5; and

FIG. 7 is a view, similar to that shown in FIG. 6, of a further embodiment of this invention in which the cathode accommodation slot is closed.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, in which like reference characters designate like parts, and particularly to FIGS. 1-3, there is shown a short are bridgewire lamp 10 constructed in accordance with this invention. Lamp 10 includes a lamp envelope 12 which is usually made of quartz for well-understood reasons. The interior 14 of envelope 12 is usually pressurized with an inert gas, such as Xenon or the like, at a pressure which is typically from between 10 to 15 atmospheres at room temperature and rises typically to about from between 15 to 30 atmospheres at the normal operating temperature. Mounted within pressurized envelope 12 are a pair of

electrodes

16 and 18 which respectively form the anode and the oathode of short are lamp 10, and which are made of some refractory metal such as swaged tungsten.

Electrodes

16 and 18 may be identical as illustrated in the drawing so that either may be selected as the cathode thereby making the lamp nonpolarized. Of course, if desirable, one elec trode may be constructed of larger diameter than the other to form the hotter anode as is usually done in arc lamps. However, the fact that the lamp of this invention may be made nonpolarized is a distinctive advantage, resulting in a substantial saving in its manufacturing cost and allowing the user to connect the supply to the lamp without regard to polarity.

Electrodes

16 and 18 are rigidly supported within envelope 12 by shrinking the envelope directly to their respective main body portions 20 and 22 and their electrode feet 24 and 26. Foot 24 of electrode 16 is connected to a conductive ribbon 28 by a spot weld to a molybdenum wire (typically .006 mil) tab 30 affixed to foot 24 in the usual manner. Ribbon 28 is typically made of molybdenum having a width of one-eighth inch and a thickness of 0.7 mil. The other end of ribbon 28 is similarly connected to a terminal lug 32 to which a flexible lead 34 is affixed. The entire assembly of ribbon 28, terminal lug 32 and, if desired, the end portion of wire 34, is encapsulated in the end portion of envelope 12 by shrinking the same utilizing conventional techniques.

In the same manner, electrode foot 26 is connected, through a conductive ribbon 36 and a terminal lug 38, to flexible lead 40, and the entire ribbon assembly has the envelope shrunk thereon. For operating the lamp, an arc sustaining power supply, not shown, is connected across leads 34 and 40 to first start and thereafter maintain arc in lamp 10. This lamp construction is generally known as a ribbon seal lamp since the seal between interior space 14 and the outside is formed around the ribbons. It is to be understood, however, that the present invention is equally applicable to rod seal? lamps in which the ribbons are replaced by rods and where the seal is formed around the rods. 1

Electrodes

16 and 18, respectively, are provided with are end portions which comprise reduced diameter cylindrical sections 42 and 44 and conical sections 46 and 48 which terminate at apexes facing one another, and defining an arc gap therebetween which is typically from between 25 to mils in length, a dimension designated as G. Bridging the arc gap is a looped bridgewire 50 which is embedded along slots in conical sections 46 and 48, respectively, and extends along cylindrical sections 42 and 44. The ends of bridgewire 50 are spot welded, or

otherwise securely affixed, to

electrodes

16 and 18 by means of platinum tabs 52 and 54 in the conventional manner. Bridgewire 50 is typically 6 mils in diameter and is made of molybdenum or the like.

Referring now more particularly to FIG. 3,

electrodes

16 and 18 are provided respectively with bridgewire accommodation slots and 62 whose width is selected to receive bridgewire 50 with a sliding fit. Slots 60 and 62 extend inwardly from the periphery into the electrodes, the slot in electrode 16 being diametrically opposed to the slot in electrode 18. The term diametrically opposed means that the slots are displaced with respect to one another to extend into the electrodes from opposite directions.

Since slots 60 and 62 are identical, the description that follows of slot 62 will be equally applicable to slot 60. Slot 62 has a bottom surface 64 which may be straight or slightly concave as shown. The primary reason for the curvature of surface 64 is its method of construction, such as by a grinding wheel, causing surface 64 to conform to the periphery of the grinding wheel. The point at which bottom surface 64 intersects the surface of conical section 48 is indicated at 66, and will be referred to as the slot exit. Similarly, the slot exit of slot 60 is indicated at 67. Slot exit 67 is located a distance A from the apex of the cone as measured along the slant surface of the cone. The intersection of bottom surface 64 with the surface of cylindrical portion 44 is not critical, and may be selected anywhere along the electrode surface so long as the portion of bridgewire 50 projecting from that intersection is a reasonably short distance from the spot welded end across tab 54.

Distance A is selected in such a manner that bridgewire 50, exiting into the arc gap, is in close proximity to the apex. Since it is desired to provide good support to and good thermal contact with bridgewire 50, a minimum dimension of A equal to at least the diameter of bridgewire 50 is necessary. Further, the distance A may be as large as four times the diameter of bridgewire 50 and still cause the formation of a stable arc. Accordingly, distance A is selected within the range extending from a minimum of about one to a maximum of about four times the diameter of the bridgewire, the optimum distance being about two diameters.

In this connection it should also be noted that the bridgewire, at the point of exit, is disposed between the exit point and the apex and, in case of burn-out, can only leave the slot by crossing the apex. Because of the sliding fit between the bridgewire and the slot side walls, good thermal contact between the bridgewire and the electrode is established, and the electrode acts as a heat sink when the lamp is started. As a consequence thereof, heat from the ends of the bridgewire immediately adjacent the electrodes is rapidly carried away. Since the center of the bridgewire is without a heat sink, it will get hot fastest and assures that the bridgewire is consumed first at the center after which there is a rapid burning back towards the electrodes.

The entire process of fusion of the center section and burn-back is very rapid which accounts for the description of such process as exploding the bridgewire. The portion of the electrodes getting hot first is the apex and since all this happens very fast, the burning back bridgewire ends are caught between the exit point and the hot apex. If the burning back bridgewire ends are disposed close to the center line of the electrodes, they fuse to close the slot across the axis thereby forming a solid and uninterrupted arc end.

If, however, the distance A is very small, say less than one diameter of the bridgewire, so that the bridgewire exits are substantially axial with the electrode axis, then after burnout of the center section the burning back bridgewire end may be flipped out of the accommodation means before the apex has become sufliciently hot to retain the same by fusion, and the bridgewire end is not consumed but may form another electrode to which the arc can move to cause arc instability. This situation is more usually encountered during vibration when the bridgewire is of straight line configuration and not tautly stretched across the electrode gap, and is sideways displaced by vibration at the very moment of starting the lamp.

The above discussion in connection with the slot slant length A is applicable to the slots in both electrodes where the lamp is nonpolarized. In case of a polarized lamp, the slot slant length A is primarily applicable to the cathode since the anode is made hotter by typicaly 700 centigrade. Accordingly, during burn-back of the bridgewire towards the anode, the bridgewire end is splattered over the conical surface and easily closes the accommodation slot. For this reason, the slant depth of the accommodation slot in the anode is not critical and may vary over a much wider range than the one given in connection with the dimension A.

The portion of bridgewire 50, which exits at 66 and 67 respectively into the arc gap from slots 60 and 62 is referred to as the free length portion of the bridgewire, and is shaped to generally conform to the letter S or a double curve or loop, as readily seen from an inspection of the drawing. The free length portion of bridgewire 50 is therefore larger than the distance between exit points 66 and 67, a distance designated in the drawing as B.

Referring now to FIGS. 4A, 4B and 4C, there are shown three different double loop free length portion configurations, respectively identified as 70, 71 and 72, disposed between

electrodes

16 and 18. Portion 70 depicts a free length configuration which has loops larger than the optimum configuration, portion 71 depicts a free length configuration which has loops regarded as the optimum configuration, and portion 72 depicts a free length configuration which has loops smaller than the optimum configuration. Even though each of the

free length portions

70, 71 and 72 depicts a configuration which has advantages over a straight line free length portion configuration stretched tightly across the arc gap between the apexes of the electrodes, the configuration of portion 71 has been found most satisfactory.

Each of the free length portion configurations depicted in FIGS. 4A, 4B and 4C comprises a looped configuration and, more particularly, a double loop in which the bridgewire exits from the electrodes at a point which is between the closed bottom of the accommodation slot and electrode apex, it being assumed that the loop is nonpolarized. Further, the looped configuration is compressed between exit points 66 and 67 so that, as long as the bridgewire is unexploded, the ends of the free length are resiliently urged against the exit points for good thermal contact with the electrodes. These features are believed to be the reason for the increased shock and vibration survival capability of the bridgewire lamp and its increased arc stability under all environmental conditions.

The length of free length portion 70 is approximately five times the arc gap width G, and its configuration may be described as an upright S with respect to the electrode axis. The two loops of the S are each about 270, and the loop commencing at one electrode approaches the other electrode a distance closer than one-half of the arc gap spacing. This configuration is regarded as the upper length limit of an optimum length range and should not normally be greater.

The length of free length portion 72 is approximately one and one-half times the arc gap width G, and its configuration may be described as an undeveloped S entirely lying on its side with respect to the electrode axis. The two undeveloped loops are each about 90. This configuration is regarded as the lower length limit of an optimum length range and should not normally be less.

The optimum configuration and length of the free length portion is shown in FIG. 4B and lies between the configuration of portions and 72. The length of the free length bridgewire portion 71 is approximately three times the arc gap width G, and its configuration may be described as a slanted S with respect to the electrode axis, the slant being approximately parallel with the conical electrode surfaces. The two loops of the S are each about 180, and the loop commencing at one electrode approaches the other electrode a distance which is almost one-half of the arc gap spacing.

Viewing free length portion 71, it can be seen that the S configuration comprises: a pair of straight line end portions 75 and 76 which are approximately perpendicular to the conical surface, a straight line center portion 77 which is substantially parallel to the end portions, and a pair of 180 curved portions 78 and 79 which connect opposite ends of center portion 77 with portions 75 and 76 respectively.

Even though it is to be understood that the exact configuration of the free length bridgewire portion across a gap is not highly critical, certain criteria for defining the optimum range of configurations can be established. Using

configurations

70 and 72 as the lower and upper limit of the optimum range, and configuration 71 as the center of the optimum range, it can be seen that the length of the free length portion should normally not be less than about one and one-half times and not more than about five times the arc gas distance G. Further, it can be seen that the S configuration, within the gap should lie between a fully upright S and a fully lying-on-its-side S. Still further, each of the two loops. forming the S should not curve more than 270 and not less than 90. This last criteria also implies, at the center of the range, that the end portions and the center portion of the S are parallel.

Referring now 0 FIG. 5, there is shown an alternate embodiment of the self starting short are lamp of this invention in which a pair of electrodes 80 and 82, disposed in an envelope (not shown), form an arc gap which is bridged by a bridgewire 84. Wire 84 is connected to electrodes 80 and 82 by spot welds indicated at 86 and 88, as before. The electrodes are provided with

accommodation slots

90 and 92 which are in all respects like the accommodation slots described in connection with the embodiment of FIG. 3 except that they are not diametrically opposed, but extend into the electrodes from the same side. In other words, they are not only parallel but have 0 angular displacement therebetween.

Bridgewire 84 is accommodate in

slots

90 and 92, and the free length portion 94 is shaped to conform to a single loop. As before, the positions of the exit point 96 on the cathode in case of a polarized lamp, and exit points 96 and 98 on the cathode and anode, respectively, in case of a nonpolarized lamp, are the same as those given previously. As far as the curvature of free length portion 94 is concerned, any curvature which offers the opportunity to compress the free length portion 94 between exit points 96 and 98 provides a greater environmental survival capability than a straight line configuration. However, the optimum range determined by tests has been found to lie between from 90 to 270 with the optimum configuration to be at the center of this range, or about 180.

FIG. 6 is a view taken along line 6-6 of FIG. 5 and shows that slot 90 is open along its entire length so that the curvature of portion 94 retains the bridgewire in the accommodation slot by being slightly compresed to resiliently urge the end portions against the exit points. FIG. 7 illustrates a further embodiment particularly applicable to polarized short arc lamps. As there shown, a cathode 100 is provided with an accommodation slot 102 in which a bridgewise 104 is embedded. As previously explained, to assure arc stability the burning back free length portion on the cathode must be captured before it has a chance to escape from the slot and form a possible alternate electrode. In all previous embodiments, such capture was provided by placing the exiting end portion close to the apex and, more particularly, by urging the exiting end against the exit point and locating the exit point past the apex so that he escaping wire would have to pass the apex. In theembodiment shown in FIG. 7, the slot 102 is closed by swaging so that the bridgewire is physically captured and cannot escape. Under such circumstances, it is desirable to place the exit point as close to the apex as possible, so that the optimum depth of the slot is one-half of the bridgewire diameter, and the wire is substantially coaxial with the apex. To assure good environmental survival, the free length portion is looped just as shown in FIG. 5. In this embodiment, since the wire is securely positioned in the cathode, the accommodation slot in the anode may have any desired angular position with respect to the accommodation slot in the cathode since only restraining contact with the exit point of the anode is necessary.

I should be clear that, in addition to the double loop and the single loop configuration illustrated, there are many other looped configurations which, upon compression, urge the ends of the free length portion resiliently into contact with suitable electrode slot exit points. Further, the relative orientation of the accommodation slots is not very cirtical so that angular displacement for the single loop configuration and 180 displacement for the double loop configuration may vary within wide limits.

There has been described hereinabove a self starting bridgewise short are lamp in which the bridgewire is of loop configuration across the arc gap, and has its ends positioned in resilient contact with the exit point of at least one of the accommodation slots located at a point which is past the apex, which combination results in improved environmental survival capability and arc stability.

What is claimed is:

1. In a self starting short are lamp, having a pair of spaced apart electrodes immovably disposed within a pressurized envelope defining an arc gap therebetween and having a bridgewire connected between said electrodes which includes a consumable free length portion unsupportedly bridging said are gap which fuses under the application of normal arc lamp sustaining power to first establish and thereafter sustain an arc across said are gap, the improvement comprising: a free length portion having a looped configuration along its length which is compressed between the electrodes to urge its ends into resilient contact against said electrodes at points approximate the electrode arc ends.

2. In a self starting short arc lamp in accordance with claim 1 in which said loop configuration comprises a single loop.

3. In a self starting short arc lamp in accordance with claim 1 in which said looped configuration comprises a double loop.

4. In a self starting short arc lamp in accordance with claim 3 in which said looped configuration conforms substantially to the shape of an S.

'5. In a self starting short arc lamp in accordance with claim 4 in which the length of said S-shaped free length portion is within the range of between three and one-half and five times the arc gap spacing.

6. In a self starting short arc lamp in accordance with claim 4 in which the arc length of each loop of said S- shaped free length portion is within the range from be tween 90 and 270 degrees.

7. In a self starting short arc lamp in accordance with claim 4 in which the S shape of said free length portion in angularly disposed with respect to the center line of said arc gap within the range of 0 to 90 degrees.

8. In a self starting short are lamp in accordance with claim 4 in which the arc length of each loop forming the S-shaped free length portion isnominally 180 degrees and in which the length of said free length portion is nominally three times the arc gap spacing.

9. In a self starting short arc lamp in accordance with claim 4 in which the extreme end portions and the extreme center portion of said S-shaped free length portion are substantially parallel to one another and connected by curved portions which are substantially semicircular.

10. In a self starting short arc lamp in accordance with claim 1 in which at least the cathode of said electrodes includes a bridgewire accommodation slot extending substantially axially from the arc end along a portion of said electrode, the width of said accommodation slot being dimensioned for slidingly receiving said bridgewire, the bottom surface of said accommodation slot intersecting said arc end to form an exit point for said free length portion, said exit point being disposed a selected distance from the arc end tip of the electrode on the side diametrically opposite to the slot opening.

11. In a self starting short arc lamp in accordance with claim 10 in which the distance between said exit point and said are end tip, as measured along the surface of the electrode, is selected to position the exiting bridgewire into close proximity to said arc tip.

12. In a self starting short are lamp in accordance with claim 11 which further includes means to close said accommodation slot to securely retain the extreme end of the exiting free length portion in contact with said exit point.

13. In a self starting short arc lamp in accordance with claim 10 in which said selected distance between said exit point and said arc end tip, as measured along the surface of the electrode arc end, is from between one to four times the diameter of the extreme adjacent end of said free length portion.

14. In a self starting short arc lamp in accordance with claim 10 in which said selected distance between said exit point and said are end tip, as measured along the surface of the electrode arc end, is substantially equal to twice the diameter of the extreme adjacent end of said free length portion.

15. In a self starting short are lamp, having a pair of spaced apart electrodes immovably disposed within a pressurized envelope defining an arc gap therebetween and having a bridgewire connected between said electrodes which includes a consumable free length portion unsupportedly bridging said are gap which fuses under the application of normal arc lamp sustaining power to first establish and thereafter sustain an arc across said arc gap, the improvement comprising: a free length portion having a looped configuration along its length; and a bridgewire accommodation slot in each electrode dimensioned to slidingly receive said bridgewire, said accommodation slots extending substantially axially along and into said electrodes and commencing proximate to the electrode arc ends, the intersection of the bottom surface of said accommodation slot with the surface of said electrode proximate the electrode arc end defines a bridgewire exit point which is disposed on the side of the electrode arc end tip opposite the accommodation slot entry, said looped free length portion being compressed between opposite exit points for resilient contact therewith.

16. In a self starting arc lamp in accordance with claim 15 in which the distance between the exit point on at least the cathode and the arc end tip is selected to position the exiting free length portion in a proximity closer than about four times the diameter of the exiting free length portion to the center line of the arc end tip.

17. In a self starting arc lamp in accordance with claim 16 in which said looped configuration comprises a single loop whose curvature is no less than and no more than 270 degrees.

18. In a self starting arc lamp in accordance with claim 16 in which said looped configuration conforms substan tially to the shape of an S and in which the curvature of each loop of the S is no less than 90 and no more than 270 deg-recs.

19. In a self starting arc lamp in accordance with claim 16 in which said free length portion conforms substantially to a semicircle and in which the accommodation slots in said electrodes are substantially parallel and extend into said electrodes from the same direction.

20. In a self starting arc lamp in accordance with claim 16 in which said free length portion conforms substantially to the shape of an S and in which the accommodation slots in said electrodes are substantially parallel and extend into said electrodes from opposite directions.

References Cited UNITED STATES PATENTS 412,599 10/1889 Coffin 219-100 2,391,611 12/1945 Baok. 3,377,497 4/1968 Keller 314-34 X BERNARD A. GILHEANY, Primary Examiner R. N. ENVALL, JR., Assistant Examiner