US2802421A

US2802421A - Static resistant electric initiator

Info

Publication number: US2802421A
Application number: US391096A
Authority: US
Inventors: Charles F Horne; Edward L Ramer
Original assignee: Hercules Powder Co
Current assignee: Hercules Powder Co
Priority date: 1953-11-09
Filing date: 1953-11-09
Publication date: 1957-08-13
Anticipated expiration: 1974-08-13
Also published as: GB739770A

Description

L A E N R O H E c.

STATIC RESISTANT ELECTRIC INITIATOR Filed Nov. .9, 1.953

FIG. 5 FIG.

CHARLES F. Momma EDWARD L mmf-R INVENTORS FIG.

AGENT.

United State srArrc nnsrsrANr ELECTRIC INIrtATon Charles F. Horne, Kingston, N. Y., and Edward L. Rainer,

Hockessin, Del., assignors to Hercules Powder Cornpany, Wilmington, Del., a corporation of Delaware Application November 9, 1953, Serial No. 391,096

24 Claims. (Cl. 102-28) This invention relates to electric initiators and more particularly to electric initiators which are highly resistant to premature ring by static electricity.

The art has long recognized the dangers inherent in accidental discharge of electric initiators by means of static electricity. Accidents, which at the time of their occurrence have seemed without explanation, have been subsequently traced to the firing of an initiator by a static discharge. Since normally used ignition compositions necessarily are highly heat sensitive, a discharge of relatively high voltage is quite capable of igniting the composition and firing the initiator. The art has Igenerally considered that such accidental iirings result from a direct discharge from a lead wire to :a metallic initiator shell in the locus of the ignition composition. Obviously, caps can also be tired by direct discharge through the bridge wire via the two lead wires. What is not so well understood, however, is the fact that an initiator can also be fired by the passage of a static voltage through the bridge wire when the discharge is from shunted lead wires to shell at a point other than through the explosives charge.

While the danger of premature tiring due to static discharge is present in all types of electric initiators, the procedures employed in seismographic prospecting and the sensitive ignition compositions employed in seismictype blasting caps have tended to make this type of initiator more susceptible to static than regular electric blasting caps and delay electric blasting caps. Until very recent years, commercial seismograph caps generally required a discharge in the order of 5000 volts to lire the cap by direct discharge through the ignition composition and a discharge in the order of 12,000 voltsto tire the caps by heating of the bridge wire when the charge was supplied by a 750 micromicrofarad capacitor and the discharge was from shunted lead wires to shell. It has been established that voltages of this magnitude may be developed by a man under proper conditions and that much higher voltages may be generated in equipment employed by seismic prospectors such as drill rigs and trucks.` Actually, conditions favoring high voltage static generation, such as low humidity, high winds, sandstorms, and the like, are common in the localities where most seismic prospecting is done. Consequently, it will be seen that a delinite danger of accidental iiring of seismic caps is present under such adverse conditions. As a result of this danger, seismographic prospecting organizations and blasting cap manufacturers have been constantly attempting to raise the -static resistance of all electric initiators and especially that of the seismic-type blasting cap.

A blasting cap structure has previously been proposed -in which a lead wire is disposed in contact or almost in contact with the metallic shell, at a point removed from the ignition composition, in order to provide for a discharge of the static electricity from the lead wire to the shell. However,` this type of structure offers lgood protection from static or tiring of the cap only when the charge passes down the one lead wire to the shell. Little or no benefit is obtained when the current passes through both lead wires. Even when both lead wires are so disposed, discharge will occur from only one Wire in many instances and protection will be limited as far as heating of bridge is concerned.

A structure has also been proposed wherein one or both of the bared lead wires :are connected to the metallic shell by means of semiconductive material outside the locus of the ignition composition. This construction affords a `considerable improvement in static resistance. However, it is very difficult with such a structure to maintain a proper balance of conductivity that will allow a discharge from both wires to the shell and still have sucient resistance for protection against the low voltage currents which attend many commercial blasting operations. Additionally, it has been found that in some instances, the static resistance of this type of structure diminishes with storage. Furthermore, such caps have been known to lire when 10-40 volts from a battery are applied between lead wires and shell.

In another proposed structure, conductive material is disposed about the bared lead wires and extends to the metallic shell. This conductive material :acts as a true resistor in that the resistance is low, normally from 10-100 ohms. The resistance of such a body of material is similar to that of a regular carbon resistor, being fairly constant, but subject to variation due to temperature. resistance does not change greatly due to passage of current until the current is suicient to cause heating. This type of cap gives good static protection in most instances. However, it has been found that in some instances discharges occur from only one wire which allow a firing of the cap by the heating of the bridge wire. Even more than in the case of the semiconductive material, this structure has a serious deficiency of insulation from shell to lead wires and can be fired in this manner with a very low voltage. In other words, this structure, while removing a `considerable part of the hazard of static electricity, has introduced an equally undesirable hazard in the form of undesirably lovv resistance between lead wires and shell.

In still another structure, it has been proposed to equip the bared lead wires with protrusions which extend toward the metallic shell. rThis structure is usually employed with a matchhead ignition element which is insulated. However, a discharge usually takes place from only one Wire and a firing of the cap by the heating of the bridge wire is thus permitted. In addition, a hotter spark is obtained when the discharge is directed from localized points. Even though the protrusions are outside the locus` of the ignition composition, such violent discharges are to be avoided when the same results can be obtained in a less violent manner.

While all of these preceding structures give an initiator a measure of static resistance, it will be seen that in each case, the protection from static is not complete and, in

.. most instances, what protection is obtained is brought about by a structure which is characterized by low voltage breakdown. Furthermore, these structures are not characterized by a sufficient resistance to heating of the bridge wire by a static discharge through the bridge wire itself. It will be further noted that the structures above described have all employed a conductive metal shell with the emphasis being placed on promoting a discharge of static from the leg wire to the shell through a conductor for such a charge or by means of arcing outside the locus of the ignition composition.

In recent years initiators employing a plastic shell have appeared on the market. It is at once apparent that a dielectric shell of plastic or equivalent material will aford protection against stray currents and at initial consideration it might also be surmised that the plastic shell would also furnish a satisfactorily complete answer to the problem of accidental rings by means of static discharge,

Aft

In fact, such claims have b en made for commercially yavailable initiators employing plastic, nonconductive shells. It is true that when a low voltage potential is builteup on a high capacity `condenser and such a potential is applied between vshunted lead wires and shell, theinitiator with the plastic shell kdoes exhibit satisfactory resist- `ance to tiring due to the insulatory characteristics of the plastic. The fallacy in such a demonstration, however, lies in .the Vfact that such 4charges are not Vthe type static charges which are encountered in actual blasting operations and'more particularly in seismic prospecting. The static charges which have causedA the accidents which the art now seeks to avoid have resultedfrom a high voltage buildup 'atlowV capacitance. It is for this reason that when determining the static resistance`of an initiator it isv customary to employ a 750 mmf. condenser and voltage ranging between 10,000 and 50,000 volts. When exposed toisuchfa realistic test 4it has been found that the initiators with plasticshellswliich are now available to the art are demonstrably more susceptible vto accidental discharge than the static resistant structures hereinabove described. In View of obvious advantages of a plastic shell, one of which is the resistance to iiring by stray currents, it is most desirable to have an initiator employing a dielectric shell which has low susceptibility to accidental firing by the high voltage staticjcharges encountered in the ield. In accordance with the present invention, initiators with dielectric shells may be readily and economically manufactured which have the necessary resistance to accidental ring either by stray currents or static electricity.

Generally described the present invention is a static resistant electric initiator having in combination a dielectric shell; an ignition assembly disposed within the shell comprising a pair of lead wires connected at their terminals by a bridge wire and an ignition composition disposed about the bridge wire; and a body of semiconductive material disposed about and in conductive relationship with bared portions of both lead wires; said shell forming a nonconductive layer between thesemiconductive body and ground of substantially less thickness than the distance between either lead Wire and ground in the locus of the ignition composition, the path from either lead Wire to grounded shell having substantially less total resistance through the Ysemiconductive ,body outside the locus of the ignition composition than the path from either lead wire to grounded shell in the locus of the ignition composition. Therefore, in the initiators of the invention, static charges vwhich can be expected to be encountered in the field are safely discharged from the semiconductive body through the shell to ground or else are discharged simultaneously from both lead wires through the semiconductivebody instead of through the bridge. Thus, the accidental iring of caps having dielectric shells may be substantially completely eliminated. The' body of dielectric material may bea separate plug disposed above the ignition plug and thus completely outside the locus of the ignition composition. To enhance manufacturing ease and to prevent undesirable elongation of the device, however, it is preferred that the semiconductive body lactually make up the ignition plug itself. In this modification the semiconductive plug will be the element in the initiator structure which is immediately adjacent theA sensitive ignition composition. Therefore, in order .to .assure complete protection against accidental ignition by a 4static discharge to ground rat the interface of plug and ignition composition, structural provision must be made .to insure discharge outside the locus of the ignition composition. employing a dielectriccoating or thin layer of dielectric material on the base of the semiconductive plug. Alternatively and preferably the semiconductive ignition plug will be stepped to provide a lower portion of reduced diameter.l A dielectric ring, ferrule, sleeve, or shell liner is then disposed about the portion of reduced diameter and thus the portion of the plug immediately adjacent the v the path to ground above such a shoulder.

l, "2,802,421 f `f Y ignition composition is spaced farther Vfrom ground than the portion above such element. Alternatively, the plastic shell may be formed or molded in such manner that a shoulder extends into the interior of the shell to produce a region of diminished inside diameter. Thus, a stepped or cylindrical ignition plug may engage this shoulder and conne the ignition composition in a portion ofthe shell where the resistance of the path to ground is greater than If desired, for ease of manufacture or any other reason, the semiconductive bodies, whether separate plugs or ignition plugs, may be made somewhat smaller than the inner diameter of the plastic sleeve and concentrically situated in the shell as long as the distance at the point of desired discharge outside the locus of the ignition composition between the semiconductive body Vand ground is less than the distance between either lead wire and ground in the locus of the ignition composition. As willbe more fully described, the semiconductive bodies or plugs are bodies` which afford high resistance to passage of currents employed to intentionally fire electric initiators but offer low resistance to high voltage static. charges Vagainst which protection is desired.

As has been initially indicated, the initiator art in its consideration of the static susceptibility of electric blasting caps has apparently overlooked the fact that accidental firing by static' may also take place by heating of the bridge wire as well as by arcing through the ignition composition to the grounded shell.- In the case of the initiator which has a plastic `or dielectric shell, the resistance to arcing to ground by employing the dielectric layer in place of the metallic casing is obviously raised somewhat. Thus, in the plastic shell cap there is an even greater tendency for accidental shots by heatingY ofthe bridge wire, since lower but still dangerousstatic potential which might ordinarily arcto a grounded metallic shell, will generously discharge by breakdown Vfrom one of the lead wires to ground vabove Vthe plastic shell. If the breakdown is from only `one lead wire through the bridge, an laccidental shot will be encountered depending on the potential involved and the heat sensitivity of the particular ignition mixture. If the bridge wire has been broken or detached from a lead wire terminal, an ignition arc may be obtained at potentials too low to arc to ground or to heat the bridge to a dangerous level. Thus, the disposition of the semiconductive body around the bared lead wires takes on even added importance as a high voltage shunt which tends in a great majority of casesV to cause breakdown from both lead wires simultaneously rather than by a unidirectional surge of current Vthrough the bridge wire from one lead wire alone.

i In copending application of Charles F. Horne, Serial No. 340,821, led March 6, 1953, now U. S. 2,658,451, issued November l0, 1953, and assigned to HerculesV Powder Company, it is disclosed that the resistanceto accidental firing ,of initiators through the heating of the bridge wire by static discharge through the bridge may also be increased greatly by employing a bridge wire of This may be accomplished by higher heat capacity. The use of a bridge wire of increased heat capacity is also included in the present invention and will be subsequently discussed.

Having generally described the invention, more specific illustration of specific embodiments will be given with reference'to the accompanying drawing in which Figs. V1, 2, and 3 are sectional views of completed electric initiators in accordance with the invention and in which Figs. 4, 5, and 6 are sectional views of'additional embodiments of ignition assemblies of initiators in' accordance with the invention.

In Fig.V l Ais shown a seismic type instantaneous initiator having a dielectric shell 10 formed from ethyl cellulose. A base charge 11 of pentaerythritoltetr'anitrate is disposed in the base of the shell V10. Above Ythe base charge 11 is a priming charge 12 of diazodinitrophenol. An ignition. composition 13. of.1diazodinitrophenol-potassium chlorate is loaded upon the priming charge 12. A plastic ignition plug 14 of ethyl cellulose is pressed upon the ignition charge. A semiconductive plug 15 is disposed above the ignition plug and contacts the shell 10. A plastic sealing plug 16 of ethyl cellulose is disposed in the mouth of the shell to provide a waterproof seal by moistening plug and shell with a mutual solvent prior to insertion of the plug. Lead wires 17 extend into the initiator through the sealing plug 16, the semiconductive plug 15, and the ignition plug 14. The lower portions of the lead wires 17 are bared for most of their passage through the semiconductive plug and all of their passage through the ignition plug 14. A bridge wire 18 joins the terminal ends of the lead wires 17 and is surrounded by the ignition composition 13. If a static charge having a voltage sufficient to fire the ignition composition 13 by arcing from the leg wire 17 through the shell 10 to ground is applied between the shunted leg wires and the grounded shell 10, discharge will `take place from the bared lead wires 17 through the semiconductive plug 15 and through the shell wall 10 to ground and thus prevent arcing in the locus of the ignition composition. The serniconductive plug 15 further acts as a high voltage shunt between the bared lead wires 17 and greatly raises the static voltage necessary to cause tiring by heating of the bridge wire 18.

In Fig. 2, a delay electric initiator is shown which has a plastic shell 20 formed from cellulose acetate. Conventional `base charge 11 and priming charge 12 are em-` ployed. A conventional delay fuse element 21 is disposed above the priming charge 12 and consists of a dielectric tube 22 containing a delay fuse train 23 of barium peroxide-selenium composition. Disposed above the delay fuse element 21 is a lead styphnate ignition composition 24. A semiconductive ignition plug 25, the base of which is coated with a dielectric iilm 26 of nitrocellulose lacquer, is pressed upon the ignition composition. A cellulose acetate sealing plug 27 is cemented into the mouth of the shell 20 to form a waterproof seal. Lead wires 17 extend through the sealing plug 27 and the semiconductive ignition plug 25. The terminals of the wires 17 extend into the ignition composition and are joined by a bridge wire 18. As in the device of Fig. 1, when a dangerous static potential is applied betweenthe shunted' lead wires 17 and the grounded shell 20, discharge will occur between a lead wire 17 and ground through the semiconductive plug 25 and shell wall 20. Again the In Fig. 3, an instantaneous blasting cap shown which.-

employs a cellulose acetate shell 30. AThe shell 30 has a tion in which a stepped semiconductive ignition plug 40 is employed. A plastic sleeve 41 is disposed about the lower portion of the stepped plug 40 having the smaller diameter and spaces the upper portion of the plug 40 slighlty from the wall of the shell 10. Ignition composition 13 is disposed in the cavity formed by the sleeve 41 and the lower surface of the plug 40. Bared portions of lead wire 17 extend through the semiconductive plug 40 into the ignition composition 13 and are joined at their terminal ends by a bridge wire 18. In this structure discharge from leg wire to ground will take place above the sleeve 41 outside the locus of the ignition composition, while the plug 40 acts as a high voltage shunt between the bared leg wires.

In Fig. 5, a cylindrical semiconductive ignition plug is employed, the lower portion of which has been dipped in a dielectric lacquer solution to produce a dielectric film 51. The plug 50 is in contact with`tl1e wall of the shell 10. Bared portions of lead wires 17 are in conductive relationship with the plug 50 and are joined at their terminals by a bridge wire 18 below the lower surface of the plug 50. A matchhead ignition element 52 is employed which consists of a bead of ignition composition 13 disposed about the bridge wire 18. Discharge from leg wire to ground will take place from leg wire 17 through the plug 50 to ground above the lm 51, while the plug 50 acts as a high voltage shunt between the bared leg wires.

In Fig. 6, a stepped semiconductive ignition plug 60 is employed and a plastic ring 61 is fitted about the portion of reduced diameter of the plug 60. The plug 60 contacts the wall4 of shell 10. Bared portions of lead wires 17 are in conductive relationship with the plug 60 and are joined at their terminals by bridge wire 18 below the surface of the plug 6l). A matchhead ignition element 52 is again employed consisting of a head ofignition composition 13 disposed about the bridge wire' 18. Static discharge to ground in this structure will occur from lead wire 17 through the plug 60 to ground above the ring 61 rather than in the locus of the matchhead 52. The plug `60 again acts as high voltage shunt between the bared lead wires 17.

In the structures described in the drawing it will be noted that the semiconductive body or plug is usually in contact with the shell wall but may be spaced therefrom as in Fig. 4, if desired. However, the plastic shell itself affords adequate protection against accidental discharge by stray currents and it is therefore preferred region of reduced inside diameter below lthe shoulder 31'.

Disposed in the region of reduced diameter is a conven-v shoulder 31 of the shell 30 and contacts the shell wall. A cellulose acetate sealing plug 33 is cemented in the mouth of the shell to produce a waterproof seal. Lead wires 17 pass through the sealing plug 33, the semiconductive plug 32, and extend into the ignition composition where their terminal ends are joined by bridge wire 1S. The lead wires are bare throughout their passage through the semiconductive ignition plug 32. In the structure shown a static discharge to ground will occur through the semiconductive plug and the thinner portionof the wall of the shell 30 above the shoulder 31 rather than in the locus of the ignition composition 13. The semiconductive plug 32 again actslas a high voltage shunt and greatly raises the static potentialrnecessary to cause breakdown that the semiconductive body or plug contact the dielectric shell wall, since such a structure is easier to manufactureV and affords a static discharge path to ground of less total resistance. Since the total resistance of path to ground is less through that portion of the semicon- `ductive plug which is not adjacent the ignition composition, static discharge will in al'l cases take place at a point safely spaced from the sensitive composition.

The semiconductive body in accordance with the invention acts as a nonconductor for the voltage employed to iire the electric initiator but is a good conductor for a high voltage static discharge. Therefore, whilethe semiconductive body or plug has no effect on the normal operation of the cap, it does, as indicated, constitute a shunt for high voltage discharge between the lead wires within the shell when the static voltage is applied to both lead wires. This effect is not only benecial in a properly constructed cap but, as already indicated, adds an additional factor of safety where a bridge wire is inadvertently broken or loosened from one of the terminals during or subsequent to manufacture.

Although the semiconductive plug does greatly increase the strength of static charge necessary to tire the cap through heating of the bridge wire, the overall susceptibility to accidental ignition in this manne-r may be still further decreased by using a bridge wire of high heat capacity which impedes the generation of sufcient heat ^7 inthe bridge to vbring about ignition of the i'griition'icomi-V po'si`tion. v The combination `of' the high4 voltagev shunt' and-the high heat capacitylbridge,wire`will prevent ring through heating of 'thebridge wire by any static; charge Waxesrand mixturesthereof; Various mixtures of these waxes may also be employed. e VInsteadlof wax, however,Y rubber or rubberl1ke I naterials,;resinous materials, vsulfur and equivalent materials which can reasonabl be ex ected to be encountered even, A may `be employed, 'Since iteis vdesirable that the body geophysicalprosptilcting, pThetheat'capacityofabridge of semlconductive material be molded about the lead Wire can be increased by increasing the length or the wires, readily moldable vnonconductivematerials are prediarneter ofthe bridge. The dimension'ofan electric ferl'ed. initiator and manufacturing considerations placca limita- The amount of a particular conductive filler employed tion onvthe permissible length of a bridge wire, and it 1n 1n the semlconductive body will depend 0n the conducis consequently preferred to increase heat capacity by iVi'iy 0f ille Particular material arid Ori the de gree 0f increasing the diameter. Antincrease in diameter will Sierre, Proteetiori deSired- The methol 0f f oirriuig elle give the desired increase in protection against static elec- SerruCOudueiiVe body aliOut the lead WireS Will alSO dietricity but for a given material the increase in diameter tate the Optimum quantityy 0f Parueulaie eeildueuye lller rwillpalso rejsu'lt 'in a corresponding increase of the minil5` eiii-ileyed-V Irl the Preferred eOmPeSi'uOilV 0f WaX and mum tiring currentof the'cap. `While this fact has no aluminum, it has been fOuiid harbest reSullS are Obj bearing upon the lreduction of static susceptibility of the airialle When between 60 and 7 07? 0f ParUelilalealU-mlinitiator, it is desired to maintain the minimum ring Dum 1S emPlOyed The ii pp ei"lir1 i1t 0f 70% 1S Primarily current of an electric initiator at aV low level in the neighdictate@ 'by the. fact that it iS desired to employ a POurborhood of 0.5 to lamp. Therefore, if, in accordance able muftlu'e for moldiug- The use of more than 70 t with afurther embodiment of the invention, the diameter of` alumirium 'Povfder usually VreSulS iu a miXrilre -Whieh of, the wire is to be increased, it is desirable to employ 1S uudesirably Viscous for Pouring OllieraueiiaVV It h aS. a metal or alloy in the bridge wire which is characterized been found that adequete Protection against Statiodie' by a high Specific resistance and a high Speeie heat c harge from t he lead wire to the shell through the igni- It has been found that with mostof the ordinary manon compositiori Learl be O btalnedwith about 60% of .terials used by the art, the minimum firing current of particulate aluminum. Lesser amounts of aluminum can the initiator is undesirably raised if the diameter of vthe b e emlloyed Wlth Veryboneclal fosults From all Cou' Bridge Wire exceeds abone 0 0015 ineh -whieh is the siderationsa 35/65 mixture of wax/aluminum has been usual diameter employed by the are It has' been found found togiveexcellent results and is preferred.V that greatly increased static resistance is obtained if a ,In @der to demonstfate tho improvement of the u ldiameter of 00020 inch is employed It is preferred itiators in accordance with the invention in terms of static to employ a bridge wire diameter of-about 0.0025 inch lp-Lggseomsllhas llislttlouglla C7225) Y 1 En@ m Order to Eil/qld an); substantlal mrease Il? .Iummlim mmf. condenser to simulate the actual static conditions rmg current 1i 1S pre @n.ed to emp Oy a n ge Wlre 35 encountered in the e'ld. The caps had vinyl plastic made from 2.1 lilckel'chromlum alloy' shells .038 Vinch in thickness, andl employed cycloniie Although 1t 1s usually preferred to employ a conducbase charges lead azide priming eharges .and-,a iead tlve-meta1 powder 3.5 the conductlfe componfmt of the styphnate ignition composition. The ignition plugs were SemiCOrldueVe gmi'ieri Plug, especially aluminum POW also of vinyl plastic. lEach cap tested was tightly wrapped der, other particulate conductive materials, such yas con- 40 in aluminum foil and the indicated potential was applied ductive carbon may be employed. The nonconductive between shunted lead wires and the foil. The following component which acts as a binder and carrier for the results were obtained:

Example 1 s Cap Number Voltage Applied* i 2 3 4 5 6 7 s 9 1o 11V S12 1a 14 15` -16 17 18 19 2o :F-Cap failed` lsDC-Cap1 btroke down, Le., a circuit was established, but cap did not detonate.

- ap s o *Voltage applied by discharging of a 750 mmf. condenser.

conductive particles is preferably wax. The wax einployed will desirably'have a melting point of 120 F. or above in order to prevent substantial cold ow of the wax subsequent to manufacture. The melting point of the wax, or any other operable carrier, will, of course, depend largely upon practical considerations. If a rub- Y ber or resin sealing plug is employed and is placed in the cap did not shoot.

In the foregoing tests, a dangerous situation existed at the voltages where breakdown occurred even though If the breakdown was from both lead wires simultaneously,` the danger of Ya shot was not great. However, if the breakdown was lfrom one lead wire through the bridge and Athe ,cap did not shoot,V the element of luck played a majorparasince caps 9, 10, 13v and 14 establish that the particular ignition composition can be shot by static discharge at voltages at least Vasv low as 15,000 volts. It`will be noted that only three of the caps tested withstood a'discharge of 40,000 volts and that thirteen of the remaining seventeen iii-'ed at 20,000 volts or below.

Example `2 Other plastic shell caps of the same manufacture were then modiiied in accordance with the invention. The plastic sealing plugs were removed and were replaced by a semiconductive plug composed of 64% particulate aluminum and 36% candelilla wax disposed about bared portions of the lead wires. Five such caps were then tested as in Example 1 at voltages of 17,000, 21,000, 26,000 and 31,000. All caps failed. These same caps were then retested at voltages of 21,000, 26,000, 31,000, and 40,000. One cap shot at 40,000 volts.`

Example 3 The semiconductive plugs were then removed from four caps which failed in the test of Example 2. When the test of Example 1 was again applied, one cap shot at 21,000 volts and three caps shot at 26,000 volts.

The initiator shells may be made from any desired dielectric material such as plastic, rubber, liber, and the like. The tough cellulosic esters and ethers are preferred such as cellulose acetate, cellulose acetate-butyrate, and ethyl cellulose. However, vinyls, such as vinyl chloride, vinyl chloride-acetate, polystyrene, acrylic polymers, etc., also are operable as are condensation resins such as the phenol aldehydes `and melamine type resins. Conceivably, conventional metal shells could be Coated with a suitable dielectric coating, layer, or casing. However, such a composite shell would be undesirably expensive unless used in an application Where great physical strength is an essential and justities the additional expense.

As indicated in the drawing, the dielectric spacing body employed may have various configurations and may be made of many different solid dielectric materials. Such materials include cardboard, paper, berboard, rubber, plastic, and the like. For most purposes, cardboard is preferred due to its ease of fabrication, economy, and excellent dielectric qualities.

`If it is desired to employ the structure of Fig. 5, any suitable material may be employed as the dielectric lm 51. Lacquer is preferred due to ease of application. Generally, however, it is preferred to employ structure in which the spacing sleeve extends below the base of the semiconductive plug unless the plug is stepped as in Figs. 4 and 6.

As will be apparent to those in the initiator art, the conventional components of the initiators may be replaced by equivalents. The lead wires may be made of any of the conventional materials such as copper and iron, and may be tinned if desired. The lead Wires may be insulated with any desired material such as cotton servings, rubber or various plastics. The base charges may be formed from any secondary detonative explosive such as pentaerythritol tetranitrate, cyclonite, tetryl, trinitrotolnene, and the like, and may be cast or pelleted as well as pressed when the nature of the explosive permits. The priming charge may be omitted if the base charge is capable of initiation by the laction of the ignition composition. When a priming charge is employed as is preferred, any primary explosive or mixture may be used such as diazodinitrophenol, diazodinitrophenol-potassium chlorate, lead azide, lead styphnate, and mercury fulminate. Any of the known ignition compositions may also be employed such as finely divided diazodinitrophenolchlorate mixture, fulrninates, leador tin-selenium mixtures, etc.

As indicated in the drawing, the ignition `assemblies may employ cavity ignition plugs, matchheads, or loose ignition charges in acordance with conventional structures employed in the blasting initiator art. The shell may be sealed by more than one sealing layer, or, as indicated, a single seal may be employed such as a cast or molded resin plug.

Since, as indicated, the initiators within the scope of the invention can be altered in many respects without changing their mode of operation, it is intended` that the inven-` tion be limited only bythe scope of the appended claims.

What we claim and desire to protect by Letters Patent 1s:

l. A static-resistant electric initiator having in combination a dielectric shell, an ignition assembly disposed within the shell comprising a pair of lead wires connected at their terminals by a bridge wire and an ignition composition disposed about the bridge wire, and a body of semiconductive materialdisposed about and in conductive relationship with bared portions of both lead wires, said shell forming a nonconductive layer between the semiconductive body and ground of substantially less thickness than the distance between either lead wire and ground in the locus of the ignition composition, the path from either lead wire to grounded shell having substantially less total resistance through the semiconductive body outside the locus of the ignition composition than the path from either lead wire to grounded shell in the locus of the ignition composition.

2. An electric initiator in accordance with claim 1 in which the semiconductive materialcomprises particulate conductive material in admixture with a nonconductive binder. L i

3. An electric initiator in accordance with claim l in which the semiconductive material comprises particulate conductive metal in admixture with a nonconductive binder.

4. An electric initiator in accordance with claim 1 in which the bridge wire has a diameter of at least 0.0020 of an inch.

5. An electric initiator in accordance with claim l in which the semiconductive body is spaced from the shell wall by a distance substantially less than the distance between either lead wire and the shell wall in the locus of the ignition composition.

6. An electric initiator in accordance with claim l in which the dielectric shell is a moldable plastic.

7. A static-resistant electric initiator having in cornbination a dielectric shell; an ignition assembly disposed within the shell comprising, a dielectric ignition plug, a pair of lead wires passing `through the ignition plug, a bridge wire connecting the terminals of the lead wires, and an ignitionA composition disposed about the bridge wire; and a body of semiconductive material disposed about and in conductive relationship with bared portions of both lead wires at a point within the shell outside the locus of the ignition composition, said shell forming a nonconductive layer between the semiconductive body and ground of substantially less thickness than the distance between either lead wire and ground in the locus of the ignition composition, the path from either lead wire to grounded shell having substantially less total resistance through the semiconductive body outside the locus of the ignition composition than the path from either lead wire to grounded shell in the locus of the ignition composition.

8. An electric initiator in accordance with claim 7 in which the semiconductive material comprises particulate Iconductive material in admixture with a nonconductive binder.

9. An electric initiator in accordance with claim 7 in which the semiconductive material comprises particulate conductive metal in admixture with a nonconductive binder.

10. An electric initiator in accordance with claim 7 in which the bridge wire has a diameter of at least 0.0020 of an inch.

1l. An electric initiator in accordance with claim 7 in which the semiconductive body is spaced from the shell wall by a distance substantially less than the distance between either lead wire and the shell wall in the locus of the ignition composition.

- I1 12.`An electric initiator in accordance Vwith claim 7 in ,which the dielectric shell is a moldable plastic.

13. Av static-resistant. electric initiator having in combination a dielectric shell, a pair of lead wires, a semi` conductive ignition plug disposed about bared portions of the lead wires within the shell, a thin layer of di.- electric material covering at least the bottom of the semiconductive ignition plug, a bridge wire connecting the terminals of the lead wires below the dielectric layer, and an ignition composition disposed about the bridge wire, said shell forming a nonconductive layer between the semiconductive body and ground of substantially less thickness than the distance between either lead wire and ground in the locus of the ignition composition, the path from either lead wire togrounded shell having substantially less totalresistance through the serniconductive body outside the locus of the ignition composition than the path from either lead wire to grounded shell in the locus of the ignition composition.

14. An electric initiator in accordance with claim 13 in which thev semiconductive material comprises particulate conductive material in admixture witha non- .conductivebinden 15. An electric initiator in accordance with claim 13 in' which the semiconductive material comprises particulate conductive metal in admixture with a nonconductive binder. f

16. An electric initiator in accordance with claim 113 in which the bridge wire has a diameter of at least 0.0020 of an inch. f

17. An electric initiator in accordance with claim 13 Vin which the semiconductive body is spaced from the conductive relationship with the plug, a bridge wire connecting the terminals of the lead Wires below the plug, and an ignition composition disposed about the bridge wire, said shell forming a nonconductive layer between the semiconductive body and ground of substantially less vthickness than the distancebetween either lead wire a"n`d ground in the locus of the ignition composition, the path from either lead wire to grounded shell having substantially less total resistance through the semiconductive body outside'the locus of the ignition composition than the path from either lead wire to grounded shell in the locus of the ignition composition. Y

20. An electric initiator in accordance with claim 19 in which the semiconductive material comprises particulate conductive material inradmixture Awithra nonconductive binder.

g 21. An electric initiator in accordanceV with claim 19 in which the serniconductivel material comprises particulate conductivevmetal in admixture with a nonconductive binder. g ,Y f f j l 22.A An electricinitiator inaccordance with claim 19 in which the bridge wirehasv a diameter of at least 0.0020 of an inch.`

2.3. A n electric initiatorvrin accordance with claim 19Vv in whichrthre yse,miconductiye body is spaced from the shell wall by a ,distancek substantially less than the distance between either lead wire and the shell wall in the locus of the ignition compositiomj f' v '2.4. An electric initiatorin-accordance with claim 19 in which the dielectric shell is a moldable plastic.;V

Refe"rences Cited inthe tile of this patentV l UNITED STATES PATENTSr Y 2,086,548 HandfQr1h' YJuly 13, 1937 2,382,921 Y Seavey Aug. 14,' 1945 2,399,034 Huyett et al 'Apr. 23, 1946 2,658,451 Horne Nov. l0, 1953 v2,685,835 Noddin et a1. 1 Aug. 10, 1954 2,718,930 ,Bazhaw sept; 27, 1955 2,721,617 Piety oct. 25, 1955 f l VFOREIGNPATENTS 501,819 Belgium v Sept. 12, 1951 664,583 Great Britain Jan. A9, 1952

Claims

1. A STATIC-RESISTANT ELECTRIC INITIATOR HAVING A COMBINATION A DIELECTRIC SHELL, AN IGNITION ASSEMBLY DISPOSED WITHIN THE SHELL COMPRISING A PAIR AOF LEAD WIRES CONNECTED AT THEIR TERMIUNALS BY A BRIDGE WIRE, AND A BODY OF POSITION DISPOSED ABOUT THE BRIDGE WIRE, AND A BODY OF SEMICONDUCTIVE MATERIAL DISPOSED ABOUT AND IN CONDUCTIVE RELATIONSHIP WITH BARED PORTIONS OF BOTH LEAD WIRES, SAID SHELL FORMING A NONCONDUCTIVE LAYER BETWEEN THE SEMICONDUCTIVE BODY AND GROUND OF SUBSTANTIALLY LESS THICKNESS THAN THE DISTANCE BETWEEN EITHER LEAD WIRE AND GROUND IN THE LOCUS OF THE IGNITION COMPOSITION, THE PATH FROM EITHER LEAD WIRE TO GROUNDED SHELL HAVING SUBSATANTIALLY LESS TOTAL RESISTANCE THROUGH THE SEMICONDUCTIVE BODY OUTSIDE THE LOCUS OF THE INGNITION COMPOSITION THAN THE PATH FROM EITHER LEAD WIRE TO GROUNDED SHELL IN THE LOCUS OF THE IGNITION COMPOSITION.