US2658451A - Static resistant electric initiator - Google Patents

Description

Nov. 10, 1953 C. F. HORNE STATIC RESISTANT ELECTRIC INITIATOR Filed March 6, 1953 CHARLES FRED HORNE INVENTOR.

' AGENT.

Patented Nov. 10, 1953 STATIC. RESISTANT ELECTRIC INITIATOR Charles F. Home, Kingston, N. Y., assignor to Hercules Powder Company, Wilmington, Del., a corporation of Delaware Application March 6, 1953, Serial No. 340,821

14 Claims.

This invention relates to electric initiators and more particularly to electric initiators which are highly resistant to premature firing 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 generally considered that such accidental firings 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 fired 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 firing due to static discharge is present inv all types of electric initiators, the procedures employed in seismographic prospecting and the sensitive ignition compositions employed in seismic-type 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 5,000 volts to fire the cap by direct discharge through the ignition composition and a discharge in the order of 12,000 volts to fir 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 calities wher most seismic prospecting is done. Consequently, it will be seen that a. definite danger of accidental firing of seismic caps is present under such adverse conditions. As a result of this danger, seismographic prospecting organizations and blasting cap manufacturers mercial blasting operations.

have been constantly attempting to raise the static resistance of all electric initiators and especially that of the seismic-typ 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 shell, at a point removed from the ignition composition, in order to provide for a discharge of the static electricity from the lead wire tov the shell. However, this type of structure offers good protection from static or firing 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 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 difiicult 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 sufiicient resistance for protection against the low voltage currents. which attend many com- 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 fire 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 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. The resistance does not change greatly due to passage of current until the currentv is suificient 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 low 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 shell. This 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 sufiicient resistance to heating of the bridge wire by a static discharge through the bridge wire itself.

The object of the present invention, therefore, is an improved initiator structure which will result in greatly improved resistance to firing by static electricity through discharge from the lead wire to the shell. An additional object of the invention is an improved initiator structure which gives adequate protection against premature firing caused by heating of the bridge wire by the passage therethrough of a static current. A further object of the invention is an initiator structure which will give one or both of these desired results and which is also characterized by high resistance to accidental firing due to stray currents.

Generally described, the invention is a static resistant electric initiator having, in combination, a metallic shell; an ignition assembly disposed within the shell comprising a pair of lead wires joined 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 at a point within the shell outside the locus of the ignition composition, said body of semiconductive material approaching the inner wall of the shell but being separated therefrom by a distance less than that between either lead wire and the shell in the locus of the ignition composition. By semiconductive body is meant a body which is an insulator at low voltage, such as -100 volts, but an extremely low resistance conductor at voltages comparable to that required for firing by static discharge. The resistance of such a body remains constant at practically infinite value at the low voltages but suddenly becomes very low when the higher voltage is applied. In a preferred embodiment the present invention will employ, as the semiconductive body, a conductive metal powder such as aluminum, substantially uniformly dispersed in a readily moldable nonconductive substance, such as wax or sulfur.

According to a further preferred embodiment, the blasting initiator of the present invention will also be equipped with a bridge wire of high 4 heat capacity in order to provide additional protection from a static discharge passing through the bridge.

The novel structure of this invention may be employed in any type of electric initiator to give substantially complete protection from the static discharges customarily encountered in blasting operations. However, in view of the fact that most difiiculty with static electricity is normally encountered with seismic-type caps, the invention will be principally described and illustrated with references to seismic blasting caps.

Having generally described the invention, the novel structure thereof will be more particularly illustrated with references to the accompanying drawing in which Fig. 1 represents a sectional view of a seismic-type electric blasting cap and Fig. 2 represents a part sectional, part elevational view of a further embodiment of a similar cap. Like symbols refer to similar structural elements.

In Fig. l, a base charge II] of detonative explosive is pressed into the base of a conductive metallic shell II. A priming charge I2 of primary explosive, such as a mixture of diazodinitrophenol and potassium chlorate, is pressed upon the base charge II). A cavity ignition plug I3 is positioned on the priming charge I2. The bared ends of insulated lead wires I4 pass through the ignition plug into the cavity I5 and the terminals thereof are joined by a bridge wire I6 of high heat capacity. Disposed in the cavity, I5 of the ignition plug I3 is a charge of ignition composition IT. The charge I! is butteredinto cavity I5 and completely surrounds the bared ends of the lead wires I4 and bridge wire I6. Positioned above the cavity ignition plug I3 and disposed about bared portions of the lead wires I4 is a cylindrical body I8 of semiconductive material. The body I8 is spaced from the inner wall of the shell II to form an annular gap I9 therebetween. A body of asphaltic sealing material 2!] is disposed above the semiconductive body I8. A top seal of sulfur 2| is disposed above the body of asphaltic material 20.

In the structure described in the drawing, it

will be noted that the body of semiconductive,

material I8 is spaced from the inner wall of the shell I I by a distance which is substantially less than the distance between the lead wires and the shell in the vicinity of the ignition composition. The semiconductive body acts as a nonconductor for the voltage employed to fire an electric initiator but is a good conductor for a high voltage static discharge. Therefore, while the semiconductive body I8 has no effect on the normal operation of the cap, it does constitute a shunt for high voltage discharges between the lead wires within the shell when the static voltage is applied to both lead wires. This effect is of particular benefit with a cap in which the bridge wire has been accidentally broken during manufacture. When the static voltage is applied across either of the wires and the shell, the charge is conducted from the lead wire through the semiconductive mass I8 and arcs across the gap between the semiconductive body I8 and the shell wall. Since the breakdown voltage of the semiconductive body I8 must be fairly low in order to give the desirable protection against static discharge, the cap as a whole would have undesirably low voltage breakdown characteristics if the body I8 were to contact the shell wall as is the case in several antistatic designs employed in prior art initiators. Because of the fact that the body is spaced from the shell wall, the voltage breakdown value of the cap is greatly increased and offers adequate protection against the type of stray current which can reasonably be expected to be found in blasting operations. For best results it is preferred to space the semieonductive body from the shell wall by a distance of between 0.005 and 0.050 inch.

When the strength of the static charge results in a passage of the current through the bridge wire, despite the shunt formed by the semiconductive body I8, the use of a high heat capacity wire impedes the generation of sufficient heat in the bridge to ignite the ignition composition II. The heat capacity of a bridge wire can be increased by increasing the length or the diameter. The dimensions of an electric initiator and manufacturing considerations place a limitation on the permissible length of a bridge wire, and it is consequently preferred to increase heat capacity by increasing the diameter. An increase in diameter will give the desired increase in protection against static electricity but for a given material the increase in diameter will also result in a corresponding increase of the minimum firing current of the cap. While this fact has no bearing upon the reduction of static susceptibility of the initiator, it is desired to maintain the minimum firing current of an electric initiator at a low level in the neighborhood of 0.5 to 1 amp. Therefore, if, in accordance with the present invention, the diameter of the wire is to be increased, it is desirable to employ a metal or alloy in the bridge wire which is characterized by a high specific resistance and a high specific heat. It has been found that with most of the ordinary materials used by the art, the minimum firing current of the initiator is undesirably raised if the diameter of the bridge wire exceeds about 0.0015 inch, which is the usual diameter employed by the art. It has been found that greatly increased static resistance is obtained if a diameter of 0.0020 inch is employed. It is preferred to employ a bridge wire diameter of about 0.0025 inch and in order to avoid any substantial increase in minimum firing current, it is preferred to employ a bridge wire made from a nickel-cromium alloy, such as that manufactured under the trade names Tophet C, Ohmax, and Jellifi 1000.

In Fig. 2, an initiator is shown which is identical to that illustrated in Fig. 1, except for the provision of a body of dielectric material in the form of a thin rubber sleeve 22 disposed about the semieonductive body 18 and substantially filling the space between the wall of the shell Ii and the semieonductive body I8. Despite the dielectric nature of the sleeve 22, static still discharges through the thin rubber to the shell wall.

Although it is usually preferred to employ a conductive metal powder as the conductive component of the semieonductive body, especially aluminum powder, other particulate conductive materials, such as conductive carbon may be employed. The nonconductive component which acts as a binder and carrier for the conductive particles is preferably wax. The Wax employed will desirably have a melting point of 120 F. or above in order to prevent substantial cold flow 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. For instance, if the waterproofing material employed is an asphaltic composition which is poured into the cap in molten form, a wax will be employed which has a melting point substantially higher than the temperature of the filling composition as it is poured into the shell. On the other hand, if a rubber or resin sealing plug is employed and is placed in position in cold form, the only consideration then is to use a wax which will not cold flow. If a molten resin is employed, the same considerations then pertain as in the case of a molten asphaltic composition. Preferred waxes include such natural waxes as candelilla, montan, and carnauba and synthetic waxes such as those manufactured under the trade names Ceramid, Acrawax, Acrawax C, Flexo Wax, and Bareco Wax. Various mixtures of these waxes may also be employed. Of this group Acrawax is preferred.

Instead of wax, however, rubber or rubber-like materials, resinous materials, sulfur and equivalent materials may be employed. Since it is desirable that the body of semieonductive material be molded about the lead wires, readily moldable nonconductive materials are preferred.

It is preferred that the annulus between the semieonductive body and the shell be substantially unfilled except by air. If the shell is sealed by a molten material which hardens in situ, some of this material may flow into the annulus. It has been found in practice, however, that this seldom occurs, probably due to the fact that the air is not readily displaced. Improved static resistance is still obtained, however, even when the annulus is filled with an insulatory material, since the static will still discharge through the thin layer. In Fig. 2, a thin rubber sleeve has been employed but other dielectric, insulatory materials may be employed such as sulfur, asphalt, synthetic resins and the like.

The amount of a particular conductive filler employed in the semieonductive body will depend on the conductivity of the particular material and on the degree of static protection desired. The method of forming the semieonductive body about the lead wires will also dictate the optimum quantity of particulate conductive filler employed. In the preferred composition of wax and aluminum, it has been found that best results are obtainable when between and 70% of particulate aluminum is employed. The upper limit of 70% is primarily dictated by the fact that it is desired to employ a pourable mixture for molding. The use of more than 70% of aluminum powder usually results in a mixture which is undesirably viscous. It has been found that adequate protection against static discharge from the lead wire to the shell through the ignition composition can be obtained with about 60% of particulate aluminum. Lesser amounts of aluminum can be employed with very beneficial results. From all considerations a 35 mixture of Acre.- wax aluminum has been found to give excellent results and is preferred.

Example 1 Seismic-type caps were prepared similar to that shown in the drawing except that no bridge wire was employed. The semieonductive body was formed from 67 powdered aluminum and 33% Acrawax and was molded directly on top of the cavity ignition plugs. The diameter of the semieonductive body was 0.20 inch and the inner diameter of the shells was 0.259 inch. These caps were tested for static susceptibility in comparison with conventional seismic caps having platinum alloy bridge wires about 0.00135 inch in diameter,

and with commercially available seismic caps having a conductive rubber plug surrounding bared portions of the lead wires and contacting the shell walls. The tests were made with a charge supplied by a I50 micromicrofarad capacitor discharged through he cap. Connections were made with the externally shunted lead wires and the shell. The following results were obtained:

inch platinum alloy bridge the maximum nonfiring voltage was 5000 volts. For the commercial cap with the conductive rubber plug in contact with the shell, the maximum nonfiring voltage was 6000 volts. For the cap with the 0.0025 inch nichrome bridge, the maximum nonfiring voltage was 14,000 volts. All three caps had a firing current of about 0.4 to 0.5 amp.

Conventional Cap Commercial Cap Invention We PP No. No. No.

Tested Shot Failed Tested Shot Failed Tested Shot Failed Example 2 Example 4 The conventional cap, the commercial cap, and the cap of the invention as employed in Example 1 (but equipped with Tophet C bridge wires 0.0025 inch in diameter) were subjected to a voltage breakdown test to determine the resistance of each cap to discharge by stray currents. In this test a variable A. C. voltage was applied between the lead wires and shell and gradually increased until breakdown occurred.

Caps from group I (platinum alloy bridge 0.00135 inch in diameter) and from group II (Nichrome bridge 0.0025 inch in diameter) were then tested in comparison by applying a static discharge supplied by a 750 micromicrofarad capacitor connected from the shunted lead wires to one bared lead wire near the cap. The maximum nonfiring voltage for the caps in group I was 10,000 volts while the maximum nonfiring Conventional Cap Commercial Cap Invention X i i pp 18 No. No. No.

Tested Shot Failed Tested Shot Failed Tested Shot Failed 1 Same 43 caps tested at progressively higher voltage.

From Example 1, it is seen that a conventional cap can be consistently shot by a static discharge from the lead wire to the shell through the ignition composition when that discharge is in the order of 5000 volts. On the other hand, the commercially available cap and the cap of the invention are both protected against discharge from lead wire to shell of static of much higher voltages than those normally encountered in the field. Example 2 illustrates, however, that the high degree of static resistance of the initiator in accordance with the invention is obtained without sacrificing to an undesirable extent the equally essential high voltage breakdown characteristics.

Example 3 Two groups of seismic-type caps were prepared without the semiconductive body. In group I, 2 platinum alloy bridge wire was employed which had a diameter of 0.00135 inch. Group II differed only in the use of a nichrome bridge (Tophet C) 0.0025 inch in diameter. These caps and the commercial cap used in Examples 1 and 2 were tested against susceptibility to firing by directing a gradually increasing static discharge directly through the bridge via the two lead wires. The charge was supplied by a 750 micromicrofarad capacitor. For the cap with the 0.00135 voltage for the caps in group II was 26,000 volts.

Example 3 illustrates that the conductive plug did not greatly increase resistance to static discharge through the bridge even though the plug actually contacted the shell.

Example 4 demonstrates that when a static discharge of over 10,000 volts is applied from shunted lead wires to one band lead wire near the shell, enough heat is generated in the bridge wire to shoot the conventional cap. When the bridge wire of higher heat capacity is employed, however, over 26,000 volts are required to generate enough heat in the bridge to fire the cap. However, it has been found that the spaced semiconductive body of the invention does assist in preventing the lower static voltages from passing through the bridge. 0n the other hand, a bridge wire of increased heat capacity will give the desired protection against this type of static discharge without a semiconductive plug of any type. Actually, insensitivity to discharge can be increased to almost any desired degree by increasing the heat capacity of the bridge. The practical limiting consideration in this regard, however, is the effect on the firing characteristics of the cap.

Example 5 Gaps in accordance with the invention were Conventional Cap Cap of Invention X i i pp 16 No. No.

Tested Shot Failed Tested Shot Failed Example 6 The two groups of caps of Example 5 were further tested by discharging a static charge supplied by a 750 micromicrofarad capacitor from the shunted lead wires to one .bared lead wire near the cap with the following results:

Conventional Cap Cap of Invention X i i pp No. No.

Tested Shot Falled Tested Shot Failed Examples 5 and 6 illustrate the excellent results which are obtainable when the spaced semiconductive body and high heat capacity bridge wire are combined. In particular, Example 6 shows that the two features complement each other in preventing accidental firing due to heating of the bridge when static discharges of higher voltage are encountered.

Example 7 Seismic-type caps were made similar to that shown in the drawing. The semiconductive body contained 37% of a 50-50 mixture of Flexowax C and Acrawax C and 63% of particulate aluminum. The semiconductive body was 0.25 inch high and 0.18 inch in diameter. The bridge wires employed were 0.00135 inch in diameter and were made from platinum alloy. The inner diameter of the cap shell was 0.259 inch. Static voltage supplied by a 750 micromicrofarad capacitor was applied from the shunted leg wires to the shell.

Voltage i Failed Shot Example 8 The tests of Example '7 were repeated with caps differing only in that the semiconductive body had a diameter of 0.20 inch.

Testd Failed Shot Mmcncqlo Ewample 9 The tests of Example 7 Were repeated with caps in which the semiconductive body was formed from a 33 67 mixture of Acrawax aluminum, had a diameter of 0.22 inch and was 0.25 inch in height. Four caps were tested at 26,000 volts and three caps at 36,000 volts. All seven caps failed.

The foregoing examples clearly illustrate that the improved initiators of the present invention are characterized by enhanced resistance to accidental firing by means of static discharge either between lead wire and shell or through heating of the [bridge as a result of the flow therethrough of static voltage.

As will be apparent to those in the initiator art, the conventional components of the initiators may be replaced by equivalents. The shells may be of any conductive metal such as brass, copper, ferrous metals and various alloys. 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, trinitrotoluene, 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 action 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 diazodinitrophenol-chlorate mixture, fulminates, leador tin-selenium mixtures, etc.

The ignition assemblies need not be the cavity ignition type illustrated. Instead, matchheads or loose ignition charges may be employed in accordance with conventional structures employed in the blasting initiator art. The shell may be sealed by more than one sealing layer as in the drawing or else a single seal may be employed such as a rubber sealing plug orv a cast or molded resin plug, and the shell suitably crimped.

The semiconductive body need. not be placed immediately above the ignition assembly as specifically illustrated but may be disposed about bared portions of the lead Wires anywhere outside the locus of the ignition composition.

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 invention be limited only by the scope of the appended claims.

This application is a continuation-in-part of my copending application for United States Letters Patent Serial No. 231,491, filed June 14, 1951.

What I claim and desire to protect by Letters Patent is:

1. A static resistant electric initiator having in combination a metallic 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 relation with bared portions of both lead wires at a point within the shell outside the locus of the ignition composition, said body of semiconductive material approaching the inner wall of the shell but being separated therefrom by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition.

2. An electric initiator in accordance with claim 1 in which the body of semiconductive material comprises particulate conductive material in admixture with a nonconductive binder.

3. An electric initiator in accordance with claim 1 in which the body of semiconductive material comprises particulate conductive metal powder in admixture with a nonconductive binder.

4. An electric initiator in accordance with claim 1 in which the body of semiconductive material comprises particulate aluminum and wax.

5. An electric initiator in accordance with claim 1 in which the body of semiconductive material comprises from 60 to 70% of particulate aluminum and 40 to 30% of wax.

6. A static resistant electric initiator having in combination a metallic 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, said bridge wire having a diameter of at least 0.0020 of an inch; and a body of semiconductive material disposed about and in conductive relation with bared portions of both lead wires at a point within the shell and outside the locus of the ignition composition, said body of semiconductive material approaching the inner wall of the shell but being separated therefrom by a distance substantiall less than that between either lead wire and the shell in the locus of the ignition composition.

7 An electric initiator in accordance with claim 6 in which the body of semiconductive material comprises particulate conductive material in admixture with a nonconductive binder.

8. An electric initiator in accordance with claim 6 in which the body of semiconductive material comprises particulate conductive metal powder in admixture with a nonconductive binder.

9. An electric initiator in accordance with claim 6 in which the body of semiconductive material comprises particulate aluminum and wax.

10. An electric initiator in accordance with claim 6 in which the body of semiconductive material comprises from 60 to 70% of particulate aluminum and 40 to 30% of wax.

11. A static resistant electric initiator having in combination a metallic shell; a base charge of secondary explosive; a priming charge of primary explosive; a cavity ignition plug, said plug having bared ends of insulated lead wires passing therethrough and terminating in the cavity, a bridge wire having a diameter of between 0.0020 and 0.0030 of an inch joining the terminals of the lead wires; an ignition composition disposed within the cavity and about the bridge wire; a substantially cylindrical, semiconductive mass containing from 60 to 70% of particulate alumi-- num and from to 30% of a wax melting above 120 F. disposed above the ignition plug and about bared portions of the lead wires, said mass having a diameter less than that of the plug and being spaced not more than 0.050 of an inch from the inner wall of the shell; and a body of sealing material disposed above said semiconductive mass.

12. A static resistant electric initiator having in combination a metallic 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; a body of semiconductive material disposed about and in conductive relation with bared portions of both lead wires at a point within the shell outside the locus of the ignition composition, said body of semiconductive material approaching the inner wall of the shell but being separated therefrom by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition; and a body of dielectric material disposed between the semiconductive body and the shell wall.

13. A static resistant electric initiator having in combination a metallic 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, said bridge wire having a diameter of at least 0.0020 of an inch; a body of semiconductive material disposed about and in conductive relation with bared portions of both lead wires at a point within the shell and outside the locus of the ignition composition, said body of semiconductive material approachmg the inner wall of the shell but being separated therefrom by a distance substantially less than that between either lead wire and the shell in the locus of the ignition composition; and a body of dielectric material disposed between the semiconductive body and the shell wall.

14. A static resistant electric initiator having in combination a metallic shell; a base charge of secondary explosive; a priming charge of primary explosive; a cavity ignition plug, said plug having bared ends of insulated lead wires passing therethrough and terminating in the cavity, a

bridge wire having a diameter of between 0.0020

and 0.0030 of an inch joining the terminals of the lead wires; an ignition composition disposed within the cavity and about the bridge wire; a substantially cylindrical, semiconductive mass containing from to of particulate aluminum and from 40 to 30% of a wax melting above F. disposed above the ignition plug and about bared portions of the lead wires, said mass having a diameter less than that of the plug and being spaced not more than 0.050 of an inch from the inner wall of the shell; a body of dielectric material disposed between the semiconductive body and the shell wall; and a body of sealing material disposed above said semiconductive mass.

CHARLES F. HORNE.

No references cited.

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