US3558976A

US3558976A - Internal resistive static discharger

Info

Publication number: US3558976A
Application number: US824570A
Authority: US
Inventors: Chester H Miller; Arthur J Brodersen
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-05-14
Filing date: 1969-05-14
Publication date: 1971-01-26
Anticipated expiration: 1988-01-26

Abstract

An internal resistive element static discharger including a conductive surface contact base, a nonconductive inner rod having an axially aligned needle discharge member affixed at the free end thereof and a conductive base contacting pin at the fixed end thereof, the said needle and the said pin being electrically interconnected by a length of low conductivity material and a hollow, nonconductive outer tube overfitting and protecting the said inner rod, the said outer rod threadedly engaging upon the base to urge the pin into reliable electrical contact with the conductive base.

Description

United States Patent Chester H. Miller 2450 SW 18th St., Miami, 33145; Arthur J. Brodersen, 2242 NW 2nd Ave., Gainesville, Fla. 32601 [72] Inventors [21] Appl. No. 824,570

[22] Filed May 14,1969 [45] Patented Jan. 26,1971

[54] INTERNAL RESISTIVE STATIC DISCHARGER 12 Claims, 13 Drawing Figs.

[52] US. Cl 317.2 [51] Int. Cl B64d 45/02, H05f 3/00 [50] Field ofSearch 317/2; 244/1R; 327/3, 4; 307/262; 174/2 [56] References Cited UNITED STATES PATENTS 3,286,141 11/1966 Plaglin 317/262 3,170,087 2/1965 Tanneretal 3,009,982 1 1/1961 Newman 174/2 2,333,144 11/1943 Bennett et al.... 244/1 2,631,189 3/1953 Sullivan et a1. 317/2 2,497,924 2/1950 Beach 317/2 2,536,818 1/1951 Lawton 317/2 Primary Examiner-J. D, Miller Assistant ExaminerC. L. Yates Att0rneyKar1 L. Spivak ABSTRACT: An internal resistive element static discharger including a conductive surface contact base, a nonconductive inner rod having an axially aligned needle discharge member affixed at the free end thereof and a conductive base contacting pin at the fixed end thereof, the said needle and the said pin being electrically interconnected by a length of low conductivity material and a hollow, nonconductive outer tube overfitting and protecting the said inner rod, the said outer rod threadedly engaging upon the base to urge the pin into reliable electrical contact with the conductive base.

PATENTEDJANZBIH?! 3.5580976 sum 1 BF 2 INVENTORS.

CHESTER H. MILLER ARTHUR J. BRODERSEN 'FIG.7 YMKW ATTORNEY PATENTEU M26 :97: 3558 976 Slim 2 0 2 PZZGI'I;

i 44' l l 68 7O 30 FIG, |2

F I G, 3

III llwilllll TORs CHESTE WLLER ARTHUR J. BRODERSEN BY MK w ATTORNEY INTERNAL RESISTIVE STATIC DISCHARGER The present invention relates to the general field of static dischargers, and more particularly, is directed to a novel improved type of static discharger wherein all functioning parts are mechanically protected.

This invention relates to the removal of static electric charges from aircraft in order to prevent corona discharge in the vicinity of radio antennae. It has been found that the corona discharge is a source of electromagnetic interference which can disrupt the normal radio communications of the aircraft. In order to minimize these effects, and the noise of corona discharge, static discharge devices have been placed at the extremities of the airframe by prior workers in the field. In particular, static dischargers are usually affixed at the trailing and lateral edges of the wings and horizontal stabilizers and at the top trailing and lateral edges of the vertical stabilizer. Most static dischargers presently in use are either of the wick type which employs a length of graphite-impregnated cotton enclosed in a plastic tube or are of the so-called null-field type which include a high impedance rod having a conductive coating thereon and carry a tungsten pin positioned at right angles to the axis of the rod. Both function to provide a localized area where static precipitation can be leaked off by corona discharge before the charge buildup is of sufficient magnitude to cause the airframe itself to discharge by corona discharge. Since this corona discharge takes place at the extremities of the aircraft remote from the radio antennae, the effect of the corona noise on the radio communications is thereby minimized.

Although the operation of the prior art static dischargers has been generally satisfactory, the mechanical and electrical designs of these prior art models are quite deficient from both a wear standpoint and from the fact that they offer a considerable danger to ground personnel from sharp, exposed portions. Further,prior art null-field models generally are fabricated with the conductive material coated to the outer surface of the discharge member. This causes exposure to the abrasive efiects of wind shear which results in a relatively short lifespan, thereby requiring frequent replacement and increased maintenance costs. Additionally, tests have shown that the exposed conductive material itself has a tendency to corona discharge thereby additionally breaking down the surface conductive material, all of which .results in a shorter period of reliable operation.

The present invention comprises an outer tube which completely surrounds the conductive element and protects the conductor from damage due to wind shear at high speeds. Further, the present mechanical designmakes the installation and replacement of the static discharger relatively simple without the need for any tools or special devices whatsoever. The outer tube, which completely'surrounds the conductive material, serves also to shield the sharp discharge needle to thereby completely eliminate all hazard to maintenance personnel. All of these mechanical advantages are obtained by utilizing the present design, and at the same time, the electrical performance achieved is equal. to or surpasses that of presently known static dischargers.

The static discharge units herein disclosed have been carefully engineered and designed so as to be completely interchangeable, with regard to weights and balances, with presently existing prior art static dischargers. Accordingly, static discharge units fabricated in accordance with the present disclosure may be readily employed or substituted without requiring a rebalance of movable aircraft surfaces.

The electrical operation of the internal resistive element static discharger relies on a sharp tungsten needle which is axially aligned and extends rearwardly to break into corona discharge before any other part of the airframe. This is accomplished simply by making the point of the needle sharper than any other part of the airframe. The high resistance between the needle and the airframe isolates the DC corona discharge from the airframe for the radio frequencies. Since the RF fields are known to have their maximum values at the extremities of the airframe, the effective isolation of the DC corona discharge from the RF fields on the airframe assures that the coupling of the noise corona into the antennae will be minimum.

The magnitude of the noise produced in the antenna by the corona discharge does not critically depend on the physical orientation of the needle. The radiation pattern of the DC corona discharge is not likely to be unidirectional thereby having a field vector whose direction is constant in space and time. Experiments on the physical mechanism of corona discharge at a point indicate that the current produced by the corona tends to be random in both direction and magnitude. The random nature of the corona has also been verified by experiments performed on actual aircraft and thus, the orientation of the needle is not of critical importance.

It is an object of the present invention to provide an improved internal resistive element static discharger of the type set forth.

It is another object of the present invention to provide an improved internal resistive element static discharger wherein the conductor and the needle are entirely surrounded by an outer rod to thereby eliminate damage from external causes such as wind shear.

It is another object of the present invention to provide a novel internal resistive element static discharger of improved design whereby installation and replacement procedures are considerably simplified.

It is another object of the present invention to provide an improved internal resistive-element static discharger wherein the discharge needle is concentrically surrounded by portions of the discharger construction.

It is another object of the present invention to provide an improved internal resistive element static discharger which 7 positions the needle point where the RF field is minimum to thereby assure that minimum noise is coupled into the aircraft antennae.

It is another object of the present invention to provide a novel internal resistive element discharger that is simple in construction, inexpensive in manufacture and trouble free when in use.

Other objects and a fuller understanding will be had by referring to the following description and claims of a preferred embodiment thereof, taken in conjunction with the accompanying drawings wherein like reference characters refer to similar partsthroughout the several views and in which:

FIG. 1 is a top plan view of an aircraft showing positions of the internal resistive element static dischargers when in use;

FIG. 2 is an exploded, perspective view of a short unit constructed in accordance with the present invention;

FIG. 3 is a partial, side elevational view of a short unit, partially broken away to show the assembled construction;

FIG. 4 is a side elevational view of a short unit adapter base, partially broken away to expose the interior construction;

FIG. 5 is a partial elevational view of a short unit adapter base, partially broken away to expose the operating elements;

FIG. 6 is a top plan view of the base of the long unit;

FIG. 7 is a side elevational view of the base of the long unit;

FIG. 8 is a side elevational view of the long unit in assembled relation, partially broken away to expose the internal construction;

FIG. 9 is a side elevational view of a long unit adapter base, partially broken away to show the internal construction;

FIG. 10 is an end elevational view thereof;

FIG. 11 is a bottom plan view thereof;

FIG. 12 is a side elevational view of the long unit adapter base applied to an existing base, parts of which are broken away to show the interior relation of the operating elements; and

FIG. 13 is a side elevational view of a short unit adapter base.

Although specific terms are used in the following description for the sake of clarity. these terms are intended to refer only to the particular structure of ourinvcntion selected for illustration in the drawings and are not intended to define or limit the scope of the invention.

Referring now to the drawings, two general types of static dischargers, embodying the present invention, are disclosed for illustration purposes. The short unit type 12 as exemplified by the embodiment of FIG. 2 is designed for and is suitable for installation on the tips or extremities of the wings and stabilizers. The long unit type 14, as exemplified by the embodiment illustrated in FIG. 8 is designedfor and is suitable for mounting upon the trailing edges of the wings and stabilizers. In accordance with well-known practices, it should be understood that the exact number and location of the internal resistive element dischargers constructed in accordance with the present invention is determined by the configuration and characteristics of the individual aircraft being so equipped and that the static dischargers herein disclosed are equally suitable for use with all types of aircraft.

Referring now to FIGS. 2 and 8, there are respectively illustrated the short internal resistive element discharge unit 12 and the long internal resistive element discharge unit 14. Each unit comprises an

inner rod

16, 16, an

outer tube

18, 18 and a relatively flat mounting

bracket

20, 20. However, for some aircraft a curved bottom is used to mount on the curved aircraft surface (not shown) whenever the surface of the aircraft so dictates. Each inner rod 16, 16' is externally respectively spirally grooved at 22, 22 to receive the resistive element 24, 24' therein about the periphery of the

inner rod

16, 16 and spiraling along the entire length thereof. The resistence element 24, 24' is sized to fit entirely within the respective grooves 22, 22' to thereby permit the

outer tube

18, 18 to slidingly overfit the inner rod 16, 16'-. A

discharge needle

26, 26 axially extends from the free end 28, 28' of the respective

inner rods

16, 16 for corona discharge purposes as hereinafter more fully set forth. A

stainless steel pin

30, 30 affixes to the

connected end

32, 32 of the respective

inner rods

16, 16 to provide positive electrical contact between each inner rod 16, 16' and the respective

conductive bases

20, 20. It should be noted that each respective needle 26, 26' and its associated

30, 30 electrically connects to the respective ends of the

resistance elements

24, 24 to thereby provide a complete electrically conductive circuit from the

pins

30, 30 to the discharge needles 26, 26. As illustrated, it should be noted that the

outer tubes

18, 18 completely cover and protect the

inner rods

16, 16, the steel pins 30, 30 and the spirally wound

resistance elements

24, 24. Additionally, each

outer tube

18, 18 extends beyond the

free end

28, 28 of the respective

inner rods

16, 16 and completely covers the extended portion of the discharge needles 26, 26 to thereby prevent physical damage and to eliminate any hazard to maintenance personnel.

As best seen in FIGS. 2 and 3, the short unit mounting bracket 20 is aerodynamically formed and fabricated with all surfaces rounded to prevent corona. The mounting bracket 20 terminates downwardly in a widened mounting base 34 which joins to the airframe of the aircraft 10 in'an electrically conductive connection by utilizing a conductive epoxy adhesive and/or by riveting in well-known manner. A mounting arm 36 which is provided with a rearwardly facing threaded socket 38 integrally rises from the base 34 to receive the threaded end 44 of the outer tube 18 as hereinaftermore fully set forth. It should be noted that the socket 38 bottoms at solid bracket metallic construction 40 whereby the inner rod affixed steel pin 30 butts against the bracket construction 40 at the bottom of the socket 38 to assure a firm r'netal-to-metal contact for electrical conductive purposes whenthe rod 18 is threaded into the socket 38.

Both the inner rod 16 and the outer tube 18 are fabricated from an insulating material such as nylon to thereby encourage static currents to follow the designed path through the bracket 12, pin 30 and thence through the resistance element 24 to the discharge needle 26. The discharge needle 26 is preferably fabricated of tungsten and terminates outwardly in sharp point 42 to thereby permit corona to occur at relatively low voltages. The pin 30 is preferably formed of stainless steel and securely affixes to the bottom of the inner rod 16 by utilizing an integral flange 46. The needle 26 and the pin 30 are electrically connected by a helix of a low conductivity material such as graphite-impregnated yarn or some other suitable conductor. We have found that a graphite-impregnated yarn having an electrical resistivity of approximate ly 50 million ohms per foot to be suitable for this purpose.

Optionally, we could employ a straight conductor. However, we prefer the helically wound embodiment inasmuch as the longer the length, the more reliable is the unit with regard to surface voltage breakdown and doping process.

Each outer tube l8, 18 is formed to a generally elongated, hollow cylindrical configuration to thereby overfit and protect the respective inner rods l6, l6 and the wound resistance elements 24, 24'. Each

outer tube

18, 18 terminates forwardly in an externally threaded

section

44, 44, which is sized to threadedly engage within the respective threaded

sockets

38, 38 of the bracket mounting arms 36, 36'. It should be noted that the

outer rods

18, 18 threadedly turn into the

respective sockets

38, 38 to thereby drive the inner rod affixed stainless steel pins 30, 30 forwardly into the socket until the respective pins bottom against the

metallic bracket construction

40, 40 to assure positive electrical contact. It is thus seen that no special wrenches, tools or other devices are required to make up a secure, rigid, electrically conductive joint between the

rods

16, 16; l8, l8 and the

brackets

20, 20.

As best seen in FIGS. 2 and 8, each outer tube 18, 18' is formed to a length sufficient to extend from the pin flange 46, 46' o the

point

42, 42 of each

respective needle

26, 26. In this manner, the free end of each outer tube l8, 18 provides a protective shield 48,. 48 which concentrically surrounds the

needle

26, 26 to thereby prevent injury to maintenance personnel from the sharp needle points 42,42.

When it is desired to utilize the short unit inner and

outer rods

16, 18, in conjunction with a mounting bracket 50 of existing design, a short unit adapter 52 may be employed. The short unit adapter 52 includes a mounting foot 54 which inserts within the existing groove of a mounting bracket 50 in tight relation and terminates rearwardly in shaped recess 56 to .receive the existing set screw 58 in the usual manner. A

threaded socket 38 integrally rises above the mounting foot 54 and provides a configuration similar to the mounting bracket socket 36 to therein threadedly receive the externally threaded section 44 of the outer tube 18. Thus, a relatively simply constructed, aerodynamically formed adapter can readily be provided to fit within an existing mounting bracket and be secured therein by the set screw 58. The threaded socket 38 threadedly receives the outer tube 18 which turns into the socket 38 and urges the pin 30 against the bottom of the socket for positive electrical contact. It is thus seen that a complete electrical circuit is assured by means of the threaded socket 38 which serves to electrically connect the existing base 50, the adapter 52 and the stainless steel pin 30. The existing set screw 58 turns into the shaped recess 56 and serves to positively electrically interconnect the existing mounting bracket 50 and the short unit adapter 52.

Referring now to FIGS 9, 10, 11 and 12, we show a long unit adapter 60 which is suitable for use with an existing mounting bracket 62. The adapter 60 is aerodynamically formed of a conductive metal such as aluminum and is provided with a bottom opening, interior recess 64 sized to overfit the central ridge 66 of the existing bracket 62. The recess 64 terminates forwardly in a locking face 68 which engages within the existing transverse slot 70 and securely affixes the forward portions of the long unit adapter 60 to the existing base 62. The adapter 60 terminates rearwardly in threaded socket 72 similar in construction to the threaded socket 38 with the exception that the socket extends completely through the adapter body and communicates with the interior recess 64. In this manner, the outer rod 18'. may be threadedly turned into the socket 72 thereby urging the stainless steel pin 30 forwardly through the recess communicating passage 73 until it contacts the central ridge 66 of the existing bracket 62. Positive electrical contact between the bracket 62 and the pin 30' may be assured by simply turning the outer tube 18' completely into the socket 72 until the pin 30' contacts the ridge 66 without utilizing special tools, wrenches or other mechanical aids. As best seen in FIG. 12, the pin 30' engages upon the rear of the central ridge 66 to securely lock the long unit adapter 60 to the existing bracket 62 by urging the locking face 68 against the transverse slot 70 and the pin 30' against the rear of the central ridge 66.

Thus it is seen that a static discharger construction has been provided wherein the

outer tube

18, 18 completely surrounds the conductive element 24, 24' to protect the conductor from damage due to wind shear at high speeds and to prevent electrical eroding of the element. The mechanical design of both the brackets 20, and the

adapters

52, 60 serve to facilitate the installation and replacement of short units 12, and long units 14 without the requirement for special tools. As best seen in FIGS. 2 and 8, the sharp tungsten point 42, 42' is concentrically surrounded by the outer rod end shield 48, 48' and thereby offers no hazard whatsoever to maintenance personnel.

In order to substantiate and prove the theoretical calculations relating to corona discharge through an internal resistive element discharger, a series of tests were conducted to determine insulation resistance of the unit, the effect of altitude, the effect of temperature cycling, the effect of vibration and the effect of shock and the electrical characteristics.

TEST NO. I

The insulation resistance of sample numbers 1 through 4 was measured using a megohm bridge at 100 V DC. The voltage was applied for a period of one minute and then measurements were taken as indicated in the following Table A. The actual result readings are set forth below at Table C.

TEST NO. II

Sample numbers I and 3 were placed in an altitude chamber wherein the pressure was reduced to a simulated altitude of 35,000 feet and maintained at that pressure for a period of 5 minutes. The samples were then returned to laboratory ambient pressure and tested for insulation resistance in accordance with the procedure of Test I. This procedure was repeated for simulated altitudes of 42,000 feet, 45,000 feet and 72,000 feet.

No physical, mechanical or electrical damage was noted as a result of this test procedure. The results of the test are set forth below in Table C.

TEST NO. III

Sample numbers 2 and 4 were tested for temperature cycling in accordance with Test Condition D, Method 102 of Mil-Std-202C utilizing the two-chamber method. One chamber was stabilized at -55C and theother chamber was stabilized at +85 C. Five continuous cycles a'were performed in accordance with the time schedule set forth below in Table B.

Measurements of insulation resistance were made following the five cycles at laboratory ambient conditions. No physical, mechanical or electrical damage was noted as a result of this test. The testrcsults are set forth below in Table C.

TABLE B Condition: Time,

rmn-

utes 55d; 0/3 .C 30 25;l: 10/5 .0 10 8 5;l; 3/0 .C 30 255: 10/5 .0 10

TEST NO. IV

Sample numbers

9, 10, 11 and 12 were subjected to vibration tests in accordance with Test Condition 102 Method 204A of Mil-Std.-202C with the specimens mounted at various angles to test the devices in various planes.

The test fixture containing the specimen dischargers was subjected to a simple harmonic motion having an amplitude of 0.06 inches BA. The frequency was varied uniformly between 10 and 55 Hz. and returned to 10 Hz. in 1 minute for 2 hours in each of three mutually perpendicular planes for a total of 6 hours. In a second test procedure, the test fixture containing the specimen'discharges was subjected to a simple harmonic motion having peak acceleration of 10g with the frequency being varied logarithmically from 55 Hz. to 2,000 Hz. The frequency range of 55 to 2,000 Hz. was transversed in 35 minutes in each of three mutually perpendicular planes. In addition, a dwell of 5 minutes was performed at each detected resonance point. A resistance check was then made after vibration in each of the planes of the first and second parts of this test to determine electrical damage.

As a result of the test procedure, no physical, mechanical or electrical damage was noted.

TEST NO. V.

TABLE C.-TEST CONDITION Sample Number Insulation resistance, Megohms Initial 120 40. 0 35K Feet" 123 42. Post, 42K Feet- 122 Altitude, 45K Feet- Post Temp Cycling 100 From the foregoing tests, it can be concluded that the internal resistive element static discharge unit with the internal resistance element 24 performed satisfactorily under all test conditions.

Although we have described our invention with a certain degree of particularity, it is understood that the present disclo- .sure has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention as hereinafterclaimed.

We claim: 1. In a static discharge unit for use in an aircraft, the combination of:

A. a mounting bracket adapted to be affixed to an aircraft in an electrically conductive connection:

1. said mounting bracket being fabricated of electrically conductive material;

2. said bracket terminating rearwardly in a threaded socket;

B. an inner rod of elongate cylindrical configuration having a connected end and a free end:

1. the said rod being removably insertable within the said socket:

2. the said inner rod being fabricated of nonconductive material;

C. an outer tube of insulation material overfitting the said inner rod and having two ends:

1. said outer tube terminating at one said end thereof in an externally threaded section, the said externally threaded section removably threadedly engaging within the said mounting bracket threaded socket;

2. said outer tube terminating at the other end thereof in a free end; and

D. electrically conductive means including means to force a portion ofthe electrically conductive means into contact with the mounting bracket by means of the tubeassociated with the said inner rod and extending throughout its length.

2. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said inner rod-connected end and a conductive discharge needle affixed at the said free end of the inner rod and extending outwardly therefrom.

3. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the connected end of the said inner rod and a discharge needle affixed at the said free end of the inner rod in axial alignment therewith.

4. The invention of claim 1 wherein the said electrically conductive means include a length of low conductivity, carhon-impregnated yarn.

5. The invention of claim 1 wherein the said electrically conductive means include a length of low conductivity, carbon-impregnated yarn, the said yarn being interposed between portions of the said inner rod and the said outer tube.

6. The invention of claim 1 wherein the said electrically conductive means include an axially positioned needle extending from the said free end of the inner rod and wherein portions of the said outer tube shield the said extended portions of the needle.

7. The invention of claim I wherein the said electrically conductive means include an axially positioned needle extending from the said free end of the inner rod and wherein pop tions of the said outer tube concentrically shield the said extended portions of the needle.

8. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said connected end ofthe inner rod and an axially aligned. conduc tive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically in terconnected by a length of carbon-impregnated yarn extending about the outer periphery of the said inner rod.

9. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said connected end of the inner rod and an axially aligned. conductive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically interconnected by a length of carbon-impregnated yarn extending through the said inner rod.

10. The invention of claim 1 wherein the said free end of the outer tube extends outwardly beyond the free end of the said inner rod.

11. The invention of claim 1 wherein the said electrically conductive means include a conductive p in affixed at the said connected end of the Inner rod and an axially aligned conductive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically interconnected by a length of carbon-impregnated yarn extending about the outer periphery of the said inner rod, the said inner rod being helically grooved to receive the said yarn therein.

12. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said connected end of the inner rod and an axially aligned, conduc' tive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically interconnected by a length of carbonimpregnated yarn extending about the outer periphery of the said inner rod, the said inner rod being helically grooved to receive the said yarn therein, the said pin being provided with the said means to force which includes a radially extending flange at the connected end of the said inner rod, the bottom of the said outer tube contacting the said flange to thereby urge the pin into electrical contact with the bottom of the said bracket threaded socket when the externally threaded section of the outer tube is turned into the socket.

Claims

1. In a static discharge unit for use in an aircraft, the combination of : A. a mounting bracket adapted to be affixed to an aircraft in an electrically conductive connection: 1. said mounting bracket being fabricated of electrically conductive material; 2. said bracket terminating rearwardly in a threaded socket; B. an inner rod of elongate cylindrical configuration having a connected end and a free end: 1. the said rod being removably insertable within the said socket; 2. the said inner rod being fabricated of nonconductive material; C. an outer tube of insulation material overfitting the said inner rod and having two ends: 1. said outer tube terminating at one said end thereof in an externally threaded section, the said externally threaded section removably threadedly engaging within the said mounting bracket threaded socket; 2. said outer tube terminating at the other end thereof in a free end; and D. electrically conductive means including means to force a portion of the electrically conductive means into contact with the mounting bracket by means of the tube associated with the said inner rod and extending throughout its length.

2. said bracket terminating rearwardly in a threaded socket; B. an inner rod of elongate cylindrical configuration having a connected end and a free end:

2. the said inner rod being fabricated of nonconductive material; C. an outer tube of insulation material overfitting the said inner rod and having two ends:

2. said outer tube terminating at the other end thereof in a free end; and D. electrically conductive means including means to force a portion of the electrically conductive means into contact with the mounting bracket by means of the tube associated with the said inner rod and extending throughout its length.

4. The invention of claim 1 wherein the said electrically conductive means include a length of low conductivity, carbon-impregnated yarn.

7. The invention of claim 1 wherein the said electrically conductive means include an axially positioned needle extending from the said free end of the inner rod and wherein portions of the said outer tube concentrically shield the said extended portions of the needle.

8. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said connected end of the inner rod and an axially aligned, conductive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically interconnected by a length of carbon-impregnated yarn extending about the outer periphery of the said inner rod.

9. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said connected end of the inner rod and an axially aligned, conductive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically interconnected by a length of carbon-impregnated yarn extending through the said inner rod.

11. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said connected end of the inner rod and an axially aligned conductive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically interconnected by a length of carbon-impregnated yarn extending about the outer periphery of the said inner rod, the said inner rod being helically grooved to receive the said yarn therein.

12. The invention of claim 1 wherein the said electrically conductive means include a conductive pin affixed at the said connected end of the inner rod and an axially aligned, conductive discharge needle extending from the said free end of the inner rod, the said pin and the said needle being electrically interconnected by a length of carbon-impregnated yarn extending about the outer periphery of the said inner rod, the said inner rod being helically grooved to receive the said yarn therein, the said pin being provided with the said means to force which includes a radially extending flange at the connected end of the said inner rod, the bottom of the said outer tube contacting the said flange to thereby urge the pin into electrical contact with the bottom of the said bracket threaded socket when the externally threaded section of the outer tube is turned into the socket.