US3320524A - Apparatus including electrode means for determining electrical condition of aerial booms for energized line working - Google Patents

Description

y 6, 1967 c. J. MILLER, JR 3,320,524

APPARATUS INCLUDING ELECTRODE MEANS FOR DETERMINING ELECTRICAL CONDITION OF AERIAL BOOMS FOR ENERGIZED LINE WORKING Filed April 29, 1965 2 Sheets-Sheet l INVENTOR. CHARLES J.MILLER JR. BY

ATTORNEY May 16,1967 c. J. MILLER, JR 3,320,524

APPARATUS INCLUDING ELECTRODE MEANS FOR DETERMINING ELECTRICAL CONDITION OF AERIAL BOOMS FOR ENERGIZED LINE WORKING Filed April 29, 1963 2 Sheets-Shem 2 wllltllallrlqlllllllall0A 5' 88 Amp.

FIG. 7 Fl 3 INVENTOR CHARLES J. MILLER JR.

ATTORNEY United States Patent 3,320,524 APPARATUS INCLUDING ELECTRODE MEANS FOR DETERMINING ELECTRICAL CONDI- TION 0F AERIAL BOOMS FOR ENERGIZED LINE WORKING Charles J. Miller, Jr., Wadsworth, Ohio, assignor to The Ohio Brass Company, Mansfield, Ohio, a corporation of New Jersey Filed Apr. 29, 1963, Ser. No. 276,388 14 Claims. (Cl. 324-54) This invention relates to aerial lifts for energized line working and to insulated booms therefor, and, more specifically, to methods of and means for determining the electrical condition of the boom.

This application is a continuation-in-part of application Ser. No. 133,200, filed Aug. 22, 1961, now Patent No. 3,159,240, by Charles J. Miller, J r.

A principal object of the invention is to ascertain the electrical condition of insulated booms of aerial lifts.

Another object of the invention is to provide useful apparatus for determining the condition of insulating devices and apparatus for working energized lines while the equipment is in the field.

Another object of the invention is to determine the presence of contaminating materials and the like on the surfaces of insulating structural members.

According to one technique of working energized electric power lines, an aerial lift is utilized to carry the workman from the earth to a working position adjacent the power line. The aerial lift is constructed with insulating means for electrically separating the working platform from the earth. According to a preferred arrangement, the aerial lift is constituted with mutually supported booms and one of the booms is constructed of a fiberglass reinforced polymeric resin along a substantial portion of its length to provide the insulation required at high line voltages and at the same time the requisite mechanical strength for carrying the workman. Various arrangements for controlling the electrical stresses on such insulating booms are described in the principal application above referred to.

I have found that the insulating quality of the boom may be adversely affected because of the accumulation of various contaminating media on the surface of the fiberglass member because of the condensation of moisture on the interior or exterior surfaces of the member or because of specific forms of deterioration of the surface structure due to tracking and the like. Under such circumstances, there is danger of cumulative breakdown of the insulation under working conditions and it is essential that the condition be detected so as to avoid the hazards to workmen, equipment, and to the transmission line and associated power equipment. The present invention relates to a method of and means for determining the insulating quality of the boom by measuring the current flow along the insulating member when the lift is energized at line voltages. More specifically, there is provided an arrangement of electrodes for intercepting currents along the insulating member together with a suitable arrangement of a meter for measuring the current flow to or from an electrode at one end of the member. The magnitude of current flow provides an indication of the conductivity and, accordingly, of the insulating quality of the boom.

More specifically, the electrode above referred to comprises a conductive ring extending peripherally about the insulating member of the boom above referred to in contact with the surface of the member. The ring is connected to ground through a sensitive current meter, suitably a microammeter of known design, which pro- 3,320,524 Patented May 16, 1967 vides a measure of the quality of the insulation in the magnitude of current flow for a given line voltage. The current meter is connected to the electrode by means of a coaxial cable with the interior conductor of the cable connected to the ring. With this arrangement, the entire connection between the ring and the meter is shielded and the random effects, which would otherwise result from capacity between the conducting lead to the meter and ground, are avoided.

One embodiment of the invention is useful in aerial lifts utilizing hydraulic or pneumatic means for transmitting power or control movements to the working platform of the aerial lift and is constituted by a metallic fitting in a fluid line otherwise constituted of electrically insulating material. With this arrangement, the condition of the line and of the hydraulic or pneumatic fluid may be monitored along with the insulating condition of the boom.

The invention, together with further objects, features, and advantages thereof, will be more fully understood from the following detailed specification and claims, taken in connection with the appended drawings.

In the drawings:

FIG. 1 illustrates the invention in connection with an aerial lift and boom of the type to which the invention pertains;

FIG. 2 shows a portion of the insulated boom of the aerial lift of FIG. 1 and illustrates one embodiment of the apparatus of the invention;

FIG. 3 illustrates the coaxial cable and indicating meter of the apparatus of FIG. 1;

FIG. 4 illustrates a second embodiment of the invention, useful in connection with the apparatus of FIG. 1;

FIG. 5 is an enlarged view of a portion of the apparatus of FIG. 4;

FIG. 6 illustrates the practice of the invention in connection with hydraulic or pneumatic lines; and

FIG. 7 illustrates an alternate embodiment of the invention in which the indicating apparatus provides an alarm signal.

In FIG. 1 the aerial lift 10 includes a mechanical arm 11 constituted by two booms 12 and 13, a pedestal 14, and a personnel carrier 15. The lower boom 12 is carried upon the pedestal 14 and supports the boom 13 which is connected at its inner end to the boom 12 by means of a pivotal joint or elbow 16. The personnel carrier 15 is supported on a head 17 at the outer end of the boom 13.

The pedestal 14 is carried upon a rotatable mount or turntable on a truck 18 so that the personnel carrier 15 may travel to any location above and about the truck by swinging movement in the horizontal plane and by pivotal movements of the two booms in the vertical plane. Suitable hydraulic apparatus is provided for turning the pedestal 14 and hydraulic cylinders (not shown) are provided for elevating the boom 12 with respect to the pedestal 14 and for extending and retracting the boom 13 with respect to the boom 12. The personnel carrier 15 is maintained in the same position with respect to the earth, whatever the positions of the booms 12 and 13, by means of a leveling system including a leveling cable 19. Control of the movement of the arm 11 is accomplished for the personnel carrier 15 by means of a system of cables which extend through the interior of the booms 12 and 13 after the manner of the leveling cable 19.

The boom 13 is constructed of insulating material along a portion of its length to insulate the personnel carrier 15 from the boom 12, the pedestal 14 and from ground. Thus, as shown in FIG. 1 and FIG. 2, the boom 13 is constituted by a hollow rectangular beam 20 of structural insulating material such as glass fiber reinforced resin (otherwise known as fiberglass). The beam is carried at its inner end by a metal end beam 21 having the same interior dimensions as the exterior of the beam 20 so that the insulating beam fits closely within the metal end beam. The beam 20 is suitably secured to the beam 21 by screw fasteners, or the two beams are bonded or welded together along the adjoining surfaces. The beam 21 is carried at the outer end of the boom 12 on the elbow 16 which is constituted by two arms or plates 22 and 23 and a shaft 24.

The head 17 is supported at the extremity of the beam 20 by a bracket 25 comprising a flat metal strap 26 which extends circumferentially about the interior of the beam, and an arm 27 which is secured to the strap 26 and extends outwardly beyond the end of the beam 20. The strap 26 is suitably attached to the wall of the beam 20 by bolts, rivets, or bonding material, and supports a main shaft 28 which carries the bucket 15 and an idler shaft 29 for the cable 19.

The cable 19 comprises an endless loop and extends about a drive pulley 30 carried by the shaft 28, a hold pulley 31 carried on a shaft 32 on the pedestal 14, a reversing pulley 33 on the shaft 24, and over idler pulleys 34, 35 and 36 associated with the respective main pulleys. The cable 19 is divided into two parts by insulating spacers 37 and 38 which comprise part of the cable loop so that the cable does not connect the boom head to the grounded metal end beam 21 and the lower boom 12 of the lift. The insulating spacers 37 and 38 are formed as rods of structural insulating material, insulated rope, strap, or the like, to provide the requisite mechanical and insulating strength, all as described in my prior application Ser. No. 133,200 idem.

The personnel carrier 15 incorporates a conductive metal screen 39 which extends about the interior of the bucket and functions to energize the body of the lineman to line voltage and as a shield means for the lineman during contact with the line conductor. The metal screen 39 is connected to the high voltage line by means of a suitable flexible connector 40 which is electrically connected to the screen 39 and to the remaining metal parts of the head 17 including the bracket 25, and is adapted to be clamped on the line conductor to maintain electroconductive contact with the conductor. The method of energized line working is described generally in United States patent application Ser. No. 105,456, filed Apr. 25, 1961, by Harold L. Rorden.

According to application Ser. No. 133,200, the insulating beam 20 is provided with means for distributing the electrical stresses longitudinally and radially of the walls of the beam. As shown in FIG. 2, such means comprises, in the apparatus herein shown, two control rings 41 and 42 which extend about the interior and exterior surfaces, respectively, of the beam 20, a suitable distance from the strap 26 of the head bracket 25. The ring 41 is carried on metal bars 43 and 44 which extend between the ring 41 and the strap 26 and electrically connect the ring to the strap 26. The ring 42 is attached to the ring 41 by rivets 44 which extend through the wall of the beam 20 so that both rings are maintained at line potential with the head 17 and the shield screen 39. Two metal rings 45 and 46 extend about the interior and exterior surfaces of the beam 20 and are attached to each other by rivets 47 extending between the rings and through the wall of the beam, and spaced from the extremity of the metal end beam 21.

The rings 41 and 42 are more closely spaced from the metal end beam 21 than the nearest approach of the metal ferrules at the associated extremity of the spacers 37 and 38, and the rings 45 and 46 are more closely adjacent the head 17 than the nearest approach of the metal ferrules at the associated extremity of the spacers 37 and 38, also as set forth in the above referenced application.

The invention relates to the arrangement of the control rings 45 and 46 about the interior and exterior surfaces of the beam 20 in spaced relation to the end of the metal end beam 21 and to the functional cooperation of the rings 45 and 46 with an external circuit including a coaxial cable 48 and a meter 49, such that the rings intercept electric charges moving along the surfaces of the beam 20 from the rings 41 and 42 toward the beam 21. It will be evident that the metal end beam 21 is at ground potential due to conductive connection through the elbow 16, the boom 12, the truck 18, and the earth connection 50, or otherwise due to the large capacitance between the truck 18 and earth. Accordingly, the rings 45 and 46 are also at ground potential, or substantially at ground potential, because of the low impedance connection between the rings and the metal beam 21 afforded by the meter 49. Thus, application of a high voltage, such as a line voltage, to the screen 39, the head 17, and the rings 41 and 42 results in a difference of potential between the end of the beam 21 and the rings 45 and 46 and the rings 41 and 42. This difference of potential or gradient, along the insulating beam 20, will produce a current flow or leakage current along the beam whenever there are conductive materials present on the surface or surfaces of the beam 20. Because of the highline voltage, small conductivities may result in substantial and excessive currents along the insulating beam.

Current flow along the surfaces of the insulating beam 20 may result from the presence of contaminant materials 011 the beam or result from conductive material imbedded in the body of the insulating material. The latter condition may result from partial electrical failure of the material, tracking and the like, or from the ionization of the contaminant materials in the presence of moisture on the surface of the beam. It is to be understood that because of the high operating voltages, as set forth above, a significant leakage current may be produced even though the contaminant materials are present in extremely low concentrations.

The rings 45 and 46 function as electrodes in intercepting charged particles moving along the surfaces of the insulating beam 20, and the meter 49 measures that current flow in the circuit from the rings 45 and 46 to the grounded beam 21. As hereinafter set forth, alternate arrangements may be utilized to intercept only currents along the exterior surface or the interior surface of the beam 20. It will be understood that the specific structural arrangement of the rings 41 and 42 is related to the stress control function described in application Ser. No. 133,200 idem, and that for the purposes of the present invention alternate arrangements of conductive parts or electrodes may be utilized. For example, in the absence of the rings 41 and 42 the bracket 26 would perform the same function inasmuch as the head 17 is energized to line voltage.

When the insulating beam is clean and dry, the meter 49 provides a measure of the charging current due to capacitance between the two sets of rings 41 and 42 and 45 and 46. This charging current provides a reference current so that measured currents may be compared with the reference current to determine that the electrical condition of the beam is other than normal. The magnitude of the current provides, of course, a measure of the abnormal condition.

In the apparatus of FIG. 1, the coaxial cable 48 extends along the interior of the beam 13, is looped about the extremities of the metal end beam 21 and the beam 12 at the elbow 16, and extends along the interior of the beam 12 to a coaxial connector 50. The cable extends from the connector 50 to the enclosure 51 of the meter 49. The central conductor 52 of the coaxial cable 48 is connected to the ring 45 and extends through the sheath 53 of the cable to the meter 49. The sheath 53 encloses the conductor 52 throughout the length of the cable 48 and is connected to the metal box 51 which supports and encloses the meter 49. The sheath 53 and box 51 are grounded to the metal end beam 21, to the body of the truck 18, and to the earth through a connection 54 and shield the current from the electric fields about the conductor, head 17, etc. With this arrangement, the effects which would otherwise exist because of capacitance between the lead conductor extending from the ring 45 and/or the ring 46 to the meter 49, due to capacity between the lead and the earth, are eliminated. Inasmuch as the current flow along the insulating section 20 is extremely small, determination of the condition of the beam would be effectively prevented in the absence of the sheath 53.

That portion of the insulating beam 20 between the rings 45 and 46 and the extremity of the metal end beam 21 must be kept free of contaminants or conducting media of any kind so that the meter circuit is not shunted by a conductive path. This may be accomplished by coating the surfaces of the beam with silicon oil or grease.

In the apparatus of FIG. 4, the invention is embodied in an arrangement wherein the indicating meter is positioned adjacent the end beam so that there is no requirement for a cable to connect the indicating meter to the electrode rings. In the apparatus of FIG. 4, a metal ring 55 extends about the interior surface of an insulating beam 56, corresponding to the beam 20 in FIG. 2, and within a metal end beam 57, corresponding to the metal end beam 21 of FIG. 2, and adjacent the extremity 58 of the beam 57. The ring 55 is secured to the beam 56 and the beam 57 by means of insulating rivets 59 which extend through the insulating beam 56. The beam 57 is provided with a receptacle 60 adjacent the extremity 58 and radially outward from the ring 55. A metal pin 61 extends from the ring 55 through the Wall of the beam 56 and into the open interior of the receptacle 60 to constitute an electrical contact for the ring 55. A meter 62 is connected to the ring 55 by means of a plug connector 63 which is engaged wit-h the receptacle 60 and is connected to the meter 62 by means of a lead 64. The meter is connected to the beam 57 by means of a lead 65 to complete the circuit between the ring 55 and the beam 57.

A control ring 66, corresponding to the ring 41 of FIG. 2, extends about the interior surface of the beam 56 at the upper end of the beam. The ring 55 and the ring 66 confine the electrical stresses to the interior surface of the beam 56. The exterior surface of the insulating beam 56 is cleaned and dried prior to use of the lift so that electrical leakage would be of importance primarily along the interior surface of the beam 56. With this arrangement, the magnitude of current flow indicated by the meter 62 results from charge and particle movement along the interior surface of the beam 56 between the electrode rings 55 and 66 and through the circuit which includes the meter 62 and the beam 57.

The receptacle 60 is illustrated in more detail in FIG. 5, wherein a shorting plug 67 is substituted for the connector plug 63. The plug 67 includes a spring clip 68 which contacts the exterior end of the pin 61 and connects the ring 55 to the metal beam 57. The plug 67 is inserted in the receptacle during periods when the current measurement otherwise provided by the meter 56 is not required, as for working in elevated locations or where there is continual movement of the booms. In the absence of the plug 67, the ring 55 floats within the beam 57 and electrical stresses are generated in the wall of the insulating beam 56 because of the radial opposition of the ring 66 and the end 58 of the metal end beam 57.

The apparatus shown in FIG. 6 illustrates the practice of the invention in connection with aerial lift apparatus in which fluid means are utilized for the transmission of power or control movements between the pedestal 14 and the head 17. Thus, a boom 70 comprises an insulating beam 71 carried by a metal end beam 72, and a line 73 for transmitting hydraulic or pneumatic fluid to hydraulic or pneumatic leveling and/ or control apparatus in the 'head 17. The line 73 includes a conduit 74 for connection to a hydraulic apparatus at the pedestal 14, a conduit 75 for connection to an apparatus at the head 17, and a spacer 76 for electrically separating the conduits 74 and 75 along the insulating beam 71. The boom 70 embodies control rings 77 and 78 which function in the same way as the rings previously described. The ring 77 may be connected to a current indicating meter for indicating the condition of the insulating beam 71.

The insulating spacer 76 comprises a conduit of electrically insulating material, preferably Teflon (trade name), i.e., tetrafluoroethylene, and connected to metal conduits and fittings 79 and 80 at the inner and outer ends of the boom. The fitting 79 comprises a conventional metal fitting which is supported from the beam 71 and the beam 72 by means such as a block 82 of electrically insulating material to separate the fitting 79 from the metal beam 72 and the associated parts. The fitting is also insulated from the conduit 74 if that conduit is made of metal or other conducting material. The fitting 79 is connected to a meter, such as the meter 49, by a coaxial cable 83 which has the center conductor 84 connected to the fitting 82 and the sheath 85 connected to the metal beam 72. The coaxial cable extends through the boom 70 and the associated boom of the lift, such as the boom 12 in FIG. 1, to the meter 49, and has the conductor 84 connected to the meter and the sheath 85 connected to the meter and the enclosure 51. In this arrangement, the fitting 79 functions as an electrode or collector for conductive particles or charges moving along the spacer 76 from the head end of the boom and indicates the insulating quality of the spacer 76 by reason of indicating the magnitude of current flow along the spacer conduit. As indicated in dashed outline at 86, the fitting 82 and the electrode ring 79 may be connected in parallel by a conductor 86. With this arrangement, the meter 49 reads the total leakage along the shunt paths constituted by the beam 71 and the spacer 76.

Typical dielectric minimal currents along the boom elements, for clean, dry elements with 266 kv. line-toground voltage applied to the head 17 and screen 39, are as follows: current along beam 20 measured at rings 45 and 46200 microamperes; current along line 76-5 microamperes (same for pneumatic lines). These currents increase rapidly where surface contaiminants are present and it is desirable to have a meter 49 with switched shunts for progressively increasing the sensitivity of the meter when first determining the condition of the boom.

Referring now to FIG. 7, there is shown an alternate embodiment of the indicating apparatus of the invention. In that arrangement, the electrode ring, e.g., the ring 45 in FIG. 2, is connected to ground through a resistor 88. The resistor 88 functions as a coupling impedance for an amplifier 89 which has the input circuit connected across the resistor 88 and the output connected to a relay 90. The contacts 91 of the relay are connected in series with a loudspeaker 92 and a source 93 of alternating voltage. With this arrangement, a current of greater than a value predetermined by the design of the apparatus operates the relay and sounds an audible alarm. Continuous monitoring is necessary or desirable in some circumstances, and the arrangement of FIG. 7 enables the lineman to devote his full attention to the work at hand.

It is to be understood that, while the invention may be practiced by connecting the current meter 49 in series between the ring 42 and the head 17, or in series with the connector 40, such arrangements are less desirable than those heretofore described because of the large component of current due to the capacitance between the various members and ground. However, such arrangements may be utilized where it is desired to indicate a large change in the steady current due to the occurrence of a fault or other event. The circuit in FIG. 7 is useful for signalling changes from a steady current since the relay 90 may be adjusted so that the contacts 91 are closed only upon a predetermined change from the steady current. In such arrangements, the resistor 88 is connected in series between the rings 41 and 42 and the strap 26, or otherwise between the rings 41 and 42 and the head 17.

It is to be understood that the foregoing description is not intended to restrict the scope of the invention and that various rearrangements of the par-ts and modifications of the design may be resorted to. The following claims are directed to combinations of elements which embody the invention or inventions of this application.

I claim:

1. In an aerial lift, boom means comprising an elongate insulating member of electrical insulating material, means at one end thereof pivotally supporting the insulating member from the said one end and electrically grounded conductive means contacting the insulating member at the said one end thereof, that improvement which comprises electrically conductive electrode means extending substantially about the insulating member adjacent the said one end thereof and spaced from the grounded means for intercepting electric current along the surface of the member, and current responsive indicating means connected between the electrode means and the grounded means for measuring dielectric and leakage current along the insulating member.

2. Apparatus in accordance with claim 1, in which there is a conductive electrode means at the remaining end of the elongate member and means for energizing the last named electrode means with high voltage.

3. Apparatus in accordance with claim 1, in which the current responsive indicating means comprises a meter, the meter is positioned remotely from the boom means'and a coaxial cable connects the meter to the electrode means.

4. In an aerial lift, boom means comprising an elongate insulating member of structural electrical insulating material, means pivotally supporting the boom means at one end thereof including an elongate metal member, means for measuring dielectric and leakage current along the insulating member comprising a metal ring extending about the insulating member in contact with a surface of the insulating member substantially about the peripheral extent thereof and spaced from the metal member and a current meter electrically connected to the ring and to the metal member.

5. Apparatus in accordance with claim 4, in which the insulating member has interior and exterior longitudinal surfaces, the said metal ring extends about the elongate member in contact with the one surface thereof, a second ring extends about the insulating member in contact with the remaining surface thereof, and the two rings are electrically connected for intercepting substantially all of the leakage currents along the member.

6. Apparatus in accordance with claim 5, in which there is a conductive electrode means at the remaining end of the insulating member and means for energizing the conductive means with high voltage.

7. Apparatus in accordance with claim 4, in which the meter is connected to the ring and the metal member by means comprising a coaxial cable extending along the boom means, the central conductor of the cable being connected to the ring and the shield conductor thereof being connected to the metal member and being effectively grounded.

8. Apparatus in accordance with claim '7, in which the boom means is supported on a base and the meter is located adjacent the base and the coaxial cable extends thereto.

9. In an aerial lift for energized line working, a personnel carrier having conductive shielding and energizing means, extensible elongate members carrying the personnel carrier at the upper end of an upper one of the elongate members with the lower one of the elongate members supporting at its upper end the lower end of the upper elongate member, an insulating section of structural electrical insulating material in the upper one of the elongate members for electrically separating the personnel carrier from ground, conductive control rings spaced apart along the said insulating section with an upper one of the control rings electrically connected to the said conductive means on the personnel carrier, a conductor extending from the lower one of the said control rings along the elongate members in electrically insulated relation thereto, a current meter, and means connecting the meter in a series circuit from the conductor to ground for determining surface leakage of the insulating section from a position at the lower end of the lower elongate member when the aerial lift is extended to a working position and the conductive means are connected to an energized line, the control rings constituting a voltage grading and stress control means for the said insulating section.

10. The invention in accordance with claim 9, in which the upper elongate member comprises a hollow metal beam at the lower end thereof supporting an insulating section in the form of a hollow beam of insulating material, and the lower control ring comprises metal parts extending along the exterior and interior surfaces of the insulating beam in spaced relation to the metal beam.

11. Apparatus in accordance with claim 9, in which the said conductor is disposed within a conductive sheath with insulation between the conductor and the sheath, and the meter is grounded to the sheath.

12. In an aerial lift, boom means comprising an elongate insulating member of structural insulating material carried at a first end thereof by an elongate metal member, means at a second end of the insulating member having a metal member adapted to be energized at high voltages, means comprising a metal ring extending substantially about the insulating member in contact with the surface thereof and spaced from one of the two named metal members toward the remaining end thereof to intercept current flow along the insulating member, and means con-nected between the ring and the associated metal member and responsive to current flow between the ring and the metal member for indicating current flow along the insulating member.

13. Apparatus in accordance with claim 12, in which the indicating means is a meter.

14. Apparatus in accordance with claim 12, in which the indicating means is an alarm signalling device.

References Cited by the Examiner UNITED STATES PATENTS 1,905,412 4/1933 Kasson 324-54 2,307,499 1/1943 Frakes 324-54 2,422,644 6/1947 Martenet 324-54 2,889,395 6/1959 Frakes 324-54 X 3,043,394 7/1962 Hall 182-2 FOREIGN PATENTS 28,059 9/ 1904 Great Britain.

WALTER L. CARLSON, Primary Examiner.

G. R. STRECKER, Assistant Examiner.

Claims (1)

1. IN AN AERIAL LIFT, BOOM MEANS COMPRISING AN ELONGATE INSULATING MEMBER OF ELECTRICAL INSULATING MATERIAL, MEANS AT ONE END THEREOF PIVOTALLY SUPPORTING THE INSULATING MEMBER FROM THE SAID ONE END AND ELECTRICALLY GROUNDED CONDUCTIVE MEANS CONTACTING THE INSULATING MEMBER AT THE SAID ONE END THEREOF, THAT IMPROVEMENT WHICH COMPRISES ELECTRICALLY CONDUCTIVE ELECTRODE MEANS EXTENDING SUBSTANTIALLY ABOUT THE INSULATING MEMBER ADJACENT THE SAID ONE END THEREOF AND SPACED FROM THE GROUNDED MEANS FOR INTERCEPTING ELECTRIC CURRENT ALONG THE SURFACE OF THE MEMBER, AND CURRENT RESPONSIVE INDICATING MEANS CON-

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