US3838242A - Surgical instrument employing electrically neutral, d.c. induced cold plasma - Google Patents

Abstract

A surgical instrument (plasma scalpel) generates an electric field plasma in a carrier gas for cutting and/or cauterizing tissue. The instrument includes a tubular conductor for carrying both current and gas to an anode. The gas flows from the tubular conductor, around the surface of an anode, and thence through an annular cathode. The electric field between the anode and cathode excites the gas to a plasma state, and the excited gas then flows in a directed stream through the cathode and out the end of the instrument. The tubular conductor is surrounded by a cooling jacket which concomitantly serves as an electrical conductor to the cathode. A dielectric sheath may be provided over the cooling jacket. This sheath may be overlaid with a conductive skin connected to earth ground to prevent an excessive electric charge from accumulating thereon. The conductive skin is preferably grounded through a circuit breaking device to cut off all power to the instrument if the potential of the skin differs greatly from the potential of the cathode.

Description

[ Sept. 24, 1974 5/1972 Sunnen et 219/121 P SURGICAL INSTRUMENT EMPLOYING ELECTRICALLY NEUTRAL, D.C. INDUCED COLD PLASMA Primary ExamirierBruce A. Reynolds [75] Inventor: Robert G. Goucher, Salt Lake City, p "l or Firm David VTraSk; Richard Utah BOJfiIlOWSkl ABSTRACT Assignee: Hogle-Kearns International, Salt Lake City, Utah l4 l n m n 3 3 1 1 B l 3% PH %6 m fl .25 9 1 2 3 12 mm; P 5M 2 l Wm 7 ""0 w m m m3 9 ""1 4 u A. l "m3 MH a x m "m6 n 00 M 2 m man u u n& 0 u n N L 0 Au d k S .Ld .1 I II F A U IF ll l .1] 2 l 2 8 2 2 5 55 l. .l. .l. [l

[56] References Cited ment. The tubular conductor is surrounded by a cool- NITED STATES PATENTS U ing jacket which concomitantly serves as an electrical conductor to the cathode. A dielectric sheath may be provided over the cooling jacket. This sheath may be overlaid with a conductive skin connected to earth Meredith 219/75 ground to prevent an excessive electric charge from accumulating thereon. The conductive skin is preferably grounded through a circuit breaking device to cut PPPSSX lllng 22296 llll 999 ll]. 2 222 2 Borneman..... off all power to the instrument if the potential of the skin differs greatly from the potential of the cathode.

219/121 219/121 P Dobbs et al. 11 Claims, 6 Drawing Figures Kemeny et a1 Wciman "C S 2m g h ammab GFGDG 37800 66677 99999 2 070 1131 06825 5208 aaaszaz PATENTED 35 241914 SIEHIWZ SURGICAL INSTRUMENT EMPLOYING ELECTRICALLY NEUTRAL, D.C. INDUCED COLD PLASMA RELATED APPLICATIONS Commonly assigned, copending application Ser. No. 79,840, filed Oct. l2, 1970, now abandoned, discloses and claims methods and apparatus for plasma surgery. According to that application, a cold plasma is established and attenuated to a small cross section. This plasma is applied to tissue to produce an incision. Commonly assigned, copending application Ser. No. 253,494, filed May 15, 1972, claims the additional discovery that an electrically neutral, D.C.-induced cold plasma is especially useful for surgical applications, and discloses surgical apparatus suitable for producing such a plasma. The present application discloses such apparatus more fully and claims the same.

BACKGROUND OF THE INVENTION 1. Field This invention relates generally to plasma scalpel devices which emit a flow of plasma for cutting or cauterizing tissue. It is specifically directed to such a device particularly useful for producing electrically neutral, D.C.-induced plasmas.

2. State of the Art The use of D.C. arc plasma for surgery is suggested by U.S. Pat. No. 3,434,476, which discloses and claims apparatus intended for use as a surgical scalpel. The apparatus thus disclosed is apparently incapable of producing plasmas which are not substantially at thermal equilibrium. This instrument is reported to be unsafe for actual clinical use because it tends to char tissue. It is recognized in the disclosure of the aforementioned application Ser. No. 79,840 that plasmas of metastable noble gas are preferred for surgical applications. The apparatus there disclosed produces rf-induced plasmas which, although cold, are not electrically neutral.

As used hereinafter, the term cold plasma includes all plasmas which evidence a low wall-heating effect, whether or not such plasmas exhibit the appearance and/or other physical characteristics normally associated with the specific cold plasmas and glow discharge phenomena heretofore recognized in the art. The term refers generally to plasmas in substantial thermal nonequilibrium; i.e., those which exhibit a much lower tactile or gas temperature than the temperature equivalent of the energy level of the free electrons in the plasma.

SUMMARY OF THE INVENTION The present invention provides a surgical instrument capable of producing electrically neutral, D.C.-induced cold plasmas in a form suitable for surgical applications. The instrument includes, inter alia, an elongate body in which is disposed a conduit made of electrically conductive material. The conduit concurrently serves to carry an electric current and a stream of surgical gas to an anode near the forward end of the instrument. The carrier gas passes around the anode, through a constrictor nozzle and then through an annular cathode which is fixed coaxially with respect to the anode to the forward end of the instrument. The electric field established between the anode and cathode generates plasma in the carrier gas which leaves the annular cathode in a directed stream. A cooling jacket surrounds the conduit and serves concomitantly as an electrical conductor to the cathode. Means are provided to convey cooling liquid into and from the jacket. In one embodiment of the invention, a sheath which includes a dielectric lining overlaid by a metallic skin is provided over the cooling jacket. A safety circuit may be connected to the skin to prevent the development of excessive potential differences between the cathode and the outer skin.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention may be better understood by referring to the following description and appended drawings which are offered by way of illustration and not in limitation of the invention, the scope of which is defined by the appended claims and equivalents. Specific reference herein to details of the illustrated embodiment is thus not intended to restrict the scope of the claims, which themselves recite those features regarded as essential to the invention.

In the drawings:

FIG. 1 is a pictorial view, partially cut away and in section, of a preferred embodiment of the plasma scalpels of the invention;

FIG. 2 is a pictorial view of the scalpel of FIG. 1 slightly modified;

FIG. 3 is a side view in section of the scalpel shown in FIG. 2;

FIG. 4 is an exploded detail, in section, of structure at the forward end of the scalpel of FIGS. 1, 2 and 3;

FIG. 5 is a schematic diagram of a safety circuit for use with the scalpel of FIGS. 2 and 3; and

FIG. 6 is a detail in cross section of a connector for use with the illustrated instruments.

DESCRIPTION OF THE PREFERRED EMBODIMENT The illustrated instrument 10 has an elongate body comprising a cooling jacket generally designated 12 in which water or other cooling liquid is circulated. The illustrated jacket 12 is a tube-within-a-tube arrangement having a cylindrical inner wall 14 spaced coaxially from a cylindrical outer wall 16 by respective, radially extending end walls 17 and 18. Together, the walls define an enclosed annular space. Preferably, the walls of the jacket are made of an electrically conductive material.

An inlet conduit 21 connects into the jacket at an aperture 23 formed through the outer wall 16 near the forward end of the instrument, and an outlet conduit 24 connects into the jacket through an aperture 26 near the rear of the instrument. Cooling liquid flows through the inlet conduit 21 into the jacket, thence circulates the length of the jacket before exiting therefrom via outlet conduit 24. The flow direction is indicated by the arrows in FIG. 1. Preferably, the conduits 21, 22 are rigid. Flexible tubes 26 (shown in FIG. 2) may be connected to the distal ends of the conduits so that the instrument can be freely moved about.

FIGS. 2 and 3 illustrate a configuration of the instrument with an insulating sheath 30 secured about the cooling jacket 12. The sheath is connected to chassis or earth ground and is isolated from the cathode which is normally at D.C. ground potential. Such isolation is desirable because it is possible for the D.C. ground potential to shift significantly from chassis ground potential. The most likely discharge path in that event would be through the body of either the surgeon or the patient.

Aside from the sheath, the instrument shown in FIGS.

2 and 3 is identical to that shown in FIG. I. The sheath 30 may include a dielectric lining 31 overlaid with a metallic skin 32. The skin 32 provides a protective cover which is readily sterilized for antiseptic use. The

dielectric lining 31 may be heat-shrinkable plastic fitted to the cooling jacket as shown. The metallic skin 32 may comprise one or more coatings of metal applied by well-known dipping, spraying or electroplating techniques. The outer surface of the skin is desirably of nickel or similar metal acceptable for hospital use.

Ordinarily, the grounded metallic skin of the sheath provides adequate protection against accidental discharges. Nevertheless, the safety circuit of FIG. is desirably incorporated in the power supply for the scape]. This circuit automatically shuts off the power supply and connects the DC. and chassis grounds in response to shifts in their relative potentials. The hazards of sparking and shocking are thereby eliminated.

The particular configuration of the sheath is of no consequence so long as the instrument can be firmly held. However, the overall shape and dimensions of the instrument are important. The utility and acceptance of any surgical instrument depends largely upon the facility with which it can be manipulated by a surgeon. An important aspect of the presently claimed instrument is that it can be constructed on a scale comparable to an ordinary scalpel, yet generate a reliable and controllable plasma suitable for surgical use.

Preferably, both cylindrical walls of the cooling jacket 12 serve as conductors for carrying an electric current to the cathode 86 of the instrument. However, the term cathode conductor as used hereinafter particularly refers to the inner cylindrical wall 14. This cathode conductor, in turn, coaxially surrounds a metallic tube 33 which carries an electric current to the anode 75 of the instrument. This metallic tube 33 is referred to hereinafter as the anode tube or anode conductor. The cathode conductor is insulated from the anode conductor, and together the two conductors comprise a coaxial electrical line. In the illustrated embodiment, the conductors are finitely spaced apart and air therebetween effects an electrical insulation.

A bushing-like insulator member 37 made of dielectric material is fitted around the anode tube 33 and spaces it from the cathode conductor 14 at the rear of the instrument. The bushing 37 has a circumferential shoulder 38 of greater diameter than the cathode conductor 14 abut the end of that conductor. The anode tube includes an integral collar 41 which abuts the forward end of the bushing 37. A cap 39 is threaded or otherwise releasably fixed over the rear of the jacket 12 and has a central aperture 43 formed therethrough which is of such size as to allow the end of the anode conductor and a portion of the bushing 37 to extend therethrough. The shoulder 38 of the bushing is, however, clamped between the rear of the jacket 12 and the cap 39. Consequently, the cap secures in position the bushing, which in turn abuts the collar 41 to prevent the anode conductor from slipping rearward.

The cap 39 is made of an electrically conductive material and serves as a lead to the cathode conductor. An

electrical wire, not shown, connects the cap to DC.

ground potential. The cap 39 is insulated from the anode conductor by the bushing 37.

While the wall of the anode tube 33 conducts an electric current to the anode of the instrument, the interior of the tube provides a closed channel for carrying a stream of gas to the tip of the instrument. The gas is supplied to the tube through a flexible hose 51 fitted to its rearwardly protruding end, entering through side ports 54. The hose houses a current-carrying wire 53 which may or may not be insulated. The wire 53 is connected, e.g., by soldering, to the wall of the anode tube 33.

The emote end 51a of the hose 51 is connected to a gas supply source (not shown) by connector assembly generally designated 57 in FIG. 6. TI-Ie connector includes a rigid block 59 having intersecting channels 61 and 62 formed therein. The end of the hose 51 is connected into channel 61 by a threaded adapter 64. The wire 53 extends through the channel 61 to another adapter 66 for connection to a power source (not shown) through an insulated wire 68. A fitting 69 leads from the connector block 59 to the gas source. The connector block may, of course, be electrically insulated from both the power source and the gas source.

Referring to the instrument proper 30, the forward end of the anode tube 33 is spaced from the inner wall of the jacket by an annular spacer 71 (see especially FIG. 3) made of insulating material. The spacer lies wholly somewhat behind the forward end of the anode tube and effects a seal between the anode tube and the inner wall of the jacket 12 to prevent gas which leaves the anode tube from flowing back into the instrument. The spacer may be epoxied or otherwise fixed to the anode tube. Ahead of the spacer 71, the wall of the anode tube is perforated and gas carried inside the tube can escape therefrom through the perforations 72.

An electrically conducting tip 75 is secured to the forward end of the anode tube and is, in fact, the anode of the instrument. Preferably, this tip prevents gas from exiting out'the anode tube end and, consequently, the total gas flow is out the perforations 72 in the tube wall.

A cylindrical, gas entry member 76, which is formed of a dielectric material, is fitted around the anode 75. The gas entry member 76 insulates the carrier gas from the inner wall of the jacket, and holds the anode in proper position with respect to the flowing gas. Accordingly, the member 76 has an enlarged central recess 77 formed in its rearward end; an axial channel extends from the recess to the forward end of the member and closely surrounds the anode 75. With the instrument assembled, the rearward end of the member 76 abuts the forward end of the spacer and the recess 77 forms a small plenum for the gas flowing out of the anode tube 33. A plurality of gas ports 79 extend from the recess to the forward face of the member 76 and carry substantially all of the gas to the tip of the anode 75. These gas ports 79 are desirably formed to impart a helical movement to the gas exiting therefrom.

Ahead of the member 76, but still within the jacket, is a cylindrical constrictor or nozzle entry section 81. The constrictor is preferably made of dielectric material, such as boron nitride, and includes a converging axial channel 82. This channel surrounds the tip of the anode, and communicates, at its enlarged end, with the gas ports in the cylindrical member 76. The wall of the converging channel maintains the gas which flows from the ports insulated from the inner wall 14 of the cooling jacket. The anode extends into, but not all the way through, the channel 82. Gas passing the anode 75 enters a nozzle throat portion 83 of the constrictor section 81.

Abutting the forward end of the constrictor is the cathode 86 of the instrument. The cathode is annular and fits within (i.e., is surrounded by) the inner wall 14 of the cooling jacket 12. The cathode may be brazed or otherwise fixed to the jacket to make a good electrical connection therewith. The orifice diameter 86 of the cathode is somewhat larger than the throat 83 of the constrictor member 81 and functions as a nozzle exit. As shown, the cathode 86 desirably terminates at the end wall 17 of the cooling jacket.

The anode tube 33 is prevented from slipping forward in the instrument by the above-described structure. That is, the spacer 71 is fixed to the anode tube and presses, via member 76 and constrictor 81, against,

the cathode 86 which is fixed to the cooling jacket. As previously described, however, the anode tube is readily removable from the rear of the instrument by releasing the cap 39.

In operation, a direct current (DC) voltage source (not shown) is connected to the instrument. The positive terminal of the source is connected to the anode lead wire 53 and the negative terminal (or ground) is connected to the cap 39. Initially, a current flows to the anode 75 and away from the cathode 86, but because the anode is spaced apart from the cathode, the current flow stops once the potential drop across these electrodes equals the potential of the DC. source. Concurrently, gas is delivered to the instrument through the hose 51. A typical gas flow is 1.0 CFH (STP). The gas flows through the anode tube 33 to its forward end thence escapes through the perforations into the plenum 77. From the plenum the gas flows through the gas ports 79 into the entry section 82 of the nozzle (81 and 86).

The voltage of the source is increased until the ionization potential of the gas is exceeded. The gas then ignites; i.e., it is transformed into a plasma state under the influence of the potential difference between the anode and the cathode. The plasma exits the orifice 87 of the cathode 86 in a directed jet stream. The characteristics of the plasma produced by the claimed instrument depend upon the amount of current available to it from the source. Increasing the power applied to the gas results in very small increases in the voltage drop across the plasma but correspondingly large increases in the current carried by plasma until the critical arcing level is exceeded; i.e., the power level at which the plasma converts to a hot plasma (at substantial thermal equilibrium). A notable characteristic of the claimed instrument is the ease with which the character of the plasma may be controlled. The arrangement of the annular cathode with respect to the anode assures an electrically neutral plasma, which is beneficial for surgical use. Cathodes of copper, tungsten or alloys thereof are generally satisfactory. A satisfactory anode material is nichrome wire.

The electric field between the anode and cathode produces a certain amount of heat. The cooling liquid which circulates through the jacket 12 as previously described effectively removes such heat. Accordingly, the

instrument is at all times comfortable to touch. Usually, the cooling liquid is ordinary tap water.

The safety circuit of FIG. 5 prevents a large net electric charge with respect to DC. ground potential from building on the metal skin of the sheath of the instrument. Lines 101 and 102 are the usual volt A.C. lines leading to a conventional transformer (not shown) which generates the voltage supplied to the instrument. A connection is made by line 106 from line 101 to line 102 through the periphery winding 107 of a conventional transformer 108. The secondary winding of the transformer 108 is connected to chassis ground at 109 by center tap line 111, and to DC. ground return 110 through a high value resistor 112. The output lines 116 and 117 of the secondary winding are connected to a conventional full wave bridge rectifier 118 which converts the alternating current in the secondary to a direct current across the bridge output lines 121 and 122. Preferably, the potential across the lines 121 and 122 is about 16 volts. Grounded capacitors 124 and 125 may be connected to the output lines 121 and 122, repsectively, to smooth the output voltage.

Each of the lines 121 and 122 lead toordinary solenoids (126 and 127) respectively; when the voltage is held constant, the solenoids are inactive. Line 121 leads from solenoid 126 to the collector of an ordinary PNP transistor 131. The emitter of that transistor is common to chassis ground (via line and to the emitter of an ordinary NPN transistor 137. The collector of this second transistor 137 connects to the output line 122 coming from the solenoid 127. The two transistors are connected in common-emitter, base-input arrangement so that the current which the transistors conduct is readily controlled by the potential applied between the transistors common base and emitter leads 134, 135.

The emitter lead 135 is at chassis potential. The base lead 141 is ordinarily at ground or zero potential; it connects through series opposed zener diodes 144 and 146 (having, for example, about 5 volt values) to a relatively small resistor, which is connected in series with the DC. ground return 110. Ordinarily, the series opposed zener diodes do not conduct current in either direction. However, if the potential difference between chassis ground and DC. ground exceeds the zener breakdown point, as when a large net charge accumulates on the skin of the sheath, a current flows through the diodes. When such a current flows, it causes the transistor output to vary considerably. Accordingly, the current through the respective solenoids 126 or 127, depending upon the polarity of the accumulated charge, changes and one or the other of the solenoids acts to break the power supply circuit, as hereinafter explained.

A line 151 is connected across the I20 volt A.C. lines 101 and 102 to control solenoid 153. That solenoid, in turn, can move to actuate a switch 154 in the main line 101. Ordinarily, the solenoid 153 retains the switch closed so that current flows through line 101. Two switches 161 and 162 are connected along line 151 (as indicated by the dotted-lines) to the solenoids 126 and 127. Ordinarily, those solenoids allow the switches 161 and 162 to remain closed so that current flows through line 151. However, when the output current of the transistors changes due to a base current in the transistors 131 and 137, at least one of the solenoids is activated and opens the switch to which it is connected. In that event, line 151 becomes an open circuit and solenoid 153 is released andvopens the switch 154 whereupon the source of power for the power supply 163 is cut.

To reset the safety circuit, a button or switch 164 is closed which short-circuits the two switches 126 and 127. Solenoid 154 is thus energized to close switch 154. A manual switch 167 may also be provided in line 151 to selectively manually cut the current in the line 101.

I claim: 1. A surgical instrument for generating an electrically neutral, D.C.-induced, cold plasma, comprising:

an elongate body member with a hollow interior;

nozzle means contained within and at the forward end of said body member, said nozzle means including a nonconductive entry portion, a nonconductive throat portion and a conductive exit portion formed as an annular cathode element;

conductor means for connecting said cathode element to the negative side of a DC. power supply;

gas supply means extending from communication with said entry portion of said nozzle, through said hollow interior and out the rearward end of said body member for connection to a source of gas;

anode means fixed to the forward end of said gas supply means and disposed within said entry portion of said nozzle in coaxial arrangement with said cathode such that gas delivered to the gas supply means flows around said anode, through the nozzle, and out through the cathode; and

conductor means for connecting said anode to the positive side of a DC. power supply.

2. An instrument according to claim 1, wherein said gas supply means includes a tube made of electrically conductive material, means are provided to electrically insulate said-tube from said body, and the wall of said tube serves as the conductor means for connecting said anode to said power supply.

3.-An instrument according to claim 1, wherein said body comprises an annular cooling jacket and including means for delivering cool liquid thereinto and for removing warmed liquid therefrom.

4. An instrument according to claim 3, wherein said cooling jacket is made of electrically conductive material and serves as conductor means for connecting said cathode to said power supply.

5. An instrument according to claim 4, wherein said gas supply means includes a tube made of electrically conductive material, means are provided to electrically insulate said tube from said body, and the wall of said tube serves as the conductor means for connecting said anode to said power supply.

6. An instrument according to claim 5, wherein said anode forms a closure at the end of said tube and perforations are formed through the wall of said tube behind said anode so that carrier gas is forced through said perforations and said perforations are disposed within a gas entry element abutting the entry portion of said nozzle and openly communicating therewith.

7. An instrument according to claim 6, wherein the gas entry element includes a plenum chamber in open communication with said perforations and longitudinal parts openly connecting said plenum chamber with said entry portion of said nozzle.

8. An instrument according to claim 5, wherein a protective sheath is provided about said electrically conductive cooling jacket and said sheath includes a lining of dielectric material. 1

9. An instrument according to claim 8, wherein said protective sheath further comprises a metallic skin which is fixed over said dielectric lining and is insulated thereby from said jacket.

10. An instrument according to claim 9, wherein said skin is connected to earth ground.

11. An instrument according to claim 10, wherein said skin is connected to ground through a safety circuit, the elements of said circuit being actuated by differences in potential between earth ground and the cathode to cut the supply of electric power to said instrument.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION luLvut Nu 3,838, 2'?- hutml SQPEQFWbGl" Inventofls) Robert G. Ceucher It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 4, line 13, change "emote" to "remote"; g g Col. 4, line 2 6,insert "agai n after-the word referting;

Col. 4, line 15, change "'I'He" to --The--',

C01. 5, line 12 chehgeLF'Se" to "em;

Col. 6, fine chetige "perieher y" t o --prirT1er y 1 Gel. 6, line 22, change "repsectively" to "respectively".

Signed and sealed this 7th day of January 1975.

(SEAL) Attest:

r-zccov-n. GIBSON JR.: c. r-mRs'HALL DANN Attesting Officer I Commissioner of Patents FORM PO-IOSD (10-69) USCIOMM-DC 6O376-P69 U.S. GOYERNMENT PRINTING OFFICE: 1969 O-366-3S4

Claims (11)

1. A surgical instrument for generating an electrically neutral, D.C.-induced, cold plasma, comprising: an elongate body member with a hollow interior; nozzle means contained within and at the forward end of said body member, said nozzle means including a nonconductive entry portion, a nonconductive throat portion and a conductive exit portion formed as an annular cathode element; conductor means for connecting said cathode element to the negative side of a D.C. power supply; gas supply means extending from communication with said entry portion of said nozzle, through said hollow interior and out the rearward end of said body member for connection to a source of gas; anode means fixed to the forward end of said gas supply means and disposed within said entry portion of said nozzle in coaxial arrangement with said cathode such that gas delivered to the gas supply means flows around said anode, through the nozzle, and out through the cathode; and conductor means for connecting said anode to the positive side of a D.C. power supply.

2. An instrument according to claim 1, wherein said gas supply means includes a tube made of electrically conductive material, means are provided to electrically insulate saId tube from said body, and the wall of said tube serves as the conductor means for connecting said anode to said power supply.

3. An instrument according to claim 1, wherein said body comprises an annular cooling jacket and including means for delivering cool liquid thereinto and for removing warmed liquid therefrom.

4. An instrument according to claim 3, wherein said cooling jacket is made of electrically conductive material and serves as conductor means for connecting said cathode to said power supply.

5. An instrument according to claim 4, wherein said gas supply means includes a tube made of electrically conductive material, means are provided to electrically insulate said tube from said body, and the wall of said tube serves as the conductor means for connecting said anode to said power supply.

6. An instrument according to claim 5, wherein said anode forms a closure at the end of said tube and perforations are formed through the wall of said tube behind said anode so that carrier gas is forced through said perforations and said perforations are disposed within a gas entry element abutting the entry portion of said nozzle and openly communicating therewith.

7. An instrument according to claim 6, wherein the gas entry element includes a plenum chamber in open communication with said perforations and longitudinal parts openly connecting said plenum chamber with said entry portion of said nozzle.

8. An instrument according to claim 5, wherein a protective sheath is provided about said electrically conductive cooling jacket and said sheath includes a lining of dielectric material.

9. An instrument according to claim 8, wherein said protective sheath further comprises a metallic skin which is fixed over said dielectric lining and is insulated thereby from said jacket.

10. An instrument according to claim 9, wherein said skin is connected to earth ground.

11. An instrument according to claim 10, wherein said skin is connected to ground through a safety circuit, the elements of said circuit being actuated by differences in potential between earth ground and the cathode to cut the supply of electric power to said instrument.

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