USRE20035E - Electric arc cutting machine - Google Patents

Description

July 14, 1936. STROBEL Re. 20,035

' v ELECTRIC ARC CUTTING MACHINE Original Filed May 9, 1932 3 Sheets-Sheet l '3 28 M zo 9 55 1.3. V g 42 -21 27 J9 7 as n I 4 1 Z A/ 3" L@ I x 67 Inventor M W v July 14, 1936. K. STROBEL ELECTRIC ARC cuwwme MACHINE Original Filed May 9, 1932 3 Shets-$heet 2 Inventor July 14, 1936. K. STROBEL rangc'rmc ARC CUTTING MACHINE Original Filed llay 1952 3 Sheets-Sheet 3 Inwentar WMW Rama-a July 14, "1936 UNITED STATES 20,035 Emc'raio l no cu'r'rmo MACHINE Karl Strobe, Whlttier, am, Strobel Corporation, Puente,

tion of California Original so. 2,007,225, dated May 9, 1932.

1936, Serial No; 81,693

No. 610,086, issue May 25,

assignor to Karl CaliL, a corpora- July 9, 1935, Serial Applicationfor re- 14 Claims." (c1. 219-1) the same. Such prior devices, howevergcannot be employed to make a relatively narrow cut,

' since it is necessary that the entire metal surrounding the cut shall be heated to the ignition point and the result is a separation between the cut sections of material Moreover, the employment of such methods for cutting requires bringingthe temperature of the metal up to the ignition point before the burning or cutting effect begins and the metal is therefore so heated that for a considerable distance on each side of the actual cut the metal is overheated. In the ;case of cutting hardened or tempered metals, such overheating causes the metal on each side of the cut to become soft, or to become crystallized, depending upon the character of the metal which is out.

It is, therefore, an object of my invention to provide a device. for. cutting metals by means of an electric arc in which the width of the cut isonly asgreat as the thickness of the electrode employed to make the cut. Another object of the invention-is to provide a device for cutting metals with an electric arc in which no oxidizing agent is required. I

Another object of my invention is to provide a device for cutting metals with an electric arc in which the heating of the metal is confined to a narrow band substantially equal to the thickness of the electrode employed, and in which the metal on each tially cold during the cutting operation.

Another object of my invention is to provide a device for cutting metals with an electric arc in which the finished cut presents a smooth line edge having the same appearance as though the metal had been cut by a contact saw. Another object of my invention is to provide a device-for cutting metals with an electric arc in which the metal to be cutmay be harder than the material ofthe electrodeemployed for-the cutting.

Another object of my invention is to provide a device for cutting metals with an electric arc in which the only mechanical contact between the cutting electrode and the workpiece to be cut is .a momentary 1. ontact necessary to start the arc.

Other objects and advantages will be apparent from a study the following specifications readin connection with the ac ompanying drawings, wherein the metal while so heated to consume I rough, erratic line of side of the cut remains substan-- Figure 1 15 a perspective view of an arc cuttingmachine constructed in accordance with my ln-' vention;

-.Figure 2 is a vertical sectional" view, taken through the electrode and work table illustrating the manner of mounting-the electrode and the manner of supplying current thereto;

Figure 3 is a horizontalsectional along line IIIIII of Figure 2;

Figure i is'a diagrammatic view of the wiring view, taken a Re. 20,035 "PATENT 7OFFICE system employed in connection with my are cut--' ting machine; and

. Figure 5 is a diagrammatic view illustrating the action of the are between the electrode and the workpiece to be cut. I

Referring to the drawings, I have illustrated my electric are cutting machine as comprising a suitable work-table I having a top 2 supported upon a plurality of le gs,3 to elevate the table toany suitable height. The table top 2 consti tutes the support for the metal which is to be cut and which may be clamped in place thereon by means of any, suitable clamping device, such as a vise structure 4 mounted upon the table top. The table top is provided with a suitable slot 5 extending therethrough over which the workpiece 6, or section of metal to be cut, may

extend. I

The cutting electrode is illustrated as comprising a circular disk 1, similar in construction to the circular saws now employed for mechanical contact sawing, the disk being mounted upon a' suitable rotatable shaft 8 permitting the disk 1 to be rotated relative to the table I.

, The disk 1 is illustrated'as .being rotatable by any suitable source of power, such as an electric motor 9, the armature of which is connected to the shaft 8 either directly or through gearing III in order to produce the desired speed of rotation of the disk 1. Since the disk 1 is to constitute oneof the electrodes employed for producing the electric arc, it is desirable to insulate the disk 1 from its shaft 8 and from the motor 9, and for this purpose I have illustrated the shaft as being 7 provided with a suitable radial flange II against which an insulating disk or cup I! may bear, presenting a plane surface' against which one side of the disk 1 may be clamped. An insulating bushing 13 may be interposed betweenthe bore ll through the center of the disk 1 and the shaft 8, while a second cup-like washer I5 may be interposed between the disk 1 and the clamp- 7 lug nut l6 employed to retain the disk upon the shaft I." In order to conduct electric current to the disk 1 a washer or ring [1 of suitable conducting material may be interposed'between the insulating washer l5 and the side of the plate or disk 1.

The disk is arranged to be operated by a motor 9 at a relatively high speed, and a guard ll may of slides 35 and 38 engaging a track .31 formed '1. The guardis preferably formed of nonmag:

netic material to prevent creation of undesired magnetic flelds around the electrode.

' The guard l8 may be employed for mounting suitable electrical conducting devices supplying current to the disk I, as, for example, by mounting a relatively large sheet 2| of insulating material upon one side of the housing or guard 18, the

' material of the guard l8 being cut .away' adjacent the inside surface of the 'plate or sheet of insulating material 2|, as is indicated at 22. A brush mounting 29 is illustrated as being secured to the plate 2|, comprising a cylindrical bushing having a bore- 24extending therethrough and through the plate 2| to mount a suitable metallic brush 25 pressed into contact withthe conducting ring .ll

under the influence of a suitable compression spring 29. The pressure exerted by thespring 29 may be suitably adjusted by means of a plug 21 threaded into the outer end of the brush mounting bushing 23.

Hence current supplied to the brush mounting. bushing 29-will be conducted through the disk 1,

while it is being rotated by the motor 9', so' that an arc may be drawn between a workpiece laid upon the table-top 2 and the rotating electrode 1.-

By referring particularly to Figures 1 and 2, it will be observed that the motor 9 is mounted upon a suitable mounting plate 28, illustrated as being formed from a section of conventional channel iron to which is secured a pair of vertically extending plates 29 and 90. The plate 29 is illustrated as having secured thereto a pair of runners or slides 3| and 92 to engage above and below a track 93. The track 33is preferably formed from a section of conventional I-beam 34 with the edgesi'of the flanges thereof beveled and machined to provide upper and lower tracks on which the slides 9| and 32'may operate. The plate 39 is also'illustrated as being provided with a pair upon the other flange of the I-beam. This construction forms amounting for the motorwhich permits the same to be moved longitudinally of the r-beam ,butwhich supports the motor rigidly in a vertical direction. An outstanding boss 99 maybe formed upon the plate 90 to constitute a nut engaging a threaded screw 29 by which the-motor and its mounting may be moved hori- ,tontally along the I-beam track 94. The I-beamtrack is, illustratedas being slidably supported upon a pair ,of uprights 49 and 4| so that the I-beam with its motor and disk may be recipr beam 34. The opposite end of the I-beammay be 9 as is illustrated herein upon a' slide 45 engaging 'rocated his vertical" direction. The upright 40 is illustrated as comprising a frameat the top mounting for the I-bea'm 34., a The screw 42 may be provided with a suitable operating handle 44 permitting the screwto be readily manipulated to elevate or' lower the I- suitablethreaded nut structure 42 forming a nut mounted in a similar manner or may bemounted the upright 4|. In order to cause the slide 45 to ,be elevated in accordance with the elevation of the nut 49, a suitable counterweight 44 may be connected by means of a flexible chain or cable 41 forward end thereof so that manipulation of the a handle 49 willcause the motor and its disk to be molten particles of the metal, and bottom of which a vertical feeding screw 42 may be journaled, the feeding screw engaging a and. the workpiece is actually to the I-beam 34', the chain or cable passing over a suitable pulley 49 supported upon the upright 4 I.

I "If desired, the upright 4| may be formed ofia tubular post with the counterweight slidable within the same. Thus the manipulation of the -single handle 44 may be employed, to elevate or I lower the motor 9 and the disk I. The lateral feed screw 39 may be manipulated by means of a suitable handle 49 secured to the screw at the moved horizontally along the I-.beam track. Thus the disk may be moved to any desired po-. sition in the vertical plane in which it rotates, this plane being aligned with the plane of the 15 slot 5 of the work-table so that the lowermost portion of the disk may pass below the level of P :the table-top 2.

It will be'observed" that the disk 1 constitutes an electrode having a relatively narrow elongated 20 edge 50, which, upon rotation of the disk by the motor, will cause the edge'of thedisk to pass continuouslyin the same direction past the work-. piece upon the table. Thus an arc is drawn be- .tween the edge of the electrode'and' the nearest 25 point of the workpiece. Thearc so drawn will heat a section of the metal of the workpiece'immediately adjacent the electrode to the point of fusion, the speed of the movementof the edge of the electrode relative to the workpiece causing 30 fused particles of the, workpiece to be drawn in the-'direotionof the movementof the electrode edge so that as soon as minute particles of the .workpiece are melted, they" are immediately ever, will're'main between the workpiece and the nearest'point of theedge of the electrode so that as eachparticle' is removed the arc continues to melt the next portion of the workpiece material.

By referring particularly to Figure 5, it will be observed that if the electrode "I is rotating in the direction indicated by the arrow, the mechanical contact of the electrode edge ill with the extreme edge SI of the workpiece 6 will cause an arc to be started between the electrode edge 50 and the workpiece'i. As the arc melts the metal, the rotation of the electrode edge will cause the particles to be drawn away from the workpiece along a line tangent to the edge 59 of the electrode I, as is indicated at 52.

By operating the feed screw 99, the electrode is drawn toward and through the workpiece in a direction transverse to the rotation ofthe edge 59 of the electrode, thus permitting the arc to v penetrate further through the workpiece until the'edge ofthe electrode has passed completely through the workpiece, severing the same into two separate parts. J In order to assist the throwing out of the the edge 50 of the 6 electrode may be provided with a plurality of serrations .or -teeth 53 similar to the teeth employed upon conventional mechanical contact saws, so that the are between the electrode edge I drawn from the points of the teeth. g 7

While the theory of the melting and'removal of the particles is not fully 'understood, it has been'observed in actual practice that the employment of teeth of the shape shown produces the most desirable results, apparently the combined tendency of the arc to follow the trailing surfaces the'cup. Also the formation of the teeth is pref-.

erably like that of mechanical contact sa'ws in that the edge of the saw is swedged out to provide a greater width at the outer edge of the teeth than the width of the electrode diskat points closer to its center. Thus the are which is drawn from the extreme edgeof the teeth will be su1-' i'iciently wide to insure removal of the metal to form a cut, the sidesoi which clear the sides of the electrode disk.

In view of the fact that the cutting operation is formed entirely by the arc, and not by mechanical contact betweenthe teeth of the electrode andthe workpiece, any metal, irrespective of its hardness, may be successfully outwith an electrode which is formed of much softer metal. The primary essential characteristic of the material from whichthe electrode is formed is that it must be sufiiciently tough to stand up under rapid rotation and must. be sufliciently rigid to maintain all parts of the edge in the same vertical plane while rotating. I have employed a relatively soft steel saw, such as may be used for wood cutting, and with such saw I have succeede'd in cutting metals having substantially diamond hardness. An interesting phenomenon is that with my cutting device the' electrode 1 does not heat up to any appreciable extent duriug the cutting operation, and, in fact, imme diately after the termination of a cut, the edge of the electrode may be readily grasped with the fingers and shows no substantial increase in temperature above the temperature of the surrounding atmosphere.

' trode tostart the are or by laying a strip of carbon or other soft conducting .material immediately in advance of theworkpiece so that the arc may be originally drawn between this mate- .rial and the electrode instead oi initially drawing they are by contact with the workpiece. Howstanta'neous that after many cuts of, extremely hard material have been made by the saw, no ap-,

preciable dulling of the tool occurs, indicating that the inttantaneous mechanical contact was of such short duration that there was little or no effective mechanical contact maintained.

.As the electrode enters the cut in the workpiece there is a tendency for the arc to travel inwardly along the sides of the electrode which ,would tend to heat the electrode and tend to.

burn away metal along the sides of theeut. To avoid this a magnetic fieldmay be created in the electrode disk which will tend to drive the arc radially from the disk and will tend to confine the arc to the edge' only of the electrode.

this purpose, any suitable means for creating a magnetic field in the electrode may be employed, though the embodiment of my invention employing a disk electrode, as shown herein, I prefor to create the magnetic field by means of a coil 54 through which'the electrode projects.

By. referring particularlyto Figures 2 and 3, it

will be observed that the coil 54 is formed by 'a continuous length of conducting material wound in the form of a tapered helix, the taper of which is directed toward the edge of the electrode; That is, the coil-54 may extend from a suitable ter-.

minal 55 around the interior of the guard l8 in The only mechanical contact I which is made between the electrode and the For -"in' line with the edge of the electrode.

the electrode 1 As the coil spaced relation to continues in the form of a second turn about the ard I8.

I find that the best resultsare achieved when the line drawn through the centers ofthe conductors'on one side of the electrode and a line drawn through the centers of the conductors on the other, side of the electrode intersect each other substantially at the edge 50 of the electrode. Such condition is illustrated by the dotted line arrows 56 and 51 inFigure 2. Thus the electrode is in effect a magnet which tends to blow the arc transversely to the direction'of constant move-' ment between the edge" ot the electrode and the work-piece and any tendency for the are to creep back along the sides of the electrode is counteracted.

For simplicity in construction, the coil il'may be formed from a bar or conducting material having a. relatively great cross-sectional area suilicient to carry the current employed in the arc, and this coil maybe connected in series relation with the electrode 1 so that all of the current which passes through the arc will also pass through the coil, thereby creating a blowing or driving force proportional at all times to the amount of the current employed in the arc. Thus the coil extends, as shown in Figure 3, from the terminal .55 and around the electrode 1 to a second terminal 59 which is bridged by means of a strap-50 to the brush mounting] bushing 23, so that current supplied to the electrode will first pass through the coil and then passto the electrode.

In view of the fact that at the instant of drawing an arc a substantially short circuit is created,

it is desirable that some current limiting device be interposed between a source of supply and the electrode. ever, inpractice this initial contact is so in- By referring 'to Figure 4, it will be observed that a typical wiring system which may be employed may comprise connection of the electrode 1 to a suitable source of supply, through the conductor 5l,-an arc control switch 52, a current limiting resistance 531mm which a conductor 54 extends to the terminal 55. Thence the circuit extends through the coil 54, through the terminal 59, bridge 50, brush 25, contact washeror ring II to the electrode disk 'I. The circuit is completed through the workpiece 6 and table top 1 which may be connected in any suitable manner, as by conductor 5'l,'to the opposite terminal of a source of electric current.

While I prefer to use supply for the arc, alternatingcurrent maybe employed with substantially equally good results.

i It will be observed that by confining thearc to the edge of the electrode, and by causing the arc to be projected outwardlyfrom the electrode edge in a direction substantially transverse to the movement between the electrode edge and the workpiece, the only portion of the workpiece ,tm-

be out which will be heated will be that directly Thus no direct current as the substantial heating of the metal on either side, of

curs beyond 1 m. m. on either side of the cut. Hence tempered materials, suchas springs .and-

the like, may be readily'cut with my saw without drawing the temper from the material to a-distance greater than 1 m. m. on either side of the cut. Also, the appearance of the sides of the cut performed by my are cutting device is smooth and straight, the appearance being identical with desiredlength. I

The automatic control of the feeding of the that of a cut made by mechanical contact saws, such as hack saws or what is known as the triotion saw.

While the cutting operation may be performed with any desired speed 0! movement of the-edge or the electrode relative to the workpiece, the best results are obtained when there .is a relatively high speed of movement between these. members, the greater the speed of movement, the

being that the electrode must have an elongated relatively narrcw'edge from which the arc may be drawn and which edge may be driven past the material or workpiece to be cut at a relatively high speed.

If desired, thefeeding movement of the disk toward and through the workpiece to 'be cut may be. automatically controlled so that thearc'between the workpiece may be maintained at any electrode may be accomplished in any desired manner, suchqas that illustrated in Figure 4,

wherein the feed screw 39, which controls the lateral movement of the electrode I and its motor 9, may be connected to' a suitable driving motor 10, the armature "ll of which may be connected across the arc, as by connecting one terminal of the armature to conductor 64 which supplies current to the electrode I, and connect ing the other terminal of the armature II to the workpiece. The-field winding 12 of the motor I0 is connected to any suitable constant source of energy so that as the arc'length increases more current will be supplied to the'armature Ii 01' the motor and it will rotate the'screw 39 to feed the electrode toward the workpiece, while, as the 'arclength decreases less current will be supplied to the armature II, and the feeding movement 01 the electrode will be reduced.

While I have shown and described the pre-' Ierred embodiment of my invention, 1 do not'desire to be limited to .any of the details' shown I claim: I 1. in a device for cutting metals with anelecexcept as defined in the appended claims. 7

} trio arc, anelectrode having an elongated edge,

means for applying electric 'potential ,between said electrode and a workpiece to be cut to pro duce an arc th'erebetween, .teeth i ormed upon said elongated edge having their extreme outer edges of greaterwidth than the remainder of said electrode, means for producing continuous unidirectional movement between said electrode edge and said workpiece,.and means for moving the edge of said electrode toward and through said workpiece in a direction transverse 'to unidirectional movement.

2. m a device torcutting metals with a. elec-Q tric are, an electrode having an endless'elon gated iedge, means for applying electric potential between said electrode and a workpiece to be cut to prod'ucean arc therebetween, teeth rm upon said elongated edge having their extreme" outer edges of greater width than the remainder of said electrode, means Ior'producing a continu-' ous unidirectional movement between said electrode" edge and said workpiece,'and means for moving the edge of said electrode toward and throughsaidworkpiece in a direction transverse to said'unidirectional movement. v

3. .In a-device for cutting metals with an elec trlc arc, an electrode having an elongated toothed edge, the peripheral edge of said elec'-" 'trode being formed of greater width than the remainder of said electrodeto confinefan arc 'to the front edge or said teeth, meansior applying electric potential between said electrode and a workpiece tobe cut to produce an are therebetween, means for producing continuous unidirectional movement between said electrode edge and said workpiece,- and means for moving the edge-oi said electrode toward andthrough said-workpiece in a direction transverse tosaid unidirectional movement. V

4. In a device for'cutting metals with an electric arc, an electrode having anelongated edge. teeth formed upon said elongated edge having. theirextreme outer edges or greater width than the remainder of 'saidelectrode, means for applying electric potential between said electrode and ,a workpie'ceto be cut to produce an are therebetween, i'or producing continuous unidirectional movement between said electrodej edge and said workpiece, means for' moving the edge of said electrode toward and through said workpiece in adirection transverse to said unidirectional-movement, and a coil in circuit with'said electrode and disposed relative to said electrode to create a magnetic field therein. one pole'ot which lies at the center of said electrode and the other poleof which lies at the peripheral edge of saidelectlode for confining the arc to the surface of the edge of said electrode.

5. In a device .for cutting metals with an electric arc, an electrode having an elongated edge,

teeth formed uponsaid elongated edge having their extreme outer edges oi greater width than the remainder of said electrode, means for applying-electric potential between said electrode and a workpiece to be cut to produce an arc therebetween, means. for producing continuous unidirectional movement between said electrode edge and said workpiece, means for moving the edge .of said electrode toward and through said I workpiece in a direction transverse to said unidi rectional -movement,'and. a coil through which I said electrode projects for creating a magnetic field in said electrode, one pole at which lies at, the center of said electrode and the other pole of which lies at the periph ral edge or said electrode for confining the ar'c to the edge or. said electrode. v J '6. In a device for cutting metals with an elec-' 'tric arc. an electro'de'h'aving'an el'ongated edge,

teeth formed upon said elongated edge having their extrem'eouter edges of greater width than the remainder of said electrode, means i'or applying electric potential between said electrode and a workpiece to'be'cut toproducepan arc therebetween, means for" producing continuous unidirectional movement between said electrode edge and said workpiece, means for moving the edge of said electrode toward and through said workpiece in a direction transverse to said unidirectional movement, and a coil through which the edge of said electrode projects, said coil being formed of a helix tapering toward the edge of said electrode for creating a magnetic field confining the arc to the edge of said electrode.

'7. In a device for cutting metals with an electric are, an electrode having an elongated edge having a plurality of saw-like teeth formed thereon, and having the outer edge of said teeth of greater width than the remainder of said electrode and confining an arc to the outer edge of said teeth, means for applying electric potential between said electrode and a workpiece to be cut to produce an arc therebetween, means for producing a continuous unidirectional movement between said electrode edge and said workpiece, and means for moving the edge of said electrode toward and through said workpiece in a direction transverse to said unidirectional movement.

8. In a device for cutting metals with an electric are, a disk electrode, teeth formed'upon the peripheral edge of said disc having their extreme outer edges of greater width than the remainder of said electrode, means for rotating said disk electrode, means for feeding said electrode toward and through said workpiece in a direction transverse to the axis of rotation of said electrode, and means for applying electric potential between said disk electrode and the workpiece to be cut to produce an arc between the edge of said electrode and said workpiece.

9. In a device for cutting metals with an electric arc, a disc electrode, teeth formed upon the peripheral edge of said disc having their extreme outer edges of greater width than the remainder of said electrode, means for rotating said disc electrode, means for feeding said electrode toward and through said workpiece in a direction transverse to the axis of rotation of said electrode, and means for applying electric potential between said disc electrode and the workpiece to be cut, and means for creating a magnetic field in said electrode with the center of said disc constituting one pole of a magnet and the edge of the disc constituting the other pole of the magnet.

10. In a device for cutting metals with an electric are, an electrode having an elongated peripheral edge, teeth formed upon and extend- 55 ing outwardly from said edge, means for applylog electrical potential between said electrode and a workpiece to be cut-to produce an arc therebetween, means for producing continuous unidirectional movement between said electrode edge and said workpiece, means for moving the edge 01' said electrode toward and through said workpiece in a direction transverse to said unidirectional movement, and a coil in circuit with said electrode and through which said electrode projects for creating a magnetic field in'said electrode, one pole of which lies at the peripheral edge of said electrode and the other pole of which lies within said electrode in spaced relation to said edge.

11. In a device for-cutting metals with an electric arc, an electrode having an elongated peripheral edge, teeth formed upon and extending outwardly .from said edge, means for applying electrical potential between said electrode and a workpiece to be out to produce an are therebetween, means for producing continuous unidirectional movement between said electrode edge and said workpiece, means for moving the edge of said electrode toward and through said workpiece in a direction transverse to said unidirectional movement, and a coil in circuit with'said electrode and through which said electrode projects for creating a magnetic -ileld in said electrode, one pole of which lies at the peripheral edge of said electrode and the other'pole of which lies within said electrode in spaced relation to said edge, said coil being formed of a helix of wire said helix tapering toward the edge of said electrode.

12. In a device for cutting metals with an electric arc,an electrode having .an elongated peripheral edge, teeth formed upon and extending outwardly from said edge, means for applying electrical potential between said electrode and a workpiece to be cut to produce an arc therebetween, means for producing continuous unidirectional movement between said electrode edge and said workpiece, means for moving the edge of said electrode toward and through said workpiece in a direction transverse to said unidirectional movement, and a coil in series circuit with said electrode and through which said electrode projects for creating a magnetic field in said electrode, one pole of which lies at the peripheral edge of said electrode and the other pole of which lies within said electrode and in spaced relation to said edge. I

13. In a device for cutting metals with an electric are, a disc electrode, teeth formed upon the peripheral edge of said disc and extending radi-- ally therefrom, means for rotating said disc electrode, means for feeding said electrode toward and through a workpiece in a direction transverse to the axis of rotation of said electrode, means for applying electric potential between said disc electrode and said workpiece to be cut to produce an are between theedge of said electrode and said workpiece and a coil disposed about said electrode and through which the edge of said electrode projects, connected in series circuit relation with said disc electrode and said workpiece for confining the arc to the peripheral edge of said disc electrode. 1

14. In a device for cutting metals with an electric arc, a'disc electrode, teeth formed upon the peripheral edge of said disc and extending radially therefrom, means for rotating said disc electrode, means for feeding said electrode toward and through a workpiece in a direction transverse to the axis of rotation of said electrode, means for applying electric potential between said disc electrode and said workpiece to be cut 60 to produce anarc between the edge of said electrode and said workpiece, and a coil disposed about said electrode and through which the edge of said electrode projects, connected in series circuit relation with said disc electrode and said 5 workpiece for confining the arc' to the peripheral edge of said disc electrode, said coil being formed of a helix of wire surrounding the said electrode, said helix tapering toward the edge of said disc electrode. 7

KARL STROBEL.

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