US1834725A - External field eliminator - Google Patents

Description

Dec. 1, 1931. E, F NORTHRUP 1,834,725

EXTERNAL FIELD ELIMINATOR Filed Feb. 18, 192B 3 Sheets-Sheet 1 Dec. 1, 1931. E. F.- NORTHRUP EXTERNAL FIELD ELIMINATOR Filed Feb. 12's, 1928 ooooooooooo ,a il) i M W 2 ooo oooooo Dec. 1, 193;.

E. F.1No-RTHRUP EXTERNAL FIELD ELIMINATOR Filed Feb. 1s. 192e s sheets-sheet s f ccc/ O VOOGOGOOOO FIELD yonnq NoYoKE USLD TURNS ON YKE LEG.

'Patented Dec. 1,L 1931 AUNITED STATES PATENT oFFlcE EDWIN E'. NORTHRUP, vOil? PRINCETON, NEW JERSEY, ASSIGNOB TO AJAX ELECTRO- THERMIC CORPORATION, OF AJAX PARK, ^NEAB. TRENTON, NEW JERSEY, A CORPO- RATION F NEW JERSEY EXTERNAL FIELD ELIMINATOR Application led February 18, 1928. Serial No. 255,248.

My invention relates to electric furnaces of the so-called coreless type in which a furnace inductor surrounds the charge being melted or heated and in which the magnetic circuit is not'completed through the inductor.

One of the purposesyof my invention is to substantially eliminate the stray magnetic field about a furnace indu'ctor coil free from interthreading of transformer iron.

A further purpose is to provide a return magnetic circuit for a coreless furnace, per.- missibly without butpreferably with excitation of the return circuit corresponding with the lines of force which are intended to flow through it.

A furthe-r purpose is toprovide a return magnetic circuit lfor a coreless furnace with a magnetizing coil or magnetizing coils helping to control the direction of the magnetic return flux of the furnace.

A further purpose is to provide a return magnetic circuit fr a furnace inductor having a multiplicity of parts and to so proportion them and the compensating windings upon them as to make the induced lines equal the total stray lines for which returnI-` is to be provided and minimize the average length of the return circuit.

A further purpose is to `permit the use of iron strengthening and supporting frame structure freely 1n furnace construction t without objectionable eect upon it by stray magnetic field. H

A further purpose isto so far improve the magnetic return circuit `of the field produced as to greatly improve the power factoiI and thus reduce the condenser capacity required to correct it.

A further purpose is by the improvement of power factor `t'o\increase the energy component of the current through a furnace coil-v` and thus increase the concentration of energy in the charge as compared with the same quantity of current passing through the inductor.

A further purpose is by improvement of power factor and concentration of the returning lines of force at the bottom of the furnaceto greatly improve the stirring effect.

A further purpose is to utilize greatly increased stirring eifect obtained to produce more eii'ective and quicker intermixing of treating or alloying ingredients and quicker practical, efficient, reliable and inexpensive,

and which at the same time well illustrates `the vprinciples of my invention.

Figure 1 is a purely diagrammatic sec-- tionai elevation of the inductor coil of a coreless induction furnace omitting power factor correction and current supply for it, but showing in one plane the general shape of the effective and stray magnetic fields produced in all planes through the axis of the coil. I

Figure 2 is a diagrammatic sectional elevation corresponding generally with Fi ure 1 but showing the effect upon the stray eld of a magnetic return circuit.

Figure 3 is 'a diagrammatic sectional ele vation simi-lar to Figure 2, but in which the magnetic return circuit is shown on both sides of the coil. l

Figure 4 is a diagrammatic top plan view of a form corresponding generally to Figure 8 but in which the magnetic return circuits have been shown as increased in number and radiating from the coil.

Figure 5 is a diagrammatic side elevation of av circular or anchor ring uniformly wound. A

` Figure 6 isa diagrammatic side elevation of a rectangular ring approximating the magnetic conditions of Figure. 5 except in so far as it has portions of different cross section and of different spacing of the winding.

Figure 7 is a diagrammatic sectional elevation of av rectanguiar ring but with .afurnace inductor coil fr. from interthreading Y section of a hea vation corresponding of transformer iron interposed instead el? part of thevwound magnetic ciL magnetic circuit corresponds gener if with that in Figure 2.

Figure 8 is a diagrammatic ser?" vation of al structure having the and having a oke circuit co with the ma etlc return circuit of L Figure 91s a diagrammatic section` vation corresponding generally to Figure except that the return circuit is shown cluding a multiplicity of wound iegs. magnetic circuit is comparable in entec; that of Figure 4.

Figure 10 is a diagrammatic sectionai elevation' corresponding with Figure 7 except that it shows the winding for the :1f: etic circuit as concentrated upon the side ci he return circuit yoke leg shown.

Figurell is a diagrammatic top plan view showing the form of Figure 9 except that the tops of 'the legs have been omitted and the winding has been concentrated upon each leg of the yoke as in Figure 10. v

Figure i2 is a diagrammatic topV plan viewv of a figure corresponding generally .with Figure 11 but showing a modification of the electricrcircuit.

Figure 13 is adiagrammatic sectional elel generall -with Figure 12 but showing a Crucible 1n place with a non-magnetic content in the Crucible.

Figure 14 is a dia ammatic sectional side elevation showing t e surface of the crucible as covered by a slag and lifted by the stillring operationil 15 is a atic longitudinal i rnace.

16 is a diagram illustrating my vention.

In the drawings similar numerals indicate `like parts.

Y There is a great ndvantage in electric furnaces in having the inductor coil directly A casings of framework Surround the charge without necessity for interthreading of transformer iron-through -the coil; Such furnaces, with the furnace inductor coil directly surrounding all or a part of unthreaded by furnace iron are for convenience called coreless furnaces.

Ccmless furnaces have been quite successful 1n smaller sizes but unless they can be strengthened externally by steel or metallic the are more dicult to build in lar sizes. Where these furnaces are to be for heating as distinguished from melting o rations(as in the case of mgots, blooms, illets, etc.) their usefulness may' be reduced because .of the lack of Strength' andruggednemv of the furnace construction. In the past the use of metal and particularly -m etic material in casing and in struct naces has not been-j permissible because of the reinforcement about or 1n Y. vel'yilargg-vcoreless induction .electric furexcessive stray' magnetic field which these furnaces have had.

This stray field has beendue to the si of magnetic return circuits for of force induced in the furnace` me stray magnetic field has been ob bie isc for the reason that it ha l ,r machinery and apparatus in the I 1 oriiood of-furnaces, requiring greatery u; ing of the coreless furnaces from othc ratus than would otherwise have quired and unduly otherwise restrimj :n .equi ment of the furnaces.

T e heating of outside metal olothe stray field has not been obi because of the damage done and rr imposed alone but calls for an magnetizing current through the cw Y, ering the furnace efficiency.

The stray eld of the furnace is highly objectionable for another reaser-L i that it lowers the power factor of th are and reduces the power input per amp Lui-n in the coil of the furnace inductor limiting the output of 'any given furnace.

I have discovered that all of the objections above can be overcome and that the objectionable stray iield can be taken care of substantially in its entirety by simple magnetic and, electromagnetic means which not only provides substantially complete paths of return for the magnetic lines but utilizes the paths effectively, to concentrate the return magnetic circuit and to greatly increase the stirring of the furnace. This increase in the stirring opens up greater possibilities of treatment of molten content such, for example,A as rapid introduction and .thoroughinter-mixtures of chemical elements either directly within the content or indirectly by Sluggig- In coreless induction furnaces as heretofore constructed the ma tic field 20 of an inductor coil 21 takes su antially the form indicated by broken lines in Figure 1 as in any straight solenoid.

That this is true most of the time notwithstanding that the induction furnace may operate upon iron, steel, nickel or other magnetic materials or alloys will be evident from this point (therecallescence int in steel) the magnetic properties may i .cred for our purpose or compromised or )usted by the return circuit winding because the general character of thestray-ield remains not-` withstanding that its value has been somewhat chan The fiel `ou 'de the inducing coil serves no useful purpose (whether the content be magnetic or non-magnetic) .and may intro-.

duce serious heating and energy losses." in solid conducting material in the surroundmg medium especially in magnetic materialsy outside of the inductor coil of low magneticreluctance (resistance) consisting of thin sheets (laminations) of transformer steel or other highly permeable materials having sides 23, top and bottom portions 24, 25 and end portions 26, 27 around the inductor coil, as in Figure 2. The stray field 20 is also altered in position by this return as shown in the figure.

The uniformity of the return and the path of return are greatly improved by dividing the circuit on opposite sides of the inductor as shown in Figure 3, reducing the cross sections required for sides 23and tops and bottoms 24', 25. The end pieces 26, 27 have been omitted' in this figure as they .seem to affect the result but little. The return is further improved by increasing the number (and permissibly reducing the cross section) of these radially disposed return circuits as shown in top plan view in Figure 4, the theoretical permeance preferred of course being reached when the-magnetic. circuit forms a shell completely surrounding the coil. A very close approximation is reached with the radially di'vided return circuit shown in Figure 4, having sides 232, tops 242 and corresponding bottoms not shown.

It is well known that when a ring core of magnetic material and of uniform cross-section is uniformly wound, a current through the coil will produce a large number of magnetic lines within the ring and yet be free from stray or external field. The apparent reason for this is that each portion of the winding independently provides magnetomotive force to supply. the requisite flux lines of its own portion of the length of the magnetic circuit. Such an arrangement is shown in the iron ringand Winding of Figure 5.

Subject to such qualifications as might be due to variations in permeability, a very close approximation to freedom from external' field 1s secured even when the crosssection of the magnetic circuit is varied asin Figure 6, provided the winding be -variedso as tomake the ampere turns per unit length of each section ofthe ring vary according to the reluctance of that portion of the ring. In Figure 6 where the ring has-been shown of varying cross-sections at 233, 243, 253 correspondingly different numbers of turns are shown `about the different cross-sections.

Because the current through the different windings must be in phase, the winding should be in series and the number of turns will vary inversely with the cross-section.

As compared with the circular form of Figure 5, the rectangular forms of Figures 2, 3, 4, 6 and 7 form very close approximations, the differences being negligible in practice.

In the diagrammatic illustration there has been no intention to makethe number of lines of force induced by the coil 29 in Figure 5 equal those induced bythe coils 30, 30', 302, 303, nor those induced by the coils used in the other figure, but merely to indicate that in any given magnetic circuit, for the best results the winding should be uniform for uniform-reluctance and that where the crosssection is not uniform `the closest approach to freedom from external magnetic field is se.

cured by winding each portion of any given circuit with turns to give throughout the circuit the same number of lines of force through the cross-section, whatever the section. Where the section of permeable material'is uniform, as at 234, 24, 254, 264, 27,.'in Figure 7, the closest approach to freedom from external field is to be secured by uniformly spaced windings as in Figure 5'.

Let us assume in Figure 6 that in xplace of the core section and coil-28, 302, an inf ductor coil 2l having an air gap be substituted as shown in Figure-7. The inductor coil will have a content equivalent magnetically to an airA core since non-magnetic ma terial would act as air would and even magnetic material would normallyv be treated in a heating or melting furnace at a temperature above that at which it would lose its magnetism.

Evidently an air gap inductor coil could be made to take the place of part of the magnetic circuit of Figure 7. provided the ampere turns of the winding be such as to roduce the same number of lines of force within this portion of the magnetic circuit as are produced by the windings of the other parts of the circuit.

Putting this the other way, and applying to the problem'at hand. a magnetic circuit with the current about it in phase with the current through. th inductor coil may be so selected as to` produce the same number of lines of force through the magnetic circuit as. are produced by the solenoidal inductor winding, with the result that nearly all of the linx through the inductor is returned through the magnetic circuit and outside or stray magnetic ,field from the inductor coil is substantially eliminated; The windings are shown at 31, 32, 33, 34 and 35.

Just as the permeable material on one side of and above the inductor coilin Figure 2 can be replaced to advantage in taking care shows in Fi of nearly all of the m 'clines of force by the construction of 3, and more .completel by the construction of Figure 4, so also e single electro-magnetic return path of Figure 7 canbe improved by placing the permeable material upon opposite sides of the inductor coil as in Figure 8(droppin the portions 26 and 27 and windings 34 and 35), and still further improved by distributing the electro-magnetic return circuits about the circumference of the inductor as in Figure 9. The windings are shown at 31', 32 and 33' in Figure 8 winding 31 only I have found that it makes very little difference in the magnetic circuit whether tbe Winding be uniformly distributed or whether itbegroupedasinFigure 10 and, asit is very much more convenient to windings at a part of the length of the magnetic return circuit Iprefer to use'a gr'rinlp or localized' lwinding. f ,h v

egrouping or o t ewinding shown at 36 in F' ocriahlxis ofcourse not advantageous in t at form alone; but is equally advantageous where the return magnetic circuit is divided u -into many parts as in Figures 8 and -9. s localization of winding is applied to the divided magnetic circuit 1n Figures 11-14.

For convenience in the mani ulation of lfurnaces built uon this princip e, the magnetically permea le top legs of magnetic material 24, 24', 24z (Figures 2 4), 24', 24, 24 and 24 (Fi 6-10) are preferably omitted as in lgures 11', l12, 13 and 14 and the increased ampere turns due to this increase in magnetic reluctance in` this part of the circuit are to be provided by the furnaceinductor coil.

This omission of the top legs produces a stray field at the top`of the furnace. However, tests have established that no objectionable results follow and in fact that a positive advantage is obtained in certain cases in that the stray field in melting down the charge where therchar has been iled u above the furnace to p P Whatever stray fie d there is in this lform is limited to acorn aratively short ace immediately above the furnace and a ut its upper end wherethere is no encasing structure and where little damage can be done to surrounding metal parts.

I have found that a close approximation to the' localized windings of the compensating coil in Figures 11, 13 Vand-14 can be obtained without the necessi for winding these compensating coils individually about the legs. In Figure 12 I have shown the compensating windings at 36 as localized and as surrounding the group legs of the return magnetic circuit as distinguished from surrounding the individual legs. Each form has its advantages but the simplicity of the form of group the Figure 12, the smaller furnace width required by ity and the fact that the lines of force set up by it affect the space between the legs as well as the actual area occupied by them suit this Figure 12 form better than the individual windings to general use. Y

In Figures 11-14 my invention is applied diagrammatically to an induction furnace of the melting type in which as a rule it is desirable to lift the furnace bodily for tilting purposes or to transfer it bodily from the position of melting to the position of pouring. In either event increasing furnace sizes bring strains u n the furnace which cannot be taken care o to the best advantage Without steel or other metal casings, supports or reinforcements for the furnaces. In these figures I have variously illustrated the strengthening or reinforcing steel at 37, 37', 37, 373, intending the illustration to be purely dia grammatic and omitting the intermediate connecting furnace structure and refractory. v

Though the casings shown in these figures are preferably of s hell form itis not the intention to indicate that any such restriction attaches to these casings, supports or reinforcements as the freedom from stray field leaves the designer free to utilize any'forln of fabricated or other metal reinforcement or support which may suit his purpose.

The capability of thus properly supporting the furnace or without regard to stray magnetic field is an important feature of my invention. n

It is also not the intention in the furnaces shown to indicate that the container 38 shall necessarily be a crucible or other preformed container as my invention is wholly free from limitations of this character, permitting the melting to be performed in a. preformed container, or in a bed or shell formed in the sand or other surrounding insulating material itself by the heat of the melt as is frequently done at the present time.

T e marked improvement in the return magnetic circuit for the lines of force produced bythe furnace inductor coil reduces the size of the magnetizing component of the current through the furnace coil, thus relatively increasing the energy component and correspondingly improving the power factor.

For the same coil winding this reduction in the percentage of the magnetizing component of the current ermits a larger energy component of the total currentthrough the coil if the combined current be kept the same,

more rapid and effective heating and ve noticeably increased stirring of the mo tent content.

As compared with the furnace when not in action, the metal surface is flat at 38 on the upper-surface of the content 39. I find that the pinch circulation is so much increased as to secure a high crowning of the center. of the metal as at 38 when the current is turned on. In some tests made to determine this the center was higher than the edges to an extent approximating the radius of the melt.-

' The metal comes up the center and down aloner the sides as in the usual pinch circulation but' to a more marked degree than I have been able to secure without the present invention. This is due not only to the excellence of the magnetic side path of return but to the positioning of the bottom of the magnetic return directly beneath the charge and close to it.

The improved pinch circulation is effective to very inuch increase the speed of mixing of added ingredients to the metal and to correspondinglyfincrease thespeed of slagging the metal by any slag 'coveringsuch as is shown at 40 in Figure 14.

The discussion has been made general and the figures have been made diagrammatic for the reason that it is recognized that the above invention is applicable to any type of coreless induction furnace whether it contain a cru;

It is recognized that in some locations, parv ticularly in heaters Where the metal is not to be melted, such as in re-heaters for ingots, it

is desirable to have the bottom of the furnace open as well as the top. Such a furnace is shown diagrammatically in Figure l5 with an ingot in place. The field-eliminator yoke used here differs slightly from that used in athe melting furnaces in that there is no bottom and that the ends of the laminated legs 41 are shawn as slightly turned in about the coil for the purpose of protecting the coil against injury from the ingot or other content which is being heated as well as to improve the magnetic return circuit. The furnace is externally protected by a steel acket 42. The furnace is shown as horizontal for the purpose of permitting feeding of the ingot 1n at one end and out at the other. In so far as the coil is intended to heat or treat a charge longer than itself the same advantage above the inductor coil in the other figures.

Experiments have been performed to gain ysome idea of the benefit secured by the use of the field-eliminator yokes. The experiments amply verify the statements made that my various departures from. the conditions of the closed ring with uniform permeability and uniformy Winding cause vso little stray field-andthat localized above the furnaceas to maintain very closevtotal, approximations to perfect return conditions.

These experiments lgive a fair general idea of the results to be obtained by the use of my return magnetic circuit without and with the compensating coils. They show that the 1aminated sheets alone secure a considerable advantage eveii without the windings and that increasing strength of compensating windings continues to improve conditions up to a point of equalizing the return field and with further increase in the number of turns the windings cause an increasing stray field which finally becomes Worse than that at the beginning. The amount of the stray field was determined by exploratory coils.

'I find that there was a slight difference between the. stray field when no yoke was used and the charge was in the furnace as compared with that when no yoke was used and the furnace was without charge, other conditions remaining the same. Field strengths appear at A and B respectively in the curve comprising Figure 16 in which the ordinates represent comparative strengths of stray magnetic fields and the abscissae represent the number of turns of compensating winding utilized on the yoke.

When the magnetic return was used Without passing any current through the coils the stray field dropped from B to C. l With currentthrough a progressively increasing number of turns on 'the' yoke the stray field reduced as will be seen nearly to zero` indicating a 'probable lzero curve position' at some such point as' D. With increasing number of turns -the stray field increased fromthe low points represented on both sides of the point D up to and finally beyond the initial stray Yl when no yoke Was used, the stray field in portion E of the curve representingl d to the excess magnetization frogmtre i the yoke beyond the strength o'f the due to the inductor coil.

It will be seen that the experiments pitttef in the curve amply confirm the claims as to reduction of stray field by the construction indicated. This makes it possible to entirely disre ard lthe `stray field in the dev sign of the urnace using iron or steel easing and structural iron without regard to its direction of extension or to yits continuity about the furnace.

' The improvement :of the path of return for the magnetic flux in, addition to substantially is found as in the case of charges extending vand output of' furnace. Also, since the heat transfer to the charge is limited by the -amount of current that can be passed through the furnace inductor coil'withlmt excessive heating, the reduction in magnetizing current component permits a larger percentage and hence a larger amount. of energy component of the `current to pass through the inductor coll. As the input of heat into the furnace is-proportionate to thev square of'the energy component of the'current the improvement just as if I had provided a magnetic return path of zero reluctance; yet with little current cost. The number of compensating turns is very small, much smaller than suggested by the figures, one or a very few turns being suicient in lar r furnaces.

less become evident to others skilled in the art, to obtain part or all of the benefits of my Because of the sm number of turns required it may be desirable to shunt part of the'n circuit about the compensator coil or coils so as to get the effect of fractional turns, either a of a turn or an intermediate position tween adjoining numbers of full turns. The shunt must be desi to vary the phase of the current through the compensating winding. The same method can be used to effect an adjustment of the compensation required for non-m etic and magnetic charges respectively, shi ingfrom one compensation tothe other as the charge becomes magnetic orceases to be magnetic, or to secure a fixed com romise compensa tion which shall reasonab y a proximate the exact com tion estimate for each. v

It will apparent that with the disa pearance of the stray magnetic field the pro lem of forming a returnmagnetic circuit for a solenoid free from interthreading with transformer iron has been solved; that a highly desirable means of power factor improvement'has been applied to solenoidal circuits and that a new means of inc the ou ut, eiiciency and effect o solenoi alv inductor furnaces dus been Each of these marked advances iii the art might be considered as a byroduct of one or. more of the others, accor regard -to their size for improvement of power factor; to furnaces dependent largely upon their inte use for iin rovement in the output and the e ect; and to la 1fsuornacesfor all df ese advantages an a to permit utilizing' magnetic maferial freely in the stiifening and strengthenw i of the furnace structure.

' view of my invention and disclosure variations and modifications' to meet indi- .vidual whims or'particular need will doubtinvention without copying the structure gned so as not to the dian rection of ap roachand each o the advancooperate-toward a unitary re consists in shown, and I, therefore, claim all such in so far as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention, what I claim as new and desire tosecure'by Letters Patent is v Y i. The process of assisting in the return of magnetic flux from a furnace coil which consists in electro-magnetically producing a iux external to the furnace coil, additional to the flux from the furnace coil and tending to.

assist in the return in direction of the flux.

2. The process of substantially eliminating the stray magnetic field about a furnace -inductor coil free from interthreading of transformer iron, which consists in providingan easier path of return for the magnetic flux than the path through the air about the coil and assistinrr the pass of the lin, of force through the easier path by surrounding it with a magnetizing current.

3. The process of substantially eliminating the stray magnetic field about a furnace inductor coil free from interthreading of transformer iron, which consists in providing an easier path return for the magnetic luxthan the path through the air about the coil and assisting the passage of the lines of force through the .easier path by surrounding it with a magnetizing current, the ma etic lines rof force induced by the current ing substantially equal to those produced by the furnace coil and in tending to assist the return ofthe magnetic lines of force from the furnace coil. Y

4. The process of eliminating the stray field from a furnace inductor coil free from interthreading of transformer iron, which consists in provi la yoke return circuit of ma etizable material about the inductor coil acompensating windi'ng aboutitin series with the' urnace coil and so connected as to assist in vreturningV the ilux of the magnetic return circuit.

5. The processl of eliminating the stray eld from a furnace inductor'coil free from interthreading of transformer iron, which providing a yoke return circuit of magnetizable material about the inductor coil and a compensating ha its current in seri with current o the in uctor coil, so connected as to assist in the ilow of the magnetic return circuit and the compensating coil being so adjusted as to induce a magnetic iux through its length of return magnetic material substantially equal to the magnetic flux induced by thevinductor coil through thev furnace itself.

6. The process of reducin the magnetiz ing component,'increasing t e energy component and thus improving the power factor of a currentthrough a corele furnace inductor coil, as compared with the totalcurrent'through the coil, which consists in prov iding magnetic return for the lines of force induced by the inductor about Vthe coil and in inducing lines of force in said return in phase with the lines of force m- 'duced by the furnace inductor coil and having the same direction as the return circuit of the lines of force from the inductor.

7. The process of improvingv the stirring of a melting furnace inductor free from interthreaded iron,l which consists in providing permeablev paths of returnr for the magnetic lines of the circuit about the side" and be uneath the bottom of the coil and electro-magnetically setting up in these magnetic paths being so located as to assist by its linx in re- 1 turnln the flux caused yby the inductor coil.'- Y

in inductor furnace coil, a magnetic1 flux in the direction of the return flux from the coil.

8. A hollow charge-surrounding inductor furnace coil and a return electro-magnetic circuit for the flux caused by the coil."

9. A hollow charge-surrounding inductor4 furnace coil and a coil outside the furnace coil in series with it, the coil outside the furnace 10. return' for the flux produced by the operation of the coil, comprising a return path of permeable material and a winding thereon as anv inductor coil adapted to produce ux Y lthrough the return magnetic circuit assisting ce" L l the flux from the coi v 11. A furnace inductor coil and a magnetic return circuit for Ithe coil extending alon the -ixlg ngitndinlly 'of the -t e rnac'e an a compensata winding forthe yoke legs surrounding of Yconnected `to receive current in phase with `comprising a plurality mg and inv I sides `and, throughout the greater part o` the the bottom of the coil and radial extent o electro-magnetizedito assist inflow of the return flux from the furnace coil.

-12. Aninductor furnace eoi1,areturnma netic circuit for the iiux caused by the coll comprislng a plurality of oke legs mace; outside of them and the current through the inductor coil.

13. "An inductor furnace coil, a return megi nete circuit for the flux caused coll ongitudinally of the outside .of the furnace and a compensating windingffor the yoke legs loo d outside of lllof the vlegs series wit the, indudtor coil.

EDWIN vNOBIIHRUP.`

of oie legs extendf.`

extend' i

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