US 2619603 A
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BY www? M Ummm 2 SHEETS- SHEET 1 CWM THERMOMAGNETIC GENERATOR AND REFRIGERATOR 1 9 l l w 5, w 2 M v. o 1 N m Nov. 25, 1952 c. cHlLowsKY THERMOMAGNETIC GENERATOR AND REFRIGERATOR 2 SHEETS-SHEET 2 Filed May 27, 1950 JNVENTOR.
ATTORZVEYS Patented Nov. 25, 1952 THERMOMAGNETIC GENERATOR AND REFRIGERATOR Constantin Chilowsky, New York, N. Y., assignor to Technical Assets, Inc., New York, N. Y., a
corporation of Delaware Application May 27, 1950, Serial No. 164,749 In France May 31, 1949 (Cl. S-4) Claims. 1
This invention relates to improvements in thermomagnetic generators and refrigerators, particularly such devices as are disclosed in applicants copending applications Serial No. 627,832, led November 10, 1945, now Patent No. 2,510,800, and Serial No. 635,980, filed December 19, 1945, now Patent No. 2,510,801, both relating to generators, and Serial No. 54,041, filed October 12, 1948, now Patent 2,589,775, dated March 18, 1952, and relating to refrigerators. In said applications there is described a particular constituent part referred to as composite laminations or a composite section occupying a gap in a magnetized armature and subjected to the action of a magnetic field the intensity of which varies with a designated frequency, for instance the frequency of a commercial alternating current. Said composite part itself is constituted by a plurality (or multitude) of ferromagnetic elements in the form of sections of laminae, plates, perforated plates, tubes, etc., having serially differing Curie point temperatures or having a progressive variation of Curie point temperatures, from one side to the other; these elements being so assembled that a number of ducts or passages are provided for the circulation of a liquid, or the oscillation of a liquid column (in a direction parallel to the progressive variation of Curie point temperatures) in the interstices between the elements, in order to effect alternating heat exchanges between the liquid and said ferromagnetic elements.
In the case of a thermomagnetic generator the alternating circulation of the liquid through the composite laminae produces their periodical heating and cooling in the region of Curie point temperatures, and an alternating current is induced in windings (coils) of the armature. In the case of the refrigerator an alternating current through the windings of the armature produces heating and cooling of the composite laminae by magnetization and demagnetization, and the circulation of the liquid serves to abduct the produced heat and cold.
A particularly important case is that where the liquid used is a metallic one, and most particularly is a molten alkaline metal or an alloy of alkaline metals, for instance of sodium and potassium. In this case some appreciable Foucault-current losses may occur in the composite section, as well as in the metallic liquid passing through it.
In order to reduce to a minimum, or eliminate said Foucault-currents, it is provided, in accordance with the present invention, that the alternating magnetic ux, coursing through the composite section part, shall be parallel to the alternating passage (through the same constituent part) of the metallic liquid; also that the said composite section shall be constituted of a number of ferromagnetic tubes (preferably flat) which are separated and insulated from each other and arranged, in the direction of length, parallel to the alternating magnetic eld. Said tubes should be, on their outer portions, electrically insulated from each other, so as to prevent any induction, across the totality of these elements, by Foucault-currents, which are perpendicular to the magnetic flux.
These ferromagnetic elements, constituting the composite section, may, for instance, have the form of a shear (cluster) of tubes of ferromagnetic material, with the metallic liquid circulating, or oscillating lengthwise on the inside of these tubes, parallel to the magnetic flux. else, and preferably, very iiat ferromagnetic tubes are used, said tubes being juxtaposed to each other and slightly insulated electrically between themselves, like the laminae of the armature of an electric transformer. In such case, the composite section will be very similar to the armature of an electric transformer, with the difference that the soft iron laminae are here replaced by hollow laminae (or flat tubes) made of a special ferromagnetic alloy, with, naturally, the essential peculiarity that the composition of the ferromagnetic alloy of each such tube (or such hollow laminae) will vary along the tube in a continuous or discontinuous manner from one end to the other, so that the Curie point tem` perature is highest at one end of the tubes, and
lowest at the other end, and that, between these limits, the Curie point temperature varies graduf ally along these tubes from one end to the other" in a continuous or discontinuous manner.
The tubular ends of these composite sectionsv are lengthened, or extended, by tightly soldering" or welding them to corresponding tubular clusters and/or chambers of non-magnetic material. The
chambers at the top and bottom communicatemaintain the oscillation of the liquid column.A
The composite section is subjected to the action of the variable magnetic field, suitably syn-j Orv chronized with the circulation of the liquid, as explained in the cited applications.
A practical embodiment of the invention is shown in the accompanying drawings, given as an example only, and in which Fig. 1 represents--diagrammatically,in longitudinal section, fthe" 'application of this improvement to the case of a magnetic refrigerator;
Fig. 2 represents a section along the line II-II.
of Fig. l;
Fig. 3 represents, in transverse sectionyade-Y tail modification of the hollow laminae;
Fig. 4 represents diagrammatically, in vertical' f section, a modied form of oscillation-maintainiy ing means;
Fig. 5 represents diagrammatically, inV planV viewy and partly in section, a Ir'ro'diiedv iorm'ofw generator Fig. 6 represents, in elevation and partlyrin.Y
section, the generator shown in Fig. 5;
' Fig. 7. represents adetailsection, :showing one... element in inthe-composite isection, takenf onthe.
Fig.-'8 representsradetail-sectionshowing a modified form` of elements, corresponding: tothe.l
element Shown in Fig.'7
Figa 9 represents a detail elevation ofthe ele-V Y ments shownin'Fig 8', and- Fig 10' represents a'detailfsection showing 'ani other `modified Vformr of element, corresponding tothe elements shown in Fig. 9. Y
Referring itoFigs: 1' and2,` thecompositesec-r tionsfl, l comprise-assemblies of lat'tubes-of`v suitable ferromagnetic alloys the-composition of Which-variesfrom the lines ma' to the-lines b, b", respectively, 'in such a Way that there is 'a progressivel decrease in Curie point temperatures alongtheilen'gthfbfsaidftubes, i. e., having thel material' withl' therriax'imumA Curie point. tem'- per'ature 'adjacent the lines? a, 1 a" and that With the minimumfsuch temperature adjacentfthe-lineV bjb The-"tubes'fcomprising"the sections l, l are'iengthen'edior extended at the"high end by' means 'of short tube assemblies 2, -2 'and at the" low end byfmeans of similar tube assemblies 3, 3.', the-assemblies?, 2', 3, 'being ofhomogene'o'u's magnetic material, such-assoit ironhavingv a Curiepoint temperature appreciab'ly higher' than'that ofl any part 'of' tlie sections! i Theftubes'i, 2"open into compartments or ably 'provided' withv generallyl annularexternal fins'5, Wfandwithelongated internal'ns 6,6; saidliins'serving'to' facilitatethe transfer of heat fromlfa'id in the chambers lY to the ambient fluidy alrlmnd:r tlie`i`ris5, f5. Beyond their finned f potionsthechambers"4, 4 areirconnected :byfa
tube fl (preferably-.of circul'ar'cross-'sectiom having-ifa thin :wall' of' .non-magnetic material;v fsuch' as stainles'sstee'l, and'vfithin the tube'l'is located a slidablelpistori 'adapted fto .oscillate freely lengthwiseofth 'tube 'and made of' a magnetic material. f'lAroundtheftubel is placeda Winding or coilfS,'-having'connections `for receiving an alter-ri'ati'n'g"current'f'l anda ring V'Il'l' of magnetic material encasing'thec'oil." The piston 8 iscenltere'd'by'`n1eans"ofspringgslf, I I* which 'are at# tacledtothe'walls of thechamberst, Il', as by a closed circuit which is filled with a column of metallic liquid terminating on opposite ends of the piston 8, the natural frequency of oscillation of said column being regulated mechanically by adjusting the tension of the springs H, Il so that .it Will equalA the frequencyf the.alternat ingcurrentfsupplied -to the' coil 9. .The piston 8, magnetically coupled to the coil 9 and ring I0, thus constitutes an electro-mechanical oscillator which assures maintenance of oscillations of the column at the frequency of the current supplied. A similar oscillator may be applied to the tube it, vif necessary, replacing all or part of the fins, but 1this .isfgenerallyl not desirable since this is thezpartofthet-system which absorbs heat at low temperature to cause the refrigerating effect, as described in'application Serial No. 54,041.
In'thisapparatus the Curie effect results from the :provision of a permanent magnet l5 having pole pieces (or shoes) I6, H, with windings I8 thereon# The ,endsof the polepiecesare-in communication With the tube. assemblies. 2, 2 and.;-l 3, -3,` respectively,4 sothat the magnetic .iluxzis .f divided in two closed circuits, completed through 1 vtl'iestube` `assemblies I I, constituting.composite..`
sections With a progressive. variation of Curie i point temperatures, `as Vdescribed above.` Alter-v nating magnetizing i and demagnetizing .currents pass through the. windings IS `(generally'fromvw the samesource which-supplies the coil. lil) .insuch a manner thatrthe magnetic fluxofthe mag-- net I5 is closed Valternately inthecircuit through` section i andinthe.- circuit through-section. l?, withv said flux varying inv intensity` in saidsec-f tionsV but .keeping Aits sign -.unchanged. The IiiuX in section .I is dephased by 1309 with. respect to the flux` insection I! and .the flow of metallic-fluid through said sections is suitably.synchronized-in. phase with thefpassageof -Vthe-magnetic flux therein.
Itis intended-that theV flat tubes constituting the sections i, l' should ble-insulatedromaeachn other by airi'spaces orlayers of insulating mate-. I rialv i i, and the-extensions of said tubes inthe assemblies 2, 2', 3, -3 `are -likewiseinsulated fromeach other,.in order to prevent the. formationpf.- Foucault-.currents in said` tubes or in the liquidw flowing -through them. The pole pieces, [55,11 are, naturally,l also laminatedfor the samefpurf.
In the modiflcation'shown Ain Fig-3, anyor-all of the flat tubes in sections i, I or theextensions .Y thereof may. be-subdividedvby providingaplurality of ribs, walls 0r spacers l! running across', the small dimensionioffthetubes anddongitudinally thereof, so as to prevent deformation lof the; tubes by internal orexternalpressure.
The same systemfcan readily be adapted4 for reducing. the Foucault-currents in .the :case` of the thermo-magnetic .generators,- lutilizing .metallic liquid, which are disclosedin applications Serial- No.527,832 and Serial, No; 635,930.1 Preferably, however, when` great-differences of theV Curie point temperatures between ends of the composite sectionc-(for instance of several hundred-degrees C.) are utilized, it is preferredtoplace the elec-H tro-mechanical oscillator Vin the cold .tubeof the generator. It. is also necessary to separate-the. end .of the. .permanentmagnet orit-s polepieces.. adjacent to .the hot ferromagnetic tubes,.by,an appropriate layer of Aa heat-proofthermal` in-v` sulating material, in order that thecorresponding pole of the magnet can be kept at a sufficiently lowtemperature,.consistentwiththe preservation of Aits .-magnetic.l properties. .i
As an alternative, the permanent magnet may be replaced by an electro-magnet, through the winding of which flows a constant current, obtained, for instance, by the rectification of the alternating current normally supplied to the refrigerator, or generated by the generator.
The invention also comprises a method and means for maintaining the liquid metal column in oscillation, without utilizing any moving mechanical parts whatsoever, but solely by means of heat and of the alternating electric current.
In order to achieve this result, the electro-mechanical oscillator previously described (8, 8, l0, Il, Il', I2, I2' of Fig. 1) and the tube 1, are omitted. The two ends of the metallic liquid column are each provided with an electric superheater, wherein the said ends of the column are subjected, alternately, to successive superheating by the alternating phases of the rectified alternating electric current. Each such superheating produces instantly a mass of metallic vapors under pressure, which act alternately on the respective ends of the liquid column; the vapors at one end being condensed as the liquid at the other end is vaporized. The work performed by these vapors serves to maintain the oscillations of the column. This mechanism is represented in Fig. 4. (wherein the portion below the chambers 4, 4 is omitted, but may be similar to that shown in Figs. 1 and 2), the two superheaters, adjoining the hot" chambers 4, 4 being formed by conical chambers 20, 20', with exterior fins 2l, 2i and interior fins 22, 22', adapted to act as condensation chambers for the metallic vapors. The small ends of the chambers 2B, 2Q are in communication with the tubes 23, 23', respectively, which tubes have relatively thin Walls of steel, with a very great specific electric resistivity.
Tubes 23, 23 are hermetically closed at the top and are preferably capped by metallic blocks 24, 24' for receiving the heavy electrodes for the rectified heating current. Annular blocks 25, 25 for receiving the opposite electrodes are located at the bottom of the tubes. Suitable thermal insulation 25, 26 may desirably be applied around the tubes 23, 23 and blocks 24, 24', 25, 25'. Thiel; walled metallic tubes 21, 21 may optionally surround and enclose the tubes 23, 23', said tubes 23, 21 and 23', 21 being insulated from each other by thin layers of electrical insulating material 28, 28', which also insulates tubes 21, 21 from the blocks 24, 24', 25, 25. Consequently, the electric current passing through tubes 23, 23 and through the liquid metal column in said tubes, does not pass through the tubes 21, 21'.
Tubes 21, 21 are intended to form a solid support for tubes 23, 23, so that, because of such a support, said latter tubes may have very thin Walls and can resist even a great internal pressure of the metallic vapors, while absorbing very little electric energy. The rectifying arrangement 29 and transformers 30, 3Q receive the alternating current and transform it into direct current pulses of low voltage and high intensity which are sent alternately to the tubes 23 and 23. A Wall 3l preferably separates the area of the chambers 4,4 from that of the superheaters and condensers of the metallic vapors.
In the case of the refrigerator, the primary current for the superheater is also the alternating current operating the refrigerator. In the case of the generator, the primary current may be the vcurrent produced by the generator itself.
` The diameter of tubes 23, 23 should be large in relationto the thicknessof the tubes walls, in
order that the cross-section of the liquid metal in the tube will be much larger than that of said walls. Since the resistivity of the metal of the tube is chosen so as to be much greater than that of the liquid metal (for instance, potassium, sodium or their alloy), it is easy to provide that the resistance of the tube walls can be, for instance, at least several scores of times greater than that of the column of liquid metal and that almost the whole electric current shall pass through the metallic liquid, instantly superheating it above its boiling point temperature.
Thus each passage of the current through the column of liquid metal will produce instantaneous ebullition in the corresponding tube. Such a passage of the current is therefore immediately followed by formation of metallic vapors under pressure, which will push the column downward and will cause the liquid metal at the other end of the column to enter the other tube, and so on, with the metallic vapor being each time condensed in the condensers 20 or 20 which are maintained at an appropriate temperature by suitable means.
This mechanism can be used for both the refrigerator and the generator. The superheating must be sufficiently great to produce pressures of the metallic vapor, which will cause the mass of the liquid column to form an oscillating liquid metallic circuit having a frequency approximating the frequency of the electric current.
Generally comparable arrangements for assuring the oscillation of the liquid column having been described and shown in the copending application of November 10, 1945, Serial No. 627,832, now Patent No. 2,510,800, (Figs. 1l, 12, 13 and particularly 14). The mechanism described in the present invention has an advantage over that shown in said Fig. 14, in that the tubes, wherein occur the instantaneous superheating and ebullition of the metallic liquid, are all metal (for instance, of highly resistant steel) and that any contact of the hot alkaline metal with any heatproof material is entirely avoided. This eliminates the possibility of any chemical action between the hot alkaline metal and the heat-proof material and permits the use of higher` superheating temperatures of the liquid metal and its vapor.
The result, in accordance with the present invention, is that each phase of the rectified alternating current, passing through the end of the liquid column, produces practically instantaneous superheating and ebullition of the metallic liquid, and thus the frequency of oscillation of the liquid column is definitely fixed by the frequency of the electric current, without necessarily having resort to the oscillation of any mechanical parts whatsoever, even auxiliary ones (piston, spring, etc). This is true both in the case of the generator, and of the refrigerator.
In the case of the generator, it is obviously possible to use, for the heating and ebullition of the ends of the liquid columns, the rectified and transformed electric current of a comparatively weak independent source of alternating current, thereby eliminating any moving parts in the generator itself.
1t is likewise possible to obtain any desired definite frequency by any other mechanical or electric means, merely by sending current impulses of the desired frequency for the heating of the metallic liquid in tubes 23, 23.
Although the thermomagnetic oscillator, represented in Fig. 4 communicates to the parts ac- 'indicated by arrows (Fig. 6).
4said free space may-'beprovided'in two :terminal bulbs attachedtoltheupper parts ofi-the tubes. 23, 23211 Said terminal bulbs, generally of metal,may also form the condensers for'the metallic vapor, 'and can bei equipped, both on the inside 'and the outside, with .appropriate ribs. In such case, the iinsA 2|, 2|', 2.2,A 22 canbe dispensed with. The said-bulbs can, ifV desiredbelled withinert gas under pressure, forming,vwiththe-liquid column, a system possessing a suitable desired natural `frequency of mechanical oscillations.V
The 'start of .operation of the system can be effected, for instance, by making thev apparatus dissymmetric, or possessing an inclination in such a mannerthatthe liquid, lling one of the tubes 23` or'-23" shall,` when at rest, touch the upper electrode.
In a modication, shown-diagrammatically in Figs. 5 and 6, the magnetic circuit, comprising the permanent magnet, the pole pieces or armatures and the composite sections, on the one hand, and the closed circuits for vthe circulation or oscillation of the metallic liquid, on the other hand, are located inplanes that are perpendicular to each other.-
In Fig. 5 thereis shown, in plan View and partlyin section, the magnetic circuit consisting of the magnet` 32,' the armatures 33, Vand the compositeasections interposed between opposite ends ofv the respective armatures and formed by flattened tubes 35, Yeach tube being made, for instance, by welding together the opposite edges of two pieces of sheet metal, having a space 36 between` them for the circulation of the metallic liquid. Adjacent tubes are insulated from each other, for instance, by `thin layers 31 of oxide or -anyother appropriate insulating material, in orribbed or plain.
Fig. '7 shows one of the tubes 35 (seen from the side); its Walls have, at one end, a maximum Curie point temperature of Tn, and at the other end the minimum Curie point temperature of T1.
The directions of flow of the metallic liquid are The tubes Wall is formed by multiple bands which are juxtaposed and-"welded together at the edges,l each successive vband having its -ownlCurie point temperature, and said temperatures rising progressively andv filling. the-interval of` tempera-tures between 'Fi-and Tm Or 6159,; the-walls may-have, from Ythe tubes ce.
one end tothe other,-a continucusevariaticn of -ternal ns d l and external ns 42 tofacilitate the thermalexchanges. The two'opposite collecting chambers iL-d are connected by a tubeY 43'containing a piston arrangement and electromagnetic'operating means (not shown) ofthe same type as shown'in Fig. l, for causing the metallicliquid column to oscillate.
- At their upper ends 'ther tubes 35 communicate individually with similar tubes 4 (outside the composite sections) which, in turn, connect with corresponding tubes in the opposite composite section. The tubes 35 are electrically insulated from each other by insulating layers 37,5'While spaces '16 are left between the adjoining tubes 44 .for the circulation of a hot, or cold, uidbetween the tubes. The walls of the tubes B4 are made of homogeneous metal, which is preferably non-magnetic (so as to avoid the parasitic deviation of the magnetic ux).
The individual tubes lid are sufiiciently spaced apart so as to leave, in the interstices t6 between them, room for the circulation of an appropriate fluid which will insure the supply of heat (or the removal of heat, as may be required) to (or"frcm) the metallic liquid, circulating inside these-tubes. The means insuring this circulation'of the nuid between the said tubes, outside the composite sections, are not shown on the drawing.
The tubes 415 can be separated by supports indicated at il in Fig. 6, and appropriatelyY placed in the spaces 6K6. An exterior wall or casing 48, similar to walls 39 and constituting, if desired. the prolongation of said walls, may be extended over The supports and the exterior wall i8 have the purpose to resist the deformation of the tubes by internal pressure. Also, if necessary, the walls of the tubes 44, not being submitted to rapid alteration of temperatures, may have essentially thicker wallsV than the tubesf'f35.
The supports @l must'be'insulat'ed from, the tubes @il by a thin layer of insulating material, for instance, a layer of oxide, in order to' oppose the passage of Foucault-currents, or these supports may be of insulating material, as shown.
Modified forms of tubes are shown in Figs. 8, 9 and l0, in which each attened tube is replaced by a range of several (or of a multitude of) square or round tubes, juxtaposed and preferably welded together along their length, the adjacent ranges of these tubes being electrically separated or insulated from each other.
Fig. 8 shows two'Y such parallel juxtaposed vertical ranges of tubes 49, forming a composite body, these vertical rangesbeing insulated from each other by layers of insulating material 5B. Such tubular ranges may-be substituted Vfor the tubes 35 of Figs. 5to 7.
Fig. l9 shows a .detailelevation of`r Yonefofitl-e Fig. 10 shows a variation in which the tubular ranges are made of two undulated metallic sheets l, provided with ribs 52, and being welded together through these ribs, in order to resist the interior pressure.
When the tubular ranges of Figs. 8, 9 and 10 are substituted for the ilat tubes 55 in the composite sections the separate tubes oi each range may be merged upwardly into the nat tubes such as tubes 44 and downwardly may open into chambers 4l) as shown in Fig. 6. Alternatively, the wide ilat tubes 35 may be merged upwardly into subdivided tubes (replacing the tubes t4) formed substantially in accordance with Fig. 10 or ranges of tubes as in Figs. 8 and 9.
As the tubes 44 or corresponding tubular ranges may have different lengths, it is provided to compensate these differences by corresponding differences in the interior cross-sections of the composing tubes, in order to equalize the dynamic and irictional resistance for the liquid in different tubes or ranges.
rEhe tubes or ranges 49 or 5l, the walls oi which comprise composite laminae, having, one end to the other, progressively varying Curie point temperatures, may be called active Their extension, in the form of tubes 44, of homogeneous metal, 'may be called passive The role of the passive parts is to insure thermal exchanges for the liquid metal which these tubes contain, such exchanges being effected without uniting the separate small streams of liquid metal in a collecting chamber, where Foucaultcurrents could form. This dissymmetry in the closed circuit of the circulation of the liquid metal is essential for the reduction of Foucaultcurrents. Said dissymmetry consists in the fact that at one end of the two composite sections there are located the passive tubes, separated from each other (so that, consequently, no Foucault-currents can be generated), and at the other end there are located the collecting chambers for the metallic liquid. It is due to this separation from each other of the adjoining tubes-active or passivethat the electromotive forces induced in the collecting chambers of the metallic liquid are of opposite sign and neutralize each other.
Obviously, the collecting tube L13, with its piston-equipped device for maintaining the liquid column in oscillation, may be replaced by the actuating device shown in Fig. 4 for maintaining the oscillation of the liquid column. If desired, in order further to reduce the Foucault-currents, the oscillating liquid column may be divided into several juxtaposed parallel columns, by substituting separate passive tubes for the chambers 40, 40 and tube 43, each such tube having its own means (of any suitable kind) for maintaining oscillations.
What I claim is:
1. In a thermomagnetic apparatus having a polarized armature in a gap of which is located a composite section of ferromagnetic elements with different Curie points arranged across the section in regular order of Curie point temperatures and adapted to be heated and cooled in the neighborhood of said temperatures by the passage of fluid heating and cooling means; means for effectively reducing the formation of Foucaultcurrents comprising, hollow tubular ferromag- 10 netic elements constituting said section and adapted to conduct said heating and cooling means in the direction of variation of Curie points, and means for insulating said elements from each other.
2. Means according to claim 1 in which said elements are substantially iiat tubes.
3. Means according to claim 1 in which the direction of variation of Curie points corresponds to the direction of magnetic iiux in the section.
4. Means according to claim 1 in which said elements are substantially flat tubes subdivided internally by one or more walls extending in the direction of variation of Curie points and in which said direction corresponds to the direction of magnetic flux in the section.
5. Means according to claim 1 which includes electromechanical means for oscillating the heating and cooling fluid through the composite section.
6. Means according to claim 1 in which the armature is bifurcated and in which there are composite sections located symmetrically in each branch of the armature.
7. Means according to claim 6 in which said elements are substantially ilat tubes.
8. Means according to claim 6 in which the direction of variation of Curie points corresponds to the direction of magnetic flux in the sections.
9. Means according to claim 6 in which said elements are substantially iiat tubes subdivided internally by one or more walls extending in the direction of variation of Curie points and in which said direction corresponds to the direction of magnetic ux in the sections.
10. Means according to claim 6 which includes electromechanical means for oscillating the heating and cooling fluid through the composite sections.
11. Means according to claim 6 in which the direction of variation of Curie points is transverse to the direction of magnetic flux in the sections.
12. Means according to claim 11 in which said elements are substantially flat tubes.
13. Means according to claim 11 in which said elements are substantially iiat tubes and in which separate insulated tubes connect the elements in one branch of the armature With respective elements in the other branch thereof.
14. Means according to claim 11 in which said elements are substantially flat tubes subdivided internally by Walls extending across the short dimension thereof and lying in planes parallel to the direction of variation of Curie points.
15. Means according to claim 11 in which said elements are substantially flat tubes, each said tube being constituted by a range of juxtaposed small individual tubes and adjacent ranges being insulated from each other.
REFERENCES CITED The following references are of record in the file of this patent:
UNITED STATES PATENTS Number Name Date 428,057 Tesla May 13, 1890 1,406,576 Murray Feb. 14, 1922 2,016,100 Schwarzkopf Oct. 1, 1935