US2474420A - High-frequency dielectric heating apparatus - Google Patents

Description

June 28, 1949. L. B. HIMMEL HIGH-FREQUENCY DIELECTRIC HEATING APPARATUS Filed July 16, 1945 J3. RM m Patented June 28, 1949 HIGH-FREQUENCY DIELECTRIC HEATING APPARATUS Loran B. Himmel, West Des Moines, Iowa, as-

signor, by mesne assignments,

Carroll to Ross M.

Application July I6, 1945, Serial No. 605,394

11- Claims. 1

This invention relates to an electrical apparatus and: particularly toa dielectric furnace system. This application. is a continuation in part of" my prior application, Ser. No. 411,711, filed September 20;. 1941,. now abandoned. In a dielectric furnace; high. frequency electrostatic fields are impressed uponsuitable material. which constitutes thefurnace load. By virtue of losses present this furnace load, heat may be generated.

I have discovered that a quarter wave open wire transmission line: having one end metall icall'y short circuited and the: other end loaded with a dielectric: furnace. has remarkable properties. A simple way: in: which oscillations may be generated in such a line is to'conncct thereto. one or more vacuum. tubes having. three or more electrodes sothat the transmission line forms a tank circuit. The line may have forming part thereof or attachedthereto at theends suitable electrodesbetween. which. processing may occur.

system of this: character has remarkable properties and characteristics; which have never been hitherto obtained in any system. Thesecharacteristics and properties briefly are as fol lows. The" radiofrequencypotential impressed across the work electrodes is always the maximum potentialgenera-ted. Itis possibleto maintainpotentials at the electrodes far greater than has hitherto been possible: The frequency at which the system operates is optimum and tendstoadapt itsel-f'to' o'hangesin the load. The transformer property of a. quarter wave lineprovides good impedance matching. Efficiency is: greater than hitherto been available. The system does not radiate power.

With reference to the maximum potential at the electrodes, a. tank circuit consisting of a quarter wave transmission line has lumped ca-- pacitance concentratedv at: the f urnace: electrodes. There is no lumped. inductance and no other capacitances through which tank currents may flow to dissipate power. The distributed inductance" and capacitance of the transmission line are inherent in. any'systerrr having a transmission line; These are not merely tolerated as is true in theprior art but deliberately used. This use imparts aremarkable operative stability to the system under various load conditions. The combination of quarter wave transmission line and 2. physical properties of. a load may change substantially and: suddenly. In a system embodying my invention, the changes in load constitute the sole variable for changing frequency during a working cycle. The system follows the load changes and. maintains maximum electrostatic potential differences across the load at one fre:- quenc resonant for the system. A highly desirable property of a quarter wavetransmission line system with suitable load is that the physical simplicity provides only one natural resonant frequency. In. systems having coil inductances or coupled circuits,- it is war known that there may be several resonant frequencies, thus making it possible: for parasitic oscillations in other modes to exist. This robs the load and tends to shorten the lifeofvarious elements.

A further highly desirable property of a system embodying. this invention resides in the fact that the open construction of the tank permits the use of any desired construction of conductor. With a conventional cjoil: type inductance, it is necessary to use conductors which can be coiled. 0n th other hand, a construction embodying this invention may utilize a conductor in any desired form designed. so that it conducts with minimum. loss. No considerations or susceptibility to coil formation need exist. It is, therefore, especially suitable for high power ratings.

A further highly'desirablc property of a quarter wave transmission line resides in its ability to function as a transformer. High and low impedances. may be matched by choosing proper portions of the line to: which connections may be made; Impedance variations are also matched. Thus, the load may go through wide impedance: variations and result in relatively smal l impedance variations where the tubes are coupled. This is: a highly important consideration. At high frequencies, matching is of vital importance for effectivetransmission of power and avoidance of unnecessary losses. Vacuum tubes have certain impedance requirements and characteristics while loads upon which the system will work may have their own impedance characteristics, which characteristics do not even remain at a constant value during a processing cycle.

Another important'characteristic inherent in a system embodyingthis invention resides in the geometry of they construction. Thus, as long as separation between the opposite portions of the line is not excessive from the point of view of radiation. the line may be shaped or its length ad-f 3 justed in any manner desired to accommodate work.

A system embodying the invention herein has substantially all components and leads at such radio frequency potentials that ground is a point of substantial symmetry at all times in the radio frequency field. The result is that there is little or no radio frequency unbalance and radiation is prevented to a degree far beyond what has hitherto been considered possible.

In practice, the quarter wave circuit and load has operated at frequencies ranging above a megacycle per second and may go up to one hundred megacycles and more. As a rule, tube losses and physical dimensions of parts determine top frequency limits. Frequency-determining load capacitances may be substantially less than the tube capacitances in this system. Thus, greater leeway in tube choice is possible while maintaining desirable tank conditions.

It is preferred to maintain a space between work and electrodes, said space having a high degree of insulation. Such space may be either air or material having low dielectric loss. Thus, the furnace capacitance consists essentially of a component which is substantially constant and the work which may vary. As a result of this, the variation in load capacitance is reduced. A system embodying the invention permits wide ranges of load conditions with resultant smaller changes at the tube terminals.

Under certain conditions. a grounded electrode may be disposed at the furnace in proximity to the work. Such an arrangement has been known to permit a noticeable increase of power absorbed by the load.

The entire tank may thus have the general shape of a U (a V shape is considered as falling within this definition), depending upon the disposition of the conductors forming the short circuit at one end of the transmission line. The arms of the tank need not necessarily be parallel or straight and, in general, the ends of the arms are adapted to be bent or shaped to accommodate work. The ends of the arms may also be adjusted a:

for length over a substantial range. While this may have a tendency to disturb the symmetry of the system, such lack of symmetry is not serious enough to alter the highly desirable normal operating characteristics of the system.

In other systems, the oscillating system is a complete entity distinct from the cables leading to the work. In such prior art systems, the cables unavoidably constitute a transmission line having substantial electrical length and require the matching of oscillator to line and line to load. It is for this purpose that prior art systems tune the oscillator so that effective transmission through the leads to the load may be provided. This requires auxiliary circuit components wherein power may be wasted. Furthermore, during processing of many materials, load changes occur too fast for tuning adjustments. In distinction to this, however, the transmission line in this invention itself functions as the seat of continuous oscillations. Thus, any shaping or variation of length of the lines adjacent to or around the load has no deleterious effect electrically. The system instantaneously adapts itself to load requirements as an inherent property.

Referring to the drawing, Figure 1 shows a system embodying one form of the invention. Figure 2 shows a modification.

Open wire line consisting of conductors I and II meet at I2 for convenience referred to as a 4 junction though the conductors may be continuous, and have free ends I3 and I4. Junction I2 is preferably at or near the geometrical center of the continuous metallic conductor extending between free ends I3 and M. This open wire line and shorted end constitutes a tank and may be of the usual construction for handling radio frequency currents. Thus, the conductor may be fabricated of one or more hollow tubes, pipes, stranded cable or the like.

However, for substantial power particularly involving heavy currents, it is preferred to form the tank at least in the neighborhood of the region around junction i2 as a plurality of separate metallic conducting elements I6. These elements are preferably spaced from each other, but disposed symmetrically to form a generally tubular cage construction. At frequent intervals, rings Il may be disposed either outside or inside of the cage structure to maintain the separate elements rigidly in position.

Rings I! may be of insulating or conducting material. Insulating material may tend to reduce losses due to differences in potential circumferentially around the cage structure. However, in practice, conducting rings are satisfactory and easier to make and install. Each element I6 may be solid wire, stranded wire, rod or tubing depending upon how finely divided the cage construction is made. To provide good surface conductivity, it is essential that copper or silver along the outer surface of the various conductors be used. It is understood that the drawing is merely illustrative of a cage structure for number and spacing of elements I6 and, in practice, an extensive conducting surface is desired.

It is well known that in a transmission line a quarter wave length long with one end short circuted and the other end open circuited (as far as a metallic connection is concerned) a voltage minimum and current maximum will occur at or near junction I2. As far as radio frequency is concerned, node I2 may be fixed and prevented from wandering along the conductor length if desired by grounding this point through condenser I5. As will be explained later, some physical adjustments of the conductors may throw node I2 off from the geometrical center of the tank. Furthermore, in some instances, some lack of symmetry in loading or attenuation may tend to move the point of electrical symmetry away from node I2. However, this has no substantial undesirable effects and, in any event, may be corrected quite simply by adjusting the line. With load, node I2 is spread out to a region of several inches along the tank.

At any suitable points preferably spaced a substantial distance from node I2, the distance being in terms of electrical wave length, conductors I0 and II may have joints 20 and 2I. These joints may be either of the universal type permitting movement in any direction or simply pivot joints permitting bending or adjustment of one portion of the conductor with respect to another portion. For ease in manufacture, joints 20 and 2! may have metallic pipe sections adjacent the pivots. Such sections may be soldered or sweated over the tubular cage construction of the conductor proper.

Beyond joints 20 and 2I may be conducting portions 22 and 23. These conducting portions need not have as great a carrying surface as is present near node I2, since currents decrease in intensity away from the nodal point. Conductors 22 and 23 may telescope with conductors 24 and 25, these conductors preferably being slidable within conductors 22 andflto adjust the over-all=conductor length. Conductors 24-1and' 25' may have pivot joints 26 and 21 near free ends 1-3 and ll ot the quarter wave :line. ='Conducting "portions 22* and 23 may be advantageously formed of perforated metal pipe or tubing. Similarly- conductors 24 and 25 maybe formed of perforated metal tubing so that the telescopicaction may be smooth.

Free ends l3 and M carry-electrodes :28 and 29 having any desired "area andcon'figuration. These electrodes are preferably formed of wire, gauze or perforated metal, it being mnderstoo'd that the metal "has good electrical conductivity. Thus, copper, brassor silverpl'ate inay' berelied upon. By virtue of the various joints and telescoping conductors, the *physical length of the quarter wave line --may "be'variedand' the shape of the line may be changedsas =desi-red. Work 30 disposed "between electrodes 28 and 2'9 may be treated and will be considered as the load. Grounded electrode may be used if desired anddisposed as desired.

The spacing between -line conductors lil'and :H should be small in terms of wave lengthito pre vent radiaticno'f power. "However, under some conditions, it may be "desirable to spread the line apart for mechanical reasons.

The actual physicalilength of line between free ends [3 and H is a matter "of design andds "determined by various factors. Thusg-an important factor is the frequency range over which the system is to operate. Another Afactor is 'the amount ofpower tobe handled. A physically shorter length of conduotor is required for mcving the frequency range higher. However, in

order to handle desired currents, the transverse dimension of conductors l'o and tl mayhave' to be quite substantial. Inasmuch as it -is highly desirable to avoid sharp bends or curves 1 in the conductors, it is A evident that the transverse dimensions of the conductors will be -a factor in the tank dimensions. In addition, excessively close spacing between the opposing parts f :the line particularly nearthe high potential-ends l3 and I 4 may induce xbreakdown' through airvor other insulating medium.

In order to excite theitransmission linaoneor two, or even more, vacuum tubes mayrbeprovided.

Thus, vacuum *tubes 31' and C3 2 .mayibe provided.

The system will operate with onlypne tube. and,

if two'tubes are used, itnisdesirablegthough not necessary, thatthe two tubes havesimilar-characteristics. In thisx-way, electrical andcphysical symmetry will result. However, dissimilaratubes may be paired and :thelconn'ections altered accordingly.

As shown here,itub'es:*3=2 andf32' :ha've. cathodes 33-and'33 of: any suitable type. Thesacathodes are energized by transformer windings 34 and? respectively whose centers .35 and -3 imay :be

grounded. Thevacuum:tubes .have-.-control grids "3 and 3'! connectedby conductors .381 and 38 to blocking condensers 3:9 and i.39'-and,thence to points 40- and 40 on'theitank. Grids 31 and'dl' are connected to :their corresponding cathodes through resistors 36 andtfir-espectively.

Tubes 32 and F32 have anodes145iand 45! con nected through leads lfi and 46. "to connectors 41 and 41' movable .over 31inesl0and2tl. Itis preferred to'haveboth grid andianode connections to the line adjustable .to permit proper choice of connecting points. A suitable source 18 of high potentiaLseither direct or-alternating,

isconnected between node i2 on the :tan-kand :ground. :It is un'derstoodthat thevtankirequency is largely determined bythe tank circuit proper. The vacuum tubes merely act to feed pulses of power into the tank.

It will be observed that grid and anode connections of a tube are on opposite sides of voltage node l2. Thus, proper phase relationship between gridand plate will result. Theexact point of connection alongthe'lineis gen erally notcritical and, atno or light loads, may-ibe varied within wide limits. As 'a rule, physical movementof the grid connection "results in a greater electrical change than the same physical movement-of the 'anode connection. This-of course, isdue to the risin potential away from node 12.

As theloading on thetankdueito furnace-op eration increases, thisvoltage rise tends to flatten out. Under some circumstances, the adjustment for the grid and anode connections :to the tank circuit must be made during such loaded conditions. With no load, the Q of the tankis so high that an adjustment of grid and anode connections satisfactory for load will generally "be tolerable to maintainoscillations atxno or light loads. Obviously, as 'the loading falls off, the power input to the tank fallsoif. This is animportant consideration in intermittent loading. Thus, the system embodying this invention -inherently has a high Q with the decrease in Q -being due solely to the'load.

As is well known, at node l2, currentand voltage are ordinarily innzphase. The twoget out of phase as points li3zand Mare approached. Thus,

adjustment of the grid and anode connections along the line not only afiect the potentials 'on these tube electrodes but also have a tendency :to affect the reactance faced by the tubes. The amount of reactance along the line varies in a manner generally well'known in the art. Thus, a reactance match between tank'and tubes isalso inherent inthe anode and grid adjustments. The

further the distance electrically from node t2, the greater'the eifective'reactanoe presented by theline. This, of course, iswell known in transbe necessary to move the grid take-off point on -two brushes may.be merged into one physical structure.

Hence, it follows that, while the grid take-01f point of one tube is oppositely phased from the anode take-off point for the same tube,

it is not necessary "that the grid .take oii point beat a lower radio frequency potential thanthe anode take-off point for the same tube. The actual radio frequencypotentials present in the grid and anode of-a tube are important. Theoperating characteristics are also important.

It is possible to dispose a pluralityof 'tubesin parallel for eachtube shown. Thus, anumber of tubes may be connected at the samepointsliin the tankor'mayiberconnected at difierent points along the tank. :If such tubes are staggered or 7 laddered along the tank, it will in general be desirable that the tubes have different characteristics, since their connection points are at diiferent distances from the voltage node. This, of course, is different than a simple paralleling of tubes to increase the power handling capacity.

The quarter wave line thus functions as a transformer upon which may be hung vacuum tubes and loads in the most effective manner possible. Between electrodes 28 and 29, suitable load 30 either stationary or movable may be disposed. Air gaps or low loss spacing between electrodes and load are desirable to stabilize the normal mode of oscillation. A grounded electrode 3| may be disposed between or adjacent the furnace electrodes under some conditions.

It is not essential that the work be disposed geometrically between the electrodes. Thus, in certain installations, the electric field between opposing electrodes may curve so that the work is outside of the geometrical region between elec trodes. By this is meant that straight lines joining the opposed electrodes will not necessarily intersect or enclose the work. It is clear, therefore, that work may be either between or adjacent the work electrodes.

A true load acts so that the lines of force are drawn toward it. This may eliminate the necessity for a grounded electrode apart from potential considerations.

In certain installations, particularly where the surface presented by the load is extensive, such as might be the case in large sheet material, it becomes impractical from both a physical and electrical consideration to have electrodes on opposite sides of work material. To dispose electrodes on opposite sides of parts of wide webs or sheets would require such long physical tanks as to reduce frequency characteristics to an undesirable value. In particular, the large physical length of such a tank may make it impossible to operate it at a desired high frequency.

To overcome such undesirable factors, it is possible to dispose the electrodes as shown in the dotted line position, so that they are more or less in the same plane and face a load as shown. The load may be a sheet or travel on a suitable conveyor or be stationary. The electrodes need not be parallel or symmetrically disposed. Be-

yond the load may be disposed grounded electrode 3|, this serving to distort the electrostatic field toward the load. The grounded electrode may be omitted if desired, or there may be several grounded electrodes.

The actual physical length of the line will naturally not correspond exactly to the theoretical length of a quarter wave line operating at the frequency at which the system actually operates. In fact, as load conditions change, the resultant variation in frequency may go through a substantial range, thus changing the electrical length without any physical variation in the length of line.

Under normal conditions of operation, there is a voltage node in the furnace system between the free ends of the line. This merely means that, at some region in the furnace, there is a point corresponding in general to node l2.

The conductors may be adjusted by movement around joints 2!) and 2| as well as adjustment of the telescoping sections. Similarly, the furnace electrodes may be adjusted on the pivot joints. In making all these adjustments, it is not necessary that precise equality of physical length on the two sides of the node be maintained. For

a permanent installation, it is preferred to have as much physical symmetry as possible. However, I have successfully adjusted the arms to various positions without impairing the satisfactory operation of the system.

Thus, the entire system under operating conditions has a high degree of symmetry to ground. This tends to eliminate radiation.

Another desirable feature of the invention resides in the fact that the furnace electrodes are both at a high positive potential to ground if the high voltage source supplies direct current. It is, of course, possible to reverse the entire system so that the furnace electrodes are at a negative potential and the cathode connections to the tubes are interchanged with anode connections to the tubes. During the operation of the system, radio frequency voltage oscillations at the furnace electrodes will tend to reduce the potential of said electrodes below the normal static value. However, the amplitude of such oscillations may be controlled so that the average potential at both furnace electrodes may be well above ground during normal operation. If such average potential is at a sufficiently high value such as of the order of five thousand volts or higher, then precipitating action on fine solids between the electrodes may result, assuming that the load is of a liquid or gas nature. Even with solids, some migration of particles may occur.

It is understood that no attempt is made to show proper proportions of physical dimensions in the drawing. This is particularly true of electrodes, their relative positions, the relative areas of the electrodes and load, the spacing between electrodes and load and between the grounded electrode if one is used, and the remaining portions of the furnace. All these are matters which must be varied for individual requirements.

The tank maybe supported on suitable posts 58 of low loss material such as quartz. The vacuum tubes and leads at high radio frequency potential such as anode and grid leads may be suitably supported by low loss material. The entire system may be considered as having an axis extending from voltage node [2 between conductors Ill and l l and through the furnace between the furnace electrodes. Such an axis should lie in an equi-potential surface extending throughout the entire system. Theoretically, in the absence of any ground surface, such an equi-potential surface would consist of a fiat plane bisecting the entire system and perpendicular to the drawing, assuming, of course, that all the parts are symmetrically disposed. The axis would be a line in this plane exceeding from node [2 to the furnace and symmetrically disposed with respect to the various portions of the system. In actual practice, the furnace system must be supported with reference to ground. The tank itself under normal conditions may be supported so that the transmission line is disposed in a horizontal plane or a vertical plane or in an intermediate position. By disposing the various portions of the system so that corresponding portions of the system on opposite sides of the axis are symmetrically disposed with respect to ground, radiation may be eliminated or reduced to such a low value as to be negligible.

It is possible to support the entire system so that electrical symmetry to ground is obtained without necessarily relying upon physical symmetry. Thus, if the tank for some reason must be supported in a vertical plane with one conductor above the other so that the conductors e1 atures ta k '9 i Figu e 1, lehh he ha ste h leh t r i 6!. To node 6|, there is also connected center: tal ransf r e ee e de 64 h vin t rminalsBE and 66. Transformer secondary 5! a Primary =6 fed b Suitab a ter ating e en Se onde y t i adapte t ev c e su tabl h a po en a s a e in s an T hhi a :65 and 55 a e eeh e d by e ds to di f e uehev e ehes and TL n h se ehe ea h eted b lead 1. and to anodes H and T5 of vacuum tuhc I16 and 11 respectively. Tubes 16 and 1;! ha\ cathodes 18' and 1.9 g un ed i be h t ltet e th t thes eat O es a sui ab y en r is d b eat rs or heat? the a -ren Grid resistors '89 and .8} are connected be.-

tween ground and control grids .8 2 and 83 of the respective tubes. Grids Q32 and 8 3 are also conheeted t hezo mi ts h ta k 69-. Th id connections be ,d r ct metallic connections to the nlr or rnay be through blocking con: densers 18.6 and 81' as shown. These blocking condensers may be formed by metal sleeves dispo e i s ee d re a i n er hhe er la e rom the a k tabl it be n hh e teee hat u ab e in u t n supports m be p o id d to a ntain th ,relatieh hi hu phrs e s id: s c nta t w en t tan and t e tak of ea s ma be ina ed if de es It is uhde hs h ef the slee e alon the e e may e adih ted to es ed ya e ahdt a the pae he et een the .eh eeee h lehser em a es m a s e adju te o desir d well ea- The sleeves may be either solid or perforated, and the physical dimensions of the sleeves will determine the effective capacitance.

Similarly, anodes 14 and are connected through blocking condensers 88 and Q9 respectively to suitable points on the tank. These blocking condensers may also be formed in a manner similar to condensers flfi and -81.

In both Figures 1 and 2 it is understood that the various leads from the tubes to the tam; and various circuit components such as grid resistors m h n los in nded shiel Similarl the vacuum tubes may be enclosed in shields if found desirable.

In operation, it is clear that the vacuum tubes will rectify the alternating current from the treh e mer- The furnace may also be modified by disposing neutral grounded electrode 9,9 between vturhat e red and @119 harrie 5 11 B111 Thus, in the absence of such a grounded electrode, there will he .a voltage .-node somewhere in .fi itl ce. 11F1 AQWW 99: Q fi tions. Without the grounded electrode, the volt-v s 1. .4 ma mer i esaee le eh ih the .natureof the work, its physical changes,and other factors. Iilowever bydisposing a grounded electrode between the furnace electrodes, the voltage node will be stabilized and fixed. The potential distribution through the node between the furnace electrodes may be controlled by the shape. size and location of the neutral electrode. It is understood that the neutral electrode may be of perforated material similar to the furnace lect des shev is .It. i eh hs het the grounded electrodeshown inFigure 1 e she ee z et eeh h iii heee ect des in a m e simi a h ha h stdes' ri 9 F ure 2.

B hih een ehse it the e s s b w ground and neutral electrode 90. Thus, electrode to wil h mended f .reei heq h denser 93 will tend to suppress damaging arcs. In e e th :t. is at hi h Pot nti t ground beee h e of d e conn ti n to a i h lta source, as in Figure 1, an arc hetween'electrode fill and ground damage the apparatus in the absence oi condenser .93. This condenser, like oi er grounded ndensers, should provide a low reactan for'radio frequency.

.l- A Qieleqtric heatin i m i h e e h e h uheh e eeh e n ctur w h Wo k e hede a .e t heh r ee etr etu e said ele heee term as a qhde e Wh se e el tri h u ie a 0 d, said ead and on ucting eihre e e atihs ua t -Wav t an mi sion l n t p t. .l. elm-h t ving a nodal m about midway aleh th s r eh t at least one vacuumtube hayi1 1g at least a cathode, eehhe t e we aired re i o b n t rid an ca hod a ze une tie th radio i e.- qu heies betwe n the ah d and said 12 .3 .e eehhe he 9 r d al fr hen es e ween th l fid shes aid he ,7 aheneeleleenneeieh hayi ne abl l n th e 'saie tehh an bein ehre i i .9 a a io i e uehe ehhectiqnh uer l. l I r and e .eQhreof h. ieetehtie be wee s d ath d and .ehede said heat be n th wis re re .el e et h he iieh y ihehe e eat e er t9 in h h ierme ieh f leere h eh ehel sewe imits of th s stem the e a ser. ihihs t ha being the domina t nfl e the amount of vae sm tribes eehlha 'ih a l a e eethede, e nirp r a d a ode es sto s b en the rid and e hes e ,q .eeeh ube nh h ns f ad o .tr lueheie between th e ht g i a d tank and between th .ehed and teh respect eir o a ime e a ano eehh et ns e a tube bein ,qpreeite y Pha an t We tube bein ppeeite l ph ed sa d eehh ehe fet 1231 1? hav n grg p len h 995 sai h. rad o r hehey e hh etieh b w sai QQP E QQ and n d l 99 @Pd .r i m of h h netehiielr etwe hs th ees he hl e sa d i h .h he ihehriee e re eiieer te hi he atelier ihdueie er .eeneeiters t m nimiz th v .,Q hehen. of Pa asit e iit and. wi h n th power limits of the s st the condenser .con" taming th ie s bein .demme .i fh e e ih hearnpuht o power the grated h ie .The s stem 9; c aim 1 wherein the a frequency connections for at least one of the electrodes of each tube includes a blocking condenser, said blocking condenser consisting of a sleeve surroundin a length of said conducting structure.

4. The system of claim 1 wherein a grounded '1 l electrode is provided adjacent the work electrodes.

5. The system of claim 1 wherein said work electrodes are disposed to face the same side of a work region and wherein a grounded electrode is disposed to face an opposing side of the work region.

6. The system of claim 1 wherein means are provided for adjusting the length and shape of said conducting structure so that said work electrodes may be disposed in any desired position with respect to the load.

7. A dielectric heating system comprising a generally U-shaped conducting structure having work electrodes at the free ends of said structure, said electrodes forming a condenser whose dielectric includes a load, said load and conducting structure operating as a quarter-wave transmission line type tank circuit having a nodal point about mid-way along the structure, an even number of vacuum tubes, each tube having at least a cathode, control grid and anode, a resistor between the grid and cathode of each tube, connections for radio frequencies between a control grid and tank and between an anode and tank respectively for each tube, each tube having its grid and anode connections to the tank oppositely phased and one-half of the tubes having their corresponding electrodes oppositely phased with respect to the corresponding electrodes of the remainin tubes, said connections on said tank having negligible length along said tank, radio frequency connections between said nodal point and said cathodes and a source of high potential between said cathodes and anodes, said tank being otherwise free of discrete high frequency inductors or capacitors to minimize the formation of parasitic circuits and, within the power limits of the system, the condenser containing the load being the dominant influence in the amount of power dissipated in the dielectric.

3. A dielectric heating system comprising a generally U-shaped conducting structure having work electrodes at the free ends of said structure,

said electrodes forming a condenser whose dielectric includes a load, said load and conducting structure operating as a quarter-wave transmission type tank circuit havin a nodal point about mid-way along the structure, at least one vacuum tube having at least a cathode, control grid and anode, a connection for radio frequencies for grounding said nodal point and said cathode, a resistor between grid and cathode of a tube, con nections for radio frequencies between the tank and control grid for a tube, a connection for radio frequencies between the tank and anode for a tube, said grid and anode connections for a tube being oppositely phased, said connections on said tank having negligible length along said tank, means for connecting a source of high potential to the anode and cathode of a tube, said tank being otherwise free of discrete high frequency inductors or capacitors to minimize the formation of parasitic circuits and, within the power limits of the system, the condenser containing the load being the dominant influence in the amount of power dissipated in the dielectric.

9. The system of claim 8 wherein the connection from an anode to the tank includes a block- 12 ing condenser, said blocking condenser consisting of a sleeve surrounding a length of conducting structure.

10. The structure of claim 8 wherein the connections from the grid and anode include blocking condensers, each blocking condenser consisting of a sleeve surrounding a length of conducting structure.

11. A dielectric heating system comprising a generally U-shaped conducting structure, said structure having a portion formed of spaced metallic tubular elements to form a generally cylindrical cage, said cage portion having extensions forming the free ends of the structure, each extension comprising a tubular conducting structure, electrodes secured to the free ends of said extensions, said electrodes forming a condenser Whose dielectric includes a load, said load and conducting structure operating as a quarter-wave transmission line type circuit having a nodal point about mid-way along the conducting structure, at least two vacuum tubes each having a cathode, control grid and anode, a radio frequency connection between the nodal point and the cathodes and ground, a radio frequency connection between each grid and tank circuit, a radio frequency connection between each anode and tank circuit, the cooperating grid and anode connections for any one tube being oppositely phased, said tubes being divided into two groups oppositely phased with respect to each other, a resistor between the grid and cathode of each tube, said anode and grid connections having negligible length along said tank, a source of high potential connected between the cathode and anode of said tubes, said tank circuit being otherwise free of discrete high frequency inductors or capacitorsto minimize the formation of parasitic circuits and, within the power limits of the system, the condenser containing the load being the dominant influence in the amount of power dissipated in said condenser dielectric.

, LORAN B. HIMNLEL.

REFERENCES CITED The following referenrces are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES A. I. E. E. Transactions, vol. 53, pages 1046- 1053, published 1934.

Hoyler: An electronic sewing machine, Electronics, August 1943, pages 90-93, 160, 162, 164, 166 and 168, particularly page 92.

Certificate of Correction Patent No. 2,474,420. June 28, 1949. LORAN B. HIMMEL It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 10, lines 69 and 7 5, column 11, lines 3 and 8, for the claim reference numeral 1 read 2;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

slgned and sealed this 29th day of November, A. D. 1949.

THOMAS F. MURPHY,

Assistant G'ommissz'oner of Patents.

Certificate of Correction Patent No. 2,474,420. June 28, 1949.

LORAN B. HIMMEL It is hereby certified that errors appear in the printed specification of the above numbered patent requiring correction as follows:

Column 10, lines 69 and 75, column 11, lines 3 and 8, for the claim reference numeral 1 read 2;

and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Ofliee.

S1gned and sealed this 29th day of November, A. D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

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