US1189725A - Method of and apparatus for producing high temperatures. - Google Patents

Description

E. F. NORTHRUP.

METHOD 0F AND APPARATUS FOR PRODUCING HIGH TEMPERATURES.

APPLICATION FILED OCT- 6, i915.

Patented July 4, 1916.

3 SHEETS-SHEET l.

E. F. NORTHRUP.

APPLICATION FILED OCT- 8, |915.

Patented July 4, 1916.

3 SHEETS-SHEET 2.

METHOD 0F AND APPARATUS FOR PRODUCING HIGH TEIVIPERATURES.

E. F. NOHTHRUP.

METHOD 0F AND APPARATUS FOR PRODUCING HIGH TEIVIPERATURES.

APPLICATION FILED OCT- 8. I9I5.

1 ,1 89,725. Patented July 4, 1916.

3 SHEETS-SHEET 3.

METEO@ @F AND APPARATUS "EUR PFUDUGN'G MGH TEMPERATURES.

risa-'rea www Specication of Letters 'Patent.

' Application led october i915. Serial No. 54,7%.

To all whom t may concern Be it known that l, EDWIN F. NOR'rHnUr, a citizen of'the United States, residing in the borough of Princeton, county of Mercer, and State of New Jersey, have invented a .new and useful Method of and Apparatus for Producing High Temperatures, of which the following is a specification.

My invention relates to electric furnaces and particularly those suitable forthe production of high temperatures.

My'invention resides in an electric furnace in which theresistor element, preferably of carbon or graphite, is confined lin a space to or from which gases cannot readily pass and in which the gas constitutes an atmosphere in which the resistor element is vchemically in equilibrium, the gas ybeing preferably generated within the furnace itself almost entirelyfroin material forming no part` of the resistor itself, particularly 'when of carbon or graphite, in which case the gas is of reducing character and consists of carbon monoXid and nitrogen,which be`.

' parts in elevation, through an electric f ur sides prolonging the life of the resistor renders the furnace as a-,whole suitable for -certain treatments where oxidation. is to be prej vented. A My invention resides alsoin a 'resistor element, preferably of carbon or graphite, preferably tubular in' forinfandlsov cut longitudi-v nally as to produce along path for current flow, making possible the usefof relatively small currents and considerable voltage, par.

-ticularly when the resistor nis of carbon' or graphite; `and a further-feature of my in vention resides in means forv mechanically strengthei'iing a resistor element so vformed as by inclosing the same between innerand outer cylinders or tubes, preferably of 'can' bon or graphite, with a illingof refractory' 'v heater units.- Fig; 81s a.

,.illustrating a systemand apparatus for proi ducing very high .temperatures by conduc- Ltion of fhigh frequency electric. energy material. v w u My invention residesalso' 1n electricfur'- nace structure whereby unusually hig'fhfteinperatures, for example 3000 .degrees-C'. andV higher, may easily'be` produced by eonduc-v tion of current through 'a solid resistor,yasA of graphite or carbon, or througha resistor of gas or vapor by conduction that is not that of an electric arc; and my Ainventioi'iresides in an'electrie furnace'in which suchv high temperatures may beproducedby they 'l nace emb dying my invention.

Patenten Jury a, raie.

not limited tothe use of a structure producing the higher temperature always in combination with a resistor v producing a lower temperature. i

My invention resides also in the method of electrically producing heat and in the apparatus therefor hereinafter described and particularly pointed .out in the claims.

For-an illustration of some of the forms my? apparatus may take reference is to be had to the accompanying drawings, 1n

which: t

Figure l is a verticalsectional view, some Fig. 2 i's a top plan view, on enlarged scale, of the heater unitof Fig. l. Fig. 3 is a view, on

in Fig. 4. Fig. 6j isa view of the resistor of Fig., 1l developed vor rolled -out into a plane. Fig.

electrica-l,i connections-which may be emplayed-for supplying electric energy Ato the through a resistor of gas orjvap'or.

diagrammatic view small scalefof the resistor element developed ig. 4 is a ver-` `.with the furnace shown in Fig. l. Fig. 5 'is a top plan view of the heaterv unit shown isl a` diagrammatic. View v illustrating vReferring to Figs. lv to 3 inclusive, 'i rep` 1 r'esentsthe outerfurnace casing and 2 an inner furnace casing, both of which may be fof metal or other suitable material, and between which is formed a compartment' filled with any suitable material 3 of poor heat conductivity, as for example, kieselgnhr. The furnace bottom 4, which may be of metal, may be supported in any suitable manner as for example, upon the legs 5.

In the inner casing 2 is a mass 6 of refractory material of poor heat conductivity which may be a body of molded refractory material fitting within the casing 2 and adapted to closely surround the heater unit structure. The body 6 may be made cfa mixture Aof quartz sand and aluminum oxid or alundum and any suitable binder, as sodium silicate. l

Disposed in the upper end of the casing 2 is the top 7 consisting of a block of material more or less refractory, as for example, alberene stone. A cover 8, which may be of metal, extends from the upper rim of the outer casing 1 to the block 7.

The resistor 9 is shown tubular in form and consists of any suitable material, as metal or Carbon, and is preferably of graphite, such as pure Acheson graphite or regraphitized graphite. As shown in Figs. 2 and 3 the resistor tube 9 has the slits or cuts 10 completely through its wall extending from one end to near the other end, and the similar slits or cuts 11 extending from the lower end to near the upper end. As seen in Fig. 3 this produces a considerable length of graphite conductor and of such an arrangement that the terminals-are both at one end of the resistor and side by side. One of these terminals 12 is shown in Figs. 1 and 2 and the other, 13, is shown in Fig. 2, these terminals 12 and 13 being attached to the resistor at 12 and 13 Fig. 3.

Because of the long path provided for the current through the resistor by the cutting or slitting referred to, the electric energy necessary to heat the resistor to a predetermined temperature may be supplied as a 'current of relatively small value and at a voltage or potential Which is correspond ingly higher than if the current path were shorter or of greater cross section.

The cross section of the graphite or resistor material at the parts 14 where the slits or cuts l0 and 11 fail to pass completely to the end of the resistor is preferably the same as the cross sectioniat any other point throughout the current path, with the result that the resistor will become heated uniformly throughout its length, developing as high a temperature at each end as at any point intermediate the ends.

The terminals 12 and 13 are of carbon or graphite, particularly when the resistor 9 is of carbon or graphite, and are attached to the resistor ends by the graphite screws 15. Each terminal has a portion 16 extending upwardly, through a hole in the block 7 and diminishing in diameter toward the top Vand having a hole 17 threaded to receive the metal bolt 18 having the A. carbon or graphite resistor formed as above described is more or less fragile. It is protected by an outer tube 28 of carbon or graphite and an inner tube 29 of carbon or graphite, the clearance space between the resistor 9 and the tubes 28 and 29 being lled with aluminum oxid 30 or other suitable refractory insulating material, as for example, zirconium oxid. extends nearly to the top of the tubes 28 and 29 above the top of the resistor 9, and is surmounted by refractory cement 30LL as a stopper. To reduce likelihood of breakage of the terminals 12 and 13 from the resistor 9 each terminal is provided with a pin or screw 3l of lavite or other suitable material embedded in the material 30. The tube 29 may have a bottom as indicated at 32.

Vithin the refractory material G is a graphite lining tube 33. The tubes 28 and 33 are cut away on one side near their tops The material 30 and 13. v

Within the tube 28 is a removable Crucible tube 34 generally of graphite but which may be of any other suitable material, as for example, Marquardt porcelain. Tube 34 forms the furnace chamber in which the temperature developed by the resistor 9 is utilized, and within the chamber 34 is placed the material to be heated. The'tube 34 has near its top an internal annular channel 35 into which may be expanded the ends of tongs for lifting the tube 34 out of the furnace through the hole 3G in the block 7 after the removal of the stopper 37, of alberene stone or other suitable material, having the holes 38 and 39, the former forming means for inspecting the furnace chamber and the latter for introducing pyrometric means.

Either direct or alternating current may be passed through the resistor 9 to raise its temperature and thereby raise the temperature within the furnace chamber.

In Fig. 7 40 and 41 represent conductors connected to a source of alternating currentI which may be connected to the primary 7) of 1 the transformer T by closing the switch 42.

legi

ductors 44 and 45 to the resistor terminals l2 and 13.

The furnace structure above described is such that the passage of gases into and out of the heating chamber is prevented or restricted and the -access of air to the resistor 9 is similarly prevented or restricted.

Carbon or graphite is readily attacked by oxygen when above a red heat or 640 deees C., and therefore to procure suitable length of life of the carbon or graphite resistor it must be effectually protected from oxidation, sincethe temperature it must produce is far above that of a red heat. rThis may be accomplished by recourse to a. vacuum or by passing through the furnace a. protecting gas, for example hydrogen. ln my furnace however, l have recourse to neither of these expedients. but produce a reducing gas or a gas neutral with respect to carbon or graphite, by the action of the oxygen of the air which may be in the furnace upon the graphite surrounding or protecting the resistor itself. `When this graphite reaches a suitable temperature the oxygen of the air confined in the inner compartment 2 of the furnace combines with the carbon of the graphite to produce carbon monoxid gas, leaving in the furnace a mixture of such gas with the remnant nitrogen of the air. These gases are practically the only ones remaining in the furnace, with the result that the resistor is effectually protected from oxida tion, even though carried to very high temperature.

The more completely the resistor is inclosed in carbon or graphite, as by the structure illustrated, the more completely will the neutral or reducing atmosphere be produced by such graphite other than that of the resistor, and any gas reaching the resistor while at high temperature will be chemically neutral with respect to the re# sistor graphite.

`While the production of a reducing or neutral atmosphere within the furnace is important as to protection of the resistor itself, the production of such an atmosphere has the further advantage of serving as a reducing atmosphere which is necessary and desirable in many furnace methods. lt will be understood therefore that by a structure of the character described, and by preventing free passage of air into the furnace. the furnace automatically produces a gas which protects its resistor and which simultaneously ser-ves as a reducing atmosphere suitable for protecting from oxidation any material heated within the furnace. And when graphite only is present in the furnace structure, no gases or vapors are generated in the furnace which might contaminate or injuriously affect any material treated in the furnace.

furnace structure of the character above described lends itself readily to an interchangeability of all parts of the furnace, and a worn out heater unit may quickly be removed and a new one inserted. rlhe furnace parts are practically indestructible with the exception of the heater unit, and it has a long life, is relatively inexpensive and is 'the only part of the furnace that must be renewed.

.By a structure asof the character above described working temperatures of 1600 degrees C., or thereabout are easily attained and higher temperatures are obtainable, but at the expense of reduced life of heater unit. The temperature produced within the heating chamber is such-that when containing a crucible a perfect black body condition or temperature is available.

l or the production of still higher temperatures a structure of the character illustrated in Figs. 4, 5 and 6 may be employed in combination with the structure hereinbefore described, though it will be understood that the structure of the character illustrated in Figs. 4, 5 and 6 is not limited to its use in such combination, and in accordance with my invention its use in any useful relation is contemplated.

Referring to Figs. 4, 5 and 6, 46 is a tubular structure of alberene stone or any Aother suitably refractory material within which are disposed the carbon or graphite electrodes 47 and 48 formed by cutting vertically through, as at 49, a carbon or graphite tube. rlhe electrodes 47 and 48 are held by screws 50 in firm contact with the inner walls of the member 46 and in electrical contact with the metallic members 5l and 52 constituting parts of the terminals 53 and 54 provided with the binding screws 55 and 56. rfhe electrodes orterminals 4T and 48 have cut in them to consid-5 erable depth the annular channels 5T and 58 in which are disposed the ends 59 vand 60 of resistor 62. Graphite screws 61 clamp the electrodes 47 and 48 snugly and in good electrical contact with the resistor ends 595 and 60. rlhe resistor '62 is a tube of graphite longitudinally slit or cut on opposite sides, as indicated at 63 and 64 from the upper end of the resistor tube to near its lower end` the lower end being circumferentially continuous at 65 to form a connection betweenthe two halves or sides of the resistor.

Secured to the tubular head 46 by screws 66 is a graphite jacket tube 6T surrounding the electrodes 4T and 48 and the resistor 62 and having in its bottom a plug 68 of graphite held in place by the transversely extending graphite pin 69. The plug 68 near its upper end is reduced in diameter, as indicated at 70, and upon the shoulder so formed rests the graphite tube 71 surrounding and spaced from the resistor 62, the tube 71 serving to confine the radiant heat to its interior and as a shield to protect the furnace parts outside of tube 71. The plug (38 has a central hole extending to the pin 09 in which is disposed the Crucible tube 72 of graphite or other material which rests upon the pin 69 and extends upwardly within the resistor 02. As shown, the crucible tube 72 may have a bottom 73 mid-way its length so that the Crucible proper is that part of the tube 72 above the bottom 73. This member may be reversed so that the lower portion may extend into the resistor and the upper portion rest upon the pin 09. The result is that the member 72 as illustrated forms a double crucible, either end of which may be employed when the other is destroyed or otherwise rendered unsuitable.

By removal of the pin G9 the plug (38 and the crucible tube 72 and tube 71 may be readily removed by withdrawing them downwardly out of the tube 67. And a new resistor element may be introduced by removing the outer tube (37 and inserting the same in place in the electrodes 47 and 48 and tightening screws 61.

This structure may be used by itself for producing` high temperatures by connecting the terminals 53 and 54 to any suitable source of current, direct or alternating, whereupon current will flow from one of the terminals, as 53, through the electrode or terminal 47, thence through the resistor 02 to the other electrode or terminal 48 and thence to the terminal 54, the current in traversing the resistor 62 raising its tem'- perature.

For the productio-n of very high temperatures, as upward of 3000 degrees C., which I have accomplished by this apparatus, as evidenced by the fact that I have melted pure tungsten therewith, which late investigations show melts at approximately 3327 degrees C., the resistor (32 should be disposed in a neutral or non-oxidizing gas which may be delivered into the interior of the apparatus or which may be therein formed as by oxidation of graphite in the presence of the small amount of air originally present.

A convenient method of using this structure is to place it in an atmosphere of neutral or non-oxidizing gas, and this may conveniently be done by inserting the same into the apparatus shown in Fig. 1, sliding the unit of Fig. 4 into the interior of the tube 29 from which the tube 34 has been withdrawn. Current is then first thrown on to the resistor 9 b v throwing the switch 43, Fig. 7, into the right hand position until a temperature of 1400 degrees C. or higher is attained; before which time the carbon monoxid and nitrogen atmosphere will have been produced which Will protect not only the resistor 9, but also the resistor 62. .After the resistor 9 has produced the temperature mentioned, the switch 43 is thrown over into the left hand position, whereupon current will flow from the transformer secondary s through the switch 43 and the Conductors 74 and 75 to the terminals 53 and 54 of the inner heating unit. Inasmuch as the resistor G2 is protected by non-oxidizing atmosphere, its temperature may be raised to practically that which is the vaporization 75 point ot' graphite at atmospheric pressure. And such temperature is easily attained and quickly by dissipating sufficient electrical energy in the resistor 62.

By the above operation there is a preheating or preliminary heating by the resistor 9 and then the temperature is stepped up to a still higher temperature in and by the I'C- sistor (3,2 and this stepping up of the temperature has been termed by me heating in t"cascade and the attachment or arrangement of Fig. 4 has been termed a cascade attachment.

I have found also that in place of conduction through a solid, as graphite, hereinbefore referred to, I can produce very high temperatures by conduction of electric energy through gases or vapors at atmospheric pressure or higher. For this purpose apparatus of the character illustrated in Fig. 8 may be employed. lNithin the hollow member 7G, which may be of graphite or other suitable material, is disposed a solid or tubular member 77 of any suitable material, for example graphite, spaced from the interior wall of the member 76, the intervening space being filled with gas or vapor, such as metallic vapor. The member 70 may have a cover 78 of refractory material, such as alberene stone, through which extends a tube 79, as of Marquardt porcelain, communicating with the interior of the member 77. The member 77 may have the removable screw threaded bottom plug 80 upon which may rest the crucible 81. By preheating the gas or vapor to a temperature upward of 1400 degrees C. conduction will occur through the gas or vapor at atmospheric pressure when the members 7G and 77 are connected to a source of current, of suitable potential, either direct or alternating. The current passed through the gas or va por may be of such magnitude as to cause the gas or vapor to rise still higher in temperature and indeed to very high temperatures, with the ,result that the member 77 is brought to a high ten'iperature, the limit of which is believed to be the .aporization temperature of the graphite of member 77. This high temperature may be utilized for heating materials placed within the member 77 or within a crucible 81 in the member 77.

The space between members 7G and 77 may contain refractory material in whose interstices is the gas. or vapor for current l" ideare@ conduction.` Ortho member 76 may have a Vrefractory lining, as a tube of Marquardt porcelain. By thus starting With a gaseous or vapor phase of matter to obtain high temperature by direct electric conduction through the gas or vapor, therev is no further state beyond the gaseous or vaporous into which matter can pass under elevation of temperature, and hence the heater medium is indestructible and therefore oers a means of producing the highest obtainable temperatures.

For the gas through which the conduction occurs l have employed air and also carbon monoXid, Which develops automatically in a graphite furnace in a closed container. l have also used the vapors of mercury and bismuth; and vapors of other metals and compounds may be used.

"W hen using direct or alternating current l have found that With a constant vapor or gas temperature an increase of potential is y -accompanied by a still more rapid increase `may be used inotlier relations,-

invention is not limited to use as a cascade y of current, and that plied potential an increase of current is accompanied by an increase of temperature.

For preliminarily raising the temperature of the gas or vapor to the above mentioned conductive state, ll have placed the members 76 and 77 Within the furnace illustrated in lFig. '1. rlherefore the ,arrangement of lig.. 8 may also be considered a cascade attachment,though it is to be understood that it and that my attachment.

, W hen using direct current or'alternating d tion being then arc conduction through a very restricted region and not Well distribthe surfaces of the members 76 To overcome this uted over and 77 as is desirable.

tendency to arcing the energy passed beductin ath of 60 'g p energy will tween the members 76and 77 through the gas or vapogr may be in the form .ofhigh frequency alternating current or in the form of. high frequency electrical oscillations 4rll`he choice of high frequency alternating current or electrical oscillations insures against the formation ofan arc, because the current of high frequency alternating current or of electrical oscillations Will not select an isolated or restricted conducting path of low resistance so readily as it Will avoid a conhigh inductance. Accordingly, by the use of high frequency alternating current or of electrical oscillations the be conducted between the members 7 6 and 77 quite uniformly over their surfaces. 7

For the production of the alternating cur- With a constant ap-` nected to the spark gap 82 connected in circuit With the condenser 83, which may be adjustable if desired, and the inductance 84:, which may be adjustable if desired, and which forms the primary of an oscillation transformer Whose adjustable secondary 85 has its terminals connected to the conductors 86 and 87, the former connected with member 76 and the latter vvith the tungsten or other vvire 88 extending through tube 79 and connecting With member 77.

As Well understood in the art of electrical oscillations there Will be produced in the circuit 82, 83, 84 high frequency oscillations Whose periodicity is largely dependent upon the capacity of the condenser 83 and the inductance of the Winding or coil Sli. As shown the transformer 84e, 85 may be a'step dovvn transformer, so that the voltage inipressed upon the members 76 and 77 may be lower than that of the oscillation producing circuit and the current greater than that in the oscillation producing circuit.

ln place of the oscillation producing circuit including va spark gap any other suitable oscillation producin means may be employed, such, for examp e, as one employingnnarc producing sustained oscillations, or any other continuously oscillating circuit may be employed or the high frequency oscillations may be generated by a dynamo-electric generator.

- lt Twill be understood that high potentials are not necessary for the operation of the gas or vapor heating device, for potentials as lov! as 100 volts or lower may be employed to secure substantial conduction through the gas or vapor.

lnasmuch as the conducting surface of the member 77 is smaller than that of the member 76 the member 77 Will attain the higher temperature because per unit area there is a greater dissipation of energy on the member 77 than there is on the member 76.

For the electrode, as 7 7, may be used refractory material such as alundum. And as a liner for the member 76 may be used Marquardt porcelain or other suitable material. Even when using these materials alundum and Marquardt porcelain l have procured conduction through the intervening gas'.

-`What l claim is:

l. An integral tubular resistor having neighboring slots extending from opposite ends and forming the resistor material into a current path extending backwardly and forwardly longitudinally.

2. An integral tubular resistor having a slot extending throughout its length, a plurality of neighboring slots extending from opposite ends and forming the resistor material into a current path extending backwardly and forwardly longitudinally, and current connections on opposite sides of and adjacent said first named slot.

3. An integral tubular resistor having neighboring slots extending from opposite ends and forming the resistor material into a current path extending backwardly and forwardly longitudinally, and terminals disposed side by side and at the same end of the resistor.

4. An integral tubular graphite resistor having neighboring slots extending from opposite ends and forming the resistor material into a current path extending backwardly and forwardly longitudinally.

5. An integral tubular graphite resistor having a slot extending throughout its length, a plurality of neighboring slots extending from opposite ends and forming the resistor material into a current path extending backwardly and forwardly longitudinally, and current connections on opposite sides of and adjacent said first namedy slots. l i n 6. An integral tubular graphite resistor having neighboring slots extending from opposite ends and forming the resistor material into a current path extending backwardly and forwardly longitudinally, and graphite terminals disposed side by side and at the same end of the graphite resistor.

7. An integral tubular graphite resistor having neighboring slots extending from opposite ends and forming the resistor material into a current path extending backwardly and forwardly longitudinally, and graphite terminals disposed side by side and at the same end of the graphite resistor, said terminals projecting beyond the end of the tubular resistor.

8. In combination, a slotted tubular resistor having neighboring slots extending from opposite ends thereof'and forming the resistor material into a current path, a tubular member surrounding said tubular resistor and spaced therefrom, a second tubular member within said tubular resistor and spaced therefrom, and means forming said resistor and said tubular members into a unit. l l

9. In combination, a slotted tubular resistor having neighboring slots extending from opposite ends thereof and forming the, resistor material into a current path, a tubular member surrounding said tubular lre- -a current path extending baclrwardly and forwardly longitudinally, the cross section of the current path at the ends of the tubular resistor having a cross section such that the passage of current therethrough produces a temperature substantially equal to the temperature produced intermediate the ends of the tubular resistor.

l1. An electric heater unit comprising ai tubular resistor, a tubular member fixed within said resistor and spaced therefrom, and refractory material in the space between said resistor and said tubular member.

12. A heater unit comprising a tubular resistor having neighboring slots extending from opposite ends thereof, a tubular inember surrounding said tubular resistor and spaced therefrom,' a tubular member disposed within said tubular resistor and spaced therefrom, and refractory material disposed in the spaces between said resistor and said members. I

13. A heater unit comprising a tubular resistor, tubular members Within and surrounding said resistor and spaced therefrom in fixed relation thereto, and refractory material disposed in the space between said resistor and said members.

14. A heater unit comprising a tubular resistor, tubular members within and surrounding said resistor and spaced therefrom, refractory material disposed in the space between said resistor -and said members, said members having greater length thansaid resistor, and refractory material extending across the end of said resistor between said members.

15. A heater unit comprising a tubular resistor, tubular members within and surrounding said resistor and spaced therefrom, refractory material disposed in the space between said resistor and said members, and a tubular member of refractory material disposed within said inner tube. Y

16. An electric heater unit comprising a tubular graphite resistor, a tubular member of graphite fixed within said resistor and spaced therefrom, and refractory insulating material disposed between said resistor and said member.

17. An electric heater unit comprising a slotted tubular graphite resistor, neighboring slots extending from opposite ends to form a current path extending backwardly and forwardly longitudinally, graphite tubes surrounding and within said tubular resistor and spaced therefrom, and refracmee tory material disposed inthe space between said resistor and saidv members.

V18. An electric heater unit comprising a tubular graphite resistor, tubular members of graphite within and surrounding said resistor and spaced therefrom in fixed relation thereto, and refractory insulating material disposed in the space between said resistor' and said members.

19. An electric heater unit comprising a tubular graphite resistor,`a tubular member of graphite surrounding and spaced from said resistor, refractory insulating material between said resistor and said member, and a terminal secured on the outer side of said resistor, said tubular member being cut away in the vicinity of said electrode and spaced therefrom.

20. An electric heater unit comprising a tubular resistor of graphite, a graphite 'terminal secured thereto, refractory material covering said resistor, and a member attached to said terminal and anchored in said refractory material.

Q1. ln an electric furnace, a heater unit comprising a tubular resistor, tubular members within and surrounding said resistor and spaced therefrom, refractory material disposed in the space between said resistor and said members, refractory material disposed at the ends of said resistor between said tubular members, a mass of refractory material, and a hollow liner therefor adapted to receive said heater unit. l

22. An electric furnace comprising a hollow mass of refractory material, a heater unit disposed within said mass and comprising a graphite resistor and a graphite inclosure for said resistor, and a graphite lining in said mass of refractory material.

23. An electric furnace comprising ahollow mass of refractory material, a heater unit comprising a graphite resistor and graphite inclosing said resistor, a terminal on said resistor disposed within said refractory material, a graphite lining in said refractory material receiving said heater unit, said graphitelining being cut away in the vicinity of said terminal and spaced therefrom. Y

24. An electric furnace comprising a mass of refractory material, a heater unit disposed in said mass and comprising a tubular resistor and tubular members withinand surrounding said resistor and spaced therefrom, and a tubular member disposed Within said inner tubular member and constituting the hot zone of the furnace.

25. rlhe combination with a resistor capable of chemical reaction with' a gas,` of refractory members spaced apart, refractory material in the space between said members, said resistor inclosed in said refractory material, saidl members heated by said resistor and. being of material capable ofreacting with said gas to produce a gas which is chemically neutralwith respect to said resistor.

26. ylfhe combination with a resistor capable of chemical reaction with a gas, of refractory members spaced apart, refractory material in the space between said members, said resistor inclosed in said refractory material, said members heated by said resistor and being of material similar to that of said resistor and reacting with said gas to produce a Gas which is chemically neutral with respect to said resistor.

27. rll"he combination carbon or graphite, of members spaced apart, refractory material between said members in which said resistor is inclosed, said members being raised in temperature by said resistor to react with surrounding gas to produce an atmosphere surrounding said members neutral with respect to said resistor. y

28. The combination with a tubular resistor of carbon or graphite, of tubular members of carbon or graphite surrounding and within said resistor and spaceditherefrom, a removable carbon or graphite tube within the inner "of said members, a mass of refractory material, a lining therein receiving the outer of said members, and a member having anJ aperture through which said tube may be withdrawn.

29.k The combination with a chamber, of a resistor for heating the same, and a second resistor in said chamber for producing a temperature different from that produced by said first named resistor.

with a resistor of carbon or graphite 30. rlhe combination wlth a chamber, of

a resistor for heating the same,'and a second. resistor in said chamber producing a temperature higher than the temperature produced by said first named resistor.

3l. 'llhe cj mbination with a chamber, of a resistor for heating the same, a second resistor in said chamber for producing a temperature different from that produced by said first named resistor, and means for enveloping said second resistor 'in an atmosphere with respect to which it is chemically neutral. y p i 32. rlhe combination with a chamber, of a resistor for heating the same, a second resistor in said chamber for producing artemperature different from that produced by said first named resistor, and means for producing an atmosphere chemically neutral with respect to said resistors.

33. An electric furnace comprising a plurality of resistors, one of said resistors constituting a preheater for another of said resistors.

34. An electric furnace comprising carbon or graphite resistors, means for passing current through one of said resistors to preheat the furnace, and means for passing curlll@ 'laov graphite resistor disposed in said non-oxidizing atmosphere and heated by said first named resistor, means for passing current through said first named resistor, and means for passing current through said second resistor to raise its temperature at higher rate.

37. A heater unit comprising a longitudinally slotted tubular resistor of carbon or graphite, terminals slotted to receive the ends of said resistor, a tubular member inclosing said resistor, a removable bottom for said tubular member, and a hollow7 memberl supported on said bottom and projecting into said resistor.

38. A heater unit comprising a longitudinally slotted tubular resistor of carbon or graphite, terminals slotted to receive the ends of said resistor, a tubular member inclosing said resistor, a removable bottom for said tubular member, a hollow member supported on said bottom and projecting into said resistor, and a tubular member supported by said bottom and intervening between said resistor and said first named tubular member.

39. The combination with a heating chamber, of`means for raising the temperature thereof, means for producing higher temperatnre disposed in said chamber and comprising a longitudinally slotted tubular resistor of carbon or graphite, terminals slotted to receive the ends of said resistor, a tubular member inclosing said resistor, and ia; removable bottom for said tubular mem-- 40. Electrical heating apparatus comprising a resistor of gas or vapor at atmosphere pressure or higher, and a source of energy alternating or fluctuating at a frequency preventing arc .conduction through said gas or' vapor.

4l. Electrical heating apparatus comprising a resistor of gas or vapor at atmospheric pressure or higher, a source of high frequency oscillations, and means for conducting the energy of said oscillations through said gas" or vapor.

42. The.method of producing heat electrically, which consists in preheating a gas or vapor at atmospheric pressure or higher, and passing electric energy of high frequency therethrough. 4

43. Electric heating apparatus comprising electrodes separated by gas or vapor, a source of current connected to said electrodes, one of said electrodes being hollow to form a heating chamber.

44. A cascade furnace comprising a low temperature resistor, a heating chamber raised in temperature thereby, and an independent heater unit disposed in said cliamber and removable therefrom comprising a closed chamber, and a graphite resistor therein for producing higher temperature.

45. A cascade furnace comprising a low temperature resistor, a heating chamber raised in temperature thereby, an independent heater unit disposed in said chamber and removable therefrom comprising a closed chamber, a graphite resistor therein for p'roducing higher temperature, a source of energy, and switching mechanism for connecting either of said resistors to said source of energy.

46. A cascade furnace comprising a heater unit, said unit comprising a resistor and a heating chamber adapted to be raised in temperature thereby, and a second heater unit disposed in and removable from said heating chamber and comprising a resistor for producing temperatures higher than said first named resistor, and a heating chamber raised in temperature by said second resistor.

47. A cascade furnace comprising heater units one of which is contained within another, one of said heater units comprising a tubular resistor, a tubular refractory member disposed within and raised in temperature by said resistor and adapted to receive another of said heater units, said other of said heater units comprising a resistor for producing higher temperatures than said first named resistor, and a hollow member constituting a heating chamber.

In testimony whereof I have hereunto affixed my signature this 6 day of October,

EDWIN F. NORTHRUP.

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