US3007236A

US3007236A - Methods of making electric heating units

Info

Publication number: US3007236A
Application number: US567849A
Authority: US
Inventors: Emmett W Barnes
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1956-02-27
Filing date: 1956-02-27
Publication date: 1961-11-07
Anticipated expiration: 1978-11-07

Description

Nov. 7, 1961 E. w. BARNES 3,007,236

METHODS OF MAKING ELECTRIC HEATING UNITS Filed Feb. 27, 1956 INVENTOR. Emma/2 W Barnes United States Patent 3,007,236 METHODS OF MAKING ELECTRIC HEATING UNITS Emmett W. Barnes, Lombard, llL, assignor to General Electric Company, a corporation of New York Filed Feb. 27, 1956, Ser. No. 567,849 4 Claims. (Cl. 29--155.67)

The present invention relates to methods of making electric heating units of the sheathed resistance conductor type.

I-Ieretofore, it has been proposed that in manufacturing an electric heating unit of the sheathed resistance conductor type, an assembly be produced that includes an elongated metallic sheath enclosing both an elongated helical resistance conductor and an elongated member formed of magnesium metal, as well as a charge or packing of crystalline magnesium oxide or other electrical insulating and heat-conducting material, and that the assembly then be subjected to heat treatment in an autoclave in order to oxidize in situ the member formed of magnesium metal to amorphous magnesium oxide, with the resulting expansion of the composite mass of refractory material enclosed in the sheath so as to obtain the desired additional compacting thereof in place between the helical resistance conductor and the sheath in the finished heating unit.

While the proposal has obvious advantages in principle, the difiiculty therewith has resided in the suggested details of the steps that are carried out in the autoclave to achieve the oxidation of the magnesium metal. More particularly, the assembly mentioned is placed in the autoclave and first subjected to the action of high temperature steam at a gauge pressure of 1000 p.s.i. for a time interval of about 12 hours, thereby to obtain complete hydration of the magnesium metal with the resulting production of magnesium hydroxide. Next, the assembly is subjected to the autoclave to a high temperature baking-out action in vacuum throughout a time interval of several days in order to obtain substantial dehydration of the magnesium hydroxide with the resulting production of amorphous and substantially anhydrous magnesium oxide.

of making an electric heating unit of the sheathed resistance conductor type, and involving an improved and simplified step for converting in situ at least the outer skin of a member formed essentially of magnesium metal into anhydrous amorphous magnesium oxide, while the member is embedded in a porous packing of refractory material enclosed in the metallic sheath of the heating unit.

A further object'of the invention is to provide a method of making an electric heating unit of the sheathed resistance conductor type, and involving an improved and simplified step for converting in situ a member formed essentially of a metal selected from the group consisting of beryllium, manganese, aluminum and titanium into the corresponding metal oxide, while the member is embedded in a porous packing of refractory material enclosed in the metallic sheath of the heating unit.

A further object of the invention is to provide a method of making an electric heating unit of the sheathed resistance conductor type that involves a simple autoclave step that may be positively and safely carried out in a simple manner, within a reasonable time interval.

3,00?,Z3fi Patented Nov. 7, 1561 ICC A still further object of the invention is to provide an improved process of the character described in which the member formed essentially of magnesium metal is oxidized directly with gaseous oxygen at a modest gauge pressure and at a controllable elevated temperature and without danger of ignition of the member.

Further features of the invention pertain to the particular arrangement of the steps of the method, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawing, in which:

FIGURE 1A is an enlarged fragmentary view, partly in section, of an assembly that is employed in making an electric heating unit of the sheathed resistance conductor type in accordance with the method of the present invention;

FIG. 1B is an enlarged fragmentary view, partly in section, of the finished electric heating unit that has been produced from the assembly of FIG. 1A;

FIG. 2A is an enlarged fragmentary view, partly in section, of a modified form of an assembly that is employed in making an electric heating unit of the type noted in accordance with the present method;

FIG. 2B is an enlargedfragmentary view, partly in section, of the finished electric heating unit that has been produced from the assembly of FIG. 2A; and

FIG. 3 is a perspective view of an electric heater, actually the inner coil of an electric hotplate, that has been produced from the finished electric heating unit of either FIG. 1B or FIG. 2B.

Referring now to H6. 18 of the drawing, the electric heating unit 10 there illustrated and made in accordance with the method of the present invention fundamentally comprises an elongated tubular metallic sheath 11 that may be formed of a suitable nickel-chromium-iron alloy and having a substantially circular cross-section, and an elongated helical electrical resistance conductor or element 12 that may be formed of a suitable nickel-chromium alloy and located substantially centrally within the sheath 11 and embedded in a body of heat-conducting and electrical-insulating material of composite structure including a central core portion 313 formed of amorphous magnesium oxide and an outer surrounding tubular portion 14 formed of c 'stalline magnesium oxide,

Also, the unit 10 comprises a pair of elongated

conductive terminals

15 and 16 respectively arranged in the opposite ends of the sheathv 11 and respectively elec trically connected at the inner ends thereof to the opposite ends of the resistance conductor 12. More particularly, the terminal 15 is provided with a shoulder adjacent to the inner end thereof defining an inwardly projecting stem-like section 15a to which several turns of the resistance conductor 12 are suitably secured at the adjacent end thereof, as by welding, or the like. The outer end of the terminal 15 projects from the adjacent outer end of the sheath 11 and is sealed in place by an insulating plug 17 formed of glass, or the like. The construction and arrangement of the terminal is is identical to that of the terminal 15; whereby the

terminals

15 and 16 are disposed along the longitudinal axis of the unit 10 and afford electrical connections to the resistance conductor 12 for the heating purpose.

In manufacturing the electric heating unit 10, shown in FIG. 1B, first an assembly is produced of the character of that shown in FIG. 1A; and specifically, the helical resistance conductor 12 is first wound upon an elongated rod 18 formed essentially of magnesium metal; and the opposite ends of the resistance conductor 12 are suitably secured to the stems 15a and 15:1 respectively provided upon the inner ends of the

terminals

15 and 16. This subassembly of the resistance conductor 12, the magnesium metal rod 13 and the

terminals

15 and 16 is arranged substantially centrally within the tubular sheath 11; and one end of the sheath 11, such, for example, as the lefthand end thereof, is suitably closed by a tubular insulating plug 59 surrounding the intermediate portion of the terminal is and by a fiber backing washer 2% arranged in the end of the sheath 11 in surrounding relation with the intermediate portion of the terminal 15 and exteriorly of the plug Then the extreme left-hand end of the sheath ill is bent over, as indicated at Ma, in order to retain the washer 249 in place.

At this time, the assembly is transferred to a loading machine of the character of that disclosed in US. Patent No, 2,316,659, granted on April 13, 1943 to John L. Andrews; the terminal 16 being arranged at the top of the loading machine and being held in place by a hook, not shown; whereby the resistance conductor 32 is in a position depending from the terminal 16 and disposed substantially centrally within the upstanding tubular sheath lit. The loading machine is then operated in a conventional manner, whereby the charge 14 of finely divided crystalline magnesium oxide is introduced into the upper end of the sheath 11 adjacent to the terminal 36 and into embedding relation with respect to the resistance conductor 12, the magnesium metal rod 18 and the inner ends of the

terminals

15 and 16. As the charge 14 of refractory material is introduced into the sheath 11, it is tamped or vibrated into place to provide a firm, but porous, packing filling the space between the centrally disposed elements l2, l8, l and it? and the surrounding tubular sheath ll.

After filling of the sheath 11, the assembly is removed from the loading machine mentioned and the upper or right-hand end of the sheath lll, as shown in FIG. 1A, is closed by an insulating plug 21 and an associated fiber backing washer, not shown, the extreme right-hand end of the sheath 11 being bent over to retain the backing Washer in place, as indicated at lib. Also, the extreme outer end of the terminal 16 that cooperates with the hook provided in the loading machine is severed. At this time, the assembly of FIG. 1A is produced, the righthand end thereof being identical to the left-hand end thereof, as illustrated; and in passing, it is mentioned that the plugs 1% and 21 may be of the frangible insulating type consisting essentially of a chalk-like material of porcelain character. However, it is noted that while the

plugs

19 and 21 retain the charge 14 in place, they are not sealed in air-tight relation to the opposite ends of the sheath 1i, and they are not sealed in air-tight relation to the respective intermediate portions of the terminals and 16.

At this time, the assembly is transferred to a rolling machine of the character of that disclosed in US. Patent No. 2,677,172, granted on May 4, 1954 to Sterling A. Oakley; and the rolling machine is operated in order to subject the assembly to a preliminary compacting step, the assembly being moved upwardly, while supported in an upright position, through the several rolling passes. Specifically, in the preliminary rolling operation, the diameter of the sheath 11 is modestly reduced so as modestly to reduce the cross-sectional area of the charge 14 and so as to crush the

frangible plugs

19 and 21 in order to obtain the corresponding modest compacting of the refractory mate-rial arranged in the sheath 11 and supporting the elements l2, l3, l5 and 16 substantially centrally thereof. At this time, following the preliminary rolling step, the charge of refractory material 14, as well as the refractory material resulting from the crushing of the

frangible plugs

19 and 21, is still porous, as explained more fully below.

Following the preliminary rolling step, the assembly, having substantially the appearance, as illustrated in FIG.

1A, is transferred to an autoclave of conventional construction including a heating chamber, an associated electric heater, a connecting vacuum pump, a connecting oxygen tank provided with a pressure regulator, pressure gauges and suitable valves and fittings, all of a conventional character.

In accordance with the method, the heating chamber of the autoclave is closed after the assembly is loaded therein; and actually the heating chamber is arranged to receive a substantial group or number of the assemblies, as a matter of production facility. After closing and sealing of the heating chamber, it is evacuated and the vacuum is held for about 15 minutes so as to remove air from the porous packing of refractory material enclosed in the sheath ll. Thereafter, gaseous oxygen is introduced into the heating chamber under gauge pressure of about p.s.i., and the assembly is soaked therein for about 15 minutes. Again the heating chamber is evacuated and the vacuum is held for about 15 minutes so as to remove residual air from the porous packing of refractory material enclosed in the sheath ll'l.

Thereafter gaseous oxygen is again introduced into the heating chamber under gauge pressure of about 70 p.s.i., and this pressure is maintained therein in order again to permeate the packing of refractory material and to contact the magnesium metal, rod 18. The temperature of the heating chamber is then elevated to a reaction temperature disposed below the ignition temperature of magnesium metal at the gauge pressure of about 70 p.s.i., which elevated reaction temperature is maintained for a suflicient time interval completely to react the magnesium metal rod 18, so as completely to convert the same to anhydrous amorphous magnesium oxide; whereby the mass of amorphous magnesium oxide comprising the core 13 is produced in the assembly, as indicated in FIG. IE.

it has been discovered that under the reaction conditions specified, the ignition temperature of magnesium metal is about 1085 F, whereby the reaction temperature is maintained in the heating chamber at about 1075 F, this reaction temperature being safely below the ignition temperature of magnesium metal, yet sufliciently close thereto to insure a high rate of reaction. It is estimated that in the manufacture of the heating unit 10, the required time interval of the reaction is about 8 hours; however, the assembly is retained in the autoclave under the reaction conditions set forth for a time interval of about 16 hours so as positively to insure the complete conversion of the rod 18 from magnesium metal to magnesium oxide. Specifically, this time interval is not critical, provided it is sufliciently long to insure the complete reaction of all of the magnesium metal and the complete conversion thereof to amorphous magnesium oxide; i.e., the maintenance of the reaction conditions in the heating chamber of the autoclave for some time interval following the complete conversion of the magnesium metal to amorphous magnesium oxide is in nowise deleterious since the reaction is automatically terminated when the conversion is complete.

After the complete reaction of the magnesium met-a1 rod 18 has been achieved in the autoclave, the heating chamber thereof is opened to the atmosphere and allowed to cool; whereupon the assembly is removed from the heating chamber and again transferred to the rolling machine; whereupon it is subjected to a final compacting step, similar to the preliminary compacting step previously described, but substantially more severe. In the final rolling operation, the diameter of the sheath 111 is substantially reduced so as substantially to reduce the cross-sectional area of the composite body of refractory material, including the core 13 of amorphous magnesium oxide and the surrounding charge 14 of crystalline magnesium oxide, so as to produce a highly compacted dense composite body of refractory material embedding the helical resistance conductor 12 and retaining the same in place in spaced-apart relation with respect to the sheath 11. The dense mass of compacted refractory material also positions and retains in place the

terminals

15 and 16.

After the final rolling operation, the assembly is removed from the rolling machine and the extreme outer ends of the sheath 11 are stripped so as to remove the backing washers 20, etc., as well as the bent-over ends 11:: and 11b of the tubular sheath 11. Finally, the portions of the compacted refractory material disposed in the extreme outer ends of the sheath 11 and respectively surrounding the

terminals

15 and 16 are removed to provide the end cavities into which the glass plugs or seals 17, etc., are ultimately cast so as firmly to retain the

terminals

15 and 16 in place and so as to seal the opposite ends of the tubular sheath 11, as shown in FIG. 113.

After manufacture of the heating unit .10, it is ordinarily subjected to the usual electrical tests in order to determine the insulation resistance, proof voltage, heat distribution, and other matters afiectiug performance and life of the unit.

As a constructional example of the manufacture of the heating unit 10, the tubular sheath 11 may have an initial diameter of 0.312; and in the preliminary rolling step, the diameter thereof may be reduced to 0.299; and thereafter in the final rolling step, the diameter thereof may be reduced to 0.270". In the heating unit 10, the resistance conductor 12 may have a diameter of about 0.0089", the spacing between the centers of the turns thereof may be about 0.0249"; and the outside diameter of the turns thereof may be about 0.110". Of course the number of turns per inch longitudinally of the resistance conductor 12 is dependent foundamentally upon the gauge thereof that, in turn, is dependent upon the desired wattage rating of the finished heating unit 10. However, as a. practical matter, it has been discovered that from 20 to 42 turns per inch longitudinally of the helix is feasible, when the resistance conductor 12 is formed respectively of relatively coarse and relatively fine resistance wire.

in a modification of the methodof making the heating unit 10, the assembly is subjected to heat treatment in the autoclave in the manner described only throughout a relatively short time interval so as to efiect the conversion of only the outer skin of the magnesium rod 18 into amorphous magnesium oxide, leaving the interior core thereof as the original magnesium metal. For example, the outer skin of the member 13 may be converted into magnesium oxide, so that the conversion of the member 18 is about complete, by treatment in the autoclave throughout a time interval of about two hours.

After the manufacture of the heating unit 1%), it is incorporated in an appliance, or the like, such, for example, as an electric hotp-late, as illustrated in FIG. 3. Specifically, the inner heating element 30 of a hotplate is ill-ustratcd in FIG. 3; and the heating element 3% is formed from the heating unitll) by appropriate bending thereof into the required configuration followed by flattening of the upper surface of the sheath 11, as indicated at 110. This flattening of the upper surface or top of the sheath ll, indicated at 110, of the heating element 30, not only accommodates the ready support ofa cooking vessel to be heated, but it also effects further tightening or compacting of the mass of refractory material enclosed in the sheath 11, as well as the elimination of any cracks or fissures in the refractory material that might be produced therein incident to the bending of the heating unit 10 into the desired configuration of the heating element 30.

Referring now to FIGv 2B of the drawing, the electric heating unit 40 there illustrated and made in accordance with the method of the present invention is fundamentally of the same construction as the heating unit 10 described above and is made fundamentally in the manner previously explained; whereby the heating unit 40 comprises the

corresponding elements

41, 42, 45, 46, 47, etc. However, in this embodiment the composite mass of refractory material enclosed in the sheath 41 comprises a central core portion 44 formed of crystalline magnesium tootsie oxide and an outer surrounding tubular portion 43 formed of anhydrous amorphous magnesium oxide. This transposition of the positions of the two

portions

43 and 44 of the composite body of refractory material is produced as a consequence of a modification in the assembly, as illustrated in FIG. 2A. More particularly, in producing the assembly of FIG. 2A from which the heating unit 40 of FIG. 2B is ultimately made, a helical coil 48 formed essentially of magnesium metal is arranged in the tubular sheath M in a position disposed exteriorly' of the helical resistance conductor 42 and in surrounding relation therewith. In producing the assembly of FIG. 2A the helical coil 48 may be formed essentially of a ribbon of magnesium metal and is slid in place in the tubular sheath 41 preceding the assembly of the resistance conductor 4?, and the terminals 45 and 46 therein. In the arrangement, the helical coil 48 may be in direct contact with the adjacent inner surface of the wall of the tubular sheath 41, the helical coil 48 being loosely fit into the tubular sheath ill; and of course, the resistance conductor 42 is placed centrally of the tubular sheath 41.

After the assembly of HG. 2A is thus produced, it is transferred to the loading machine; whereby the finely divided crystalline magnesium oxide 44 is charged thereinto and in embedding relation with respect to the resistance conductor 42 and the helical coil it? so as to produce the central arrangement of the crystalline magnesium oxide 44, as indicated in FIG. 2A. Thereafter, the other end of the sheath 41 is closed, in the manner previously explained; whereupon the assembly, as shown in FIG. 2A, is subjected to the preliminary rolling step, as previously described.

Then the assembly is subjected to heat treatment in the autoclave, in the manner previously explained; whereby the helical coil 48 is converted from magnesium metal to anhydrous amorphous magnesium oxide producingthe outer portion 43 of refractory material disposed in contact with the inner surface of the wall of the tubular sheath 41 and surrounding the central portion 44 of crystalline magnesium oxide. Still subsequently, the assembly is subjected to the final rolling step in the rolling machine; and thereafter the sheath 41 is trimmed and the glass plugs 47, etc, are cast in the opposite ends thereof so as to produce the finished heating unit 40, as illustrated in FIG. 2B.

In a modification of the method of making the heating unit 40, the assembly is subjected to heat treatment in the autoclave in the manner described only throughout a relatively short timeinterval so as to effect the conversion of only the outer skin of the magnesium helix 48 into amorphous magnesium oxide, leaving the interior core thereof as the original magnesium metal. For example, the outer skin of the helix 43 may be converted into magnesium oxide, so that the conversion of the helix 48 is about 10% complete, by treatment in the autoclave throughout a time interval of about two hours.

Of course, the finished heating unit 40 may be appropriately worked in the manner of the finished heating.

unit 10, described above, in order to produce the heating element 30.

In the manufacture of either of the

heating units

10 or 40, it will be understood thatwhen the corresponding

member

13 or 48 formed of magnesium metal is oxidized into anhydrous amorphous magnesium oxide, there is a corresponding expansion of the volume occupied thereby within the corresponding sheath 11 or 41, since there, is an expansion of about 200% when .a given mass of magnesium metal is converted into a corresponding mass of anhydrous amorphous magnesium oxide by oxidation. This action, of course, producesfurther compacting of the resulting mass of refractory materialin the sheath of the heatingunit further eliminating voids therein and further contributing toward the production of a dense mass of refractory material in thefinished heating unitembedcling the resistance conductor and retaining the same in place. This feature is very advantageous, as it will be understood that the composite mass of refractory material mentioned not only serves the mechanical functions described with respect to holding the resistance conductor in place in insulated condition withrespect to the surrounding sheath, but it also serves the function of transmitting the heat produced in the resistance conductor to the sheath for the useful heating purpose. It is emphasized that the last-mentioned function is very important, as it not only contributes to eiiiciency of the finished electric heating unit, but it also prevents excessive temperatures of the resistance conductor, thereby materially contributing to desirable long life of the heating unit.

Further, in conjunction with the operation of the autoclave, the processor is cautioned that the reaction temperature maintained in the heating chamber thereof must not be permitted to rise to the ignition temperature of magnesium metal, since it will be apparent that the ignition of the magnesium metal will bring about the production of an exceedingly high temperature, with the consequent melting of the adjacent resistance conductor, or even the enclosing sheath of the heating unit undergoing the heat treatment. Fortunately the ignition temperature of magnesium metal in gaseous oxygen at a gauge pressure of about 70 p.s.i. is well defined at 1085 F., whereby the reaction temperature of 1075 F. is entirely safe for this step. In this connection, it is noted that the ignition temperature of the magnesium metal is related to the pressure of the atmosphere of gaseous oxygen, the ignition temperature increasing with increasing pressures of the gaseous oxygen atmosphere. Thus it will be appreciated that the reaction 'temperature-gauge-pressure relationship mentioned is capable of appropriae variation dependent upon the factors noted; however, from a practical standpoint, the relatively low gauge pressure of about 70 p.s.i. and the readily controllable reaction temperature of about 1075 F. are recommended for coinercial production of the heating units in accordance with the present method.

In the foregoing explanation of the present method, the member 18 of the assembly of FIG. 1A and the member 43 of the assembly of FIG. 2A were described as being formed essentially of magnesium metal; however, a modification is contemplated, wherein these members are formed essentially of beryllium, magnesium, aluminum or titanium, or alloys thereof, as it will be understood that the elements named comprise a well-defined group of metals that may be readily converted from the metallic form into the corresponding metal oxide by oxidation with gaseous oxygen under gauge pressure and at an elevated temperature in the autoclave in a manner substantially identical to that described, and wherein each of the corresponding metal oxides constitutes a refractory material having good electrical-insulating and good heat-conducting properties. In each case, the operating temperature of the heating chamber of the autoclave is established somewhat below the ignition temperature of the corresponding metal or alloy so as to prevent ignition of the metal member in the atmosphere of oxygen gas under gauge pressure at the elevated temperature.

In view of the foregoing, it is apparent that there has been provided an improved and simplified method of making an electric heating unit that may be readily carried out upon a mass-production basis for commercial purposes.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. The method of making an electric heating unit of the sheathed conductor type, which comprises: providing an assembly of an elongated member formed essentially of magnesium, an elongated helical resistance conductor wound upon and supported by said member, an elongated tubular metallic sheath enclosing both said resistance conductor and said member and spaced from said resistance conductor, and a porous packing of finely divided electrical-insulating and heat-conducting refractory material arranged in said sheath and embedding both said resistance conductor and said member and retaining the same in place in said sheath with said resistance conductor in spaced relation with said sheath; confining said assembly in an autoclave; subjecting said confined assembly to gaseous oxygen at a gauge pressure so as to contact said member and to permeate said porous packing of refractory material with said gaseous oxygen; and simultaneously heating said confined assembly to an elevated temperature disposed below the ignition temperature of magnesium metal and throughout a sulficient time interval to convert said member into a charge of magnesium oxide while it is thus embedded in said porous packing of refractory material.

2. The method of making an electric heating unit of the sheathed resistance conductor type, which comprises: providing an assembly of an elongated member formed essentially of magnesium, an elongated helical resistance conductor Wound upon and supported by said member, an elongated tubular metallic sheath enclosing both said resistance conductor and said member and spaced from said resistance conductor, and a porous packing of finely divided electrical-insulating and heat-conducting refractory material arranged in said sheath and embedding both said resistance conductor and said member and retaining the same in place in said sheath with said resistance conductor in spaced relation with said sheath; confining said assembly in an autoclave; subjecting said confined assembly to gaseous oxygen at a range pressure of about 70 p.s.i. so as to contact said member and to permeate said porous packing of refractory material with said gaseous oxygen; and simultaneously heating said confined assembly to an elevated temperature disposed below the ignition temperature of magnesium metal and throughout a sufficient time interval to convert said member into a charge of magnesium oxide while it is thus embedded in said porous packing of refractory material.

3. The method of making an electric heating unit of the sheathed resistance conductor type, which comprises: providing an assembly of an elongated member formed essentially of magnesium, an elongated helical resistance conductor wound upon and supported by said member, an elongated tubular metallic sheath enclosing both said resistance conductor and said member and spaced from said resistance conductor, and a porous packing of finely divided electrical-insulating and heat-conducting refractory material arranged in said sheath and embedding both said resistance conductor and said member and retaining the same in place in said sheath with said resistance conductor in spaced relation with said sheath; confining said assembly in an autoclave; subjecting said confined assembly to gaseous oxygen at a gauge pressure so as to contact said member and to permeate said porous packing of refractory material with said gaseous oxygen; and simultaneously heating said confined assembly to an elevated temperature of about 1075" F. throughout a suificient time interval to convert said member into a charge of magnesium oxide while it is thus embedded in said porous packing of refractory material.

4. The method of making an electric heating unit of the sheathed resistance conductor type, which comprises: providing an assembly of an elongated member formed essentially of magnesium, an elongated helical resistance conductor wound upon and supported by said member, an elongated tubular metallic sheath enclosing both said resistance conductor and said member and spaced from said resistance conductor, and a porous packing of finely divided electrical-insulating and heat-conducting refractory material arranged in said sheath and embedding both said resistance conductor and said member and retaining the same in place in said sheath with said resistance conductor in spaced relation With said sheath; confining said assembly in an autoclave; subjecting said confined assembly to gaseous oxygen at a gauge pressure of about 70 p.s.i. so as to contact said member and to permeate said porous packing of refractory material With said gaseous oxygen; and simultaneously heating said confined assembly to an elevated temperature of about 1075 F. throughout a sufiicient time interval to convert said member into a charge of magnesium oxide While it is thus embedded in said porous packing of refractory material.

References Cited in the file of this patent UNITED STATES PATENTS Woodson June 10, 1930 Backer May 10, 1932 Deroche May 7, 1940 Heath et a1 Feb. 17, 1942 Dahl Nov. 23, 1948 Thomas Mar. 2, 1954 Huck Feb. 21, 1956 Andrews Oct. 28, 1958 Andrews Aug. 11, 1959 FOREIGN PATENTS Great Britain Jan. 15, 1931 Great Britain 1 Nov. 2, 1942 Great Britain Nov. 3, 1948 UNITED STATES-PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, -3 OO'Z 2S6 November 7 I961 Emmett W. Barnes It is hereby certified that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as corrected below.

Column 7 line 74!; after "sheathed" insert resistance column 8 line 35 for "range read gauge Signed and sealed this 1st day of May 19620 (SEAL) Attest:

ERNEST w; SWIDER AVID L. LADD Attesting Officer Commissioner of Patents UNITED STATESPA'TENT OFFICE CERTIFICATE OF CORRECTION Patent No, 3 OO7 236 November "I 1961 Emmett Wa Barnes It is hereby certified that error appears in the above numbered pat ent requiring correction and that the said Letters Patent should read as corrected below.

Column 'Z line 74 after "sheathed" insert resistance -5 column 8 line 35 for "range" reed gauge Signed and sealed this 1st day of May 1962,

(SEAL) Attest:

ERNEST w; swmza, I DAVID L. LADD Attesting Officer Commissioner of Patents