US2271975A

US2271975A - Temperature responsive device

Info

Publication number: US2271975A
Application number: US327901A
Authority: US
Inventors: Chester I Hall
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1940-04-04
Filing date: 1940-04-04
Publication date: 1942-02-03
Anticipated expiration: 1959-02-03

Description

Feb. 3, 1942. l

u 'TEMPERATURE RESPONSIVE DEVICE Filed April 4, 1940 @bgg mmc@ C. l.` HALL i 3 Sheets-Sheet 1 [LINE/V7' RESISTANCE@ @HHS n HiaAAttor-neg,

REFERENCE Feb. 3, 1942. c. l. HALL 2,271,975" TEQPERATURE RESPONSIVE DEvIcE l Filled Ap'ril 4. 1940 s sheets-sheet 2 Inventor: Chester- I. Hall,

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"CROSS REFERENCE" Feb. 3, 1942. c. l. HALL 4 TEMPERATURE RESPONSIVE DEVICE l "Fi1ed April 4, 1940 3 Sheets-Sheet 3 600 1000 15mm me: oran-:s F

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Patented Feb. 3, 1942 f mums assronsrvs navos chuter l. nali, scnenemay. N. Y., minor to General El New York eetric Company, a corporation of annue-suon Apen i, me, sensi No. :zam

(ci. :o1-5m C 3 .12 Claims.

This invention relates to temperature responsive devices. and it has for its object the provision of an improved device ci this character and method of making it.

This application is a continuation-in-part of my copending application, Serial No. 281,856, illed June 29, 1939, which is assigned' to the same assignee as this application.

More specifically, this invention contemplates the provision of a temperature responsive device which may be used, for example. to control an electric circuit in accordance with temperature, the responsive device being provided with an element the resistance of' which varies with temperature, that is. one having, for example. a high initial resistance that is reduced'as the temperature of the element increases. ind this invention contemplates the provision of an improved temperature responsive device of this character which is extremely sensitive to temperature changes; which may be operated through a very wide temperature range, and at very high temperatures; one which will provide extremely large variations in resistance with small temperature changes: one that will give constant resistance values and characteristics when manufactured and also after long continued service, that is, a control device which is reliable and stable in its operation; and one which is simple, rugged, and mechanically strong; and it further contemplates an improved method of making the control device that insures a reliable and uniform product, and which may be manufactured cheaply with comparatively simple manufacturing methods.

In accordance with this invention, a suitable temperature sensitive resistance material is highly compacted, and is intimately bonded with its electrodes that are suitably prepared'in accordance with this invention to insure the intimate relationship between the highly compacted resistance material and their surfaces that contact the material. One of the electrodes may be in the form of a casing housing the insulating material, and preferably will be `arranged so that it'may be placed directly in contact with the medium or body to whose temperature the device is responsive. In this manner, the time required for the transfer of heat from the body or lmedium to the temperature sensitive material is 'greatly reduced.

The resistance material in granular form is highly compressed, preferably in relatively short sections, and while in contact with the electrodes.

` ance material so compact, coherent mass having uniform density throughout and having considerable mechanical strength. This mass is bonded to the electrodes so as to be in intimate thermal and electrical contact with them. For this purpose. the electrodes before the sintering operation will have been provided with coatilgls of a flux and an oxide of the electro ng the sintering operation, these coating coal ,ith the resistn eilect the resistance material is sintered into a coherent mass with the electrodes.

The temperature sensitive device forming the subject matter of this application may be employed either as an element of a pyrometer or thermostat, that is either as a device to record temperature or to control temperature. Any suitable means may be used to'determine the resistance of'the device and hence inferentially the temperature to which it is subjected. By way of example, if the device is connected in series circuit with a source of potential, the current flow in the circuit will vary in functional relation to the resistance. Hence, a 4current measuring device connected in the circuit may be graduated to read directly in temperature.

Likewise, the device may be used in conjunc- 'tion with control devices to regulate automatically the temperature to which it is exposed or to control an agent or medium in accordance with that temperature. Such control devices may be actuated directly or in conjunction with the measuring devices previously referred to.

For a more complete understanding of this invention, reference should be had to the accompanying drawings in which Fig. l is an elevation of a temperature responsive device arranged in accordance with this invention; Fig. 2 is an enlarged fragmentary sectional view of the device of Fig. 1; Fig. 3 is a diagrammatic view illustrating the resistance-temperature relationships for various'resistance materials used as the temperature 'sensitive element; Figs. 4, 5 and 6 are elevations illustrating certain devices used in making the control device of Figs. 1 and 2, the elevation in Fig. 4 being in section; Fig. 7 illustrates diagrammatlcally a. heating member used to heat-treat the temperature sensitive element in accordance with this invention; Fig. 8 is a diagrammatic representation showing the relationship between the resistance of the control device and time that is effected in one of the heat-treat- It is' suitably heat-treated to sinter it into s. 55

ing steps used in making the control device; Fig. 9 is a view similar to Fig. l, but showing a modined form of this invention, and having parts in section; Fig. 10 is an enlarged sectional view 'of the modified formof the invention shown in Fig. 9; Figs. 1l, l2 and 13 are diagrammatic views illustrating certain means for modifying the CXDBDSIOB. Mixtures f u 1.1!

sodium silicate 519ml wi out`any copper "oxide, may 'used in different proportions to obtain different desired characteristic curves and coemfundamental relationship between temperature 6 cients of expansion; and still other variations and resistance of the control device of this invention; and Figs. 14, 15 and i6 are curves showing the changes in the fundamental resistance characteristics of the control device elfected by the controlling elements shown in Figs. 11, 12 and 13 respectively.

Referring to Figs. l and 2. this invention has been shown in one form in connection with a temperature control device for controlling the circuit of an electric furnace (not shown). As shown, the control device comprises a cartridge III which is provided with a pair of electrodes II and I2 spaced from each other. The inner electrode II is inthe form of a core. while the outer electrode I2 is in the form of a cylinder. closed at one end, as shown. Interposed between the electrodes and embedding the inner end of the electrode II is a mass I3 formed of resistance material having a negative temperature coeillcient.

The material of which the inner and outer electrodes II and I2 is made may-vary widely, the maximum temperature at which the control device is to operate being one of the chief factors to be considered in selecting the material that will be used; for example, for temperatures ranging between zero and 800 F. a metal known as Monel" may be used, while if the temperature range should vary between 100 and 1000 F. it is preferable to use nickel.

The material Il will be chosen in accordance with the characteristic temperature-resistance curve desired, and also so as to provide an insulating mass having substantially the same thermal coemcient of expansion as have the electrodes that will be used. For a particular desired temperature-resistance curve and where nickel electrodes are used, a mixture of m nesium oxide, sodium silicate and co r oxide Vwi be used Having a ratio 5y weight of .2 of copper oxide to 2.52 of maesium oxide and 1.17 of cium sili te. The sodium silicate is made up of iicon oxide and sodium oxide, and it is preferable that these materials shall be in the ratio of approximately l:3.22, although this ratio may be varied to give erent desirable physi cal characteristics.

Different temperature-resistance characteristice and coemcients of thermal expansion may be obtained yby varying the proportions of the ingredlents used. Generally it is Ipreferable to vary the percentage of the copper oxide in the mixture. Fig. 3 illustrates how the resistance characteristics will vary with temperature with dinerent ratios of copper oxide. Curve a corresponds to the above example where the resistance has a ratio by weight of .25 of copper gmde, to A 2.52 of magnesium oxirle and TA? of silicate; curve"b"is`the""curve of a "where the ratio by weight of coppergwxide k changed to 1; c where itis changed to 3; and d where it is changed to 6. When the ratio of copper oxide isvaried tocontrglmtne the o vmagnesium oxide to sodium silicate should be substantially constant for the various resistances.

However. the percentages of magnesium oxide, sodium silicate and copper oxide all may be varied to obtain different temperatureqesistancc charmayv be had by substituting zirco aluminum oxide and the like for the magnesium oxide, an using zinc oxide. nickel oxig chromium oxide and' the like in place of the coppe'r ox e. A wide range of temperature-resistance characteristic curves, and coefiicients of expansion may be obtained by selecting different ones of these materials and using them in different proportions.

The resistance I3 is highly compacted and the various ingredients of the resistance are sintered to each other and are also intimately bonded to the outer surfaces of the inner electrode II and to the inner surfaces of the outer electrode I2.

The control device thus far described is made as follows. The inner surface of the outer electo formfawe of the oxide and silicate on the su aces of the electrodes.

The outer electrode I2 is then mounted in a form Il (Fig. 4) having great mechanical strength. This form Il, as shown, is provided with a hole or recess I5 which will have such a diameter that the outer electrode I2 will have a sliding fit with it. A measured quantity of the resistance material I3 in granular form is then poured into the outer electrode I2.

acteristics. and to provide dinerent coefncients of "5 It is to be understood that it is desirable that the material Il be in the form of fine granules that will pass through a 200 mesh screen. Moreover. the granular ingredients which make up the material Il musi'I be thoroughly mixed. Furthermore. it is desirable that these materials be chemically pure and anhydrous. Before the granular mixture be used. it is preferable that it be heated for about 3 hours at around 400 F. in order to drive oi! any moisture that may have been absorbed by the mixture while it is being handled. and preferably it will be introduced into the electrode I2 while still hot. After the measured quantity ofthe mixture has been placed with in the outer electrode I2, it is subjected to a great pressure by means of a suitable plunger. such as the plunger I6 shown in Fig. 5. Preferably. the mixture will be compressed at about ten tons per square inch. This forms a small tablet I1 in the inner end of the electrode I2. as shown in Fig. 4 and as indicated in dotted lines in Fig. 2. It will be observed that the lower end of the plunger Il which contacts the resistance granules is conical so that the tablet I'I will be provided with an upper coni- """fcRo'sS REFERENCE shown-inlilg.ti,willbeused. Asshown.this

plunger is 'provided with a passageway il throughout its length: this passageway receives sembly and the form I4 is provided with an aperture 20 through which a suitable instrument (not shown) may be inserted to drive the assembly from the form. f

'Ihe assembly of the two electrodes Il and I2 and the compressed resistance material Il is then placed in a suitable furnace, such as an electric' furnace, where it is heated for about one hour at a temperature of around 950 I'. It is brought up to this temperature from room temperature relatively slowly so that any moisture or water which might be contained in.the powder will not generate steam which might blow out or disturb 4portions of the powder. Preferably. it should require about one hour to bring the unit up to this temperature of 950 F. 'Ihisheatlng operation drives out any moisture which may` have been picked up by the resistance material, or which may not have been driven out by the previous drying out step referred to above.

The assembly of the electrodes and resistance powder maybe heated in this manner in a tube Il as shown in Fig. 1. This tube has one end closed and the other end open, and is sumciently long to receive the entire cartridge unit il.

, Preferably, the walls will be made of a suitable high temperature porcelain material. The tube and cartridge assembly preferably will be heated in a high temperature electric furnace.

,After the assembly has been heated for one hour at 950 F. the energy rateto the furnace is increased so that in a period of about two hours a high sintering temperature will be reached. I'his temperature while high should be well below the melting temperatures of the materials. -l'br the above specific example having a ratio by weight of .25 of copper oxide to 2.52 of magnesium oxide and 1.47 of sodium silicate a sintering temperature of aroundggglgkwill be used. The

' bly is held at temperature for about ten minutes and then is permitted to cool down slowly to atemperature of about 1100:. F. where 1t is nele for about 1s hours, af erw@ it n permutedweqol.: u

One end o`f tle furnace 2l is left open during the above-described heating steps so as to expose the resistance material il within the outer electrode i2 to a normal air atmosphere. while the outer surfaces of the electrode I2 are in an atmosphere substantially free from oxygen. It is important that the contents of the electrode i! be thus exposed to air, particularly where one Manganime tustfthsee formcuoshssatendencytoreduceto it te fee particlesofthegrsnuls'mixturesothatin the resistance material It is formed intoa medium high temperature and for a relatively long period of time.

an air atmosphere. The same furnace 2| may be yused for mixture of the above example, the assembly is aged at a temperature of around 1000 to 1100 and for a period of about 40 to 50 hours. The

aging temperature is selected so as to be some-- what higher than the ultimate temperature at whiclr the element is to be operated. While the v miiliamperes. The aging process causes all of vFil. 8.

the copper oxide in the form of CuO, which has not reverted to CuOs. to go back to this stable form. This stablizes the resistance. 'nie effect of this aging treatment is shown graphically in Here the resistance at the end of the nring is likely. to be higher than the stabilized resistance. as shown by the curve, but after about 40to50hoursofagingitwilldropdownand will become stable due to the conversion of the copper oxide from the form CuO to CuOs. Moreover. I have found that the aging process greatly improves the Joint or bond between the electrodes and the resistance mass. i

The assembly after "aging is then assembled with an extension tube 22 which is used to support the cartridge Il ih assembled relation with the medium whose 'temperature is to be controlled, and which tube may be formedof any suitable material. such as steel. Por example, if the control device is to be'used to control the temperature of an electric furnace. the tube l! willbepassedthroughthefurnacewallsoasto hold the cartridge Il in position to be immersed in the furnace atmosphere. The length of the supporting tube Il will depend. of course, upon the particular application of the control element Il. Preferably, the tube 22 will be threaded on the electrode i2, as shown, the electrode and tube having relatively fine inferfitting threads. The threaded joint between the electrode and tube will be sealed by a suitable high temperature cement which is placed on the threads before they are threaded together..

A suitable lead- 23. which preferably will be formed of nickel. is brazed to the inner electrode of the ingredients is copper' oxide, because in 7g I2 and extends outwardly through the tube 22. 'Ihis lead is provided with a covering 24 of a suitw XANHNI able electrically insulating material, such as asbestos.

I'he outer end of the tube 22, that is its righthand end as viewed in Figs. l and 2 is closed by means of an electrically insulating terminal block 26 which, as shown, is fitted over this end `like a cap. The block is made of any suitable material. such as a suitable asbestos board. This terminal block is provided with an aperture 2t through which the lead 2l is passed and it carries a terminal 2l (Fig. 1) to which the lead is brazed. The terminal block 25 also carries a terminal 2l to which the lead 29 is electrically connected. This lead 29 is directed through an aperture I0 provided for it in the terminal block and it is welded, as shown. to the outer surface of the tube 22, the tube 22, of course, constituting a continuation of the outer electrode i2.

The terminal block is secured by means of a set screw 30a; and the Joint between the block 25 and the outer surface of the tube 22 and between the inner walls of the channel 26 and the lead 23 are covered with a suitable electrically insulating sealing cement, such as a suitable alkyd resin.

In the form of the invention shown in Figs. 9 and 10, the cartridge Il is provided with inner and

outer electrodes

32 and 33 which are both in the form of cylinders closed at one end, as shown.

Here the two electrodes are separated by a mass 2l formed of the resistance material having a negative temperature coefficient and which ma terial will be the same as the material I3 of the first form.

The cartridge 3l is made in substantially the same way as the cartridge I0. Thus, the `inner walls of the cuter electrode and the outer walls of the inner are provided with coatings comprising the oxides of the electrode walls and a suitable flux, such as sodium silicate; the resistance material 34 is prepared in the same way as the material i3; the electrode 3| is placed in a form and a tablet of the resistance mat'erial, similarto the tablet I1, is formed in its inner end; the inner electrode is inserted with its tapered end fitted in the tablet; and the rest of the resistance material 3l is then compressed into place. The assembly is then given the treatments described in connection with the first form, whereby acoherent mass of sintered resistance material 34 is lprovided embedding the inner end of the electrode 32 and integrally united with the surfaces of the electrodes which it contacts. Here, a supporting steel tube 35 is secured t0 the outer end of the electrode 33- In this case the tube 25 is welded to the electrode. An insulated lead 26 has its inner conducting end welded to the inner electrode 32 while its outer end is passed through a conducting bolt 31 which has a central passageway, as shown, for receiving the lead. This outer end of the lead is soldered to the outer end of the stud 31. 'I'his stud 31 passes through an insulating terminal Abloclr or head 28 formed of any suitable electrically insulating material, such as asbestos board. As shown, this member 38 has a centrally arranged aperture through which the ably will be sealedto the inner walls of -the tube 1o l5 and tothe stud Il by means of a suitable electrically insulating celrient,- such as alkyd resin. l t

It will be understood that for my given temperature responsive cartridge having a particular temperature sensitive resistance the cartridge will have a fundamental temperature-resistance relationship. That is, the resistance of theelement wilidrop rapidly with temperature lncrease. along a nxed curve, which is incapable of modification after the cartridge has once been produced since the slope of the curve and the actual values of the coordinates are dependent entirely upon the quality and kind. of resistance materialused and the process through which it has been carried. For example, with a specific cartridge, the resistance of the element will vary with temperature as shown by the solid curve in Fig. 14. At times, it is desirable to modify the shape of this curve and this may be accomplished by suitable controlling resistances inserted in the controlled circuit in which the controlling cartridae is placed. Referring to Fig. ll, the rechanges. Moreover, the resistance mass is bond -on the fundamental sistance of the cartridge is indicated Ri- If a series resistance Rz be placed in the controlled circuit, as shown diagrammatically in Fig. ll, the high temperature end of the curve alone is affected, as indicated in dotted lines in Fig. 14. This is becausea fixed series resistance becomes lproportionately a lower ratio of the total resistance as the temperature is reduced.

On the other hand, a parallel resistance will affect only the cold end of the curve since its ratio of effectiveness is entirely negligible when' the control element resistance Ri is low. Fig. 12 shows a resistance R: connected in parallel with the resistance Ri, and Fig. 15 shows in dotted lines the eiiect of the addition of this resistance temperature-resistance curve.

In Fig. 13 both a series resistance R5 and a parallel resistance R4 are used. In this case one resistance is effective at one end of the curve and the other at the other end so as to modify` the slope of the curve throughout, as shown in Fig. 16.

It will be understood, of course, that the effectiveness of the controlling resistances may be varied as desired by any suitable adjusting means so that the slope of the fundamental characteristic curve 'for any particular cartridge elemen may be varied.

It will be observed that I have provided a temperature control element which is extremely sensitive to temperature changes. 'I'he sensitive resistance is in intimate thermal relation with its outer electrode which functions as a casing to bring it into contact directly with the medium whose temperature is to be controlled; the particles forming the resistance sensitive element are compacted under a tremendous pressure and are sintered so as to form a compact, coherent mass. This coherentcompacted body is bonded and substantially integrally united with the surfaces of the outer electrode -which is immersed in 'the medium whose temperature is to be controlled which insures a minimum time lag in the response of the'control device to temperature ed with the outer electrode land also with the inner electrode to eil'ect a very good electrical connection between these members. In addition, the unit is mechanically strong and will withstand considerable abusage before it will be damaged, far more abusage than will be encountered in its controlling applications: it is stable and durable: and is physically small so that its range of application is materially extended. It will also be observed in view of the foregoing description .of the method of making this unit that relatively simple manufacturing steps are used. In addition to this, the construction of the element in its various parts and the method whereby these While I have shown particular embodiments of my invention, it will be understood. of course. that I do not wish to be limited thereto since many modifications may be made, and I, therefore, contemplate by the appended claims to cover any such modifications as tall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A temperature responsive device comprising a tubular metallic electrode closed at one end, a second metallic electrode inserted in said tubular electrode and spaced from its walls. and a temperature sensitive resistance material within said tubular electrode embedding the inner end of said second electrode formed of a preselected mixture of granular magnesium oxide, sodium silicate and copper oxide. the granular particles of said mixture being highly compressed and sintered into a coherent mass and bonded intimately with the surfaces of said two electrodes.

-2. A temperature sensitive device comprising spaced metallic electrodes, a sintered mass of temperature sensitive granular resistance material between surfaces of said electrodes, said surfaces being provided with iiuxed coatings intimately bonded with said surfaces and also sintered into a coherent mass with said sintered granular resistance material.

3. A temperature responsive device comprising a tubular metallic electrode closed at one end, a second metallic electrode inserted in said tubular electrode and spaced from its walls. granular resistance material within said tubular electrode embedding the 'inner end portion of said second electrode sintered into a coherent mass that is bonded intimately with surfaces of said electrodes. a tubular supporting extension supporting said outer electrode. a lead connected to said inner electrode extending through said tubular extension, a lead connected to said tubular extension, an insulating member closing said tubular extension and having apertures through which said leads pass, and means for sealing the joints between said electrode and said extension.

and between said extension and said closing member, and also for sealing said closing member to said lead that extends from said inner electrode.

4. The method of making a temperature responsive device which comprises inserting a metallic electrode into a tubular metallic electrode that has one end closed so that the ilrstnamed electrode is in spaced relation with the 4walls of the latter, inserting granular temperature sensitive resistance material inw said tubular electrode in relatively small quantities at a time and highly compressing each quantity of the material after it has been added and before the next quantity has been added, and then nring at a high temperature to sinte'r said granular resistance material.

5. The method of making a temperature responsive device having a pair of spaced electrodes joined by a temperature sensitive resistance material formed of magnesium oxide. sodium silicate, and copper oxide which comprises mixing granules of said materials together. placing them between said electrodes. compressing said granules to highly compact them while in contact with said electrodes, heating said compacted granules slowly until a temperature of around 950 Il'. is reached in about one hour. holding said temperature for about one hour, increasingl ance material formed of magnesium oxide, so`

dium silicate. and copper oxide which comprises mixing granules of said materials together, heating said granules at a temperature of aroundy 400 F. for about three hours, placing said gran'- ules whileheated between said electrodes, compressing said granules Vto highly compact them while in contact with said electrodes. then gradually raising the temperature of said compacted granules in about onelhour to a temperature of about 950 F. and holding said temperature for about one hour, increasing the temperature of said compacted granules so that in about two hours it reaches a sintering temperature of around 1975 F. and holding said temperature for around ten minutes. and then permitting said insulating material to cool to a temperature of around 110,0 Il'. and holding said temperature for about sixteen hours. v

7. The method of making a temperature responsive device having spaced metallic electrodes and a temperature responsive element formed of a sintered granular temperature sensitive resistance material between them comprising forming on the surfaces of said electrodes that shall contact said resistance material coatings consisting of a mixture of a metallic oxide and a iiux that are capable of being sintered to said granular resistance material. compressing granules of said resistance material into a dense mass in contact with said coatings, and then sintering said granules to cause them to coalesce with each other A and with said coatings.

A ings 8. The method of making a temperature responsive device having spaced metallic electrodes and a temperature responsive element formed of a temperature sensitive resistance material between them comprising heating surfaces of said 9..'I'he method of making a temperature re- 75 sponsive device having spaced electrodes and a temperature sensitive resistance element joining 4 said electrode which comprises forming a tubular electrode, and also a second lelectrode adapted -to be inserted in said first electrode in spaced relationwith its walls, inserting a quantity of said resistance material in said first electrode, highly compressing said material to form a relativelyl of pressure steps between said electrodes. and

then highly heating the assembly of electrodes and resistance material so as to forma substantially coherent mass of said resistance material.

10. The method of making a temperature responsive device having spaced electrodes and a temperature sensitive resistance element Joining said electrodes which comprises forming a ltubular electrode and a second electrode adapted to be inserted in said first electrode in spaced relation with its walls. inserting a quantity of a mixture loi magnesium oxide, sodium silicate 'and copper oxide granules in predetermined proportions in said iirst electrode and highly compact.- ing said granules in the spaces between said electrodes. and highly heating the assembly o! velectrodes and granules to sinter the granules into a coherent mass while maintaining a non-oxidizing atmosphere outside o! said assembly and an oxidizing atmosphere within said first electrode.

11. The method o! making a temperature re-A sponsive device having a pair o! spaced electrodes Joined by a temperature sensitive resistance material formed o! magnesium oxide. sodium silicate. and copper oxide which comprises mixing granules of said materials together, placing them between said electrodes. compressing said granules to highly compact them while in contact with, said electrodes, heating said compacted granules slowlyvuntil a temperature of around 950 F. is reached in about one hour. holding said temperature for about one hour, increasing said temperature in about two hours to around 1915 F. and holding said temperature for around ten minutes, and then permitting said granules to cool down to a temperature of around 1100" F. and holding it at this temperature for around sixteen hours, and then aging" said resistance material from around forty to iifty hours at a temperature of around 1000 to 1100 F.

12. The method ot making a temperature responsive device having a pair oi.' spaced electrodes Joined by a temperature sensitive resistance material formed oi' magnesium oxide, sodium silicate, and copper oxide which comprises mixing granules o! said materials together, placing them between said electrodes. compressing said granules to highly compact them while in contact with said electrodes. heating said compacted granules to sinter them into a coherent mass. and

CHESTER I. HALL.

f caimrrcirs or consorten.

Patent No. 2,271,9Y5.v

crmsrsn I. HALL. f

^ February 5, 19112.

Itis hereby certified thaterror'appears 1ni the printed. specification ofthe above numbered patent requiring correction as follows: Rage'Z, first column, 11ne 555lt, for "characteristice" readcharacteristics-Qg 'page il,

second column, line 1|., forv "my'i 'rnd 'W....

page 6, 'first column, -line 2, claim 9, for "electrode" read l-e1eci:ro'iesv'; artiA 'that the said Letters Patentshould be read with this correction that the smemiey conform to the record of the case in. the Patent Office. s

signed and sealed this alim dey or web', A. o. 19112. f

` (seall Henry Ven lirsdale, Acting Commissioner of Patents.

a y cEmFIcATE'oF conmcnon. l Patent No. 2,271,975. February 5, l9l|.2. .I

CHESTER I. HALI. o

Itis hereby certified tknterrorappears in the printed specification ofthe above numbered patent Arequiring correction as follows: Rage'2,.first column,V 1ine 5551|., for "characteristics" read --characteristice-;fpage h., second column, line h, for "my". fread --any--glpage 6, first column, -line 2, claim 9, for l"electrode" read `--electrode's--g and. that the said Lettere Patentshould be read with this correction that the eamemay conform to `the record of the case inthe Patent Office.

d signed and sealed this aim; day or nag-ch, A. D. 19142.

Henry Van Arsdale, {Seal} Acting Conmissioner of Patents.