US 1695302 A
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Description (OCR text may contain errors)
ec F. w. THOMPSON 1695302 ELECTRIC HEATER Allorneyo Dec 1'8, 1928. l 1,695,302
F. w. 'rHoMPsoN ELECTRIC HEATER Filed April `12. 1922 'r sheets-Sheet 2 y llorneyo Dee; 1s, 192s, 1,695,302 1- F. W. THOMPSON ELECTRIC HEATER Filed April 12. 1922 '7 Sheets-Sheet 3 Inventor Fra/a6 14( Thompson B (Swain I im# M Alfomcya Dec. 18, 1928.
1,695,302 F. w. THOMPSON ELECTRIC HEATER Filed April 12. `1922 y 7 Sheets-Sheet 4 llorncya F. w.` 'rHoMPsoN ELECTRIC HEATER Filed April 12. 1922 7 Sheets-Sheet 5 HIM Invenlor Allorneya Dec. 18, 19m28.
Dec. 18, 1928. 1,695,302
F. w. 'rHoMPsoN ELECTRIC HEATER Filed April 12, 1922 7 Sheets-Sheet 6 Allorneyo De@n '18, 192s. 1,695,302
F. W. THOMPSON ELECTRIC HEATER A Attorney Patented. Dea 1s, 192s.
. 1,695,332 Nr omer.,
rmx w. 'rnoxrsoia or wiis'r HAVEN, ycoNNiicarcirr.
. n LnG'rnIc narran.
Application ma apra 12,
This inventionl relates to electric heating, and moreparticularly to a methodfor'heating electro-conductive articles comprising the passage ofelectricity therethrough, and to an 5 ap aratus for carrying out the new method.`
' Y n certain manufacturing o erations, it is desirable to heat-metal b a shapes, and whether of uniformV cross sectional area or of irregular cross-section', to a 19 .uniform temperature,l or tov heat specified pora .tions of the blanks to *certain temperatures, ,i 'i while other portions are heated to different temperatures or are notheated at all, as the casemaybe.:
.An importantobject of this invention v1s to provide a metho and means for heating electro-conductiveblanks of varying or uniform cross-section throughout an `specified portions to any number of di erent tem era- 20 tures, by the passage of electricity throug the blanks. l
In heating short metal bars such as rivets, it hasbeen thepractice, in .order to. obtains uniform ow of current through the bar and good electrical contact between the bar and the electrodes, to' compress the bar lbetween two electrodes 'of opposite polarity, each end of the bar abutting against an electrode.. Al though this method of holding a bar to vbe v heated is reasonably satisfactory for short thick bars having prepared'ends, it isof comparatively little vuse with Vmany'articles of. less strength orhaving rough or irregular ends, .since the pressure ofthe electrodes nec- ,essary to vestablish good electrical contact, even with prepared ends, may be so great as 'to deform Vsuch articles and mushroom the ends abutting the electrodes when a high temperatureis reached; while itlwould be diicult, if not impossible, to establish good elec- "trical contact with the irregular-and unpreardd ends foundon many articlesv to be ea e f f It is another obj ect this invention-prof.)
a. vide 4means forjh'oldingany article he ed without'materallydeformin" thesauie a any te 'peraturebelowj-its meltingpoint which it maybe heatedi, A .furthfrf` object-'isl .toprovide holdin' MI; means which willcoiitrol lthe distributionfo heat throughout any desired portion f the article; -For example, difficulties' hayeybe'enf encountered in .heating that; 'rtio'nrof anl'article which contacts with 't e electrodes, 0.6 audit is a particular object of the invention:
Y of various A temperature at one rate wvor cooling the article at vvvai'furce"of'higher potential and applying a source oflwerpotential 1922. Seritl No. 551,864.
to provide electrodes whereb the contacting portions orf-,the article may e heated `to an approximately uniform .temperature or iven any desired temperature gradient, or w ereby the'he'ating of such portions may be controlled at will.
In numerous industrial processes, for example in forging, itis necessary to heat a series of articles uniformly to a definite predetermined temperature, repeatingthe process over and over with a large number'of substantially identical articles. A Moreover .A theoperator is not always ready to perform the next step .upon the article at the instant l.when it attains the required temperature, and good results would not be accomplished if the heat were merely .shut oil, since cooling would begin at once and the article would not be at the proper tem rature when the operator was ready to per orm the next step. Again in heat treating certainarticles, it is desirable to heat them atone rateto a given temperature and thereafter either to maintain such temperature constant,jor to increase or decrease the temperature at a different rate until a final desired value is reached, which temperature. maybe maintained constant. A .further obj ect of the invention is to provide means vfor automatically .heating an electro-conductive article, to a predetermined then further heating a diierent rate until v ajinal predetermined temperature isreached, and thereafter maintining such temperature substantially constant for any desired period,
or for maintaining constant. the predetermined 'temperature top which .the article was heatedat first. f
` It is st'll another object of the invention to provide a methodv of controllintgh'electric .circuits in response to a. -hagein e resistance of an 'article beingfheated resulting'from;
a change. ofthe temperature hf the article.
y In accomplishing 'these objects'the're may be provided twogor more sources dielectric .power 'kiferentjjpotentiah means vfor applying source articletobe heats an easfres'ponsiveto apredetermined increase fresistance. ofthearticle for A.cutting 'oil a infpractice that this 'nethod ofhe'a an ,A
ai'ticle may 'be most economically `out heated.
In order to apply the current to the article to be heated without undue losses at the point of contact, or overheating of the electrode, and to avoid deforming the article to be heate'd, each electrode may advantageously comprise a clamp having two or more relatively movable jaws, which may be forced together to grip a strong portion of the article sutiiciently tightly to give good electrical contact without tending to mushroom the portion gripped. Since the article is supported b v two such clamps, it is not. thereby subjected to any force which might tend to deform it, as would be the case it' it were compressed between electrodes at opposite ends. The use of clamps gripping the sides ot the article requires specially shaped electrodes of special composition, since otherwise the ditl'erence of resistance of the currentpath through the article being heated between ditl'erent portions of the electrodes Vof opposite polarity would cause an uneven distribution of the current and of'the heat. The details ot a number of satisfactory electrodes will be given in the following description.
One embodiment of the invention as applied to the heating of metal bars for forgin 0'.
as been illustrated in the accompanying drawings and will be described in detail, but it is to be understood that the invention is applicable to the heating of other articles, that the new method can be carried out by other means than those speciiically described, and that the description is merely by way of eX- ample and not to limit the invention as defined in the appended claims.
In the drawings:
Fig. 1 is a circuit diagram of one. electric heater for carrying out. the invention;
Fig. 2 is a plan view with certain parts broken away of such an electric heater;
Fig. 3 is a front sectional elevation taken on the line 3-3 of Fig. 2 as seen in the direction of the arrows.I showing certain paits of the heater and of the operating mechanism for a circuit changer;
Fig. 4 is an elevation showing on a larger scale the circuit changer operating mechanism seen in Fig. 3, together with the right hand end of a circuit changer operated thereby; part of the operating mechanism appearing in normalposition in solid lines and in an intermediate position in dotted lines;
Fig. 5 is a view of the elements shown in Fig. 4 in operated position in solid lines, and in another intermediate position in dotted lines; v
Fig. 6 is a plan View of a transformer forming part ofthe heater;
Fig. 7 is a front elevation of the transformer shown in Fig. 6;
F ig. 8 is a section taken on the line 8 8 of Fig. 7 as seen in the direction of the arrows;
Fig. 9 is an elevation showing the front.
o't` the circuit changer in normal position as viewed from the left of Fig. 4;
Fig. 10 is an elevation showing in operated position certain of the parts shown in Fig. 9;
Fig. 11 is an elevation showing in operated position the left` hand end of the circuit changer together with a controlling magnet;
Fig. 12 is a section taken on the line 12h12 of Fig. 9 as seen in the direction of the arrows Fig. 13 is a section taken on the line 13-13 of Fig. 2 showing a detail of the heater:
- Fig. 14 is a view on a larger scale and partl v in section of the electrode mounting shown in Fig. 3;
Fig. 15 is a section taken on the line 15-15 ot Fig. 16, showing on a still larger scale the electrode shown in Fig. 14;
Fig. 1G is a sectional plan view of a bar to be heated abutting an electrode;
Figs. 17, 18, 21 and 22 are views similar to Fig. 16 showing various modifications of the electrode;
Fig. 19 is a view similar to Fig. 15 showing another modification;
Fig. 20 is a section takenon the line 20-20 of Fig. 19;
Fig. 23 is a plan view of certain portions of a heater and an article to be heated having a reduced central portion;
Fig. 24 is a central transverse section taken on the line 24-24 of Fig. 23;
Fig. 25 is a plan.view of certain portions of a heater and an article to be heated having an enlarged central portion, and
Fig. 26 is a transverse section of a dashpot to cooperate with a solenoid.
The purpose of the various structures to be :described will be more readily understood after a brief consideration of the circuit diagram in Fig. 1. In the diagram, 1 and 2 indicate conductors connected to a suitable source of alternating current which may be connected to conductors 3 and 4 by the closure of a switch 5. At the right of the diagram is shown a step-down transformer having two. vprimary windings, I1 consisting of a relatively large number of turns of fine wire, P2 consisting of a relatively small number of turns of a larger wire and a secondary winding S consisting of a single open turn which preferably comprises a heavy bar of negligible resistance as compared with that of the primary windings. To vary the number of turns of primary winding P1, which ma at any time be included in the circuit, the winding is tapped at suitable intervals and the successive taps connected to successive iiXed contacts of a rotary switch 6; Similarly,
winding P the number of turns in primari'l l t 1e position may be varied in accordance wit of a switch 7.
In order to control the connection of the transformer to the source of power, there are i v provided a circuit changer 8 and a solenoid switch 9. The circuit chan er in the present embodiment comprises a cy indrical drum 10 of wood or other suitable insulating material, carried by a shaft 11 rotatably supported on standards 12. Drum 10 carries a contact 1-3, a contact 14 and a row vof contacts 15, each comprising a strip of copper secured "to the drum, the strips 13,14 and 15 being arranged to interconnect pairs of contact springs 16, 17 and 18, respectively'. The contacts are shown in solid lines intheir': normal or first operative position, in -which spring pairs 18- are interconnected by strips 15,v while spring pairs 16 and G,17 are open. 'When drum 10 is rotated in the direction of the arrow by operating mechanism to be' described, the contacts move to the positions indicated in dotted lines, first opening springs 18, next closing springs 17 and a fraction of a second later closingsprings 16. To hold the drum 10,- in this second operative position against the force of a spiral sprin 19 constantlyurging it into the normal or rst operative position, there is provided a pawl 20 carried by armature lever 21 of arelease magnet'22.
Tov heat anarticle of uniform cross-section such as metal bar' A, the electrical and thermal characteristics of which' have been determined, 'the switch 6'is set to include that number of turns of primary transformer over a circuitwhich ma bet'aced from' con'-` ductor11, right-hand side ofiewi'tch; conductor 3,"co'nductor 23 through theupper o fwin f-P,`.c onductor 24 spring's 18 by leftand vstx'ip, 15,lconductor 25,
" remaining'- turns o f.- thef-.pper half ofv windmg P?, conductor 26, .second pair of springs"- 18 and contact 15, 27, upper portion of .lower half of windingP, conductor 28,
pair of springs 181`and cgntact 15,'"conductor. 29, remainin jportionlfrofz lower Jhalf of-.winding P,'switc .6, conductor-30, right-` to the large number'o turns in windin`g.P?,in Y duces a relatively low potential inthe secondf.
lary winding, so that a current so weak as to heat bar A but slowly flows through the secondary circuit S and .bar Ain series. When,
as is usually the case, it is desired to raise the temperature of the ba-rrapidl the winding P1 1s disconnected, and win ing P2 whic i has fewer turns and hence induces a higher secondary voltage is connected. To eiect the substitution of the transformer windings the Vcircuit changer '8 is operated in a manner to be described to rotate `drum 10 in the directionof the arrow, opening contact springs 1 8 to'interrupt the circuit just traced, then closing springs 17 and a moment thereafter closing springs 16. When springs 17 ride into engagement with contact 14a circuit is coinpleted for winding P2 which may be traced as follows: Conductor `1, lright-hand side of l switch, conductor 3, Ispring 17 and contact 14, conductor 31, winding P2 (upper and lower to conductor 2.
The heavy fiow of current in this circuitl halvesin series), switch 7, winding ofsolenoid 9, conductor 4,1eft-hand side of 'switcli' l tacts 34 upon which they normally rest.`
Drum 10 is locked in operated position by .thel vengagement of pawl 20 with a tooth 35 on shaft 11, and remains in. this position'until release magnet '22 is operated over a'circuit pre-pared by the closing of springs 16 by conytact 13, but interrupted by the opening of solenoid contacts `34s The release magnet circuit may be traced from left-hand stationary solenoid contact 34, through springs 16 and. contact 13, winding'of magnet'22, conductors 3 and 23,'through a few turnsof primary winding Plfnow out of the main electricV circuit but acting as a low voltage secondary winding of the transformer, and thence over conductor 36 to the right-handstationarycontact 34 of solenoid 9. l f
This circuit 'will vbe completed whenever the current ilowin through primary winding P. Aand solenoi 9 is too weak to oiset the action of a compression spring 37 which tends d to .hold contacts 33 and 34'closed. The current flowing. throughp imary 'winding over the circuitfjust trace sets up a very high current in lthe secondary winding S which passes through the coldf metal bar A- and .starts to heat the same; The rapidity with I lwhich. the A isgheated depends von 'the v voltage .impressedi-on, the secondaryv circuit f 'which is in turn controlled by the position of` Switch l7 so,thatthe.temperatureof the bar maybe v'increased at*an'y vdesired rate 'by a I i y vvproper..( lesgiifof the ',transformer and thei f--' hand springs 18 'and contact 15,'conductor 4, 'zleft-hand'side of switch 5 toconduetor 2L 1 The currentr flowin *in this circuit', owing inclusion of the prfo'per number :of turns 'of windingl?, As bar Al-'becomes hot its-.resistance increases appreciably, thereby incrcasv xiv current How in the primary circuit. If the predetermined temperature to which bar A is to be heat-ed lies below or only a little above the critical point of the material of which the bar is composed, it is merely necessary to adjust the tension of solenoid spring 37 and the vertical position of stationary contacts 34, so that when bar A has reached the predetermined temperature the eiiect of the current flowing in the primary circuit and through solenoid 9 will be just overcome byv spring 37, which thereupon orcesplunger 32 vdownward until movable contacts 33 encounter stationary contacts 34. Contacts 33, being carried by a metal bridge 38, close the circuit previously traced to operate releasemagnet 22, thereby withdrawing pawl 20 from engagement with tooth 35. Spring 19 immediately returns drum 10 to its normal position, indicated in solid lines in Fig. l, thereby interrupting the circuit through transformer winding P2 and completing the circuit previously traced through winding P1. rlhe number of turns included in winding P1 has been arranged by proper positioning oli' switch G- to induce the proper voltage in the secondary circuit to cause just sutilcient curren'. to flow through bar A to produce heatI equivalent to the losses from the bar by radiation, convection and conduction, and thereby to maintain the temperature of bar A constant at a predetermined value which may be below, equal to or above the temperature at which the solenoid operated to cut out winding P2. The bar is then slowly cooled, maintained with unchanged temperature or heated. as the case may be, until the final temperature resuliing from the particular position of switch is reached. lVhen the operator is ready to use the hot bar, he removes it and substitutes a cold one. The circuit changer 8 is again operated, the new bar is brought to exactly the predetermined temperature, the circuit through winding P2 is then automatically interrupted and the bar retained at or brought toits iinaltemperature, and so on for as many bars as are to be heated. Thus a single adjustment of switches 6 and 7 insures uniform heat treatment of any number of identical articles which may be heated one after the other.
It frequently occurs that the predetermined temperature to which it is desired t`o heat a bar will materially higher than the critical point for the metal composing the bar to be heated. When this is the case, it would not ordinarily be possible, to operate the heater in precisely the manner described, since when the critical point of the bar is reached the resistance does not increase in proportion to increase in temperature, and very nearly the minimum value of the current iiowing through winding P2 would be attained before the bar reached the predeterminedtemperature. It is therefore desirable I three straight bars 41, 42 and 43, the
to introduce a time interval between the attaining of approximately the minimum value of the current through primary winding P2 and the closure of solenoid contacts 33, 34, during which time interval the bar .is heated beyond its critical point to the predetermined temperature by the current flowing in the secondary winding S, which is induced by the current still flowing in primary winding P2. To introduce the necessary time interval, there may be provided a dash pot 39 which may be regulated in conjunctionl with compression spring 37 to delay the closure of contacts 33, 34 for any desired interval after the bar has reached its critical temperature and the current in P2 has reached its approximate minimum value. Thus, when the bar has been heated to its critical point, plunger 32 begins to move out of the solenoid winding 9, but this movement is opposed by dash pot 39 until when contacts 33, 34 finally close, bar A will have reached the predetermined temperature. As before, the closure of the solenoid contacts operates the release magnet to return the circuit changer to normal position, opening the heating circuit through transformer winding P2 and closing the constant temperature circuit previously traced through winding P1. When the rate at which the bar is heated is very great, it is frequently unnecessary to employ a dash pot to provide an interval between the attaining of the approximate minimum value of the current through winding P2and the disconnection of said winding, since the time inherently required for operation of the solenoid, release magnet and circuit changer may be sufficient to allow heating of the bar A well beyond its critical point.
Considering now the structure of the heater as shown in Figs. 2 and 3, there is provided a table 40 of wood or other suitable insulating material at a convenient height from the ground upon which is set the heater, which comprises the transformer having primary windings P1 and P2 and the open secondary circuit S, which consists of a heavy copper bar magnetically linked with the primary windings of .the transformer. Secondary turn S is most conveniently built up from ars 4l and 43 being supported at'either end on sleeves 44 resting on table 40 and secured thereto by bolts 45, while bar 42 rests on the left hand ends of bars 41 and 43 and is held in close electrical contact therewith by any suitable means such as cap screws 46 (see Figs. 6 and 7). If an article to he heated were merely laid across the right hand ends of bars 4l and 43, while some current would iiow through the secondary circuit, the contact between the article to be heated and 'the copperbars would be very poor, moreover the heat losses by conduction through the parts of the article in contact with the bars of the yas 1,ees,son`
secondary circuit would be so eat as t0 a1.- most wholl prevent heating o such portions of the artic e. Againlif as is usually the case, the article to be heated .was composed of a material of higher resistivity than that of the secondar circuit, the resistance of the portions of t e article in contact with the secondary bars would be so much higher than that of the bars themselves, that most-of the current would enter the article at the interior edges of the bars andthe small current flowing through those ortions of the article contacting with the ars would be insuflicient to heat the article materially. In order therefore to establish goed electrical contact, to control the'path ta en bythe current in entering the article to be heated, and to control the heat losses by conduction there are rovided special electrodes 47 and '48 an special mountings therefor. In order thattheelectrodes may be relatively movable to accommodate articles of various lengths, the base for electrode 48 comprises a copper slab 49 which lies flat upon the right end vof bar 41 of the secondary winding and is secured thereto by screws 50 passing through slots 51 and provided with friction washers 52 to engage the margins of said slots. B loosening screws 50 it is therefore possib e to adjust the position of slab 49 throughout a wide range, and to clamp it against accidental displacement when properly positioned. v
Owing to the expansion which will take place in any article to be heated as its temperature rises, provision must be made for such linear expansion and the base for electrode 47`is therefore allowed a limited movement relative to its bar 43. The base for electrode-47 consists of a copper slab 53 somewhat narrower than slab 49gto support which slab there are provided two copper bearing bars 54, parallel/to each other and to slots 51, which bars are secured in any desired manner to the upper surface of bar 43. Slab 53 is provided with grooves, asshown, to cooper-p ate with the bearing bars 54-and is thus in good electrical contact with the secondary bar 43 while capable of movement transverse thereto. To give still better electricalA contact, slab 5,3 may be connected to bar 43 by suitable flexible conductors, not shown. To further control the movement of base 53 there are provided at the opposite ends thereof copper angle pieces 55 rigidly secured to bar 43 and having grooves to cooperate with similar grooves in the ends of base 53 in carrying balls 56 Whi ch limit the movement of base 53 on its bearing bars andv ball bearings described. Two brackets 57 (see Fig. 13) are vrigidly secured to the inner vertical face of the bar 43 and carry set screws 58 which limit the travel of base 53 towards base/49. To normally hold base 53 in contact with the set screws, two springs 59 are secured on the outer vertical face of bar 43 with. their fbree ends' bearing against `the outer face of base 53. Thus when the article being heated expands longitudinally, owing to itsincreasing temperature, it rneed not slip in its electrode 47 but base 53 will move against the tension of springs 59. The same result may be accomplished by making bar 43 of yieldin construction, as bydbuilding itup from exible sheets of copper. j
`Since the standards carried by bases 49 and 53 are identical, the description of those carried by base53 will also serve as a description of those on base 49. Cast integrally with or rigidly secured to base 53 are two copper standards 60 and 61, through the heads of which are drilled holes at right angles to the direction of movement of base 53 and in alignment with one another. A rod 62 is slidably mounted in the hole through standard V60 The mountin s for the two parts of elec-v trode 47 being i entical it will be necessary to describe onl the right hand mounting (clearly shown in i0'. 14). y ball 66 on whic a water jacket electrode base 67 is carried by a knuckle joint consisting of tapered plug 68 which closes thehole through which the core used in casting water acket' 67 was withdrawn and which plug is s aped to bear against ball 66. Plug 68 is held aga-inst the ball, with just suiiicient pressure to ordinarily prevent relative movement by a ma e9 which is heid in position by a 106i; ma
70. The two parts of electrode 47 are each dovetailed into aI groove 71 provided in one lface of water jacket 67. Sincevthe knuckle -j ointdescribed might not afford entirely lsatisfactory electrical connection between standards 60 andr 61 and their respective Water.
jackets, the standards are conductively united to the water jackets 'by copper braid 72 suitably secured to the standards and jackets, respectively, and having sufficient stiffness to return the water jackets to their normal opposed position when they havevbeen moved ontheir knuckle joints, owingftol some irregularity in shape of the article to be heated. Other means than braids 72 may be employed to straighten'out the water jackets, or such means may be dispensed with entirely, since the compression of the-v electrodes against each bar will of' itself move. the jackets until the surface of the bar is in intimate Contact,
with the electrode faces. To'supply a flow electrodes, rubber or -other non-conducting of cooling fluid through jackets 67 to cool the as oil is brought about in any desired manner.
The relative movement of the two halves of electrodes 47 and 48 is controlled in accordance with the position of rods 63 carrying the right hand halves of the electrodes. Each rod 63 terminates at its outer end in a rigidly secured collar 74, which may be integral with or otherwise fixed to a disc 75. A compression spring 76, bearing against the right hand face of standard 61 and the left of collar 74, constantly urges rod 63 to the right. A disc 77 is separated from disc 75 by three compression springs 7 8, the combined strength ot' which is considerably greater than that of spring 76, which they oppose. The faces of discs 7 5 and 77 are maintained parallel by an alignment pin 79 secured at the center of disc 77 and traveling in a cooperating hole in disc in a pair of ears perfot iorni vfcmshings for pins on which are rigldy mounted cams 82 and 83. The pin for cam is coextensive with its bushings while the pin carrying cam 82 comprises a shaft extending inwardly far enough to clear base 53. As seen in Fig. 3, in which it is shown in operated position, cam 82 is circular throughout its lower half up to the point of contact with disc 77 from which point the radius of curvature of the cam surface gradually increases until the surface becomes flat at 85. Cam 83 is of the same shape as cam 82 and the cams are operated by levers 86 and 87, the inner ends of which fit snugly into recesses provided in the cams to receive them. Rigidly secured at the outer end of lever 86 is a ring 88 through which is mounted a handle 89 of suitable insulating material eXtending to a semi-circular ring 90 open at its top and secured to the outer end of lever 87.
Thus when cams 82 and 83 are in their normal position with levers 86 and 87 rising vertically and discs 77 resting against flat portions of the cams, springs 7 6 will force collars 74 and rods 63 to their extreme righthand position, so that the opposite jaws of electrodes 47 and 48 will be wide open to receive the article to be heated, in this case the bar A. After a bar has been inserted between the opposite faces of the electrodes, handle 89 is pusher1 down from its vertical position to the horizontal position shown in Fig. 3, during which motion cams 82 and 8S first compress springs 76, moving rods 63 to the left until the bar A ist-ightly gripped between the jaws of the electrodesl and thereafter springs 78 are compressed until the full radius of the cams comes in contact with discs 77. Later when handle 89Yisglifted it raises lever 86 and unclamps the jaws of electrode 47, but there being no top" tering 90, handle 89 in being raised allows lever 87 to remain in its operated position and electrode 48 continues to grip the bar A firmly until lever 87 is itself lifted. The releasing of cam 82 allows springs 7 6 and 78, which had been compressed by the operation of said cam, to expand and the friction between the cam face and disc 77 .is so little that once the cam has started its return movement the expansion of these springs will cause it to resume its normal position. The removal of the right-hand jaw of electrode 47 from contact with bar A ail'ects in nowise however the contact of said bar with the face of the left-hand jaw of the electrode since the bar is still firmly grasped by electrode 48. Upon the removal of pressure from the jaws of electrode 47 springs 59 return base 53 to its normal position abutting set screws 58.
Referring now to the structure of circuit changer 8 and its operating mechanism, illustrated in detail in Figs. 4, 5, 9, 10, 11 and 12. there is provided a suitable base 91 whose surface is located at a convenient height between table 40 and the floor. Mounted in the right-hand front corner of base 91 is a standard l2. A similar standard 12 is mounted at. the same distance from the rightlhand edge of the base but farther back thereon, as clearly shown in Fig. 9. The upper surface of each.
`'standard 12 is shaped to provide a bearing surface for a shaft 11 which carries the wooden drum 10 of circuit changer 8. Shaft 11 is retained on its bearings by fastonings 92 suitably secured to standards 12 as by the bolts and nuts shown. Drum 10 is rigidly secured for rotation with shaft 11 b collars 93 and 94, located at each end. Le t-hand collar 94 has a cam surface to provide a tooth 35 to cooperate with apawl 20. Behinddrum 10 (to the left in Fig. 11 and at the left end of said drum is mounte a lmagnet 22 having an armature lever 21 pivotally mountedon an upward extension 95 of left-hand standard 12, armature lever 21 carrying at its front end the pawl 20 which cooperates with tooth 35 and being normally held in retracted position by a tension spring 96. Thus when drum 10 is operated it moves in an anti-clockwise direction from the position shown in dotted lines in Fig. 11 to that shown in full lines, and is held in such operated position by pawl 20. The rotation of drum 10 in this direction is limited by the abutment of a pin 96 fixed to collar 94 against a stop 97 set in base 91. Upon the energization of magnet 22 and the consequent attraction of its armature lever 21, pawl 20 is disengaged from tooth 35 to permit a spiral spring 19 to rotate drum 10 (in a cloclowise direction as seen in Fig. 11) until a pin 98 comes in contact with a spring cushioned stop 99 secured to base 91.
Drum 10 carries the copper contact strips 18, 14 and 15, previously described in connection with Fig. 1, while their cooperating contact springs 16, 17 and 18 are secured at their bases to blocks 100 of suitable insulating material by binding posts 101. 'When contact strips 13, 14 and 15 are of copper, it
' may be described to provide carbon strips (not shown) similarly positioned on drum` 10 and arranged to be engaged by contact springs connected in parallel with the regular service springs 16, 17 and 18, or to lprovide some other conventional arrangement to prevent ar'cing upon the breaking of the metal contacts. To'cooperate with other drum operating mechanism to be described, a disc 102 is rigidly secured to the right-end of shaft 11 and secured to the outer surface thereof is a lever 103.
As described in connection with the circuits in Fig. l, it is desirable that the clamping of bar A between electrodes 47 and 48 should be simultaneous with the operation of the cirposition, in
y contacta 15, to its operated position in which said brushes are disconnected and brushes 16 and 17 are connected. The downward movementof handle 89 in clamping bar A causes shaft 84 of cam 82 to rotate. The rotation of shaft 84 is employed through suitable linkage to operate circuit chan er 8. As shown, a lever 104 is mounted at't e front end of shaft 84 and is caused to rotate therewith by' means such as a set screw 105. As seen in Figs. 2
and 3, the outer end of lever 104 terminates in two ears 106 between which there fits the upper perforated end of a link 107 which may be held in place by a pin 108 passing through ears 106 and link 107. .To insulate the sec'- ondary circuit S, which is in electrical contact with lever 104 and thus with the upper end of link 107, an insulating block 109 is inserted between the upper and lower portions of link 107. A universal joint 110 connects the lower end of link 107 to the upper end of a rod 111 which is made rotatable relative to joint 110 by the pin and slot connection shown at 112, a spiral spring 113 being provided to maintain the normal relative position between rod 111 and joint 110. Rod 1.11 is sildably mounted for vertical movement in supports 114 secured bto convenient parts of table 40 and base 91 and carries a roller 115 which normally rests upon thefouter end of circuit changer operating lever 103, as seen in Fig. 4.
When handle 89 is depressed to clamp thel .travel the reaction oflever A103 on roller 115 causesrotationof pod-111 'a theY action of spring 113 until roller 115 rides over the end of lever 103 as shown in Fi 10, whereupon spring 113 rotates the ro and roller back to its normal position with respect to joint 110, as shown insolid lines in Fig. 5. Roller 115 is now below lever 163, so that upon the operation of release magnet 22 the roller will not interfere with the return to normal of drum 10. Rod 111 is retained in its lowermost' position by a catch 116 which engages a collar 117 rigidly secured to the rod as by the set screw shown. Catch 116 is mounted on the upper end of a lever 118 pivotally mounted at119 and normally held in the position shown in Fie. 5 by-a spring 120.
Upon the operation of re ease magnet 22 ('see Fig.` 11) spiral spring 19 causes the return to return to normal position of cam 82 and the operating mechanism for circuit changer 8, including rod 111. Just before rod 111 reaches its normal position, pin 98 comes in contact with cushion stop 99 and compresses said stop sufliciently to allow roller 115 to ride from below lever-` 103 around the outerr with the upper face of said lever. j
In connection' with the structure of the circuit changer 8 and its relation to the electric heater described, it is tobe noted that when the bar A is first inserted in position to be heated, circuit changer 8 is in normal position and the currents flowing through windend thereof to its normal position in contact ing P1 with its relatively large number ot" turns induces a, relatively low potential in the secondary circuit S, so that there is no appreciable tendency to arcing as bar A isl placed in contact with the left-hand stationary jaws of electrodes 47 and 48. In depressing handle 89 the drum 10 of the circuit changer has rotated far enough to move contacts 15 out of engagement with springs 18, thereby breaking the circuit for winding P1, but not .far enough to move contact 14 into engagement with spring 17 at the time when cams 80 and 82 bring the right-hand movable -aws of electrodes 47 and 48 into contact with ar A. Thus there is no current flowing in either priniary 'winding of the transformer at the time Abar A is clamped between the jaws of. the electrodes and hence no tendency for arcs to occur at this time. Itis only afterY handle 89 has been fully depressed and the bar is tightly clamped that contact 14 on the circuit. changer drum 10 completes `the llO circuit for primary winding P2 which therehy induces a heavy flow of current through the secondary circuit including the bar to be heated. Conversely, when the current flowing through winding P2 has decreased suffiiently to allow solenoid 9 to complete the circuit for release magnet 22, contact 14 on drum 10 will have moved out of engagement with spring 17 before cam 82 has been turned sufficiently to break the contact between the right-hand movable jaw of electrode 47 and the bar A and, as there is no current flowing in either primary winding at the moment when the right-hand jaw of electrode 47 is removed from bar A. there is no tendency to are at this time. Then it is desired to take bar out of the heater only the weak current induced by winding P1 will be flowing through the secondary and lever 87 may be lifted to open the jaws of electrode 48 without causing any appreciable arcing. lf, however, it is desired to avoid any arcing what soever at bar A, switch 5 may be opened bclore lever 37 is raised to release the bar.
In connection with the provision of a plurality of contacts 15 and cooperating` springs 1S on the circuit changer drum, it is to be noted that these contacts and their cooperatingsprings constitute a break-up switch for primary transformer winding P1 so that upon the operation of the circuit changer to close the circuit for winding P2, winding Pl is broken-up into as many electrically separate parts as there are provided contact-s 15. rlhc purpose of thus breaking up winding P1 is to avoid the existence of the high potential difference which would exist between the ter-v minals of winding P1, acting as a step up secondary winding owing to its large numlrer of turns, while current was flowing through winding P2. Thus-by providing the required number of contacts 15 and pairs of springs 1S winding Pl may be separated into elements cach comprising so Vfew t'rns that" the difference of potential between terminals of each element will not be dangerously high while current is flowing through transformer winding P2.
teferring now to the structure of the transformer as seen in Figs. 2, 3, 6, 7 and 8 no particular structure is required, but a suitable transformer may be conveniently made up by assembling a laminated iron core 123 between four U-shaped cast iron heads 124, there being provided an upper and a lower head at each end of the core. Six tic-bolts 125 pass through lugs 126 to hold the core in assembly, while an upper and a lower channel iron 127 hold the core against vertical mo vement. To reduce magnetic leakage, the primary and secondary windings preferably pass around the same leg of the core, secondary bars 41 and 43 being supported by sleeves 44, as' has been described, while primary winding P2 is made up of two". pancakes 12S held adjacent to the secondaryv bars by pins 129 and wedging blocks 130. The two upper elements of'winding P1 may be conveniently wound together in a pancake 131 while the two lower elements are wound in a pancake 132, which pancakes are likewise held in position adjacent to those comprising winding P2 by pins 129 and blocks 130. It is to be understood, however, that the inven tion is not to be restricted to this particular transformer since many different structures might be used. For example, bars 41 and i3 might extend further to the left than as seen in -Figs. G and 7, and connecting bar 42 might be removed and electrode mechanism similar to that shown at the right-hand of Fig. 2 might be supported on such extensions of bars 41 and 43. With this arrangement two bars A. could be included in the secondary circuit simultaneously and the rapidity of production of heated `o the electric heater thereby doubled. more than the one set of secomarj; f wwn might be magnetically linked with the primary windings by encircling either leg of the transformer core or more than two articles to he heated might be connected in series with a single set of secondary bars if it were desired to heat simultaneously more than two bars A. In fact the number of identical articles which may be heated simultaneously is only limited by mechanical considerations in designing the transformer.
As has been pointed out there are many classes of work to be heated with which it would not be satisfactory to employ endtouch electrodes, that is electrodes which hold the article by contact with its opposite ends. For example such electrodes could not be used where it was desired to heat only certain portions of an article, or where the article had irregular ends 'which would not make good electrical contact with the electrode, or where the pressure necessary to give good electrical contact would be sufficient to bend the article when hot. In lthese and other instances it is desirable to provide side-touch electrodes Which-contact with the article to be heated in the manner illustrated by electrodes 47 and 48 and bar A in Figs. 2 and Y3. In order that the electrodes may support the work as Well as conduct electricity to it, each electrode preferably comprises a clamp having at least two re atively movable jaws, as seen at 47, Figs. 3 and 12. To insure good electrical contact between the jaws ofthe electrode and the article to be heated, the surface of each jaw is most desirabl shaped to fit closely against said article. n the present case the work to be heated has been illustrated as a cylindrical bar A so that the contacting faces of the electrodes are of cylindrical configuration. If the work were hexagonal or some other shape, the electrode contact surface should be hexagonal or of whatelectrodes itis to be noted that if the elecn trode 47 consisted solely ofea single metal block 134 withoutthe various other elements indicated in Fig. 15, the electrical resistance ofthe path between the surface 133 of the electrode, which contacts vwith the face of` water-jacket 67, through the electrode and bar A to electrode 48 which contacts with a lower portion of the bar A, would not bev uniform. The path of least electrical resistance would l extend from water-jacket surface 133 along the lower edge of block 134 and thence through the shortest extent of bar A included between electrodes 47 and 48, while the path of greatest resistance would extend from surface 133 along the upper edge of block 134 and through the portion of the bar A which is in contact with the surface of blocks 134. The increased resistance of this second path due to the inclusion of the portion ofbar A which is between blocks 134- `would be so eat that in the assumed case, in which bleef-134 constituted the entire electrode, practically all the current would flow alon the lower ed e of the block 134 passing into ar A at the le -hand lower corner of the right-hand block as seen in Fig. 16, and only a negligible amount of-current would pass through the remaining contact surface between the work and the electrode. The great `current density at the lower edge of block 134 would tend to roduce an unduly high temperature alon t e lower edge of the block and could easily ring about a welding condition atthe meetin point with the work, whilethe ve the end of t e bar would be insuilicient to raise its temperature appreciabl When, therefore, it is desired to heat wor not only between the adjacent edges of electrodes of opposite polarity but a so throughout the portions which contact with the electrode faces, 'special means must 'be provided to cause the current to flow through the portions of the work in contact with the electrode. and special attention must be .'vento the thermal conductance and electrode, so that eat will not b'e 4dissipated j through the water-'acketsas fast as it'is roduced in the end o the bar being hea In 'order to heat' the tip-oan article 'auch` as bar A to a temperature uniform with` that!!v of the remainder ofthe which'isfntin y contact with the electrode, it sobvioustliat either substantially `all the.' "eurieuntl the bar must'pass along that edgeof1 thefelectrode which is adjacent the ti 'of the bar 'further removed from' the :e ectrode rvof opor heat must be sup lied .by
posite polarity, n
to the conduction from the small ow of current through plhysica dimensions ofthe p flhercu Anot er *electrode compose by oxidation or else soften and mush-V room out of shapeor melt away so that re. l
placement of the electrode would be necessary each time an article was heated if not more often.v Since this would not be practical a compromise is struckby providing an electrodewith sufficient surface in contact with the work to carry the heating current without destruction of the electrode, while the current density through the tip side of the electrode is made as high as possible and the current density through the side of the electrode adjacent t'he electrode of opposite polarity (the lower edge of block 134 in Fig. 16) is kept as low as possible. A largenumber of diierent structures for accomplishing this result have been found, all working however on the principle that theelectiical resistance of the portion of the electrode near the tip of the bar is to be less than the resistance of the portion`` removed from the tip.
One electrode constructed on this principle is shownv in Fig. 21, and consists of a base 170 of highly conductive material such as cog t e per, havin an extension which forms to strip o the electrode, the base being slotte to receive strips 171,172, 173, and 174 of identical dimensions, but each composed of metal or alloy of lower conductivity than the precedin one. For example strip 171 may e nicke v'1752. nickel and, manganese, 173 Mone'l metal, and 174 manganin. It is sometimes desirable to further vary the resistance of the different strips by v in their dimensions as shown in Fig. 22. e e ectrode there shown com rises a short thckstrip 175, and
iio-y l progressive ylonger and thinner strips 17 6,
'177, 178,` and i179. When the; electrode is used in -heating articles the resistivity of certain articles these strips maybe of the same material, while with-,other f the various strips `is also varied. Thevelectrode as just describedl is composed ofl a plurality of structurallyv sep' `arate conducting elements assembled to form a built-u electrode and to ensure intimatev contact o themutual contacting surfaces of the said elements, the1 latter are vcoated- Vwith s material, preferably 1 a suitable''eondue amalgam in Figs. 19- udea'lajtn `plm' 135 each' sucmding'shea 136, 133,138 mi 139 ii. mm1..
y i @at .up at an femm 'the offhsgeieetmde udgannhe tip of t e bar, wile the resistance o f l; plish the same purpose may be made up asprogressively greater by cutting out a small central portion from sheet 136; and an increasingly larger portion from each of the remaining sheets 137, 138 and 139, to pro essively decrease the cross sectional area o the conducting path from base surface 133 to bar A. The increased resistance ma also be brought about by using metals an alloys of dlerent resistivity for the diilerent sheets, with solid or cut-out centers as required. As shown in Fig. 2O an electrode of this type may be iven greater strength without substantial y varying its electrical characteristics by lling the interstices `with a suitable refractory cement such as corundum or carborundum which also helps to cool the electrode,fbeing a better heat conductor than air.
A still more durable electrode to accomshown in Fi s. 15, 16, 17 and 18 from a number of e ements comprising sheets or blocks of different metals held in physical contact with one another by suitable means such as pins 140 which are driven through' holes drilled transversely through the assembled elementsnear the base of the electrode. The electrode shown in Figs. 15 and 16 is composed of a block 134 of diicultly oxidizable metal of relatively high electrical resistivity and lowthermal conductivity, and strips 141, 142, 143, 144, 145 and 146 composed of metal of negligible electrical resistivity as compared with that of the block `134. For example, in an electrode to be used in heating a bar A of medium carbon steel, the block 134 may advantageously be composed of a silicon chromium resistance alloy of iron having a resistivity of about 115 microhms per cm3 while the other elementsfof the electrode may be made of copper which has a resistivity of only about 1.7 microhms per cm3. As has been pointed out, if the electrode consisted merely of the iron alloy block 134 and copper base made up of strips 143, 144 and 145, a relatively dense current would flow along the lower edge of the block while only a weak current would flow through the upper edge, so that the Joulean or IR heat developed in the tip of the bar A, which is in Contact with blocks 134, would be much smaller than that developed farther down the bar where the total current is flowing. In order, therefore, to cut down the resistance along the upper edge of block 134 between base surface 133 of the electrode and the bar to be heated, and thereby to cause a more'v 31; of an inch; while its exact distance is a matter of design in each case and is determined in accordance with the electrical resistance required at that point. In some cases it is easier to control the electrical characteristics of the electrode by adding the further strip 141 of low resistance metal, but it is not essential since a single strip 142 may be designed to bring about the desired dense flow of current through the upper portion of 4block 134. The block 134 being of low thermal-conductivity would not under operating conditions in which a series of bars were rapidly heated one after the other, have sufficient time to cool off between the removal of a hot bar and the insertion of a cold one, so that for cooling purposes it is desirable to provide a strip 146 of high thermal-conductivity in contact with the lower surface of block 134. Strip 146 also affects the flow of current through ythe block 134, and its lefthand edge is considerably farther removed from the bar A than is that of strip 142, so that a less dense current'will pass through the lower rtion of the contact surface of block 134 t an passes through the upper portion of said contactv surface. To further aid in the conduction of heat from block 134, strip 1440i high thermal-conductivity preferably forms a iin fitting snugly into a central groove out along the base of block 134,
Lwhile for manufacturing reasons strips 143 and 145 of high thermal-conductivity are fitted between strips 142, 144 and 146 to give a smooth bearin surface 133 which will make good electrica contact with the face of the copper water-jacket 67. For ease in changing the electrodes all the stri s are preterably dovetailed to fit against shoulders 71 of the previously described groove in the face of the water-'acket, so that it is merely necessary to sli e out one electrode and slide in another when a change is to be made. When an electrode is to be used in heating an article to a high temperature, it is desirable to insert a thin carbon strip 147 between strip 142 and the upper surface of block 134 where the current density is the greatest, and contacting with the article being heated, so that a'reducing atmosphere will be produced by the corbon to counteract any tendency to oxidization at the contact surface between block 134 and the work A, as shown in Fig. 17. Additional carbon strips may be inserted between other sheets ofthe electrode, or transversely of the electrode elements and parallel to the bar A, thereb extending the reducing area.
To urther aid in cooling the block 134, it is desirable under certain working conditions to enclose the strip 146 and the exposed lower surface of block 134 with a cement of refractory material, such as oorundum or carborundum as seen at 148, having embedded therein a cooper fin 149 and supported at its lower fore be supplied to the; ti
pass from the-bar to the water- I ance-,allo having edge by a retaining strip 150. The refractory cement is of such low electrical conductivity -as not to materially influence the-electrical characteristics of the electrode, and invsulates therefrom the copper fin 149 and rewater-jacket, and further will vary according to the temperature gradient whichV 1s to be produced inthe work itself when the:
electrode is not designed to heat the tip .of the work uniformly with the portion between electrodes of opposite polarity. For ex-l ample, in producmg a uniform -temperature in a given time throughout a bar A, it is obvious that no matter what the relative dimensions or resistivities of block 134 andstri 141, 142 and 146 maybe, theportion of tii bar below the lower'surface of block 134 carries all the current flowing in the work circuit, while the upper part of the portion of the bar gripped between the electrode blocks'v does not carry the whole current, since there issome current passing from block 134 into the bar'at'all points on the surface of the block. Hence, it follows that the PR heat developed in the ti of the bar will be less per unit volume than t at devel'o electrodes'47 and 48. In or er toI raise the temperature of the tip uniformly withlthat of the remainder of the bar heat must there# by conduction from the electrodes. If t e PR heat developed within the electrode were -pro r.
tionately greater than the heat conducte by the electrode from the bar to the water# jacket, a condition of thermal equilibrium would exist at .the contact surface of blockv 134 and. the tem rature of the bar'f would not be influenced y its contact-with the elect-rode. v Since it is necessary in the ,instant case to` sup ly `heat to thetip by conduction, it follows t electrode must` be determinately but proportionately greater than'the heat uction through the conditions under which the electrode is to' be used. The best resultshave obtained by making the contactblockj134 cfa resista resistivi considerably greater t an that of the wor to be' heate `d between. the A at the -IR heat developed in the v which wouldv ack'et by conelectro e, the exact. amount being governed by -the partic'zulx'tr"v but having a far lower temperature coeiiicient of resistivity. All the othermetal in the'electrode'should be of very low electrical and thermal resistivity. By employing an electrode made up of such metals itis possible to design the electrode so that when a cold bar is put in the heater and current begins to flow through the Work circuit heat will be supplied by conduction from the electrode to the tip ot' the bar at a rate suiicient to equalize the lesser 12R heat developed in the tip of the bar; while as the work and electrode are heated up by the continued flow of current, the resistance of the work increases faster than that of the electrode, un til at'the point when the desired predetermined vtemperature Vof the work has been reached thermal equilibrium will exist at the contact surface between block 134 and the work, at -which time the 12R Vloss in lthe electrode is considerably greater than the conductance loss between the bar and waterjacket.`
It is to be noted that with the mechanicalY arrangements of the electric heater previously described, when the desired temperature has been reached by the work the electrical potential applied to the work circuit is decreased -soas taweaken the current flowing through'said circuit to a point where no further increase in temperature occurs in the Iwork, but the current supplied just equalizes the heat losses by conduction, convection and radiation from the' work. Suchweaker current would not develop enough heat in the electrodes to i .maintain the thermal equilibrium' existing at the contact surface between blocks 134 and the work, so that there would bea tendency to lower the` temperature of the tip of the work. In order to prevent such a result .thpcontact area between. theelectrode and the workis decreased by opening the jaws of the electrode whereby 1,
'aw of electrode 47, as seen in Figs. 2 and 3,
.and is out of contact with the right-hand jaw.
B roperlychoosing the relative dimensions ofy t e two jaws of the electrode, or by using more than two aws and removing theI proper number when the lowered p tential is applied to the work circuit, it fo lows that thermal equilibrium may be maintained at the 'contact surface between blocks 134 which remain '1n contact with the work after the current" flowing through the work vcircuithas been decreased. Y i
" lillctr'odes' of the type previously described and shown in Figs; 15722 may also be made to oduce .any desired temperature gradient wit in the portion of work engaged by the electrode `jaws, a result which is often desirable in heat treating portions of certain articles where no'sharp line of demarcation 1s desired' between thetreated and untreated the tip remains in contact with the left-hand portions. Where a temperature gradient is to be produced over a considerable distance, an electrode of the type shown in Figs. 19, 20 or 21 may be used, the sheets being spaced much farther apart than as shown in Fig. 20, so as to include the entire portion of the work throughout which the gradient is to exist.
In heating electro-conductive articles of varying cross-section such as the bars 151 and 159 seen in Figs. 23, 24 and 25, a uniform temperature would obviously not be produced in the article if it were merely inserted in the heater between electrodes 47 and 48 in the manner indicated by bar A in Fig. 2. Since if the only points of contact with the source of current were said electrodes, the current passing through any section of the bar between electrodes 47 and 48 would be vuniform and therefore the current density would be greater in the portion of the article having a smaller cross-sectional area than in the portion having =a larger area, the smaller portion would therefore heat faster and in a given time reach a higher temperature than the larger portion. Special means must therefore be provided to produce uniform heating ofi an article having varying cross-section.
The particular means employed in producing uniform heat throughout articles of varying cross-section will differ in accordance with the particular conditions Ato be met in any given article, but the underlyin principle is the-same regardless of the con guration of the article 1n question, namely that current of approximately uniform density should flow through all parts of the article.
In order to heat a bar having a reduced central portion such as bar 151, shown in Figs. 23 and 24, it is merely necessary to provide suitable means for carrying the excess current around the reduced portion. In the present case there are provided moderately fiexible leaf s rings 152 havin end pieces 153 of diiicultiyl oxidizable con ucting material which abut against the. shoulders 154 formed by the decrease in diameter of the reduced central portion 155. By properly proportioning the resistance of the shunt springs 152, it is obvious that the current density in the vreduced portion 155 may be maintained substantially equal to that in the enlarged portions 156. Any convenient means may be employed to hold the' shunt springs in position during the heating operation such for example as the clamp 157 having'- jaws facedwith -blockt 158 of non-conducting refractory material, as seen in Fig.
48 by brackets 166. From the foregoing def scription it "is obvious that by constructing auxiliary jaws 163 and 164 and brackets 165 and 166 of materials of suitable resistivity, and by properly dimensioning such auxiliary conducting ath, it will be possible to establish a num er of parallel circuits between electrode 47 and electrode 48 and thereby to maintain approximately equal current density throughout the article 159 being heated. For example. as shown in Fig. 25, one path of heating current would extend from electrode 47 through bar 159 to electrode 48, whilea parallel path would existfrom electrode 47, base 67, bracket 165, jaw 163, shoulder 162 of the enlarged portion 160, shoulder 161, jaw 164. bracket 166, base 67 and electrode 48. Under certain operating conditions the auxiliary electrode jaws 163 and' 164 would most desirably be cooled by some circulating fiuid in which case it is obvious that suitable means, such as water-jackets, might easily be provided for this urpose.
ile one elect-ric heater embo ying a number of types of electrodes and one method of operating such heater have been described with considerable detail, it is to be understood that thel invention contem lates the provision of heaters of quite di erent structure and of different operating mechanism. For example, all the functions performed by the circuit changer and its operating mechanism might be performed by manually actuated switches, or the s'ame or 110 a different circuit changer might be o erated by entirely different mechanism. n particular it is to be noted that solenoid 9 with its dash-pot 39 which is directly responsive to current flow in the primary circuit of the transformer may properly be considered as means for operating the circuit changer when a predetermined time has elapsed after the Work circuit S has been closed b the in: clusion of an article A to be heate andpotential has been applied to such circuit by the completion of the circuit through winding P2 of the transformer. This follows from a consideration of the fact that a. series of identical bars A will all reach a predetermined temperature and therefore cause apredetermined increase in the resistance of the work circuit and consequent decrease in iis 1,eon,soa
current flow in the primary circuit and through t-he solenoid in the same time interval. Such being the case, it will be apparent that for. the particular means previously j described (solenoid and dash-pot) to coiitrol the operation of the circuit changer there might be substituted time control means such as clock work or escapement devices icontrol the time during which the' circuit higher resistivity than that of an article to be heated and having Ia face shaped to contact with such article, and a strip of material of v lower resistivity than that of the block, said strip overlying and contacting with one side of -said block but cut away from the face whereby it is out of contact with the article to be heated.
2. In an electric heater, an electrode com prising a clamp havin a plurality of jaws adapted to grip the si es of an electro-conductive article to be heated, one of the jaws' being connectible to a source of electric cur- -rent and comprising conducting material arranged when so connected to cause a current of high density to flow through one edge of f the jaw into the article to be heated and current of density diminishing in proportion to the distance from said edge to flow through the remainder of the `surface gripping the article to be heated.
3. In an electric heater, an electrode comprising the combination with a metal block of relatively high resistivity and a strip of reducing material such .as carbon arranged to contact with an article to `behheated, of a strip of metal of relatively low resistivity in contact with the reducing strip'and out of contact with the article.
4. In an electric heater, in combination, anv electrode com rising two relatively movable jaws adapte to receive an article to be heated, one of the 'aws having a surface shaped to fit the article to beheated, and a universal joint to carry said jaw whereg of the jaws will bring-the shap clampin the article surface into intimate contact with tobeheated.
, l 5. In an electric heater, in combination, an
electrode comprising two relatively movable ljaws having face portions adapted to receive an article to be heated, a mounting compris- `ing a universal jointfor, one of said jaws whereby movement thereof towards the other onu jaw will bring its face vrtion into intimate contact with the artic e top be heated, and
j means operable upon movement of said jaw out of such contact for returing said j aw to its normal position relative to its mounting.
6; 'In an electric heater, an electrode comprising two jaws adapted toV grip an article to be heated, a base for each jaw, a mounting for each base, a joint to permit'limited universal movement b etween each base and its mounting, a longitudinally movable member to carry one of said joints, and means limiting the movement of the joint carrying member whereby the locus of the center of movement of the carried joint' will be a straight line intersecting the center of movement of said other joint.
.7. In an electric heater, an electrode comprising a block of conducting material of one resistivity to contact with an article to be heated, and a block of material of lower resistivity in contact with the first block and separated from the article.
8. In an electric heater, a composite elec trode comprising a plurality of elements composed of material of different electrical and thermal conductivity, and an additional element providing a protecting face for the electrode adapted to contact with an article to be heated and said rotecting face consisting of a thin strip of iliicultly oxidizable electroconductive material.
9. In an electric heater, an electrode comprising a plurality of electro-conductive jaws to grip an article to be heated, means-for passing electric current through the jaws into the article to heat the same and means operable upon a predetermined-change in the flow of current to change the number of jaws in contact with the article. l
10. In an electric heater, an open work circuit having terminals', an electrode comprising a plurality of relatively movable jaws secured to terminals of one polarity and adapted to grip an article to be heated, means impressing an electromotiveforce upon the work circuit, means responsive to a predetermined increase in temperature for discontinu- Bzi ing the application ofelectromotive force and means responsive to the operation of said discontinuin means for removing certain of laidtgiws rom contact with the article being lL-In an"electric heater, an electrode l adapted to contact rwithfthe side of an article to be heated near the tipv of the article and with a work circuit terminal, said electrode comprising a path of varying electrical resistance to conduct relatively 4dense current through the the ti *of t main er of the electrode current of density diminishing `as the distance from such porportion of the electrode adj acont e 'article and through the 're-,
ytion increases, whereby the tip of the article may be heated to anydesired degree relative to the remainder of the article.' 12. In electric heating, the combination with a conductor whose resistance is a function of its temperature, the conductor being i included in an electric circuit associated with means for a potential in the circuitto cause to dow through the coniio