US2040760A - Heating method and apparatus - Google Patents

Description

May 12, 1936. E. NQRTHRUP 2,040,760

HEATING METHOD AND API ARATUS Filed Sept. 24. 1950 s Sheets-Sheet 1 May 12, 1936.

E. F. NORTHRUP HEATING METHOD AND APPARATUS Filed Sept. 24. 1930 3 Sheets-Sheet 2 Imma er- May 12, 1936. NQRTHRUP 2,040,760

HEATING METHOD AND APPARATUS Filed $ept. 24. 1950 3 sheets-sheet 5 M an M n Patented May 12, 1936 UNITED STATES PATENT OFFICE to Ajax Electrothermic Corporation,

Ajax

Park, N. J., a corporation of New Jersey Application September 24, 1930, Serial No. 484,120

12 Claims.

My invention relates to electric furnaces of the inductor type.

One purpose of my invention is to apply heat to one or to a plurality of sheets, plates or strips by means of heat distributors by which heat applied directly or indirectly at the distributor edges is conducted and diffused throughout the sheet or pile by the distributor.

A further purpose is to interleave electrical resistors of high heat conductivity between sheets or groups of sheets of relatively low heat conductivity, preferably in'contact with them, and to generate heat electrically in the resistors.

A further purpose is to interleave sheets or groups of sheets to be heated and resistor sheets of high heat conductivity and to heat the interleaved sheets at the edges, preferably by electrical induction.

A further purpose is to heat a pile of metal charge sheets or plates, hereinafter called sheets, having relatively inferior heat conductivity, such, for example, as steel sheets used for automobile bodies, by inducing electric current in interleaved sheets of high heat conductivity desirably projecting beyond the charge sheets to provide a conducting path for electric current whose resultant heat will travel by thermal conductivity to the interior of the pile. v

A further purpose is to provide interleaving 30 heating resistance of relatively flat form or of other shape conforming to the shape of sheet material to be heated, forming the resistors of copper or other highly heat conductive material, and to heat the resistors by passage of alternating 35 current about the edges of the plates.

A further purpose is to transmit heat quickly to the interior of a poorly heat conducting pile by a distributor of high heat conductivity and concurrently and subsequently to transmit heat lat- 40 orally from the distributor to the pile. Though capable of being operated from other sources of heat about the outer surface of the pile, I prefer to generate the heat in the outer portions of the distributor, in the adjoining edges of the pile or charge sheets. The concentration is secured preferably by improving the inductive coupling with the distributor sheets, as, for example, by extending their edges beyond the charge sheet edges. 5

A further purpose is to increase the power consumed by a resistor sheet to be used in the inductive heating of charge sheets by artificially increasing its resistance as, for example, by slitting the edges of the resistor sheet so that the resist- 10 ance of the secondary will increase without increasing the reactance and the optimum conditions for power absorption will be approached.

A further purpose is to regulate the number of slits in the resistor sheets and/or their depth so 15 that the resistance is comparable with the inductance of the sheets to obtain maximum power input.

A further purpose is to slit resistor sheets to prevent buckling during heating.

A further purpose is to insulate inductivelyheated resistor sheets from charge sheets,

especially where the resistor sheets are deeply slitted, so that excessive direct heat conduction from hot spots and, where the sheet is slitted, arcing from the resistor sheets to the charge sheets will be avoided.

A further purpose is to apply an inducing current. about the edges of a pile of charge sheets mingled with heat-distributor sheets, regulating or selecting the frequency used to determine by the frequency the depth of penetration of the induced current and, hence, the width of the edge section of the sheets within which the heat is developed.

A further purpose is to anneal iron or steel sheets by conveying heat to them from adjacent heat-distributor sheets while insulating the charge sheets against loss of heat, and by cutting off the source of heat and permitting slow 40 cooling of the charge sheets.

A further purpose is to pile sheets, some of iron or steel and others of copper, desirably distributing the copper sheets throughout the pile, and to place the pile, with the large surface of the sheets perpendicular to the coil axis, within an inductor coil or to place an inductor coil about the pile to inductively heat the pile as desired.

A further purpose is to insert such a composite pile into an induction furnace from below, desirably closing the top of the space within the inductor to permit the use of a protective or treating atmosphere lighter than air.

A further purpose is to insert from below a charge to be heated within a closed-top furnace and to use the closed top and furnace as a gas bell to contain protective or treating gas lighter than air.

Further purposes will appear in the specification and in the claims,

My invention relates to methods or processes disclosed as well as to apparatus by which they may be carried out.

I have preferred to illustrate my invention by one main form only, with variations, among the many forms in which it may appear, selecting a form which is practical, effective and highly desirable but which has been chosen primarily because it well illustrates the principles of the invention.

Figure 1 is a sectional side elevation showing a heating mechanism capable of carrying out the processes described herein. As shown the mechanism is in position for insertion of the charge within a coil.

Figure 2 is a top plan view of a mechanism such as shownv in Figure 1, omitting the furnace cover and the mechanism belowthe coil.

Figure 3 is a section corresponding otherwise to line 3 -3 on Figure 2 but having a different proportion of charge sheets to distributor sheets.

Figure 3a is a fragmentary section of a modification of Figure 3.

Figures 4, 5 and 6 are top plan views of heat distributor plates modified with regard to that shown in Figure 2.

Figure 7 is a sectional elevation of a muille form of furnace for heating a charge pile.

Figure 7a is a sectional elevation of a furnace for heating piles of charge material, in which the gases of combustion come in direct contact with the charge.

Figure 8 is a fragmentary section to illustrate diagrammatically the travel of the heat. In position it corresponds generally to Figure 3.

Figure 9 is a fragmentary section similar to part of Figure 3 but showing insulation between the heat distributor sheets and the charge sheets.

Figure 10 is a fragmentary view of an indi vidual slitted distributor showing electrical insulation about the slit. The view is a top plan.

In the drawings similar numerals indicate like parts.-

The demand for heat treatment of thin metal sheets for annealing and other purposes is large and is increasing. To indicate this it will not be necessary to cite more than one example; namely, the sheets of steel used for automobile bodies, and other automobile parts, which often measure four-or more feet in their smallest surface dimension. The thickness of these sheets is of the order of one-sixteenth of an inch, making the sheets extremely sensitive to excessive temperatures, and requiring the greatest care in their heating and heat treatment.

If under-heated, the heat treatment is ineffective. If over-heated, the sheets burn, weld together or buckle. If unevenly heated, the sheets buckle. Danger of buckling has been reduced by extremely slow because of the low rate of heat transfer through iron and steel-true also of many other metals-and heating to any considerable depth from the sides is diflicult because of the decrease of heat conductivity from sheet to sheet.

The delay due to relatively poor heat conductivity has been further aggravated by the fact that it has not been possiblein the past to heat edges or sides of the sheets directly, on account of the danger of oxidation and of excessive heating.

Experiments. with steel sheets have indicated that the heat will travel from one point at a maintained given temperature, to raise a new point to the same temperature at a rate of about an inch and a half an hour, evenunder favorable conditions.

As a result of the above difficulties, in one plant of which I have knowledge, sheets intended for use for automobile bodies require 18 hours of heating before they reach a uniform annealing temperature.

I have discovered that the speed of conduction of the heat within such a pile of sheets can be tremendously increased by interspersing within the sheets, what for convenienceI have called heat distributor sheets, intended to be heated at their outer edges and capable of conducting heat within the body of the pile at a much more rapid rate than is true of conduction in the charge sheets composing the rest of the pile. By properly selecting the thickness and spacing of distributor sheets as well as, desirably, their accommodation at their edges to the form of heating selected, heat applied to or developed in them at the edges can be conducted rapidly into the interior of the pile and from their spaced interior surfaces can be conducted transversely across relatively short distances through the intervening charge sheets.

Where the surface conditions, thickness or other characteristics of the charge sheets make heat conduction difiicult, the heat distributor sheets can be alternated with the charge sheets, but I have found that this is usually not necessary and that the heat distributor sheets need not be interspersed more frequently than one to, say five or more of charge sheets of the same thickness as the distributor sheets.

In my preferred form shown in Figures 1, 2 and 3 I have shown charge sheets In between which are interspersed heat distributor sheets I l which in this case extend beyond the charge sheets as exaggerated at l2. These form a pile which rests upon any suitable base I3 capable of being lifted by elevator mechanism [4 supplied with fluid pressure connections and exhaust connections at I5 and IS. The heat-insulating furnace bottom I3 is normally supported in its lowermost position upon a car floor I! which is carried by a frame [8 and wheels 19. The wheels move laterally on track 20 supported by ties 2| from beams 22. This mechanism provides for lateral transfer of the entire pile from a suitable sheet loading point to a point from which the pile may be charged bodily within the heating furnace, and from which when discharged it may be removed to a suitable point for the next operation.

When the elevator is operated the pile is lifted to a position in inductive relation with a coil 23 whose turns 24 correspond in interior contour rather closely with the contour of the sheets, here shown as rectangular. The reason for special desirability of correspondence of these contours is that the coupling is thus made uniform about the sheets with the result that the edges of the sheets are heated to approximately the same temperatures.

A source of current for the coil is shown at and conventional power factor correction is shown at 26.

The edges of the base l3 are formed at 21 to engage and to seal the walls of a corresponding opening 28 in a platform 28 upon which the coil is supported.

Within the coil I show electrical insulation 29, and also heat insulation which may be of several different characters. As illustrated it consists of a sheet of solid heat insulation such as asbestos board 30 which has mechanical strength "as well as heat resisting properties and a filling of finely divided heat insulator. There are many such heat insulations which would serve the purpose, including zircon sand, but the best of them known to me is diatonaceous earth which is commercially available under the name of silocel.

The voltage drop between individual turns of the inductor coil need not be large with the result that the requirements of electrical insulation are small. It is not the intention in showing the insulation 29 to indicate that any electrical insulation is required further than that which wouldbe provided by the heat insulation. Furthermore the finely divided heat insulation serves the heat insulation purpose well so that further heat insulation is not required. The heat insu-' lation 30, however, serves a purpose which the coil itself or the electrical insulation or both may not effect in an individual case of mechanically holding the finely divided heat insulation 3|.

The cover 32 may be permanently applied or removable. In either event, with the coil if this be reasonably atmosphere tight, or with the electrical insulation or solid heat insulation, it forms the furnace into a bell which is closed at the top and into which treating atmosphere may be inserted by pipe 33, for example. As the treating atmosphere will ordinarily be lighter than air, escape of air should be provided at the bottom during the insertion of the treating atmosphere, but the point of insertion of this atmosphere is not material. Preferably the atmosphere is inserted after the charge-bearing furnace bottom has been lifted nearly or wholly into position and before the admission of the finely divided heat insulation so that the atmosphere may displace the air from about the edges of the plates.

The finely divided heat insulation may be inserted through any suitable covered openings 34,

suitable measures being taken to prevent escape of the special atmosphere through them.

As the finely divided heat insulation is poured in it is desirable to allow escape of air which has been poured in along with the insulation.

When the heating operation has been concluded and the base I3 is lowered the finely divided heat insulation will fall out around the edges of the base and will settle in pit 35.

Though five sheets of charge are shown in Figure 1 to each distributor sheet this is not intended to indicate that any such proportion need exist, as the number of sheets heated from each distributor may be greater than this, as seen in Figure 3a or less as shown in Figure 3. In Figure 3 the sheets alternate while in Figure 3a there is one heat distributorsheet for each ten ad joining charge sheets. It will be recognized that this is a matter affecting the speed of heat transfer and that heating will take place much more rapidly where the charge and heat distributor for example in Figure 1.

sheet must be heated by each distributor sheet.

Figure 3a. differs from Figure 1 in another particular in that the heat distributor sheets are not there extended marginally beyond the charge sheets.

Where the heat distributors do not project preferential heating of the heat distributor sheets must depend primarily upon the metal of which the heat distributor is composedas compared with that of the charge sheets.

Where the distributor sheets do not extend beyond the charge sheets and heating is effected by electrical induction, more current will be induced in the charge sheets than in the structure It is then desirable to use a smaller number of charge sheets as compared with the heat distributor sheets and preferably to interleave them alternately, so that any local heating of the edge of the charge sheets may findv relief in rapid distribution of the heat through distributor sheets which immediately engage their surfaces.

. In operation of the preferred form shown in Figures 13:--

The alternating current passed through the inductor coil about the edges of the sheets in- ,duces a corresponding current in the opposite direction about the circumferences of the sheets. Because of the projection of the heat distributor sheets beyond the charge sheets, which results in better coupling between the heat distributor sheet edges and the coil than between the edges of the charge sheets and the coil, and also because of the preferential induction of current within copper rather than within steel under equal conditions, the induced current flows so largely through the edges of the heat distributor sheets as to raise them to the desired temperature without any approach to this temperature in the steel sheets alternate than where more than one charge charge sheets. The current path in the heat disature. When the desired temperature is attained,

the input of heat through the current induced from the coil should be reduced or stopped.

Whatever the induction of current within the edges of the charge sheets, and even if they be heated up to the same temperature as the edges of the heat distributor sheets and maintained at this temperature, there is such slow transfer of heat toward the interiors along the material of the charge sheets that such transfer plays very little part in the heating of the interiors of the pile of sheets. The rate of heat transfer along copper is approximately ten times the rate of heat transfer along steel, with the result that in copper the rate of change of the temperature at one point to the maintained temperature at another point is approximately ten times that in steel.

As a result of this higher rate of conduction in copper, heat applied to or generated in the edge of a copper sheet will penetrate to the middie of that sheet with much greater rapidity than in the case of steel, and this rapidity of heat penetration is but little affected by the concurrent lateral heat transfer to the adjacent contacting steel sheets. Each individual copper sheet is, therefore, quickly raised to a temperature .be made very short, and is largely within the control of the designer. This is true notwithstanding that the heat transfer across the surface of juncture between the copper and steel and then through the steel will be slower than if the copper and steel were in integral relationship.

Assuming for the lack of exact information that the travel would take place across the sheets of steel with the same speed as through the length or width of the. steelnotwithstanding this introduces an error-I have for the sake of illustration of the theory given a diagrammatic view in Figure 8 to compare the rates of travel of heat in the two metals.

At the end of an assumed time, heat in the copper heat distributor sheet II will have reached a point 31 (Figure 8) in its travel toward the interior. At the beginning of this time, heat from the copper sheet will have begun to travel laterally through the charge sheets at point 38, and at the end of the assumed time will have reached the point 39 in these sheets.

At the point 31, where nearly the desired temperature has been reached at the very end of the time assumed, there will have been no time for the adjacent steel to have become correspondingly heated. At intermediate points along the copper heat distributor sheet, the heat will not have penetrated so far, the extent of corresponding temperature rise being dependent upon the distance and, hence, upon the time.

The line 40 connecting the points 31 and 39 therefore indicates roughly the outer border of steel which has been raised to approximately the same temperature. The same conditions will obtain on either side of the heat distributor sheet, as indicated by the line 40' connecting the points 31 and 39. For convenience in visualizing the condition at the end of the assumed time interval, the portions of the sheets raised to approximately the same temperature have been shaded.

As the time interval increases further the bases of the triangles will ultimately overlap, forming a mutually included area which would be supplied with heat from heat distributor sheets on opposite sides. When this condition exists, lateral heat transfer near the outside of the heat distributor sheets will become greatly reduced, and heat will be carried almost entirely to the interior where the higher temperature has not yet been attained.

As fast as the distributor sheets are raised in temperature from the outside toward their centers they transfer heat laterally, but complete uniformity of temperature in the steel will not be reached at intermediate points until the distributor sheets have reached approximately that temperature all over.

The triangles represent areas of attainment of nearly the temperature sought. Of course the areas outside of the triangles will have been heated also, only not so hot.

The overhang or projection l2 of the heat distributor sheets beyond the charge sheets has been greatly exaggerated in the view. With a frequency of 2,000 cycles it has been found that an inch overhang is ample on sheets having a width of more than a yard. The extent of overhang can be nicely determined in view of the fact that there is no object in making it greater than three times the depth of penetration of the current at the frequency chosen. The question of depth of penetration has been discussed at length in my Patent No. 1,694,792, granted December 11, 1928. Not only is there a relation between the extent of the overhang l2 and the frequency of the inducing current used, but the frequency is important for another reason to be treated at more length later; namely, that the resistance of the copper sheet to the induced current alters with the frequency as does also the inductance of the 'induced circuit within the sheet.

Where the heat distributor sheet does not extend beyond the charge sheets, or does not extend beyond them to suflicient distance for substantially the entire induced current to lie within this extension, as in Figure 3a, for example. where thereis no extension, the frequency becomes of importance because of the depth of penetration. With a lower frequency and increased depth of penetration, the depth of the edge heated is increased and the intensity of heat induction within a given'volume of metal of the heat distributor need not be as great to induce the total heat required to maintain the temperature at the outer edge notwithstanding the transmission of heat into the interior of the pile.

By selection of the frequency, therefore, the volume as well as the depth of the annulus at the edge of the heat distributor which is to receive inductive heating may be adjusted and determined to avoid excessive temperature at the edge of the sheet.

For maximum power consumption, the secondary (induced) circuit resistance should equal the reactance. In copper sheets such as we have been discussing, the resistance is normally quite low (lower than the reactance), even when high frequency current is used and the skin eifect increases the effective resistance. The disproportion between inductive reactance and resistance is less pronounced with increase in temperature because the resistance increases as the temperature rises. Therefore the power consumption will progressively increase to a maximum as the copper heats to the desired temperature.

I have found that resistance can be artificially increased without undesirably affecting the inductance and without affecting the heat transfer quality of the copper. Means for accomplishing this result is shown in Figures 4, 5 and 6, in which the edges of the copper sheet are cut to provide non-inductive enlargements of the path of the induced current in the copper.

In Figure 4 these cuts are short and. appear at 4| from opposite edges at the sides of the sheet, and at 42 from opposite edges at the ends of the sheet. The end cuts 42' are made shorter than the others to avoid undue reduction of heat conduction cross section from the corners to the body of the distributors. In Figures 5 and 6 the cuts are much deeper and are made from the side edges only. In Figure 5 the cuts 4| extend over from side edges approximately to the center of the sheet, leaving a portion of the metal at 43 uncut between opposite cuts in line. In Figure 6 the cuts 4| are made wholly from one side edge and extend almost the entire distance across the sheet. In eachof Figures 4, 5 and 6 the path of induced current flow in the sheet is shown by in potential between the opposite sides of the cuts is low because it is due wholly to RI drop and R. is low. Where the cuts are deep and the extension of the current path is correspondingly great, R increases with a consequent increase in voltage drop between the opposite sides of the cut and most pronounced at the mouth of the cut.

While this is true to some extent in the form,

I of Figure 5, it is more noticeable in the form of for a short distance at the edges of the cut, and

this may be done by placing sheet insulation there or by coating over the copper, or the steel, with an insulating material, such as awhite porcelain of the type now on the market under several trade names, one of which is Insalute, or an-enamel.

In Figure 9 I show insulation 41 on either side of the heat distributor sheets, which have been cut at 48. It will be understood that insulation may be used in connection with uncut heat distributor sheets if desired.

In Figure 10 white porcelain is shown at 41' about the cut 48 to insulate the distributor sheet at this point.

My invention is by no means restricted to inductive heating, nor to heating in a special atmosphere, but may be applied with fuel heating,

\ direct or indirect, with non-inductive electric heating, with heat transfer from a heated medium, or with any practicable means of heat application. Also, my invention may be employed in structure in which the gases of combustion come in direct contact with the charge, in structure in which the gases of combustion are shielded from the charge by the walls of a muflie or otherwise, and in structure in which no gases of combustion are present.

Because of the requirement of the statute that I describe my best embodiment, I have first described the inductive form. I will now proceed to illustrate by way of example two other structures which might be used.

In Figure 7 the pile of charge sheets i0 mingled with heat distributor sheets II are contained in a muiiie 49 covered by a top 50 and resting on a support 5|. The walls of a flue 52 carry gases of combustion into contact with the muffle.

Heat present in the atmosphere within the muffle, whether in a special atmosphere or in air, will heat the edges of the heat distributor sheets, which in turn will carry the heat into the interior of the pile and difluse'it throughout the charge in the manner previously described.

I In Figure 7a a pile of charge sheets l0 mingled with heat distributor sheets II rests on the floor 53 of a fuel heated furnace 54 in direct contact with the gases of combustion. An oil jet 55 is supplied with oil at 56 and air at 51. The gases of combustion are removed through the stack 58.

Heat from the sensible heat of the gases of combustion will heat the edges of the heat distributor sheets and by them be conducted throughout'the pile.

' Thus it ,will be seen that, no matter how heat is applied to or produced in the edges of my heat distributor sheets, they will conduct heat into the center of the pile in substantially the same man- Where the heat distributor sheets may come in contact with oxidizing gases, and desirably in all'installations in order to permit the removal of the heat distributor sheets from the muiiie orheating furnace into contact with the atmosphere before they have fully cooled, I will plate the heat distributor sheets with a suitable metal resisting oxidation, as for example nickel or chroimum.

My invention will be useful wherever objects must be heated which have relatively great dimensions intw'o directions as compared with their dimension in the third direction, whether they be extremely thin (of the order of one-sixteenth inch or less up. to one-half inch) and are known as sheets, or are somewhat thicker (from one-half inch to two or three inches) and are known as plates, or are deformed and are known as pressed metal shapes. In any of these cases, my invention will be desirable for most advantageous heating. No matter what the thickness, if the thickness be relatively slight by compari son to the other dimensions, I elsewhere refer to the objects as sheets, not meaning to imply by that term that they are necessarily fiat, though this would normally be the case.

In the case of heating material which is not substantially fiat, I will, of course, preferably change the shape of the heat distributors to conform approximately to the shape of the objects to be heated, providing effective lateral contact with them.

By effective contact I do not mean necessarily that the heatdistributor must make an absolutely tight fit with the charge at all points, or even that it must be in immediate contact with the charge, since, for example, an electric insulator might be interposed between, but I mean to indicate that the heat distributor sheet is in lateral heat transfer relationship with the charge so that heat may be carried from the point of application to the heat distributor to some otherpoint on the heat distributor, and there diffused through the charge.

While the most usual purpose for which sheets are heated is in order to anneal them, my invention is not restricted to heating with this end in view, since it would be equally eflfective to accomplish the heating step no matter what the subsequent operations might be. For example, where relatively heavy stock is to be pressed through dies, or where the extent of deformation makes this necessary, I may heat sheets in weight of the pile will prevent any buckling incase the energy input is so rapid that theheating is at all uneven.

I have shown a further precaution against buckling in the slits of Figures i, 5 and 6, since the slits will avoid any possibility of edge curl in the heat distributor sheets even though they be heated considerably hotter at their outer edges than in the interior By making the slits of substantial width, 1 also avoid. danger of buckling through uneven expansion and engagement of the opposite walls of the slits.

Where the sheets are to be heated in some other position than that of a horizontal pile, as ior example, when stacked vertically or when nested after deformation, heating should be gradual in order to insure uniformity and avoid buckling.

Where heating is to be inductive, the inductor coil could, oi course, be placed vertically or horizontally and the charge can be inserted continuously or intermittently. Where the inductor coil axis is vertical, the charges may be placed within the inductor coil as in the form of Figures 2 and 3, or the inductor coil could be raised or lowered into a position about the charges as desired. These features I regard as relatively immaterial to my broad invention.

With inductive heating the determination of the frequency will depend upon a number of factors. Where any substantial part of the inductive heating is to talre place in a magnetic charge, and that charge is to be heated above decalescence, high frequency is much to be preferred. On the other hand, with low frequency the lines of force have an increased depth of penetration, and therefore will inductively heat a larger area about the edges of the charge. Also the cost of the generator is lower with low frequency. On

' the other hand, with high frequency, power factor correction is a smaller matter. The various items of expense and the demands of the particular installation will in each case determine the frequency to be used in accordance with the factors here outlined.

In view of my invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the to obtain all or part of the benefits of my invention without copying the structure shown, and I, therefore, claim all such in so far as they fall within the reasonable spirit and scope of my invention.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent lsz- 1. The method of edgewise heating a metal sheet with maximum power input, which consists in passing an alternating current about its outside circumference to induce current within the periphery of the sheet and in artificially increasing the resistance of the portion of the sheet within which the induced current travels by cutting the sheet part way across.

2. The method of edgewise heating a metal sheet with maximum power input, which consists in passing an alternating current about its outside circumference to induce current within the periphery of the sheet and in artificially increasing the resistance of the portion of the sheet within which the induced current travels to approach the reactance of the path to the induced current by cutting the sheet. part way across.

3. The method of edgewise heating a metal sheet with high power input, which consists in passing an alternating current about its outside circumference to induce current within the pei'f'phery of. the sheet and in artificially increasing the resistance of the portion 01 the sheet within ace-once v passing an alternating current about its outside circumference to induce current within the perlphery of the sheet and in diverting the induced current from its normal path along a noninductive path by cutting the sheet part way across to increase the resistance without increasing the inductance of the path.

5. In mechanism for heating metallic charge sheets having relatively low heat conductivity, such as iron or steel, a distributor sheet of metal of relatively high heat conductivity, such as copper, placed in eflective lateral contact with one or more of the sheets to be heated, the distribu tor sheet having cuts in its periphery to increase its resistance to circumferential currents, an inductor coll about the edge of the distributor sheet and a source of alternating current for the coil.

6. In mechanism for heating metallic charge sheets having relatively low heat conductivity, such as iron or steel, a distributor sheet of metal of relatively high heat conductivity such as copper placed in effective lateral contact with one or more of the sheets to be heated, the distributor sheet having cuts in its periphery to increase its resistance to circumferential currents, an inductor coil about the edge of the distributor sheet and a source of alternating current for the coil, the frequency and the cuts being so proportioned that the resistance will approach in value the reactance in the distributor sheet for the alternating current induced in it.

7. In mechanism for heating metallic charge sheets having relatively low heat conductivity,

such as iron or steel, a distributor sheet of metal of relatively high heat conductivity, such as copper placed in effective lateral contact with one or more of the sheets to be heated, the distributor sheet having cuts in its periphery to increase itsof relatively high heat conductivity, such as cop-- per placed in eflective lateral contact with one or more of the sheets to be heated, the distributor sheet having cuts in its periphery to increase its resistance to circumferential currents, an inductor coil about the edge of the distributor sheet, a source of alternating current for the coil, the cuts being deep enough for there to be considerable drop of potential across the cuts at their mouths and-insulation between the charge sheets and the adjacent heat distributor sheet to prevent arcing at the cuts. 1

9. In mechanism for heating metallic charge sheets having relatively low heat conductivity, such as iron or steel, 9. distributorsheet of metal of relativelyhigh heat conductivity, such as cop per placed in efl'eotive lateral contact with one or more of the sheets to be heated, the heat distributing sheets being cut inwardly from their ends to a considerable distance toward the oppositeedge of the sheet, a; coil surrounding the edge of the distributor sheet, a source of current for the coil and insulation between the edges of the cut in the distributor sheet and the adjacent charge sheets.

10. In mechanism for heating metallic charge sheets having relatively low heat conductivity, such as iron or steel, a distributor sheet of metal of relatively high heat conductivity, such as copper placed in effective lateral contact with one or more of the sheets to be heated, the distributor sheet being cut at intervals along its edge to a substantial width to permit expansion on the edge of the sheet without buckling of the sheet.

11. In mechanism for heating metallic charge sheets having relatively low heat conductivity, such as iron or steel, a distributor sheet of metal of relatively high heat conductivity, such as copper having an interrupted edge placed in efiective lateral contact with one or more 01' the sheets to be heated, electrical insulation between the edges of the heat distributor sheet and the adjoining charge sheets, an inductor coil about the edges of the heat distributor sheet and. a source of alternatlng current energy for said coil.

12. In mechanism forheating metallic charge sheets having relatively low heat conductivity, such as iron or steel, a distributor sheet of metal of relatively high heat conductivity, such as copper having an interrupted edge placed in effective lateral contact with one or more of the sheets to be heated, an insulating coating between the edges of the heat distributor sheet and the adjoining charge sheets, an inductor coil about the edges of the heat distributor sheets and a source of alternating current energy for said coll.

EDWIN FITCH NORTHRUP.

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