US3124425A

US3124425A - Richelsen

Info

Publication number: US3124425A
Authority: US
Publication date: 1964-03-10
Anticipated expiration: 1981-03-10

Description

March 10, 1964 M. RlcHELsEN REACTION FURNACE 2 Sheets-Sheet 1 Filed Oct. 30. 1958 l we Wm w p( w Ml G 5 mw 2 @/w 5 l /5 3 fo 6 AI \/\.Wv- ,71 l

March 10, 1964 M, RlcHELsEN 3,124,425

REACTION FURNACE.'

Filed Oct. l30. 1958 y 2 Sheets-Sheet 2 United States Patent Oice 3,124,425 Patented Mar. 1G, 1964 3,124,425 REACTEN FURNACE Mark Richelsen, 401i Prospect Arve., Medina, N.Y. Filed Oct. 3i), 1953, Ser. No. 770,740 9 ciaims. (ci. 2st- 277) This invention relates to gas heating apparatus and .is particularly concerned with apparatus for heating nonoX-idizing gases to high temperatures and for utilizing such hot gases in chemical reactions.

It is an object of the present invention to provide apparatus that can be used to heat non-oxidizing gases to high temperatures conveniently and eciently.

Another object of the invention is to provide apparatus of the character described that Will withstand -attack by hot, non-oxidizing gases.

Another object of the invention is to provide apparatus that may be conveniently used for kcarrying out chemical reactions involving or utilizing hot, non-oxidizing gases.

A further object oi. the invention is to provide apparatus of the character described that may be easily and inexpensively assembled and may be conveniently operated.

Another object of the invention is to provide a process for producing high temperature gas reactions.

Still another object of the present invention is to provide a process for halogenating materials at high temperatu'res.

Other objects and advantages of the invention will be apparent from the following description taken in conjunction with the accompanying drawings in which:

FIGURE 1 is a vertical sectional view through apparatus embodying the principles of the invention;

' FIGURE?. is an elevation (partly in section) on a reduced scale of apparatus like that illustrated in FIGURE 1 but with a modied form of gas heating device;

FIGURES 3 and 4 are fragmentary sectional views illustra-ting other modified tormso-f gas heating devices; and

FIGURE is a horizontal sectional View of apparatus according to the invention utilizing a plurality of gas heating devices.

In FIGURE l there is shown a preferred form of gas f heating device in a casing that constitutes a reactor within which various lreactions involving the hot gas may be carried out. The gas heating device comprises a graphite tube 11, a graphite rod 12, axially aligned with the tube 11, and `a carbon block 13 between the tube 11 and the rod 12.

The tube 11 and the block 13 are joined by a graphite nipple 14 that is threadedly engaged in lrecesses in the tube and the block. Similarly the rod. k12 and the block 13 are joined by a graphite stud E15 threadedly engaging in recesses in the rod and the block. The nipple and stud are also 'axially aligned with the tubey 11 and the rod 12 and are of such lengths that the tube and the rod may be tightly drawn, respectively, against the upper and lower surfaces of the block 13. l Extending downwardly (as viewed in FIGURE l) through the bore 17 of the tube 11 and the nipple 14 is a gas-impervious tube 18, the lower end of which is exteriorly threaded land engages in the interiorly threaded, enlarged, upper end of a well 21 that extends downwardly in the carbon block 13 from the nipple 14 to a point somewhat below the center of the block. In the upper end of the nipple 14, around the tube 518, there is provided a packing gland which consists of a mass 22,y of flake or folated graphite compressed by a threaded, annular, graphite plug 23. This prevents *leakage of gas around the tube 18. Electrode joint compound applied to the threads of the nipple 14 will also minimize leakage of gas therethrough. 'n

At its upper end the tube 18 projects from the tube 11 and is threadedly connected to one arm of a T-tittingZS.

The other -arm of the tit-ting is provided with a cap 26 in which there is mounted a sight glass 27 of quartz. A gas supply line 28 is threadedly attached to the leg of the T.

Adjacent the outer ends of the tube 11 and the rod 12 there are tirrnly clamped, by suitable means such as plates 3d and sere-Ws 31, conductors or bus bars 32 which are connected to a suitable source of elect-ric current. Care is taken to have the surfaces of the carbon block 13 which are in cont-act with the graplnte tube 11 and the graphite rod 12 and the corresponding surfaces of the tube and rod suiiiciently smooth and flat that good electrical contact will be obtained between the three elements.

t is well recognized that both graphite and amorphous carbon of the types used for blocks, electrodes, tubes and other shapes are quite porous. The degree of porosity varies somewhat with the method and conditions `of production but porosities of 2G%-4% are common, the sizes of the pores usually ranging from about lilmicrons in diameter. There-fore, when a non-oxidizing gas is passed into the cavity or well 221 of the block 13 through the supply line 2S, iitting'ZS land tube 2S, it will readily escape through the pores of the block. If -at the'same time electrical current is supplied through the bus bars 32 to the tube 11 and the rod 12, the block 13 will be intensely heated since the carbonhas an electrical resistance several times as great as that of the graphite and the gas passing therethrough will also be heated to a high temperature as a result of its extremely intimate contact with the intensely heated carbon surrounding the minute pores in the block.

Obviously, a gas heating device of the type described above can be used in many ways. In FIGURES l and 2 there is illustrated an apparatus adapted to carry out certain chemical reactions with the highly heated gas issuing from the gasheating device described above. The `apparatus Ior reactor shown is specically designed for the halogenation of various solid materials and accordingly is constructed to resist attack by hot halogen gases as well as to prevent leakage of such gases -into the atmosphere.

The illustra-ted reactor comprises an outer, preferably cylindrical casing 3d which may be formed of ordinary steel, l.a top cover 37 and a bottom cover 3S. The covers are provided with annular gaskets 41 and are secured to the peripheral iianges S and itl at the upper and lower ends of the casing 36 by suitable means `such as'bolts 42 spaced `around their peripheries. Within the casing 36 extending from the cover 37 to the cover 3d is an annuf lar wall 44, which, as shown, may be outwardly flared in its upper portion. The wall is preferably concentric with the casing 3d and formed or" carbon. While a unitary carbon wall structure may be used, one formed of carbon blocks or shaped carbonbricks is quite satisfactory and less expensive. To ensure tightness carbon cement may be used in the joints. The space between the wall 44 and the casing 36 is packed with carbon black as indicated at 45. v

The space within the wall i4 receives the gas heating device described above and constitutes a reaction chamber 46. The graphite ,tube 11 and the graphite rod 12, which serve .as electrodes, project outwardly through yaxial orifices 47 and 48 in the covers 37 and 33, respectively, and the bus bars 32 are clamped to the electrodes outside the reactor. A water jacket Sti is provided around each of the

electrodesy

11 and 12, cooling water being suppliedthereto through a pipe 51 and being `drawn ott through a pipe 52. '[hese jackets keep the ends of the electrodes cool and thereby prevent Ltheir oxidation and the undesirable heat-ing of the bus bars 32. On theexterior of each of the jackets 5@ adjacent the inner end thereof is an annular ring or ange 53. y

At spaced intervals around the hole 47 at the top of the reactor there are secured a plurality of outwardly projecting studs S5, the outer ends of which extend through holes in an annular disc 56. The hole in the center of the disc 56 is just large enough to slidably tit the exterior of the graphite tube 11. A sealing ring 57 is provided between the flange 53 and the inwardly inclined edge of the hole 47 and another sealing ring 5S is placed in the annular recess 59 formed at the outer end of the water jacket Si) and is engaged by the plate 56. The

rings

57 and 58 are conveniently formed of graphitized asbestos. Tightening of the nuts 6d on the studs exerts pressure on the packing ring 58 and seals the space 1ne'tween the tube 11 and the water jacket Si). At the same time the packing ring 57 is compressed between the flange 53 and the top cover 37 of the casing, thus sealing the central opening through the cover. The lower cover and the space between the electrode 12 and the lower water jacket are also provided with sealing means which may be, as shown, identical with the sealing means described above.

An inlet pipe or chute 61 extends through the top cover 37 and may be used for feeding material into the reaction chamber 46. There is also provided on the cover 37 an outlet duct 62 through which gaseous products of reaction may be withdrawn. An illustrative example of the use of the reactor and the gas heating device for the chlorination of rutile to produce titanium tetrachloride is described below.

A briquetted mixture of granular rutile and about carbon is fed into the reaction chamber 46 through the inlet chute 61 substantially filling the chamber. Suitable means (not shown) is provided for closing the inlet to prevent access of air therethrough. A non-oxidizing, inert gas is then introduced under pressure through the supply line 28 and the tube 18 into the cavity or well 21 in the interior of the carbon block 13. Flow of the nonoxidizing, inert gas, which may be argon, helium, or even nitrogen, is continued until substantially all of the air is displaced from the interior of the reaction chamber 46 and the gas heating device. Electrical current is then supplied to the graphite tube 11 and graphite rod 12 and cooling water to the jackets 50. A voltage from about 3 volts to about 24 volts is preferred and suiiicient current is supplied to quickly raise the temperature of the carbon block 13 to the point of incandescence. Chlorine gas is then passed into the interior of the block 13 through the passages provided.

The chlorine is preheated as it passes through the tube 18 since the hollow electrode 11 is also, of course, hot. The temperature of the chlorine increases as it approaches the carbon resistance block 13 and reaches a maximum as it percolates outwardly through the ne pores of the block into the chamber 46. The charge of briquettes in the chamber is also heated by the incandescent resistance block and the chlorination of the TiOg with formation of TiCl4 proceeds rapidly.

For proper chlorination the charge should be heated to at least about l475 F. and preferably to about l850 F. Even higher temperatures may be employed. The temperature within the chamber can be determined with thermocouples (not shown). The temperature of the carbon block 13 may be readily ascertained by temporarily interrupting the flow of chlorine to the reactor and introducing an inert gas such as nitrogen or argon through the gas inlet system to clear the system. An optical pyrometer may then be sighted at the interior of the Well 21 through the quartz sight glass 27.

As the reaction proceeds the TiCl4 formed is withdrawn along with other products of the reaction through the outlet duct 62 and may be condensed and purified in known manner. Additional charge is admitted from time to time through the inlet chute 61 as the level in the reaction chamber falls. The process can thus be carried on for prolonged periods of time. When the level of unchlorinatable impurities and debris in the reaction charnber 46 reaches an undesirable level, the process can be discontinued while the impurities are removed. This may be accomplished conveniently through a clean-out hole communicating with the reaction chamber 46 that may be provided in therreactor at any desired and sultable place. If desired, a plurality of inlet chutes and outlet ducts may be provided for the reactor.

Apparatus of the type described above can be built in any desired size and accordingly, it is possible to carry on large scale reactions with production of large quantities of product. The chlorine or other gas may be 1ntroduced at a high rate since any tendency to cool the block 13 can be offset by merely increasing the current through the block. While the construction of the reactor can be varied in many respects, as pointed out below, the use of carbon or graphite for the portions thereof that contact chlorine or other halogens at high temperatures is practically essential as no other materials will satisfactorily resist attack.

It will be understood that the example above of the chlorination of rutile is only one of many halogenation reactions that can be carried on conveniently in apparatus of the type described. Reactions involving uorination and iodination may also be carried on and halides of many metals and metalloids such, for example, as zirconium, silicon, chromium, aluminum, uranium, boron, as well as others, may be produced.

The construction of the novel gas heating device ofthe present invention may be varied and modied in` a number of ways. Examples are illustrated in FIGURES 2, 3 and 4. In FIGURE 2, although the structure of the reactor and the remainder of the gas heating device are like that in FIGURE 1, the electrodes and resistance block are different. As shown, the porous resistance block 66 -is of smaller diameter than the tube 11 and rod 12. This is to demonstrate that the size of the block is not critical per se, but may vary in accordance with the relative resistivities of the several elements of the heating device and the current available for heating the block. It will also be seen that the ends of the block 66 are threaded for connection in the ends of the graphite tube 67 and graphite rod 68. FIGURE 4 shows that the shape of the block is not critical. It also indicates that a tapered or partially tapered outer surface on the block 71 may be advantageous to prevent accumulation of charged material on the top thereof. In this connection, it may be noted that the upper, outer edge of the block 13 in FIGURE ll has been bevelled for the same reason. As will be seen, the graphite tube 7 2 in FIGURE 4 is threaded directly into the block 71 as is also the graphite rod 73.

In FIGURE 3 there is shown a portion of a graphite rod 76 which can serve as a single electrode substituting for the tube 11, rod 12 and block 13 of FIGURE 1. At a point between the ends of the rod 76 is a reduced portion 77 which will, of course, have a greater resistance than the remainder of the rod. A bore 7 S is provided from one end of the rod 76 to the reduced portion 77 and a gas impervious inlet tube 79 is inserted through the bore and threadedly engaged therein adjacent the inner end, leaving a well or cavity from which the gas may escape through the porous wall.

Other possible variations, too numerous to illustrate, are possible in the construction of gas heating devices according to the present invention. Thus, merely for example, the electrodes may be secured to the same side of a porous carbon resistance block which thus forms a bridge across the two electrodes. Alternatively, one electrode may be secured to an end of a porous carbon block while the other electrode is secured on the side of the block. In both cases mentioned one of the electrodes will preferably be solid while the other electrode will be tubular to permit the supplying of gas to a cavity or hollow portion in the interior of the porous block.

Still another possible construction involves the use of two concentric tubular electrodes with a porous block, one or the other of the electrodes carrying gas to a cavity in the block. If desired, the cavities or wells in the porous electrodes or blocks may be enlarged in diameter or otherwise formed to increase their surface areas, thereby to facilitate flow of gas therefrom through the walls. On the other hand, when very porous material is used for the diffusion portion of the device and low gas flow rates are employed, it may be unnecessary to provide a well extending beyond the end of the gas feed tube. It will be understood that the porous blocks and/or the electrodes may be of any necessary size. While preferably in most cases they are circular in cross-section because of the lower cost of production, they may also be of any shape desired. As will be observed from the foregoing discussion, gas heating devices according to the present invention essentially comprise a porous diffusion section and a pair of electrode sections electrically connected thereto, one of which includes gas-impervious means for conducting a gas to the interior of the diffusion section. The resistance of the electrode sections is normally intended to be less than that of the diffusion section. It is, therefore, usually convenient to use graphite electrodes with an amorphous carbon diifusion block since the resistivity of graphite shapes is usually less than that of similar amorphous carbon shapes. However, as illustrated in FGURES 2 and 3, this is not necessary and any desired variation in this respect can be employed. The term carbon, therefore, is meant to include graphite unless the opposite intention appears.

In the several illustrations in the accompanying drawings gas is conducted to the interior of the resistance block or, as in FIGURE 3, to the interior of the hollow electrode, by a gas-imprevious carbon or graphite tube. Ordinary carbon or graphite tubes are, as indicated above, relatively porous. They may, however, be rendered substantially impervious to gases, vapors and liquids even at elevated temperatures and pressures at least of the order of 20 p.s.i. gage by impregnation with carbon to fill the pores. Graphite and carbon shapes so treated are commercially available, being sold under the trademark Graph-i-tite. Other high density carbon and graphite tubes having negligible porosity are also obtainable. If desired, instead of using a separate gas-impervious tube within a tubular electrode, the latter may itself be formed of gas-impervious graphite or carbon.

It will be apparent from the foregoing that by proper design and choice of shape and material, gas heating devices suitable and convenient for use in any type of furnace, reactor or other apparatus may be constructed. Obviously, they may be operated vertically, horizontally, or in any other desired position.

Although reactors containing only a single gas heating device can be used satisfactorily in many cases, it is also possible to use a plurality of such devices in a single reactor Where large volume output is desired. In FIGURE 5 there is illustrated, somewhat diagrammatically, a construction of this type. The apparatus, as shown in a construction suitable for producing metal or metalloid halides, comprises a casing S5 having therein a concentric Wall 86 preferably formed of carbon blocks. The annular space between the casing and wall is filled with loose carbon black 87. Here, as also in the reactor illustrated in FIGURES l and 2, the mass of carbon black not only serves as an efficient insulator to prevent loss of heat from the reactor, but also, because of its resistance to attack by hot halogens, as a barrier to any halogen leakage through the wall 86 from the reaction chamber 88.

In order to operate at a high level of production a plurality of gas heating devices are arranged in the reaction chamber 88. These are arranged transversely of the chamber and, as indicated, may have porous resistance blocks 91 of different sizes thereby providing a major portion of the reaction chamber cross-section with heating surfaces from which heated gas is emitted. Each of the gas heaters may be constructed similarly to those hitherto described. However, the ends of the electrodes 92 extend through cooling chests 93 arranged on opposite sides of the reactor, suitable packing glands being provided in the chests around each electrode to prevent leakage. The

chests 93 are preferably formed of nickel or nickel-clad steel to avoid or minimize corrosion and are suitably sealed in the wall of the reactor. The outer end of each electrode will, of course, be provided with a conductor or bus bar (not shown) to supply electrical current to the resistance blocks and at least one of each aligned pair of electrodes 92 will be provided with a bore through which a halogen gas may be introduced into the interior cavity of the associated porous resistance block 91.

In a construction like that just described, when a suitable charge, for example briquetted aluminum oxide or Zircon and carbon, is introduced into the reactor above the gas heaters the reaction with the hot halogen gas is very rapid. In general, the greater the flow of hot halogen gas that can be brought into intimate contact with the material to be halogenated, the higher the rate of production. This rate is increased by the high temperature of both the charge and the halogen. Obviously, many other arrangements of multiple gas heaters in a reactor can be made and, since as set forth above the gas heating devices can take various shapes and have a number of yditferent electrode arrangements, the construction of reactors particularly designed for most efliciently carrying on specific types of reactions is possible. In this connection, it may be noted that reactors embodying the principles of the present invention can be so constructed that the iiow of material therein is vertical, horizontal, or inclined. While, in general, when using a solid as one of the reacting materials, a vertical or inclined feed is preferred, reactors utilizing internal conveying means to move the reactive material into proximity to the porous resistance blocks can be constructed. It will also be clear that reactors of rthe present general type can be used with moving bedsy or lluidized beds of reactive, solid material, employing, if desired, the hot reactive gases issuing from the porous resistance elements as carrier gases.

Gas heating devices in accordance with the present invention may be used in a wide range ofways. Thus, as described above, they may be employed to heat gases to make them more reactive and may, at the'same time, also heat the material either as a solid or as a vapor or gas with which the heated gas issuing from'the porous resistance block is to react. In other cases, they may be used to dissociate gases. As examples, ammonia may be dissociated to nitrogen and hydrogen, and hydrocarbon gases orvapors may be cracked in such devices. If desired, such dissociated gases or some of them may be reacted with other heated gases or solids in a surrounding reactor. Further, in gas heating devices of the type described a mixture of gases may be heated in the porous resistance block to cause the formation of other compounds either with or without dissociation or cracking of one or both of said gases. In some types of reactions it may be found useful to employ a modification in which two sources of gas feed into a single cavity in aporous Iresistance block. Also, of course, when a reaction between gases in the reaction chamber around a gas heating device is contemplated the gas inlet to the chamber should be so chosen as to feed the gas at a point where the desired conditions of reaction will be attained. rfhus, depending upon the temperature required, the gas may be fed to the chamber below or above the hot, porous, resistance element.

Other types of reactions will suggest themselves in which the present gas heating devices may be advantageously used. In addition, the intense heat of the resistance block and the high temperatures to which the gas can be raised permit use of gas heating devices according to the invention to cause great volume expansion of the gas for propulsion or other use with or without the reaction. It will be understood that in the present specification and the appended claims the term reaction is intended to include not only reactions in which chemical compounds are formed by combination, but also' Sasa-225 by dissociation or decomposition. It will also be apevident, the ynon-oxidizing gases with which the present i' invention is used may be fed to the gas heating device as a gas orv vapor or, if convenient, as a liquid, it being understood thatin the latter case the liquid will be vaporized inthe inlet system before reaching the porous resistance yelement through which it diffuses.

The materials of which gas heating devices and/.or associated reactors are constructed may vary yconsiderably in. accordance with the conditions and requirements ofv use. Thus, whilecarbon and/or graphite are essential for most portions of apparatus in which hot halogens are present, the cooler portions of the reactor, for example, the` water jackets around the electrodes may be formed of nickel or halogen-resistant nickel alloy and the covers for the reactor may be in some cases nickel clad steel or-even plain steel if the halogen concentration of gases in-contacttherewith is negligible. Where less corrosive gases` are in use, the materials employed may be less inert. j In such cases, if desired, other materials such, fork example, as silicon carbide may be used for the electrodes and/or resistance blocks and other suitable materials may be employed for construction of the reactors and other elements.

As previously pointed out, the diffusion portions of gas heating devicesl according to the present invention are readilyheated to incandescence by passage of electrical currentl therethrough. The temperature reached may be controlled by theamount of current supplied and accordingly thereactions can be easily controlled. Temperatures may be raised to the sublimation point of carbon or the decomposition temperature of other porous materials used, although somewhat lower temperatures are advisable to prolong the useful life of the porous sections of the heaters. The use of extremely hot reactive gases will, ofcourse, increase greatly the speed of reactions involving them and many reactions are feasible` with reactors according to the present invention which are not readily carried on otherwise. As will be evident from the foregoing description, elements of a gas heating device may be easily replaced when necessary. In this connection it will be recognized that in such replacement there may be substitution of parts of different shapes and/or sizes so as to vary the operation. For example, electrodes of different lengths may be installed which will result in a shifting of the position of the diffusiony block.

I- claim:

1. A gas heating device comprising a porous diffusion section of carbon having a cavity therein'l having an en, trance on the exterior thereof, a tubular carbon electrode section electrically joined to said diffusion section adjacent the entrance to said cavity, and a solid carbon electrode section electrically joined to said diffusion section at a point removed from said entrance, said electrode sections having lower resistance than said diffusion section and being adapted for separate connection to a source of electrical current.

2. A gas heating device as set forth in claim l in which said electrode sections are separate from said diffusion section and are formed of graphite.

3. A gas heating device as set forth in claim 1 in which said tubular electrode section is gas-impervious.

4. A gas heating device asset forth in claim 1 in which there is provided lwithin said tubular electrode section 'a gas-impervious tube communicating with the entrance to said cavity.

5. Areactor comprising a closed shell having a gas heating device therein, said device comprising a porous diffusion section having a vrelatively high electrical resistance, a pair ofy current carrying sections electrically joined to said diffusionv section and adapted for separate connection to a source of electrical current, said diffusion section having a cavity therein, said cavity having an entrance on the exterior of said diffusion section, and gas-impervious conduit means within one of said current carrying sections communicating with said cavity through said entrance for supplying gas to the interior of said diffusion section, the outer ends of said current carrying sections projecting outwardly through said shell; a cooling jacket surrounding each of said projecting ends; and

sealing means for preventing leakage of gas between said shell and said cooling means and between said cooling means and said projecting ends.

6V. A reactor as set forth in claim 5 in which said gasimpervious conduit means is a separate tube.

7. A reactor comprising a shell having a body portion and covers for the ends of said body portion; an inlet to the interior ofy said shell; an outlet from said shell; a gas heating device, said device comprising a porous diffusion section having a relatively high electrical resistance, a pair of current carrying sections electrically joined to said diffusion section and adapted for separate connection toa source of electrical current, said diffusion section having a cavity therein, said cavity having an entrance on the lexterior of said diffusion section,` and gas-impervious conduit means within one of said current carrying sections communicating with said cavity through said entrancejfor supplying gas to the interior of said diffusion section, the louter ends of said current carrying sections projecting outwardly through said shell; a cooling jacket surrounding each of said projecting ends; and sealing means for preventing leakage of gas between said shell and said cooling means and between said cooling means and said projecting ends.

8. A reactor as set forth in claim 7 in which said gas heating device is arranged axially within said shell.

9. A reactor as set forth in claim 7 in which said shell is provided with insulation.

References Cited in the tile of this patent UNITED STATES PATENTS 1,339,225 Rose May 4, 1920 1,880,306 Wulff Oct. 4, 1932 2,237,503 Ridgway Apr. 8, 1941 2,378,675 Agnew et al. .Tune 19, 1945 2,436,282 Bennett Feb. 17, 1948 2,447,809 Niguet et al Aug. 24, 1948 2,708,156 Paoloni May 10, 1955 2,790,703 Frey Apr. 30, 1957 2,855,273 Evans et al. Oct. 7, 1958

Claims

1. A GAS HEATING DEVICE COMPRISING A POROUS DIFFUSION SECTION OF CARBON HAVING A CAVITY THEREIN HAVING AN ENTRANCE ON THE EXTERIOR THEREOF, A TUBULAR CARBON ELECTRODE SECTION ELECTRICALLY JOINED TO SAID DIFFUSION SECTION ADJACENT THE ENTRANCE TO SAID CAVITY, AND A SOLID CARBON ELECTRODE SECTION ELECTRICALLY JOINED TO SAID DIFFUSION SECTION AT A POINT REMOVED FROM SAID ENTRANCE, SAID ELECTRODE SECTIONS HAVING LOWER RESISTANCE THAN SAID DIFFUSION SECTION AND BEING ADAPTED FOR SEPARATE CONNECTION TO A SOURCE OF ELECTRICAL CURRENT.