US2796545A - Electronic discharge tube - Google Patents

Description

A. BRASCH ET AL 2,796,545

ELECTRONIC DISCHARGE TUBE June 18, 1957 Filed Deo. 21, 1949 7 Sheets-Sheet 1 June 18, 1957 A. BRAscH Er AL ELECTRONIC DISCHARGE TUBE 7 Sheets-Sheet 2 Filed Dec. 2l. 1949 IN VEN TOR. ,4e/v0 BRA sa# WOLFGANG H055@ ,4a/VAN Hmy -BY A free/wey June 18, 1957 A. BRASCH Er AL 2,796,545

ELECTRONIC DISCHARGE TUBE Filed Dec. 21, 1949 '7 Sheets-Sheet 3 INVENTOR. ,4e/vo sense WQLFA/VG H0854? June 18, 1957 A. BRASCH ET AL ELECTRONIC DISCHARGE TUBE 7 Sheets-Sheet 4 Filed Deo. 2l. 1949 INVENTOR. APA/a (SR/15CH.

June 18, 1957 A. BRAscH ET AL ELECTRONIC DISCHARGE TUBE 7 Sheets-Sheet 5 Filed Dec. 21. 1949 June 18, 1957 A. BRAscH Er AL ELECTRONIC DISCHARGE TUBE 7 Sheets-Sheet 6 R 0 NM la A Filed Dec. 21. 1949 June 18, 1957 A BRAscl-i Er AL ELECTRONIC DISCHARGE TUBE 7 Sheets-Sheet 7 Filed Dec. 21. 199

y s non. N mit f Vx@ r mga, H .A a wm M E en my P United States ,Patent 'C "i par ELEC'rRoNrC DISCHARGE rtm1:

Arno Brasch, Wolfgang Huber, and Adnan Wely, Broeiklyn, N. Y., assignors to Electronized Chemicals Corporation, a corporation oiNew York Application December 21, 1949, Serial No. 134,271

2 Claims. (Cl. 313-74) The present invention deals with the problem of utilizing in electric discharge tubes the extremely high current intensities caused by high voltage impulses especially of the type derived from condenser discharges as generated, for instance, by a capacitron.

It is known in the operation of X-ray tubes, cathode ray tubes, ion tubes or the like, to apply individual condenser impulses of a duration short compared to the interval between two subsequent impulses, see for eX- ample Patent No. 1,957,008. The discharge tubes used for such application are principally of the laminated type such as described in Patents Nos. 2,099,328, 2,043,733, 2,005,021.

Depending on the size of the condensers and the voltage used, the overall current intensities released in such an impulse are well within the order of hundreds of thousands of amperes.

Evidently it is diicult, if not impossible, to operate a hot filament cathode at such high current intensities. Electric current consumption as well as cooling concentrated in a rather limited space would pose too intricate problems.

Discharge tubes such as disclosed above were usually i operated as ion tubes wherein a suitable gas, nitrogen,

oxygen, or the like, or a mixture of such gases was injected into the discharge space to initiate electronic discharge by the collision of ions.

Experiments underlying this invention have shown that, while the collision method in principle is sound, in actual practice the efficiency, that is the ratio between overall current energies stored in the impulse generator and the amount of radiation energies recovered, in the case of X-rays at the anticathode, or in the case of cathode rays, outside the exit window of the tube, was not satisfactory.

lt was found that there is a very sensitive optimum in the performance of a capacitron operated tube with respect to the tolerance threshold permitted between too soft and two hard radiation. It the amount of gas, measured for instance with a flowmeter and blown into the discharge tube, is too large, then the discharge breaks down at very low voltages in a short-circuit like manner. On the other hand, if not enough gas is applied, the tube will become so hard that practically no emission at all will take place.

As a matter of fact, in order to create useful emission at any given moment of operation, a suihcient concentration of gas molecules must be maintained. However, too large an amount of gas concentration may cause so many collisions that the necessary free passages for charged particles will not exist any more. In this case, even if the diameter of the discharge Vessel were so chosen that the ratio between diameter and length would be very favorable, i. e. the diameter would be relatively wide compared with overall length, it will not be possible to solve this passage problem to practical satisfaction. The discharge will have a tendency to concentrate in the center of the tube or discharge channel, no matter what geometricconlguration is applied.

Patented June 18, 1957 In order to use the high voltage impulse source to the fullest possible extent, in accordance with the invention, a multitude of discharge channels separated from each other and each operated as an individual unit are connected to that voltage impulse source.

It has been found, while a single discharge channel permits recovery usually of not more than 5% of radiation from energy stored at the impulse generator, the provision of several channels permits multiplication of such recovery with the result, for example, that a parallel connection of ten of such channels would permit recovery of about 50% of the overall input energy.

One of the objects of the invention is a single tube body containing a number of substantially separate discharge paths, spaces or channels.

These paths, spaces, or channels may be physically separate-i. e., they may consist of completely divided Vevacuation spaces with individual cathodes and individual anodes. However, according to another object of the invention, while there are provided substantially divided evacuation spaces, the cathodes or anodes or both electrodes of the diierent channels each form a single unit for the different spaces or channels.

According to still another embodiment of the invention, the different evacuation spaces communicate with each other to a greater or lesser extent without affecting the over-all eciency of the arrangement, thus eliminating the necessity of providing separate evacuation mechanisms or pressure gauge inlets.

According to another object of the invention, the different channels are t0 be arranged in parallel or at different angles symmetrically or asymmetrcally with respect to a predetermined axis.

The cross sections of the diierent channels may also be alike or different, all these dilerent features representing various embodiments of the invention depending upon the use to which the invention is put.

lt is a further object of the invention to provide communicating spaces at or near the anodes or cathodes or at both these electrodes.

A more specic object of the invention is to provide large and relatively homogeneous radiation areas which would permit, with cathode ray type tubes, to electronize relatively large surfaced objects.

To increase homogeneity of the radiation area, it is another object of the invention to spread the cathode rays emerging from the diierent channels for example by defocussing means as of the magnetic type so as to compensate for the loss in radiation surface caused by the walls between the diierent discharge channels of the tube.

A further specific object of the invention is to provide walls between the diierent discharge channels which are thin compared to the outer wall of the tube-i. e., the length wall separating the discharge channels from the outside space or atmosphere.

Another advantage of the single multi-channel tube is in that the entire unit may easily be placed under oil. Otherwise, with several separate discharge tubes the necessary oil containers would become heavy and clumsy, and assembly becomes dilcult. Y

Thus, according to the invention, practically within the space of a single discharge tube, a number of discharge channels are provided in such a manner that despite the increase in radiation output the unit will be used like a single tube and that a relatively large portion, if not practically all the energy available in the pulse generator or in other high voltage supply means, such as a condenser battery, will be transformed into effective radiation of sucient hardness.

In order to compensate for the inhomogeneous distribution of radiation over the exit area or over any. area near said exit area, caused by the inner walls or partitions separating the different channels, according to another object of the invention, defocussing means are provrded along the path of the electrons Vbefore or after their exrt from the tube. These defocussing means also serve the purpose to mix more effectively the radiations of different hardness produced in the tube.

These and other'objects of the invention will be apparent from the drawings annexed here a description of which follows: Y

Figure 1 shows graphically the radiation spectrum or the eiiiciency of the tube in percentage of output over input as a function of radiation hardness or gas concentration. Y

Figure 2 shows diagrammatically a' number of discharge spaces incorporated in a single tube in accordance with the invention and connected in parallel to a com'- mon impulse generator. Y

Figures 3, `4 and 5 show, in two top elevations and one front elevation, respectively, a circular type discharge tube having three parallel circular type discharge channels.

respectively, show a laminated type tube body 17 (Figure 3). Tube 17 consists generally of a number of Bakelite discs 18 (Figures 3 and 5). Each disc 18 has three apertures 19 symmetrically arranged (Figure 5). .Between two adjacent discs 18 there is arranged a metal disc 20 of substantially the same outer diameter as disc 18, but with apertures 20 which are smaller than but coaxial with the corresponding apertures 19 of disc 18. Metal disc 20 is sandwiched between two rubber or neoprene gaskets 21, 22 to facilitate tightening of the entire device. Y

The end plates of tube or stack 17 indicated schematically at 23 and 24 respectively, are of stainless steel.

Figures 6, 7 and 8 showin two top elevations and one Vfront elevation, respectively, the arrangement of three divergent discharge channels of circular cross section.

Figures 9 and l0 show in perspective and front elevation, respectively, the arrangement of several quadrangular section channels as parts of a single cubical type tube body.

Figures 1l and 12 show also in perspective and front elevation, respectively, the arrangement of sector shape channels within a cylindrical type tube body.

Figure 13 in front elevation only and in section shows a modiiication of the exit portion of the tube shown in Figures 9 and 10.

Figure 14 represents an exploded view of a tube having a great number of honey-comb section discharge channels.

Figure 15 represents another modification of the invention providing a number of parallel discharge channels Without physical partitions.

Figures 16 and 17 represent modifications of Figure l5.

From Figure 1, it is apparent that for constant energy input with decreasing gas concentration 'the radiation becomes harder but at the same time more monochromatic, and of lesser quantity.

At about 5% of output, the radiation is, sufficiently hard and of relatively maximum quantity. If an attempt is made to increase vacuum, hardness increases but overall output diminishes.

On the other hand, an attempt to increase the gas supply although causing the overall output to increase is accompanied by a decrease in average hardness with the result that Vthe tube becomes useless for the purpose of the invention such as sterilization.

Figure 2 shows diagrammatically a number of discharge spaces 1, 2, 3 suchas described in Patent 2,429,217, connected in parallel to a common impulse generator consisting of a rectifier bridge 4. A Marx type circuit consisting of a number of condensers 6 charged in parallel over resistors or preferably inductors 7 or combinations of both` elements discharged in series over spark gaps 5; the entire circuit being connected over main gaps 8, 9, and 10 in parallel to Vdischarge spaces 1, 2, f3, respectively, which in accordance with this invention are incorporated in a single tube body as will be explained further below. v

Any type of discharge space and any type of impulse generator may be used in accordance with this invention without exceeding the scope thereof.

Separate gas inlets and vacuum gauges Vor flow meters 'are indicated at 11, 12, 13, and 14, 15, 16, respectively.

, Figures 3, 4, and 5 in one front and two top elevations,

They serve to support cathodes or as in the example illustrated a common cathode plate 25 and anodes as the separate anode plates 26, 27, and 28, respectively.

Stack 18, 20, 21, 22, 23, 24 is held together by clamping screws (not shown) or in anypother way as is well known to anyone skilled in the art or as will be apparent from Patent 2,429,217 cited above.

Cathode plate 25 isof the type shown in Patent 2,429,217 with separate cathodes or cathode tips of the type shown in Figure 25 at 412 of the above mentioned patent. One of the tips for the diterent cathode or cath- =ode portions of the different discharge spaces formed by the stack tube 17 is indicated at 29 in Figure 3. The discharge spaces themselves are indicated in Figures 3, 4, and 5 at 30, 31, and 32 respectively. The common cathode plate or cathode support 25 is connected to an impulse generator in accordance with the diagram of Figure 2 or in any other manner in accordance with the invention.

The separate anode plates 26, 27 and 28 are connected in parallel to ground as also indicated in the diagram of Figure 2. Y A

Instead of connecting all the discharge chains in parallel to a common impulse source, the channels may also be connected to different impulse sources or some channels in parallel to one source, and another or others in parallel to another source, without exceeding the scope of this invention.

Figures 6, 7 and 8 show in one front and two top elevations respectively, a conical type discharge tube wherein there are provided three discharge spaces 32, 33, 34. Spaces 32, 33, 34 have equal cross sections of conical shape and which of course may be different depending upon the purpose of the tube. Each channel has a relatively small circular cross section at the cathode end and a relatively large circular cross section at the yanode end of the discharge space. As in Figures 4, 5, and 6 the conical tube is composed of stacked Bakelite discs 3S separated by metal discs 36 sandwiched between rubber gaskets 37 and 38 respectively. Parts 35, 36, 37, and 38 are provided in symmetrical arrangement with concentric openings symmetrically arranged with respect tothe axis of tube cone 39. The different openings gradually increase in diameter and radial disposition so as to form the three conical discharge spaces 32, 33, 34.

Similar to the .arrangement shown in Figures 3, 4 and 5, the apertures in metal discs 36 are smaller in diameter than the concentric apertures in rubber gaskets 37 and 38 and in Bakelite discs 35. End discs 43 and 44 of tube 39 are of metal and support separate cathode plates 45, 46, 47 provided with supporting grills such asV shown at 48 and :a common anode plate 49 provided with cold cathode tips such as shown yat 51,V respectively.

The separation of anodes or cathodes, or both, has

. the advantage that it permits exchange of anodes as well as cathodes separately and the relatively independent re pair of discharge spaces which 'have become inoperative -for any reason whatsoever.l 1

Figures 9 and 10 show a quadrangular or approximately cubic type of discharge tube. In Figure 9, perspectively, the discharge tube 52 is shown to consist of square Bakelite discs 53 separated from each other by metal plates 54,sandwi ched between gaskets of corresponding squareshape 55 and 56. Bakelite discs 53, metal plates 54, and gaskets 55, 56 are provided with quadrangular openings schematically indicated at 57, 58, 59 and 60 Yforming parallel parallelepipedic cross section channels extending from a common cathode schematically indicated at 61 to a common anode indicated at 62. Metal plates 54 have quadrangular openings in Bakelite discs 53 and gaskets 55 and 56 to project predeterminedly into the discharge space in' accordance withV well known principles in the construction of such lgas discharge tubes.

Anode 62 and cathode 61 are also of well known construction. Anodes as well as cathodes are disclosed in Patents 2,429,217 and 2,449,872 in detail and there is no necessity to discuss this construction further here, although any type and arrangement of electrodes may be used without exceeding the scope of this invention.

The electrical connections are not shown in Figures 9 and l0. They are similar to those disclosed in Figure 2 or may be of any type and arrangement in accordance with the principle of the invention.

Figures 1l and l2 show a tube arrangement which permits industrial sterilization of food or any other product in an extremely practical manner. The stacked tube body is of hollow cylindrical shape. The elements of the stacks consist of concentric cylinders of Bakelite of various diameters such as shown at 66 and 67. The space between adjacent Bakelite cylinders 66 and 67 is iilled with a metal cylinder 68 which is also concentric with cylinders 66 and 67 and which is sandwiched between cylinder gaskets 69 and 70. Cylinders 66 through 70 are provided with quadrangular openings schematically indicated which gradually increase in size from the outer surface 71 to the inner surface 72 of the tube. The end cylinders of the tube indicated at 73 and 74 respectively consist of metal and serve t support separate cylindrically shaped cathodes 75 and anodes 76, respectively.

As in the embodiments of the invention described above, the metal cylinders 68 have quadrangular openings 77 which are smaller than the corresponding quadrangular openings in Bakelite discs 66, 67, which in turn substantially coincide with those in the cylindrically shaped rubber gaskets 69 and 70.

In Figure 1l, there are shown six substantially sector shaped discharge spaces 79, S0, 81, 82, 83, 84 in radial arrangement. In this way, it is possible to feed the objects or matter to be sterilized radially through the internal space 85 formed by anodes 76 and it is also possible to repair any of the individual discharge spaces or channels 79 through 84 without interfering substantially with the operation of the remaining channels.

Each discharge space may be provided with a separate gas inlet such as shown at 86, 87, S8, 89, 90, 91, and separate vacuum gauges or other measuring devices such as shown at 92, 93, 94, 95, 96, 97. The electrical connections are not shown in detail. The separate cathodes and the separate anodes are `all connected in parallel to the high potential and low potential terminals of the impulse generator, respectively, as is shown diagrammatically in Figure 2 and as has been discussed with respect to the other embodiments of the invention.

While in the above mentioned embodiments use is made of a laminated body of ring shape whereby the circumference of the ring is divided in a number of concentric radial sectors with the diirerent aperture laminae of Bakelite, metal, and rubber being manufactured separately, it is also possible within the framework of this invention to provide a multitude of separate channels within a single structural enclosure whereby linsulated as well as conductive parts and the gasket materials, if gaskets are used at all, are assembled first or made out of one piece and the necessary holes and openings are drilled or milled into the body.

The necessary amount of gas can be blown in each of the concentric divisions either separately as shown in Figures 9 and l0 with a number of connecting distributors or from a central source. It is also provided that if a central source is used special limiter valves are built into each of the conducting pipe lines to adjust the amount of gas according to the diierent volumes with concentric divisions formed and according to any operational requirements which may differ from point to point of the body to be treated.

In all these cases, it is preferred to use a type of lamination such as shown in patents mentioned above.

There the metallic parts are arranged to overlap the insulating material which in many cases is made of organic material.

Furthermore, in order to make the creep path between two laminations as long as possible, the inside diameters for the gasket material are diierent from those of the insulating and conductive parts.

ln order to avoid a glide ydischarge starting from the contact point between conductor and insulator, as shown in Figure 3, the insulating material 23 has a step like recession 98. Good eifects may be achieved by making the discharge channels exceedingly small, for instance, one inch or less, and it is also possible to make the walls between the channels very thin, since all the channels are evacuated and there is no pressure diierence therebetween. Only the channels bordering on the outside atmosphere have to support some pressure diiterence.

It is, of course, not necessary for the purpose of the invention to use round or quadrangular or square cross sections, but also octangular or any other form may be selected.

Furthermore, any type of discharge channels may be used for the purpose of the invention. If cathode rays are caused to penetrate into the open atmosphere, the exit window may be either a single unit and cover all the exits from the different discharge channels or each channel may be equipped with an individual exit window. Since small dead spaces may occur due to the spacing of the channels, the materials to be irradiated should be placed a certain distance from the exit Windows. At this distance due to the normal scattering etect in air the distribution of radiation will become sufficiently homogeneous.

Also if necessary special ray deviation devices such as magnets of the permanent -or electromagnetic type may be used to scatter the electrons articially, as shown in Figure 9 at 99, 100.

One of the means for producing defocussing and also for enhancing the mixture of radiations of diierent hardness is shown in Figure 13 which represents a modification of Figure 10 and more specifically a modiication of end plate 101 and anode 62 of Figure l0. In Figure l0, end plate 101 is shown to be substantially of disc shape. In Figure 13, the corresponding end plate 102 is of more complex shape as shown in section. Plate or piece 102 has a conical inner portion 103 which permits the electrons or other corpuscular radiation emerging from channels 104, 105, etc., to scatter. A common window or anode 106 is attached to piece 102 which in turn is attached to the laminated body 109 which is similar to that shown in Figures 9 and 10. To enhance scattering and mixture of electrons, end piece 102 may be made of magnetic material and provided (as shown) with a space 110 containing electromagnetic winding 111. This permits the use of relatively weak defocussing elds While homogeneous radiation of great intensity will be obtained over a relatively large exit area.

It is well understood that the voltage applied to a tube should be as low as possible in vorder to prevent the occurrence of undesired radiation or reduce such occurrence to a minimum. One of the advantages of the invention is in that such voltage reduction can be achieved without substantially affecting the output of the tube. In accordance with the invention, for example, it is possible to have an eight channel tube connected to a seven million volt source of the same output as a single channel tube connected to an eight times seven--fty-six million volt source. y

The advantages of the invention are particularlyapparent when kconsidering the treatment of vaccines -with a circular type tube such as indicated in Figures 12 and 13. In order'to kill microbes or viruses existing in such vaccines with absolute certainty, it is necessary to apply a mortal dose to a vaccine or a certain point of the body only.

In accordance with the invention, by applying radiation from several channels arranged in circular array around said point, it is possible to concentrate or to obtain such mortal dose at a desired point or space of application, while surrounded by or adjacent parts, points or spaces are exposed to relatively low intensity radiation only. In a channel array such as provided by the invention concentration of high intensity at a desired point or space is assured while surrounding or adjoining portions are affected only by fractions of such radiation corresponding to the intensities of radiation emerging from the different channels individually.

Figure 14 gives an exploded view of a multi-channel tubeY with a great number of channels arranged in a honey-comb type structure shown at 113, In this modiiication of the invention in addition to the honey-comb type assembly 113 a iield free space of considerable length is provided to permit defocussing of the electrons emerging from the various channels and an appropriate mixture of electrons to produce as homogeneous an exit area as possible.

Body or tube 113 is composed of a structure or stack of alternating insulating and metal discs similar to those shown and described with respect to the embodiments of Figures 3 through 12. The honey-comb channels shown at 114 are provided at their entry with a single cathode plate 115. At the exit of tube 113 there is provided a magnetic winding 118. The magnetic eld lines trans-V versing slot 117 cause a deflection of the electrons emerging from honey-comb channels 114.

In order to maintain such deflection, there is provided a iield free space of conical shape such as shown at 119 adjoining the magnetic body 116.

In order to prevent negative ions from reaching the exit window shown at 120 there is also provided an additional intermediate window 123 provided with a supporting grill 121 and an interposition plastic protection layer 122, preferably, as shown, at the entrance of iield free cone 119.

However, such protective window 123 may be arranged in any part of field free cone 119 without exceeding the scope of the invention, provided, however, such intermediate window should be at a distance suticient to effect separation f harmful ions and also not to be affected by the heat occurring at exit window 126. Protective Window 123 may have any thickness structure and other dimension or configuration appropriate to prevent passage or cause absorption Vof undesired gas ions produced in the discharge channels.

Figure 15 shows a modification of the invention in which a great number of parallel discharge channels are provided Without physical separation of such channels. This is achieved by proper configuration yof the cross section of the discharge space which as shown in Figure 15 is `of substantially longitudinal shape. This longitudinal extension may be straight as shown or circular or otherwise curved without exceeding the scope of the invention.

In Figure 15, 124 represents the common cathode, 125 the discharge tube composed of a metal top piece 126 supporting the cathode 124 and a conical bottom piece 127 forming a eld free space and supporting anode 128. Tube 125 is composed, as in the previous embodiments of the invention, of insulating layers 129, between which there are inserted metal plates 130 sandwiched between rubber gaskets 131, 132. The discharge space as formed by slots in insulating pieces 129 is indicated in full line at 133. The space formed by metal plates 130 '8 is indicated at 134. Cross section 133, 134 is .of substantially quadrangular shape.

Experiments have shown that a relation of the order of 1:5 results in satisfactory division of the dischargespace, while a relation of at least 1:10 results in a still better subdivision.

Figure 16 shows'the combination or parallel arrangement of a number of slot shaped discharge spaces such as indicated at 135, 136, 137 and 138.

Figure 16 represents a top view on a tube with a cornmon cathode plate removed therefrom. At the bottom of tube body 139 which is laminated in a way similar to tube of Figure 15, there are apparent Ithe cathode supporting grills 140, 141, 142, 143, respectively.

Figure 17 shows a modification of Figure 16 in which these slots, instead of `being straight, are arranged circularly in the form of a number of concentric cylinders of diiierent diameters indicated at 144, 145, 146, 147, 148.

Each of these cylindrical spaces, of course, may be,if desired, divided into radial sectors to further enhance if necessary the subdivision of each cylinder into separate discharge channels in accordance with the invention.

Any other configuration, however, may be provided in accordance -with this invention to eiect subdivision of discharges and thereby a better exploitation of the energy applied to the tube, -without exceeding the scope of this invention.

It is obvious that the discharge space may be given any desired configuration and any convenient number and that any number of electrodes of any appropriate shape may be used Without exceeding the scope of this invention.

The invention may also be applied to any type of radiation, alpha, beta, lgamma type or any other corpuscular type radiation.

We claim:

1. A high e'iciency electron lbombardment tube of the type which produces and accelerates electrons to an extremely high velocity when energized at potentials of the order of a million or more volts and projects at least a portion of these electrons from the tube through electron permeable window means, said tube comprising: a laminated insulating structure forming -a generally tubular space closed at both ends with a cathode at one end and an anode at the other end, electron permeable window means at the anode end, said space being evacuated to very low pressure, and partitioning means within said space dividing the space into a plurality of smaller generally tubular spaces which are separated from each other yby the partitioning means, said partitioning means extending from the region of the cathode at least the major portion of the distance to said window so that when the tube is energized electrons are drawn from a plurality of spaced regions of said cathode rather than largely from the center thereof.

2. A high efficiency electron bombardment tube of the type which produces and accelerates electrons to an extremely high velocity when energized at high electrical potentials and projects lat least a portion of these electrons from the tube through electron permeable window means, said tube comprising: an insulating kstructure forming a generally tubular space closed at both ends with a cathode at one end and an anode at the other end, electron permeable window means at the anode end, said space being evacuated to very low pressure, and partitioning means within said space extending from the region of the cathode and dividing the space into a plurality of smaller generally tubular channels which are `separated from each other rby the partitioning means, said partitioning means extending from the cathode region at least the major portion of the distance to said window but being interrupted before reaching said window so that when the tube is energized electrons are drawn from a plurality of spaced regions of said cathode rather than largely from the center thereof, `but are permitted to spread at least partially from the individual smaller spaces before reaching said Window.

References Cited in the le of this patent UNTED STATES PATENTS 10 Y Fodor July 13, 1937 Brasch et al. Nov. 16, 1937 Lndenblad June 7, 1938 Lloyd Jan. 3l, 1939 Hare Apr. 2, 1946 Thomas Dec. 9, 1947 Thomas Ian. 4, 1949 Trump et al. Mar. 28, 1950 Van De Graaff et al. Aug. l, 1950

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