US2832005A

US2832005A - Electron-discharge devices

Info

Publication number: US2832005A
Application number: US214121A
Authority: US
Inventors: William C Brown
Original assignee: Raytheon Manufacturing Co
Current assignee: Raytheon Co
Priority date: 1951-03-06
Filing date: 1951-03-06
Publication date: 1958-04-22
Anticipated expiration: 1975-04-22

Description

April 22, 1958 w. c. BROWN ELECTRON-DISCHARGE DEVICES 3 Sheets-Sheet 1 Filed March 6, 1951 SIGNAL OUTPUT TO LOAD S IGNA L lNPU T I /NVENTOR WILLIAM C. BROWN BY Arm/aver SQPPL April 22, 1958 w, B OWN 2,832,005

ELECTRON-DISCHARGE DEVICES Filed March 6, 1951 3 Sheets-Sheet 2 INVENTOR W/L, LIAM C. BROWN A rum/Ev April 22, 1958 w. c. BROWN ELECTRON-DISCHARGE DEVICES 3 Sheets-Sheet 3 Filed March 6, 1951 I INVENTOI? WILLIAM C. BROWN ATTORNEY vice.

2,832,005 ELECTRON-DISCHARGE nnvioas William C. Brown, Lincoln, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass, 21 corporation of Delaware This invention relates to electron-discharge devices, and more particularly to amplifying devices of the traveling-wave type.

In the copending application of Edward C. Bench and Winston M. Gottschalk, Serial No. 86,956, filed April 12, 1949, now abandoned, entitled Traveling Wave Amplifier, there is disclosed an electron-discharge device wherein signals are applied to an electron-discharge device of the magnetron type, said signals being applied to the electron stream at the cathode of said discharge de- These signals cause perturbations of the electron stream which encircles the cathode, said perturbations being strongest in the areas of the cathode stream immediately adjacent the cathode. The perturbations are then amplified within the electron stream as said streamjarogresses from the cathode to the anode. As the electron stream passes the anode members of the anode structure, the amplifier perturbations therein will induce alternating currents between adjacent anode members, thereby extracting the amplified signal from the electron stream.

This invention discloses that, for optimum operation, the number of cathode members of the cathode structure between which signals are applied to the electron stream should be substantially equal to the number of anode members. By this structure, the perturbations in the electron stream will, upon reaching the area adjacent the anode structure, have the correct spacing to induce signals in the anode structure which will "be cumulative of each other rather than of a type which will cancel out.

Specifically, one embodiment of the invention discloses a cathode structure comprising a plurality of interdigital cathode members, one of the cathode members being spaced radially inwardly from each of the anode mem bers.

In another embodiment of the invention, the cathode members comprise small rods substantially parallel to the axis of the anode structure, said rods being substantially a half wave length long at the center frequency of the band to be amplified by said device, the ends of the cathode members being connected by conductive rings, and

an anode member being spaced radially outwardly from each of said cathodemembers. Alternate cathode members of said cathode structure are connected to the inner conductor of a coaxial line, the outside conductor of which is connected to one of the rings connecting the ends of said cathode members.

This invention further discloses that tendency of the magnetron-anode structure to interact with the electron stream to produce oscillations may be substantially reduced by heavily loading the anode structure, and by means of a feedback path which feeds back signals to the cathode signal input substantially out of phase with the signals at the oscillation frequency. The amplitude of United States Patent the signal feedback is made substantially equal to the regenerative tendency produced by the interaction between the anode structure and the electron stream.

Other and further objects of this invention will be ap- 2,832,005 Patented Apr. 22, 1958 ice parent as the description thereof progresses, reference being had to the accompanying drawings, wherein:

Fig. 1 illustrates a longitudinal, crosssectional View of a magnetron-amplifying device embodying this invention, and illustrating a degenerative signal feed-back mechanism;

Fig. 2 illustrates an enlarged longitudinal, cross-sectional view, taken along line 2-2 of Fig. 3, and showing details of the cathode structure of the device shown in Fig. 1;

Fig. 3 illustrates a transverse, cross-sectional view of the cathode structure shown in Fig. 2, taken along line 3-3 of Fig. 2;

Fig. 4 illustrates a longitudinal, cross-sectional view of an electron-discharge device illustrating another embodiment of the invention;

Fig. 5 illustrates a partially, broken-away, cross-sectional view of the device shown in Fig. .1, taken along line 5-5 of Fig. 4, and showing the details of the interdigital cathode structure; and

Fig. 6 illustrates a perspective view of the interdigital cathode structure illustrated in Figs. 4 and 5.

Referring now to Figs. 1, 2 and 3, there is shown an anode structure 10 substantially similar to a conventional cavity magnetron structure comprising a cylindrical metallic envelope member 11 which may be, for example,

copper. Extending radially inwardly from cylinder 11 is a plurality of anode vane members 12 which, as shown here, comprise substantially rectangular planar members, adjacent members 12, together with the space therebetween, defining cavities. The inner ends of the members 12 are connected at their upper and lower edges by conductive strapping 13 in a well-known manner such that alternate anode members are connected together by said strapping. A signal output coupling 14 is provided, said coupling comprising a central conductor 15 which is connected to one of the outer straps 13 and extends through an opening in the anode cylinder '11. An outer conductor 16 surrounds conductor 15, and is rigidly attached to anode cylinder 11. A dielectric seal 17 is provided between

conductors

15 and 16 to rigidly support conductor 15 in spaced relation to conductor 16, and to provide a vacuum seal for the discharge device. A coaxial line 18 of any desired type is connected to the output structure to feed the output signal to the load, not shown, which may be of any desired type, such as an antenna.

Within the space defined by the inner ends of the anode members 12, there is positioned a cathode structure 19. Cathode structure 19 comprises a plurality of cylindrical cathode members 20 whose axes are parallel to the axis of anode cylinder 11, and are spaced in a circle about said axis, one of said cylinders 20 being spaced radially inwardly from the end of each of anode members 12. While the device illustrated herein discloses eight cathode members 20 and eight anode vane members 12, any desired number, for example, twenty or more anode and cathode members, can be used, if desired.

The upper and lower endsof cathode cylinders 20 are connected together by conductive members, :for example, as shown herein; the lower ends of the cathode cylinders 20 are connected to a conductive plate 21, while the upper ends of the cathode cylinders 20 are connected to a conductive ring 22. Each of the cathode cylinders 20 contains a heating coil 23, the upper ends of heating coils 23 being attached to the annular ring 22, and thereby to the conductive cylinders 20, while the lower ends of the material attached, as by soldering, to'anode cylinder 11; The pole piece 26 has a frustro conical shape and extends upwardly from the lower end of anode cylinder 11 to a point somewhat below the inner ends of anode vane members 12. Member 26 has a hole in the center thereof coaxial with the axis of cylinder 11 of sufficient diameter to admit a portion of the lower conductive plate 21.

A cylindrical metallic member 27 is sealed into the hole in member 26, said member 27 having a reduced portion 23 surrounding lead-in member 2.5. A ceramic seal 29 connects the reduced metallic portion to the lead-in member 23, thereby rigidly and insulatedly supporting the member 25 with respect to the member 28.

Member 2?. which connects the upper ends of cathode cylinders 26* is attached to one end of the outer cond cr 3'1 of a coaxial line 31, said. outer conductor extending upwardly through an upper pole piece 32 sealed to anode cylinder ll. Upper pole piece 32 is substantially similar to lower pole piece 26 and has a hole in the center thereof in which is sealed a metallic cylindrical member 33 having a reduced section which surrounds outer conductor of coaxial line 31, and is insulated-ly sealed thereto by a ceramic seal 35. Coaxial line 31 has a central conductor 36 which extends out beyond the end of the outside conductor in which is connected to the member .22, said central conductor 36 being connected to a spider 37 which, in turn, is connected to alternate of the cathode cylinders 2d at points somewhat above the mid points thereof.

if desired, cathode cylinders 2h may be made substantially a half wave length long at the center frequency of the signals to be amplified by the device, and the point of attachment of the spider 37 to the alternate cathode cylinders will be determined by the impedance matching considerations dictated by the characteristc impedance of the coaxial line 31 and the cumulative impedances presented to said coaxial line by the cathode cylinders.

A ceramic seal 38 is provided between central conductor 36 and outer conductor 3% in order to rigidly support central conductor 36 with respect to outer conductor 35? and to provide a vacuum seal for the discharge device. If a source of heater voltage 39 is connected between outer conductor 3t and conductor 25, a current will flow in the heating coils 23 to heat the cathode cylinders 26). The outer surfaces of cathode cylinders 29 may be coated with eleetron-emissive material ll which, upon heating, will emit clouds of electrons in the areas surrounding the cathode structure 19. if a high-voltage supply 4 1 is connected between the anode structure, for example, as shown here, anode cylinder 11, and the cathode structure, for example, by connection to outer conductor 3b of the coaxial line 31, these electrons will be attracted toward the anode structure. In the presence of a magnetic field produced by a magnet applied between the

pole pieces

26 and 32, the electrons will circle the cathode structure.

If a signal input is applied between adjacent cathode members, for example, by feeding said signal into the cathode structure through coaxial line 31, perturbations in the electron stream will be produced in those portions of the stream immediately adjacent the anode structure. As the electrons move outwardly in larger and larger circles, the perturbations will be amplified by the ditlering velocities of adjacent strata. Upon reaching the portions of the electron stream adjacent the inner ends of the anode vane members 12, the perturbations in the electron stream will, upon passing the ends of said anode vane members, induce voltages between adjacent anode members, thereby producing a signal in said anode structure. Said signal is coupled out of said anode structure, as previously described, by the coupling device 14.

Since there will be a certain amount of interaction between the electron stream and the signal in the anode structure, the device will have a tendency to oscillate. This tendency may be substantially reduced by heavily loading the output device 14, for example, by a relatively yin low impedance load fed by the coaxial line 18. in addition, if desired, means may be provided for selectively feeding back a portion of the output signal to the input side of the disch srge device through coaxial line 31. This feed-back path may be, for example, as shown here, a coaaiul line One end of the central conductor 43 of coaxial line extends through a slot cut in the wall of the outer conductor of coaxial line The amount of energy coupled out of the output coaxial line into the F will be determined by the depth which the c inner conductor is extends through the slot 4d, said depth being adjustable by moving the line 42 axially in the bushing 46 surrounding said line adjacent the line 18.

The phase of the energy coupled out of the output line may be adjusted by sliding the coaxial line 42 along the line 15 such that the end of the inner conductor 43 extending into the line moves along slot dd. Slot 4 is made wide enough such that the central conductor 43 does not touch the outer conductor of line 18.

While the lines shown lIre are of the metallic cylindrical type, for ready adjustment the coaxial lines may, if desired, be of the flexible type wherein the outer conductors are, for example, braided metallic strands.

The other end of the central conductor from that which extends into the line is connected to the central conductor 47' of a coaxial. line which feeds the coaxial line which, as previously set forth, feeds the input signal to the cathode structure Coaxial line 48 is connected to coaxial line through an insulating coupling, as at 4.7 said insulating coupling comprising an expanded pouch of the conductors of the line coaxially overlapping portions of the respective similar conductors of the line 531 with the spaces between the overlapping portions being filled v/ith insulating material, as at do, to provide substantial insulation for D. C. potentials between the cathode structure which is connected to the coaxial line 31 and the anode structure which is connected to the coaxial line By this structure the positive feedback, due to the into; action between the anode structure and the electron stream, as well as within the electron stream itself, may be compensated for by a negative feedback through the coaxial line theamplitude of the negative feedback being adjusted by adjustment of the depth of penetration of central conductor 4% into slot 44 such that it is substantially equal to the magnitude of the positive feedback, and the phase of the negative feedback being adjusted by movement of the end of conductor d3 along the slot 44 until it is substantially out of phase with the positive'feedbaclt.

Referring now to Figs. 4, 5 and 6, there is illustrated a second embodiment of this invention wherein the cathode structure comprises cathode members arranged in interdigitai relation. in these figures, the anode structure, comprising the anode cylinder 13., anode vane members 12, strapping l3, and upper lower pole pieces 32 and is substantially similar to that illustrated in Fig. l. However, in this embodiment sixteen anode and cathode members are shown, while Fi t. 1 only eight were used. In addition, the output device is substantially similar to that shown in Fig. l.

Extending downwardly from lower pole piece 2d and attached to the hole therein is a metallic cylinder 51 which is sealed by a c sleeve 52 to a second inetab lic cylinder 53 coaxial with cylind r and of substan tially the same diameter cyl e Attached to the inner surface of cylinder 53 15 av member which is of a modified cylindrical form. Member Ed extends upwardly through ceramic sleeve 5?. and cylinder 51. member 54 being reduced in diameter prior to passage through members 513. and such that there is no contact therebetween; Member 54- extends upwardly to a point just above the upper edges of anode vane members 12. The portion of the member 54- which extends. past the inner ends of anode vane members 12 is partially cut away to provide a plurality of fingers 55 extending up from a cylindrical base portion, the upper end of said cylindrical base portion terminating somewhat below the lower edges of members 12. One of the fingers 55 is positioned adjacent the end of each alternate anode memher, and the space between the fingers 55 is somewhat greater than the thickness of said fingers. The fingers may be, for example, substantially the same as the thickness of anode vane members 12.

Positioned inside member 54 coaxial therewith is a cylindrical member on, the outer diameter of member 56 being somewhat less than the inner diameter of member 54- such that members 54- and 56 form a portion of a coaxial line. Member d6 extends downwardly beyond the end of member 54 and is connected, as by soldering, to a member 57 of substantially cylindrical form closely surrounding member 56. A raised annular portion of member 57 is connected, as by soldering, to a cylindrical member 5'8 which, in turn, is insulatedly sealed to cylindrical member 53 by means of a ceramic sleeve 59, thereby rigidly supporting cylinder 56 with respect to the anode structure. Cylinder 55 extends upwardly through cylinder and fingers 55 to a point somewhat above the upper end of the upper pole piece where it rigidly engages a cylindrical member 60 surrounding member 56 and extending downwardly through upper pole piece i At a point somewhat above the upper ends of anode vane members 121, member 60 is machined to produce a plurality of fingers 61 which extends downwardly past the inner ends of anode members 12 to a point near the lower edge of said anode members. Fingers 61 are substantially similar to fingers 55, each of fingers 61 being positioned in between a pair of adjacent fingers 55 such that fingers 611 do not touch fingers 55 either at the sides or ends thereof. This produces an interdigital cathode structure with the central conductor 56 of the coaxial line comprising conductor 56 and member 54 feeding the members 61 extending down from the upper end of the discharge device, and the outer conductor 54 of said coaxial line feeding the fingers extending from the bottom 'of the discharge device. The surfaces of the fingers adjacent the inner edges of the anode vane mem bers 12 have electron-emissive material 62 positioned thereon, for example, in grooves on the outer surfaces thereof.

Inside cylinder 56 at a point

adjacent fingers

61 and 55, there is positioned a heat-er coil, not shown, the upper end of which is connected to the cylinder 56, and the lower end of which extends out through a lead 63 which is sealed to cylinder 56 by a ceramic bead 64;. End shields are provided for the cathode structure by means of cylinders 55 and as, respectively, cylinder 65 extending from a point just above anode vane members 12 and surrounding cylinder as but spaced therefrom at this point upwardly for a distance on the order of a half wave length of the operating frequency of the device to a point above upper pole piece 32. Here cylinder n5 is rigidly attached to cylinder 56.

Similarly, lower cylinder 66 extends from a point adjacent the lower edges of anode vane members 12 downwardly through pole piece 26, surrounding member 54 and spaced therefrom, for a distance equal to substantially a half wave of the operating frequency to a polnt Where cylinder 66 threadedly engages a. shoulder of cylinder 54.

A plate member 67 is attached to the upper edge of upper pole piece 32, said member having an annular struck-up portion 6i! thereof which surrounds

cylinders

61 and 65, but is spaced therefrom. Sealed to the struckup annular portion 68 is a ceramic dome 69 which covers the end of the cylindrical members so and his and seals the upper end of the discharge device. An input signal may be applied to the device by applying the signal between

metallic cylinders

53 and 58 by any desired means, such as by connecting a parallel wire line to the invention described herein.

said cylinders or by inserting said cylinders in a wave guide with their axes parallel to the electrostatic field thereof.

This completes the description of the embodiments of However, many medi fications thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. For example, any configuration of anode structure and magnetic pole structure could be used. The number of anode and cathode members may be increased or decreased, as desired, and the input and output circuit connections may be wave guides instead of coaxial lines, if so desired. Accordingly, it is desired that this invention be not limited by the description of the embodiments described herein, except as defined by the appended claims.

What is claimed is:

1. An electron-discharge device comprising an envelope containing an anode structure, a cathode structure spaced from said anode structure, said cathode structure including first and second sets of interdigital cathode members, high frequency input signal coupling means coupled between said first and second sets of cathode members, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said cathode and anode structure, and output means coupled to said anode structure for deriving from said anode structure an output signal of greater amplitude than that of said input signal.

2. An electron-discharge device comprising an envelope containing an anode structure including a plurality of anode members, a cathode structure spaced from said anode structure, said cathode structure comprising first and second sets of interdigital cathode members substarn tially equal in number to the number of anode members, a high frequency input signal coupling structure coupled between said sets of cathode members, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the region between said cathode and anode structures, and output means coupled to said anode structure for deriving from said anode structure an output signal of greater amplitude than that of said input signal..

3. An electron-discharge device comprising an envelope containing an anode structure, a cathode structure including first and second sets of elongated electricallyconductive cathode members, the length of said conductors being of the order of a half wave length at the desired operating frequency of said device, high frequency signal input coupling means coupled between said sets of cathode members, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said anode and cathode structures, and output means coupled to said anode structure for deriving from said anode structure an output signal of greater amplitude than that of said input signal.

4..An electron-discharge device comprising an envelope containing an anode structure, a cathode structure including first and second sets of cathode members, said cathode members comprising elongated conductors, the ends of adjacent conductors being conductively connected together, the length of said conductors being of the order of a half wavelength at the desired operating frequency of said device, a high frequency input signal con-- pling means coupled between said sets of cathode members, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said anode and cathode structures, and output means coupled assaoos anode structure. said cathode structure comprising firstand second sets of cathode members, one of said cathode members being spaced radially from each of said anode members, the number of said anode members and cathode members being substantially equal, high frenueni y input signal coupling means coupled between said sets of cathode members, said cathode members comprising elongated conductors, the ends of adjacent conductors being conductively connected together, the length of said cathode .rembers being of the order of a half wavelength at t c desired operating frequency of said device, means for ducing an electric field between said anode cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said anode and cathode structures, and output means coupled to said anode structure for deriving therefrom an output signal of greater amplitude than that of said signal.

6. An electron-discharge device comprising an envelope con ining an s ructure including a plurality of anode members, a cathode structure spaced from said anode structure, said cathode structure comprising first and second sets of elongated electricallyconductive cathode members, the length of said cathode members he a of the order or" a half wavelength at the desired operating frequency of the device, the number of said cathode bers being substantially equal to the number of said .e members, high frequency input signal coupling men s coupled between said sets of said cathode members, met its for producing an electric field between said anode and cathode structures, means for producing magnetic field directed substantially perpendicular to the directi of the electric field in the space between said anode cathode structures, and output means coupled to said anode structure for deriving an output signal from said anode structure of greater amplitude than that of said input signal.

7. An electron-discharge device comprising an envelope containing an anode structure including a plurality of anode members, a cathode structure spaced from sai anode structure, said cathode structure including a f member having a cylindrical base portion fr m which extends a first set of elongated spaced cathode second member having a cylindrical b portion from which extends a second set of elongated spaced fingers, each of said fingers of said second set being position-er. between corresponding adjacent fingers of said irs set, the number of said cathode members being ssbsta'ntia-lly equal to the number of said tie members, means for producing an electric held between said anode and sai" de structures, a 1 money .i ,al coupl ,5 structure coupled to said cathode members, said high frequency coupling structure including i 'er conductor extending Within and spaced from said iner er and an outer conductor electrically nnected to aid second member, a heater within said inner conductor, me i for producing an electric field I: tnode cathode structures, means for directed subs an electr field between said one o and catl'iode structures, and output ins coupled to said structure for deriving therefrom an output signal. of ater amplitude than that of said input signal.

An. device cot'nprising an o clope on structure including a pl of anode m. :s, a cathode structure r spaced from said a; structuiz, said cathode strn comprising first and second sets of interdigital cathode members, a high frequency input co pling structure coupled between said sets of said cathode members, means e direc ion of the for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said cathode and anode structures, and output means coupled to said anode structure for deriving therefrom an output signal of greater amplitude than that of said input signal.

9 An electron-discharge device comprising an envelope containing an anode structure, a cathode structure spaced from said anode structure, said cathode structure including a first member from which extends a first set of elongated cathode elements and a second member from which extends a second set of elongated cathode members, said elements being arranged interdigitally, a high frequency input signal coupling structure coupled between said sets of cathode members, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said anode and cathode structures, and output means coupled to said anode structure for deriving therefrom an output signal of greater amplitude than that of said input signal.

10. An electron-discharge device comprising an envelope containing an anode structure including a plurality of anode cavity resonators, a cathode structure spaced from said anode structure, said cathode structure comprising first and second sets of interdigital cathode members, a high frequency input signal coupling structure coupled between said sets of said cathode members, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said anode and cathode structures, and output means coupled to said anode structure for deriving therefrom an output signal of greater amplitude than that of said input signal.

ll. Ari electron-discharge device comprising an envelope containing an anode structure, a cathode structure comprising first and second sets of elongated electricallyconductive cathode members, the length of said conductors being of the order of a half wavelength at the desired operating frequency of said device, a high frequency input signal coupling structure coupled between said sets of said cathode members for producing an input signal voltage between said sets of cathode members, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially perpendicular to the direction of the electric field in the space between said anode and cathode structures, output coupling means coupled to said anode structure for deriving therefrom an output signal of amplitude greater than that of said input signal, and means for feeding a portion of the output signal from said device back to said input signal coupling structure out of phase with the input signal.

12, An electron-discharge device comprising an envelope containing an anode structure, a cathode structure comprising a first set of alternate elongated electrically conductive cathode members and a second set of alternate elongated electrically conductive cathode members, said cathode members being of the order of a half wavelength at the desired operating frequency of said device, means for producing an electric field between said anode and cathode structures, means for producing a magnetic field directed substantially normal to the direction of the electric field in the space between said anode and cathode structures, a high frequency input signal coaxial coupling structure having an inner conductor connected to said first set of said cathode members and an outer conductor connected to said second set of said cathode members for producing an input signal voltage between said sets of cathode members, each of said cathode members having mounted therein a heater element, output means coupled to said anode structure for deriving therefrom an output signal of greater amplitude than that of said signal, and means for feeding a portion of the output signal from said device back to said input signal structure out of phase with the input signal.

13. An electron-discharge device comprising an envelope containing an anode structure including a plurality of anode cavity resonators substantially resonant at the operating frequency of said device, a cathode structure spaced from said anode structure, said cathode structure comprising a plurality of cathode members, the number of said cathode members being substantially equal to the number of said anode cavities, means for providing an electric field between said anode and said cathode structures, a high frequency input signal coupling structure coupled to said cathode members, and means for producing a magnetic field directed substantially perpendicular References Cited in the file of this patent UNlTED STATES PATENTS 2,057,170 Usselman Oct. 13, 1936 2,084,867 Prinz et al June 22, 1937 2,211,859 Percival Aug. 20, 1940 2,252,565 Haelf Aug. 12, 1941 2,414,085 Hartman Ian. 14, 194-7 2,414,121 Pierce -1 Jan. 14, 1947 2,416,298 Fisk Feb. 25, 1947 2,423,443 Fay July 8, 1947 2,428,612 Blewett Oct. 7, 1947 2,443,179 Beniofl June 15, 1948 2,595,677 Law May 6, 1952