US3028516A

US3028516A - Electron tube and socket therefor

Info

Publication number: US3028516A
Application number: US793010A
Authority: US
Inventors: Willis B Foote; Jr Jackson W Kendall
Original assignee: Eitel Mccullough Inc
Current assignee: Varian Medical Systems Inc
Priority date: 1959-02-13
Filing date: 1959-02-13
Publication date: 1962-04-03
Anticipated expiration: 1979-04-03

Description

A ril 3, 1962 w. B. FOOTE ETAL 3,028,516

ELECTRON TUBE AND SOCKET THEREFOR Filed Feb. 13, 1959 2 Sheets-Sheet 1 INVENTOR.

WIL LIS B. FOOTE BY JACKSON w KENDALL, JR.

ATTORNEYS A ril 3, 1962 w. s. FOOTE ETAL ELECTRON TUBE AND SOCKET THEREFOR 2 Sheets-Sheet 2 Filed Feb. 13, 1959 INVENTOR. WILLIS a FOOTE BY JACKSON w KENDALL,JR.

1&2. ATTORNEYS U d S '0 cc 3,028,516

' brazing material having a higher melting temperature than 3,028,516 the operating temperature of the tubeand lower melting ELECTRON TUBE AND SOCKET THEREFOR Willis B. Foote, Miilbrae, and Jackson W. Kendall, J12, Los Altos, Calih, assignors to Eitel-McCuilough, Inc, San Bruno, Caiifi, a corporation of California Filed Feb. 13, 1959, Ser. No. 793,010 10 Claims. (Cl. 313-51) This invention relates generally to an electron tube and socket therefor, and more particularly to an electron tube and socket suitable for operation under conditions of high temperature, vibration, shock and acceleration.

It is a general object of the present invention to provide an electron tube having an anode which is light in construction whereby forces produced by inertia during vibration, shock and acceleration tend to be minimized.

It is another object of the present invention to provide a vacuum tube having a ceramic envelope with an internal radiation cooled anode.

It is another object of the present invention to provide a tube of the above character including a rigidifying element which serves to rigidify the tube elements whereby they maintain their position relative to one another under acceleration, vibration and shock.

It is still another object of the present invention to provide an electron tube of the above character which includes a mounting element near its center of gravity for mounting on a platform with the tube terminals mating with terminal stacks disposed below the platform.

It is a further object of the present invention to provide an electron tube and socket therefor in which the tube socket includes a platform on which the tube is mounted with the terminal leads extending below the platform whereby certain of the tube elements are electrically shielded one from the other.

These and other objects of the invention will become more clearly apparent from the following description when taken in conjunction with the accompanying drawmg.

Referring to the drawing:

FIGURE 1 is a side elevational view, in cross section, of an electron tube constructed in accordance with the invention;

FIGURE 2 is a perspective view showing the vacuum tube and socket therefor;

FIGURE 3 is a side elevational view, partly in section, showing the electron tube and socket; the view is taken generally along the line 3-3 of FIGURE-4;

FIGURE 4 is a bottom view of a suitable socket; and

FIGURE 5 is an enlarged sectional view taken along the line 55 of FIGURE 4.

The electron tube shown in FIGURE 1 is a ceramic tetrode capable of handling substantial amounts of power under conditions of high temperature, acceleration, vibration and shock. The tube includes an evacuated'envelope of generally cylindrical construction with an internal anode of relatively light construction.

The side walls of the envelope comprise stacked

ceramic rings

11, 12, 13 and 14 with

metal terminal rings

16, 17, 18 and 19 bonded thereto. The terminal rings 16-19 provide desired lead-in conductors through the envelope to the tube elements carried wthin the envelope. A metal dome 2 forms the upper portion of the envelope and is supported on the sealing ring 22 by the sealing ring 23 which is arc welded to the ring 22 and brazed to the peripheral edge of the dome 21. A contact ring 24 is brazed to the upper surface of ring 23. The welded connection 26 forms a suitable vacuum seal. The end-27 of the sealing ring 22 curves inwardly and is bonded to the upper end of the ceramic ring 14 and to the ceramic backing ring 28. For mechanical rigidity, the other edge of the backing ring 28 is brazed to the metal dome 21 with Spaced channels 29 are formed on the upper edge of the ceramic backing ring 28. The channels serve to connect the space 31 enclosed by the rings 22 and 23 with the interior portion of the envelope. Consequently, a pressure differential is not produced between the space 31 and the interior of the envelope when the tube is heated in an oven, evacuated and the various parts assembled.

The lead-in for the screen grid is part of a seal assembly which permits forming the tube in two parts. The Seal assembly comprises the terminal ring 19 which is bonded to the upper end of the ceramic ring 13 and which extends outwardly to receive a mounting ring 37. A seal ring 38 is brazed to the ring 19 adjacent the inner margin of the mounting ring 37 and extends upwardly therefrom. A sealing ring 39 is bonded to the lower end of the ceramic ring 14. The sealing ring 39 has a portion which extends upwardly to cooperate with the adjacent portion of the ring 38. Ceramic backing rings 43 and 44 are suitably bonded to the

rings

19 and 39, respectively.

It is seen that the sealing ring 39 and ceramic backing ring 44 are part of the upper envelope portion which includes the anode. The terminal ring 19, mounting ring 37, seal ring 38 and ceramic backing ring 43 are part of the lower portion of the envelope which includes the other tube elements. In manufacture, the elements in each portion are mounted and the two portions assembled. For mechanical rigidity, the ceramic backing rings 43 and 44 are brazed to one another, in a manner and with alloys similar to those used for the dome 21 and backing ring 28. To form a vacuum seal, the adjacent portions of the

sealing rings

38 and 39 are are welded. Spaced channels 45 connect the space 46 with the interior of the tube to eliminate pressuredifierentials. v

The

ceramic rings

11, 12, 13 and 14 which form the envelope are preferably of insulating material such as high alumina ceramic. Both ends of the rings are metallized by a suitable metallizing procedure, such as by molybdenum manganese powder sintering process. The

metallic rings

16, 17, 18 and 19 are relatively thin and have high electrical conductivity.

Metallic rings

39 and 22 are relatively thin and preferably made of kovar. The

sandwiched parts are brazed together using high tempera- V ture brazing alloys such as copper-silver eutectic. The metal bonds at the brazed joints form strong mechanical connections and provide a vacuum seal so that the side walls of the envelope provide a solid impervious cylinder of rugged construction.

The electrodes disposed within the envelope include a cathode assembly 51 having a cylindrical cathode 52 heated by means of a filamentary heater 53. An oxide coating on the exterior of the side wall of the cylindrical cathode 52 provides the emissive surface. One end of the cylindrical cathode 52 is closed by an end wall 54. A tubular center post 55 is mounted coaxially within the cylindrical cathode 52 by means of a metallic flange 56 at one end thereof which is spot welded to the end wall 54 of the cathode cylinder 52. The end wall 54 of the cathode cylinder 52 is dished inwardly to receive thermal insulation. 7

The filamentary heater 53 is formed and shaped'to lie along a cylindrical surface. As shown, the heater 53 comprises a filament which zigzags up and down along 3 heater 53 within the cup-like cathode member formed by cylinder 52 and end wall 54. The

ceramic washers

58 and 59 each have an annular groove 62 formed on their facing surfaces near the periphery of the same which receives the adjacent bight portions of the filament.

In assembly, the cup-like member is oriented with the end wall 54 down and the first ceramic washer 58 is slid over the post 55 and seats itself against the end wall of the cup-like member. The filamentary heater 53 is then loaded into the cup and the bight portions are seated in the accommodating groove 62 formed in the ceramic washer 58. The second ceramic washer is then inserted over the post so that the groove 62 engages the adjacent bight portion of the filamentary heater 53. A flat ring 63 is then placed over the extending end portions of the post 55 and seated against the ceramic washer 59. A flaring tool is then inserted in the post 55. The flaring tool includes an elongated tine portion which extends through the post and cuts an opening in the end wall 54 so that it may receive a pin to be presently described. The flaring tool also includes a flaring means which serves to flare the lower end of the post S outwardly to engage the ring 63 and hold the above mentioned cathode parts in assembled relationship.

The free ends of the filamentary heater 53 extend through the

openings

64 and 65 formed in the lower ceramic washer 59 and provide means for making electrical connection thereto. It will be seen that the heater cathode assembly above described will have considerable mechanical strength due to the mounting of the filamentary heater 53 between the ceramic washers and the mountings of the washers within the cup-like cathode member.

Thermal insulating material 66 may be mounted within the dished end wall 54. The insulating material 66 may, for example, comprise a plurality of circular sheets having corrugated sheets interposed therebetween. A flat ring 67 may overlie the insulating material and be held in place by means of tabs (not shown) which may be formed at the outer periphery of the dished wall 54 of the cathode member. The open end of the cylindrical cathode 52 is provided with a cylindrical support 68 which has its lower end suitably secured to support cone 69 which has a radially extending flange portion which forms the terminal ring 17.

Heat insulating assembly 71 of the type previously described is carried by the disc 72 which is secured to the support 68. An electrical insulating sleeve 73 extends through the heat insulation 71 and ring 63 in alignment with the hole 65 in the ceramic washer 59 and serves to electrically insulate the first end of the filamentary heater from the heat insulating material 71 and ring 63. The other end of the filamentary heater extends through the hole 64 of the ceramic member 59 and through an aligned hole formed in the heat insulation and is electrically connected at 74 to the supporting cylinder 68. It is observed that the heater cathode assembly is of rugged construction. Relatively large filament wires zigzag between a pair of ceramic washers spaced at opposite ends of a post 55. Thus, the heater is not subject to breakage under mechanical vibrations, shock or acceleration.

A truncated cone 76 is coaxially mounted within the cone 69 and in spaced relationship thereto. A flange on the enlarged end of the cone 76 provides the terminal ring 16. The ceramic ring 11 is bonded to the terminal rings 16 and 17. A center post 77 is electrically and hermetically attached to the apex of the cone 76 and extends both upwardly and downwardly therefrom. The cone 76 and the center post 77 serve as an end wall for the evacuated envelope. The center post 77 serves also as a holding device to hold a ceramic pin which is employed to rigidity the internal parts as will be described hereinafter. A flexible lead 78 is connected between the post and the first end of the filamentary heater 53. The

terminal means 16 and 17 provide for the application of heating current to the filamentary heater 53.

The lower portion of the envelope includes a disc-like support 81 which has its peripheral edge brazed to the ceramic ring 82. The ceramic ring is added primarily to reinforce the braze between the ceramic ring 11 and the terminal ring 16. Ceramic and metallic parts have different coefficients of expansion and without the ring 82, the joint between 11 and 16 would not be strong and gas tight. Holes 83 may be provided in the support 81 since it does not form part of the air-tight envelope. These holes may be employed for mounting the tube if it is so desired. The post 77 is secured at its lower end to the ring 81. The upper end of the post 77 extends into the evacuated envelope and is provided with a plurality of ports 86. A tubulation 87 may be suitably secured to the lower end of the post to provide means for evacuating the envelope.

A control grid 91 is mounted coaxially with respect to the cathode emitting surface 52. The grid may be of vertical wire cage construction with the wire elements 92 being attached at one end to a cylinder 93 which forms the upper portion of a support cone 94. The lower end of the cone has a flange which extends outwardly to form the terminal ring 18. The upper ends of the wire elements 92 are attached to a control grid cap 95 having a central aperture therethrough. The grid cap 95 has formed therein radial ribs to provide rigidity.

A screen grid 98 of vertical wire cage construction is mounted coaxial with respect to the control grid 91. The wire elements 98 of the screen grid have their lower ends suitably attached to the support cone 102 which has an outwardly extending flange portion forming the terminal ring 19. The upper ends of the screen grid elements 98 are attached to a screen grid cap having a central aperture and radial ribs.

Both grids have circumferential reinforcing wire 104 to rigidity the vertical grid wires. After the grids are assembled by spot welding, the grids are preferably brazed with gold or a gold alloy to insure against spot weld failures.

It is observed that the conical supports 69, 94 and 102 include

apertures

195, 106 and 107, respectively. The apertures are provided so that the spaces between the various elements are evacuated during evacuation of the tube.

According to the present invention, there is provided an internal radiation cooled anode 111 which surrounds the cathode assembly. The walls of the anode are cylindrical and relatively thin. The anode is light and inertia forces due to shock, acceleration and vibration are minimized. The lower end of the cylindrical anode 111 is rounded as indicated at 112. The upper portion is suitably secured to a support 113, as for example, by means of screws 114 which are locked in position with safety wire to prevent loosening under severe vibration. The anode support 113, in turn, may be brazed or otherwise bonded to the dome 21. The anode support 113 aids in conducting heat away from the anode to the dome where it is dissipated to the surrounds by radiation and convection. The heat is, however, dissipated primarily by radiation from the anode surface to the surrounding ceramic cylinder 14 which, in turn, re-radiates to the surrounds. By employing a ceramic-type tube, the temperature at which the tube may be operated is considerably higher than the conventional internal anode glass-type tubes. Even though transmission of radiant energy may be somewhat less through the ceramic envelope, the re-radiation from the ceramic envelope at the higher operating temperatures more than compensates for the difference in transmission. The ceramic envelope provides a more rugged envelope which can withstand greater stresses and higher temperatures.

A lower cylindrical shield 115 surrounds the lower portion of the anode to prevent bombardment of the ceramic envelope portion 14 by electrons which are emitted by the cathode surface and'accelerated towards the anode, but which in transit miss the anode.

Another feature of the present invention is the provision of a rigidifying element which serves to maintain the various tube elements concentric to one another. In accordance with the present invention, an insulating pin 116 extends coaxially within the tube. It is supported at one end by the center post 77 carried by the bottom portion of the tube. Preferably, the pin 116 is a hollow ceramic rod which is light in construction. The inertia effects due to acceleration, vibration and the like are minimized and yet there is provided a relatively rigid element. The insulating pin 116 extends upwardly through the insulating assembly 71, the center post 55, the insulating assembly 66 and through the center apertures formed in the grid cap 95 and screen cap 103. The ceramic pin is preferably processed prior to insertion to form a metallized coating over restricted regions which coincide with the grid and screen caps. The grid and screen caps are then brazed to the pin to prevent vibration. The pin may be terminated just beyond the screen cap 103 and in such event will serve to rigidify the various elements associated with the lower portion of the electron tube. However, it is preferable to extend the pin upwardly and to form the anode support 113 in such a manner that it includes a downwardly extending portion 117 which receives the other end of the pin 116. With the additional support 117, the pin is supported at both ends and serves to more rigidly maintain the various elements in coaxial relationship.

Referring to FIGURES 2 and 3, it is seen that the terminal-rings have outwardly extending tabs designated generally by the reference numeral 122. Each of the rings may have a plurality of such tabs spaced circumferentially about the tube. The tabs for the various rings are arranged one above another in vertical rows. The outwardly extending mounting ring 37 is provided with a plurality of cut-outs 123 for positioning the tube on the platform of an associated socket, as Will be presently described.

The tube socket includes a support platform 124 be-v neath which are mounted one or more contact stacks 125. As seen best in FIGURES 3 and 4, each of the stacks includes spaced metallic contact segments 126 which extend radially inward to form contact fingers 127. The contact segments include tabs 128 for connection to external circuits. The contact segments 126 are arranged one above another to form rows similar to the rows of the terminal tabs on the tube. The individual fingers 127 are each flared at their side edges (FIGURE 5) to facilitate entrance of an associated tube tab 122. The contact segments are preferably formed by placing two sheets of metal in abutting contact so that their associated contact fingers will engage both sides of an associated tube tab.

The contact segments include pairs of spaced holes which accommodate insulating sleeves 129, FIGURE 5. Securing means such as metal bolts 131 pass through the sleeve and secure the stack to the platform 124. Insulating spacer segments 132 are placed over the insulating sleeves. The spacer segments 132 are placed be- Lceramic tube and socket therefor.

tween contact segments 126 to space the latter axially a distance corresponding to the spacing of the terminal tabs 122 of the associated tube.

The platform includes a plurality of circumferentially spaced slots 133 which accommodate the terminal tabs 122 as the tube is inserted into the socket. An annular plate 134 having inwardly extending tabs 135 is disposed beneath the platform (FIGURE 5) and aids in centering the tube. In assembly, the tube is rotated until the tabs are aligned with the slots 133 and the tube is lowered into position until the mounting ring 37 abuts the platform 124. The tube is then rotated so that the terminal tabs in the platform 124. The apertures and threaded holes are located so that they register when the tube is mounted in the socket. fix the tube in the socket.

It is observed with respect to FIGURES 1 and2 that the mounting ring 37 is located substantially at the center of gravity of the tube whereby the tendency of the tube to tilt in the socket due to inertia forces when it is subjected to shock, acceleration or vibration is minimized. It is further observed with respect to FIGURE 1 that the mounting ring 37 is secured to the tube envelope at axially spaced points by the lead-in terminal 19 and the sealing ring 39 to thereby give a rigid spaced support. This also contributes to the minimization of any tilting or flexing motion of the tube with respect to the mounting member One further feature of the tube mounting in accordance with the present invention is that the anode does not see the grid of the. tube since there is interposed between the two a conducting plane. Thus, the anode is shielded from the grid and the anode to grid capacitance is minimized.

Thus, it is seen that there is provided an improved The tube includes an internal anode which is relatively light in construction whereby the forces produced by inertia during acceleration, shock or vibration tend to be minimized. The internal elements including at least the cathode assembly, control grid and screen grid are supported and maintained coaxial within one another by a rigidifying element securely mounted to the base of the tube. The rigidifying element prevents relative movement between the parts. Improved shielding between the anode and control grid is .provided by the tube and associated socket.

We claim:

1. An electron tube having a ceramic envelope, a thin cylindrical anode disposed entirely within said envelope and spaced therefrom, means for supporting said mode from one end of said envelope, a cathode assembly disposed coaxially within said anode, a cylindrical grid structure surrounding said cathode and spaced therefrom, an insulating pin extending coaxially of the tube and engaging said grid and cathode assembly to hold the same in coaxial relation, and means mounted at the other end of said envelope serving to support rigidly one end of said pin.

2. An electron tube having a cylindrical ceramic envelope, a thin cylindrical anode disposed entirely within said envelope coaxially thereof and spaced therefrom, a dish shaped metal member closing one end of said cylindrical ceramic envelope and serving to support said anode therein, a cathode assembly including a cylindrical cathode surface disposed coaxially within said anode, a

cylindrical grid structure surrounding said cathode and spaced therefrom, a hollow insulating pin extending coaxially of the tube and engaging said cathode assembly and said grid to hold the same against movement with respect to one another, and a post rigidly supported on the other end of said envelope and serving to engage rigidly one end of said hollow insulating pin.

3. An electron tube having a cylindrical ceramic envelope portion, a dish shaped metal member closing one end of said cylindrical ceramic envelope portion and forming part of the envelope, a metal support member secured to said dish shaped member extending into said envelope, a thin cylindrical anode secured to said support member and disposed coaxially within the ceramic envelope portion and spaced therefrom, a cathode assembly Apertures 137 may be formed in the Screws may then be inserted to firmly' including a cylindrical cathode surface disposed coaxially within said anode, a cylindrical grid structure surrounding said cathode and coaxial therewith, a pin extending coaxially of the tube and engaging said cathode assembly and grid structure to hold the same in coaxial relation and to limit movement of the same within the tube, means at the other end of said ceramic envelope serving to support one end of said pin, said pin supported at its other end by said support member.

4. An electron tube having a cylindrical ceramic envelope, a dish shaped dome secured to one end of said envelope, an anode support secured to said dome and extending into said envelope, a thin cylindrical anode supported by said support extending coaxially of said envelope and spaced therefrom, a cathode assembly supported coaxially within said anode, first and second cylindrical grid structures surrounding said cathode and spaced therefrom and from one another, and means carried at the other end of said envelope serving to support said cathode assembly and said first and second cylindrical grid structures.

5. An electron tube having an envelope comprising alternate ceramic and metal rings bonded together in a stacked relationship to form a cylindrical envelope portion, a dish shaped disc secured to one end of said cylindrical envelope portion, an anode support secured to said dish shaped member and extending into said tube, a thin cylindrical anode extending into said tube and spaced from the cylindrical envelope portion supported by the anode support, a post supported from the other end of the envelope on the tube axis and connected to a first one of said metal rings, a second one of said metal rings being positioned nearer said anode than the first ring, said post being attached to said second ring at a position spaced towards the anode from the connection between said post and said first ring, a third one of said metal rings being positioned nearer the anode than said second ring, a cathode assembly mounted on said third ring, a fourth one of said metal rings being positioned nearer the anode than said third ring and serving to support a cylindrical grid structure.

6. An electron tube having an envelope comprising alternate ceramic and metal rings bonded together in a stacked relationship to form a cylindrical envelope portion, a dish shaped disc secured to one end of said cylindrical envelope portion, an anode support secured to said dish shaped member and extending into said tube, a thin cylindrical anode extending into said tube and spaced from the cylindrical envelope portion and supported by the anode support, a post supported from the other end of the envelope on the tube axis and connected to a first one of said metal rings, a second one of said metal rings being positioned nearer said anode than the first ring, said post being attached to said second ring at a position spaced towards the anode from the connection between said post and said first ring, a third one of said metal rings being positioned nearer the anode than said second ring, a cathode assembly mounted on said third ring, a fourth one of said metal rings being positioned nearer the anode than said third ring and serving to support a cylindrical grid structure, and a hollow ceramic pin extending through said cathode assembly and cylindrical grid structure and serving to rigidify the same, one end of said pin being supported by said post.

7. An electron tube having an envelope comprising alternate ceramic and metal rings bonded together in a stacked relationship to form a cylindrical envelope portion, a dish shaped disc supported at the upper end of said envelope portion and forming a vacuum seal therewith, an anode support secured to said disc extending into said envelope, a thin cylindrical anode extending into said cylindrical envelope portion, a support post disposed on the tube axis at the other end of said tube and connected to a first one of said metal rings, a second one of said metal rings being positioned nearer said anode than said first ring, said post being attached to said second ring at a position spaced toward said anode from the connection between said post and said first ring, a third one of said metal rings being positioned nearer the anode than said second ring, a cathode assembly including a cylindrical cathode mounted on said third metal ring, a fourth ring positioned nearer the anode than said third ring, a cylindrical grid structure supported on said fourth ring, a fifth ring spaced closer to the anode than said fourth ring, a cylindrical screen grid structure supported on said fifth ring, said fifth ring extending outwardly, and a mounting ring secured to said fifth ring.

8. An electron tube having an envelope comprising alternate ceramic and metal rings bonded together in a stacked relationship to form a cylindrical envelope portion, a dish shaped disc supported at the upper end of said envelope portion and forming a vacuum seal therewith, an anode support secured to said disc extending into said envelope, a thin cylindrical anode extending into said envelope portion supported by said anode support, a support post disposed on the tube axis at the other end of said tube and connected to a first one of said metal rings, a second one of said metal rings being positioned nearer said anode than said first ring, said post being attached to said second ring at a position spaced toward said anode from the connection between said post and said first ring, a third one of said metal rings being positioned nearer the anode than said second ring, a cathode assembly including a cylindrical cathode mounted on said third metal ring, a fourth ring positioned nearer the anode than said third ring, a cylindrical grid structure supported on said fourth ring, a fifth ring spaced closer to the anode than said fourth ring, a cylindrical screen grid structure supported on said fifth ring, and a hollow ceramic pin extending coaxially in said cylindrical envelope supported at one end on said post, said hollow ceramic pin serving to engage the cathode assembly and the grid structures to maintain the same coaxial Within one another.

9. An electron tube having an envelope comprising cylindrical ceramic insulating sections, metal terminal rings sealed to the ends of certain of said ceramic sections, terminal tabs projecting outwardly from said terminal rings, said tabs being disposed in rows along the length of said envelope, said tube comprising electrodes including at least a cathode connected to said terminal rings and an anode, and a supporting ring structure projecting outwardly from said envelope, said supporting ring structure comprising a first metal ring sealed to the end of one of said ceramic sections, a second metal ring sealed to the end of another of said ceramic sections, spacing means holding said first and second metal rings substantially separated along the length of said envelope, said first and second rings being bonded together externally of said spacing means, and the distance between the axis of the tube and the outer edge of said supporting ring structure being substantially greater than the distance between the axis of the tube and the outer edges of said terminal tabs,

10. An electron tube as claimed in claim 9 in combination with a socket comprising an apertured support platform, a plurality of contact fingers mounted on said platform in circumferentially spaced rows extending parallel to the axis of the socket, each of said rows containing a plurality of contact fingers spaced along the row and mounted to project radially inwardly toward the axis of the socket, said tube being received in said socket with said supporting ring structure in abutment with said platform and said terminal tabs in contact with said contact fingers, said terminal tabs extending radially beyond the inner edge of said contact fingers so that the terminal tabs and contact fingers overlap, the aperture in said support platform being large enough to permit passage therethrough of said terminal tabs and small enough to prevent passage therethrough of said supporting ring structure.

References (Jited in the file of this patent 10 Drieschman et a1. .....1 Jan. 15, 1952 Bell Dec. 9, 1952 Sorg et a1. -5 July 28, 1953 Polese Nov. 26, 1957 Stangl Nov. 4, 1958