US1951143A - Gaseous electric discharge device - Google Patents

Description

March 13 1934. r. E. .FOULKE GASEOUS ELECTRIC DISCHARGE DEVICE Filed July 15; 1 955 INVENTOR TORNEY 4 Patented More. E3, l34

insists oasnoos zsnnc'rmo nrsonaiton novice ll'ed 1E. Foulk Nutley, N. 3., assignor to General Electric Vapor Lamp Company, tllobolsen, N. 3., a corporation off New Hersey Application July 13, 1933, Serial Noa 6%,3135 8 Claims. (Cl. 176-122) The present invention relates to electric gaseous discharge devices, and particularly to tubular devices having glow supporting cathodes of appreciable length.

Ihe invention consists in an electric gaseous discharge device having the novel structure and characteristics hereinafter set forth and claimed.

A particular object of the invention is to provide an electric gaseous discharge device of the cathode glow type in which the extent of the cathode glow indicates the current flow therethrough. A further objectof the invention is to provide a device of this type wherein the length of the glow along the cathode is substantially a linear function of the current flow thereto. A further object of the invention is to reduce the sputtering from the cathode of such a device. Another object of the invention is to provide a discharge device of low impedance. Another object or" the invention is to provide a discharge device having the foregoing characteristics which will have a long useful life. Stfll other objects and advantages of the invention will appear from the following detailed specification, or from an inspection of the accompanying drawing.

It has been proposed heretofore to use a cathode glow device as a visual ammeter for very small currents, and attempts been made to use a tubular device having a small anode and a longwire cathode for this purpose, the tendency of a cathode glow to maintain a norrnal cathode fall and a constant current density then being depended upon to the glow en tend up the cathode for a distance which is a function of the ctu'rent. These devices have not gone into any extensive practical use, however, despite the great need for a device of this type for various purposes, such to facilitate accurate tuning in radio sets having automatic volume control, or in series with a high resistance as a visual voltmeter due to the fact that they possess several inherent faults. Thus the sputtering rate from th a ode of one oi these devices, with its attei .o'ackening and gas clean-up, is so high the useful life of such a device is but a few I .sing them entirely impractical for genera-r use. Furthermore the distance which the moves down the cathode for a given increase in current decreases more and more rapidly as the current increases, so that the sensitivity of thedevice is least for the maximum current. Hence the device is of little use for sharp tuning, due to this lack of sensitivity at the current peak. In

addition the impedance of these devices is so high that it interferes unduly with the electri cal characteristics oi the circuit in which the device is inserted, and attempts to lower this impedance by the use of cathode coatings of to low work function have been unsuccessful because-of the high sputtering rate. i have now discovered that all of these faults arise from a single cause which I have now eliminated by means of a novel structure of my invention. This cause is the abnormal cathode fall at' the end of the cathode which is adjacent to the anode which results from the fact that the cathode is all at the same potential,'whereas the space potential. in a relatively small tubular to envelope varies from one end of the cathode to the other, due to the resistance to current flow therethrough to the far end of the cathode. Thus since the normal cathode fall must be maintained, as a minimum, at the far end of 25 the cathode in order to produce a glow discharge at that point it is obvious that the space potential. at the near end of the cathode must. exceed the normal cathode fall by the potential drop in th gas column. As a result the near St; end of the cathode is subjected to an extremely heavy ionic bombardment which rapidly removes the base metal or any activating coating of low work iuncticn thereon by sputtering, thus black ening the envelope and cleaning up the gas. g5 Furthermore this abnormal cathode fall causes the previously mentioned deviation from a linear relation between the current and the distance covered by the glow along the cathode, due to the fact that the glow discharge is of abnor= really high current density in the region 'of ab-- normal cathode fall. I have now discovered that this cause, with all its attendant difiiculties, can be eliminated by making the cathode of a higlr resistance material, provided the current lead to said cathode is connected to the end thereof which is remote from the anode. With this construction the cathode fall is made to closely approximate the normal value throughout the length of the cathode and the sputtering is vir- 10o tually eliminated. As a result it is now possible to produce devices having a useful life many times that heretofore obtainable in devices of this type. Moreover, activating coatings of low work function may now be provided on the cathode, when desired, with assurance that the coating will remain intact throughout a long useful life. As a result the impedance of the devices isgreatly reduced, as is also the potential required to initiate the discharge. The use of a preferred structure, with a typical operating" circuit therefor, 7 Fig. 2 is an enlarged view, in part section, of the cathode shown in Fig. 1,

Fig. 3 is an elevational view, in part section, of a modification of the structure of Fig. 1, and Fig. 4 is a similar view of another modifica tion of the structure shown in Fig. 1, together with a schematic diagram of a typical operating 7 circuit therefor.

In this drawing, with particular reference to Figs. 1 and 2, there is shown an amm'eter lamp having a tubular sealed enveiope l of glass or other suitable material having an outside diam eter of the order of inch into one end 'of which a pair of inleads 2 and 3 are sealed through a conventional pinch seal. Said inlead 2 terminates in a short wire 4 of nickel or the like which serves as an anode, while the inlead 3 extends almost to the opposite end of the envelope 1 through a vitreous tube 5 which is fused into the aforesaid pinch seal. Starting at a point near the anode 4 a fine wire 6 which serves as the cathode is closely wound in a single layer along the tube 5, until at the far end of said tube it is electrically connected to the projecting end of the inlead 3.' This wire 6 preferably has a relatively high resistance, of the order of 250-5000 ohms per linear inch along the tube 5, since when the resistance is of this order the glow extends, rather than moves; along the cathode as the current is increased. The higher the resistance, however, the further a given current will cause the glow to extend along the cathode. Thus variation in this resistance affords a means of varying the current range measurable with a cathode of a given length. While I can use iron, tungsten, or the like for the wire 6 I prefer to make it of a *nickel-chrome alloy, such as the one known to the trade as Nichrome, since due to the higher specific resistance thereof thewire 6 need not be so fragile for a given resistance. For example; I have obtained very satisfactory results using '7 mil Nichrome wire closely wound on a tube 5 which is approximately 2 mm. in diameter, full glow being obtained over a cathode 2 inches long with a cathode of this construction upon flow of a current of 10 milliamperes. The surface film which is normally present on this 'wire is ample insulation between turns, and hence no special separation need be provided. The end of the inlead 3 is preferably covered with a bead '7 of glass or the like after the wire 6 has been attached thereto in order to prevent transfer of the discharge to said inlead, and also to hold the end of said wire 6. The other end of said wire 6 may be held in any suitable manner, as by winding the first few turns of the wire over each other, or by affixing it to the tube 5 by heat, or in any other suitable manner. Theenvelope 1 may contain any suitable gaseous atmosphere, such as neon, helium, argon, mercury vapor or the like. I prefer, however, to use neon containing about 0.4% of argon because of the relatively low potential required for the initiation and maintenance of a discharge therein, and because of the color of the light emitted. This gas may be at a pressure of the order of from 1040 mm. of mercury, although I prefer to use a. pressure of say 20 mm. "In order to further reduce the starting and operating potential, and thus the impedance of the device, the wire 6 is preferably coated with a material having a relatively low work function, such as an alkali or an alkaline earth metal, or a compound thereof. For example, in practice I coat these Wires with a mixture of barium azide and about 72, of caesium azide which is reduced by heating after the envelope has been thoroughly exhausted to produce a film of barium and caesium metal together with the nitrides thereof, according to the novel process set forth in detail in my copending application; Serial No. 611,177, filed May 13, 1932; When so coated my; device has a breakdown potential of but '75 volts, D. C., and a discharge maintaining potential of the order of 60-65 volts, D. C., so that its impedance is far less than that of any of the prior devices of this type.

As shown in Fig. 1 this novel device may be connected in series in the output circuit of one of the tubes in a radio set or the like. In this case the cathode inlead 3 is connected to the plate'of the electron tube 8, while the anode inlead 2 is connected through the primary of the output transformer 9 and the B battery 10 to the filament of said electron tube 8. I

In the use andjoperation of the foregoing device, theicurrent output of the electron tube 8 traverses the gas in the envelope 1, prodncing'a gaseous discharge of the cathode glow type between the anode:4 and the cathode 6. While this discharge starts at the point on said cathode 6 which is nearest to the anode 4' the well known :tendency' of a cathode glow discharge to establish a characteristic normal cathode fall and a glow of uniform current density causes the glow discharge to extend up the cathode 6, without leaving the end thereof, until these conditions are fulfilled. The length of this glow along the oathode 6 is thus a measure of the current flow in the output circuit of the electron tubes. Due to the voltage drop produced in the cathode 6 as a result of the current flowing therethrough each point on the glow covered portion of said cathode is at substantially the'same potential with respect to the gas adjacent thereto, so that an abnormal cathode falleat the end of the cathode which is near the anode is avoided, despite the added voltage necessary to pass the current through the gas column to the most remote point on said cathode. As a result there is no appreciable sputtering of the cathode at any point, so that my novel device has a long useful life. Likewise I now find it practicable for the first time to use barium, caesium, or other low work function material on the cathode, whereby the impedance of the device is greatly reduced. Furthermore, due to the uniform normal cathode fall the current intensity is likewise substantially uniform throughout the glow area, so that the length of the glow is approximately a linear function of the current, giving equal sensitivity at low and full range values of the current.

In the device of Fig. 3 the cathode inlead 3 extends in a similar manner through a glass tube 5 toward the end of said envelope 1 which is remote from said seal. A rod 16 of high resistance material, such as the coated glass rods which have been used for grid leaks, is attached in any suit able manner to the free end of the inlead 3 and extends parallel to the tube 5 to a point near the anode 4. All conducting parts of the support for said rod and the exposed end of the inlead 3 are preferably coated with any substance which will prevent the striking of the discharge thereto. Said resistance rod 16 in some cases has a resistance of the order of 10,000 ohms where it is 2 inches long, one milliampere in this case being s'ufiicient to cover the entire rod with a cathode glow. This rod is likewise preferably coated with the same coating of low work function which is employed on the cathode 6. The functioning of this device is, of course, entirely analogous to that described in connection with the device shown in Fig. 1, and hence need not be further described.

The modified structure shown in Fig. 4 is in all respects similar to that of Fig. 1, except for the addition of a third electrode 13, which is used in the circuit illustrated as an auxiliary cathode, although various other circuit arrangements are possible therefor, and will produce the same results. An auxiliary source of direct current, such as the battery 11, is connected in series with a resistance 12 between the anode inlead 3 and said auxiliary cathode 13. Said battery has sufiicient potential to initiate a glow discharge between the anode 4 and said auxiliary cathode 13, while the resistance 12 is of sufficient value to limit this discharge to a very small current. This discharge results in ionization of the gas adjacent to the wire 6, so that a discharge can be readily initiated between said anode 4 and the cathode resistance wire 6 whenever even a small current tends .to flow in the plate circuit of the electron tube 8, thus making the device more accurately indicate the circuit conditions under certain circumstances.

With each of the foregoing structures the resistance of the cathode obviates the abnormal cathode fall and thus eliminates the difficulties heretofore encountered with this type of device. It is to be understood, however, that my invention is not limited to these structures, but that various changes, substitutions, or omissions may be made therein, within the scope of the appended claims,

without departing from the spirit of my invention.

I claim as my invention:-

1. An electric gaseous discharge device com-' prising a tubular sealed envelope, a gaseous atmosphere therein, an anode near one end of said envelope, a cathode having a high resistance per unit length extending within said envelope from a point near said anode to a point near the other end of said envelope, and current leads for said anode and cathode, the lead for said cathode being connected to the end thereof which is remote from said anode, said cathode having a resistance of such a value that the voltage gradient therein when covered with a substantially uniform cathode glow is approximately the same as the voltage gradient in the surrounding gas column.

2. An electric gaseous discharge device comprisinga tubular sealed envelope, a gaseous atmosphere therein, an anode near one end of said envelope, a cathode having a high resistance per unit length extending within said envelope from a point near said anode to a point near the other end of said envelope, current leads for said anode and cathode, the lead for said cathode being connected to the end thereof which is remote from said anode, and means to prevent the transfer of a discharge from said cathode to the lead thereto.

3.'An electric gaseous discharge device comprising a tubular sealed envelope, a gaseous atmosphere therein, an anode near one end of said envelope, a cathode extending within said envelope from a point near said anode to a point near the other end of said envelope, said cathode having a resistance of the order of 250-4000 ohms per linear inch, and current leads for said anode and cathode, the lead for said cathode being connected to the end thereof which is remote from said anode. I

4. An electric gaseous discharge device comprising a tubular sealed envelope, a gaseous atmosphere therein, an anode near one end of said envelope, a cathode having a high resistance per unit length extending within said envelope from a point near said anode to a point near the other end of said envelope, said cathode having a coating of a material of low work function thereon, and current leads for said anode and cathode,

the lead for said cathode being connected to the end thereof which is remote from said anode.

5. An electric gaseous discharge device comprising a tubular sealed envelope, a gaseous atmosphere therein, an anode near one end of said envelope, a cathode having a high resistance per unit length extending within said envelope from a point near said anode to a point near the other end of said envelope, said cathode having a coating of barium and caesium intermixed with the nitrides thereof, and current leads for said anode and cathode, the lead for said cathode being connected to the end thereof which is remote from said anode.

6. An electric gaseous discharge device comprising a tubular sealed envelope, a gaseous atmosphere therein, an anode near one end of said envelope, a cathode having a high resistance per" unit length extending within said envelope from unit length extending within said envelope from a point near said anode to a point near the other end of said envelope, current leads for said anode and cathode, the lead for said'cathode being connected tothe end thereof which is remote from said anode, and an auxiliary electrode adjacent to said anode, said cathode having a resistance of such "value that the voltage gradient therein when covered with a substantially uniform cathode glow is approximately the same as the voltage gradient in the surrounding gas column.

8. An electric gaseous discharge device comprising a tubular sealed envelope, a gaseous atmosphere therein, an anode near one end of said envelope, a cathode having a high resistance per.

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