US1997693A - Electrical discharge device - Google Patents

Description

April 16, 1935. E. F. LOWRY ELECTRICAL DISCHARGE DEVICE Filed Nov. 16, 1929 lNVENTOR Erwin F? Lowry.

ATTORNEY Patented Apr. 16, 1935 PATENT OFFICE ELECTRICAL DISCHARGE DEVICE Erwin F. Lowry, Forest ,Hills, Pa., assignor to Westinghouse Electric & ManufacturingCompany, a corporation of Pennsylvania Application November 16, 1929, SerialNo. 407,690

1 Claim. (01. 250-27.5)

. surface.

Another object of my invention is to provide an electron-emissive cathode in which the ratio of the electron emission to theheat dissipation, by unit area of the cathode, is relativelylarge.

Another object of my invention is to provide an electrical-discharge device with an electronemissive cathode in which the rate of heat'dissipation per unit area of surface, at a given operating temperature, isminimized.

A further object of my invention is to'provide an electron-emissive cathode for electrical-discharge devices which shall havea larger surface area than cathodesof the prior art but which shall be strong and rigidly mountedl Other objects of my invention will become apparent upon reading the following specification, taken in connection with the drawing, in which Figure 1 is a view, in elevation, of an electrical-discharge device having an electron-emissive cathode embodying one form of my invention, and Fig. 2 is a sectional view taken on the line 11-11 of the tube shown in Fig. 1.

Fig. 3 is a view, in elevation, of another form of electron-emissive cathode of my invention.

Fig. 4 is a sectional View takenonthe line IV--IV of the tube shown in Fig. 3.

o It is desirable, for certain, purposes, to provide electrical-discharge devices with electron-emissive cathodes having relatively large surfaces and consequently capable of emittinglarge electron currents. Such cathodes. are. desirable both for high-vacuum electrical-discharge tubes and for electrical-discharge devices having gaseous atmospheres at appreciable pressures. Devices in which mercury vapor is present, as an atmosphere, about thecathode' are examplesof the latter type. of .devices. However, when it is attempted to ensure a large electron current by extending the surface area of the cathode to large dimensions, the energy'dissipated as heat from the cathode surface [becomes a serious source of loss and decreasedelectrical efficiency. One of the principal objects of my invention has been, accordingly, to devise methods of constructing hot cathodes which shall have relatively extensive surface areas but which shall require only moderate amounts of power to maintain them at their proper operating temperatures The foregoing objectI achieve by constructing the cathode of a relatively thin sheet material which is so bent back and forth or pleated that a large fraction of its surface is so positioned that it.faces other portions of its surface, In this way, the heat radiated from the first-mentioned portion of surface is absorbed by some other portion of the surface, and the amount dissipated to the surrounding surfaces and lost entirely is relatively small. Thus, the heating current has to supply only the relatively small losses last mentioned, and the amount of cathode heating power required to maintain the cathode at its operating temperature is minimized.

It is, of course, necessary to devise a structure for the cathode which shall be sufiiciently strong and rigid to maintain its shape, even when heated to high temperatures, andthe' cathode structures which I describe herein have been found to be satisfactory in this regard.

With the foregoing principles and purposes of invention in mind, reference may be made to Fig. l in which is shown a Vacuum-tight container I of conventional form in which is positioned an anode 2 and a control electrode 3 of conventional type. The container I may be evacuated to the highest vacuum attainable but my invention will be found to be well adapted for use in discharge tubes inwhich a gaseous atmosphere of appreciable pressure, such, for example, as mercury vapor, neon or argon is present.

The electrical-discharge device l is provided with a cathode 4 which consists of a thin ribbon, preferably of a high-resistance metal, which is bent back and forth on itself or pleated in the manner indicated in the drawing to form a pill.- rality of planes in the major portion of its surface. A specific material which I-have found to be desirable is the alloy of cobalt,,nickel and ferrotitanium, commonly known by the trade-mark name Konel, and described and claimed in my copending application Serial No. 144,911 filed October 28, 1926 and assigned to the Westinghouse Electric & Manufacturing Company. The pleated ribbon just described is preferably bent into the form of a cylinder, as indicated in Fig. 1, and is supported at its two ends on two rigid inleading wires 5 and 6 sealed through the wall of the container I.

After being pleated and bent to approximately its final form, the cathode ribbon may be coated over its entire surface, except at the ends which are attached to the inleading wires 5 and B, by a material adapted to emit electrons freely when heated. A mixture of equal parts of barium oxide and strontium oxide made into a paste by moistening with barium nitrate and painted and dried on the cathode ribbon is one example of an electron-emissive coating suitable for my cathode. The ribbon thus coated may then be welded to the inleading wires 5 and 6, which have been previously mounted in the press but which have not been sealed in place therein. The stem, with the cathode thus attached, may then be sealed into the containing bulb, and the tube thus formed exhausted and provided with the desired gaseous atmosphere by methods well known in the lamp art.

It is usually desirable that the cathode shall operate with a comparatively small potential difference between its ends and it will be readily apparent that materials having a proper resistivity are available so that the cathode which I have described will so operate. It will likewise be apparent that the pleating or multiple-folding of the cathode renders it more rigid and better adapted to support itself by reason of the corrugations thus introduced into its structure. It may, in certain instances, be desirable to bridge across the adjacent ends of successive folds of the metal by means of high-resistance struts, which may be of silica or of some high-resistance alloy of small diameter in order to provide a mechanically strong construction.

It will also be recognized that the bending of the pleated ribbon into a cylindrical form is desirable, though not necessary, and that, for certain purposes, it may be more desirable to form the pleated ribbon into a different shape. For example, Fig. 3 shows a cathode comprising a pleated ribbon which is disposed in a straight line between its supports, a grid electrode and a plate anode being disposed to surround the cathode thus formed, The, mode of operation is believed to be too obvious to require any extended description.

Instead of coating the cathode ribbon with barium and strontium oxides, in the manner above described, it may be desirable to make the ribbon itself from an alloy which embodies electronemissive substances incorporated within it. Thus, alloys of nickel and metallic barium may be employed to constitute the cathode, these having the property of emitting electrons freely when heated. Such cathodes are, however, not being claimed in this application, but are described and claimed in my joint application Serial No. 404,196 with G. B. Halliwell, filed November 1, 1929 and assigned to the Westinghouse Electric & Manufacturing Company. Another form of cathode which will be found useful in the present connection consists of a ribbon of the alloy designated by the trade name Konel on which has been deposited a surface layer containing metallic barium. This may be done by placing the k0nel" Within an exhausted container where it will come into contact with the heated vapor of barium trinitride. The form of electron-emissive cathode just described is also described and claimed in my copending application Serial No. 424,745, filed January 30, 1930 and assigned to the Westinghouse Electric & Manufacturing Company.

When a cobalt-nickel base alloy is employed as the core material, the electron-emitting metal is in intimate contact or bond with the base metal and is, at least partially, alloyed therewith.

Cathodes prepared in the manner specified are considerably superior to platinum filaments coated with an alkaline-earth metal because they give satisfactory emission at a lower temperature, are less expensive, and have considerably longer life in service. In fact, their electronemissive propetries compare favorably with oxidecoated filaments. Such filaments may be readily and uniformly reproduced, however, and are not subject to the other inherent defects which are characteristics of oxide coated filaments.

The alloy which I employ has a base or core metal containing nickel and cobalt in the aggregate of to in which the nickel and cobalt are present in proportions ranging from 95% to 5% nickel and from 5% to 95% cobalt, the remainder of the alloy consisting of iron and an additional ingredient which is effective in increasing the strength of the alloy, such as tungsten, manganese, titanium, vanadium or molybdenum. The additional ingredient is preferably added in the form of a ferrous alloy, the total amount of which must be maintained below 30%. The additional ingredient constitutes from /3 to A1 of the ferrous alloy so that its content will not exceed 10% of the final material.

While the proportionsabove specified have been stated broadly, the nickel is preferably maintained in excess of the cobalt because nickel is less susceptible tooxidation than cobalt and, at the present time, is less expensive. I have found that the alloys known under the trade-mark Konel which contain cobalt, nickel, iron and titanium are especially suitable, and particularly the alloy containing 80 parts nickel, 20 parts cobalt and 7 parts iron and /2 parts titanium.

It will be evident that, if desired, the cathode ribbon above described may not itself constitute the heating element but that it may be employed as an enclosure or wall about an independent heater of am! convenient form from which it may be wholly or partially insulated. It will also be recognized that the articulation of the cathode surface may be effected otherwise than by pleating a plane ribbon as herein described. Thus, if desired, cross walls between the pleats dividing the cathode into a series of nearly closed chambers will be found to be a desirable .construction .for certain purposes. The presence of an appreciable pressure of gas in the container is found to prevent space-charge effects from producing a high internal impedance, inthe case of cathodes having convoluted surfaces, such as are here described. By reason of the fact that the convolutions or chambers are nearly closed, the angle between their sides being less than a right angle, most energy radiated from any one area of a chamber wall strikes some other portion of the chamber wall and is there either absorbed or reflected. When a relatively small portion of the radiant energy takes the direction of the open end of the chamber and is thus dissipated'to the space surrounding the cathode, the cathode walls, under these conditions, may be described as introspective. As is well known, the heat radiation in the interior of a nearly enclosed chamber is black-body radiation.

The electrons in Fig. 1 travel through the space between the cathode 4 and anode 2. The aver- Due to the area of the deep pleats in the cathode, the area of the cathode is many times the area of this average cross sectional area.

One of the principal advantages of the oathode which I have here described is that the heat dissipation to the surrounding surfaces is minimized and that, consequently, the amount of heating energy required to maintain them in a condition capable of emitting an electron current of large magnitude is relatively small. In order to further this effect, it may be desirable to wholly or partially surround the cathodes by reflecting walls or by walls which are heat insulated. The current which flows from anode to cathode must, of course, traverse the'body of the cathode ribbon and its heating eiTect therein may be very appreciable. In certain instances, it may, in fact, be suchthat, once the tube is started in operation, all other heating current for the cathode may be dispensed with. The provision of reflecting and heat-insulating enclosures of the cathode will assist in producing this result.

In certain instances, it may be found desirable to position the anode wholly or partially within an enclosure comprising the cathode surface. For example, the anode may occupy the center of a cylindrical cathode, such as that illustrated in Fig. 1.

In accordance with the patent statutes, I have described particular embodiments of my invention, but it will be recognized that these are intended to be illustrative only and that the broad principle thereof will be capableof alternative embodiments which will be evident to those skilled in the art. I desire, accordingly, that the claim shall be given the broadest construction of which its terms are susceptible in view of the prior art.

I claim as my invention:

An electrical-discharge device having a gaseous atmosphere and comprising an anode and a substantially annular cathode of closely pleated sheet metal coated with an electron emissive layer, the angle between the sides of successive pleats being less than ninety degrees.

ERWIN F. LOWRY.

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