US2014787A - Thermionic cathode - Google Patents

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-se t. 17;, 1935.

C. J. SMITHELLS ET AL THERMIONI C CATHODE Filed May 29, 1934 Patented Sept. 17, 1935 UNITED STATES PATENT OFFIQE 2,014,787 'rnEaMioNIo cArnonE London, England Application May 29, 1934, Serial No. 728,062 In Great Britain June 24, 1933 4 Claims.

This invention relates to indirectly heated cathodes for thermionic tubes and, more particularly, to those suitable for receiving valves.

In one known method of making indirectly 5 heated cathodes the heating wire, usually a tungsten filament, is covered with insulating refractory material, such. as alumina, and is then placed in a metal (usually nickel) casing coated with alkaline earths, which coating forms the cathode surface. In this method there is a very definite lower limit to the ratio between the diameter and the length of the cathode; if this ratio is less than this limit, the heating wire cannot be pushed into the casing, except possibly by elaborate and therefore inconvenient methods. Consequently it is impossible to manufacture long lengths of cathode which can subsequently be cut up in order to provide the cathodes of individual valves. The object of this invention is to overcome this difficulty and to provide a method of manufacturing indirectly heated cathodes, suitable for mass production, in which cathodes for many valves are made at the same time.

The limitation on length can be removed by depositing the casing on the insulated heater rather than placing the insulated heater within a preformed casing.

When indirectly heated cathodes are to be heated by alternating current and used in receiving valves, it is necessary to avoid the hum that arises from the magnetic field of the heating current. This hum can be avoided, as is well known, by winding the heating element noninductively, that is to say, by forming it into a loop with closely adjacent parallel members traversed by oppositely directed currents.

But if hum is avoided by enclosing two parallel elements Within the same cathode surface, the object of the present invention is not attained;

for if such a cathode is to be made in long lengths, subsequently cut into sections, one pair of ends of the cut heating element will have to be joined together in the very small space available in actual receiving valves at one end of the cathode. Since tungsten is the most suitable material for the heating element, this process will be very difiicult. Accordingly a proposal of this kind provides no solution of our problem; and we declare that the use of parallel heating 50 elements, both enclosed in the same cathode surface, is specifically excluded from the scope of this invention.

We have found however that the problem can be solved, at least in part, by making the cathode first in the form of a helical wire wound on a single straight heating element of continuous indefinite length and then cutting off the desired length for the cathode and doubling it on itself so that successive limbs are adjacent and substantially parallel. The magnetic fields of the 5 parts of the heating elements within the successive limbs compensate each other, not perhaps as completely as when they are within the same cathode surface, but sufiiciently for many purposes. 10

It might be thought that, in the proposed method of construction, the turns of the spiral must be in contact at least when the cathode is very long; for if they were separated, so that current flowing through the wire has to pass along 18 its length, the wire would be heated unequally by the thermionic current drawn from it; consequently the wire would be much hotter at one end than at other. But we have found that this expectation is not fulfilled, probably because 20. the wire is in good thermal contact with the insulating layer on the heater and the alkaline earth layer deposited on it (which enters into thermal contact with the insulating layer), so that the temperature of the wire cannot rise appreciably above that of the insulating layer.

Actually a small separation between the turns of the spiral is desirable in order that the structure first formed may be readily bent so that its successive branches are adjacent; but the pitch 30 is not at all critical. The function of the wire spiral is, of course, to convey current to the oathode proper which (according to the theory accepted at present) is a monomolecular layer of alkaline earth metal on the surface of the 0xides. It appears that the conductivity of the cathode surface in the direction parallel to its surface is so great that an appreciable distance between points on its surface and the nearest part of the wire spiral does not introduce unde- 4O sirable large differences of potential over neigh' bouring parts of the cathode surface.

On the other hand, if the cathode is long there may be a considerable drop of potential along the spiral wire when thermionic current is drawn from the cathode surface. To overcome this difiiculty we have found that it is usually sulficient to earth the wire spirals (i. e. to connect it to the conductor at mean cathode potential) at several points along its length and not only at its ends. Thus when the cathode has been bent into loops, an earth connection may be conveniently welded to the nickel wire at each bend.

Again the insulating layer between the heater and the spiral must be of a suitable character in order that it may not crack off when the cathode is bent. We have found that a suitable layer can be provided by depositing on the heater finely divided insulating material, such as alumina, from a suspension.

One embodiment of the invention is shown by way of example in Figures 1 and 2 of the accompanying drawing, which show successive stages of manufacture. Here I is the heater consisting of a. tungsten wire 0.1 mm. in diameter. On this is deposited a layer 2 of alumina between 0.1 and 0.2 mm. thick, either by spraying or drawing the wire of indefinite length continuously through a coating bath. On this layer 2 is wound a spiral 3 of nickel wire 0.1 mm. in diameter, the average pitch of the spiral being about 0.11 mm. The whole is then sprayed with a suspension of barium and strontium carbonates, forming a coating 4 which is afterwards converted into oxides in the usual manner.

The structure so formed and shown as a broken away section in Figure 1 after being formed in a continuous indefinite length is cut into appropriate lengths and each length is doubled on it self, as shown in Figure 2, so that successive branches are adjacent. A plurality of leads 5 connected to each other and to earth are joined to the spiral 3 at its two ends and at one or more intermediate points. The branches of the oathode may be supported in the usual or any preferred manner.

We claim:

1. In a thermionic tube an indirectly heated cathode comprising a continuous metal core adapted to serve as the heating element, a continuous layer of refractory insulating material covering said core, a continuous wire helix on said insulating material, a continuous thermionically emitting layer on the wire and on the insulating material not covered by said wire, said continuous cathode being doubled on itself so that successive limbs are adjacent and substantially parallel and conductors connecting said helix at least at one end and at least at one point intermediate between its ends to an external conductor for earthing the same.

2. In a thermionic tube an indirectly heated cathode comprising a continuous metal core doubled upon itself so that successive limbs thereof are adjacent and approximately parallel, said core serving as the heating element, a continuous layer of refractory insulating material covering and adhering to said core throughout its length, a continuous wire helix wound with successive turns in spaced relation on said insulating material, the helix windings being continuous over the bends to successive limbs and a layer of electron emitting material adhering to and continuously coating the wire of the helix and the insulating material between successive turns so as to be in conducting engagement therewith and leads connected with said helix.

3. In the manufacture of thermionic tubes having an indirectly heated cathode bent upon itself so that adjacent lengths are substantially parallel, the process which includes coating a metal core with a layer of finely divided insulating material deposited thereon from suspension,

winding upon the insulating material a continuous wire helix having successive turns slightly separated, then coating the helix and the insulating material between successive turns thereof with a layer of electron emitting material, the respective coatings being so constituted that with the spacing of the turns of the helix the core may be bent to bring portions of the cathode into parallel relation without rupturing the coatings at the bends.

4. In the manufacture of thermionic tubes having an indirectly heated cathode bent upon itself so that adjacent lengths are substantially parallel, the process which includes coating a metal core with a layer of finely divided insulating material deposited thereon from suspension, winding upon the insulating material a continuous wire helix having successive turns slightly separated, then coating the helix and the insulating material between successive turns thereof with a layer of electron emitting material, the respective coatings being so constituted that with the spacing of the turns of the helix the core may be bent to bring portions of the cathode into parallel relation without rupturing the coatings at the bends, and connecting conductors to at least one end and to at least one intermediate point between the ends of said helix.

COLIN JAMES SMITHELLS. LESLIE RONALD GEORGE TRELOAR.

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