US1896614A - Gas content rectifier - Google Patents

Description

Patented Feb. 7, 1933 UNITED STATES PATENT OFFIC AUGUST GEHRTS, OF BERLIN-FRIEDENAU, GERMANY, ASSIGNOR TO SIEMENS & HALSKE,

AKTIENGESELLSCHAFT, OF SIEMENSSTADT NEAR BERLIN, GERMANY, A CORPORA- TION OF GERMANY GAS CONTENT RECTIFIER No Drawing. Application filed April 24, 1928, Serial No. 272,570, and in Germany April 25, 1927.

This invention relates to a gas-filled rectifier.

It is well known that thoriated tungsten wires may be put into a highly active, electron emitting condition by means of a suitable heating process. Such process involves the heating of the filament in the finished valve to 3000 0. Such a heating is, however, not always realizable, since the heating current required for this end may disturb the filament lead seals, more particularly in the case of thick tungsten wires. Furthermore, it has not yet been possible to maintain the working condition of a thoriated tungsten filament of the type described within the rare gas atmosphere. The workable heating current being sometimes very high with rectifiers, it is just in this case that the abovenamed difiiculties are encountered.

These drawbacks may, however, be overcome in accordance with the present invention, if as the cathode in a gas-filled electronic emission tube, more particularly a rectifier, there is employed a filament composed of a metal having a very high melting point, such as tungsten, which is alloyed at least at its surface with a high percentage of a'metal, such as thorium, uranium and the like, adapted to increase the electronic emission of the filament.

A filament or cathode of this type has at its surface an alloy of thorium or uranium, or the like, with the supporting metal filament, which alloy is composed preponderatingly of thorium or uranium (almost 100 per cent), so that in contradistinction to the thorium-film (thorium-coated) cathodes used heretofore, the thorium content at the surface of the cathode is not destroyed or rendered ineifective by the ionic bombardment.

Cathodes composed of tungsten wire whose surface is coated with an alloy of tungsten and thorium, are known. Their use has, however, been limited to highly evacuated tubes in which the non-sensitivity against bombardment could not show itself.

A filament or cathode particularly adapted for the purposes of the present invention may be produced by coating the surface of cause the nitrate to become disintegrated or decomposed, liberating the metal (thorium) which alloys itself with the tungsten at the surface of the filament. It will, of course, be understood that instead of thorium nitrate, I may employ the nitrate of a metal having properties similar to thorium, and a particularly uranium. By the treatment described there is formed a filament which at its surface has a practically pure layer of 100% thorium (or uranium), while at increasing distances from the surface the thorium (or uranium) content diminishes until at the core of the filament only tungsten is present. The heating involved in this process is, as stated, of very short duration; it

may amount to only a fraction of a second, 7

and need not exceed 2000 C. It is therefore preferably applied to filaments already in position in the valves or bulbs.

Contrasted with known filaments whose surface is coated with an alloy high in thorium, a filament produced in accordance with my new process has the advantage that it comprises a product which in a certain sense may be regarded as a homogeneous filament. Where, as in known processes, there is appliedto the supporting wire a coating com posed of an alloy of thorium and a refractory material, such as tungsten, there is produced an encased filament having a core composed of tungsten and covered with a casing of an alloy of tungsten and thorium. Such encased filaments have, as is known, low strength and have a tendency to lose their casing. These disadvantages can not appear in a filament constructed in accordance with the present invention as in such filament the \Lil tungsten is intimately alloyed with the thorium of the coating, so that a filament is produced which gradually varies in composition from thorium at the surface to tungsten at the core.

The new filament is particularly suitable for rectifier Valves filled with rare gas or inercury vapour. With a high vapour pressure (1100 111/111 mercury) a very long life for such filament has been observed.

1 claim A method of manufacturing a filament of highly rafractory metal containing on its surface a high percentage of thorium, which consists in applying thorium nitrate and boraX to the highly refractory metal, and disintegrating such nitrate by the application of heat.

In testimony whereof I affix my signature.

AUGUST GEHRTS.