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The Use of Thoria

Allan Wyatt and Stig Comstedt, The Valve Museum, May, 2016
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This short article is to provide a vehicle for original patent documents relating to the use of thoria in lamp filament production and subsequently as a way of making a more emissive cathode than pure tungsten.

Thoria is a white powdery oxide of thorium. The chemical formula is ThO2. Thorium is a radioactive metal atomic number 90 and atomic weight 232.

The work that resulted in the patents was carried out by researchers working for the American company General Electric.

The first patent document was filed in June 1912 and passed on December 30, 1913. The author was William David Coolidge, a physicist and engineer, who became the director of the General Electric Research Laboratory. His claim to fame was the development of ductile tungsten in or around 1909. The ductility came from the use of thoria mixed with the tungsten powder. The improved ductility made the drawing of tungsten wire easier and thus increased the efficiency by which the filaments for incandescent lamps could be made.

The next development, the subject of a patent application in July 1914, was by Irvine Langmuir of General Electric. His work on electron emissivity discovered the fact that thorium has a much greater emission than tungsten and his patent description gives the details. Here is his apparatus, here is page 1 of the document, here page 2 and here page three.

The final PDF here examines tungsten, thoriated-tungsten and thoria emitters and gives a clear description of the metallurgy. There is a description of the powder metallurgy and subsequent techniques involved in the working of tungsten. See also The production of tungsten wire for valve making and valve cathodes.

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