M. Kawakami and N. Ogasawara
From the result in the 1st report, we found that it was suitable to adopt the range of 2.45~3.16% as the carbon content in the carbide layer. Theoretically, we led the following formula representing the relation between the degree of carburization (S) and the ratio of increase in cold electric resistance (λ),
where ρW and ρW2C are the
cold electric specific resistances in the uncarburized core and the carburized
If the carbide layer is all of W2C composition, the value of ρW/ρW2C is 0.07, but ρW/ρW2C is experimentally about 0.3 . The operating temperature of the carburized filament is not directly determined , because the total emissivity of W2C has not been correctly found heretofore, so that using the actual data in the transmitting tube working, the following formula was theoretically determined.
where Rθ fθ is either the ratio of the hot-to-cold electric resistance in a Th-W filament carburized, or not respectively, and ρW(T), ρW2C(T) is ρW,ρW2C at operating temperature T respectively. At any time, the degree of carburization is obtained from the formula (1)by measuring the cold electric resistance and the operating temperature of the carburized Th-W filament is known from the formula (2). The expansion of filament by carburization is increasing as ThO2 content in W is decreasing, but the amount is negligibly small.
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