| 摘要 |
PROBLEM TO BE SOLVED: To reduce scattering due to the sputtering of a tungsten in case of abnormal lead heat of a filament at the end of service life, so as to prevent current-carrying between a pair of internal lead wires by forming a part of the filament provided on a pair of internal lead wires inside a light emitting tube of a tungsten alloy having a specified fusing point. SOLUTION: A pair of internal lead wires are made to pass airtightly through the sealed end part of a light emitting tube, and the end parts of the paried internal lead wires inside the light emitting tube are provided with a filament 4 made by a tungsten wire 4a. An alloy part 5 made of nickel and tungsten is provided on a part of the tungsten wire 4a, the fusing point of which is adjusted to 1,500-2,000 deg.C by the alloy proportion of nickel and tungsten, and provided at a suitable position, and electron emitting material such as barium oxide or the like is applied to the filament 4. Thus, even if the filament 4 is heated red abnormally at the end of its service life, the alloy part 5 is fused earlier and then the filament 4 is broken.
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