| 摘要 |
To protect a niobium tube (9) forming a current supply lead through a through-opening (14) in an end plug (10) in a ceramic discharge vessel against attack by halides or condensed sodium, the through-opening is formed in two portions, of different diameters. The outer portion (16), remote from the discharge vessel, has a diameter which is matched to the diameter of the tube (9), leaving only a capillary between the plug opening and the tube; the second, inner portion (17) is narrower than the first one, that is, outer portion, and surrounds the shaft of the electrode by a sufficient distance to permit expansion of the electrode, typically of tungsten, under operating conditions, without cracking the plug by forced engagement with the wall of the inner portion (17) of the aperture. A sealing glass, melt-sealed into the capillary of the outer portion, seals the niobium tube. Preferably, the niobium tube terminates inwardly in a dome-shaped end (20) seated in a similarly shaped cavity forming the transition between the two diameters of the through-opening, and also providing a capillary space for ingress of molten sealing glass. The inner diameter of the outer portion (16) of the through-opening differs, preferably, by at most about 0.05 mm from the outside diameter of the tube (9), whereas the diameter of the inner portion (17) of the through-opening (14) differs by at least 0.5 mm from the diameter of the electrode shaft.
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