joint of extraneous material such as sealing frit between them the plug portion includes a generally cylindrical pedestal portion 7 rising up from the region of commonality with the wall and defining with the wall an annular chamber or compartment 8 for holding unvaporized excess sodium mercury amalgam shown at 9 the ceramic tube 2 with integral apertured plug 5 is made into an arc tube by first sealing into the aperture an inlead electrode assembly comprising niobium wire 10 to which an electrode 11 is attached by a weld knot 12 the electrode conventionally comprises a tungsten shank 13 having one or more layers of tungsten wire 14 coiled around it and retaining an electron emissive material such as barium calcium tungstate ba 2 cawo 3 between turns the niobium wire is upset at 15 to provide a shoulder which serves to locate the electrode with respect to the top of the pedestal the lower electrode inlead assembly is sealed in while the tube is held with the plugged end up that is upside down relative to the illustration of fig2 a cross wire 16 is spot welded to the niobium inlead wire to retain it in place and prevent it from falling out during sealing the sealing frit or glass may be provided as a powder surrounding inlead wire 10 where it comes out of the plugged end of the tube or preferably in the form of a washer of pressed powder which is threaded over the projecting portion of the wire.