Internal Seals.—It is convenient to class those cases in which a glass tube passes through the wall of another tube or bulb under the heading of "Internal Seals." These are met with in barometers, spray arresters, and filter pumps, in condensers and some forms of vacuum tube. The two principal methods of making such seals will be considered first and their special application afterwards.
An Air Trap on a Barometer Tube.—This involves the use of the first method, and is perhaps the simplest example that can be given. Fig. 9, a, a1 and a2, show the stages by which this form of internal seal is made. For the first trials, it is well to work with fairly thick-walled tubing, which should be cut into two pieces, each being about eight inches long.
Fig. 9
First seal the end of one tube as described on page 13, heat the sealed end and expand to a thick walled bulb. Fuse the end of the other tube, attach a piece of glass rod to serve as a handle, and draw out; cut off the drawn-out portion: leaving an end like a.
Now heat a small spot at the end of the bulb, blow, burst out, and remove the thin fragments of glass. Heat a zone on the other tube at the point where the drawn-out portion commences and expand as shown by a1.
The next stage is to join the tubes. Heat the ragged edges of the burst-out portion until they are thoroughly rounded. At the same time heat the drawn-out tube to just below softening point. Then, while the rounded edges of the burst-out portion are still soft, insert the other tube; rotate the join in the blowpipe flame until it is quite soft, and expand by blowing. If necessary, re-heat and expand again. The finished seal, which should be slightly annealed by smoking in a sooty flame, is shown by a2.
A Spray Arrester.—This is made by the second method, in which the piece of tube which projects inside the bulb is fused in position first and the outer tube is then joined on. The various stages of making are illustrated by b, b1 and b2, Fig. 9.
A bulb is blown between two tubes by the method given on page 22, the larger tube is then cut off and the small piece of tube introduced into the bulb after having been shaped as shown in by b, Fig. 9. The opening in the bulb is sealed as shown by b1. The sealed part is now heated and the bulb inclined downwards until the inner tube comes in contact with the seal and is fused in position. This operation requires some practice in order to prevent the inner tube either falling through the soft glass or becoming unsymmetrical. The end of the bulb, where the inner tube comes in contact with it, is now perforated by heating and blowing, thus giving the form shown by b2, and the outer tube is joined on. The finished spray arrester is shown by b3. Practice alone will give the power to produce a symmetrical and stable piece of work.
Two Forms of Filter Pump.—That illustrated by d, Fig. 9, is made by the method explained under "An Air Trap on a Barometer Tube." That illustrated by c is made by the method explained under "A Spray Arrester." No new manipulation is involved, and the construction should be clear from a study of the drawings.