A handbook of laboratory glass-blowing

by Bernard D. Bolas

Fig. 11

The syphon tube is now joined on to the large tube as shown by a, Fig. 11, care being taken to seal the other end of the syphon tube before joining. The details of the final and re-entrant joint of the syphon tube are shown at the lower part of a. This join is made by expanding the sealed end of the syphon tube into a small, thick-walled bulb, and the bottom of this bulb is burst out by local heating and blowing; the fragments of glass are removed and the edges made smooth by melting. A similar operation is carried out on the side of the tube to which the syphon tube is to be joined. This stage is shown by a. Now heat the syphon tube at the upper bend until it is flexible, and press the bulb at its end into the opening on the side of the other tube. Hold the glass thus until the syphon is no longer flexible. The final join is made by heating the two contacting surfaces, if necessary pressing the edges in contact with the end of a turn-pin, fusing together and expanding. The finished apparatus is shown by c.

Electrodes.—A thin platinum wire may be sealed into a capillary tube without any special precautions being necessary. The capillary tube may be drawn out from the side of a larger tube by heating a spot on the glass, touching with a glass rod and drawing the rod away; or the exhaustion branch described on page 18 may be used for the introduction of an electrode. It is convenient sometimes to carry out the exhaustion through the same tube that will afterwards serve for the electrode. The electrode wire is laid inside the branch before connecting to the exhaustion pump. When exhaustion is completed the tube is heated until the soft glass flows round the platinum and makes the seal air-tight. The branch is now cut off close to the seal on the pump side, a loop is made in the projecting end of the platinum wire, and the seal is finished by melting the cut-off end.

Platinum is usually employed for such work, but if care is taken to avoid oxidation it is not impossible to make fairly satisfactory seals with clean iron or nickel wire. Hard rods of fine graphite, such as are used in some pencils, may also be sealed into glass, but it seems probable that air would diffuse through the graphite in the course of time.

Another method for the introduction of an electrode is illustrated by d, e, f and g, Fig. 11. In this case the bulb or thin-walled tube into which the electrode is to be sealed is perforated by a quick stab with an intensely heated wire—preferably of platinum—which is then withdrawn before the glass has had time to harden, and thus a minute circular hole is made. The electrode is coated with a layer of similar glass, or of the specially made enamel which is sold for this purpose, inserted into the bulb or tube by any convenient opening, and adjusted by careful shaking until the platinum wire projects through the small hole. The bulb or tube is then fused to the coating of the electrode and the whole spot expanded slightly by blowing. The appearance of the finished seal is shown by g. It is well to anneal slightly by smoking.

Thermometers.—Apart from the notes on page 20 with respect to the blowing of a suitable bulb on capillary tubing there is little to say in connection with the glass working needed in making a plain thermometer. The size desirable for the bulb will be determined by the bore of the capillary tube, the coefficient of expansion of the liquid used for filling, and the range of temperature for which the thermometer is intended.

Filling may be carried out as follows:—Fit a small funnel to the open end of the capillary by means of a rubber tube, and pour into the funnel rather more than enough of the liquid to be used than is required to fill the bulb. Mercury or alcohol will be used in practice, most probably. Warm the bulb until a few air bubbles have escaped through the liquid and then allow to cool. This will suck a certain amount of liquid into the bulb. Now heat the bulb again, and at the same time heat the capillary tube over a second burner. The liquid will boil and sweep out the residual air, but it is necessary to heat the capillary tube as well in order to prevent condensation. Allow the bulb and tube to cool, then repeat the heating once more. By this time the bulb and tube should be free from air, and cooling should give a completely filled thermometer. Remove the funnel and heat the thermometer to a few degrees above the maximum temperature for which it is to be used; the mercury or other filling liquid will overflow from the top, and, as the temperature falls, will recede, thus allowing the end of the capillary to be drawn out. Reheat again until the liquid rises to the top of the tube, then seal by means of the blowpipe flame. The thermometer is now finished except for graduation; this is dealt with on page 75.

An Alarm Thermometer.—A thermometer which will complete an electric circuit when a certain temperature is reached may be made by sealing an electrode in the bulb and introducing a wire into the top, which in this case is not sealed. Naturally, this thermometer will be filled with mercury. There is considerable difficulty in filling such a bulb without causing it to crack.

Several elaborations of this form are made, in which electrodes are sealed through the walls of the capillary tube, thus making it possible to detect electrically the variation of temperature when it exceeds any given limits.