CHAPTER IV.
MAKING THISTLE FUNNELS, U-TUBES, ETC.—COMBINING THE PARTS OF COMPLICATED APPARATUS—MERCURY, AND OTHER AIR-TIGHT JOINTS—VACUUM TAPS—SAFETY TAPS—AIR-TRAPS.
In Chapter III. the simpler operations used in making the separate parts of which apparatus is composed have been described. In this Chapter finished apparatus will be described, and the combination of the separate parts into the more or less complicated arrangements used in experiments will be so far explained as to enable the student to set up such apparatus as he is likely to require. I have thought it would be useful that I should add a short account of various contrivances that have come much into use of late years for experimenting under reduced pressure, such as safety taps, air-traps, vacuum joints, etc.
Fig. 24.
Electrodes.—On [page 38] ([Fig. 13]) is shown a simple form of electrode sealed into a glass tube, which for many purposes answers very well. But frequently, in order that there may be less risk of leakage between the glass and the metal, the latter is covered for a considerable part of its length with solid glass, which at one extremity is united to the apparatus. In [Fig. 24] W is the metal core of the electrode, and G the glass covering around it. The wire is fused into the glass, and the glass is then united to the apparatus; a little white enamel should be applied at one end and combined with the glass by fusion.
U-Tubes.—A U-tube is but a particular case of a bent glass tube. It is scarcely possible when bending very large tubes in the manner described on [p. 29] to produce regular curves of sufficient strength.
To make a U-tube, or to bend a large tube, close one end of the tube selected with a cork, soften and compress the glass in the flame at the part where it is to be bent till a sufficient mass of glass for the bend is collected, then remove the mass of glass from the flame, let it cool a little, and simultaneously draw out the thickened glass, bend it to the proper form, and blow the bend into shape from the open end of the tube. Small irregularities may be partly corrected afterwards.