On Laboratory Arts

by Richard Threlfall

More intense effects are often got by making the cathode slightly concave, but in this case the risk of melting the thin glass is considerably increased. No doubt, Bohemian glass might be used throughout instead of soft soda glass, and this would not melt so easily; the difficulties of manipulating the glass are, however, more pronounced.

It will be shown directly that the best Roentgen effects are got with a high vacuum, and it is for this reason that the glass near the cathode seal requires to be strong. The potential right up to the cathode is strongly positive inside the tube, and this causes the glass to be exposed to a strong electric stress in the neighbourhood of the seal.

Although the GLASS-BLOWING involved in the making of a so-called focus tube is rather more difficult than in the case just described, there is no reason why such a difficulty should not be overcome; I will therefore explain how a focus tube may be made.

Fig.

41.

A bulb about 3 inches in diameter is blown from a bit of tube of a little more than 1 inch diameter. Unless the walls of the tube are about one-eighth of an inch in thickness, this will involve a preliminary thickening up of the glass. This is not difficult if care be taken to avoid making the glass too hot. The larger gas jet described in connection with the soda-GLASS-BLOWING table must be employed. In blowing a bulb of this size it must not be forgotten that draughts exercise a very injurious influence by causing the glass to cool unequally; this leads to bulbs of irregular shape.

In the method of construction shown in Fig. 41, the anode is put in first. This anode simply consists of a square bit of platinum or platinum-iridium foil, measuring about 0.75 inch by 1 inch, and riveted on to a bent aluminium wire stem.

As soon as the anode is fused in, and while the glass is still hot, the side tube is put on. The whole of the anode end is then carefully annealed. When the annealing is finished the side tube is bent as shown to serve as a handle when the time comes to mount the cathode. Before placing the cathode in position, and while the main tube is still wide open, the anode is adjusted by means of a tool thrust in through this open end. This is necessary in view of the fact that the platinum foil is occasionally bent during the operation of forcing the anode into the bulb.

The cathode is a portion of a spherical surface of polished aluminium, a mode of preparing which will be given directly. The cathode having been placed inside the bulb, the wide glass tube is carefully drawn down and cut off at such a point that when the cathode is in position its centre of curvature will lie slightly in front of the anode plate. For instance, if the radius of curvature of the cathode be 1.5 inches, the centre of curvature may lie something like an eighth of an inch or less in front of the anode.