The cathode as shown in Fig. 41 is rather smaller than is advantageous. To make it much larger than is shown, however, the opening into the bulb would require to be considerably widened, and though this is not really a difficult operation, still it requires more practice than my readers are likely to have had. The difficulty is not so much in widening out the entry as in closing it down again neatly.
Now as to making the anode. A disc of aluminium is cut from a sheet which must not be too thick — one twenty-fifth of an inch is quite thick enough. This disc is bored at the centre to allow of the stem being riveted in position. The disc is then annealed in the Bunsen flame and the stem riveted on.
The curvature is best got by striking between steel dies (see Figs. 39 and 40). Two bits of tool steel are softened and turned on the lathe, one convex and the other concave. The concave die has a small hole drilled up the centre to admit the stem. The desired radius of curvature is easily attained by cutting out templates from sheet zinc and using them to gauge the turning. The two dies are slightly ground together on the lathe with emery and oil and are then polished, or rather the convex die is polished — the other one does not matter. The polishing is most easily done by using graded emery and oil and polishing with a rag. The method of grading emery will be described in the chapter on glass-grinding.
The aluminium disc is now struck between the dies by means of a hammer. If the radius of curvature is anything more than one inch and the disc not more than one inch in diameter the cathode can be struck at once from the flat as described. For very deep curves no doubt it will be better to make an intermediate pair of dies and to re-anneal the aluminium after the first striking.
When the tube is successfully prepared so far as the glassblowing goes it may be rinsed with strong pure alcohol both inside and out, and dried. The straight part of the side tube is then constricted ready for fusing off and the whole affair is placed on the vacuum pump.
In spite of the great improvements made during recent years in the construction of so-called Geissler vacuum pumps — i.e. pumps in which a Torricellian vacuum is continually reproduced — I am of opinion that Sprengel pumps are, on the whole, more convenient for exhausting Crooke's tubes. A full discussion of the subject of vacuum pumps will be found in a work by Mr. G. S. Ram (The Incandescent Lamp and its Manufacture), published by the Electrician Publishing Company, and it is not my intention to deal with the matter here; the simplest kind of Sprengel pump will be found quite adequate for our purpose, provided that it is well made.
Fig. 42 is intended to represent a modification of a pump based on the model manufactured by Hicks of Hatton Garden, and arranged to suit the amateur glass-blower. The only point of importance is the construction of the head of the fall tube, of which a separate and enlarged diagram is given. The fall tubes may have an internal diameter up to 2 mm. (two millimetres) and an effective length of 120 cm.
Free use is made of rubber tube connections in the part of the pump exposed to the passage of mercury. The rubber employed should be black and of the highest quality, having the walls strengthened by a layer of canvas. If such tube cannot be easily obtained, a very good substitute may be made by placing a bit of ordinary black tube inside another and rather larger bit and binding the outer tube with tape or ribbon. In any case the tubing which comes in contact with the mercury should be boiled in strong caustic potash or soda solution for at least ten minutes to get rid of free sulphur, which fouls the mercury directly it comes in contact with it. The tubing is well washed, rinsed with alcohol, and carefully dried.
Fig. 42.