n 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.

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 rad