ipe till it
forms a bead just large enough to pass into the hole drilled up the
stem of the electrode. The junction between the stein and the
platinum wire is then made permanent by squeezing the aluminium down
upon the platinum wire with the help of a pair of pliers. It is also
possible to fuse the aluminium round the platinum, but as I have had
several breakages of such joints, I prefer the mechanical connection
described.

Fig. 39. Sets for striking aluminium electrodes

Fig. 40.

i. Aluminium electrode.

ii. Aluminium electrode connected to platinum wire.

iii. Aluminium electrode connected to platinum wire and protected by
glass.

iv. Detail of fastening platinum wire.

The stem and platinum wire may now be protected by covering them with
a little flint glass. For this purpose the flint-glass tube is pulled
down till it will just slip over the stem and wire, and is cut off so
as to leave about half a centimetre of platinum wire projecting. The
flint-glass tube is then fused down upon the platinum wire, care being
taken to avoid the presence of air bubbles. At the close of the
operation a single drop of white enamel glass is fused round the
platinum wire at a high temperature, so as to make a good joint with
the protecting flint-glass tube.

The negative electrode being nearly as large as the main tube, it must
be introduced before the latter is drawn down for sealing. After
drawing down the main tube in the usual manner, taking care not to
make it less than a millimetre in wall-thickness, it is cut off so as
to leave a hole not quite big enough for the enamel drop to pass
through. By heating and opening, the aperture is got just large
enough to allow the enamel drop to pass into it, and when this is the
case the joint is sealed, pulled, and blown out until the electrode
occupies the right position--viz. in the centre of the tube and with
its face normal to the axis of the tube.

The glass walls near the negative electrode must not be less than a
millimetre thick, and may be rather more with advantage, the glass
must be even, and the joint between the flint glass and the soda
glass, or between the wire and the soda glass, must be wholly through
the enamel. The "seal" must be well annealed. It will be found that
the sealing-in process is much easier when the stem of the electrode
is short and when the glass coating is not too heavy. Half a
millimetre of glass thickness round the stein is qui