for the connection to the leading-in wire w. This terminal must be
well insulated from the metal tube S, therefore, if the cement used is
conducting--and most generally it is sufficiently so--the space
between the plug P and the neck of the bulb should be filled with some
good insulating material, as mica powder.

[Illustration: FIG. 20.--BULB FOR EXPERIMENTS WITH CONDUCTING TUBE.]

Fig. 20 shows a bulb made for experimental purposes. In this bulb the
aluminium tube is provided with an external connection, which serves
to investigate the effect of the tube under various conditions. It is
referred to chiefly to suggest a line of experiment followed.

Since the bombardment against the stem containing the leading-in wire
is due to the inductive action of the latter upon the rarefied gas, it
is of advantage to reduce this action as far as practicable by
employing a very thin wire, surrounded by a very thick insulation of
glass or other material, and by making the wire passing through the
rarefied gas as short as practicable. To combine these features I
employ a large tube T (Fig. 21), which protrudes into the bulb to some
distance, and carries on the top a very short glass stem s, into which
is sealed the leading-in wire w, and I protect the top of the glass
stem against the heat by a small, aluminium tube a and a layer of mica
underneath the same, as usual. The wire w, passing through the large
tube to the outside of the bulb, should be well insulated--with a
glass tube, for instance--and the space between ought to be filled out
with some excellent insulator. Among many insulating powders I have
tried, I have found that mica powder is the best to employ. If this
precaution is not taken, the tube T, protruding into the bulb, will
surely be cracked in consequence of the heating by the brushes which