sealed off. The principal advantage of this
construction was that it allowed of reaching extremely high vacua,
and, at the same time use a large bulb. It was found, in the course of
experiences with bulbs such as illustrated in Fig. 29, that it was
well to make the stem s near the seal at e very thick, and the
leading-in wire w thin, as it occurred sometimes that the stem at e
was heated and the bulb was cracked. Often the outer globe L was
exhausted only just enough to allow the discharge to pass through, and
the space between the bulbs appeared crimson, producing a curious
effect. In some cases, when the exhaustion in globe L was very low,
and the air good conducting, it was found necessary, in order to bring
the button m to high incandescence, to place, preferably on the upper
part of the neck of the globe, a tinfoil coating which was connected
to an insulated body, to the ground, or to the other terminal of the
coil, as the highly conducting air weakened the effect somewhat,
probably by being acted upon inductively from the wire w, where it
entered the bulb at e. Another difficulty--which, however, is always
present when the refractory button is mounted in a very small
bulb--existed in the construction illustrated in Fig. 29, namely, the
vacuum in the bulb b would be impaired in a comparatively short time.

[Illustration: FIG. 29.--LAMP WITH INDEPENDENT AUXILIARY BULB.]

The chief idea in the two last described constructions was to confine
the heat to the central portion of the globe by preventing the
exchange of air. An advantage is secured, but owing to the heating of
the inside bulb and slow evaporation of the glass the vacuum is hard
to maintain, even if the construction illustrated in Fig. 28 be
chosen, in which both bulbs communicate.

But by far the better way--the ideal way--would be to reach
sufficiently high frequencies. The higher the frequency the slower
would be the exchange of the air, and I think that a frequency may be
reached at which there would be no exchange whatever of the air
molecules around the terminal. We would then produce a flame in which
there would be no carrying away of material, and a queer flame it
would be, for it would be rigid! With such high frequencies the
inertia of the particles would come into play. As the brush, or flame,
would gain rigidity in virtue of the inertia of the particles, the
exchange of the latter would be prevented. This would necessarily
occur, for, the nu