cracking of the neck by the rapid heating of the aluminium tube upon a
sudden turning on of the current. The inside bulb should be as small
as possible when it is desired to obtain light only by incandescence
of the electrode. If it is desired to produce phosphorescence, the
bulb should be larger, else it would be apt to get too hot, and the
phosphorescence would cease. In this arrangement usually only the
small bulb shows phosphorescence, as there is practically no
bombardment against the outer globe. In some of these bulbs
constructed as illustrated in Fig. 28 the small tube was coated with
phosphorescent paint, and beautiful effects were obtained. Instead of
making the inside bulb large, in order to avoid undue heating, it
answers the purpose to make the electrode m larger. In this case the
bombardment is weakened by reason of the smaller electric density.
Many bulbs were constructed on the plan illustrated in Fig. 29. Here a
small bulb b, containing the refractory button m, upon being exhausted
to a very high degree was sealed in a large globe L, which was then
moderately exhausted and 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.]
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