Fig. 22 illustrates a similar arrangement, with a large tube T
protruding in to the part of the bulb containing the refractors button
m. In this case the wire leading from the outside into the bulb is
omitted, the energy required being supplied through condenser coatings
CC. The insulating packing P should in this construction be tightly
fitting to the glass, and rather wide, or otherwise the discharge
might avoid passing through the wire w, which connects the inside
condenser coating to the incandescent button m. The molecular
bombardment against the glass stem in the bulb is a source of great
trouble. As illustration I will cite a phenomenon only too frequently
and unwillingly observed. A bulb, preferably a large one, may be
taken, and a good conducting body, such as a piece of carbon, may be
mounted in it upon a platinum wire sealed in the glass stem. The bulb
may be exhausted to a fairly high degree, nearly to the point when
phosphorescence begins to appear.
[Illustration: FIG. 21.--IMPROVED BULB WITH NON-CONDUCTING BUTTON.]
[Illustration: FIG. 22.--TYPE OF BULB WITHOUT LEADING-IN WIRE.]
When the bulb is connected with the coil, the piece of carbon, if
small, may become highly incandescent at first, but its brightness
immediately diminishes, and then the discharge may break through the
glass somewhere in the middle of the stem, in the form of bright
sparks, in spite of the fact that the platinum wire is in good
electrical connection with the rarefied gas through the piece of
carbon or metal at the top. The first sparks are singularly bright,
recalling those drawn from a clear surface of mercury. But, as they
heat the glass rapidly, they, of course, lose their brightness, and
cease when the glass at the ruptured place becomes incandescent, or
generally sufficiently hot to conduct. When observed for the first
time the phenomenon must appear very curious, and shows in a striking
manner how radically different alternate currents, or impulses, of
high frequency behave, as compared with steady currents, or currents
of low frequency. With such currents--namely, the latter--the
phenomenon would of course not occur. When frequencies such as are
obtained by mechanical means are used, I think that the rupture of the
glass is more or less the consequence of the bombardment, which warms
it up and impairs its insulating power; but with frequencies
obtainable with condensers I have no doubt that the glass may give way
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