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Tuesday, February 27, 2007


Fig. 22 illustrates a similar arrangement, with a large tube T protruding into the part of the bulb containing the
refractory button m. In this case the wire leading from the outside into the bulb is omitted, the energy required
being supplied through condenser coatings C C The insulating packing P should in this construction be tightly
fitting to the glass, and rather wide, or otherwise the discharge Bright 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.
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 con-
nection 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 without previous heating.
Although this appears most singular at first, it is in reality what we might expect to occur. The energy supplied to
the wire leading into the bulb is given off partly by direct action through the carbon button, and partly by
inductive action through the glass surrounding the wire. The case is thus analogous to that in which a condenser
shunted by a conductor of low resistance is connected to a source of alternating currents. As long as the frequencies
are low" the conductor gets the most, and the condenser is perfectly safe; but when the frequency becomes
excessive, the role of the conductor may become quite insignificant. In the latter case the difference of potential at
the terminals of the condenser may become so great as to rupture the dielectric, notwithstanding the fact that the
terminals are joined by a conductor of low resistance.
It is, of course, not necessary, when it is desired to produce the incandescence of a body inclosed in a bulb by
means of these currents, that the body should be a conductor, for even a perfect non-conductor may be quite as
readily heated. For this purpose it is sufficient to surround a conducting electrode with a non-conducting material,
as, for instance, in the bulb described before in Fig. 21, in which a thin incandescent lamp filament is coated with a
non-conductor, and supports a button of the same material on the top. At the start the bombardment goes on by
inductive action through the non-conductor, until the same is sufficiently heated to become conducting, when the
bombardment continues in the ordinary way.

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