Like
the first it is a closed cylinder of glass, with a central indentation
forming a kind of hanging pocket and almost dividing the tube into two
compartments. This pocket, silvered on the air side, forms a hollow
glass diaphragm that can be connected electrically from the outside,
forming the negative pole, A; the two ends of the tube, also outwardly
silvered, form the positive poles, B B. I pass the current, and you
will see the dark space distinctly visible. The pressure here is 0.076
millimeter, or 100 M. The next stage, dealing with more rarefied
matter, is that of phosphorescence. Here is an egg-shaped bulb, shown
in Fig 19, containing some pure yttria and a few rough rubies. The
positive electrode, B, is on the bottom of the tube under the
phosphorescent material; the negative, A, is on the upper part of the
tube. See how well the rubies and yttria phosphorescence shows under
molecular bombardment, at an internal pressure of 0.00068 millimeter,
or 0.9 M.
[Illustration: FIG. 18.--PRESSURE = 0.076 MM. = 100 M.]
A shadow of an object inside a bulb can also be projected on to the
opposite wall of the bulb by means of an outside pole. A mica cross is
supported in the middle of the bulb (Fig. 20), and on connecting a
small silvered patch, A, on one side of the bulb with the negative
pole of the induction coil, and putting the positive pole to another
patch of silver, B, at the top, the opposite side of the bulb glows
with a phosphorescent light, on which the black shadow of the cross
seems sharply cut out.
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