coatings, C and C_1 respectively, and is filled as far as the coatings
reach with conducting, and the space above with insulating powder.
These coatings are merely used to enable me to perform two experiments
with the tube--namely, to produce the effect desired either by direct
connection of the body of the experimenter or of another body to the
wire w, or by acting inductively through the glass. The stem s is
provided with an aluminium tube a, for purposes before explained, and
only a small part of the filament reaches out of this tube. By holding
the tube T_1 anywhere in the electrostatic field the filament is
rendered incandescent.
[Illustration: FIG. 34.--TUBE WITH FILAMENT RENDERED INCANDESCENT IN
AN ELECTROSTATIC FIELD.]
[Illustration: FIG. 35.--CROOKES' EXPERIMENT IN ELECTROSTATIC FIELD.]
A more interesting piece of apparatus is illustrated in Fig. 35. The
construction is the same as before, only instead of the lamp filament
a small platinum wire p, sealed in a stem s, and bent above it in a
circle, is connected to the copper wire w, which is joined to an
inside coating C. A small stem s_1 is provided with a needle, on the
point of which is arranged to rotate very freely a very light fan of
mica v. To prevent the fan from falling out, a thin stem of glass g is
bent properly and fastened to the aluminium tube. When the glass tube
is held anywhere in the electrostatic field the platinum wire becomes
incandescent, and the mica vanes are rotated very fast.
Intense phosphorescence may be excited in a bulb by merely connecting
it to a plate within the field, and the plate need not be any larger
than an ordinary lamp shade. The phosphorescence excited with these
currents is incomparably more powerful than with ordinary apparatus. A
small phosphorescent bulb, when attached to a wire connected to a
coil, emits sufficient light to allow reading ordinary print at a
distance of five to six paces. It was of interest to see how some of
the phosphorescent bulbs of Professor Crookes would behave with these
currents, and he has had the kindness to lend me a few for the
occasion. The effects produced are magnificent, especially by the
sulphide of calcium and sulphide of zinc. From the disruptive
discharge coil they glow intensely merely by holding them in the hand
and connecting the body to the terminal of the coil.
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