the latter being in a line at right angles to the plane of the wire
circle and passing through its centre. When the discharge is
established under proper conditions, a luminous hollow cone is formed,
and in the dark one-half of the brass sphere is strongly illuminated,
as shown in the cut.
By some artifice or other, it is easy to concentrate the streams upon
small surfaces and to produce very strong light effects. Two thin
wires may thus be rendered intensely luminous.
In order to intensify the streams the wires should be very thin and
short; but as in this case their capacity would be generally too small
for the coil--at least, for such a one as the present--it is necessary
to augment the capacity to the required value, while, at the same
time, the surface of the wires remains very small. This may be done in
many ways.
Here, for instance, I have two plates, RR, of hard rubber (Fig. 9),
upon which I have glued two very thin wires ww, so as to form a name.
The wires may be bare or covered with the best insulation--it is
immaterial for the success of the experiment. Well insulated wires, if
anything, are preferable. On the back of each plate, indicated by the
shaded portion, is a tinfoil coating tt. The plates are placed in line
at a sufficient distance to prevent a spark passing from one to the
other wire. The two tinfoil coatings I have joined by a conductor C,
and the two wires I presently connect to the terminals of the coil. It
is now easy, by varying the strength and frequency of the currents
through the primary, to find a point at which, the capacity of the
system is best suited to the conditions, and the wires become so
strongly luminous that, when the light in the room is turned off the
name formed by them appears in brilliant letters.
[Illustration: FIG. 9.--WIRES RENDERED INTENSELY LUMINOUS.]
It is perhaps preferable to perform this experiment with a coil
operated from an alternator of high frequency, as then, owing to the
harmonic rise and fall, the streams are very uniform, though they are
less abundant then when produced with such a coil as the present. This
experiment, however, may be performed with low frequencies, but much
less satisfactorily.
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