[Illustration: FIG. 10.--LUMINOUS DISCS.]
When two wires, attached to the terminals of the coil, are set at the
proper distance, the streams between them may be so intense as to
produce a continuous luminous sheet. To show this phenomenon I have
here two circles, C and c (Fig. 10), of rather stout wire, one being
about 80 centimetres and the other 30 centimetres in diameter. To each
of the terminals of the coil I attach one of the circles. The
supporting wires are so bent that the circles may be placed in the
same plane, coinciding as nearly as possible. When the light in the
room is turned off and the coil set to work, you see the whole space
between the wires uniformly filled with streams, forming a luminous
disc, which could be seen from a considerable distance, such is the
intensity of the streams. The outer circle could have been much larger
than the present one; in fact, with this coil I have used much larger
circles, and I have been able to produce a strongly luminous sheet,
covering an area of more than one square metre, which is a remarkable
effect with this very small coil. To avoid uncertainty, the circle has
been taken smaller, and the area is now about 0.43 square metre.
The frequency of the vibration, and the quickness of succession of the
sparks between the knobs, affect to a marked degree the appearance of
the streams. When the frequency is very low, the air gives way in more
or less the same manner, as by a steady difference of potential, and
the streams consist of distinct threads, generally mingled with thin
sparks, which probably correspond to the successive discharges
occurring between the knobs. But when the frequency is extremely high,
and the arc of the discharge produces a very _loud_ but _smooth_
sound--showing both that oscillation takes place and that the sparks
succeed each other with great rapidity--then the luminous streams
formed are perfectly uniform. To reach this result very small coils
and jars of small capacity should be used. I take two tubes of thick
Bohemian glass, about 5 centimetres in diameter and 20 centimetres
long. In each of the tubes I slip a primary of very thick copper wire.
On the top of each tube I wind a secondary of much thinner
gutta-percha covered wire. The two secondaries I connect in series,
the primaries preferably in multiple arc. The tubes are then placed in
a large glass vessel, at a distance of 10 to 15 centimetres from each
other, on insulating supports, and the vessel is filled with boiled
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