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.

[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 d