the discharge phenomena observed, the most pleasing to the
eye, and the most instructive, are those observed with a coil which is
operated by means of the disruptive discharge of a condenser. The
power of the brushes, the abundance of the sparks, when the conditions
are patiently adjusted, is often amazing. With even a very small coil,
if it be so well insulated as to stand a difference of potential of
several thousand volts per turn, the sparks may be so abundant that
the whole coil may appear a complete mass of fire.

Curiously enough the sparks, when the terminals of the coil are set at
a considerable distance, seem to dart in every possible direction as
though the terminals were perfectly independent of each other. As the
sparks would soon destroy the insulation it is necessary to prevent
them. This is best done by immersing the coil in a good liquid
insulator, such as boiled-out oil. Immersion in a liquid may be
considered almost an absolute necessity for the continued and
successful working of such a coil.

It is of course out of the question, in an experimental lecture, with
only a few minutes at disposal for the performance of each experiment,
to show these discharge phenomena to advantage, as to produce each
phenomenon at its best a very careful adjustment is required. But even
if imperfectly produced, as they are likely to be this evening, they
are sufficiently striking to interest an intelligent audience.

Before showing some of these curious effects I must, for the sake of
completeness, give a short description of the coil and other apparatus
used in the experiments with the disruptive discharge this evening.

[Illustration: FIG. 3.--DISRUPTIVE DISCHARGE COIL.]

It is contained in a box B (Fig. 3) of thick boards of hard wood,
covered on the outside with zinc sheet Z, which is carefully soldered
all around. It might be advisable, in a strictly scientific
investigation, when accuracy is of great importance, to do away with
the metal cover, as it might introduce many errors, principally on
account of its complex action upon the coil, as a condenser of very
small capacity and as an electrostatic and electromagnetic screen.
When the coil is used for such experiments as are here contemplated,
the employment of the metal cover offers some practical advantages,
but these are not of sufficient importance to be dwelt upon.

The coil should be placed symmetrically to the metal cover, and the
space between should