When a coil is operated with currents of very high frequency,
beautiful brush effects may be produced, even if the coil be of
comparatively small dimensions. The experimenter may vary them in
many ways, and, if it were nothing else, they afford a pleasing sight.
What adds to their interest is that they may be produced with one
single terminal as well as with two--in fact, often better with one
than with two.

But of all 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