t is made to flow through the
secondary, though its terminals are insulated from each other. To one
entirely unacquainted with the properties of alternating currents
nothing will look more puzzling. This feature was illustrated in the
experiment performed at the beginning with the top plates of wire
gauze attached to the terminals and the rubber plate. When the plates
of wire gauze were close together, and a small arc passed between
them, the arc _prevented_ a strong current from passing through the
secondary, because it did away with the capacity on the terminals;
when the rubber plate was inserted between, the capacity of the
condenser formed counteracted the self-induction of the secondary, a
stronger current passed now, the coil performed more work, and the
discharge was by far more powerful.

The first thing, then, in operating the induction coil is to combine
capacity with the secondary to overcome the self-induction. If the
frequencies and potentials are very high gaseous matter should be
carefully kept away from the charged surfaces. If Leyden jars are
used, they should be immersed in oil, as otherwise considerable
dissipation may occur if the jars are greatly strained. When high
frequencies are used, it is of equal importance to combine a condenser
with the primary. One may use a condenser connected to the ends of the
primary or to the terminals of the alternator, but the latter is not
to be recommended, as the machine might be injured. The best way is
undoubtedly to use the condenser in series with the primary and with
the alternator, and to adjust its capacity so as to annul the
self-induction of both the latter. The condenser should be adjustable
by very small steps, and for a finer adjustment a small oil condenser
with movable plates may be used conveniently.

I think it best at this juncture to bring before you a phenomenon,
observed by me some time ago, which to the purely scientific
investigator may perhaps appear more interesting than any of the
results which I have the privilege to present to you this evening.

It may be quite properly ranked among the brush phenomena--in fact, it
is a brush, formed at, or near, a single terminal in high vacuum.

In bulbs provided with a conducting terminal, though it be of
aluminium, the brush has but an ephemeral existence, and cannot,
unfortunately, be indefinitely preserved in its most sensitive state,
even in a bulb devoid of any conducting electrode. In study