I cover the secondary with wax, and turn it off in a lathe to a
diameter slightly smaller than the inside of the primary coil. The
latter I provide with a handle reaching out of the oil, which serves
to shift it in any position along the secondary.

I will now venture to make, in regard to the general manipulation of
induction coils, a few observations bearing upon points which have not
been fully appreciated in earlier experiments with such coils, and are
even now often overlooked.

The secondary of the coil possesses usually such a high self-induction
that the current through the wire is inappreciable, and may be so even
when the terminals are joined by a conductor of small resistance. If
capacity is added to the terminals, the self-induction is
counteracted, and a stronger current 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