The induction coils operated from the machine are rather small,
containing from 5,000 to 15,000 turns in the secondary. They are
immersed in boiled-out linseed oil, contained in wooden boxes covered
with zinc sheet.
I have found it advantageous to reverse the usual position of the
wires, and to wind, in these coils, the primaries on the top; this
allowing the use of a much bigger primary, which, of course, reduces
the danger of overheating and increases the output of the coil. I make
the primary on each side at least one centimetre shorter than the
secondary, to prevent the breaking through on the ends, which would
surely occur unless the insulation on the top of the secondary be very
thick, and this, of course, would be disadvantageous.
When the primary is made movable, which is necessary in some
experiments, and many times convenient for the purposes of adjustment,
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
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