I have obtained in commerce two kinds of gutta-percha wire: in one the
insulation sticks tightly to the metal, in the other it does not.
Unless a special method is followed to expel all air, it is much safer
to use the first kind. I wind the coil within an oil tank so that all
interstices are filled up with the oil. Between the layers I use cloth
boiled out thoroughly in oil, calculating the thickness according to
the difference of potential between the turns. There seems not to be a
very great difference whatever kind of oil is used; I use paraffine or
linseed oil.
To exclude more perfectly the air, an excellent way to proceed, and
easily practicable with small coils, is the following: Construct a box
of hard wood of very thick boards which have been for a long time
boiled in oil. The boards should be so joined as to safely withstand
the external air pressure. The coil being placed and fastened in
position within the box, the latter is closed with a strong lid, and
covered with closely fitting metal sheets, the joints of which are
soldered very carefully. On the top two small holes are drilled,
passing through the metal sheet and the wood, and in these holes two
small glass tubes are inserted and the joints made air-tight. One of
the tubes is connected to a vacuum pump, and the other with a vessel
containing a sufficient quantity of boiled-out oil. The latter tube
has a very small hole at the bottom, and is provided with a stopcock.
When a fairly good vacuum has been obtained, the stopcock is opened
and the oil slowly fed in. Proceeding in this manner, it is impossible
that any big bubbles, which are the principal danger, should remain
between the turns. The air is most completely excluded, probably
better than by boiling out, which, however, when gutta-percha coated
wires are used, is not practicable.
For the primaries I use ordinary line wire with a thick cotton
coating. Strands of very thin insulated wires properly interlaced
would, of course, be the best to employ for the primaries, but they
are not to be had.
In an experimental coil the size of the wires is not of great
importance. In the coil here used the primary is No. 12 and the
secondary No. 24 Brown & Sharpe gauge wire; but the sections may be
varied considerably. It would only imply different adjustments; the
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