es not seem to be the case with oil. Perhaps it is
because the dielectric losses in the liquid are smaller.

I have tried at first silk and cotton covered wires with oil
immersion, but I have been gradually led to use gutta-percha covered
wires, which proved most satisfactory. Gutta-percha insulation adds,
of course, to the capacity of the coil, and this, especially if the
coil be large, is a great disadvantage when extreme frequencies are
desired; but on the other hand, gutta-percha will withstand much more
than an equal thickness of oil, and this advantage should be secured
at any price. Once the coil has been immersed, it should never be
taken out of the oil for more than a few hours, else the gutta-percha
will crack up and the coil will not be worth half as much as before.
Gutta-percha is probably slowly attacked by the oil, but after an
immersion of eight to nine months I have found no ill effects.

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 slo