of course, not necessary, when it is desired to produce the
incandescence of a body inclosed in a bulb by means of these currents,
that the body should be a conductor, for even a perfect non-conductor
may be quite as readily heated. For this purpose it is sufficient to
surround a conducting electrode with a non-conducting material, as,
for instance, in the bulb described before in Fig. 21, in which a thin
incandescent lamp filament is coated with a non-conductor, and
supports a button of the same material on the top. At the start the
bombardment goes on by inductive action through the non-conductor,
until the same is sufficiently heated to become conducting, when the
bombardment continues in the ordinary way.

A different arrangement used in some of the bulbs constructed is
illustrated in Fig. 23. In this instance a non-conductor m is
mounted in a piece of common arc light carbon so as to project some
small distance above the latter. The carbon piece is connected to the
leading-in wire passing through a glass stem, which is wrapped with
several layers of mica. An aluminium tube a is employed as usual for
screening. It is so arranged that it reaches very nearly as high as
the carbon and only the non-conductor m projects a little above it.
The bombardment goes at first against the upper surface of carbon, the
lower parts being protected by the aluminium tube. As soon, however,
as the non-conductor m is heated it is rendered good conducting, and
then it becomes the centre of the bombardment, being most exposed to
the same.

I have also constructed during these experiments many such single-wire
bulbs with or without internal electrode, in which the radiant matter
was projected against, or focused upon, the body to be rendered
incandescent. Fig. 24 illustrates one of the bulbs used. It consists
of a spherical globe L, provided with a long neck n, on the top, for
increasing the action in some cases by the application of an external
conducting coating. The globe L is blown out on the bottom into a very
small bulb b, which serves to hold it firmly in a socket S of
insulating material into which it is cemented. A fine lamp filament f,
supported on a wire w, passes through the centre of the globe L. The
filament is rendered incandescent in the middle portion, where the
bombardment proceeding from the lower inside surface of the globe is
most intense. The lower portion of the globe, as far as the socket S
reaches, is rendered c