It is, 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