without previous heating. Although this appears most singular at
first, it is in reality what we might expect to occur. The energy
supplied to the wire leading into the bulb is given off partly by
direct action through the carbon button, and partly by inductive
action through the glass surrounding the wire. The case is thus
analogous to that in which a condenser shunted by a conductor of low
resistance is connected to a source of alternating currents. As long
as the frequencies are low, the conductor gets the most, and the
condenser is perfectly safe: but when the frequency becomes excessive,
the _role_ of the conductor may become quite insignificant. In the
latter case the difference of potential at the terminals of the
condenser may become so great as to rupture the dielectric,
notwithstanding the fact that the terminals are joined by a conductor
of low resistance.
[Illustration: FIG. 23.--EFFECT PRODUCED BY A RUBY DROP.]
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
52