were used to protect the electrode, or to economize the energy by
confining the heat. But as such means are not at disposal, it becomes
necessary to place the terminal in a bulb and rarefy the air in the
same. This is done merely to enable the apparatus to perform the work
which it is not capable of performing at ordinary air pressure. In the
bulb we are able to intensify the action to any degree--so far that
the brush emits a powerful light.

The intensity of the light emitted depends principally on the
frequency and potential of the impulses, and on the electric density
of the surface of the electrode. It is of the greatest importance to
employ the smallest possible button, in order to push the density very
far. Under the violent impact of the molecules of the gas surrounding
it, the small electrode is of course brought to an extremely high
temperature, but around it is a mass of highly incandescent gas, a
flame photosphere, many hundred times the volume of the electrode.
With a diamond, carborundum or zirconia button the photosphere can be
as much as one thousand times the volume of the button. Without much
reflecting one would think that in pushing so far the incandescence of
the electrode it would be instantly volatilized. But after a careful
consideration he would find that, theoretically, it should not occur,
and in this fact--which, however, is experimentally demonstrated--lies
principally the future value of such a lamp.

At first, when the bombardment begins, most of the work is performed
on the surface of the button, but when a highly conducting photosphere
is formed the button is comparatively relieved. The higher the
incandescence of the photosphere the more it approaches in
conductivity to that of the electrode, and the more, therefore, the
solid and the gas form one conducting body. The consequence is that