The heating of a conducting body inclosed in a bulb, and connected to
a source of rapidly alternating electric impulses, is dependent on so
many things of a different nature, that it would be difficult to give
a generally applicable rule under which the maximum heating occurs. As
regards the size of the vessel, I have lately found that at ordinary
or only slightly differing atmospheric pressures, when air is a good
insulator, and hence practically the same amount of energy by a
certain potential and frequency is given off from the body, whether
the bulb be small or large, the body is brought to a higher
temperature if inclosed in a small bulb, because of the better
confinement of heat in this case.

At lower pressures, when air becomes more or less conducting, or if
the air be sufficiently warmed as to become conducting, the body is
rendered more intensely incandescent in a large bulb, obviously
because, under otherwise equal conditions of test, more energy may be
given off from the body when the bulb is large.

At very high degrees of exhaustion, when the matter in the bulb
becomes "radiant," a large bulb has still an advantage, but a
comparatively slight one, over the small bulb.

Finally, at excessively high degrees of exhaustion, which cannot be
reached except by the employment of special means, there seems to be,
beyond a certain and rather small size of vessel, no perceptible
difference in the heating.

These observations were the result of a number of experiments, of
which one, showing the effect of the size of the bulb at a high degree