the residual atmosphere, and partly the atoms, molecules, or lumps
thrown off from the electrode. If the electrode is composed of bodies
of different character, and if one of these is more easily
disintegrated than the others, most of the electricity supplied is
carried off from that body, which is then brought to a higher
temperature than the others, and this the more, as upon an increase of
the temperature the body is still more easily disintegrated.
It seems to me quite probable that a similar process takes place in
the bulb even with a homogeneous electrode, and I think it to be the
principal cause of the disintegration. There is bound to be some
irregularity, even if the surface is highly polished, which, of
course, is impossible with most of the refractory bodies employed as
electrodes. Assume that a point of the electrode gets hotter,
instantly most of the discharge passes through that point, and a
minute patch is probably fused and evaporated. It is now possible that
in consequence of the violent disintegration the spot attacked sinks
in temperature, or that a counter force is created, as in an arc; at
any rate, the local tearing off meets with the limitations incident to
the experiment, whereupon the same process occurs on another place. To
the eye the electrode appears uniformly brilliant, but there are upon
it points constantly shifting and wandering around, of a temperature
far above the mean, and this materially hastens the process of
deterioration. That some such thing occurs, at least when the
electrode is at a lower temperature, sufficient experimental evidence
can be obtained in the following manner: Exhaust a bulb to a very high
degree, so that with a fairly high potential the discharge cannot
pass--that is, not a _luminous_ one, for a weak invisible discharge
occurs always, in all probability. Now raise slowly and carefully the
potential, leaving the primary current on no more than for an instant.
At a certain point, two, three, or half a dozen phosphorescent spots
will appear on the globe. These places of the glass are evidently more
violently bombarded than others, this being due to the unevenly
distributed electric density, necessitated, of course, by sharp
projections, or, generally speaking, irregularities of the electrode.
But the luminous patches are constantly changing in position, which is
especially well observable if one manages to produce very few, and
this indicates that the configuration of the electrode is rapidly
changing.
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