At a somewhat lower degree of exhaustion, which is about the ordinary
"non-striking" vacuum, and generally as long as the matter moves
predominantly in straight lines, the screen still does well. In
elucidation of the preceding remark it is necessary to state that what
is a "non-striking" vacuum for a coil operated, as ordinarily, by
impulses, or currents, of low-frequency, is not, by far, so when the
coil is operated by currents of very high frequency. In such case the
discharge may pass with great freedom through the rarefied gas through
which a low-frequency discharge may not pass, even though the
potential be much higher. At ordinary atmospheric pressures just the
reverse rule holds good: the higher the frequency, the less the spark
discharge is able to jump between the terminals, especially if they
are knobs or spheres of some size.
Finally, at very low degrees of exhaustion, when the gas is well
conducting, the metal tube not only does not act as an electrostatic
screen, but even is a drawback, aiding to a considerable extent the
dissipation of the energy laterally from the leading-in wire. This, of
course, is to be expected. In this case, namely, the metal tube is in
good electrical connection with the leading-in wire, and most of the
bombardment is directed upon the tube. As long as the electrical
connection is not good, the conducting tube is always of some
advantage, for although it may not greatly economize energy, still it
protects the support of the refractory button, and is a means for
concentrating more energy upon the same.
To whatever extent the aluminium tube performs the function of a
screen, its usefulness is therefore limited to very high degrees of
exhaustion when it is insulated from the electrode--that is, when the
gas as a whole is non-conducting, and the molecules, or atoms, act as
independent carriers of electric charges.
In addition to acting as a more or less effective screen, in the true
meaning of the word, the conducting tube or coating may also act, by
reason of its conductivity, as a sort of equalizer or dampener of the
bombardment against the stem. To be explicit, I assume the action as
follows: Suppose a rhythmical bombardment to occur against the
conducting tube by reason of its imperfect action as a screen, it
certainly must happen that some molecules, or atoms, strike the tube
sooner than others. Those which come first in contact with it give up
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