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
their superfluous charge, and the tube is electrified, the
electrification instantly spreading over its surface. But this must
diminish the energy lost in the bombardment for two reasons: first,
the charge given up by the atoms spreads over a great area, and hence
the electric density at any point is small, and the atoms are repelled
with less energy than they would be if they would strike against a
good insulator: secondly, as the tube is electrified by the atoms
which first come in contact with it, the progress of the following
atoms against the tube is more or less checked by the repulsion which
the electrified tube must exert upon the similarly electrified atoms.
This repulsion may perhaps be sufficient to prevent a large portion of
the atoms from striking the tube, but at any rate it must diminish the
energy of their impact. It is clear that when the exhaustion is very
low, and the rarefied gas well conducting, neither of the above
effects can occur, and, on the other hand, the fewer the atoms, with
the greater freedom they move; in other words, the higher the degree