pump--when the matter in the globe is in the ultra-radiant state, it
acts most perfectly. The shadow of the upper edge of the tube is then
sharply defined upon the bulb.

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.