it is desirable that the vibration be harmonic, for any suddenness
hastens the process of destruction. An electrode lasts much longer
when kept at incandescence by currents, or impulses, obtained from a
high-frequency alternator, which rise and fall more or less
harmonically, than by impulses obtained from a disruptive discharge
coil. In the latter case there is no doubt that most of the damage is
done by the fundamental sudden discharges.

One of the elements of loss in such a lamp is the bombardment of the
globe. As the potential is very high, the molecules are projected with
great speed; they strike the glass, and usually excite a strong
phosphorescence. The effect produced is very pretty, but for
economical reasons it would be perhaps preferable to prevent, or at
least reduce to the minimum, the bombardment against the globe, as in
such case it is, as a rule, not the object to excite phosphorescence,
and as some loss of energy results from the bombardment. This loss in
the bulb is principally dependent on the potential of the impulses and
on the electric density on the surface of the electrode. In employing
very high frequencies the loss of energy by the bombardment is greatly
reduced, for, first, the potential needed to perform a given amount of
work is much smaller; and, secondly, by producing a highly conducting
photosphere around the electrode, the same result is obtained as
though the electrode were much larger, which is equivalent to a
smaller electric density. But be it by the diminution of the maximum
potential or of the density, the gain is effected in the same manner,
namely, by avoiding violent shocks, which strain the glass much beyond
its limit of elasticity. If the frequency could be brought high
enough, the loss due to the imperfect elasticity of the glass would be
entirely negligible. The loss due to bombardment of the globe may,
however, be reduced by using two electrodes instead of one. In such