Fig. 17, has one of its ends connected to one of the terminals of the
induction coil, and the other to a metal plate, or, for the sake of
simplicity, a sphere, insulated in space. When the coil is set to
work, the potential of the sphere is alternated, and the small helix
now behaves as though its free end were connected to the other
terminal of the induction coil. If an iron rod be held within the
small helix it is quickly brought to a high temperature, indicating
the passage of a strong current through the helix. How does the
insulated sphere act in this case? It can be a condenser, storing and
returning the energy supplied to it, or it can be a mere sink of
energy, and the conditions of the experiment determine whether it is
more one or the other. The sphere being charged to a high potential,
it acts inductively upon the surrounding air, or whatever gaseous
medium there might be. The molecules, or atoms, which are near the
sphere are of course more attracted, and move through a greater
distance than the farther ones. When the nearest molecules strike the
sphere they are repelled, and collisions occur at all distances within
the inductive action of the sphere. It is now clear that, if the
potential be steady, but little loss of energy can be caused in this
way, for the molecules which are nearest to the sphere, having had an
additional charge imparted to them by contact, are not attracted until
they have parted, if not with all, at least with most of the
additional charge, which can be accomplished only after a great many
collisions. From the fact that with a steady potential there is but
little loss in dry air, one must come to such a conclusion. When the
potential of the sphere, instead of being steady, is alternating, the
conditions are entirely different. In this case a rhythmical
bombardment occurs, no matter whether the molecules after coming in
contact with the sphere lose the imparted charge or not; what is more,
if the charge is not lost, the impacts are only the more violent.