of aluminium sheet, fastened to a needle and arranged to rotate freely
in a metal piece screwed to one of the terminals of the coil. When the
coil is set to work, the molecules of the air are rhythmically
attracted and repelled. As the force with which they are repelled is
greater than that with which they are attracted, it results that there
is a repulsion exerted on the surfaces of the fan. If the fan were
made simply of a metal sheet, the repulsion would be equal on the
opposite sides, and would produce no effect. But if one of the
opposing surfaces is screened, or if, generally speaking, the
bombardment on this side is weakened in some way or other, there
remains the repulsion exerted upon the other, and the fan is set in
rotation. The screening is best effected by fastening upon one of the
opposing sides of the fan insulated conducting coatings, or, if the
fan is made in the shape of an ordinary propeller screw, by fastening
on one side, and close to it, an insulated metal plate. The static
screen may, however, be omitted, and simply a thickness of insulating
material fastened to one of the sides of the fan.

To show the behavior of the coil, the fan may be placed upon the
terminal and it will readily rotate when the coil is operated by
currents of very high frequency. With a steady potential, of course,
and even with alternating currents of very low frequency, it would not
turn, because of the very slow exchange of air and, consequently,
smaller bombardment; but in the latter case it might turn if the
potential were excessive. With a pin wheel, quite the opposite rule
holds good; it rotates best with a steady potential, and the effort is
the smaller the higher the frequency. Now, it is very easy to adjust
the conditions so that the potential is normally not sufficient to
turn the fan, but that by connecting the other terminal of the coil
with an insulated body it rises to a much greater value, so as to