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
rotate the fan, and it is likewise possible to stop the rotation by
connecting to the terminal a body of different size, thereby
diminishing the potential.
Instead of using the fan in this experiment, we may use the "electric"
radiometer with similar effect. But in this case it will be found that
the vanes will rotate only at high exhaustion or at ordinary
pressures; they will not rotate at moderate pressures, when the air is
highly conducting. This curious observation was made conjointly by
Professor Crookes and myself. I attribute the result to the high
conductivity of the air, the molecules of which then do not act as
independent carriers of electric charges, but act all together as a
single conducting body. In such case, of course, if there is any
repulsion at all of the molecules from the vanes, it must be very
small. It is possible, however, that the result is in part due to the
fact that the greater part of the discharge passes from the leading-in
wire through the highly conducting gas, instead of passing off from
the conducting vanes.
In trying the preceding experiment with the electric radiometer the
potential should not exceed a certain limit, as then the electrostatic
attraction between the vanes and the glass of the bulb may be so great
as to stop the rotation.
A most curious feature of alternate currents of high frequencies and
potentials is that they enable us to perform many experiments by the
use of one wire only. In many respects this feature is of great
interest.
In a type of alternate current motor invented by me some years ago I
produced rotation by inducing, by means of a single alternating
current passed through a motor circuit, in the mass or other circuits
of the motor, secondary currents, which, jointly with the primary or
inducing current, created a moving field of force. A simple but crude
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